Plastic Injection Defects: Voids or Bubbles

Bubbles are either pockets of trapped gas or vacuum voids. It is important to determine which type of bubble exists in your part to more quickly pinpoint the source and determine the correct approach. A simple test of warming the part area containing the bubble until it softens can be used to determine its bubble type as trapped air or void. With the test, if there is gas in the bubble, the gas will want to expand the bubble as the part softens. If there is no air in the bubble and a vacuum void exists, the bubble will collapse due to the atmospheric pressure. A hot air gun is best to heat the area. Trapped air is often a root cause of bubbles as well as blisters. Trapped gas may result from flowfront issues such as seperate fronts knitting together, jetting, or even equipment and production problems such as non-vented core pins, poor venting, too much decompression, or materials. The air or gas may appear from water vapor or contamination in the resin. Air may be trapped in ribs, threads or non-vented extensions off nominal walls. Flowfront path can be a major cause of bubbles. Processors should examine each part's flow path via shot profiling to see if the flow front is coming around on itself. Note: jetting can cause air to become trapped in the polymer. Other causes for trapped air leading to bubbles or blisters include inadequate venting, material flow path design, or gas traveling across a part's surface during the fill or pack stage. Flowfront issues may indicate a need to change gate location to avoid air or gas trapping or to promote uniform filling. Equipment can also be a cause for bubbles. When working with a hot runner mold, the possibility exists that a venturi effect can suck air from between the plates into the hot runner, pulling air into the melt. To check for this, disassemble the mold and apply bluing near the drops, being careful not to apply any in the flow path. If the bluing agent shows up in the part then you have found the cause of the defect. Tool venting is another critical consideration, so vent properly or use a porous steel to eliminate gas traps. Check the number of vents as well as vent depth. Check vents with pressure sensitive paper. Clean all parting line and core vents. This can prevent the opportunity for trapping air. Voids occur during cooling, usually in thick sections of the part where there can be a significant cooling rate difference in the material forming the core and skin of the part. Voids are signs of internal stress and warning signs that the part may not perform as required. Insufficient shot size can be a main reason for voids so packing more plastic into the cavity is recommended. Molders should maintain a consistent cushion , making sure you are not bottoming out the screw. You should aim for higher pressures/duration in the hold/pack stage times. To solve voids, try slow fill rates or increased backpressure. You can open the gate for longer gate seal times to allow more packing during the second stage. Molders can also reduce melt temperatures. From a tooling standpoint, you can increase the runner diameter. You should determine where the sink is. Is it near the gate or farther down the flow part? If near the gate, perform a gate seal study. If it is farther down the flow part, increase injection speed to decrease peak pressure viscosity and allow more packing pressure. Gas can be created by the degradation of the resin or additive, so try a new lot of material and/or use virgin material. It is best to check the melt temperature process range recommended by the resin supplier. Molders should minimize material residence time, and one way is to use the correct barrel size for the shot.