3.1 Polymer Materials

Key characteristic of AM polymers and base monomers/chemistries

The key characteristic that must be considered for the polymers used in AM is the method by which they are shaped and under what conditions can the AM process yield the required resolution and properties required for a given material and application. For example, the only way to process a polyamide (Nylon) is by thermal shaping, as this material cannot be photopolymerized. Therefore, if polyamides are the desired end-use material, thermal AM processes, such as ME and PBF are the main pathways for production of these parts. Similarly, if a thermosetting acrylate is required, the only way to access this polymer composition is through vat photopolymerization. Using these conventional pathways for deploying polymers in AM, processes can be paired with the requisite materials, or vice-versa. From there, dimensional tolerances, surface finish, and post-processing schemes can be considered. However, new research breakthroughs are occurring in hybrid polymer processing where the shaping technology is complemented by a post-processing scheme to facilitate post-shaping chemistry or conversion of the material. Materials including crosslinked polyurethanes, cyanate ester thermosets, and liquid silicone rubber are all being deployed in multistage processes to access both the advantages of AM shaping technology and robust engineering properties of these materials.

Polymer Feedstock Types and Processes

Based on the two general classes of polymers, thermoplastics and thermosets, there are two basic types of polymer feedstocks and printing processes that form the basis for understanding the constellation of polymer materials for AM. For thermoplastics printing, Material Extrusion AM, is ubiquitous. This type of printing employs a filament or pellet feedstock that is thermally extruded through a simple orifice to build a component from roads. Readers versed in the art will know the ever-present MakerBot and similar printers as a Material Extrusion AM thermal process. While ME is still the predominant technology for hobbyists and desktop printing, it has been adapted successfully to a number of commercial systems including units by Stratasys and Cincinnati Incorporated. ME is inexpensive in concept and can be successfully used to process a wide variety of thermoplastics at many different size scales.

Powder bed fusion of thermoplastics, while laser driven, is a thermal process. Long wavelength (1-10 μm) fiber or diode lasers are used to heat thermoplastic powder that is often darkened with carbon black or other additives to increase the heating of the powder for rapid fusion. This technology is deployed industrially for a few different types of polymers, namely PA11/12 and PEEK. The polymer PBF equipment setup is similar to that used for metals, however, the physics of fusion is much different between the liquid melt pool needed for metals and the fusion of viscous polymer powders above their melting temperature. Polymers have two thermal transitions, the Tg, or glass transition temperature, and the Tm, or crystalline melting temperature (Figure 3.2). The Tg, where a polymer turns from a glassy solid into a rubbery solid, can be observed for nearly all polymers and is defined as the temperature at which chains can diffuse past one another and the material can undergo flow when exposed to shear. At the Tg, the viscosity of the material drops in the transition from a glass to a rubber. To achieve reasonable flow, amorphous polymers must generally be processed 50 °C or more above their Tg. The Tm only exists for semi-crystalline polymers that have a mixture of amorphous and crystalline regions in their solid-state structure. Polymers with a Tm must be processed 20-50 °C above Tm to induce rapid flow and fusion. Still, even above Tm, polymers are viscoelastic and do not easily undergo rapid fusion. Thus, only a small number of polymers have been employed widely in PBF processes.

Figure 3.2 Viscosity/Modulus of thermoplastic polymers as a function of temperature

PBF generally requires a polymer to have a Tm and a rapid viscosity drop above Tm to achieve the temperature spike, flow, fusion, and solidification of the material. While some success has been achieved in the PBF of amorphous thermoplastics, the transient temperature spikes and rapid processing times of PBF are not compatible with a highly viscous polymer above Tg. Thus, material introductions to PBF are generally slow and the limited availability of development machines and suitable polymers has held back the widespread adoption of this technology outside of specialized applications. Additionally, polymer powders are highly explosive and pose a serious environmental safety and health risk due to their low density and ability to circulate in the air for extended periods.

For thermosets, light-driven vat photopolymerization including the variations of SLA, DLP, CLIP, (as described in Ch 2) etc. are employed. The feedstocks for these processes are all UV-cured photopolymers where the rapid liquid to solid curing kinetics are leveraged for component fabrication using high-intensity photopolymer curing methods such as UV DMD chips and diode lasers. While some heat is liberated from photocuring reactions, the primary mechanism of the liquid to solid transition is photochemical, although heat must be managed during rapid curing. Material jetting of UV curable polymers is widely employed to obtain highly detailed models with resolution rivalling the best VP systems.

Material preparation for polymers in AM is relatively straightforward in the industry, although production of well-dimensioned filaments, polymer powder, and shelf-stable liquid polymer resins is challenging for non-specialists. These materials can be prepared for processing under mild conditions which lends itself to widespread adoption and experimentation across the community.

Photopolymers

Photopolymers or light-cured materials are the feedstocks used in vat photopolymerization and MJ processes, among other light-driven AM processes. Photopolymers are a wide class of materials, but industrially, photopolymerization or photocrosslinking (a technique that uses light to create bonds) of liquid precursors to crosslinked solids is usually achieved through UV curing of acrylates. This photopolymer crosslinking chemistry is fast, inexpensive, and easy to deploy for a wide range of polymers to yield a tremendous variety of materials. In fact, nearly all commercially sold vat photopolymerization and ME resins are the same crosslinking chemistry.

The different properties derived from light-cured acrylate resins are due to the chemical composition of the groups between acrylate crosslinking sites, the crosslinking density, and other additives, such as monomer diluents. Using photopolymerization, elastomers, tough engineering plastics, and even highly filled ceramic resins can be accessed. Many feedstocks for vat photopolymerization are sold with descriptors such as “polyethylene like” or “polycarbonate like” properties. In fact, these materials may not contain any of the noted polymers in their descriptions, but the resin formulations are tuned to display thermal and mechanical properties that mimic more familiar thermoplastic polymeric materials. This approach is typified in Table 3.2. Many of these resins contain 20-30 components including UV absorbers and stabilizers and are not usually formulated by the manufacturer listed on the label, these materials are generally formulated by specialty chemical suppliers for private label reselling. Generally, the formulation of these materials is confidential. This type of feedstock formulation is still in the art phase although advanced experimental design and data analytics can be used to optimize these materials.

The advent of advanced vat polymerization processes and chemistries that complement this rapid production of engineering-grade parts has ushered in a new approach of rapid light shaping and dual cure mechanisms. Companies like Carbon have leveraged new chemical approaches to VP feedstocks and post-processing schemes to achieve properties in photopolymerization materials that have not been possible previously. The resolution and surface finish of VP makes it alluring for direct production of consumer and industrial grade parts and new materials, coupled with improvements in printing technology, have brought a number of successful products forward.

Thermosets

Thermosets tend to refer to any crosslinked polymer system, including photopolymers (or perhaps photosets). Familiar thermosetting materials include epoxies, crosslinked polyurethanes, and thermally cured or room temperature vulcanizate (RTV) silicones, as examples. Thermoset usually implies thermal curing at temperatures greater than room temperature, but this does not have to be the case. Superglue is a thermoset, along with bathroom caulk. Once hardened, thermosets cannot be reprocessed due to the crosslinking of polymer chains and their inability to flow. By far, the largest use of thermosets in AM is in vat photopolymerization that leverages the liquid to solid transition that occurs due to the rapid increase in molecular weight and crosslinking of polymer chains during acrylate photopolymerization. As described above, these acrylate crosslinking systems can achieve a wide range of properties that are dependent on the polymer characteristics between acrylate groups. In addition to vat photpolymerization, MJ printing leverages thermosetting liquid acrylate resins for building voxel-based designs.

Thermosetting polymers such as two-part epoxies or thermally cured silicones can be deposited using direct ink writing, which is a form of material extrusion AM. In many cases, these thermally cured thermosets are post-processed to achieve the desired properties. For instance, many long pot life silicones and epoxies tend to cure at temperatures above 100 °C. These materials also have Bingham plastic behavior where they can support a finite stress at zero shear. This unique property of these highly viscous thermosetting precursors can be used to stabilize liquid direct ink writing printing before thermal post-curing.

Thermoplastics

Thermoplastics are ubiquitous in ME and PBF and there are at least 20 different types of thermoplastics used routinely in AM processes that can be sourced from any of the major manufacturers and polymer distribution houses. Thermoplastic polymers are characterized by their ability to be shaped under high temperatures (between 150-400 °C) and moderate pressures. All of the common AM polymers are thermoplastics, aside from those that are used in vat photopolymerization and liquid resin-based printing. These include Nylons (polyamides), PE, PP, PLA, ABS, PET, PC, and PEEK, among others, see Table 3.3. All of the thermoplastics available for AM are sourced from the plastics processing industry where extrusion and molding are the mass-production technologies of choice. Thermoplastics are generally used for inexpensive, moderately demanding applications. Composites of thermoplastics are widely used in consumer and automotive components and are the result of mass-production from injection molding. However, injection molding is prohibitively expensive for small part counts and machining of many plastics can be challenging. Therefore, AM of thermoplastics has filled an important gap in prototyping of plastic parts. These materials can also be ground and reprocessed, although degradation in performance usually results from repeated recycling.

One of the key drawbacks of current hobby and prosumer printers is the need for thermoplastic filament. The main feedstock used in the polymer industry is pellets. So, pellets must be converted through extrusion into a well-gauged filament. The consistency of the filament dimensions is of utmost importance as most machines feed a certain length of filament in the build and do not monitor variations in thickness or flowrate. Pellet printers that directly use widely available materials from the plastics extrusion industry are highly desirable, but are not common for small-scale printing. While there are drawbacks of deploying thermoplastics in relatively low-resolution printing processes, their engineering properties and low-cost still make thermoplastics desirable materials.

Thermoplastics can have a wide variety of properties, including water solubility. Many ME builds employ water soluble supports in dual printing schemes to enable complexity that is difficult to achieve with single-material builds. Water soluble thermoplastics such as poly(vinyl alcohol) (PVA) can be deployed in many dual extrusion thermal ME printers and have the melt-flow characteristics needed for support structures. By the same method of dual or multiple extrusion, multicomponent builds using thermoplastics are possible, although material fusion can be a challenge. Additionally, a number of dual head printers exist on the market, but more than two materials integrated into a fused filament fabrication scheme is not common. ME of thermoplastics yields the characteristic striations of the extruded, layered build of fused filament fabrication, along with the associated z-direction anisotropy (as described explicitly in ch 2).

PBF of PA11/12 and PEEK has begun to disrupt the production of high-performance polymer parts as a consequence of the high-fidelity manufacturing available from PBF and 100 or even 1000s of parts accessible in short run times. However, while PBF does incur anisotropy, its part count per build, reasonable surface finish, and acceptable tolerances make PBF one of the scalable routes to large-scale manufacture of polymer parts. PA11/12 (Nylon variations) and PEEK are also highly desirable thermoplastics that can be used in demanding applications from consumer goods to biomedical devices. Production of thermoplastic powders is usually accomplished by cryogrinding polymer pellets, although different methods of polymer powder production and the powder characteristics needed for effective polymer PBF are an area of active research.

Composites

Composites can be composed of a thermosetting or thermoplastic polymer and a “filler” or reinforcing inorganic phase, typically glass, carbon fiber, carbon black or ceramic particles like silica. There are many types of composite materials, but for the purposes of this discussion, we will confine the description of composites to those that contain polymer and fiber or particles and are processed in the realm of filled thermoplastic polymers. Because polymers are generally softer than other engineering materials, filling them with inorganic phases greatly increases their moduli and rigidity. Key to formulating composites is a coupling agent between the filler phase and the polymer matrix. Silane coupling agents or sizing agents have been specifically designed for currently available composite systems and are being leveraged to produce robust composite materials for AM. Chopped fiber or particle filled polymers are widely available, but can be abrasive during thermal ME, which is the most common method of processing composite polymeric materials. Some SLA resins are composites, but these are generally classified as ceramic photopolymer resins where the composite properties are less important than using the photopolymer function to shape the material.

While composite materials are critically important and widely used, their advance in the AM industry has been slow, primarily due to their difficult processing and the different formulations needed between molding and extrusion as compared to AM. Adding a polymer filler increases a polymer’s melt viscosity and can drastically reduce interlayer adhesion. Additionally, many composites are processed under vacuum conditions to compact the material and avoid air pocket defects, which is difficult to deploy in AM. Composites have played a major role in the development of Big Area Additive Manufacturing (BAAM). Because most polymeric materials used in structure applications are composites, a process that produced filled composite materials was critical to large-scale printing. Currently, most BAAM materials are 10-30 wt % chopped glass or carbon fiber in ABS or other medium performance thermoplastics. As the adoption of BAAM and similar platforms has progressed, major plastics manufacturers like SABIC have introduced AM-specific lines in addition to boutique polymer compounding house’s products, such as those available from TechmerPM, see Table 3.4. As an advantage of these composite feedstock materials, stiffer materials can combat warping during printing. Oak Ridge National Laboratory showed that carbon fiber filled materials resist thermal warping during printing compared to more flexible thermoplastics. So, even though composite materials are more difficult to process, there can be unique properties that can be leveraged for innovative processing schemes. Another example of unique facets of composite feedstocks is the use of external IR heating to promote interlayer adhesion. This type of pre-heating is helpful to promote interlayer adhesion, especially in large builds. Due to the presence of carbon fiber in many of these materials, the heating at the surface can be accomplished rapidly without long hold times at temperature that may oxidize the material.

Finally, continuous fiber composites such as fiberglass and carbon fiber composites are some of the highest-performance widespread materials available for industrial and consumer applications. Aerospace components, automobile panels, wind turbine blades, sporting goods, and other lightweight, strong components are all composed of these continuous fiber composites. However, due to the layup procedures and vacuum consolidation used in production of these types of parts, processing continuous fiber composites remains a challenge. Markforged has pioneered continuous fiber AM using ME with layers of thermoplastic and thermoplastic coated Kevlar, glass, or carbon fibers. These parts have shown promise in light-duty structural aluminum replacement applications where loads are reasonable, but the intrinsically layered structure of this method and the presets in Markforged software presents challenges to custom part design. Conventional high-performance composites also have some interwoven character to the reinforcement, which is difficult to envision being deployed in standard AM. Z-extrusion and interlayer stitching methodologies have been reported in the literature, but have not made it out of the research laboratory, yet.

There have been attempts to adapt automated fiber layup machines, such as the Ingersoll Machine Tools MongooseTM machine. These types of concepts have been successful in printing on curved mandrels in sizes up to those of 100 m wind turbine blades, although free-forming large-scale components continues to be an unmet challenge.

Elastomers

Elastomers are a high-value class of polymers that can have hardnesses and strengths ranging from nearly glassy polymers with high impact strengths and high loading resistance to the softest silicones for vibration dampening and all familiar rubbery materials in between. Interestingly, elastomers can be composed of either thermoplastics, as in thermoplastic elastomers, or thermosets, as in thermally cured liquid silicone rubbers. There are a few key thermoplastic elastomers (TPE) on the market for AM, including the NinjaTek (Fenner, Inc.) family of products which are generally thermoplastic polyurethanes (TPU) with durometers of Shore 85A to 95A. These ME filament feedstocks are derived from injection molding grades of TPUs. Other TPEs on the market include Kraton® (Kraton Corp.) or Pebax® (Arkema Group)-based filaments sold under a variety of trade names. Since all of these materials were initially invented for traditional extrusion-based processing of polymeric materials, like injection molding, much is known about their performance across a wide range of applications and they are in common use. Soft elastomers with Shore 60A or lower durometers are difficult to adapt to traditional filament ME processes due to the mechanical deformation of the filament as it is fed into the extrusion die. Therefore, pellet-based ME of elastomers enables the use of a wide range of materials, although elastomer parts tend to be small with fine detail and the larger pellet-based ME technology is not necessarily suitable for these components.

Flexible SLA resins and MJ flexible photopolymer materials are widespread on the market. What is interesting about MJ elastomers is that due to the voxelization of the MJ process, the durometer of these materials can be dialed in during the design of the part – just by mixing hard and soft voxels, an element of volume in 3-dimensional space. Combined with the architectural complexity of additive manufacturing, material variations on the fly are an important advantage of this technology, albeit the photopolymerization of elastomers generally does not yield the properties of melt-processed materials.

Photopolymer elastomers can be widely customized during blending in the manufacture of the material or at the point of use. Because of the multicomponent photopolymer formulations, a wide range of properties can be achieved, however the elongational properties of these thermoset elastomers are not as good, in general, as the elongation of TPEs or thermally cured elastomers from direct in writing. Regardless, SLA and its vat polymerization variants give the surface finish and fine detail required for consumer products such as silicone watch bands and soft-touch headphone components. Direct ink writing (DIW) ME has been demonstrated for a variety of silicones from Wacker Chemie AG and Dow although due to the drawbacks of DIW, there are not commercial applications of these processes, yet.

TPUs are also being investigated in PBF and HP Multi-Jet Fusion processes as the powder feedstock. TPU powder is still under development by a number of companies, including Covestro. While PBF and HP MJF processes are routine for PA11 and other thermoplastics, the unique powder rheology and fusion characteristics of elastomers need to be revisited in terms of these known processes. Additionally, highly fused elastomer powders will likely suffer from low elongation to break and high defect concentrations which will limit their high-strain properties and fatigue resistance.

Polymer summary

Figure 3.3 Polymer performance pyramid.

Additionally, there are some polymers that are widely used that have not been deployed in additive processes. Materials such at polytetrafluoroethylene (PTFE), polybenzimidazole (PBI), and other common polymeric materials, such as Kevlar, are not necessarily melt processible or cannot be formulated as a polymerizable liquid resin and will be difficult to integrate into AM feedstocks except in the case of composites.

The availability of high-performance, finished materials directly from the AM process (and associated post-processing) enables low-run production of end-use parts, full-functionality advanced prototypes, and one-of-one customization without sacrificing the materials performance that is expected from high-fidelity manufacturing techniques.