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Tech Tip January 1997

Part Breakage:


Though I typically talk about processing, we have to consider part design and also mold design so that as processors we have the largest possible molding window. Sharp inside corners are not good. I recently was at a molders where we were trying various grades of materials to prevent breakage, but upon examination of the part we found that no mater the material it broke. This than aided us in convincing the OEM that it was a design problem for what and how they were using the part.

The #2 pencil is your tool. If you can take the pencil with a good point on it ( I know this subjective) and run the point of the pencil down the inside corner of the part. If we get two parallel lines on the adjoining surfaces we have too sharp of an inside corner. If we get a single line the radius is good. Please note that what is happening is tangent lines of the radius of the pencil to the part.

Sharp inside corners are stress raisers and need to be controlled. If you are a part designer please don't put radiuses into the notes section, call it out on the print at the points. If you are the processor and get parts back look to see if they are all breaking in the same area, this is one heck-of a clue.

You may ask why the example parts had sharp corners. It was the only way an inexpensive design could be used and the parts were used in a product that did not exert stress on the area. When the OEM decided to use the same parts on a new product they failed, this being due to the stress applied to the parts on occasion.


Thanks for the time.

Steven L Silvey
Sr. Technical Service
General Polymers