Why Does Your Plastic Pelletizing Machine Have Black Spots and Contamination?

Many factory supervisors assume the screw is badly damaged when they see black specks. In most Ghanaian workshops, though, black spots and contamination usually come from three areas: material sorting, carbonization, and machine maintenance.

The first cause is poor sorting and washing of the feedstock before it reaches the hopper. When you process locally collected plastics — discarded LDPE water bags, greasy HDPE jerry cans — any sand, dust, paper labels, or metal wires that were not fully washed off bake under the heat inside the barrel and turn into hard black specks in the melt.

The second is carbonization from stagnation. If a sudden power cut (Dumsor) hits, or the machine is switched off at the end of the day without a proper purge, the plastic left inside the barrel stays hot too long. It degrades, burns, and sticks to the screw flights as a hard black crust, then flakes off into fresh material on the next restart.

The third is contamination from an incomplete material change. When operators switch from a high-melting plastic like ABS to a lower-melting one like LDPE without cleaning the machine properly, the remnants of the old material over-bake and char at the new settings, leaving a continuous streak of dark specks. A temperature profile set too hot for the resin can do the same thing, so it is worth checking the zones as well as the changeover.

Step 1: Upgrading Raw Material Sorting and Installing Magnetic Separators

Tighten the quality control at the sorting and washing stage. Have workers separate paper labels and heavily soiled plastics before they go to the crusher.

Because local scrap yards collect plastics from the ground, metallic debris is common — a heavy-duty magnetic separator mounted right above the feed hopper catches iron filings, screws, and metallic dust before they enter the barrel. That keeps them from burning into black spots or scratching the screw and barrel liner.

Step 2: Implementing Proper Purging Procedures Before Shutdown

If carbonized crust is causing dark streaks, change how the machine is shut down. Do not let the team simply cut the power and walk away while the barrel is full of recycled plastic.

Before a planned shutdown or weekend maintenance, run a high-viscosity purging compound — or a batch of clean, stable raw material — through the line to push out the degraded scraps. That keeps the screw flights clean so nothing sits and chars during the next startup heating cycle.

Step 3: Pulling the Screw for Manual Cleaning and Calibrating Heat Zones

If black spots persist after careful sorting and purging, a layer of carbonized plastic has baked onto the hardware and needs a deep clean. Schedule it during weekend downtime, and treat it as a serious maintenance job: pulling a long screw is heavy work, so have an experienced mechanic do it, keep the screw and barrel aligned during removal and reinstallation, and if you are not confident, contact the machine supplier first.

For the carbonized crust on the screw root and die head channels, prefer brass scrapers or a purging-compound soak over a wire wheel. A wire wheel can wear the flight surfaces and hurt plasticizing performance later; if you do use one, keep it to non-mating surfaces only. Work gently and check that the steel is not scratched.

Once the hardware is clean and reassembled, calibrate the temperature control panel so no single zone overshoots its limit and cooks the plastic into charcoal.

Summary: Keep Your Output Pure and Command the Best Market Prices

With strict material washing, a disciplined purging routine, and periodic screw deep-cleaning, you can bring black spots and contamination under control. That keeps your plant turning out bright, clean recycled granules that local manufacturers will buy at good rates across West Africa.