Patent Application
20230382055
The invention refers to a method of manufacturing an object by stereolithographic 3D printing, wherein the object is built on a building platform layer-by-layer to obtain a stack of structured layers, wherein each structured layer is obtained by the steps of:
Further, the invention refers to a device for manufacturing an object by stereolithographic 3D printing, comprising:
As used herein, “light” may include any electromagnetic radiation that is able to induce polymerization of a photosensitive resin. The term “light” needs not be restricted to visible light, e.g., the portion of the spectrum that can be perceived by the human eye. The radiation may have a wavelength in the range of 10 nm to 10,000 nm, preferably 100 to 500 nm.
The term “photosensitive resin” may refer to a material that conforms into a hardened polymeric material through a curing process. A photosensitive resin, may include, but is not limited to, a mixture of monomers, oligomers, and photoinitiators. A photosensitive resin may also be referred to as an uncured photopolymer.
“Curing” the photosensitive resin is a process, wherein the photosensitive resin is polymerized or cross-linked as a result of being irradiated by light.
As used herein, a “light engine” is a device that is able to generate dynamic light information according to a predetermined pattern. As an example, LCD-displays, Digital Light Processing (DLP), other active mask projection systems, and/or laser-scanner based systems may be used to selectively project light information on the surface of the photosensitive resin.
Compared to other additive manufacturing technologies, commercially available stereolithography (SLA) printers provide high resolution and surface quality. Further, with certain process adaptions like heating or thin film coating (see, e.g., EP 3284583 A1 and EP 3418033 A1), highly viscous materials, e.g. resins, can be printed, which cannot be processed with any other additive manufacturing technique. Highly viscous materials can lead to advanced mechanical properties of parts printed with stereolithography.
Current stereolithographic processes, however, can only process one material at a time. Another drawback of stereolithography is the necessity of support structures. Both, for top-down and for bottom-up systems, support structures serve to support overhanging parts, undercuts or other segments which are not supported by the main building part. In stereolithography, support structures inherently consist of the same material as the building material. Therefore, the support structures cannot be easily removed by washing or melting but have to be removed mechanically and in many cases manually, e.g. broken off or cut away, leaving marks on the part's surface. Besides a potential damage of the surface, this involves extra time and work requirement. Especially for parts which need debindering and/or sintering after the printing process, e.g. ceramic or metal filled photopolymers, the manual removal of the support structure is time-consuming as well as labor- and tool-intensive and adds further risk to damage the part over the process chain.
Other additive manufacturing technologies, such as Inkjet printing or Fused Filament Fabrication (FFF), can process multiple materials at once. Those systems can often provide a support material for printing the support structures, which is inherently different from the building material. There are several ways to remove this support material, including melting the support material or using a solvent to wash it away. This allows to remove the support material without damaging the printed part's surface. However, printing systems for Fused Filament Fabrication currently lack the accuracy, layer adhesion and surface quality, which is necessary for most industrial applications. Inkjet printing technologies on the other hand have significant limitations regarding viscosity parameters of the photosensitive resin, commonly limited to around 20 mPa s due to the fluidic restrictions of available print heads. This has a significant influence on the achievable mechanical properties as low viscous monomers usually feature short polymer chains and form dense polymer networks thus resulting in brittle polymer parts.
To overcome certain limitations of the printing processes, different printing strategies can be combined. There are approaches to combine thermoplastics (FFF) with photopolymers from inkjet printheads (e.g. U.S. Pat. No. 10,486,364 B2) as well as SLA systems and inkjet printheads (e.g. U.S. Pat. No. 10,486,364 B2, U.S. Pat. No. 10,449,715 B2, US 2015/131074 A1) for coloring purposes. Multi-material approaches for SLA often imply multiple vats with different resins each (e.g. US 2017/100899 A1). Those approaches, however, often suffer cross-contamination problems between the different materials.
It is an object of the invention to improve a stereolithography based printing method and a device to allow for the placement of support structures that can be easily and controllably removed in a post-processing step without damaging the printed object's surface.
To solve this and other objects, the invention in a first aspect thereof provides for a method of manufacturing an object by stereolithographic 3D printing, wherein the object is built on a building platform layer-by-layer to obtain a stack of structured layers, wherein each structured layer is obtained by the steps of:
The invention uses a stereolithography based printing process which allows for the processing of mechanically favorable photopolymers and is based on the idea to selectively add an additional substance during the printing process, the additional substance being a removable material. In particular, the invention is based on the idea to arrange a film of a removable material between structured layers of the cured photosensitive resin and thereby provide a temporary interface between different parts of the 3D-printed object, which can easily be disintegrated and removed in a post-processing step. Such a removable interface can be used to separate the 3D-printed object into two or more parts or to remove a supporting structure from the object without negatively affecting its surface.
Since the removable material is not the same material as the photosensitive resin, a post-processing step to remove the removable material can be designed as a process to which only the removable material is responsive, thereby leaving the cured photosensitive resin unaffected. In particular, the post-processing step is designed to subject the film of removable material to a physical and/or chemical process that causes the first film of removable material to disintegrate or lose its stability.
According to a preferred embodiment, the physical and/or chemical process is selected from the group consisting of melting, evaporating, sublimating, swelling and dissolving in a solvent. Thereby, the post-processing step takes advantage of the different physical and/or chemical properties of the cured photosensitive resin and the removable material, and applies a physical and/or chemical process that produces a disintegration reaction, such as melting, evaporating, sublimating, swelling and dissolving in a solvent, only in the removable material, but not in the cured photosensitive resin.
As mentioned in the introduction, using multiple materials in stereolithography process usually requires the use of multiple material reservoirs, such as vats, one for each material, wherein the building platform or the object at least partially built thereon is alternately immersed into said reservoirs. This, however, results in cross-contamination between the different materials, because an un-cured amount of a first material still adhering to the object after a structured layer of the first material has been produced will be transferred into the second material when the object is immersed into the second material, and vice versa.
In order to avoid cross-contamination between the two different materials, the removable material is applied by an ejector that ejects drops of the removable material onto the structured layer made from the photosensitive resin. In particular, the drops are selectively placed at one or more locations of the structured layer, where the removable interface is needed to fulfill the desired function. Preferably, the ejector is designed as an inkjet printhead or as a nozzle or droplet dispenser.
The interface made of the removable material may serve to separate selected parts of the cured photosensitive resin from the 3D-printed object. If the selected parts to be removed are confined by structured layers on both, the upper and the lower side, a removable interface would be advantageous on both sides. Therefore, a preferred embodiment of the invention provides that the stack of structured layers comprises a second pair of adjoining structured layers,
An important application of the invention is the removal of support structures that are printed during the manufacturing of the object in order to support overhanging parts or undercuts of the object. The removal is enabled by confining the support structure with a film of removable material on its upper and its lower side. Accordingly, a preferred embodiment of the process of the invention provides that a support structure consisting of the photosensitive resin is created between the first film of removable material and the second film of removable material, wherein in the post-processing step said support structure is removed from the object.
As an alternative or in addition to a support structure that is confined by structured layers on both, its upper and lower sides, each being provided with a film of removable material, a support structure may also be arranged between a structured layer of the object and the building platform. Here, a preferred embodiment of the process of the invention provides that a support structure consisting of the photosensitive resin is created between the first film of removable material and the building platform, wherein in the post-processing step said support structure is removed from the object.
In order to facilitate the separation of the support structure from the building platform, a third film of removable material may be placed onto the building platform by ejecting drops of the removable material from an ejector, before building a first structured layer of photosensitive resin on the building platform. In this way, an additional interface made of the removable material is created between the building platform and the support structure.
The support structure can fill the entire space below an overhanging part or an undercut. However, in order to minimize the amount of photosensitive resin needed for building the support structure, the support structure may comprise a plurality of support elements, such as a plurality of columns.
As commonly practiced in stereolithography 3D printing processes, the support structure is built concurrently with the actual object, so that a preferred embodiment provides that the support structure is built form a plurality of layers of cured photosensitive resin. To this end, a section of the pattern may each be used to build the support structure layer-by-layer.
According to a particularly preferred embodiment, a film of removable material is each arranged between said layers of cured photosensitive resin of the support structure. In this way, the support structure consists of alternating sections made of cured photosensitive resin and of removable material. In other words, the support structure is divided into at least two sections by at least one film of removable material. During the post-processing step, when the removable material is disintegrated or loses its stability, the support structure separates or collapses into at least two sections, i.e. into smaller parts that can be removed more easily. This allows the removal of the support structures from areas of the object that are hard to access or from cavities of the object. Further, this method allows to integrate the removal of the support structure in an automated post-processing and cleaning step, since the removal can be highly independent of the printed part's geometry.
Further, this gives the ability to create movable joints, gears or other types of bearing or moving solutions between parts, subparts or assembly groups in part assemblies of a single printing job or even in assembly groups which cannot be disassembled after printing.
As to the nature of the removable material, any material may be used that can be applied by the ejector in drops and that differs from the cured photosensitive resin in at least one physical and/or chemical property, such as the melting point, the solubility, the boiling point etc.
Preferably, the removable material is a material that is polymerizable and, in its polymerized and pre-polymerized state, can be dissolved or swollen in a solvent. Such a material may therefore consist of at least a polymerizable group, such as acrylates, methacrylates, acrylamides, vinyl ethers or vinyl esters, maleimides, cyclic ethers, isocyanates, amines or other polymerizable unsaturated or saturated groups, and may optionally further comprise at least one hydrophilic or oleophilic group. The polymerized material may be dissolvable or swellable in a solvent, such as water or alcohol or oil or other organic solvents. Such materials for example comprise hydroxy, carbonyl and carboxy groups and derivatives with other electronegative hetero atoms, amines, ionic liquids and salts, for example hydroxyethylmethacrylate (HEMA), 2-(2-ethoxyethoxy)ethyl acrylate (EOEOEA), acryloyl morpholine (ACMO), polyethylene glycol derivates, polyethers, hydroxy ethylene or lauryl acrylates.
According to an alternative preferred embodiment, the removable material is solidified and meltable after the printing process and therefore is a material having a melting point that is below the decomposition temperature of the cured photosensitive resin, such as a melting point of <200° C., such as long-chain alcohols or waxes.
In order to improve the adhesion of the removable material on the cured photosensitive resin, the structured layer may be pre-treated by means of surface conditioning, e.g. plasma or Corona treatment, before ejecting drops of the removable material from the ejector onto the structured layer.
Since the removable material is applied onto the cured photosensitive resin layer in liquid form, a preferred embodiment provides that the film of removable material is solidified, before a structured layer is built on the film of removable material. Preferably, the film of removable material is subjected to a drying step, solidifying step or a light-curing step before a structured layer is built on the film of removable material. Alternatively, the film of removable material is subjected to hardening or curing by a chemical reaction process which is triggered by surrounding gases like ambient atmosphere or special process gases or process atmospheres or by contact with a second removable material compound which may be ejected by an additional ejector device. In another approach, the film of removable material stays liquid on top of the surface of the cured photosensitive resin layer and might be cured later, at least partially, by the electromagnetic radiation pattern which forms the next layer of cured photosensitive resin. In an alternative approach the film of removable material continues to stay liquid on top of the surface of the cured photosensitive resin layer and even between multiple layers of cured photosensitive resin. However, in such a configuration, the viscosity of the film of removable material might vary or stay the same over the whole printing process and might be influenced in its viscosity by its physical (e.g. process temperature) or chemical (e.g. surrounding gas atmosphere) process environment.
Further, the structured layer may be structured to have a cavity, into which the drops of the removable material are ejected from the ejector. This improves the positioning of the removable material at the desired location.
According to a further aspect of the invention, a device for manufacturing an object by stereolithographic 3D printing is provided, comprising:
Preferably, the control unit is configured and programmed to change between the first and the second position according to a predetermined sequence so that a second film of removable material is placed onto a first structured layer of a second pair of adjoining structured layers, whereafter a second structured layer of said second pair of adjoining structured layers is built on the second film of removable material, thereby arranging the second film of removable material between the first and the second structured layers of said second pair of adjoining structured layers.
Preferably, the control unit is configured and programmed to change between the first and the second position according to a predetermined sequence so that a support structure consisting of at least one structured layer of the photosensitive resin is created between the first film of removable material and the second film of removable material.
Preferably, the control unit is configured and programmed to change between the first and the second position according to a predetermined sequence so that a support structure consisting of at least one structured layer of the photosensitive resin is created between the first film of removable material and the building platform.
Preferably, the control unit is configured and programmed to change between the first and the second position according to a predetermined sequence so that a third film of removable material is placed onto the building platform, before building a first structured layer of photosensitive resin on the building platform.
Preferably, the control unit is configured and programmed to change between the first and the second position according to a predetermined sequence so that the support structure consists of a plurality of structured layers of the photosensitive resin.
Preferably, the control unit is configured and programmed to change between the first and the second position according to a predetermined sequence so that a film of removable material is each arranged between said plurality of structured layers of the support structure.
In the following, the invention will be described in more detail by reference to specific preferred embodiments of the invention.
The present invention is a method of additively manufacture parts by selective exposure to light of a filled or unfilled photosensitive resin with a further phase of selectively placed removable material to facilitate support structure removal. Stereolithographic printing systems allow for the processing and selective curing of photosensitive resins of various viscosities, preferably dynamic viscosities between 0.1 Pa.s and 20 Pa.s at predefined process temperature, with or without fillers, with the same material for the printed part itself, as well as for the necessary support structure. To overcome the disadvantage of commonly available printers of being limited to only one material, the multi-material stereolithography printing is achieved by integrating a drop generating device, e.g. an inkjet printhead, a nozzle or pipette, into the printing process, which can selectively place at least one additional material, including a plurality of additional materials. The invention of introducing removable materials as a second or additional phase into the printed object can be applied for all types of stereolithographic printing processes.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20020447.7 | Oct 2020 | EP | regional |
The present application is a national phase application of PCT Application No. PCT/IB2021/058407, filed Sep. 15, 2021, entitled “METHOD AND DEVICE FOR MANUFACTURING AN OBJECT BY STEREOLITHOGRAPHIC 3D PRINTING”, which claims the benefit of European Patent Application No. 20020447.7, filed Oct. 2, 2020, each of which is incorporated by reference in its entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2021/058407 | 9/15/2021 | WO |
