Why injection molded parts lack surface gloss

Injection molded parts sometimes appear matte, lack gloss or have insufficient transparency. The mould cavity first needs a suitable surface finish. A common production cause is that the melt cools too much during injection: its surface loses gloss, its flow deteriorates, and it no longer makes close contact with the polished mould.

For a glossy or transparent surface, the melt should retain good flow while filling the cavity. Both melt and mould temperatures are important.

Raising the melt temperature by a moderate amount, slightly increasing back pressure and avoiding an excessively slow injection can help. Without the right mould temperature, however, those changes may not achieve the intended result.

The old-site article reports that with a mould temperature above 50 °C, common materials such as ABS, HIPS, PP and PVC can often develop a glossy or transparent surface at normal melt temperatures. PC generally needs a higher mould temperature, preferably above 80 °C.

Mould temperature is therefore a major control for appearance. Raising melt temperature alone has limits: too much heat can cause shrinkage, scorching or discolouration.

Mould temperature also has an upper practical limit. The article cautions that temperatures above about 80 °C for rigid plastics such as ABS and HIPS, or above about 120 °C for PC, can increase shrinkage and lengthen the production cycle.

If a matte finish is required instead, reducing melt and mould temperatures may help, but sandblasting or texturing the mould surface remains the key prerequisite.