Injection Molding: Defect and Solution

Plastic injection molding may cause various kinds of defect, the reasons are not only the malfunction of the machine, but also performance of the machine, design of the mold, and feature of the material, etc.


Molding conditions (such as temperature, pressure, additive, venting, etc.) are key factors for injection molding as well.
The different molding conditions may spend different time tending towards stability, and the results can be taken as references upon steady.
e.g. If you just adjusted the temperature, it is necessary to keep observing for at least ten minutes until it has become stable.

Therefore, once adjusting the molding condition, here are few things which should consider:

1. Do not adjust more than one molding condition at one time.

2. After adjusting the molding condition, it is necessary to observe until it becomes steady.


See below form for the reasons and solutions of injection defect:

Ⅰ . Short Shot

Reason Solution
Temperature of the material is too low Raise the temperature
Insufficient injection pressure Raise the injection pressure
Insufficient plastic material Add in extra plastic material
Insufficient injection time Extend the time period
Speed of inject is too slow Faster the speed of injection
Temperature of the mold is too low Raise the temperature of the mold
Uneven temperature of mold Adjust the position of the cooling water jiffy plug
The ventilation of the mold is poor Applied additional vent hole on appropriated position
Clogged injection nozzle Remove and clean
Filling uneven Redo the position of the gate
The gate is too small Enlarge the gate
Lubricants not enough Add in extra lubricants
Poor fluidity of plastic material Add in extra lubricants or use another material
Check ring was worn out Remove and repair
Performance of the machine is poor Use another suitable machine

Ⅱ. Shrink

Reason Solution
Insufficient plastic material Add up extra plastic material
Insufficient injection pressure Raise the injection pressure
Insufficient injection time Extend the time period
Injection speed is too fast Slower the speed of injection
Filling uneven Redo the position and the size of the gate
Clogged injection nozzle Remove and clean
Temperature is too high Reduce the temperature
Insufficient cooling time Extend the cooling time
The ventilation of the mold is poor Applied additional vent hole according to the position which shrinked
Undersize runner Replace with larger runner
Under packing Extend packing time
Check ring was worn out Remove and repair the mold
Matrial is too soft Use another material
Excessive shrinkage rate Use another material

Ⅲ. Sticking on Mold

Reason Solution
Injection pressure is too high Reduce the injection pressure
Material injected too much (Overpack) Using adequate mold release agent
Close time is too long Reduce the close time
Injection is too fast Slower the speed of injection
Temperature of the material is too high Reduce the temperature
Some parts are overpack due to the uneven material feeding Redo the position and the size of the gate
Insufficient cooling time Extend the cooling time
Temperature of the mold is too high or too low Adjust the temperature of male part and female part
Under-cut Modify the mold and remove the undercut
Mold interior walls is not glossy Polish the mold

Ⅳ. Sprue Sticking

Reason Solution
Injection pressure is too high Reduce the injection pressure
Temperature of material is too high Reduce the temperature of the meterial
Oversize runner Modify the mold
Insufficient cooling time of runner 1. Extend the cooling time
2. Reduce the temperature of runner
The angle of the runner is too low Modify the mold to increase the angle
Sprue bushing is not aligned with injection nozzle Redo or adjust the mold
Rough runner interior walls Maintenance and repair the mold
Under-cut Maintenance and repair the mold
Broken runner external hole Maintenance and repair the mold
No snatch pin Add in extra snatch pin
Too much material injected (Overpack) 1. Reduce usage of material
2. Reduce injection time
3. Slow down speed of injection

Ⅴ. Flash / Burr

Reason Solution
Temperature of the material is too high 1. Reduce the temperature of material
2. Reduce the temperature of mold
Injection pressure is too high Reduce the injection pressure
Too much material injected (Overpack) 1. Reduce usage of material
2. Reduce injection time
3. Slow down speed of injection
Parting line of mold is untight Maintenance and repair the mold
Insufficient locking pressure  Raise locking pressure
Close time is too long Reduce the close time
Injection speed is too fast Slower the speed of injection
Excess fluidity of plastic material Use another material

Ⅵ. Broken / Cracked

Reason Solution
Too much material injected (Overpack) 1. Reduce usage of material
2. Reduce injection time
3. Slow down speed of injection
4. Decrease injection pressure
Close time is too long Reduce the close time
Temperature of the mold is too low Raise the temperature of the mold
Draft angle is too small Maintenance and repair the mold
The gate is too small Maintenance and repair the mold
Ejector pin doesn't work because of position or quantity Maintenance and repair the mold
Under-cut Maintenance and repair the mold
Material has excessive hygroscopicity 1. Dry the material
2. Use another kind of material
Mold became vacuum during ejecting 1. Slower the speed of opening or ejecting
2. Use air-intake equipment

Ⅶ. Welding Line (Knit Line, Parting Line)

Reason Solution
Poor fluidity of plastic material 1. Raise temperature
2. Raise back pressure to faster the rotating speed of the screw
3. Use another kind of material
Temperature of the mold is too low Raise the temperature of the mold
Injection speed is too slow Faster the speed of injection
Injection pressure is too low Raise the pressure of injecting
Residual impurities in plastic material Check the material
Too much mold release oil Reduce or prevent using mold release oil
Undersize runner or gate Using adequate mold release agent
Not able to vent the air Check the vents to see if they are clogged, or add extra vents
Lack of overflow well Add extra overflow well under the welding line

Ⅷ. Flow Lines

Reason Solution
Material is not melted appropriately 1. Raise temperature
2. Raise back pressure to faster the rotating speed of the screw
Temperature of the mold is too low Raise the temperature of the mold
Injection speed is too slow or too fast Adjust the speed of injection
Pressure of injection is too high or too low Adjust pressure of injection
Accidentally mixed with other materials Check the material
Undersized runner Enlarge the runner
Thickness difference of cutting surface is substantial Change the design of the finished product or the gate position

Ⅸ. Silver Streaks (Slivery Line)

Reason Solution
The material is moist 1. Dry the material completely
2. Raise the back pressure
The temperature of the material or the mold is too high, and result in generation of gas Reduce the temperature of the material or the mold
Material is mixed with volatile addictives such as mold release agent, dye, etc. Reduce the usage of addictives or replace with alternatives which is heat resistance
The material and the didn't mixed well Mixed completely
Injection speed is too fast  Slower the injection speed
Temperature of the mold is too low Raise the temperature of the mold
Uneven mixed material grain Use material which has even grained size
Inappropriate mold flow 1. Adjust the position of gate
2. Maintain suitable mold temperature

Ⅹ. Lack of Gloss

Reason Solution
Temperature of the mold is too low Raise the temperature of the mold
Insufficient material 1. Incrase usage of material
2. Extend injecting time
3. Increase injecting pressure
Too much mold release oil Wipe the mold clean
Water in mold interior Wipe and check if there's water leakage
Mold interior walls is not glossy Polish the mold

XXI. Warpage, Part Distortion

Reason Solution
The finished product has not cooled down when it is ejected 1. Reduce the temperature of the mold or material
2. Extend the cooling time
The shape and the thickness of the molded plastic part is asymmetric 1. Fix the shape with styling frame
2. Modify the design of finished product
Injection pressure or speed is not suitable (Overpack) 1. Reduce usage of material
2. Reduce injection time
3. Slow down speed of injection
4. Decrease injection pressure
Filling uneven Redo the gate
The ecjector pin is not balance Improve the ejector system
Temperature of the material is too high Reduce the temperature
Structure of the plastic article is too friable or too tight Increase or decrease the close time
Uneven temperaure between male mold and female mold Balance the temperature of male mold and female mold
Pressure of the mold is not suitable Balance the pressure of the mold

XXIX. Air Trap (Air Bubbles/Voids)

Reason Solution
Cutting surface of finish product is too thick Modify the design of finish product or position of gate
Injection pressure is too low Raise the injection pressure
Insufficient injection time Extend the injection time
Runner or gate is too small Enlarge size of runner or gate
Injection speed is too fast  Slower the injection speed
The material is moist Dry the material completely, or raise the back pressure
Temperature of material is too high and result in decomposition Reduce temperature of material
Unven mold temperature Adjust mold temperature
Cooling time is not suitable Reduce or extend cooling time
Insufficient back pressure Raise back pressure
Inappropriate temperature of feed tube Reduce temperature of injection nozzle and forepart, and raise temperature of rear part

XXX. Black Streaks

Reason Solution
Temperature of the material is too high Reduce the temperature of the runner
Rotating speed of screw is too fast Slow down rotating speed of screw
Residual impurities in plastic material Check the material and clean feed tube
Undersized hole size of injection nozzle Modify the hole size
Scorched material in feed tube Clean the feed tube

XLV. Brittleness

Reason Solution
Temperature of feed tube is too high Reduce the temperature of feed tube
Back pressure is too high Reduce the back pressure
Rotating speed of screw is too fast(Result in fracture of resin molecular chain) Slow down the rotating speed of screw
Resin is not dried completely Dry with suitable time and temperature
Using too much re-grinding material Reduce the usage of the re-grinding material
Different kinds of re-grinding material was used Use the same kind of re-grinding material