Plastics, whether in powder, filament, or pellet form, are one of the main material types used in 3D printing/ additive manufacturing (3DP/AM). As 3DP/AM matures, developers continue to explore plastic chemistries and capabilities to offer materials that rival more traditional versions. For example, polypropylene and ABS can have higher tensile strength than traditional injection-molded plastics. Other interesting information of note include:
— Photopolymers continue to be one of the larger groups of plastic materials purchased for use in 3D printing. Known as thermoset plastics, photopolymers cure (harden) with exposure to some type of light source. Frequent light sources include lasers, LEDs, lamps, and even visible light. Once hardened, these materials cannot be remelted and reused as the photoinitiators in them (oligomers or monomers) bind into three-dimensional forms during light exposure. Photopolymer developers will take advantage of this characteristic and alter the mix of oligomers and monomers to develop specific physical properties such as stiffness or viscosity.
–Variations of photopolymer materials have characteristics that closely match those of injection-molded thermoplastics. These versions can maintain shape and mechanical properties for months. One example is CeraMAX from 3D Systems. It is a ceramic-reinforced composite with good temperature, chemical, moisture and abrasion resistance characteristics.
— So what’s the difference between plastics and resins? The length of the carbon chains within them. Plastics have long carbon chains; resins have short carbon chains. Exposure to UV light starts a process where photoinitiators create much longer and stiffer chains.
— The main 3DP/AM technologies that work with photopolymers are:
–vat photopolymerization, also known as stereolithography (SL);
–digital light processing (DLP), which includes Continuous Digital Light Manufacturing (cDLM) technology from EnvisionTEC and the MOVINGlight process from Prodways;
–Continuous Liquid Interface Production (CLIP) from Carbon;
–and various material jetting processes, such as PolyJet from Stratasys Ltd. and MultiJet Printing (MJP) from 3D Systems.
— To find a material’s properties, consult with ASTM International, which publishes information on various standard test methods used to determine material properties.
–Of the seven or so major 3D printing technologies, the Stereolithography (SL) 3D printing process produces parts that come the closest to resembling plastic parts made through injection molding, as far as finish goes. In some cases, depending on the material used, the SL process produces parts with better mechanical properties than those produced through injection molding.
Most SL 3D printing processes use custom techniques to modulate the cure rate of the plastic material, putting the material in a semi-reactive state. In this state, the material retains enough pliability to bond with previous layers without demarcation lines between the layers. The semi-reactive state is also known as the “green state.”
SL materials come in a range of tensile strengths, tensile and flexural moduli, and color and opacity options. Parts made through SL exhibit strength regardless of build orientation, a property known as isotropic.
In general, the impact strengths and heat deflection temperatures (HDTs) are lower than those of common injection-molded plastics. However, newer formulations of resins and plastics are challenging injection molding.
Carbon, for example, uses custom resins that can produce physical characteristics that rival those made with injection molding. Carbon’s CLIP technology uses a specific process to build parts quickly.
Like most SL parts, Carbon parts are isotropic. Post process heating is a finishing step that can activate other properties in the materials.
The EPU 60 material from Carbon, for example, is an elastomeric polyurethane (EPU). Characterisics include high elasticity, resilience, and tear resistance.
Another material for SL is Somos WaterShed XC 11122. The characteristics of it include low moisture absorption (0.35%) and near-colorless transparency. This material is suitable for general-purpose applications and for use in flow-visualization applications. It’s tensile strength, compared to average injection-molded ABS values, is 7.8 ksi vs. 6.0 ksi, with HDT at 130°F vs. 215°F.
Accura SL 5530A is a material offered by Proto Labs with a thermal post-cure. It has tensile and flexural moduli of 545 ksi and 527 ksi, respectively. Postcure processes can make 5530 less durable with an impact strength of 0.4 ft-lb/in. Without the thermal post-cure, 5530 retains its tensile strength and becomes more flexible.
Selective laser sintering
Selective laser sintering (SLS) can use metals or plastics in powder form. The plastic materials are usually thermoplastic powders such as PA (polyamide).
Parts made of polyamide in this process exhibit good toughness, impact strength, and high heat deflection temperatures similar to those of injection molded PA materials. However, the mechanical properties differ between AM and injection molding. The surface finish of an SLS part may need post processing, depending on the final application of the part. And the SLS process is not known for delivering fine surface details.
Proto Labs offers a fiber filled polymide material, DuraForm HST composite, that exceeds a mineral-filled PA 12 in all areas except tensile strength. It is similar to a 25% mineral-filled PA 12. The fiber content increases strength, stiffness and HDT to temperatures above 300°F. It is a good choice for functional applications. The material is somewhat brittle, however, with an elongation at break of 4.5%. Proto Labs offers several plastic materials, which can be found on its website.
The Polyjet printers from Stratasys and the Multi-jetting printers from 3D Systems use photopolymers in the form of photosensitive inks, which are cured under a UV lamp.
The photosensitive inks tend to be more reactive and cure more quickly than photopolymers used for other additive/3D printing processes. In jetting processes, the photosensitive inks can be precisely placed in the build of a part to develop a variety of physical properties in or on the object. Support material may consist of a type of hydrogel that can be washed away with water.