It seems in additive manufacturing (AM) we spend a lot of time talking about advanced solutions and sometimes the basics get forgotten behind. I was reminded of this when I interviewed Zehavit Reisin from Stratasys who told me how early on in her career she explained material properties to Objet’s resellers. My immediate thought was it might be handy to have that in writing, for others. That started me thinking about this new blog series (to appear once every month or two) – #AMbasics – and today I’ll start from Zehavit’s topic: AM plastic material properties – what do they mean and how are they measured. This is not a regular blog post, more of a lookup table if you need something but I will give some general info up front. Most importantly, we must remember that the part material properties are not 1-to-1 with the raw material so the same raw material with different processing can result in different part properties, so when you nail your process make sure to lock it in (with LEO Lane, for example so you have repeatability and consistency in this as well. If there are terms I missed, I will add them as I realize it (or someone tells me) – there are many general plastics material properties but let’s focus on those currently relevant for production materials in AM. Often people 3D print what’s called “bones” in various locations on the printer bed to test these properties on – orientation is another thing that can sometimes affect polymer material properties and even bed location (location is even more critical in metal, but that’s for another post….).
Basic Terms Used
Stress – internal force particles exert on each other (e.g., when a sample is bent)
Strain – the deformation of a sample (how much it is displaced from its original shape)
Pascal – The pascal (Pa) is a unit of pressure used to quantify internal pressure, stress, Young’s modulus, and ultimate tensile strength. A standard atmosphere is 101,325 Pa. MPa denotes mega pascal (1,000,000 pascals).
Joule – a unit of energy that is dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.
Basic Mechanical AM Polymer Material Properties (and Their Tests)
Strength (Tensile Strength)
Strength is the force you need to pull a material in opposite directions in order to break it. Tensile strength is measured in pascals (Pa). There are additional measures such as the Elongation to Break test which checks how much a sample stretches before it breaks (measured in %: the ratio of elongated sample just before breaking to its original length), and the Tensile Modulus of Elasticity which is the ratio of stress to strain just before breakage from pulling.
Stiffness (Flexural Strength)
A measure of the bending properties of a material. It is determined by subjecting a sample to three-point bending. The sample is deflected until it breaks and a continuous stress-strain curve is plotted (the ratio of stress to strain is called Flexural Modulus of Elasticity). The Flexural Strength is the maximum stress sustained by the sample before it breaks – it is expressed in mega pascals (MPa).
Hardness (Fingernail Test)
Resistance of a sample to indentation, e.g., fingernail scratches.
Toughness (Izod Impact Notched)
The energy a sample can absorb before it breaks. It is determined by an Izod Impact (Notched) test: A notched sample is hit with a swinging pendulum – the energy level that breaks the sample divided by the cross sectional area at the notch is the Izod Impact Notched. It is expressed in Joules per meter squared (J/m²). There is a version of this test that is not notched and it is measured in Joules per meter (based on the weight of the pendulum and the distance it is swung from).
More AM Polymer Material Properties
Squishiness (Compressive Strength)
The load at which there is a set (say 10%) permanent deformation in the sample (it doesn’t bounce back). Measured in pascals or pounds per square inch (lbs/in²).
The density of the material compared with water density (water density = 1).
Porosity / Water Absorption (Immersion Test)
The % increase in sample weight after 24 hour immersion in water.
Insulation (Dielectric Strength)
The voltage needed to puncture the plastic insulation measured in volts per millimeter (V/mm).
As you see, there is room above to add more properties and tests as needed – let me know if I missed something and stay tuned for more #AMbasics posts by following us on LinkedIn or subscribing to our newsletter for weekly updates and to make sure you don’t miss a post. Photos: EOS powder (top), SRAirsoft stress test, and airwolf3d‘s test of filaments