Troubleshooting

Plastic Injection mold troubleshooting

Burn Spots on Molded Part

Burn Marks at Gate

Discolored Streaks or Chunks

Distorted Parts

Flash on Parting Line of Part

Jetting

Material Drools from Nozzle

Flash on Parting Line of Part

Part Dimensions are Too Large

Part Dimensions are Too Small

Parts Stick in Sprue/Cavity

Poor Finish

Poor Surface Detail

Poor Weld Lines on Part

Short Shots Prevent Complete Mold Filling

Silver Streaks

Sink Marks

Voids in Part

Warping

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 of Part

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

Flash on Parting Line of Part

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

Part Dimensions are Too Large

Possible Cause	Possible Solution
Mold is over packed	Decrease hold pressure Decrease overall injection speed Decrease final injection speed Decrease back pressure Decrease front barrel zone temperature Increase mold temperature
Mold designed incorrectly	Verify mold shrinkage factor used
Material is wet	Pre-dry the material

Part Dimensions are Too Small

Possible Cause	Possible Solution
Mold is under-packed	Verify injection fill time Increase overall injection speed Increase hold pressure and hold time Reduce cushion in cylinder Increase the back pressure Decease mold temperature Increase melt temperature Increase cavity venting Increase the cooling time
Mold designed incorrectly	Verify mold shrinkage factor used adjust mold design if necessary Add additional gates

Parts Stick in Sprue/Cavity

Possible Cause	Possible Solution
Part is not sufficiently cooled	Reduce stock temperature Decrease second stage injection time Reduce injection pressure Reduce screw RPM Add decompression Increase overall cycle time Decrease mold temperature
Mold design is incorrect	Seal nozzle correctly in sprue bushing Reduce length of sprue Increase draft on side walls of part to assure smooth release Draw polish cavity surface in direction of ejection
Cores are too slender	Make cores stronger to prevent misalignment by the injection pressure
Overpacking problem	Reduce injection speed Back off transfer point Reduce pack/hold pressure

Poor Finish

Possible Cause	Possible Solution
Mold fill is too slow	Increase injection pressure Increase gate size Increase injection speed Increase melt temperature
Mold function is incorrect	Make sure venting is adequate Increase mold temperature Do not use mold lubricants Clean cavity surfaces
Part design is incorrect	Reduce abrupt changes in section thicknesses Avoid ribbing that is too thick

Poor Surface Detail

Possible Cause	Possible Solution
Mold is filling improperly	Increase shot size Increase overall injection speed Increase back pressure Increase mold temperature
Packing conditions are improper	Increase injection fill time Increase mold temperature Increase second stage pressure and time

Poor Weld Lines on Part

Possible Cause	Possible Solution
Material flow is cooling too quickly	Increase melt and mold temperature in cavity Increase injection pressure Increase ram speed Increase nozzle temperature Lengthen cycle time
Mold design and/or function are improper	Make sure vents are not plugged Increase gate size; relocate for most efficient melt flow Raise mold temperature Increase size of runner Decrease fill speed

Short Shots Prevent Complete Mold Filling

Possible Cause	Possible Solution
Available melt volume is insuffiecient	Increase injection fill time Make sure nozzle and runners are clear of obstructions Increase front barrel zone temperature Increase nozzle temperature Verify operation of non-return check valve on screw
Mold has high pressure drop	Increase overall injection speed Increase back pressure Increase hold pressure Try a higher flow grade Add gates
Mold function is incorrect	In multi-cavity molds, be sure all gate sizes are uniform to prevent unbalanced melt flow Check part design to make sure there are no restrictions Make sure mold is vented correctly and vents are clear

Silver Streaks

Possible Cause	Possible Solution
Material has trapped moisture	Pre-dry material according to instructions Make sure mold is vented properly
Melt temperature is too high	Reduce barrel beating to prevent decomposition Reduce screw RPM to stop excessive shear heating
Cold mold causes moisture to condesnse	Increase mold temperature; make sure cavity is dry.

Sink Marks

Possible Cause	Possible Solution
Mold is not packed enough	Increase overall injection speed Increase boost cut off set point Increase hold pressure and temperature Increase cushion in cylinder Increase shot size Increase barrel heating Increase back pressure Add or increase nozzle heating
Mold function is incorrect	Be sure gates are large enough and are correctly located Reduce length of sprue bushing, increase bushing diameter Increase mold temperature Make sure vents are large enough and are clear Increase mold cooling cycle time
Part design is incorrect	Redesign part to remove sharp thickness changes in section

Voids in Part

Possible Cause	Possible Solution
Material in core sets up more slowly than surface material, especially in thick cross sections	Increase injection pressure and time Increase back pressure Increase mold temperature Make sure sprue, runner and gates are are large enough to assure free flow Increase cooling time
There is moisture in pellets (raw material)	Pre-dry material/pellets according to instructions

Warping

Possible Cause	Possible Solution
Material is oriented during injection	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 faster Increase screw RPM
Molded part has stresses	Increase melt temp 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
Material flow into mold is not uniform	With multi-cavity molds, make sure flow is balanced
Part is to hot when it's ejected	Reduce mold temperature Increase cooling time Reduce melt temperature
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