Problem

Possible Causes

Possible Solutions

Rough extrudate surface

Melt or Die temperature is too low

1. Increase extruder temperature profile.

2. Increase die temperature.

3. Check heater/thermocouple functioning

Melting is not homogeneous/complete

1. Increase melt shear (Use screw with higher compression ratio or screw with mixing zone.)

2. Use a more restrictive screen pack

Moisture

1. Pre-dry material properly

Die is not Streamlined

1. Check die alignment

Excessive drawdown ratio

1. Modify die design or drawing speed to reduce drawdown

Poor die design

1. Reduce land length.

2. Reduce shear in die and taper entrance.

3. Reduce die length.

4. Used polished die

5. Check wear on the die

Improper screw design

1. Use thermoplastic screws with longer L/D ratio and higher compression ratio

Logitudinal dimension variations

Surging

1. Reduce throughput.

2. Use screw with longer feed or metering section.

3. Use more restrictive/tighter screen pack to increase back pressure.

4. Check wear on screw/barrel.

5. Check die and melt temperature.

6. Use shorter land die

Uneven Take-off speed

1. Check take off device.

Porosity and internal voids

 Moisture content too high

1. Dry pellets in dehumidifying dryer for specified time at specified temperature; check for possible condensation water in hopper throat

Melt temperature too high/uneven cooling

1. Reduce barrel temperature set points

2. Adjust quenching conditions (air gap, water temperature)

Too much screw shear

1. Change screw to one with lower shear (deeper metering channel or lower compression ratio)

Air in screw feed

1. Reduce screw speed, reduce barrel temperature in screw feed zone, increase feed throat cooling.

Black specks / Undispersed particles

Contamination

1. Purge with low-MFI PP or HDPE.

2. Check the compatibility of the color concentrate.

3. Check for dirt and foreign particles

Odor or yellowing

Melt too hot/Degradation

1. Reduce barrel temperature.

2. Reduce die temperature.

3. Check heater/thermocouple functioning

4. Reduce shear (reduce screw speed, use less restrictive/coarser screen pack to reduce back pressure)

5. Use screw with lower compression ratio.

6. Streamline melt flow

Edge tearing along the profile

Die temperatures is too low

1. Increase die temperature

2. Check for malfunctioning heater band or thermocouple

Melt viscosity is too high

1. Reduce viscosity by increasing screw rpm or by increasing barrel temperature profile

Die flow is improperly balanced

1. Extend the compression zone to insure that material flow reaches the edges

Die lines in extrusion direction

Die temperature too hot, cold or not uniform

1. Check die temperature controller, adjust temperature

High moisture content

1. Dry resin carefully ; check dryer for proper operation

Melt temperature too low

1. Increase melt temperature

Material build-up in die

1. Clean and polish die

Radial dimension variations

Die is worn

1. Replace die

Extrudate deforms due to insufficient cooling

1. Reduce melt temperature.

2. Increase cooling (Reduce length of air gap and cooling bath temperature)

3. Reduce extrusion rate

High pressure /

low throughput

in extruder

Melt too cold

1. Increase extruder temperature.

2. Increase die temperature.

Obstructed screens

1. Clean or replace screens.