Many injection moulding factories will encounter similar problems: product quality is not a problem, but the gloss of the product is unqualified, which ultimately leads to injection moulded parts processed out of the scrap. In addition to the problem of the plastic itself, there are such as: injection moulds, production, design and other aspects of the problem.

1. In the injection moulding production process

There are several options to try that can be altered to affect gloss by adjusting the mould temperature, make-up/holding pressure, fill rate and material temperature. In most cases, these adjustments will not have a large impact and will reduce the process window for the entire production run, thereby increasing the likelihood of other problems. Therefore, it is best to find the most robust process for the part and maintain the surface finish of the cavity mould.

2. In the case of injection moulds

When dealing with gloss issues, don't just go ahead and change the surface finish of the mould steel. Instead, start by making adjustments to the process parameters to change the gloss of the product, a lower temperature mould, colder melt, lower make-up/holding pressure and lower fill rate may all result in a brighter gloss on your plastic part. This occurs because the plastic does not replicate the microscopic details of the surface finish of the mould steel due to the lower mould temperatures and less applied pressure.

Conversely, higher injection mould and melt temperatures, higher shrinkage/holding pressures and faster filling speeds will give your part a duller lustre, with melt temperature not having as much of an effect as filling speed, as the filling speed and resulting shear forces have a much greater effect on the viscosity of the material in relative terms.

On the other hand, if the surface gloss of the product is too high, this can be achieved by considering reducing the surface finish of the mould steel surface or by sandblasting the cavity. Both of these methods will create small pits in the steel, thus adding more surface area, which will allow the injection moulded product to absorb more light, thus appearing to make your part darker.

3. In injection moulded product design

Another major cause of gloss issues involved is the design of the product, especially where the wall thickness of the product changes changes. When the wall thickness changes, it is difficult to go about maintaining a consistent gloss level on the part. Due to differences in flow patterns, thinner wall segments are not under as much plasticising pressure, resulting in higher gloss in that area.

Inadequate venting can also produce inconsistent surface gloss. Depending on the material and process, insufficient outgassing can result in both dark and bright spots.

The above three points are important factors affecting the gloss of injection moulding processed products, as long as the injection moulding processor puts these issues in place before producing the product, the problem of gloss of injection moulded products can be avoided.