Analysis of the cause of stress cracking of ABS

Q: a new kind of flame retardant ABS product has been developed. The customer says the internal stress can not reach the requirement, and the carbon tetrachloride is cracked by 5 min with carbon tetrachloride, and some stress cracking will occur after the injection. What's the reason for it? How to prevent it?

Answer: the general injection molding grade ABS rubber content is about 18%. ABS is amorphous polymer, and the internal stress comes from the difference between orientation and thermal shrinkage. The cracking of the product in the processing of the injection products includes the filamentous crack, the crack, the top white, the cracking and the cracks caused by the adhesive die of the parts and the viscous die of the flow channel, and the cracking and stress cracking are divided according to the time of the cracking. The main reasons are the following.

ABS analysis of internal stress cracking mainly includes:

(1) too much processing pressure, too fast speed, too much filling, too long injection and holding time, will cause the internal stress to crack too much.

(2) adjust the speed and pressure of the mold opening to prevent the quick drawing parts from cracking.

(3) appropriately raise the mold temperature, make the parts easy to release; adjust the material temperature appropriately and prevent decomposition.

(4) prevent cracking due to the deterioration of mechanical properties due to weld marks and plastic degradation.

(5) residual stress of the workpiece can be eliminated by internal heat treatment immediately after molding, so as to reduce the formation of cracks.

The internal stress cracking of ABS is mainly analyzed from the aspects of mold.

(1) the top out of the balance, such as the number of top rod and the section area should be enough, the stripping slope is enough, the cavity surface should be smooth enough to prevent the external force to lead to the residual stress and crack;

(2) the structure of the parts should not be too thin, and the transition part should adopt circular transition as far as possible to avoid the stress concentration caused by sharp corners and chamfering.

(3) to minimize the use of metal inserts to prevent the increase of internal stress caused by different shrinkage of inserts and parts.

(4) a suitable demoulding intake duct should be set up for deep bottom parts to prevent vacuum negative pressure.

(5) the main channel is large enough to make the gate material not ready for demoulding during curing, so that it is easy to release.