Sheet Extrusion Troubleshooting

Problems Possible Causes Suggested Solutions
Applesauce, gels, poor sheet clarity 1. Melt Temperature too high 1. Reduce melt temperature.
a. Correct malfunctioning thermocouple and controllers.
b. Reduce temperature settings.
c. Install a lower shear screw.
d. Reduce head pressure by using a less restrictive breaker plate and screen pack.e. Use a higher flow GPPS resin for blends
.f. Sharpen grinder blades and reduce blade clearance as needed.
2. Excessive residence time at process temperature 2. Eliminate melt hangup.
a. Change screw or temperature profile if material hangs in vent.
b. Correct fit of transition sections so no ledges exist.
c. Eliminate deckle bars or reduce and plate heater temperatures on die.
Black Specks 1. Foreign material 1. Eliminate source of problem.
a. Use liners when storing resin or regrind in gaylords.
b. Keep paper bag fibers out of resin.
2. Degraded polymer 2. Disassemble and clean extruder barrel, screw and die to remove deposits
Milky areas of poor clarity 1. Contamination by incompatible polymer 1. Prevent contamination.
a. Clean loader, hopper and dryer.
b. Purge extruder.
c. Disassemble and clean barrel, screw and die if needed.
Bubbles in sheet 1. Air entrapment 1. Improve melting and mixing function of extruder.
a. Increase head pressure by using a more restrictive breaker plate and screen pack.
b. Set inverse temperature profile on extruder.
c. Use a higher compression screw.
Silvery streaks 1. Moisture on resin 1. Prevent or remove moisture.
a. Dry copolymer at 140°F (60°C) for one to two hours.
b. Melt resin more efficiently per recommendations shown in “air entrapment” section above.
Dull surface overentire sheet 1. Poor polishing due to insufficient contact with chill rolls 1. Fill both nips to ensure contact. Do not, however, allow rolling bank of material to occur.
Dull surface in moving, diagonal bands 1. Nip pressure too low, so polish rolls move up and down 1. Provide force of at least 300 pli (pounds per linear inch) or 5 newtons per linear meter of sheet width to hold rolls steady. Repair damaged or leaking polish roll cylinders as required.
Dull surface areas in transverse direction with sheet width varying 1. Extruder output surging 1. Provide uniform melting and material feed.
a. Ensure regrind/virgin ratio is consistent.
b. Cool feed zones & hopper zone to prevent bridging.
c. Invert temperature profile if head pressure is lower than 1000 psi (7.0 Pa).
d. Check operation of all heaters, thermocouples and controllers.
e. Eliminate extruder drive and line speed variations.
f. Change extruder screw or evaluate different GPPS for blending.
Dull surface areas in machine direction 1. Varying sheet gauge 1. Adjust gauge.
a. Line out sheet die.
b. Fix faulty heaters, thermocouples and controllers in die.
c. Eliminate air currents or uneven die heat profile.
Edge curl or poorsheet flatness 2. Polish rolls have poor TIR (tolerance in radius) 2. Repair as needed.
1. Polish roll temperatures not balanced 1. Correct with roll temperature. Sheet will move toward a roll as heat is increased and away from it as it is cooled.
2. Non-uniform roll temperature 2. Maintain temperature variation across polish roll surface to less than 5°F (3°C).
a. Increase system pressure and coolant flow rate to manufacturer’s design recommendations.
b. Remove scale by acidizing rolls as needed.
3. Uneven heat transfer 3. Increase sheet tension to improve contact with polish rolls
Surface scratches or lines in the machine direction of extrusion 1. Charred resin on die lips 1. Scrape die with brass tool. Do not use hard steel or damage to die surfaces may result.
2. Scratched, burred or dirty die 2. Clean and repair die sections as necessary.
3. Scored polish rolls 3. Polish or resurface roll face.
4. Moisture in sheet (sheet will have silvery cast) 4. Dry resin.