By Brad Cleveland, Protomold
Comparative Strength and Finish
If prototypes are to be used for functional testing, they should meet the performance characteristics of production parts. And if they are to be used for market testing, they certainly should approximate the finish of production parts.
We’ve often referred to the relative strength of molded plastic prototypes compared to that of layered rapid prototypes, but we’ve never actually offered any quantitative measures. Nor have we presented actual comparisons of part finish. No one has actually challenged us on these points but we thought we owed the market more than unsupported assertions, so we hired Stork Materials Technology, a network of independent, accredited laboratories, to perform strength tests. Later in this article we’ll show you comparative finishes and let you judge for yourself.
Strength testing was done on six types of samples. Because gating can affect the strength of injection molded parts, three different gate configurations were tested. The lab also tested CNC machined parts made by First Cut Prototype. Fused deposition modeling (FDM) was chosen to represent additive prototyping methods since it produces the strongest parts of any major Rapid Prototyping (RP) process. All test parts were made from the same CAD model using ABS (or, in the case of FDM, ABS-like) plastic. Table 1 shows the results of standard ASTM D790 testing, measuring flexural properties. Table 2 shows results of ASTM D638 testing, measuring tensile properties.
Note that Average Strain at Break and Average Break Stress are significantly higher for the machined and molded parts than for parts built using the additive process. The highlighted figures for Average Strain at Break, % for two of the three molded parts demonstrate the importance of gate placement. The molded part with one end gate shows superior results for Average Strain at Break, %.
Obviously, these figures do not represent all parts in all plastics. They do, however, suggest that while additive rapid prototyping processes have a place in the development process they may not be suitable for functional testing of production parts.
|The following photographs show parts made with each of the three processes. All were made from the same 3D CAD model.Clearly, the molded and CNC machined parts have a measurable advantage in strength and a visible advantage in appearance over parts made by the additive RP process. Additive processes certainly have their place in the development process and may be perfectly acceptable for determining the fit of a part. They may, however, not be suitable once testing begins and performance and appearance become considerations.|
|Cut, Mold, or Build|
|Part I||Three Ways to Make a Horse|
|Part II||Comparative Strength and Finish|
|Part III||Comparative Speed|
|Part IV||Comparative Cost|
About the Author
|Brad Cleveland, President and CEO
The Protomold Company, Inc.
1757 Halgren Rd.
Maple Plain, MN 55359 – USAPhone: 763 479 3680
Fax: 763 479 2679
|Brad Cleveland has been the president and CEO of The Protomold Company, Inc. since November of 2001. Prior to Protomold he was cofounder and vice president of AeroMet Corporation, a laser additive manufacturing company and subsidiary of MTS Systems Corporation.Protomold®, a Proto Labs service, is the world’s fastest source for custom injection molded parts. The company fills a unique niche in the manufacturing of plastic parts, using a combination of advanced, proprietary software and sophisticated equipment to produce prototype and low volume runs of custom injection molded parts. The material properties of Protomold’s real molded parts surpass those of parts produced by additive prototyping processes. At the same time, delivery is faster and costs are lower than those associated with traditional injection molding.|
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