Cracks are a commonly seen defect in plastic injection molded products, of which the main causes are stress and deformation, mainly including residual stress, external stress and product deformation caused by external environment.
(I) Cracking caused by residue stress
The residual stress is mainly generated in the following three scenarios, i.e., overfilling, product ejection and metal inserts. The cracking generated in the case of overfilling can be solved from the following aspects: (1) Since the direct gate is able to minimize pressure loss, if cracking is mainly generated around the direct gate, you can opt for the multi-point gate, the side gate or the tab gate. (2) On condition that the resin material does not decompose or deteriorate, appropriately increasing the resin temperature is able to reduce melt viscosity and improve fluidity, while reducing the injection pressure, so as to lower the stress. (3) Usually, stress easily occurs when mold temperature is low, so the temperature should be appropriately increased. However, when the injection speed is high enough, stress can also be reduced even though the mold temperature is relatively low. (4) Stress will also occur when the injection and pressure holding time is too long. It would be better to reduce the time or perform pressure switch appropriately. (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. Inappropriate design can lead to failure and subsequent cracks.
(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. Environmental factors can degrade plastic parts and cause cracks.