Patent Application
20210094225
The present invention relates to a device to add pressure in a laser sintering process. The present invention further relates to the production process of a part made of e.g., ultra-high molecular weight polyethylene with a different porosity index and the part therefore produced.
Parts made of ultra-high molecular weight polyethylene (UHMWPE) have a high commercial value due to their special properties such as low density, chemical resistance, very high toughness, impact resistance, excellent wear resistance and low coefficient of friction, along with a relatively low price. On the other hand, the costs involved in UHMWPE processing are still high, because this polymer does not flow. In order to overcome this characteristic, sintering methods have been used, such as thermo-compression processes and ram extrusion processing. However, those processes result in a large block or conceptually infinite dowels and profiles. Nevertheless, when a more complex part is needed, a post-machining step will be necessary. In those processes, a solid and non-porous part is obtained due to the presence of heat and pressure.
UHMWPE does not flow due to its very high molecular weight. In the melted state, UHMWPE molecules have a very high entanglement level, resulting in a high viscosity, hindering the processability in the ordinary processing methods widely used in thermoplastics. However, when a sintering method is used, heat, pressure and time are necessary for producing a solid part having good mechanical properties.
In general, UHMWPE is sold in powder form. UHMWPE particles are highly porous and thus need heat and pressure to achieve enough molecular mobility and interfacial contact so that the reptation process may take place. The reptation model was originally developed by P. G. de Gennes (1971) and explains that a polymeric molecule in the melted state diffuses into an imaginary tube until a thermodynamically entangled state is achieved. The reptation theory is used to understand the sintering behavior of UHMWPE particles at temperatures greater than the melting point and at high pressure. In this process, high pressure is needed to ensure that no voids from the porosity remain in the final part, allowing a highly interfacial contact so that the reptation process can occur. In this process, molecules from different particles pass through the interfacial surface, making a well-linked interface.
UHMWPE is a semi-crystalline polymer that melts similarly to ordinary polyethylene. In a DSC (differential scanning calorimetry) experiment, nascent UHMWPE powder has a first melting point in a temperature range between from 140° C. to 146° C., whereas in the second fusion, the melting point range is from 132° C. to 135° C. This observed decrease in melting point in the second compared to the first melting event can be explained by a lower entangled level of UHMWPE molecules when crystalized in catalyst sites during synthesis. In a sintering process, the diffusion mechanism between interface walls is possible just above the melting point, because crystals work as anchoring sites, hindering the reptation phenomenon.
The nascent UHMWPE is a very porous particle. Even in a melted state, pressure is necessary to collapse the porous particle, and therefore allow close contact among interfaces. Thus, a temperature greater than the melting point and pressure are necessary to reduce the porosity in the final part. The minimum pressure needed to produce acceptable parts depends on molecular weight. The typical pressure range used to produce acceptable parts ranges from 5 to 30 MPa. ISO Standard recommends a pressure of 10 MPa in a full pressure step, so that specimens can be repeatedly obtained.
Besides heating and pressure, time is the third key aspect in UHMWPE molding. The higher the molecular weight, the lower will be the molecular diffusion velocity in a sintering process. Time is therefore an important cost component in the UHMWPE molding operation. Technical knowledge in this industry is achieved by mastering those three processing parameters: temperature, pressure and time.
There are two main processes to sinter UHMWPE and thus produce acceptable parts. The first is thermal compression, where in general a large block is produced. The final parts having different geometries are obtained using the general machining methods commonly used in metals. This processing can be considered as batch processing, and is work and time intensive. The time spent is in part due to a very slow reptation process and in part due to a very low heating conductivity of UHMWPE. The time required for a plate core to achieve the desired temperature, passing by a very thick path, is quite high.
The second sintering process commonly used is ram extrusion. In this method, a conceptually infinite profile having different cross-sectional geometries is obtained. The powder is fed in a piston cavity. Ram or plunger extruders are simple in design, having an essentially positive displacement, being able to generate very high pressures. In their intermittent operation, the polymer is rammed in the die direction while it is molded. Due to the back pressure generated by high polymeric viscosity, the pressure achieved can reach 300 MPa in this kind of extruder. Ram extrusion can be considered as a semi-continuous process to sinter UHMWPE.
Additive manufacturing is the official term used to describe the process to produce parts layer-by-layer using a similar concept used in printers. However, in additive manufacturing, a volume element is added instead. In this process, these volume units are commonly called voxels.
In additive manufacturing, the voxels are added layer-by-layer to form a final three-dimensional (3D) piece, and for this reason the term 3D printing became commonplace. An advantage of this process is the possibility of obtaining very complex geometries which are difficult to be made through the ordinary molding process.
3D printing was invented almost 50 years ago, and the first commercial system was commercially available in the late 1980s. In this process, seven process categories were developed: 1. Material extrusion, 2. Material Jetting, 3. Binder Jetting, 4. Sheet Lamination, 5. Vat Photopolymerization, 6. Powder Bed Fusion and 7. Directed Energy Deposition.
Due to UHMWPE characteristics, Powder Bed Fusion is very promising, because no flow is necessary in this process. Powder Bed Fusion, where 3D laser sintering is by far the most popular method, uses a highly energetic beam to melt a specific region of the surface of polymeric powder. In this method, there are four key components: a laser scanning system, a powder delivery system, a roller or rake and a fabricated piston, as shown in
In the first step, the powder reservoir (2) is full while the powder bed (1) is empty. The powder delivery piston (3) is moved up one layer and then the roller (8) passes, dragging the powder to fill the first layer in the powder bed (1). At that moment, the laser source (7) is switched on and the scanner (5) starts to melt a 2D surface in a powder bed (9), moving the laser beam (6) in a pre-defined path. At the end of this step, a new step begins with a concomitant opposite layer movement with both a powder delivery piston (3) and a fabrication piston (4). A new fresh powder layer is charged over the powder bed, and the process starts again. The part is made, layer-by-layer until the powder reservoir becomes empty. The part can then be finished. The powder in excess accumulates in the reservoir (10).
Additive manufacturing processes have opened a new range of possibilities in generating parts with very complex geometries using UHMWPE, previously not possible using classic processing methods. Additive manufacturing allows producing new part geometries with unique UHMWPE properties, what can be very valuable for many different applications.
The ordinary additive manufacturing process, more specifically the laser sintering process, can be used for producing parts using UHMWPE. However, due to the fact that UHMWPE does not flow under heating conditions, the final parts produced are highly porous. That porosity decreases the mechanical properties of UHMWPE, resulting in a poor final part.
Thus, producing parts made of UHMWPE using an additive manufacturing method remains a challenge.
The present invention relates to a device able to apply pressure during laser sintering.
The present invention further relates to the process to produce that part with a different degree of porosity and therefore different mechanical property levels.
The present invention further relates to parts made of e.g., UHMWPE using laser sintering with controllable pressure levels, in this way being able to produce parts with different porosity levels, not obtainable using an ordinary laser sintering method.
The present invention is best understood from the following detailed description when read in conjunction with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not necessarily to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Like reference numerals denote like features throughout specification and drawings.
The present invention relates to a device able to apply pressure during the laser sintering process. The device introduces pressure in an ordinary sintering process, allowing for porosity control during production of parts made with UHMWPE.
Unlike temperature and time, pressure is the single key parameter important to produce solid parts of UHMWPE, that is not present in an ordinary laser sintering process.
Pressure is necessary for collapsing voids and allowing enough contact among porosity interfaces, important considerations for achieving reptation.
In an embodiment of the present invention, the bulkhead is comprised of any material transparent to a laser beam such as, but not limited to, germanium (Ge), zinc selenite (ZnSe), gallium arsenide (GaAs), or any material transparent to a CO2 laser beam.
In further embodiments of the present invention, other materials can be used depending on the type of laser used.
In an additional embodiment of the present invention, the bulkhead can be comprised of a non-transparent material as shown in
In this embodiment, the bulkhead is composed of a mechanically resistant and insulating material (15), containing an isotropic heating conductor (16). In this exemplary device, the laser shines each conductor point (14) in the bulkhead's top surface (12). In this way, heat will propagate along the isotropic conductor (16) to the bulkhead's bottom surface, heating a very restricted region of powder under pressure. This device was developed as an option to a transparent bulkhead. CO2 transparent materials are in general brittle and/or expensive.
In a further embodiment of present invention, the isotropic heating conductor (16) can be any oriented material having a high thermal conductivity in its main axis direction. Examples of oriented materials include, but are not limited to, carbon fiber, metal filament, graphite fiber, etc.
In a further embodiment of present invention, the insulating material (15) can be any mechanically resistant and insulating material such as, but not limited to an epoxy resin.
In order to apply pressure over the top of the powder bed (1), the bulkhead (11) is fixed in the D position by means of a clamp (not shown) to bear pressure imposed by a fabrication piston (4). The fabrication piston (4) is moved by any suitable driver such as a servo-hydraulic system, electro-fuse system, etc. The pressure is set according to the following Equation 1.
P=F/S Equation 1
P is the pressure, in MPa.
F (
S (
In an additional embodiment of the present invention, the process to produce parts made of UHMWPE is described by the following steps:
The present invention describes a part produced for any powder that can be sintered, such as metals, ceramics, vitreous materials, polymeric materials, and combinations thereof.
In a preferred embodiment, any polymeric powder can be used, such as polyolefins, polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE), UHMWPE, and combinations thereof.
In a particularly preferred embodiment, an UHMWPE is used.
The present invention further relates to a part made of UHMWPE that is produced by laser sintering under different pressure levels. The pressure will define the amount of porosity of the final part.
In a further embodiment of the present invention, a pressure range from 0 to 300 MPa is desirable, with a range of 5 to 80 MPa preferred, and a range from 5 to 30 MPa particularly preferred.
In an additional embodiment of the present invention, the Porosity Index (PI), according to the following Equation 2, defines the level of part porosity:
Where:
The effect of pressure on the mechanical and tribological properties of UHMWPE has been previously studied. The mechanical and tribological properties increase asymptotically with applied pressure. The pressure is necessary to keep the porous wall in contact, allowing the reptation process to occur.
In an additional embodiment of the present invention, a part made of UHMWPE has a porosity index (PI) from 0 to 1, with a porosity index from 0.3 to 1 preferred, and a porosity index from 0.6 to 1 particularly preferred.
The present invention further relates to a method of producing a three-dimensional object comprising the steps of: (a) disposing a layer of a powder material on a target surface; (b) applying pressure to the powder material layer; (c) directing an energy beam over a selected area of the powder material layer, wherein the powder is sintered or melted; and (d) repeating steps (a)-(c) to form the three-dimensional object. This method may further comprise the step of disposing a bulkhead over the powder material after disposing the layer of the powder material on a target surface. Step (c) may occur under pressure, steps (b) and (c) may occur sequentially, and the bulkhead may be transparent to the energy beam.
The foregoing is provided for purposes of illustrating, explaining, and describing embodiments of this invention. Modifications and adaptations to these embodiments will be apparent to those skilled in the art and may be made without departing from the scope or spirit of this invention.
Although the subject matter has been described in terms of exemplary embodiments, it is not limited thereto. Rather, the appended claims should be construed broadly, to include other variants and embodiments, which may be made by those skilled in the art.
The present application claims benefit of U.S. Provisional Patent Application No. 62/608,957, filed on Dec. 21, 2017, the entirety of which is incorporated herein by reference.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2018/066503 | 12/19/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62608957 | Dec 2017 | US |
