What causes cracking in plastic injection molded parts? -2

(5) It should be noted that amorphous resins, such as AS, ABS and PMMA, are more prone to residual stress than crystalline resins, such as PE and POM.

During product ejection, stress is generated due to the high ejection force caused by the small draft angle, rough mold core / cavity. Sometimes, even ejector marks or cracking occurs around the ejector pin. Just look closely at the location of cracking to identify the cause.

Stress is the most likely to occur when metal inserts are involved in the plastic injection molding process. And, the cracking usually occurs after a period of time, so it is extremely harmful. The stress is mainly caused by the large difference in thermal expansion coefficient between the metal and the resin materials. 

As time goes by, the stress exceeds the strength of the resin material that gradually deteriorates, so cracking occurs. In order to prevent the cracking thus caused, as a rule of thumb, the general-purpose PS is basically not suitable for inserts, while the impact of the inserts on the nylon material is minimal. Thanks to the small thermal expansion coefficient, the glass fiber reinforced resin is more suitable for the application of inserts.

In addition, a better effect can be achieved if you pre-heat the metal insert before injection molding.

(II) Cracking caused by external stress

The external stress mainly refers to the stress concentration caused by inappropriate design, especially the sharp corners.

(III) Cracking caused by external environment

Chemicals, water degradation caused by moisture absorption, and excessive application of recycled materials will degrade the physical properties, and thus cause cracking.

(The end.)