Introduction

Black specks and carbon deposits in thermoplastic products fall into two categories: those present before molding, and those generated during the molding process.

Pre-Molding Black Specks: Source Control

For black specks existing prior to molding, the primary approach is to identify the contamination source and implement strict cleanliness controls across all handling stages—from production, packaging, storage, and transport, to bag opening, blending, and feeding.

- Install brushes at bag-opening stations to remove external contaminants from packaging surfaces.

- Avoid using regrind materials for transparent or light-colored products, as the regrinding process itself tends to generate black specks.

- Evaluate the necessity of dust-controlled or cleanroom environments in the production area.

Molding-Generated Black Specks: Process Optimization

For black specks generated during the molding process, the following measures are critical:

1. Precise Temperature Control

Displayed temperature differs from actual melt temperature. On the same machine, variations in back pressure, cycle time, shot size, and thermocouple positioning can significantly affect degradation and carbonization tendencies. Lower back pressure, shorter cycles, larger shots, and thermocouples placed on the upper barrel all reduce degradation risks.

2. Standardized Operating Practices

- Limit residence time for flame-retardant materials during standby to ≤15 minutes.

- Use dedicated screw purging compounds for heat-sensitive materials during startups and shutdowns.

3. Screw & Mold Design Optimization

Eliminate dead zones and material stagnation points in the barrel, nozzle, and runners. Ensure smooth, gradual transitions at all turns to prevent material hold-up.

4. Scheduled Maintenance with Purging Compounds

Carbon deposits and additive degradation residues on screw flights and barrel walls are gradually sheared off and discharged during production. To minimize impact on product quality, schedule regular cleaning using professional purging compounds.

Purging Methods by Application

While traditional cleaning methods are straightforward, they are often wasteful and less effective. The recommended approach is using specialized screw purging compounds, with adjustments based on equipment type:

- Open-Nozzle Purge-to-Air Method - Fill the barrel with purging material and purge to atmosphere; repeat as needed. For abrasive-type purging compounds, high back pressure enhances shear and cleaning efficiency. Set back pressure as high as possible—up to the point just before shear heating causes the barrel temperature to rise.

- Hot-Runner Safe Method– The open-nozzle method with abrasive purging compounds is not suitable for machines equipped with hot runners, as abrasives can damage precision components. Use chemical-type purging compounds instead, which break the bond between carbon/pigment deposits and metal surfaces through a chemical soaking action, allowing contaminants to be flushed out with the melt.

- Mold-Related Burn Marks – For molds with burn damage, first identify the root cause and location. Moving components such as slides, core pins, ejector pins, and ejector blocks are susceptible. Repair damaged surfaces, reduce sliding friction, and ensure adequate lubrication.

- Large Flake Deposits – Carbon deposits may be purged out in large sheets, particularly those formed during startups and shutdowns.

Sanp Tech – Professional Purging Compounds for Black Speck & Scrap Reduction

Sanp Tech offers high-performance screw purging compounds specifically formulated to eliminate black specks and reduce scrap rates in thermoplastic processing.

- Pellet form – smokeless, low odor

- Chemical cleaning action – no abrasives, non-damaging to screws

- Hot-runner safe – suitable for hot runners and precision dies

- Versatile application – effectively cleans and prevents carbon buildup in both injection molding and extrusion screws

Sanp Tech purging compounds – the ideal choice for maintaining clean, efficient, and high-quality thermoplastic production.