Black specks and bubbles appear in modified plastics? Common Compounding System Problems and Solutions

1. Black speck problems in plastic compounding and modification systems

Identifying the source of black specks

Possible causes:

Raw material contamination: impurities in recycle material, pigment agglomeration, carrier resin degradation

Equipment contamination: carbon deposits in the screw/barrel, clogged filters, incomplete cleaning

Improper process: carbonization caused by excessive temperatures (e.g., sensitive materials like PVC and PA)

Solutions

Raw material control

Recycled material pretreatment:

Screening (40 mesh or higher) to remove particulate matter

Pre-drying (to prevent moisture-induced degradation)

New material testing:

Spot-check melt index (MFI) stability

Avoid mixing raw materials from different batches

Equipment maintenance

Regular cleaning:

Clean the screw/barrel every shift (using PP or PE cleaning material)

Replace damaged filters (80-120 mesh recommended)

Carbon deposit treatment:

Use a dedicated screw cleaner before shutting down High-temperature flushing

Physically remove stubborn carbides with a copper brush

Process optimization

Temperature adjustment:

Lower the melting zone temperature (5-10°C below the theoretical value)

Use a step-by-step temperature ramp for sensitive materials (e.g., PA)

Screw speed:

Avoid excessively high speeds (>80% of maximum speed can cause shear overheating).

2. Bubble Problem Analysis and Solutions

Causes of Bubbles

Main Causes:

Raw Material Water Content: Hygroscopic materials such as PET and PC are not fully dried

Residual Volatile Matter: Decomposition of additives (such as lubricants) produces gases

Poor Venting: Vacuum system failure or improper venting section design

Solutions

Raw Material Handling

Drying Process:

PET: 120°C x 4 hours (dew point ≤ -40°C)

PC: 120°C x 6 hours (moisture content <0.02%)

Additive Screening:

Use a low-volatile lubricant (such as silicone instead of paraffin)

Equipment Improvements

Vacuum Venting:

Check the vacuum pump (maintain a pressure above -0.08 MPa)

Clean the vent blockage (once a week)

Screw Optimization:

Add a venting section (two-stage screw design)

Use a barrier screw Reduce melt backflow

Process adjustments

Injection/extrusion parameters:

Reduce injection speed (reduce shear heating)

Increase back pressure (5-15 MPa to promote degassing)

Mold temperature control:

Raise mold temperature by 10-20°C (reduce condensation bubbles)

Emergency Response Plan

Sudden black spots: Immediately shut down the machine and check the raw material batch and temperature profile.

Continuous bubbles: Suspend production and inspect the drying system and vacuum lines.

3. Common Problems and Solutions for Plastic Compounding and Modification Systems

Material Blockage (Bridging/Screw Stuck)

Phenomenon:

Raw material accumulation at the feed port

Abnormally high main unit current

Solution:

Emergency stop, reverse the screw to remove residual material

Inspect raw materials:

Agglomeration (hygroscopic materials such as PA/ABS require pre-drying)

Particle size uniformity

Process adjustments:

Increase feed zone temperature (5-10°C above theoretical value)

Reduce feed rate (matching main unit speed)

Black specks/yellowing

Root cause analysis:

Raw material degradation (excessive temperature or excessive residence time)

Equipment contamination (carbon deposits in the screw/barrel)

Solution:

Short-term: High-temperature flushing with PP cleaning compound (+5% calcium carbonate)

Long-term:

Clean the filter (80-120 mesh) every shift

Use a low-temperature, high-speed process for sensitive materials (such as PVC)

Screw/Barrel Wear

Signs of Wear:

Decreased Output (Extrusion Volume Reduction >10% Under the Same Parameters)

Increased Melt Pressure Fluctuation

Countermeasures:

Minor Wear: Adjust the process (increase the temperature by 5°C to compensate for plasticizing capacity)

Severe Wear:

Nitride Layer Damage: Screw/Barrel Replacement Required

Bimetallic Alloy Material Preferred (Lifespan Increased by 3x)

Abnormal Temperature Control

Troubleshooting:

Vacuum exhaust failure

Symptoms:

Increased product bubble rate

Vacuum gauge displays > -0.06 MPa

Handling procedures:

Check vacuum pump oil (replace if turbid)

Clean exhaust port filter (once a month)

Test seal (apply soapy water to check for leaks)

gearbox abnormal noise

Diagnosis methods:

Regular abnormal noise: Bearing damage (replace required)

Irregular noise: Gear wear (check lubricant oil for impurities)