Automated Sample Inspection
and Detection of Metallic
Contamination in Plastic Pellets
Even smallest metallic contamination in plastic
pellets can cause significant problems during
the production and processing of plastics.
Whether the pellets will be used for the
production of injection molded parts, cables,
films or profiles – fault prevention almost
always means a major effort and even more,
when fault correction becomes necessary. The
dismantling of injection molding tools, the
exchange of hot runner systems and the
reworking of cavities is time consuming as well
as expensive.
The reasons for metal contamination in raw material
are manifold: During production, for instance, an
abrasion of the metal screws in the extruder and
of the barrel occurs, or of other machine parts
that come in contact with the product (picture
1). Abrasion can also occur at conveyer pipes and
pipe bends; this is enhanced by abrasive material.
Furthermore, metal contamination can get into the
pellets due to damaged melt filters or a defective
granulator. Neither with melt filters nor with
inductive metal separators or permanent magnets, a
full protection against metal contamination can be
ensured. Therefore, for example, high voltage cables
have to be coated with a 100 % metal free plastic
insulator. Otherwise, the costs to recover and repair a
submarine cable after a breakdown (short circuit) due
to metal contamination can amount to a mid-seven-
figure sum. Also the dismantling of bigger injection
molding tools with several damaged cavities and/or a
blocked hot runner distribution system can be costly.
The above points show that only with absolute
pure raw material a highest possible quality of the
final product can be guaranteed. In this respect, a
continuous quality control in the production process
is essential. Thus, plastic producers and processors
6 WIRE NEWS December 2019
Picture 1: Metal inclusions in plastic pellets, caused
by a damaged extruder during material production