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			Impact Testing Prototyping Situation Many injection-molded components must survive impact loads such as a drop. The objective of this prototyping situation is to verify that the design will survive normal impact loads. It is not intended to determine the maximum impact load or the effect of impact loading on the life of the component. Key Material Properties The primary material property that controls performance in this situation is stiffness. Stiffness will determine magnitude of the force spike and the amount of deformation as a result. Since most plastic parts are stressed in flexure as opposed to tension or compression, the appropriate measure of stiffness is flexural modulus. (To see a comparison of flexural modulus for common rapid prototyping materials, click here) A second key material property is some measure of impact resistance. There are several available and some evidence that none are great indicators of impact performance. However, the most common, and the one for which most information is available in Izod Notched Impact Rating. (To see a comparison of Izod notched impact rating of common rapid prototyping materials, click here) Approximation Requirements How close must we approximate the material properties of an injection-molded plastic to be able to simulate them? We believe that flex modulus must be within 20% of that of the injection molded plastic. That way, deflections will be within 20% of those in actual use. The izod notched rating for the SLA resin should be greater than half that for the proposed plastic. We assume that some factor of safety is included in the design to account for greater than intended impact loads. Since the test is only to accommodate intended loading, we do not need to include that factor of safety here. Selection Chart The chart below plots flexural modulus against izod notched impact rating. Each of the SLA resins are plotted as a point on that chart. A shaded rectangle surrounds each point, which represents the range of material properties that that resin can approximate as determined by the approximation requirements above. The chart also includes several common injection molded plastics. If a plastic falls within a shaded rectangle, that resin for this prototyping situation can then adequately approximate it. click here to see full view It is very clear that SLA resins will not adequately simulate injection-molded plastics for the purposes of impact testing. Other RP materials do not fare significantly better. All fall short in their ability to simulate injection molded plastics for impact testing.
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