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How to solve bubble problem in TPU injection molded part
January 29 , 2021

Bubbles are the one of major defects in TPU molded parts during injection. Bubbles in TPU molded parts not only results in appearance problems but impairs physical properties as well.

There are two types of bubbles, it may be a voided area trapped within a molded plastic part or trapped gases in a molded plastic part. There are three major reason that cause bubbles in injection products.


Bubbles caused by too thick plastic wall. Where plastic wall is too thick, the cooling rate of the outer surface is faster than that of the center part. Therefore, the resin in the center part shrinks and expands toward the surface, it cause insufficient filling and voided area in the center part .

1)Determine the reasonable gate and runner size according to the wall thickness. Generally, the gate height should be 50% to 60% of the product wall thickness.     
2)A certain amount of supplementary injection material is left until the gate is sealed.  
3)The injection time should be slightly longer than the gate sealing time.
4)Reduce the injection speed, increase the injection pressure
5)Use materials with high melt viscosity grades.    
Bubbles caused by volatile gases or trapped gas. non-vented core pins, poor venting, too much decompression air or water vapor all can cause gas.

1)Fully pre-dry.
2)Lower the resin temperature to avoid the generation of decomposition gas
3)Increase the injection pressure or the hold time to help force the gases out.
Bubbles caused by low mobility of TPU.

1)Increasing the temperature of the resin and mold
2)If bubbles appear, slow down the injection fill speed. If short shots appear, speed up the injection fill speed.


Reason analyze
Improving method
1. Back pressure is low or the melt temperature is too high.Increase back pressure or make the melt temperature low. 
2. Raw materials are not fully dried.Increase the intensity of the drying process
3. The screw rotate speed or the injection speed is too fast.Decrease the speed of the screw or the injection pressure
4. The gas cannot expel quickly enough.Make the air vent larger to improve the exhaust effect
5. The size of gate is incorrectDecrease the size of gate or change the shape of gate

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