SLA material 101: designers comparison to molded thermoplastics
- Posted by Nick Greusel
- On December 5, 2022
This article provides an important and simple tool for R&D groups using 3D printed prototypes for engineering testing.
When a product team asks “How are we going to prototype this product?”, one of the first options they usually consider is 3D printing. 3D printing is simple, fast, and inexpensive, and some 3D printing methods can yield parts that look ready for mass production. But looks can be deceiving. The dirty secret of 3D printing is that while a 3D-printed plastic part can look like a mass-produced part, the materials often have very different properties from injection-molded plastics that will be used in production. Those differences in material properties often limit how much a design team can learn from testing 3D printed prototypes.
Engineers often need to run verification tests to answer questions like “Will the product break when it’s dropped?”, “Will this springy feature have the right combination of strength and flexibility?”, or “Will this part deform if it gets cooked at 170°F in a hot car?” If you want to use a single 3D-printed material to answer all of these questions, then that material needs to match the planned production material’s properties as closely as possible. In practice, that’s a difficult or sometimes impossible goal to meet.
Matching one or two properties isn’t too difficult, but there are often half a dozen or more properties that are important to match, and matching that many properties is just not possible in many cases. To make matters worse, comparing properties from material datasheets can be a tedious process. They are usually just tables of numbers, and finding the right material becomes a long repetitive game of “spot the difference”.
Enter: the spider chart! (aka web chart, radar chart, star plot… it has a lot of names.) The spider chart is a useful tool for comparing multiple properties at a glance, finding the best matches, and quickly understanding the tradeoffs. Senior Mechanical Engineer Nick Greusel from Engenious Design has created a spider chart based on one created by Formlabs, the manufacturer of some of Engenious’s most-used 3D printing materials.
This chart compares 6 important material properties:
- Tensile Strength – the maximum tensile stress the material can withstand before it breaks
- Tensile Modulus – how “stiff” the material is: how much force it takes to stretch it a certain amount
- Flexural Modulus – similar to tensile modulus, but in bending instead of pure tension
- Heat Deflection Temperature – the temperature above which it starts losing stiffness
- Izod Impact Strength – how resistant it is to breaking in a fast impact
- Elongation at break – how far the material can stretch before breaking, as a percentage of its original length
As you can see, the injection molded materials often outperform the 3D-printed materials. Close matches of one or two properties can often be found between an injection molded material and a 3D printed material, but the 3D printed material often falls significantly short in some of the other properties.
For example: ABS versus Formlabs Tough 2000. Both have tensile strength around 50 MPa and Tensile and Flexural Modulus around 20 GPa, but ABS is 5 or 10 times stronger in Izod Impact, even though Tough 2000 is advertised as “ABS-like”, and as being tougher than Formlab’s standard resins. Tough 2000 might be a reasonable substitute for ABS when testing general strength or flexible features, but in a drop test, it’s far more likely to break than ABS. It’s also not as heat resistant. The story is similar with many other “matches” between 3D-printed and injection-molded materials.
For this reason, it’s best practice to develop a test plan before choosing prototyping methods or materials. With almost any 3D-printed plastic material, compromises of some kind will need to be made. If you choose a single 3D-printed material for all your verification tests, you will likely need to accept that some properties won’t be up to production standards and won’t yield useful information from certain tests that involve poorly matched properties. Alternatively, you could choose to make prototypes in multiple materials for different tests, matching the most relevant properties for each specific test, but this may add cost and complexity.
Prototypes are never a perfect analogue to mass-produced products. Tradeoffs and compromises are an inherent part of the prototyping process. The key to an efficient development cycle is awareness and strategic management of those tradeoffs, and the spider chart is a helpful tool for fast and intuitive comparison and selection of materials.
Good luck! And we are here at Engenious Design if you need help designing, prototyping or testing parts; it’s how we make the world a better place through design.
ABOUT THE AUTHOR(S)
Nick has been designing products for 12 years in the consumer electronics and medical industries, from rugged outdoor GPS systems to delicate surgical instruments and dozens of things in between. He designs, tests, and optimizes products for reliable mechanical performance in whatever conditions and uses it might encounter.