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 |
Jetting
| 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 |