Recycled Polymers for 3D Printing

Abstract

Waste materials from various processes (injection molding, selective laser sintering) are processed to form powders with optimal particle size and distribution for the 3D printing technology known as Composite Based Additive Manufacturing (CBAM). Alternative recycling processes include grinding and sieving, emulsion extrusion, and liquid-liquid phase separation.

Description

[Technical Field]

[0001] <Cross-reference to related applications> This application claims priority to U.S. Provisional Patent Application Nos. 63 / 133,666 (filed January 4, 2021) and 63 / 184,755 (filed May 5, 2021), each of which is incorporated by reference in its entirety.

[0002] The present invention is in the field of 3D printing processes and systems. More specifically, it relates to improvements in material inputs and feedstocks for systems that create 3D printed parts. These improvements originate with the assignee and are within a subfield of 3D printing that the assignee calls Composite-Based Additive Manufacturing (CBAM). [Background technology]

[0003] 3D printing processes have many limitations, particularly with regard to materials. Similar methods, such as selective laser sintering or high-speed sintering, as well as processes based on them, such as HP's MultiJet Fusion and Stratasys' Selective Absorption Fusion, primarily use materials known as PA12 or PA11. This is because PA12 and PA11 have a large thermal processing window unavailable for many thermoplastic polymers. Nevertheless, the sintering process requires a polymer that melts within a 10°C processing window. In the case of PA12 and PA11, these are expensive resins, making typical powder production by precipitation expensive. This limits the number of resins available and increases costs. The polymer sintering process requires the polymer powder to be kept in a bed at high temperatures (just below the polymer's melting point) for extended periods. For polymers like PA12 and PA11, this high-temperature heating can cause polymer chain extension and an increase in molecular weight. These phenomena can render previously usable materials unusable under standard sintering parameters. Therefore, the waste powder must be thoroughly processed or blended with virgin material. This means that the sintering process requires, on average, twice the amount of powder contained in the parts used in production, doubling the material cost of the part. This powder must be disposed of, resulting in significant waste. These same problems are exacerbated in stereolithography, where resins and photopolymers are more expensive than thermoplastics, and spoilage is also a significant issue, as with two-component systems such as those used by Carbon, which have limited pot life. Furthermore, stereolithography requires expensive photoinitiators, and thermoset resins produced by stereolithography are difficult to recycle.

[0004] Similarly, injection molding, blow molding, and thermoforming processes also generate waste, such as rejects and sprues, among other things. These processed materials cannot consistently achieve the same performance as virgin materials due to a variety of reasons, including inclusions, impurities, and variations in melt flow and crystallinity. This means that recycled materials cannot consistently achieve the performance required for mass production in these manufacturing processes. Increasing the utilization of recycled materials offers benefits, including environmental improvements and reduced polymer feedstock costs. Incorporating recycled polymers into 3D printing processes would be beneficial, and processing recycled materials for such applications as powders, eliminating impurities as a by-product of processing, would be even more beneficial. In addition to the above issues, post-consumer recycled materials, such as PET beverage bottles, are difficult to use in their recycled state due to contamination, colorants, and inconsistencies in the material and its properties.

[0005] Impossible Objects' CBAM process (Composite Based Additive Manufacturing) does not have these problems. CBAM is disclosed in numerous patents, including 10,967,577, 10,046,552, and 9,827,754, each of which is incorporated herein by reference. It can be used with most thermoplastic resin powders. Excess powder is captured and reused during printing, resulting in little waste. Additionally, heating is performed in a separate press from printing, eliminating the constraints of heat treatment windows.

[0006] In CBAM, a computer model slices the part to be printed into cross sections. A printing technique (such as inkjet printing) is used to print a liquid onto a porous sheet in a shape corresponding to one of the object's cross sections. The porous sheet is typically carbon fiber, but can also be made of fiberglass or other suitable substrates. Alternatively, printing can occur at the end of a supplied roll (or web), with cutting performed downstream. A powder (usually a thermoplastic powder) is applied to the printed sheet in large quantities to ensure that it adheres only to the printed areas and not to the non-printed areas. Various means (e.g., vacuum, vibration, air knife) are used to remove the loose powder from the sheet. The sheet then moves to a stacking process, where it is placed on top of a previous sheet (if any) that has undergone a similar process for the adjacent object's cross section. A stacker aligns the sheets by fitting tapered resist pins into holes drilled in the sheet by the upstream printing process. This process is repeated for the required number of cross sections, creating a build block by stacking multiple substrate sheets in the exact order necessary to represent all cross sections of the 3D object. The build block is then compressed, heated, or otherwise processed to melt the powder in the printed areas and fuse it around the fibers. The compressed and heated build block is then ablated to remove the substrate, e.g., brittle carbon fiber, from the non-printed areas. The fused / fused areas can withstand this abrasion, and the process results in the intended shape of the final 3D printed part, as defined in the computer model. Advantageously, this use of carbon fiber and thermoplastic powder results in a part that is highly durable and suitable for the high tolerances required in industrial applications (hence, it is a "composite-based" 3D printed part). The '552 patent describes various aspects of the aforementioned system and embodiments of subsystems that perform each step (e.g., material supply, printing on a platen, powdering, powder removal, layering, etc.). For non-brittle substrates, such as PET, a chemical removal process can be used, as described in the referenced patent.

[0007] One advantage of Impossible Objects' CBAM process is that it enables the use of high-performance thermoplastic materials such as PEEK, PEKK, PAEK, PPS, and PEI. PEEK, for example, is a very expensive material, costing approximately $65 per pound. Traditional PEEK is difficult to grind into a smooth powder because the grinding process generates faceted particles, as seen in Figures 1A and 1B (Prior Art). Figure 1A shows the agglomeration of virgin PEEK (not an issue with the recycled PEEK discussed here), and Figure 1B (the same as Figure 1A but enlarged 500x) shows the variation in particle size. These powders have a wide particle size distribution, with a significant prevalence of small particles, and an overall average particle size D50 of 30 microns.

[0008] One of the challenges of the CBAM process is getting a sufficient amount of polymer (thermoplastic resin powder) onto the sheet. Since particle mass is proportional to the cube of the particle diameter, larger particles significantly increase the mass of polymer deposited. A larger deposited mass produces a stronger part and eliminates the need for compression, allowing for thicker layers and therefore a faster process.

[0009] For many powders, flow aids such as Aerosil 200 (Evonik Industries, Essen, Germany; "Aerosil" is a registered trademark; note omitted below) are sometimes added to the powder to improve flow. For example, in the case of recycled PEEK, the angle of repose of the untreated powder is over 40 degrees, but with Aerosil 200 the angle of repose is less than 35 degrees (measured according to ASTM C1444). This allows the powder to flow well during the CBAM process.

[0010] Take PEEK, for example. Many processes, such as injection molding, produce significant quantities of PEEK that are discarded. PEEK waste is often generated in industries such as medical and aerospace, where regulations require only virgin material. For a variety of reasons, many molders do not regrind rejected parts, sprues, or runners for reuse. Typically, injection molding allows only a small portion of the regrind material to be used to create new parts. Summary of the Invention

[0011] It has been discovered that PEEK injection molding waste can be reclaimed in CBAM by regrinding a high percentage, or even completely. Furthermore, unexpectedly, grinding this reground injection molding waste produces a better particle size distribution and larger particles than virgin ground material purchased from manufacturers like Solvay. Discarded injection molding material (e.g., sprue, runners, rejects, etc.) can be ground and sieved to the appropriate particle size distribution for use in the CBAM process.

[0012] They also discovered that waste polymers from polyolefins, PET, and other non-CBAM technologies can be reprocessed and reused using emulsion technology, which involves heating and mixing thermoplastic materials in an immiscible liquid, followed by high-speed stirring or agitation to form droplets in the mixture, followed by cooling to solidify the thermoplastic materials, and then recovering the powder from the mixture. [Brief explanation of the drawings]

[0013] [Figure 1A] FIG. 1A (prior art) shows a block of virgin PEEK at (low) optical resolution. [Figure 1B] Figure IB (Prior Art) is a close-up photograph of virgin PEEK at 500x magnification, showing the relative non-uniformity of the powder particles. [Figure 2] FIG. 2 shows recycled PEEK powder according to a grinding embodiment of the present invention, with a D50 of 80-100 microns. [Figure 3]FIG. 3 is a plan view of an example structure (solder pallet) made by a CBAM process using recycled PEEK according to a grinding embodiment of the present invention. [Figure 4] FIG. 4 illustrates a workflow for recycling PEEK according to a grinding embodiment of the present invention. [Figure 5] FIG. 5 (Prior Art) shows the Russell Ultrasonic Finnex device for high precision sieving. [Figure 6] FIG. 6 shows recycled PEEK powder according to a grinding embodiment of the present invention, after the addition of a flow agent. [Figure 7] FIG. 7 illustrates a workflow for recycling polymers according to an emulsion embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] Improving particle size and distribution (D50 between 80 and 100 microns) results in better (stronger, denser) parts with CBAM. Figure 2 shows the output of a grinding embodiment described herein with such a size and distribution. Because the new thermoplastic feedstock is typically discarded, and the grinding costs a fraction of the raw material, material costs are lower for this new, high-performance PEEK than for a lower-performance material like PA12. This allows for parts with substantially improved material properties to be produced more cheaply.

[0015] The recycled PEEK material used in CBAM has a tensile strength of 140 MPa and a tensile modulus of 14 GPa, very close to the performance of conventional PEEK powder. Furthermore, using recycled materials reduces the carbon footprint of the final part. Nearly all thermoplastics can be used as recycled powders. These include PET (polyethylene terephthalate) from beverage bottles, polyolefins such as polyethylene terephthalate (PE) and polypropylene, polystyrene, polycarbonate, and thermoplastic elastomers.

[0016] An example of a final product made from recycled PEEK in the CBAM process is a solder pallet. Figure 3 shows both sides of a solder pallet made from recycled PEEK powder according to the teachings herein.

[0017] In CBAM, changes or fluctuations in melting point or melt viscosity (melt index) do not significantly impact the finished product. Changes in polymer melt flow and viscosity generally have less impact in CBAM processes than in sintering-based processes. This is because the final processing step in CBAM involves heating all materials above their melting point and compressing the build block to a predetermined thickness, forcing the molten thermoplastic to flow and solidify. Thus, the process and results described herein demonstrate that recycled PEEK (and others) not only possesses improved powder properties (i.e., large particle size and excellent particle distribution, maximizing substrate sheet coverage during the powdering process), but can also be achieved using common waste products from more conventional manufacturing processes. Positive environmental impacts, such as reduced carbon dioxide emissions, inevitably result, expanding the use of post-consumer recycled materials, such as injection molding waste and packaging materials like polyethylene and polypropylene, which are largely not recycled, and PET from beverage bottles.

[0018] Efforts to use recycled materials are not limited to high-performance materials like PEEK, but can also include a variety of other materials, such as PET. PET is used in beverage containers, a material with a high recycling demand. Recycled PET materials cost approximately half the cost of virgin materials. Impurities and / or colorants in recycled materials are not a major issue for CBAM. This is true for most thermoplastics, including polyolefins such as polyethylene and polypropylene, engineering plastics such as PA12, as well as PEEK, PAEK, PEKK, PPS, and PEI. The PA12 and PA11 powders generated during the sintering process can be recovered, re-extruded, and converted into powder using, for example, emulsion or other processes, and reused in the CBAM process.

[0019] Substrates made from recycled materials, such as nonwoven PET from Freudenberg Group (Weinheim, Germany), can also be used to manufacture parts with CBAM. Powder materials, such as EVA, can be used with excess material chemically removed.

[0020] Example #1: Recycled PEEK by Grinding With reference to FIG. 4, the following procedure describes the steps required to prepare a production batch of recycled PEEK material for a CBAM system. In step 10, PEEK-based injection molding scrap is collected. In step 20, operators grind the scrap into particles having approximately the preferred size and distribution described above: D50 of 30 microns or greater, more preferably 70 microns or greater, and most preferably 80-100 microns. In step 30, operators sieve the particles to obtain the desired range of size and distribution. In step 40, operators then add a flow aid. In step 50, operators sieve the particles a second time (this time including the flow aid), resulting in 60—the desired final powder with the desired particle size and distribution, along with the flow aid.

[0021] More specifically, regarding steps 10, 20, and 30, used PEEK resin is sourced from an existing industrial injection molding operation. Sprues, runners, and other waste materials are collected and run through a mill to reduce them to a fine powder. This granules are then processed through a Hosokawa Micro Air Jet Sieve Tester equipped with a 140-mesh sieve to produce the initial powder batch.

[0022] More specifically, with respect to step 40, the material is treated with a 1% concentration of Aerosil 200 silica flow aid. The flow aid particles are mixed with the PEEK powder in a high-speed variable speed blender to achieve an angle of repose of 28 to 34 degrees as measured per ASTM C1444.

[0023] More specifically with respect to step 50, the PEEK and Aerosil blend is subjected to a secondary sieving process using a Russell Finnex sieving station (shown in Figure 5) equipped with a 140 mesh sieve and an ultrasonic head.

[0024] More specifically, with respect to step 60, the twice-sieved material (FIG. 6) is then fed into the CBAM system for production, for example, into a powderization subsystem.

[0025] The general reason why recycled PEEK from injection molding has been largely rejected by industry as a primary source of bulk raw material to date has to do with the polymer changes that occur in materials heated to process temperatures. When such materials (now waste) are heated to relatively high temperatures, melted, and cooled, the polymer chains are likely to become longer than in the virgin material they were made from. The injection molding process requires materials with consistent molecular weights, melt flow, and viscosity. While this is possible with virgin powder resins, it cannot be guaranteed with reground materials. On the other hand, as previously mentioned, the CBAM process is effective for longer chain polymers and polymers with non-uniform chain length distributions.

[0026] Similarly, many preheated materials used in conventional sintering processes experience changes in viscosity and melt flow. As a result, sintering waste is typically discarded. Once heated, the materials are inconsistent and unpredictable in viscosity and melt flow, preventing widespread reuse. The CBAM process can tolerate these variations and variations in viscosity and melt flow while still producing satisfactory workpieces.

[0027] Example #2: Recycled Polymer from the Marshon Process Aside from the grinding embodiment, the emulsion embodiment also uses recycled polymers to achieve satisfactory end-product results. In this embodiment, a waste polymer (which may be PEEK, olefins, PA12, etc., or other polymers described in this patent document) is melted and mixed with an immiscible liquid. The heated mixture is then stirred or agitated at high speed to form small droplets within the mixture. While the term "emulsion" is used, the resulting heated mixture may or may not exactly meet the most technical definition of an emulsion, as long as it is a bonded (here, molten) material containing a polymer suspended therein.

[0028] After cooling the two-phase fluid mixture, the polymer solidifies into a powder, which is then removed. This operation can take several forms. Continuous processes, such as extrusion, can be employed. An advantage of emulsion processes is that they simultaneously produce powder and remove impurities and contaminants from the recycled material. This means that post-consumer recycled polyethylene, polypropylene, and PET of any color can be used in the process. Furthermore, because polymer melt flow is not critical in CBAM, changes or variations in melt flow and molecular weight have little or no effect on the process or the finished product. Similarly, unlike many other processes where unblemished (and more expensive) raw materials are considered more desirable as feedstocks, both unblemished and colored raw materials are generally equally acceptable for CBAM products.

[0029] As one non-limiting example of the foregoing discussion of the present invention, the extrusion substep may be performed using a prior art process for converting PBT granules into a powder (RG Kleinjnen, M Schmid, K. Wegener: Production and Processing of a Spherical Polybutylene Terephthlate Powder for Laser Sintering; Applied Science, 2019, Vol. 9, pp. 1308 ff.). In the previously published discussion, 6 kg of PBT TORAYCON 1200M granules (Toray Industries, Tokyo, Japan) were blended with 9 kg of PEG polyglycol 3500S flakes (Clariant, Muttenz, Switzerland) in a laboratory-scale single-screw extruder (Brabender Extrusiograph GmbH, Duisburg, Germany) with a barrel temperature profile of 230°C to 250°C. The extrudate was passed through a slit die and then cooled to room temperature; during this cooling step, molten PBT domains (T m = 225 °C) is melted into the PEG matrix (T m It solidified within 10 minutes (=68°C).

[0030] Water was used to dissolve the PEO solid matrix phase in a 2.5 kg batch of the blend, which was stirred in a concrete mixer. The water-insoluble PBT particles settled and were then washed two more times. The resulting wet PBT was then dried. Microscopic observation revealed that 75% of the PBT particles were nearly spherical (aspect ratio A = 1.22), while the remaining 25% retained a fibrous shape induced by the shear field in the extruder. The spherical PBT particles were separated by settling to remove the smallest particles (less than 10 microns), then dried and sieved through a 150-micron sieve. The particle size distribution was D10 9 microns, D50 35 microns, and D90 110 microns.

[0031] The classified spherical PBT particles were blended with 0.05 wt% Aerosil R812 flow aid (Evonik, Essen, Germany) and used to print tensile bars in a Superstation 2000 laser sintering machine (DTM, Austin, Texas, USA). Contrary to the intended use in prior art disclosures, it was discovered that the powder obtained by the emulsion process crystallizes at higher temperatures for unknown reasons. This change has a negative impact on SLS parts, but for the reasons mentioned above, it unexpectedly provides a satisfactory feedstock for CBAM.

[0032] A second liquid mixing process may also be advantageously used. When scrap / waste thermoplastic materials are combined with a solvent (rather than a non-solvent), powder particles can be formed by a process of liquid-liquid phase separation, or LLPS. Briefly, a polymer-solvent system is heated to form a homogeneous single phase—a solution. By appropriately selecting the polymer molecular weight and concentration, the single solution phase separates upon cooling into polymer-rich liquid domains dispersed in a polymer-poor matrix. Due to interfacial surface energy, the dispersed domains assume a spherical shape. Further cooling of the two-phase mixture causes the polymer-rich droplets to crystallize, fixing their shape and allowing for powder recovery by simple filtration. Compared to emulsion methods, LLPS requires higher temperatures (dry T) for particle formation. m (exceeding 1000 Hz) or strong mechanical shear fields.

[0033] Prior art sources provide a descriptive example of an LLPS process for producing powders currently considered suitable for CBAM 3D printing (MA Dechet, et al. a: Production of polyamide 11 microparticles for additive manufacturing by liquid-liquid phase separation and precipitation. Chemical Engineering Science, Vol. 197 (2019), pp. 11-25). In a DBA-3 autoclave (Berghof) with magnetic stirring at 100 rpm and a capacity of 200 Bar, 20 grams of PA11 (Rislan BMNO, Arkema) was blended with 80 grams of 99.5% ethanol modified with 0.1% MEK. The temperature was increased to 190 °C, held for 15 minutes, and then cooled at a rate of 0.5–3 °C / min. When the temperature reached 50 °C, the reactor was opened and the PA11 powder particles were collected by filtration through Whatman #1 filter paper ("Whatman" is a registered trademark) placed on a Buchner funnel. The molecular weight by gel permeation chromatography was reduced by approximately 20% in this process.

[0034] The dried powder was treated with 0.5 wt% hydrophobic fumed silica (Aerosil, Evonik). Under electron microscopy, the particles appear as irregular equiaxed granular agglomerates; particle size distribution by light scattering yields a volume average D503 of 150 microns. Continuous cooling interrupted by a 30-minute isothermal hold at 120°C or 130°C yields a D503 of 90 microns and a D503 of 50 microns, respectively. The LLPS powder had an SLS process window of 15°C to 19°C, depending on the cooling conditions. Bed temperature: 170°C, power: 0.6 J / mm 2 Tensile specimens were prepared on a DTM Sinterstation 2000. The parts were cosmetically good, but no tensile data was reported.

[0035] The LLPS process is a batch method that requires high-pressure agitation. Particle size can be controlled by the cooling rate. LLPS is easier to remove impurities and additives than emulsion processes.

[0036] Additionally, waste materials, particularly from high-speed sintering and Hewlett-Packard's Multi-Jet Fusion processes (e.g., PA12, which would otherwise need to be discarded), can be recycled and reused in this emulsion embodiment because molecular weight changes and crystallization do not act as obstacles to CBAM. The same is true for PET recycled nonwoven substrates, as well as cellulose and other natural fibers, water-soluble paper, and olefins such as thermoplastic polymers like PET and polylactic acid (which can be derived from corn).

[0037] With reference to FIG. 7, the following procedure describes the steps required to prepare a production batch of recycled polymer material for a CBAM system according to an emulsion embodiment. In step 110, an operator obtains waste resin from other processes or products, such as PET bottles. This could also be a collection of PA12 discarded after a laser sintering process. In step 120, the operator converts the waste into flakes (if not already in a semi-ground or fully ground state). This can be done using an industrial grinder, such as those used in the grinding embodiment. In step 130, the operator then performs a emulsion process to convert the flakes into a powder. As previously mentioned, this process involves stirring or agitation while heating with an immiscible liquid. Finally, in step 140, the resulting powder is placed into a CBAM printing process to create the finished artifact.

[0038] Just as an injection molded part is structurally different from a welded or sculpted part with the same overall shape, one skilled in the art will understand that a 3D printed CBAM part constructed using any of the methods described above will be structurally different from a 3D printed CBAM part made from virgin polymer. At a microscopic level, the use of powdered feedstock polymer material according to any of the above descriptions will result in different properties and characteristics due to the large variations in (for example) polymer chain length, crystallization structure, etc. across the final part.

[0039] While the above specification and examples provide a description of the present invention, many embodiments of the present invention are possible without departing from the spirit and scope of the present invention. It should be understood that the foregoing embodiments are provided by way of example only and are not intended to limit or define the scope of the present invention. Various other embodiments are also within the scope of the following claims.

Claims

1. a. Converting polymeric materials used in industrial processes or consumer products into recycled powders; b) inserting said recycled powder into a powderization step of a 3D printing process that deposits said recycled powder onto a substrate sheet to create cross-sectional layers corresponding to planar layers of a 3D object; 1. A process for making 3D objects from recycled materials, comprising: First, grinding the polymer material into particles; Second, sieving the particles a first time to obtain sieved particles; Third, adding a flow aid to the sieved particles; fourth, sieving the sieved particles a second time to obtain double-sieved particles.

2. The process of claim 1 , wherein the adding step comprises adding a flow aid.

3. The process of claim 1, wherein the particle distribution D50 is greater than 70 microns.

4. a. Converting polymeric materials used in industrial processes or consumer products into recycled powders; b) inserting said recycled powder into a powderization step of a 3D printing process that deposits said recycled powder onto a substrate sheet to create cross-sectional layers corresponding to planar layers of a 3D object; 1. A process for making 3D objects from recycled materials, comprising: The converting step includes: First, processing the polymeric material into a modified polymer consisting of smaller particles than those originally contained in the polymeric material, and then melting the smaller particles to form a molten polymer; second, stirring or agitating the modified polymer in an immiscible liquid set at a temperature above the melting point of the polymeric material but having a lower melting point than the polymeric material to provide sufficient shear to form a polymer-liquid mixture including additional modified polymer suspended in the liquid; third, outputting the polymer-liquid mixture while above the melting point temperatures of both the polymer material and the immiscible liquid to form an output mixture; fourth, cooling the output mixture below the melting point temperature of the polymeric material to form polymer particles suspended in the immiscible liquid; Fifth, removing the immiscible liquid to obtain particles of polymer formed during stirring or agitation; Sixth, drying the polymer particles; A process having

5. 5. The process of claim 4, wherein the processing step produces the modified polymer in the form of particles, flakes, pellets, or granules.

6. a. Converting polymeric materials used in industrial processes or consumer products into recycled powders; b) inserting said recycled powder into a powderization step of a 3D printing process that deposits said recycled powder onto a substrate sheet to create cross-sectional layers corresponding to planar layers of a 3D object; 1. A process for making 3D objects from recycled materials, comprising: The converting step includes: First, mixing the polymeric material with a solvent liquid at an elevated temperature to form a polymer-liquid mixture; second, heating the polymer-liquid mixture with stirring to form a solution; Third, cooling the solution to form two liquid phases having polymer-rich droplets in a polymer-poor matrix; Fourth, continuing to cool the two liquid phases until substantially all of the polymer has precipitated; Fifth, filtering the solvent-precipitation mixture to recover a powder; Sixth, the process includes drying the powder.

7. 7. The process of claim 6, wherein the step of cooling the solution to form two liquid phases is carried out at a cooling rate of about 0.5 to 3° C. per minute.

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