What are the causes of injection defects and how can they be solved when using recycled polymers?

There are aesthetic defects that could form during the injection work using regenerated granules, which are from recycled post-consumer or post-industrial origin.

The post-consumption granule lends itself more, in any case, to the possible creation of aesthetic defects since the polymeric composition of the granule itself can comprise fractions of materials that are not completely homogeneous (for example PP / PE).

The aesthetic deficiencies expressed in superficial streaks, also called marble streaks, normally do not cause a technical defect of the printed product, but usually an aesthetic defect which, in any case, can lead to the refusal of the product by the end customer.

We have already dealt with the topic concerning the reconsideration of the aesthetic aspects of products made with a recycled granule, in the presence of small defects, with a view to increasing the circularity of plastic waste, precisely to have a correct judgment on the aesthetic expectations of products that employ recycled plastic. (read article).

In the molding phase, the plastic used as raw material reaches temperatures between 175 ° and 400 °, depending on the material used, creating various transformation processes within the melt.

The water is vaporized, and some low molecular weight additives and polymers could degrade producing volatile substances that will accompany the melt inside the mold. Furthermore, the molding speed could act on the polymeric molecules creating a certain percentage of plastic degradation.

Due to the difference in density between the molten mass, the volatile and the degraded substances, there will be a separation between the heaviest and the lightest parts inside the mold, where the latter will arrive first towards the walls of the mold itself, then followed by the molten mass, of which they will get dirty.

Therefore, any volatile and / or degraded part that will be pushed towards the mold wall by the melted recycled polymer, will create streaks or marbled parts on the walls of the finished product which may be unsightly.

The causes of these defects can be summarized in:

• Moisture of the material

• Degradation of plastic parts due to high temperature

• Excessive molding time with degradation of polymers

• Counter pressure too low

• Too small injection points that could degrade the raw material

• Excessive wear of the spindle

• Dirt near the gas ejection points in the mold or insufficient number of points

In consideration of the above, to avoid or reduce these unsightly phenomena, it is necessary to take all the necessary precautions for the adjustment of the machine and mold parameters, in addition to checking, through the study of the DSC of the recycled polymer to be used, the weight of the components that could degrade.