Polycarbonate (PC) is a very versatile engineering polymer but who has ever moulded this resin is aware of the possibility of occurrence of black specks. Black specks are especially critical when producing transparent parts. Those experienced with the material know that if certain rules are followed in processing, the risk of the occurrence of black specks can be minimised.

The reason for this material to be so prone to the formation of black specks is it’s apparent “fondness” for metal surfaces. For that reason PC based materials often contain a release agent to facilitate demoulding.
During processing a thin layer of material sticks to the surface of part of the plasticising screw and carbonizes over time. This carbonized layer is in principle quite stable specifically when running the same grade of material for prolonged periods. There are certain conditions however that cause the layer of carbonized material to break of from the screw’s surface. Running excessively wet material may be such a condition that causes black specks to occur. The duration of this issue can take several hours leading to significant amounts of rejected parts.
Suppliers of the resin are able to provide instructions on the start-up and shut down procedures when running PC. When shutting down a machine it is generally recommended to purge the machine by f.i. polypropylene before a shut-down. If PC is left in the machine during shut-down, upon cooling the solidifying melt contracts and as it adheres, it can disturb the carbonized layer resulting in black specks during start-up. Polypropylene will also contract on cooling down, it will however not exhibit the same adhesion to the carbonized layer and thus will not disturb it.

In extreme cases the adhesion of polycarbonate to the metal surface when solidifying is so strong, it can result in damage to the hardened layer of the screw. How bad can it get?

One interesting example occurred when a moulder experienced a power outage lasting several hours when running a PC/PBT blend on one of their production lines. As the machine had complete cooled down, it was decided to pull the screw rather than trying to restart production. What they found was remarkable; as it solidified in the barrel the material had pulled of large parts of the hardened layer of the screw evidenced by the material pulled off the screw. The picture below of the actual material shows the extend of the issue.
Resuming production without checking may have caused further damage to the equipment and production; needles to say they made the right call.