It goes without saying that different manufacturing materials have fundamentally different properties. Certain materials are excellent in certain contexts, but are poorly suited for others. Glass is an excellent material for creating transparent windows, but a poor choice for children’s toys. Right?
What’s maybe a bit less obvious is that the same kind of material can exhibit different properties depending upon the production method. The properties of an object will be determined both by the materials it’s made from as well as the process by which it was made. Thermoplastics that are machined have different properties than those that are injection molded or 3D printed. In this article, we’ll go over some considerations to help you decide what materials are appropriate to your project, and we’ll go over the four major kinds of materials currently available.
3D printing, or additive manufacturing, is a unique manufacturing technique, and while it has many exciting advantages, it also has its own set of challenges. Because of the unique method by which objects are put together in a layer-additive process, the physical characteristics of the produced part can be different in certain ways from a similar part made from the same alloy or thermoplastic by a different method.
This isn’t to say that parts that are printed are better or worse. They are simply different, and the relative advantages and disadvantages of various material and production options will depend upon the nature of your project. When you’re planning on 3D printing your next prototype of your new product, the selection of materials needs to be an important consideration.
Challenges with 3D Printing
3D printing technology has been advancing rapidly over the past few years, and it continues to improve every day. This is great for product designers, as additive manufacturing has gone beyond the realm of models and prototypes and is now a viable technology for functional testing, industrial use, and even for production parts. With its capacity to create complex geometries and for easy iteration, this is an exciting development for innovators.
That said, there are some challenges specific to 3D printing. The available data pertaining to the performance of various materials exposed to different conditions is somewhat lacking in additive manufacturing compared with other production methods like casting or molding. This is largely because experimentation is somewhat more difficult given the peculiarities of 3D printing and the considerable effect the design has on the physical properties of the part.
The data that 3D printing users do have access to tends to be a bit more limited than with other manufacturing options, and tends to represent a best-case scenario, which may not necessarily reflect your experience. For example, material information that is based on testing “virgin material” may not correspond accurately to a recycled form of the same material.
Finally, perhaps the most complicated aspect of 3D printing is that the material properties can vary significantly across the X, Y and Z axes. Because of the way that materials are built up layer by layer, 3D printed objects are generally not isotropic. Isotropy means being uniform in all directions. 3D printed objects are anisotropic, meaning that they are not uniform in all directions. If you have identical parts, one that is printed and the other being cast, the printed part will be anisotropic even if the cast part is isotropic. This has a significant impact on the mechanical properties of the object.
Different 3D printing technologies will be more or less uniform. Direct metal laser sintering produces parts that are the closest to isotropic, but they still aren’t quite.
These deficiencies or challenges with 3D printed can be accommodated and dealt with effectively at the design stage. Parts must be designed specifically with additive manufacturing in mind. Working with an experienced designer who understands 3D printing is important. Bringing in outside expertise is a great idea if you lack the knowledge and experience internally.
Choosing a Material
With such a wide range of materials now available for 3D printing, the best strategy to approach material selection is to first determine the kinds of properties you product or part needs to have. Once you have a good idea of the mechanical and thermal characteristics that are important for your project, you can then narrow down the list of materials and eventually select the most appropriate candidate(s). Once you’ve narrowed the list down to a smallish selection, you can further refine your material requirements or look at secondary characteristics that you would prefer but which aren’t critical.
There can be a tendency to want to simply take a material that one is familiar with from molding or machining and just transfer it over to 3D printing. This isn’t the best strategy. While the same material may be available for 3D printing, the processes are sufficiently different that the material which was ideal for molding, for example, may not be the best choice when it comes to additive manufacturing. To a certain extent, you want to forget what you think you know about manufacturing materials so that you can approach 3D printing with fresh eyes. Evaluate the material options in order to find the best solution — don’t assume that because it worked with other techniques that it is the best option here.
A Wealth of Options: Considerations When Selecting a Material
The variety of material choices available now for 3D printing is really quite incredible. Thermoplastics like those used in injection molding are commonly used in Fused Deposition Modeling (FDM). Materials that have traditionally required machining are also available. Direct Metal Laser Sintering (DMLS) can now be used with stainless steel and titanium. Biocompatible materials like ULTEM 1010 and polycarbonate that can be used for surgeries, joint replacements, heart valves, etc are also available for Laser Sintering and FDM.
You need a strategy for narrowing the choices down. To do so, there are some important considerations to make.
Primary Application: Depending upon how your product will be used, there may be certain important characteristics that are only available using certain materials. Considering whether your product must meet any of the following conditions will help you narrow down your options:
- FDA certification
- Heat certification
- Toxicity certification
- Fire retardant certification
- Chemical resistance
- Other certifications
Geometry/Shape: The shape of your product or part should play a big role in your selection both of material and 3D printing technology. Different methods have varying strengths and weaknesses when it comes to the types of geometries they can produce. This needs to be considered alongside your mechanical and other material considerations. You need to think carefully about the stress tolerances and wall thickness that your design requires and compare that with the materials available that are appropriate for your application (above).
Post Processing: Different materials and additive techniques require varying levels of post-processing once the part has actually been printed. Stereolithography, for example, produces extremely detailed parts with a high level of surface accuracy, but often requires the use of support structures in the design in order to hold the part together during assembly. These need to be removed after the fact. Other techniques like SLS that user powder-based materials may or may not require support structures depending on the geometry of your design. On the other end of the spectrum, 3D printed metal parts from DMSL require a significant amount of post-processing that must be undertaken by experts with the proper equipment.
Let’s look at some of the most popular kinds of materials currently available.
Photopolymers begin as liquid resins, which harden when exposed to special UV light to produce plastic parts. Photopolymers are used primarily for PolyJet and Stereolithography. These materials can create very high-resolution models, and are great for master patterns for casting. They are also regularly used for precise form and fit testing. Because of their high degree of accuracy, they are often used as models for medical devices, anatomical models, and other education purposes.
Both PolyJet and SLA have considerably lower HDT than other 3D printing methods and are more likely to warp when exposed to heat or UV rays. This limits the kinds of applications to which they can be put.
PolyJet creates a smooth surface and offers a variety of colors. These materials achieve the highest resolution of any 3D printing technology, with layers as thin as 16 microns. A major advantage of PolyJet is that it is currently the only process that can print in multiple materials with different durometers. Common PolyJet materials include VeroWhitePLus, VeroBlue, VeryGray, Amber Clear, GreenFire, and Flex.
SLA offers easily sanded surfaces for cosmetic paint finishes. Dimensional tolerance is between 0.020″ or±0.004″ along the X/Y axes and ± 0.005” or ± 0.002 in Z. There are a variety of materials available, and selection will depend on the speed, shrinkage, and feature details required for your design. Common SLA materials with low shrinkage rates and higher print speeds include SC 4500, SL7810, Accura25, Next, and Somos18420.
Laser Sintering (LS) works by heating powdered plastic nylons with a CO2 laser, which causes them to fuse or “sinter”. The result is a dense plastic part with good strength and elongation properties. These nylons deflect heat. Because the process does not require support materials, it is great for parts with complex internal geometries that would be difficult or impossible to achieve using other methods. Laser Sintering can create the most complex designs of all 3D printing technologies.
The nylon composites are derived primarily from Nylon 11 and Nylon 12, and are enhanced with various different kinds of fillers to provide varying properties. Fillers include glass, aluminum, and carbon fiber. Again, there is a wide variety of nylon composites available. Some are certified for toxicity, smoke, and heat resistance. FAR 25.853 is a nylon composite which is certified for fire, smoke, and toxicity and is regularly used in aircraft interiors and other applications where burn tests and smoke/toxicity certifications are required.
Layer thickness is usually between 0.004″ and 0.006″, with tolerances ranging from ± 0.020” or ± 0.003″, whichever is greater.
LS parts are likely to experience some amount of shrinkage. Plan your design with potential shrinkage in mind. Work with experienced LS designers if in doubt.
3D printing metals functions according to a similar principle as powdered plastics (above). Powdered metals are heated using a Yb-fibre laser, which melts the powder, fusing it layer-by-layer to form the part. 3D printing with metal is incredibly useful, but it does require a significant amount of post-processing and is a bit more technically involved than other methods. You must work with experienced engineers and manufacturers when 3D printing metal.
Direct Metal Laser Sintering (DMLS) can produce highly complex parts. This method allows for the consolidation into a single part what otherwise would have to be assembled from various castings. This is a tremendous design advantage. Because of this, DMLS has become popular in the aerospace, oil and gas, medical, and transportation industries.
DMLS is extremely precise. Layers as small as 40 microns can be produced. The minimum feature size for metal parts is 0.012″, with tolerances at roughly ± 0.005” for the first inch and ± 0.002”/” after. Available materials include various stainless steels, aluminum, inconel 625 and inconel 718, titanium, and cobalt chrome. The materials have similar strength and welding characteristics as conventionally built materials (though strength must be considered relative to the axes and to wall thickness at any given point, as noted earlier).
While the properties of metals produced through additive manufacturing will not always be identical to those produced through machining or casting, they can be further heat treated or coated to meet the required specifications. 3D printed metals are dense, strong, and corrosion-resistant.
Support structures are required when printing metals. Because these supports are made from the same material as the part itself, they are not easy to remove. Post-processing requires expertise and specialized equipment.
Thermoplastics are some of the most commonly used materials in 3D printing, largely because they are available for hobbyists and smaller firms. Thermoplastic printing is typically done using Fused Deposition Modeling (FDM), which is the method adopted by the RepRap project any many home-use systems. It is also used in commercial/industrial contexts, though.
The thermoplastics used in FDM are high performance, engineering-grade production materials. They are characteristically very similar to parts made through injection molding. Thermoplastic materials available for 3D printing include polycarbonate (PC), acrylonitrile butadiene styrene (ABS), and acrylonitrile styrene acrylate (ASA). ULTEM, a high-performance polyetherimide thermoplastic used in medical and chemical instrumentation, is also available.
Other specialized FDM thermoplastics include ABS-ESD7 which prevents static build-up, biocompatible PC-ISO, and ABS-M30i which is designed for food packaging. ULTEM 9085 is flame retardant and chemically resistance and suitable for aerospace design applications.
FDM thermoplastics are widely used in medical packaging and instruments, aerospace, electronics, and other specialized low-volume production applications. However, it is also widely used for general purposes when functional, durable plastic parts are needed.
Your material choices will have a tremendous impact on the ultimate characteristics of your final product. Material selection should be undertaken carefully, and should be taken into account throughout the product development process. We offer contract 3D printing services to our clients looking to connect with leading 3D printing firms in the U.S. We also offer expertise during the design process through our top-tier 3D printing design freelancers. Let us know about your project, and we’ll provide you with a free quote, and connect you with the design expertise you need to meet your project goals.