Patent Application
20240326119
The invention relates to photopolymerizable slurries, methods for manufacturing such photopolymerizable slurries, methods for manufacturing sintered ceramic articles with such photopolymerizable slurries, and a method for casting articles.
Shaping of matter into complex structures has been greatly advanced through the advent of additive manufacturing techniques. A crucial development in this field has been the design and formulation of resins or inks with a broad variety of material compositions [11, 24, 35, 36]. This has turned 3D printing from a prototyping tool into a manufacturing platform for functional objects and devices in health [37], energy [38], architecture [39], and robotic applications [40]. Using inks with self-assembling building blocks has expanded the capabilities of 3D printing techniques to the fabrication of structures with intricate hierarchical architectures and feature sizes below accessible printer resolution [11, 41, 42, 43, 44]. Despite these enticing achievements and promising prospects, many materials are still not directly 3D printable due to challenges in fulfilling the rheological, physical and chemical requirements of extrusion- and light-based printing techniques [45]. In addition, the lack of reproducibility and reliability of additive processing has made it difficult to produce parts for critical applications such as aerospace and medical, where certification of microstructure and part integrity is paramount.
The manufacturing of structures with controlled chemical composition, complex shape and porosity is crucial for many applications in medicine, engineering, and architecture. Because shaping technologies are not always available for the materials/chemical compositions of interest, it is sometimes convenient to manufacture templating molds into which materials with desired chemistry can be cast. Additive manufacturing has been extensively employed to fabricate such complex-shaped molds. However, removal of the mold after casting might be challenging or impossible when the final articles feature complex shapes or internal porosity.
3D printed sacrificial molds and templates offer a universal approach to shape matter into complex architectures without rheological or chemical modification of the material of interest. The idea of such indirect additive manufacturing is to first print the sacrificial template into the desired negative geometry and then cast or deposit the material of interest before final removal of the template [22, 46-48,49-52]. This manufacturing strategy has enabled the fabrication of intricate structures from a wide range of materials at very different length scales. At the centimeter scale, binder-jetting printed sand or stereographically printed polymer molds have been utilized to fabricate complex metal parts by investment casting [22,52] and elastomer parts by casting silicone resins, respectively [48,50]. At the millimeter scale and below, two-photon polymerization has been exploited to print three-dimensional polymer templates for the creation of metal and ceramic micro-lattices with exquisite architectures and mechanical properties [53,54]. In many of these processes, the template material needs to be dissolved with chemicals or thermally degraded at temperatures above 400° C. [55,56]. This often makes the process energy demanding and/or increases the probability of cracking or deformation of the templated material.
Porosity is an essential feature in a wide range of applications that combine light weight with high surface area and tunable density. Porosity is desired in a broad range of materials for applications such as catalytic supports and lightweight structures. Particularly, porous materials are of high interest in the biomedical field, especially for their use in tissue engineering and bioresorbable implants, such as implants for osteosynthesis.
One approach to manufacture porous casts in shown in CN 110407604. Gypsum is cast into a sponge, and after solidification of the gypsum cast the sponge is burned out, resulting in a lost mold that can then be used to cast a porous article. Similar methods are known from U.S. Pat. Nos. 3,616,841 and 3,946,039. Slurries used in said methods for producing the ceramic lost mold do not comprise a binding agent.
Highly porous materials can be obtained by several approaches such as salt leaching, gas foaming, freeze-drying and sintering, and phase separation [1]. A cost-effective and sustainable strategy to circumvent the above-mentioned processing issues is to use readily soluble inorganic salts such as sodium chloride (NaCl), as molds or templates. Because NaCl is non-toxic and readily dissolvable in water at ambient temperature, salt templates do not require excessive heating or toxic solvents to be removed and are broadly studied for biomedical applications. These features also make them suitable templates for shaping temperature-sensitive materials, such as living materials, hydrogels and polymers. Alternatively, the physical and mechanical stability of NaCl up to its melting temperature of 801° C. enables infiltration of materials at elevated temperature in the molten state. Such versatility has allowed for the use of salt particles as pore-forming templates for the fabrication of a broad range of materials, from aluminum foams for structural applications [6] to porous hydrogels for tissue engineering [57,58]. With the salt leaching technique, porous materials based on all material classes, i.e. metals, ceramics or polymers, can be obtained [2]. Typically, salt particles are used as leachable powder to produce a compacted preform, which is then infiltrated with the material of interest. Upon solidification of the infiltrate, the salt is removed by leaching in a suitable solvent, and the solidified, infiltrated material remains. The salt leaching technique has been used to achieve porosity in a wide variety of materials, including natural polymers such as silk fibroin [3], synthetic polymers such as poly(l-lactic acid) [4], and bulk metallic glasses [5], and crystalline metals such as aluminum [6] and magnesium [7]. In all these approaches, the pore size of the final part is defined by the size of the original salt particles, or the salt particle aggregates used as the template. The pore geometry of the resulting material is random, with broad pore-size distributions, which reflects the polydisperse nature of the salt particle template matrix. This limits this technique's ability to control the porous architecture and achievable overall porosity of the final scaffold.
Beyond templating particles, salt has also been deposited around 3D printed polymeric templates to create castable molds upon polymer removal [22,52]. The direct printing of NaCl into three-dimensional grid-like structures for the fabrication of magnesium with unique porous architectures has been described [22]. However, 3D printing of NaCl for extrusion-based direct ink writing is limited in both the attainable resolution and the freedom of architectural design. Novel approaches to manufacture complex-shaped salt templates are highly demanded.
Additive manufacturing allows to manufacture complex three-dimensional shapes and is also used in the manufacturing of porous materials, where it allows tuning of the pore geometries and sizes. Additive manufacturing techniques allow to produce three-dimensional structures with well-controlled porosity and pore sizes from nanoscale to macroscale [8, 9, 10, 11, 12]. Various additive manufacturing techniques such as selective laser sintering (SLS), fused deposition modeling (FDM), selective laser melting (SLM) or stereolithography (SL) have been applied. SLS and FDM have been mainly applied for polymeric materials and as a combination with ceramics, e.g. polycaprolactone (PCL), polylactic acid (PLA), hydroxyapatite (HA), and calcium phosphate (CaP). SLM has been used for the fabrication of metal parts from metal powder, e.g. Mg. Moreover, porous parts from PCL and CaP were also achieved with stereolithography printing. [13,14,15]
While there is a broad choice of materials available for additive manufacturing, it is difficult to find materials that are both suitable for common AM techniques and result in a product having a mechanical strength suitable for the end application (e.g. as bone replacement) and still being biodegradable. It also remains a challenge to shape materials that possess a high chemical reactivity, such as for example magnesium, using additive manufacturing techniques.
Magnesium (Mg) is receiving increasing attention as a metallic biodegradable implant material for temporary bone replacement or osteosynthesis [16]. Magnesium has similar mechanical properties as bone and is able to induce new bone formation [17] and is bioresorbable [18]. It is widely accepted that pore size, pore shape, pore directionality, and the degree of porosity of Mg scaffolds are factors that strongly influence cell viability and growth. To guide bone-tissue growth, large open porosity with pore sizes >300 μm in combination with surface roughness appears to be most successful. [19, 20] Thus, the ability to shape the magnesium into structures with controlled porosity and pore size, and in a patient-specific outer geometry, is highly desired.
A three-step process has been reported for indirect additive manufacturing of metallic magnesium [21, 22]. A 3D polymer structure is produced via AM. The resulting polymer structure is then infiltrated with a NaCl paste. Upon removal of the polymer structure, the NaCl acts as a template for Mg infiltration. In this technique, the additional processing step required to generate the polymer template increases the processing time and results in enhanced risk of imperfect structure replication. Furthermore, the technique is limited to geometries that allow the necessary infiltration of the highly viscous NaCl paste into the polymer template.
WO 2020/046687 A1 discloses a method of making a non-oxide ceramic part. A photopolymerizable slurry, containing non-oxide ceramic particles with high melting point such a silicon carbide or boron nitride, is selectively cured to obtain a gelled article. The gelled article is dried to form an aerogel article or a xerogel article. The aerogel article or xerogel article is heat treated to form a porous ceramic article, which is then sintered to obtain a sintered ceramic article.
N. Kleger et al. [23] describe a method for directly 3D printing a NaCl template for Mg infiltration, using an ink consisting of NaCl particles dispersed in paraffin oil. In this method, a three-dimensional structure with controlled pore geometry can be printed via the direct ink writing technique. After removal of the paraffin oil, the resulting green body is calcined and sintered to a NaCl template. The template is infiltrated with melted magnesium. After leaching the NaCl template structure, the solid magnesium scaffold remains. The spatial resolution of the disclosed method is limited. Moreover, mechanical stability issues of the green body limit the available geometries.
Manufacturing processes used for the fabrication of inorganic materials often involve the addition to organic binders to the inorganic particles of interest and the formation of a green body that is later subjected to debinding and sintering at high temperatures. A common problem when debinding green bodies consisting of organic binder and ceramic particles is crack formation. While debinding is thought to be a common cause of cracking, polymerization shrinkage, low particle concentrations and the presence of unreacted monomer can also lead to the formation of cracks during the manufacturing process [24, 25, 26, 27]. No general solution has been found so far to avoid such crack formation.
When such leachable structures are intended to be used as templates, the infiltration of the templates with the material of interest is an important step. There exists a variety of techniques for infiltration, such as pressure and vacuum infiltration, chemical vapor infiltration, and injection molding. Infiltration processes are also commonly used to fabricate composite materials.
For the production of metal matrix composites (MMC), mostly vacuum infiltration is used. Typically, a porous preform and the metal to be infiltrated are first evacuated in an oven before the metal melts. Then, pressurized, inert gas is filled into the oven, forcing the molten metal into the preform. The surface tension of the metal and its wettability on the preform material play an important role for the quality of the final product.
Chemical vapor infiltration works similar to the aforementioned infiltration, however the infiltrate is a reactant gas flowing through the preform (mixture of carrier gas along with the matrix material of interest). This is achieved through diffusion or a pressure difference. The matrix material reacts chemically with the preform and hence results in a composite material.
Injection molding is most popular for the processing of polymeric material but can also be used for metals and ceramics. Typically, the molten cast material is injected under high pressure into a heated mold. After cooling, the molded part is removed from the mold. The mold contains small air vents, placed at appropriate places in the mold wall, which allow the air to escape when the material is injected. Injection molding is not an infiltration technique per se but can be used as such. Vacuum pressure infiltration as well as injection molding are both techniques that can be used for the infiltration of sintered ceramic articles.
There is a general need for improvements in this field.
It is the overall objective of the present invention to provide improvements in manufacturing cast objects with template molds.
One object of the present invention is to provide advantageous photopolymerizable slurries that allow to manufacture molds made of soluble inorganic salts, with higher spatial resolution and/or increased mechanical stability.
Another object of the invention is to provide advantageous methods for manufacturing such photopolymerizable slurries, methods for manufacturing sintered ceramic articles made of soluble inorganic salts, as well as methods for manufacturing cast articles.
These and other objects are substantially achieved through the features of the independent claims. Advantageous embodiments and variants are set forth in the dependent claims.
Further aspects of the present invention become evident as this description proceeds.
The present invention may comprise one or more of the features recited in the attached claims, and/or one or more of the following features and combinations thereof.
One aspect of the invention concerns a photopolymerizable slurry.
A photopolymerizable slurry according to the invention comprises a plurality of particles of an inorganic salt and at least one polymerizable monomer or oligomer. The cation of the inorganic salt is a metal cation. The anion of the inorganic salt is neither oxide nor hydroxide. The inorganic salt has a melting point of above 250° C. at atmospheric pressure and has a solubility in water above 9% w/w at room temperature.
Metal oxides and metal hydroxides are not suitable for a photopolymerizable slurry according to the invention, particularly due to their highly caustic properties, which can negatively affect the intended use of the photopolymerizable slurry, for example the polymerization reaction.
The term “polymerizable monomer or oligomer” shall comprise molecules that can react together with other monomer or oligomer molecules to form a larger polymer.
The term “photopolymerizable” shall refer to the ability of a chemical composition to a polymerization reaction that requires actinic radiation (e.g. UV or visible light) for the propagation step.
As used herein, the term “polymerizable slurry” means a mixture of solid material suspended or dispersed in a liquid, of which composition at least one component can undergo polymerization upon initiation, e.g. free-radical polymerization initiation. As a result, the mixture undergoes gelation. Typically, prior to gelation, the polymerizable slurry has a viscosity profile consistent with the requirements and parameters for the additive manufacturing method it is intended to be used for (e.g. 3D printing). Irradiation with actinic radiation having sufficient energy to initiate a polymerization or cross-linking reaction, for instance ultraviolet (UV) radiation or electron beam radiation, or both, can be used for this purpose. By exposing the photopolymerizable slurry to a suitable light source, the radiation-curable monomer or oligomer can be polymerized. The resulting polymer matrix acts as a binder for the inorganic salt particles, resulting in a green body.
The term “inorganic salt” shall comprise salts consisting of inorganic ions, including carbonate and hydrogen carbonate ions.
Advantageously, in a photopolymerizable slurry according to the invention, the anion of the inorganic salt is selected from a group consisting of bromide, chloride, fluoride, iodide, sulfate, nitrate, nitrite, carbonate, and cyanide.
Advantageously, in a photopolymerizable slurry according to the invention, the metal cation of the inorganic salt is selected from a group consisting of barium, beryllium, cadmium, calcium, cesium, cobalt, copper, iron, lead, lithium, magnesium, manganese, nickel, potassium, rubidium, silver, sodium, and zinc.
A melting point above 250° C. of the inorganic salt allows a sufficient increase in temperature to remove the polymer binder and other carbonaceous compounds by pyrolysis, without melting the particles and destroying the binderless body.
For an efficient removal of a template mold after a casting process by dissolving the mold in water or an aqueous solution, the inorganic salt used for producing the mold should have a sufficiently high solubility, above 9% w/w in water.
Inorganic salts that can be used for such a photopolymerizable slurries, and that have a melting point of above 250° C. at atmospheric pressure and a solubility in water above 9% w/w at room temperature are listed Table 1.
It is also possible to use mixtures of inorganic salts. Interactions of different salt compounds, particularly interactions of particles of different salts, can influence the local and/or the overall behavior of the salt particles, particularly the melting temperatures, and the sintering behavior.
Advantageously, the used inorganic salt consists of ions that are physiologically acceptable. For example, sodium, potassium, calcium, and magnesium can be used as cations, and chloride and carbonate can be used as anions. Sodium chloride (NaCl) and potassium chloride (KCl) are particular advantageous choices for the inorganic salt in view of physiological safety, melting temperature, solubility in water without the need of organic solvents, costs, biocompatibility, high thermal and chemical stability, and applicability to a vast range of scaffold materials.
The polymerization reaction of the monomer in the photopolymerizable slurry can be initiated by irradiation with suitable actinic radiation.
Advantageously, the photopolymerizable slurry can comprise a photoinitiator.
A photoinitiator is a molecule that creates reactive species (free radicals, cations or anions) when exposed to actinic radiation (UV or visible light). The photoinitiator must be suitable for the applied system of polymerizable monomer or oligomer,
By irradiation of a certain volume of the photopolymerizable slurry with light, the polymerization reaction in this volume is initiated, and the photopolymerizable slurry in this volume solidifies.
In an advantageous variant of a photopolymerizable slurry according to the invention, the inorganic salt particles are coated or functionalized with a dispersant.
This surface coating or functionalization of the inorganic salt particles is advantageous in regard to constant and reproducible properties of the slurry and the green body produced from it.
An advantageous variant of a photopolymerizable slurry according to the invention further comprises an inhibitor.
The inhibitor can for example be a UV blocker, a compound that absorbs UV light. This is advantageous in regard to increased spatial resolution during the additive manufacturing process.
The inhibitor can be a chemical additive that inhibits or retard the degradation (oxidation, thermal degradation, etc.) of the polymerizable slurry or the solidified green body. This is advantageous e.g. in regard shelf life, mechanical stability of the polymer matrix during debindering, etc.
An advantageous variant of a photopolymerizable slurry according to the invention further comprises a diluent.
A diluent can be used to adjust the viscosity of a photopolymerizable slurry.
A diluent can also be used to improve the mechanical stability of the green body or the binderless body.
At room temperature, the diluent can be a liquid, or a solid compound that is dissolved in another component of the photopolymerizable slurry, e.g. in a liquid monomer.
Advantageously, camphor is used as the diluent.
An advantageous variant of a photopolymerizable slurry according to the invention further comprises a sintering aid.
Advantageously, the sintering aid improves the sintering step. For example, Na2SO4 can be used to decrease the melting point of NaCl used as the inorganic salt.
Another aspect of the invention concerns a method for manufacturing a photopolymerizable slurry.
A method for manufacturing a photopolymerizable slurry according to the invention comprises the steps:
In an advantageous variant of such a manufacturing method according to the invention, the inorganic salt particles are produced by milling an amount of inorganic salt together with a dispersant in a liquid phase, thereby obtaining a dispersion of inorganic salt particles coated or functionalized with the dispersant; and removing the liquid phase.
The milling may for example be carried out in a suitable ball mill.
The liquid phase can be removed for example by evaporation of the liquid phase, or by spray drying.
In an advantageous variant of such a manufacturing method according to the invention, a photo initiator, and/or an inhibitor, and/or a diluent, and/or a sintering aid are provided.
More advantageously, the provided photo initiator, inhibitor, diluent, or sintering aid are in a liquid state, or are solids that are dissolved in the monomer or oligomer, obtaining a liquid composition.
A further aspect of the invention concerns a method for manufacturing a sintered ceramic article.
A method for manufacturing a sintered ceramic article according to the invention comprises the steps:
The term “binderless body” designates a green body after debinding, namely the removal of the binder, i.e. the polymer matrix, e.g. by pyrolysis. The removal of the binder matrix can be incomplete. The rest of the binder will burn off during the sintering step.
The remaining polymer can help to connect the inorganic salt particles until in the subsequent sintering step the individual particles are sintered together. Thus, a binderless body may comprise less than 10%, advantageously less than 5%, even more advantageously less than 2% of the original weight of the polymer in the green body.
In an advantageous variant of such a manufacturing method according to the invention, the selective curing of the photopolymerizable slurry to obtain a green body article is carried out within an additive manufacturing process.
Advantageous additive manufacturing methods that can be applied for the additive manufacturing process in such a manufacturing method are for example stereolithography (SLA) 3D printing, 2-photon polymerization (2PP), or digital light projection (DLP) 3D printing.
Another aspect of the invention concerns a green body article as it can be obtained after step b) of a method for manufacturing a sintered ceramic article according to the invention.
Another aspect of the invention concerns a binderless body article, as it can be obtained after step c) of a method for manufacturing a sintered ceramic article according to the invention.
Another aspect of the invention concerns a sintered ceramic article, manufactured with a method for manufacturing a sintered ceramic article according to the invention.
A further aspect of the invention concerns a method for casting articles.
A method for manufacturing cast articles according to the invention comprises
Since the first template mold is destroyed in the separation step, the first template mold is a so-called lost mold.
A particularly advantageous variant of such a method for manufacturing cast articles comprises the steps:
In an advantageous variant of such a manufacturing method according to the invention, the solid article and the first template mold are removed from the second mold before dissolving the sintered ceramic article of the first template mold.
In another advantageous variant of such a manufacturing method according to the invention, the solid article is positively locked in the first template mold.
In a further advantageous variant of such a manufacturing method according to the invention, the second mold is a permanent mold or a lost mold.
In yet another variant of such a manufacturing method according to the invention, a metallic material or a polymer material or a ceramic material or a composite material is used as the casting material.
Another aspect of the invention concerns cast solid articles, manufactured with a method according to the invention as discussed above.
Such a cast article can be a tissue scaffold and/or a medical implant and/or a medical device.
In order to facilitate a fuller understanding of the present invention, reference is now made to the appended drawings and figures. These references should not be construed as limiting the present invention and are intended to be exemplary only.
Components that are identical, or that are identical at least in terms of their function, are designated below by identical or similar reference numbers.
A process to prepare a salt-based slurry for photopolymerization is described.
The obtained slurry is gelled by radiation curing, particularly in a DLP stereolithography printing step, and a green body is obtained. Non-gelled slurry is further removed, such as by rinsing and/or sonication in a solvent with low salt solubility. The cleaned article is further heat treated, so that the polymer matrix of the green body is pyrolyzed, and a binderless object consisting essentially of inorganic salt particles is obtained. The binderless article is then sintered to obtain a mechanically stable mold with a dense surface. During sintering potential residuals of the binder are removed. The sintered mold is used as a template for infiltration with a fluid material such as a powder, liquid or molten material, which can be further solidified by temperature, light or chemical reaction. Alternatively, the sintered article can also be used as a core for dip coating, spray coating or wrapping. Finally, the sintered mold is dissolved by an appropriate polar solvent, preferably an aqueous solution, and the final cast product is obtained.
The following compounds have been used for the tests and experiments described in this description:
Additive manufacturing (AM, also known under the synonymous term 3D printing) is a manufacturing technique where an object is built up layer by layer in an additive manner. It enables the production of highly complex structures, which cannot always be achieved with conventional manufacturing techniques. Various additive manufacturing techniques have been developed over the past years, such as direct ink writing (DIW), selective laser melting (SLM), and stereolithography (SL). Stereolithography is the method that is advantageously applied for the given invention.
To produce an object with additive manufacturing, a three-dimensional model of the object, for example in the form of computer-aided design (CAD) file, is sliced into a series of 2D cross-sectional layers before it can be printed. In stereolithography, each of these thin layers of photocurable resin will be cured consecutively on top of each other. The resin usually consists of photosensitive monomers such as acrylates or epoxides, which will polymerize upon illumination [27, 28].
Stereolithography can be divided into two classes, with the main difference being how the actinic radiation (typically UV light) is applied. Either selected parts of the whole layer are cured at once using a light mask, or a laser beam scans over the surface, only polymerizing small volumes, so called strands, one by one. These two methods are called projection-based stereolithography (PSL), also known as direct light processing (DLP), and scanning-based stereolithography (SSL), respectively. SSL cannot reach a resolution as high as PSL because the scanning laser beam has an under width limit. Moreover, PSL is less time consuming because each layer is solidified in one step. However, SSL has higher radiation intensities and is advantageous for larger objects, although some of the resolution is lost.
A stereolithography apparatus (SLA), generally comprises five parts: resin tank, recoater, print platform, optics, and control systems. The recoater is mainly needed for high viscosity resins because they do not flow back and level in the tank. The printing platform is coupled to an elevator which controls the up- and downward movements of the platform while printing. The light source is usually a laser (Diode, He—Cd, Ar) coupled to an acoustic optical modulator to quickly turn on and off the laser. For direct light processing, usually a LED-LCD system is used as the light source.
In this description, a stereolithography apparatus will alternatively be named as stereolithography printer or 3D printer.
The photopolymerizable resin used for stereolithography will alternatively be named as photopolymerizable slurry, photocurable resin, printing resin, or 3D printing ink.
There are two possible setups for a stereolithography printer, namely top-down or bottom-up.
For the top-down orientation, the light source is placed on top, and the printing platform is inserted in the tank just below the resin surface. After each exposure, the platform moves downward by one layer height. The resulting objects are thus printed in upward direction. This approach has some disadvantages such as the need for large amount of resin and the extensive time for the resin to equilibrate again after the stage moved downward. Further, it is very challenging to control the layer thickness because only gravitational forces act on the fresh layer of resin on top of the already cured one. The use of the recoater blade is crucial in mitigating this problem as well as the use of a rather low-viscosity resin.
In contrast, the bottom-up approach circumvents most of these issues. The light source is placed on the bottom and the print head is placed on top. The printing platform is moved to the bottom of the resin bath where a thin layer of resin is polymerized on the platform surface through a transparent, anti-adhesive window in the bottom of the tank. After illumination, the tank is tilted in order to detach the cured layer from the tank and to allow the resin to flow back and form a fresh layer on the tank bottom. This allows for the use of considerably less resin compared to the top-down approach. After each layer, the print head is moved upwards by one layer height to cure the next layer. The resulting objects are hanging from the print head upside down. Another advantage compared to the top-down setup is the avoidance of a height limit of the object due to the tank size. The layer thickness can be controlled better, because it depends on the platform elevator and not on the fluid properties of the resin, which leads to higher vertical resolution as well as better surface quality of the object. However, the major drawback of the bottom-up approach is the requirement for an anti-adhesive and transparent bottom surface of the tank. Coatings such as Teflon or silicone are needed to easily detach the cured resin from the bottom, while it continues to adhere to the print platform.
In order to print complex geometries with high spatial resolution, it is crucial to understand and analyze the influence of the fundamental parameters in stereolithographic printing that determine the width and depth of the exposed layer and how they depend on the applied energy dose. There are two groups of parameters, a first group related to the printing process such as the laser power, layer thickness and hatch spacing, and a second group determined by the resin, including penetration depth and critical energy dose.
During printing, the photoinitiator turns into a radical upon irradiation with a suitable light source and hence initiates a free radical polymerization reaction of the monomer. This reaction continues as long as the resin is illuminated and will lead to a soft solid once the gelation point of the resin is reached. The intensity of the light decreases in z direction and xy direction of the resin away from the focal point on the surface. The gelation point can only be reached if the energy dose at that depth D(z) is above a critical energy dose (Dc). The polymerized thickness zp, also called cure depth, is the depth z at which the energy dose is just sufficiently high that the resin composition reaches the gelation point. The absorption of the energy of the light beam in the resin follows a modified version of the Beer-Lambert law:
z
p
=h
a ln(D/Dc), (1)
where D is the applied energy dose, zp is the resulting cure depth, ha is the penetration depth (the cure depth when D=Dc), and Dc the critical energy dose.
The parameters Dc and ha depend on the resin composition and the spectrum of the applied actinic radiation and should be determined before printing, to assess the optimal printing parameters for a certain composition. Similarly, photoinitiator and UV blocker concentrations can be optimized for a given energy dose and a desired layer thickness.
The use of a UV blocker in a photocurable resin allows to reduce the penetration depth ha and xy resolution for a given critical energy dose Dc, since part of the actinic radiation is absorbed by the UV blocker when passing the resin. The decreased penetration depth ha leads to a decreased production speed, since either the illumination time has to be increased to reach the same polymerized thickness or more layers have to be produced. However, the decreased penetration depth also reduces the sensitivity of the cure depth towards slight changes of the light intensity. Such changes can occur for example due to clouding of the non-adhesive film of the resin tray or due to slight variations in the illumination time, changing illumination intensity and time, respectively. Therefore, an intermediate UV blocker concentration is often desired in the resin to accelerate the printing process without making the resin too sensitivity to unavoidable variations in illumination intensity.
The monomer conversion rate depends on the depth z (distance from illuminated surface), as predicted from equation (1). The degree of polymerization is largest at the surface and decreases exponentially thereafter. This irregularity within each layer can cause problems in further processing steps due to shrinkage during polymerization. As a result, mechanical stress is present and deformations appear, which may lead to crack formation of the objects. This defect is known as print-through. Furthermore, there needs to be some overcure, meaning that each strand should slightly penetrate the adjacent layers in order to avoid layer delamination, i.e. the separation of adjacent printed layers. It is advantageous to overcure 10%-35% in z height to avoid common printing defects such as delamination or print-through.
For a transparent resin, a cured line of the resin has the same shape as the intensity distribution of the light source, which corresponds to a Gaussian curve. However, if the resin contains inert particles, as in the photopolymerizable slurries according to this invention, the profile of the cured line is altered significantly due to scattering of the light beam. This leads to a broadening of the beam, especially for large differences in the refractive index of the particles and the monomeric phase [27,29]. Hence, a lower resolution is obtained. This effect can be mitigated for example by matching the refractive indices of the particles and the monomer, reducing the solid loading and adjusting the size distribution of the particles.
Method: The additive manufacturing of complex-shaped objects using salt templates involves a series of processing steps that includes light-based printing (i.e. vat photopolymerization), debinding, sintering, infiltration and leaching processes (
In the context of the invention, 3D printing of the structures was carried out with a commercial bottom-up direct light processing (DLP) device (Original Prusa SL1, Prusa Research a.s.). The objects were designed in Solidworks (Solidworks 2020-2021, Dassault Systemes SolidWorks Corporation) and further edited in the original slicer software of the 3D printer (PrusaSlicer Version 2.2.0, Prusa Research a.s.).
Before printing, the printing resin was always vortexed until it was well homogenized (vortex-genie 2, Scientific Industries, Inc.). Alternatively, the printing resin can be homogenized in a mixer (Thinky Mixer ARE-250, Thinky cooperation).
The 3D printer was calibrated prior to every use and the illumination time was set to 40 s for the first four layers and to 22.5 s for the consecutive layers (for resins with 0.125 wt % Sudan I and 3 wt % BAPO. Alternatively, a minimally required exposure time to reach 80 μm cure depth was calculated from the resulting light penetration depth (ha) and critical energy dose (Dc) for each ink individually. In order to increase the adhesion at the printhead, the initial layers were exposed for 30-50 s with 10 fading layers to reach the desired illumination time. The layer thickness was set to 50 μm.
The required illumination time heavily depends on the slurry composition such as type, concentration and reactivity of monomer, initiator and inhibitor. The illumination time can be calculated from the working curve measured from an exposure test of the slurry according to a pre-established protocol [30].
The printing was performed at room temperature. However, slight heating may be applied to further decrease the viscosity facilitating the printing process.
Particles of commercially inorganic salts, such as for example regular NaCl particles, are generally not suitable to be directly incorporated into a 3D printing ink, because the particle size exceeds the printing layer height, which is usually between 25 μm and 100 μm. Therefore, the particle size needs to be decreased.
In ceramic stereolithography (CSL), it is common to use particles with a size range of 0.05 μm to 10 μm. Such particle size ranges can be readily obtained by planetary ball milling. The advantage of ball milling over a direct synthesis of particles with the required size is the high efficiency of ball milling, and the limited possibilities for synthesizing salt particles with a certain, small particle size.
Typically, ceramic milling balls are used to produce enough kinetic energy to break the particles into smaller ones. The size of the milling balls has a large influence on the resulting particle size and particle size distribution. Finding the optimum diameter of milling balls is a compromise between the kinetic energy available for breaking the particles and the number of contact points of milling balls where particles can be broken. For a given weight of milling balls, the number of milling balls n increases with decreasing ball diameter d, with the relation n∂d−3. The number of contact points between the milling balls can be understood as the coordination number N arising from studies about dense packing of spheres. Since the coordination number is independent of the sphere diameter, the number of milling balls for a fixed weight of milling balls can be assumed to be proportional n∝N. This results in the combined relation n∝N∝d−3. The kinetic energy depends on the mass of the milling balls as well as the rotation speed of the ball mill. It is advantageous to use a mix of various milling ball sizes, in order to get μm sized particles with a narrow particle size distribution. Nevertheless, all the milling parameters have to be considered to find the optimal ball size variation for the desired particle size and particle size distribution. For the use of such particles in a resin, a narrow size distribution is favorable for high spatial resolution and a fine and smooth surface of the ceramic in the end.
In order to obtain a stable suspension and keep the particles from sedimenting and agglomerating, a surfactant or dispersant, respectively, can be used to modify the surface of the particles and to stabilize them in the liquid components of a photopolymerizable slurry. The surfactant can either be added after a (dry) milling process, or before milling. The latter case is more advantageous, since it reduces one step in the process.
In the context of this description, the terms “surfactant” and “dispersant” will be used interchangeably.
In one advantageous approach for producing homogeneous, fine-grained NaCl particles for use in a polymerizable slurry that can be used as a printing resin for 3D printing, the NaCl particles are reduced in size and are functionalized with the surfactant dioctyl sulfosuccinate sodium salt (AOT). In a typical process, the NaCl and the AOT are dispersed in isopropanol and added to an alumina milling jar. The dispersion is wet milled with zirconia mixing balls in four different sizes for 2 hours at 200 rpm with 5 min milling and 5 min break (planetary ball mill PM100, Retsch GmbH). The diameters of the mixing balls are 9.7 mm, 7.6 mm, 4.9 mm, and 2.4 mm.
For a batch of 200 g NaCl, the total mass of the added mixing balls is 575 g. The obtained slurry of AOT-functionalized NaCl particles in isopropanol is dried overnight in the oven at 60° C. For the de-agglomeration of the dried, functionalized NaCl particles, the obtained powder is dry ball milled with half of the zirconia mixing balls used for the wet milling process (5 min, 200 rpm).
Alternatively, the NaCl particles may also be produced by precipitation or spray drying [31].
A salt particle loaded, photocurable resin used for stereolithography typically consists of salt particles, a continuous monomeric phase, a photoinitiator, a UV blocker, and a dispersant. The dispersant is used to prevent the particles from agglomerating and keeping them dispersed and stable in the resin. Additionally, a diluent can be added to decrease the viscosity for easier printing [13, 24, 32].
There are two main groups of photocurable resins, namely aqueous and non-aqueous resins. Currently, mostly non-aqueous systems are used owing to several advantages such as higher strength of the green body and a lower difference between the refractive indices Δn of the particles and the continuous liquid phase. Typically, the liquid phase mainly consists of epoxy or acrylate monomer as suitable polymer building blocks. For good printability of a particle-loaded resin with DLP stereolithography, it is important that the solid loading (i.e. the particle concentration) is sufficiently high while the viscosity of the resin is kept low. High solid loading is advantageous due to enhanced form and structure stability. High solid loading decreases crack formation, geometrical deformation, pore formation, and general shrinkage during sintering. Further, it is required to have a printing resin with a sufficiently small yield point, because the resin needs to flow back below the printing platform, in order to cure the subsequent layer. Additionally, a diluent can help to reduce the viscosity due to high solid loading. It is possible to add as much as 40 wt % diluent, this upper limit being given by the requirement to still have enough of the monomeric phase to ensure sufficient mechanical stability of the printed part and limited shrinkage.
A key requirement of the process is to produce a dense, crack-free salt template at the end of the sintering step. Cracking is a common challenge in ceramic printing, and may result from internal mechanical stresses developed during shrinkage of the printed object upon calcination and sintering. To reduce shrinkage and minimize cracking, the concentration of salt particles in the initial resin should be maximized without impairing the rheological properties needed for the stereolithographic printing process. More specifically, in terms of rheological behavior, the ink must be sufficiently fluid to replenish the print stage under the action of gravity. Previous studies have shown that this flowability is achieved if at a shear rate of 30 s−1 the yield stress and apparent viscosity of the ink are kept below 10 Pa and 60 Pa·s, respectively [26, 59].
In
where F the force acting on the surface area A. ρresin, mresin and Vresin are the density, mass and volume of the resin, respectively, g is the gravitational acceleration and σflow is the flow stress. Technically, all flow points from the formulated resins are well below 17.2 Pa as depicted in FIG. 1(b). By fitting Equation (2) to the experimental data, a solid NaCl loading of 65 wt % was found to meet the threshold stress (σflow) for a printing resin.
To reduce crack formation, it is advantageous to use a monomer or a monomer mixture that exhibits a low polymerization shrinkage. The polymerization shrinkage for a variety of acrylate monomers is listed in Table 2 (cf. [34])
The photopolymerizable slurries are prepared by first dissolving diluent (camphor), UV blocker (Sudan I), and photoinitiator (BAPO) in the monomer composition through vigorous stirring, followed by the addition of the functionalized NaCl particles. The slurry is then thoroughly mixed two times, for 5 min at 800 rpm (Thinky Mixer ARE-250, Thinky cooperation). A zirconia ball (d=7.6 mm) was added to enhance the mixing process.
An exemplary composition of a photopolymerizable slurry is given in Table 3, with a possible range of alternative concentrations, and alternative compounds.
ESOA: epoxidized soybean oil acrylate (CAS No. 91722-14-4); IDA: Isodecyl acrylate; PPTTA: Pentaerythritol-tetraacrylate (CAS No. 51728-26-8); TPO: Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (CAS No. 75980-60-8)
Two advantageous variants of photopolymerizable slurries are discussed below. In a first variant a) 100 wt % HDODA monomer is used, while in a second variant b) a monomer mixture of 85 wt % TMPTA and 15 wt % IBOA is used.
Printing resin with both monomer compositions were first tested without diluent, a NaCl particle concentration of 60 wt %, and a photo initiator concentration of 1 wt % with respect to the monomer content.
The composition of three advantageous polymerizable slurries is given in Table 4.
In DLP stereolithography, the resin has to fulfill two main rheological requirements. First, it should have no or only a small yield point. Additionally, a low viscosity (below 5000 mPa·s) is advantageous [33]. While these two requirements are best met for low solid loadings, high loadings are beneficial for limiting the crack formation during pyrolysis and sintering and reduce risk of (anisotropic) shrinkage. Hence, a compromise between a high solid content and a low viscosity has to be found for maximizing the solid content in the printing resin, yet ensuring good printability.
The printing resin was characterized with rheological measurements in order to check its printability. During DLP printing, the resin is required to flow back below the print head to replenish the new layer. Rheological measurements of the resins containing varying concentrations of AOT or NaCl were carried out by stress-controlled oscillatory measurements on a rheometer (Anton Paar MCR 302, Anton Paar GmbH). A stainless-steel parallel plate with a diameter of 25 mm and a gap size of 1 mm was used. The plates were sand blasted to minimize wall slip. For very low viscous resins (low NaCl particle concentration), a gap size of 0.5 mm was chosen. All amplitude sweeps were carried out at 1 rad/s with a logarithmically increasing shear stress from 0.001 Pa to 100 Pa. The apparent yield stress σflow was determined from the crossover point from the storage modulus G′ and loss modulus G″ for each resin composition. For determining the viscosity of the resins with increasing diluent concentration, steady state measurements were conducted. The shear stress was logarithmically increased from 0.01 Pa to 1000 Pa. All of the rheological measurements were performed at 25° C. at a sample size n=1.
Influence of solid loading: Oscillatory rheological data was acquired for NaCl particle concentrations varied between 30 wt % and 70 wt %. HDODA was used as photosensitive monomer in all compositions.
where σflow is the yield stress and p the solid loading of NaCl.
Influence of Surfactant on Flow Behavior: Because the NaCl particles are hydrophilic and have to be dispersed in the hydrophobic monomeric phase, the surfactant AOT is added to the printing resin. The surfactant modifies the surface of the particles to make them hydrophobic and hence decrease the particle-particle attractive interactions. In order to quantify the reduction of attractive interactions, the rheological behavior of the printing resin was investigated. Oscillatory amplitude sweeps were conducted to investigate the influence of the AOT concentration on the rheological properties.
Influence of Diluent on Flow Behavior: The composition of the printing resin is a compromise between high solid loading and low viscosity. Since the solid content strongly increases the viscosity of the printing resin, a diluent was added to counteract this effect. Camphor was used as diluent. An advantage of camphor is its melting temperature above room temperature and its quick dissolution in acrylates. Further, the printed parts gain additional strength after the solidification of camphor after the polymerization reaction. Data from steady state rheological measurements were acquired to investigate the influence of camphor as a diluent on the viscosity.
The concentration of camphor was altered from 0% to 40 wt % with respect to the total amount of liquid phase, as displayed in
A similar analysis was performed for a resin containing the monomers IBA and TMPTA, the diluent camphor as well as AOT functionalized NaCl particles. The rheological properties of inks with varying salt, surfactant and diluent concentrations were analyzed by viscosity measurements on a stress-controlled rheometer (Anton Paar MCR 302, Anton Paar GmbH). All measurements were performed at 25° C. with a parallel-plate setup using sand-blasted plates to minimize wall slip. The plates had a diameter of 25 mm and a gap size of 1 mm was employed. Steady-state measurements were performed by increasing the shear stress logarithmically from 0.01 to 200 Pa. The yield stress was defined as the point of sudden increase of shear stress in a double logarithmic shear strain versus shear stress representation of the data.
The rheological measurements revealed that the yield stress and the apparent viscosity, defined as the ratio between shear stress and shear rate, of the ink strongly depend on the surfactant (AOT) and salt concentration (
The photoinitiator and UV blocker concentrations on the polymerization reaction have to be adjusted to maximize the resolution of the 3D printed parts, while avoiding print-through and delamination. While the initiator dominates the energy needed to initiate the polymerization, the UV blocker primarily affects the penetration depth.
To find the optimal photoinitiator and UV blocker concentrations for the photopolymerizable slurry, exposure tests have been carried out. Printing resins with a composition according to Table 4 have been used. Regular glass slides were covered with a piece of fluorinated ethylene propylene (FEP) film and further placed directly onto the liquid crystal display (LCD) of the 3D printer (Original Prusa SL1, Prusa Research a.s.). A sufficiently thick layer of printing resin was evenly spread on top of the FEP film, fully covering the glass slide. A rectangular area was split into 32 small rectangles which were exposed to near UV light (λ=405 nm) for different time periods, between 2 s and 64 s, with 2 s increments. After exposure, excess resin was removed, and the glass slides were rinsed with isopropanol and water. They were left to dry overnight before the thickness of each rectangle was measured with a micrometer (IP 54, Helios-Preisser GmbH).
Photoinitiator Concentration: BAPO was used as a photoinitiator. The photoinitiator concentration is relevant because the photoinitiator generates the free radicals for the polymerization reaction. If the initiator concentration is too low, the printing resin will not solidify or will not form a sufficiently deep layer, increasing risk of delamination or print-through. Contrarily, if there is too much initiator, overcuring takes place and reduces the resolution of the final part.
The photoinitiator (BAPO) concentration was varied from 1 wt % to 5 wt % with respect to the monomer concentration, while the UV blocker concentration was kept constant at 0.125 wt %. The layer thickness of each rectangle was determined after exposure according to the exposure method explained above. The energy dose was obtained by multiplying the exposure time with the light intensity of the light source (0.1 mW/cm2).
Likewise, exposure tests with 0 wt % and 0.05 wt % UV blocker and 1 wt % to 5 wt % photoinitiator (with respect to the monomer concentration) were performed (
In terms of critical dose, inks with low Dc are often desired to reduce the printing time. Since the printer operates at a fixed illumination intensity, a lower critical dose reduces the time the ink needs to be illuminated to generate the concentration of reactive species required for polymerization. With regards to the light penetration depth, formulations with low ha show a weaker dependence of the cure depth on the illumination dose. This makes the inks more stable against possible variations in the illumination dose resulting from manufacturing issues, potentially improving the reproducibility of the printing process.
UV Blocker Concentration: Due to the light scattering particles, the light penetration is in principle limited also in absence of a UV blocker. However, the very high penetration depth in such a system directly results in high sensitivity of the cure depth to slight process changes in illumination time or illumination intensity—both affecting the irradiation dose. Reducing the penetration depth for higher process stability will inevitably result in longer print duration. It is hence essential to find a good balance between stable and efficient printing for a desired resolution. Such a balance is usually found at a penetration depth of 2-4× targeted layer height, with a factor 2 favoring high resolution prints. For a layer thickness of 50 □m, a penetration depth of 100-200 □m is required. UV blockers must hence absorb light at the wavelength of the light used for printing. In the discussed 3D printing resins, Sudan I was used as a UV blocker.
For finding the optimal value and keeping the resolution high, exposure tests were conducted for different UV blocker (Sudan I) concentrations, in analogy to the exposure tests in
To evaluate the effect of the resin formulation on the fidelity and reproducibility of the SLA printing process, model parts with positive or negative features were printed and compared the experimentally obtained sizes with their nominal theoretical values (
Example 1: An intermediate slurry was prepared by ball milling a mixture consisting of 28.6 g HDODA, 69.6 g NaCl, and 1.8 g AOT. To prepare the slurry, 1.7 g HDODA, 0.024 g BAPO, and 0.00013 g Sudan I were added to 10 g of the intermediate slurry. The slurry was printed at a light intensity of 20 mW/cm2, with an exposure time of 3 s and a layer thickness of 50 μm.
Example 2: Surface-modified NaCl particles were prepared by ball milling NaCl with 1.75 w/w AOT in isopropyl alcohol followed by drying at 60° C. overnight. A slurry was prepared by mixing 24.5 wt % HDODA, 10.5 wt % camphor, 3 wt % BAPO (with respect to monomer concentration), and surface-modified NaCl particles as balance. The critical energy dose and the penetration depth were determined from an exposure test as 0.26 mJ/cm2 and 151 μm, respectively. Using a 3D printer with a light intensity of 0.1 mW/cm2, an illumination time of 5 s was applied for each layer. The base layer illumination time was set to 15 s.
Example 3: Surface-modified NaCl particles were prepared as in example 2. A slurry was prepared by mixing 20.7 wt % IBA, 3.7 wt % TMPTA, 10.4 wt % camphor, 2 wt % Omnicure 819 (with respect to monomer concentration), 0.05 wt % Sudan I (with respect to monomer concentration), and surface-modified NaCl particles as balance. The critical energy dose and the penetration depth were determined from an exposure test resulting in 0.51 mJ/cm2 and 119 μm, respectively. Using a 3D printer with a light intensity of 0.1 mW/cm2, an illumination time of 10 s per layer was applied. The initial exposure time was 30 s.
Example 4: Surface-modified NaCl particles were prepared as in example 2. A slurry was prepared by mixing 20.0 wt % IBA, 6.7 wt % AESO, 8 wt % camphor, 3.9 wt % BAPO (with respect to monomer concentration), 0.125 wt % Sudan I (with respect to monomer concentration), and surface-modified NaCl particles as balance. The critical energy dose and the penetration depth were determined from an exposure test resulting in 0.18 mJ/cm2 and 51 μm, respectively. Using a 3D printer with a light intensity of 0.1 mW/cm2, an illumination time of 22.5 s per layer was applied. The initial exposure time was 30 s.
Example 5: Surface-modified KCl particles were prepared by ball milling KCl with 1.75 w/w AOT in isopropyl alcohol followed by drying at 60° C. overnight. A slurry was prepared by mixing 20.7 wt % IBA, 3.7 wt % TMPTA, 10.4 wt % camphor, 2 wt % Omnicure 819 (with respect to monomer concentration), 0.05 wt % Sudan I (with respect to monomer concentration), and KCl as balance. The critical energy dose and the penetration depth were determined from an exposure test resulting in 0.63 mJ/cm2 and 197 μm, respectively. Using a 3D printer with a light intensity of 0.1 mW/cm2, an illumination time of 9 s per layer was applied. The initial exposure time was 30 s.
Example 6: Sodium chloride (NaCl), bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT) and 2-propanol (99.8% purity) were all purchased from Sigma Aldrich. To decrease the size of the as-received particles to an average particle size below 2 μm, 50 g of NaCl and 0.875 g AOT (1.75 wt % with respect to NaCl, except noted otherwise) were dispersed in 100 mL of 2-propanol (99.8%, Sigma-Aldrich). The dispersion was ball-milled in a planetary ball mill (PM100, Retsch GmbH) at 200 rpm for 2 h (including cooling breaks), at intervals of 5 min milling spaced by 5 min breaks. The milling was performed in a custom-made alumina jar using 575 g zirconia balls with diameters of 9.7 mm, 7.6 mm, 4.9 mm, and 2.4 mm. The milled slurry was dried at 60° C. overnight. To form the photopolymerizable resin, the non-reactive diluent camphor ((1R)-(+), 98% Alfa Aesar), the UV blocker Sudan I (Acros Organics) and the photoinitiator Omnirad 819 (iGM Resins) were fully dissolved in the monomers isobornyl acrylate (IBOA, technical grade, Sigma Aldrich) and 1,1,1-trimethylolpropane triacrylate (TMPTA, technical grade, Sigma Aldrich). The monomers were used at an IBOA-to-TMPTA weight ratio of 85:15. The concentration of camphor is always indicated as weight percentage of the liquid phase. UV blocker and -initiator were added at 0.05 wt % and 2 wt % with respect to monomer concentration, respectively (unless stated otherwise). To prepare the final ink, 65 wt % of milled NaCl particles were homogeneously dispersed in the photopolymerizable resin at 800 rpm for 10 min by using a planetary centrifugal mixer (Thinky Mixer ARE-250, Thinky Cooperation).
A minimally required exposure time to reach 80 μm cure depth was calculated from the resulting light penetration depth (ha) and critical energy dose (Dc) for each ink individually. In order to increase the adhesion at the printhead, the initial layers were exposed for 30-50 s with 10 fading layers to reach the desired illumination time. The layer thickness was fixed at 50 μm, unless stated otherwise. The print accuracy of negative and positive features was analyzed for various UV blocker concentrations to investigate its role on the attainable resolution. All inks were printed under exposure conditions to reach a targeted cure depth of 80 μm. Negative holes of varying diameter were printed in a 2 mm solid bar, parallel to the build plate. Positive bars of varying width and 1 mm height were printed on a baseplate. The samples were visualized with a Keyence microscope and the resulting images analyzed with ImageJ (ImageJ2, version 2.3.0/1.53f). Three measurements were conducted for each data point. Digital designs were oversized by 15% in the x- and y-direction, and 25% in the z-direction to compensate for the shrinkage due to the heat treatment. After printing, the samples were cleaned with 99.8% pure 2-propanol by 3×3 min sonication in an ultrasound bath. The samples were further dried at ambient conditions before characterization.
Washing after Printing
After 3D printing, excess printing resin with unreacted monomer has to be removed from the printed object, by washing the object in a solvent. Several combinations of various solvents for washing or sonication have been tested (see Table 5).
The test objects were hollow cylinders with cross-beams. After printing, the printed objects were immersed in a first solvent A, sonication was applied for 6 minutes, and the printed objects were subsequently rinsed with a second solvent B. Drying was conducted at ambient temperature and atmosphere. Moreover, the samples were dried with their openings up-side down for the remaining solvent to flow out by gravity.
The results of the different washing routine tests and techniques were qualitatively analyzed based on the images displayed in
It is clearly visible that the overall quality of the sintered samples is higher with the printing resin composition with IBA 85 wt %/TMPTA 15 wt %. It is assumed that this is the result of the more advantageous cross-linking effect of TMPTA in combination with IBA (which does not cross-link), compared to the cross-linking HDODA alone.
Visual inspection revealed that washing routine No. 2 is most detrimental, and leads to thinned out walls, and largely changed dimensions (see
The sample objects treated according to washing routines No. 4 and 5 had few small cracks and were not as stable as the sample objects treated according to washing routines No. 1 and 3.
Sample objects No. 4 and 5 in
After visual inspection of the samples, it thus was found that washing routines No. 1 and No. 3 rendered the best results. Both routines contain sonication in isopropanol. The only difference is the rinsing after the sonication, which is done with isopropanol and water for routine 1 and 3, respectively (see Table 5).
Based on these findings, the washing routine was further improved. The most advantageous washing routine was found to be washing the printed objects with isopropanol directly after printing and sonicate them as long and as many times as needed until the isopropanol remained transparent. After extensive ultrasonication, both rinsing with either isopropanol or isopropanol followed by water worked well. The latter yielded slightly better results because less salt was crystallized on the surface upon drying.
In a particularly advantageous washing routine, the objects are washed in isopropanol for 3 to 4 times to remove the samples from unreacted monomer and lose salt particles. After ultrasonication in isopropanol for 6 min and removal from the ultrasonic bath, the objects were washed again with isopropanol, followed by water, to remove remaining salt particles on the surface. A final rinsing with isopropanol prevents recrystallization of dissolved NaCl traces on the sample surface. The objects were then left upside down for drying at ambient temperature. It is also advantageous to dry complex structures in an air stream after the last rinsing.
With such a washing method, defect-free objects as shown in
Crack formation can occur during and after various steps of the manufacturing process. Particularly during the debinding and sintering steps, the green bodies and binderless bodies are subject to thermal and mechanical stress, which can lead to crack formation. Cracks often are oriented along the printing planes, as was determined by 3D printing sample objects in different spatial orientations.
To reduce possible sources for crack formation, the above-discussed washing routine can be applied to remove unreacted monomer compounds prior to pyrolysis, in order to avoid thermally induced polymerization of remaining monomer. It has been experimentally shown that sonication of a sample object in isopropanol for 3×6 min strongly reduced crack formation, compared to sonication in isopropanol for 6 min.
Furthermore, post-curing with UV light can be used to react remaining monomer.
Finally, the fabrication of crack-free salt templates after calcination and sintering was experimentally found to require the presence of 30 wt % of camphor as diluent in the ink (with respect to monomer). To better understand the role of camphor in preventing cracking of the printed object during calcination, the microstructure of the polymerized ink after heat treatment at 200 and 690° C. was examined. In these experiments, cracks were quantified by image analysis of cracked area in printed cubes of different sizes imaged under transmitted light (
Cubes of different side length (0.25-1 cm) were printed as described previously. All cubes were printed with a targeted cure depth of 80 μm and post-processed as described above. No cracks were observed for any of the cubes after printing and cleaning. For the first heating step, the cubes were heated to 200° C. at 0.67° C./min in a Nabertherm LT furnace. The temperature was then held at 200° C. for 4 h, before cooling freely back down to room temperature (no active cooling or heating). For the second heating step, the samples were heated up to 200° C. at 3.3° C./min to allow for the continuation of the thermal treatment as described above. After each heating step, the samples were imaged optically in transmission on an optical microscope (Keyence, VHX-5000, Keyence). The images were analyzed by determining the pixel ratio of crack area (white in transmission micrographs) to cube side area in ImageJ (ImageJ2, version 2.3.0/1.53f). Since the samples are intrinsically anisotropic due to the layered build-up approach of 3D printing, only the planes of the cubes parallel to the z-axis were considered. These four sides showed the largest cracking due to the reduced material strength at the layer interface. All measurements were performed in triplicates.
In line with previous studies [24,60], the addition of 30 wt % diluent was found to significantly reduce cracking of the sample upon heating, decreasing the imaged cracked area of sintered 1 cm cubes from approximately 2.9% to values below 0.6% (
Surprisingly, it has been discovered that the beneficial effect of camphor does not arise predominantly from its expected role in generating open pores upon sublimation to facilitate the removal of thermally degraded polymer at higher temperatures. Instead, experiments with the camphor-free inks show that extensive cracking is already observed when the sample is heated up to 200° C. (
Controlled experiments in single printed layers provide insightful hints into the possible effect of camphor in preventing extensive cracking during calcination at relatively low temperatures. The inks were individually exposed to light for 40 s in strips with a dimension of 20 mm×5 mm. No printhead was used, allowing the light to freely propagate through the ink or resin. The printed layers were wiped with a damp tissue with 2-propanol to remove unreacted ink. The samples were dried overnight (dried) and finally placed into the oven for the heat treatment at 200° C. The oven was heated from room temperature to 200° C. in 1 h, followed by an isothermal hold at 200° C. for another hour. Finally, the samples were left to cool back to room temperature. It is hypothesized that internal stresses manifested as warpage in these model experiments may cause cracking of the multi-layered printed object during thermal treatment.
Photographs of a polymerized sample without salt particles and camphor reveal strong warpage of the single layer upon drying at RT, an effect that becomes even more pronounced at 200° C. (
The addition of salt particles to the camphor-free polymerized samples leads to even stronger warpage after room-temperature drying. Interestingly, the high initial warpage of such layer reduces significantly when the sample is further heated to 200° C. This effect is interpreted as a result of jamming of the salt particles at the not directly illuminated top side of the layer, which restricts the shrinking during monomer evaporation due to the formation of a compressed load-bearing particle network. This leads to preferential shrinkage of the directly illuminated bottom side of the layer, partially compensating for the strong initial warpage. Importantly, our experiments show that the initial warpage of the layer can be fully reversed when camphor is added to the formulation. It is assumed that sublimation of camphor leads to sufficient shrinkage of the sample, such that particles at both sides of the layer become jammed. By providing a locking mechanism that is triggered via the sublimation of camphor, the salt particles prevent the build-up of differential stresses across the sample cross section that cause cracking in the thermally treated printed objects.
SEM images from samples dried for 20 h at 200° C. confirm the strong shrinkage of the polymer phase and the formation of pores between the salt particles after the sublimation of the diluent (
The proposed microstructural locking effect induced by the removal of camphor is reflected in the evolution of the mechanical properties of the printed object upon heating (
After 3D printing and subsequent washing the object, the polymer binder matrix holding the salt particle structure together has to be pyrolyzed to obtain the binderless body. The binderless body has to be sintered, in order to connect the particles to obtain sufficient mechanical stability for the future use as a mold.
Pyrolysis and sintering were carried out in one cycle using a conventional electrical furnace (HT 08/17, Nabertherm GmbH). However, the debinding and sintering can also be conducted in separate steps. Debinding is carried out in ambient atmosphere. Sintering can be conducted in inert, reducing or ambient atmosphere. Depending on the slurry composition and the sintering temperature, atmosphere and time, the above mentioned heat treatment will result in a partially to fully dense sintered body.
The debinding step is critical because all of the organic material has to be pyrolyzed and leaves the ceramic material as gas phase, which subjects the green body to mechanical stress and may lead to crack formation.
A heating rate that is too high does not leave enough time for the volatile decomposition products to diffuse out of the material, resulting in pressure buildup, which leads to mechanical defects such as cracking or delamination.
It is also advantageous to obtain a mold that is sufficiently dense. For a proper infiltration of casting materials with low viscosity into the molds, the mold or at least its surface has to be dense or not wettable by the casting material. Otherwise, the liquid casting material will penetrate into the NaCl microstructure resulting in poor surface quality and impeding the dissolution of the NaCl in the subsequent manufacturing step.
The optimal conditions for debinding cycle are typically determined by thermogravimetric analysis (TGA, STA 449 C, Netzsch-Gerätebau GmbH).
Isothermal treatments are usually performed at temperatures at which the differential thermal analysis (DTA) shows peaks, i.e. when the decomposition processes occur. For objects with large cross sections, diffusion of decomposition products out of the structure is hindered, resulting in stresses. A way to reduce these stresses is to add a suitable non-reacting diluent. This reduces the overall concentration of photosensitive material and leaves some voids or channels prior to the decomposition of the polymeric material, since the diluent evaporates or decomposes at lower temperatures. Moreover, remaining unreacted monomer can induce mechanical stress during debinding due to shrinkage upon heat-induced polymerization of the remaining monomer.
To find suitable conditions for the debinding and sintering cycle adjusted to the given system, several characterization methods were applied. The influence of the AOT concentration, sintering time, and sintering temperature on the final density has been investigated.
The sample objects were cylinders with a diameter of 3 mm and a height of 5 mm and were prepared from composition Variant b, Table 4.
Shrinkage during sintering was assessed by measuring the weight, height and diameter of the green body and comparing this data to the same sample after sintering. Several parameters were investigated, namely the sintering time (i.e. duration of isotherm at sintering temperature; 0 h, 4 h, 8 h), the sintering temperature (650° C., 690° C., 730° C.) and the surfactant concentration (0.1 wt % to 5 wt % AOT during salt particle preparation).
Thermogravimetric analysis was carried out for assessing suitable conditions for debinding, namely determining temperatures of highest mass loss, in order to determine the temperature of isotherms for the debinding profile. The decomposition reaction and products were not studied in detail. As depicted in
A similar analysis, combined with dilatometry measurements, was performed on a TGA/DSC 3+, Mettler Toledo with printed cylinders with 30 wt % camphor or without (0 wt %) camphor, 0.05 wt % Sudan I and 2 wt % BAPO (
In order to find suitable sintering parameters for this system, dilatometric data was acquired, providing information about the length change of the sample over time for a distinct temperature profile.
The temperatures at the mass loss peaks were chosen as isotherms in the pyrolysis cycle in order to assure complete polymer removal. The corresponding debinding cycle is shown in
The sintering can be carried out separately, as indicated by the dashed line in
All the differential changes in length follow the same curve up to approximately 9 h, as displayed in
The influence of the sintering temperature and sintering time for different AOT surfactant concentrations on the relative density (in relation to the maximal density of crystalline NaCl) and shrinkage was studied.
The density of the sintered samples was determined with the Archimedes principle (Used device: XS205 DualRange, Mettler-Toledo GmbH). Ethylene glycol was used as the medium to measure the buoyancy of the samples, and hence determine their density. Using the density of NaCl (2.16 g/cm3), the relative density of the sintered sample was calculated.
The experimental data in
z-shrinkage only exhibits a trend for the lowest AOT concentration and remains nearly constant for higher AOT concentrations, as can be seen in
It is assumed that Na2SO4 acts as sintering aid, yielding higher final densities compared to the influence of sintering temperature Ts and sintering time ts. One decomposition product of AOT during pyrolysis is sodium sulfate Na2SO4, which together with NaCl forms a eutectic composition.
The defect-free infiltration of NaCl printed templates requires molds with liquid-tight surfaces. However, the pressureless sintering of pure NaCl to high density is very challenging [61]. Dense structures can be obtained by optimizing any of these three parameters. However, the increase in AOT concentration renders the densest microstructure (
Since it is assumed that sodium sulfate Na2SO4 is responsible for the enhanced sintering, energy dispersive X-ray spectroscopy (EDS) measurements were conducted on the particles found on the NaCl mold surface to prove its presence (5 kV, LEO 1530, Zeiss). In
Further evidence of the sintering aid like behavior of AOT due to its decomposition into Na2SO4 is provided in
The weight of the sintered samples was recorded in dry (wd), saturated (wsat) and fully immersed (wA) states. Soaking and immersion were performed in NaCl-saturated ethylene glycol. The open porosity (f) was then calculated using the relation [cf. 62]:
Combined with SEM imaging, our measurements revealed that samples processed without additives had an interconnected pore network with a high remaining porosity of 11.4 vol % and small grain size (
These findings further support the hypothesis, that AOT has a dual role as particle dispersant during printing, and as sintering aid after its decomposition into Na2SO4.
The infiltration or coating of the samples depend on the infiltration material and geometries. In general, the material is infiltrated by a pressure difference, with or without simultaneous temperature aid to melt a material and/or lower its viscosity. An illustrative description of varying infiltration processes is shown in
The ability to obtain crack-free salt objects after the drying and sintering steps opens the way towards the manufacturing of complex-shaped structures for a variety of materials by simple infiltration and leaching of the salt template. To illustrate this process, a salt object with a complex gyroid geometry was generated and used it as a template to create polymer scaffolds with unique three-dimensional architecture. The fidelity of the manufacturing process is evaluated by performing microcomputed tomography (microCT) of the salt-based object after each of the printing, sintering and infiltration steps (
Digitally visualized cross-sections obtained from the microCT of the object demonstrate that the manufacturing process preserves with high fidelity the morphology of the original digital design throughout each of the multiple steps. Importantly, the shrinkage of the object along the process needs to be considered in the original design to reach the desired final dimensions. Because of the layer-by-layer nature of the printing process, the object was found to shrink predominantly along the vertical (z) direction during the post-printing drying and sintering steps. This is well in-line with previous sintering studies of particle-based stereolithography printing [63,64]. Linear shrinkage in the z-direction reached 27.9% after complete heat treatment, as opposed to a value of 19.1% along the x- and y-directions (
The open porosity of the chosen gyroid structure allows for facile infiltration of the sintered salt template by simple casting of a thermal curable resin or injection-molding of a molten polymer.
The compatibility of our developed salt templates with these two infiltration approaches is demonstrated (
Low viscosity materials such as epoxy, radiation curable monomers or low viscosity silicones were infiltrated by assistance of vacuum (
For metal infiltration, the salt mold 1 is placed into an outer mold 2′ e.g. made from graphite, which in turn is placed in a graphite tube 7 inside a heating cylinder 6. This gas-tight set-up is placed inside an oven. After vacuum 4 application, the set-up is heated, and the molten metal 3′ is infiltrated by the application of inert gas 5 pressure. Molten magnesium metal was casted at 700° C. and an argon pressure of 5 bar, after vacuum application (
For more viscous materials, injection molding can be used for infiltration. Injection molding can be performed by a small hand-operated injection molding device, or by an industrial injection molding machine. For both cases, the salt mold 1 is placed into a metallic outer mold 2″. Polycaprolactone 3″ (PCL, Mw=40′000) was injection molded at a pressure of 2 bar. Low molecular weight polypropylene (PP) was injection molded at 60 bar (
Scaffolds with such digitally programmable pore size and morphology are highly desired for the ingrowth of cells and tissues in biomedical applications. Since the material of interest is directly infiltrated into the salt, it does not contain unreacted monomers or additional chemicals typically required for the light-based printing of polymers. This beneficial feature allows for the fabrication of complex-shaped biocompatible structures without toxic residues that could be harmful for living cells. While salt templating is often used to fabricate bio-scaffolds, the poor geometrical control over the pore structure of existing processes challenges the cell seeding efficiency and vascularization thereof.
To demonstrate the suitability of the proposed process in fabricating scaffolds for biomedical applications, the viability of pre-osteoblast cells on silicone and PCL scaffolds displaying a gyroid structure with distinct pore sizes in the range 150-500 μm was measured (
Beyond silicone and PCL, a broad range of materials can be shaped into complex architectures using the proposed manufacturing platform. This universal nature is demonstrated by creating intricate 3D structures from various materials as diverse as chocolate, aluminum, magnesium, carbon fiber composites and degradable polymers (
Complex-shaped structures were generated by casting or infiltration of the salt templates following different procedures depending on the chemical and physical nature of the infiltrating material. Polymeric stent—The pre-polymers of randomly polymerized D,L-lactide (DLLA) and ε-caprolactone(CL) was prepared as previously reported [37]. In short, a four-armed copolymer of 15000 g/mol and a linear copolymer of 650 g/mol were synthesized using a monomer ratio CL/DLLA of 7/3 and 2/2, respectively. The obtained copolymers were mixed by combining 75 wt % four-armed and 25 wt % linear molecules. This pre-polymer mixture and 3 wt % of initiator (TPO-L, Speedcure) were heated to 60° C. to decrease the viscosity and facilitate homogenization. The resulting photosensitive resin was manually pressed into the sintered salt mold and cured by UV-light (Omnicure S1000, Lumen Dynamics) for 3×10 min. The mold was finally dissolved in water. Aluminum lattice—The salt templates were infiltrated as described previously [65]. A refractory crucible with the NaCl templates and a piece of AlSi 12.6 wt % aluminum alloy was heated to 710° C. under vacuum. After a holding time of 3 min, an Ar pressure of 15 bar was applied to enable metal infiltration. Finally, the directionally cooled samples were cut to gain access to the NaCl templates, which were subsequently leached in water. Chocolate bunny—Swiss dark chocolate was molten, filled into the salt mold and placed into the fridge for full solidification. The mold was dissolved in ice-water to prevent the chocolate from re-melting. Carbon fiber composite—The printed and heat-treated NaCl cores were wrapped with two layers of a braided carbon fiber sleeve (5 mm diameter, Suter Kunststoffe AG), infiltrated with a low viscosity epoxy resin (EPIKOTE™ Resin MGS®, Suter Kunststoffe AG) and cured at room temperature with a shrink tape for compression (R&G Filament 160, Suter Kunststoffe AG, Switzerland) before final leaching in water.
The complex mold geometries enabled by light-based printing allows to shape these materials into 3D structures that would be very challenging to achieve using conventional manufacturing technologies. For example, biodegradable high molecular weight poly(D,L-lactide-co-8-caprolactone) methacrylate (poly(DLLA-co-CL) MA) copolymers that are too viscous for light-based direct printing [37] were easily molded into stent-like geometries using printed salt templates (
This plethora of examples demonstrate the potential of this light-based printing technology in leveraging salt templating for the shaping of so far inaccessible materials and geometries.
All methods result in an infiltrated body 8, which is further leached in aqueous solution to obtain the final part 9.
For carbon fiber composites, the sintered articles are wrapped with the carbon fabric and coated with an epoxy resin. The fabric was further impregnated with the epoxy by vacuum application in a vacuum bag.
Upon solidification of the materials or epoxy, the sintered salt article was removed by immersion in a polar solvent, such as water. The demolding is further facilitated by ultrasonication, solvent jetting or mechanical force.
An example of a cast epoxy resin object is shown on the right side of
The present invention is not to be limited in scope by the specific embodiments de-scribed herein. Indeed, various modifications of the present invention, in addition to those described herein, will be apparent to those skilled in the art from the foregoing description and accompanying drawings. Thus, such modifications are intended to fall within the scope of the appended claims. Additionally, various references are cited throughout the specification, the disclosures of which are each incorporated herein by reference in their entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 21186780.9 | Jul 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/061334 | 4/28/2022 | WO |
