Plastic Injection Defects: Voids or Bubbles

Garrett MacKenzie: plastic411 Editor

There are two primary causes of a bubble/ void.. trapped gas or vacuum voids. It is simple to test the defect to quickly pinpoint the defect cause and determine how to correct the condition. Warm the part area containing the bubble using a heat gun until it softens. This will help you to determine what type it is.. whether trapped air or a gas void. If there is gas in the bubble, it will expand the bubble as the part softens. If a vacuum void exists, the bubble will collapse due to atmospheric pressure.

Trapped air and blisters are often the root cause of bubbles (voids). The speed and direction of flowfronts can create unwanted trapped gas pockets.Separate flowfronts knitting together too quickly or jetting can be primary causes of these types of defect.

Even equipment and mechanical failures might create voids and bubbles. This condition can result from non-vented core pins, poor venting, or a setpoint allowing too much decompression after screw completes rotation. When working with a hot runner mold, the possibility exists that, due to the venturi effect air can be drawn from between the plates into the hot runner. This can pull air into the melt, creating a void or bubble. To check for this condition, disassemble the mold and apply bluing near the drops,being careful not to apply any to the flow path. If the bluing agent shows up in the part then you have located the cause of the defect.

The air or gas may also appear from water vapor or contamination in the resin... so it is important to be aware of material moisture content using moisture analysis equipment. Take the time to make a visual inspection of your material for potential contaminants. If regrind is being used, determine the percentage, and check to see if you have recently changed to a new lot of material from the supplier, or a new batch from in-house.

Air trapped in ribs, threads or non-vented extensions off nominal walls can be a common flaw. Flowfront path can be a major cause of bubbles. Processors should analyze each part's flow path via injection profiling to see if flowfront travel is creating an undesired defect. If via profiling analysis, you identify this as the primary cause, you should consider whether the part can be gated differently, or if you may be able to profile velocities in a way that will eliminate the issues you are facing. By changing gate location you may avoid air or gas trapping and promote a more uniform fill approach.



Tool venting is another critical consideration. It is important to note where the void is located. Is at the end of fill, or near a parting line? Venting properly should be standard in your facility. The use of a porous steel is another consideration that can reduce or eliminate "gas trap" defects. Check the number of vents, as well as vent depth. Consider what type of material you are using. Is this material prone to gassing? Check vents with pressure sensitive paper. Is the void located on a parting line or at the end of fill? If so, install a paper vent to see if the void (bubble) is eliminated. If by installing a "paper vent", the condition is corrected, your solution is to have a vent installed where the paper vent was tested. If the condition is abnormal to normal run conditions, clean all parting line and core vents. This can prevent the opportunity for trapping air.


Voids can be signs of internal stress and warning signs that the part may not perform as required. Insufficient shot size can be a reason for voids, so perform a normal decoupled molding approach. Assuring that shot size is 95-98% of your overall fill is always your first approach. Maintaining a consistent cushion is always a primary consideration , making sure you are not bottoming out the screw, or that the cushion exceeds normal operations, which can lead to melt temperature variations. You should aim for higher pressures/duration in the hold/pack stage times...but as stated above, take the time to perform the proper gate seal study. To solve voids, try slower fill rates or increase back pressure.You can open the gate for longer gate seal times and increase packing during the second stage.

From a tooling standpoint, increasing the runner diameter may be a consideration. Where does the runner configuration stand in regards to total shot weight?

Gas can be created by the degradation of the resin or additive, so try a new lot of material and/or use virgin material, or decrease your regrind percentage. It is best to check the melt temperature process range based on recommended the resin supplier's processing window. Molders should always consider material residence time,assuring they are using the correct barrel size for the shot, as well as hopper residence time, so that the materials they use have been dried properly.

There are many larger reference books that can provide more detailed information of how to acheive standardization of your molding operation. Here are some of the molding resources that we highly recommend:

Scientific & Injection Molding

Lean Manufacturing