By Jon Eric Van Roekel, Protolabs

Advancements in 3D printing have made it a proven, industrial-grade manufacturing practice for end-use parts that is shaping product design. At Protolabs, we are seeing this evolution firsthand as customers are leveraging new additive manufacturing (AM) processes and expanding material options to make a more complex, lightweight, or sustainable product.

We are in an exciting time for manufacturing. While we are a constantly evolving industry, it is not every day we have an emerging practice like 3D printing making the previously impossible, possible.

However, the integration of additive manufacturing (AM) into the product design process is not without some tradeoffs along the way. A common decision point to overcome for many is choosing the right material, an evaluation step that is more complex with 3D printing practices. Or at least requires a new set of considerations.

A material must be well-suited for the application in order to achieve design goals. And as we see 3D printing emerge as a manufacturing method for end-use part production, the properties of any material become increasingly important. With that being said, AM benefits tremendously from recent investment and R&D efforts dedicated to the creation of materials specifically designed for industrial-grade 3D printers. In terms of mechanical and physical properties, material selection hinges on the prioritization of design and desired quality requirements.

Compared to traditional manufacturing practices, materials for 3D printing are still advancing to include rich sets of performance data that characterize materials over a range of conditions. Another factor to consider is that 3D printing produces anisotropic properties where the values differ for the X, Y, and Z axes. The degree of anisotropy varies by technology, but it should always be a consideration. Both can be overcome by designing for additive manufacturing and leveraging the experience of knowledgeable service bureaus that have worked on millions of parts.

What to consider with 3D printing materials
One or two material properties can distinguish one additive material from another. The first step in identifying a material is defining the mechanical and physical properties that are critical for the application. Common measured properties of interest to designers can include:
• Ultimate tensile strength (UTS): The maximum stress the material can withstand before breaking.
• Tensile modulus, or elastic modulus: Measures stiffness, the higher the modulus, the stiffer the material.
• Elongation (%): Measuring ductility, a higher elongation percentage indicates a material is more likely to be able to stretch or elongate into a thin wire shape.
• Hardness: The higher the number, the harder the material. Hardness is typically measured and reported in HRC or HRB on the Rockwell scale for metals. For polymers, like PolyJet materials, durometers are typically reported.
• Heat Deflection Temperature (HDT): Sometimes called heat distortion temperature, the temperature at which deformation occurs when a rigid material is placed under a specific load.

The following takes a look at the information to consider in the selection of materials for the most popular 3D printing processes: direct metal laser sintering (DMLS), stereolithography (SLA), Carbon DLS, selective laser sintering (SLS), Multi Jet Fusion (MJF), and PolyJet (PJ).

Direct Metal Laser Sintering (DMLS) materials
DMLS, a type of metal 3D printing, uses pure metal powder to produce parts with properties that are generally accepted to be comparable to wrought metals when comparing them in the heat-treated condition. Unique in 3D printing, it also produces parts with material properties that approach an isotropic state, meaning similar properties independent from direction of measurement.

Some commonly used materials to consider:
• Aluminum AlSi10Mg is comparable to a 360.0F aluminum alloy, which is commonly used for die-casting. The material features good strength-to-weight ratio, high temperature and corrosion resistance, and good fatigue, creep, and rupture strength.
• Stainless steel is available in two grades at Protolabs: 17-4 PH and 316L. Select 17-4 PH (precipitation hardened) for its significantly higher tensile strength and yield strength, but recognize that it has less elongation at break than 316L, which means that 17-4 is less malleable than 316L.

Stereolithography (SLA) & Carbon DLS materials
SLA offers the broadest selection of 3D-printable plastics with a large range of mechanical properties. It is also a go-to process for parts that require fine features and cosmetics, as well as a quality surface finish. Compared to injection-molded plastics, impact strengths are often lower, and exposure to moisture and UV light may alter the appearance, size, and mechanical properties of SLA-printed parts over time.

Some commonly used, all-purpose materials to consider:
• ABS-like White and ABS-like Gray are two of the more widely used durable SLA materials. In terms of flexibility and strength, both materials fall between molded polypropylene and molded ABS, which makes them an easy choice for functional prototypes.
• Ceramic-like Advanced HighTemp (PerFORM) is a stiff material option that is superior to similar injection-molded thermoplastics when HDT (heat deflection) is critical.
• RPU 70 Rigid Polyurethane is a tough, all-purpose material. When manufactured using Carbon DLS, it can be categorized as an ABS-like material. Due to the post-build thermal baking process, materials paired with Carbon DLS can achieve heightened mechanical properties.

Selective Laser Sintering (SLS) & Multi Jet Fusion (MJF) materials
Primarily using polyamide nylon materials, both processes offer the most economical choices with often greater toughness and high impact strengths when compared to SLA. The density of SLS/MJF parts are also closer to traditionally manufactured parts; however, both processes lack the ability to provide the surface finish and fine feature details offered with SLA. When compared to injection-molded materials, polyamide has similar HDT values but less robust mechanical properties. SLS/MJF material properties also have a known degree of anisotropism when measured in the x-y plane or the z plane.

Some durable materials to consider:
• Nylon PA 11 Black is a popular, general-purpose material that delivers on ductility and flexibility without sacrificing tensile strength and temperature resistance. It also offers the highest elongation of all AM nylons. A stiffer option is PA 12 White, which features slightly higher elastic modulus. These PA nylons can be mineral or glass-filled to further improve stiffness.

PolyJet (PJ) materials
The PJ printing process produces digital photopolymer parts with varying flexibilities, durometers, and colors. Compared to injection-molded liquid silicone rubber (LSR), the mechanical properties are similar with one important distinction: digital photopolymers show more viscoelastic creep than an equivalent LSR material, meaning it may feel softer over time when under constant stress.

In conclusion, the wealth of 3D printing materials available today across all processes make additive manufacturing more accessible than ever, and that library of materials is ever-expanding. As more companies turn to additive manufacturing for production parts, the ecosystem is also better defining the capabilities and development opportunities for each material.

Jon Eric Van Roekel is a process engineering manager for 3D printing at Protolabs.

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