Thursday, October 7, 2021

Injection moulding defects & their remedies | QSQTECH.COM


Hello friends, 
Today, We'll discuss on Injection molding defects & how to fix them. Hopefully, you'll understand this topic.
First of all, thanks for visiting.
Now, below topics that we are covering today. 


Defects is an error which comes in any machine, mold, material, system and also anything else.  In other words, Defects or problem is an opportunity for improvement. 
According to customer point of view, if any product or service are not meet their specifications, then the product called defective or defect. 
Defect is an error that hit on any system fitment and functional.


Injection molding defects are as given below:
  2. FLASH 



Short molding occurs when the flow of molten material cannot completely fill the cavities in the mold. Short molding is comes at that place where material flowing into cavity reach at last filling area. 

Short molding

Ⅰ. Cause: 

  1. The flow of molten material into the mold is restricted by narrow or blocked gates.
  2. If the material is too viscous or the mold is too cold, the molten material is not allowed to completely fill the mold.
  3. Sometimes the trapped air pockets into mold may restrictions proper flow.
  4. The injection pressure may be insufficient or low as per specification. 
  5. Auxiliary equipment like MTC or HRTC malfunctioning & no alarm implement for when temperature disturb. 
  6. Process was not running as per standard set parameters or going out of spec. as compared to standard set parameters. 
  7. Molten material was leakage from nozzle tip as nozzle tip was not matching properly with sprue bush and dent mark on tip or wear out of nozzle tip dia. 
  8. Sprue bush rotate from its actual position. 

Ⅱ. Countermeasures

<Molding equipment>
  1. Make sure that the nozzle is not clogged. If the nozzle clogs frequently, increase the mold temperature or shorten the cycle time. 
  2. Increase injection speed
  3. Increase cylinder temperature setting. Also increase the nozzle temperature. 
  4. Increase the feeding amount of material. If material feeding is still insufficient at maximum material feed capacity, change to a larger capacity machine. 
  5. Install a screw with a back-flow check valve.
  6. Raise the cylinder temperature setting. Also raise the nozzle temperature.
  7. Make sure there are no cut lines in the heater. 
  8. To prevent material from cooling too rapidly, increase the mold temperature. 
  9. Make sure that nozzle tip is properly match with sprue bush. If any leakage found then check the allignment of nozzle tip and sprue bush. Check the nozzle tip dia size. 

  1. Raise the mold temperature. 
  2. Increase the mold gas release. 
  3. Increase gate cross-section area. 
  4. Increase the molded product thickness. 
  5. Add ribs to the molded product design to enhance flow capacity. 
  6. Check the sprue bush and it should not be rotate from its actual position. 

  1. Choose a low-viscosity high flow material type. 
  2. Apply surface lubricant. (Add 0.05-0.1% by weight). 

2. FLASH: 

Flash is a defect in an injection molding part, occurring only where two cavities of the mold meet. Flash is found at the location where the mold separates.
It looks like a thin layer on the molded part. 


Ⅰ. Cause

1. Low clamping force.
2. Insufficient injection pressure. 
3. Mold cavity wear & tear. 
4. Mold life complete as per its capacity. 
5. Improper venting 
6. Gap between core and cavity of the mold.
7. High barrel temperature

Ⅱ. Countermeasures 

<Molding equipment> 
1. Increasing the plate clamping force or moving the mold to a machine with higher clamping force can solve this issue. 
2. Ensure that the mold is properly maintained and cleaned.
3. Ensure that injection pressure is lower than clamping force.


Burn mark is an injection molding defect that comes generally in injection molding process. This defect is generally creates due to the burn of the material at a particular place on last filling area in the mold due to some reasons. It may be a small, dark and black spot in appearance. Sometimes, this is happening because when air or gases are not release properly through air vent. 
The air remaining during the pouring of the material into the cavity of the mold is compressed and produces a burn mark on the last filling area. 

Burn mark injection molding defect

Ⅰ. Cause 

  1. Insufficient injection speed
  2. Insufficient injection pressure
  3. High Melt/barrel temperature and mold temperature
  4. Air vent blockage or air not release through venting into mold. 
  5. Foreign material mixing
  6. Presence of Moisture in material 

Ⅱ. Countermeasures 

<Molding equipment> 
  1. Decrease injection speed 
  2. Decrease injection pressure 
  3. Decrease mold/melt temperature 
  4. To improve venting, reduce the clamp force. 
  1. Use the material after pre drying into hopper drier. 
  2. Each material having its own different property, place all material separately. 


Flow mark is an injection molding defect that is
generally, comes when molten material flowing into large variation of cooling speed between sections into mold. It means lack of compensated material during packing stage.
Flow mark is wavy lines or streaks and off tone pattern in appearance. It is also known as Flow line. 

Flow mark

Ⅰ. Cause

  1. Low injection speed 
  2. Low injection pressure 
  3. Low melting temperature 
  4. Low mold temperature 
  5. Small runner stem and gate 

Ⅱ. Countermeasures 

  1. Increase injection pressure 
  2. Increase injection speed 
  3. Increase gate size and runner system for specific mold and material 


Air bubble is an injection molding defect that is similar to blister in appearance in that there is air trapped into the molded part. Material over heating into mould is general root cause of air bubble.
There are many reasons to occur this defect. 

Air bubble injection molding defect

Ⅰ. Cause

  1. High mould temperature 
  2. Material over heat 
  3. Air vent blockage 
  4. Cooling trapped into mold 
  5. Material contamination or foreign material mixing 
  6. Cooling line restricted 
  7. High injection speed 
  8. Moisture in material
  9. Small shot size

Ⅱ. Countermeasures 

  1. Decrease injection speed
  2. Decrease injection temperature
  3. Decrease mold temperature
  4. Increase shot size
  5. Pre drying material before molding 
We hope you will understand this topic.

Thank you. 

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