With this blog we wanted to dive in with some popular contamination issues that we have seen over the past few years. Below are steps that you can take if you happen to have an issue when molding.  These are only suggestions that we have learned through technicians during our time in the industry.


Contaminates often show up as black specks or color streaks.

  • Review resin handling system to ensure no foreign materials are mixed with virgin or regrind.
  • Switch to all virgin to isolate source.
  • Purge barrel and clean hopper, dryer and hose lines.
  • Clean grinders, filters and containers.
  • Ensure magnet is in place and no objects are attached to it.


Black Specs are non- miscible particles only seen on the surface of an opaque part because pigments tend to hide them. With transparent parts they are visible throughout.

  • Increase melt temperatures to help remove degraded resin
  • Purge well with Thermally Stable resin or purging compound (click here to look at what products we provide that can help if you are having these issues)
  • Remove screw and clean inspect screw, screw tip and check ring for dead spots (I.E hang up areas).
  • Lower melt temperatures, injection speeds, screw speed and back pressure to reduce shear and thermal input.
  • Verify that heater bands are not overriding.
  • Clean the mold surface.
  • Examine virgin, regrind and concentrates for foreign material.


Discoloration can be caused by several factors and can occur to the actual pellets prior to molding as well as the final part.

  • Purge out injection barrel until resin runs clean.
  • Lower melt temperature, screw speed and/or back pressure. Also, decrease injection velocity.
  • Reduce cycle time, use small volume barrel. Use a reverse temperature profile.
  • Verify feed throat temperatures and reduce feed zone temperature.
  • Shot capacity is too small, move to large barrel volume to aid dispersion of additives and reduce severe shear heating.
  • Too much shear from screw. Verify proper screw design, raise rear zone temperatures to lower the viscosity of the resin and shear effects.