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Three methods to detect internal stress of injection molded parts

Plastic internal stress refers to an internal stress generated during the plastic melting process due to factors such as the orientation of macromolecular chains and cooling shrinkage.
When the forces and entanglements between macromolecular chains cannot withstand this kinetic energy, the internal stress balance will be destroyed, and plastic products will produce stress cracking and warping deformation.

        solvent method

1. Acetic acid immersion

The acetic acid (CH3COOH) used must be more than 95% acetic acid and the number of repeated uses must not exceed 10 tests.

① Surface stress test: Pour acetic acid (glacial acetic acid) into the glassware, and completely immerse the product in the acetic acid for 30 seconds. After 30 seconds, use a clamp to take out the sample and immediately rinse it with clean water (tap water is sufficient). Check whether there are any whitening or cracks on the surface of the sample. Judgment: There should be no cracking, and a slight whitening on the surface is allowed.

② Internal stress test: Dry the sample that has passed the surface stress test and completely immerse it in acetic acid for 2 minutes. After 2 minutes, take out the sample and rinse it immediately with clean water (tap water is sufficient), and inspect the sample for whitening and cracks. Judgment: There must be no breakage. Slight cracks and surface whitening on the insert are allowed.

2. Methyl ethyl ketone + acetone immersion method
   Completely immerse the whole machine in a 1:1 mixture of methyl ethyl ketone + acetone at 21 degrees Celsius, take it out and spin dry immediately, and check according to the above method.

Principle: Based on the phenomenon of medium stress rupture, that is, after the solvent molecules penetrate into the macromolecules of the resin, the interaction force between the molecules is reduced. In areas with large internal stress, the force between molecules was originally weakened before immersion. After immersion in solution, these weakened areas are further weakened, causing cracking. Areas with small internal stress will not crack in a short period of time. Therefore, the size and location of the internal stress of the plated parts can be determined from the time and degree of cracking on the surface of the plated parts. To determine whether the plastic parts are electroplated.

        Instrumental method

Use polarized light to illuminate plastic parts and analyze the intensity of internal stress depending on the number of colored light bands. It is only suitable for transparent parts. The instruments required for the polarized light method are expensive, the operation is complex, and the accuracy is not high. Since the change of the part before and after treatment is not obvious, the light bands appearing in the spectral band are not necessarily affected by internal stress, such as ripples on the surface of the part. affect the results of the inspection. However, this method has no impact on the performance of the parts. It is a non-destructive test, and the inspected parts can continue to be electroplated and used.

        sudden temperature change method

This method is to repeatedly expose the plastic parts to be plated to cold and heat, and evaluate the internal stress based on the length of time the cracks appear. It is suitable for all types of plastic formed parts. The equipment required for the sudden temperature change method is simple, but the test time is long. The plastic parts that have been repaired have been damaged and cannot be used continuously.

It is very important to understand the knowledge of internal stress when we design and making the moulds for plastic parts.