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Q: We are having a problem with one of our parts cracking about 2 days after molding. What would cause this?
A: When an injection molded part cracks or breaks there are three possible reasons:
- the internal molded-in stress has increased
- the external stress has increased
- the strength of the plastic has decreased
Whenever the total stress exceeds the strength, the part fails. When plastic part failure occurs, you must determine which of the 3 possible causes is the reason. If you run your injection molding operations using the principles of scientific molding, you will be familiar with our structured, step-by-step approach to solving the problem.
Part designers usually specify a plastic part strength of 2 to 3 times the expected external stress. Part designers rarely attempt to estimate the amount of internal molded-in stress because this stress can vary widely due conditions during the injection molding cycle.
Of the 3 causes of injection molded part failure, increased internal stress, increased external stress, and reduced plastic strength, the molder has control over 2 of them. The strength of the plastic can be degraded by overheating, or by contamination of the raw material. The internal stress can be increased having incorrect injection molding machine control setting which leads to unfavorable molding conditions.
Checking For Stress
To minimize any loss of strength by degradation of the plastic raw material, injection molders should avoid overheating the plastic. They should also avoid using an excessive percentages of regrind.
To check for changes in the internal stress level of molded parts, there’s a couple of methods used.
On clear plastic parts, you can check for changes in the orientation and stress of parts by putting the part between cross-polarized plastic sheets. The combination of stress and molecular orientation will show as color patterns. The more patterns, the more orientation and stress in the molded parts.
You can also use a heat distortion test. If you heat plastic parts above their heat distortion temperature in an oven, the parts will deform in proportion to the amount of stress and orientation they contain. The highly stressed and oriented plastic parts will distort more.
Several additional tests related to stress and orientation have also been developed over the years; including tensile strength, impact strength, and chemical resistance tests.