Burn Spots on Molded Part

Possible Cause Possible Solution
Mold is insufficiently vented Make sure vents are clear of obstruction
Insure vents are deep
Add vents if necessary to assure good operation
Injection rate is too high Reduce injection speed
Screw RPM is too high Decrease screw RPM
Back pressure is too high Reduce back pressure. Minimum is 0.35MPa (50psi)
Clamp pressure is too high Reduce clamp pressure

Burn Marks at Gate

Possible Cause Possible Solution
Melt flow across gate restriction;
area causes excess shear heat
Reduce injection speed
Decrease nozzle temp
Decrease front barrel zone temp
Decrease mold temp. in gate area
Check for resin contamination

Discolored Streaks or Chunks

Possible Cause Possible Solution
Additives or colorant have incompatible carriers Change to an additive or colorant material
Never use additive or colorant materials that contain polyvinyl chloride carriers.
Prior runs have contaminated the plastication system Physically clean the entire system-hopper, barrel, shut-off valve, screw nozzle and mold
Purge the system with polyethylene or polypropylene
Melt has insufficient color concentrate blending. Increase back pressure
Increase barrel temp and reduce rear zone temperature
Change screw speed
Decrease nozzle orifice diameter
Replace screw / nozzle with one that has

Distorted Parts

Possible Cause Possible Solution
Material is oriented during injection process Decrease injection velocity for slower filling
Increase mold temperature Adjust velocity profile to provide homogeneous melt.
There is a difference in packing density Increase hold pressure
Increase injection fill speed to fill mold faster
Increase screw RPM.
Material flow into mold is not uniform. With multi-cavity molds, make sure flow is balanced.
Part is too hot during ejection Reduce melt temperature
Increase cooling time
Reduce mold temperature
Molded part has stresses Increase melt temperature
Prevent overpacking at gate by reducing
hold pressure or reducing hold time
Lengthen cooling time
Delay knockout actuation
Increase injection forward time
Reduce temperature differential in mold.
Knockout is operating improperly Knockout engaging too fast, slow down ejection
Non-uniform operation of pins
Bearing area of knockout pins is too small
Increase diameter and number of knockout pins
Relocate knockout pins to strong area of part
With delicate parts change to air blow-off
Part design is incorrect Make sure part contains no sharp variations in cross sections
Correct taper or draft insufficiencies.

Flash On Parting Line

Possible Cause Possible Solution
Excessive injection pressure causes mold to part Verify the injection fill time
Decrease final injection speed
Reduce back pressure
Move transfer point back
Increase clamp pressure.
Melt temperature is too high Reduce nozzle temperature
Reduce front barrel zone temperature
Reduce back pressure
Reduce screw RPM’s
Decrease mold temperature
Mold function is incorrect Make sure mold faces are in correct alignment
Remove foreign matter from mold faces
Decrease size of vents
Decrease mold temperature
Check platens for square.
Mold release agent causing flash on part Mold surface lubricant should not be used


Possible Cause Possible Solution
Mold design has insufficient impingement Relocation of gate to create impingement

Material Drools from Nozzle

Possible Cause Possible Solution
Pressure build up Add decompression
Barrel front zone is too hot Reduce front barrel zone temp
Nozzle is too hot Reduce nozzle temp