The appearance of bubbles, voids, sinks or blisters on an injection-molded part make up the most significant causes for part rejects due to cosmetic requirements. These troublesome “features”, although not the most common of problems when injection-molding parts, can inhibit part performance and are problematic to solve.
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 adjustment to make. 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 trapped in the bubble, the gas will warm up and 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 pushing on the softened walls of the part. A hot air gun is best to heat the area, a small lighter is next, and a torch if you know what you are doing.
Trapped Air Issue
Trapped air is a root cause of bubbles as well as blisters. Trapped gas may stem from flow front issues such as converging fronts, or jetting, to equipment and production problems such as non-vented core pins, poor venting, too much decompression, or resin degradation. The air or gas may appear from water vapor, volatiles from the resin, or decomposing by-products. The air may be trapped in ribs, threads or non-vented projections off the nominal wall. Melt flow pattern is a major cause of bubbles. Processors should examine each parts flow pattern via short shots to see if the plastic flow front is coming around on itself. Note if there is a race-tracking effect or jetting that can cause air to become trapped in the polymer.
Observe flow path for back flow or trapped air in blind ribs. Examine the part to determine if the rib or support areas of a part are covered before part is completely filled. If it is a flow pattern issue, run a short shot molding sequence, changing the transfer position or shot size to make various sized short shot parts ranging from 10 percent to 95 percent of the full part. Use this to find out where and how the bubble occurs. This test requires the process to be velocity controlled for the first stage (injection). This cannot be done if the first stage pressure and velocity is reduced.
Other causes for trapped air leading to bubbles or blisters include inadequate venting, material flow pattern design, and gas traveling across the part surface during the fill or pack stage. Melt flow issues may indicate a need to change the gate location to avoid race tracking, trapping air or to promote uniform filling
Equipment can also be a cause for bubbles. If you are working with a hot runner tool it is possible that a venturi effect can suck air from between the plates into the hot runner, pulling air into the melt, forming a bubble. To check for this the tool must be disassembled, and a bluing agent is applied 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 source of the problem.
One can also examine the purging of a normal shot to see if the bubble originates from the barrel or screw. General-purpose screws with dimensions of 18:1 L/D or lower can be the culprits for a bubble or blister. One solution is to raise the backpressure to 1000 to 1500 psi melt pressure. Another solution may be to pull a vacuum on the mold just before injection, so that air is pulled out. Moisture in the molding system can also be a source of trouble.
Tool venting is another big issue, 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.
Avoiding Voids and Sinks
A void occurs 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. A sink is a depression impacting the surface of a part that does not mimic the mold steel surface.
Voids and sinks are signs of internal stress and are warning signs that the part may not perform as required.
Insufficient plastic can be a main reason for sinks or voids so packing more plastic into the cavity is recommended. Molders should have a consistent cushion on the press, making sure you are not bottoming out the screw. You should go for higher pressures in the hold/pack stage and longer stage time. To solve voids or sinks, trial slow fill rates, the use of gas counter-pressure, and increasing backpressure. You can open the gate for longer gate seal times to allow more packing during the second stage. Molders can also try reducing the melt temperature.
Mechanically, 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, check the gate seal time. If it is farther down the flow part, increase injection speed to decrease viscosity and allow more packing pressure.
Another approach to eliminating voids or sinks is to “thin-up” the nominal wall. Thicker is not always stronger in plastic parts. Thick nominal walls should be redesigned with ribs if strength is needed. This will save plastic and cycle time also. Molders can core out the thick section if possible. Changing the gate location to fill thicker areas in the mold first, this may allow more polymer into the part (before the gate freezes). Molders may also try raising the mold temperature significantly and/or ejecting the part sooner, which can avoid voids by allowing the outside walls to collapse during cooling.
For sinks, users may try cooling the part in water or between aluminum sheets rather than air. Here, inner section of the thick-sectioned part can reheat or re-melt the outside surface of the part once ejected, allowing the surface to collapse. However, this may cause a vacuum void. You might be able to diminish the vacuum void by not cooling the part surface and keeping it warm by placing on wood or insulating foam. One should expect sinks with this approach.
Be Aware of Blisters
Blisters, a thin film of plastic that bubbles up from the part surface, can also ruin the aesthetics of a part. Like bubbles they can be caused by gas traveling across the surface during fill or pack, or due to trapped air issues (inadequate venting, melt flow pattern or screw L/D), and the solutions are the same. However, blisters can also originate from process problems or degradation of the resin or additive package. Delamination is a serious part defect.
Excessively high injection rates can cause blisters, as they can develop a highly oriented thin layer or film on the surface of a molded part. Sometimes sticking tape to the part and lifting it pulls this layer off. Try injecting the melt at a slower rate. High temperatures of steel near the gate can cause blisters also and should be made lower if possible. If the decompression of the screw is excessive, it is possible that air will be pulled into the nozzle and remains trapped. This air pocket enters the melt stream during nozzle contact with the sprue. If using a hot runner system, at injection, the trapped air is moved through the hot runner system and can create the bubble. In a cold runner system the air is ahead of the flow front and gets vented.
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.
Be sure that the plasticating screw be at least 20:1 L/D. If you are require longer cycle times and higher backpressure to reduce the occurrence of blisters, then it is possibly a screw design issue.
About the Author
Injection Molding (IM) Solutions
1019 Balfour St.
Midland, MI 48640-3227
|John Bozzelli is a graduate of Marietta College (BS) and Ohio University (MS). His studies were interrupted for a stint in Vietnam (US Army, Purple Heart; Silver Star). Twenty years in Dow Plastics provided extensive experience in polymer synthesis, development, production, and processing. John has been a seminar leader with RJG Associates, Injection Molding Magazine, University of Wisconsin Milwaukee, General Polymers and John Klees. Competent in resin characterization and analysis, his specialty is practical, hands-on injection molding training with both small and large machines. National recognition has come through ten patents, over 60 papers covering plastics, processing, machine specifications, and over 12 years on the national seminar circuit. Feature articles such as the “Productivity”; by Plastics World and ”Scientific Molding” by Injection Molding Magazine October, November and December 1997, have highlighted a couple of exemplar case histories. Check out the August 2001 issue for applications of The Universal Set Up Sheet.John is the initiator of Scientific Injection Molding and teaches the plastic’s point of view for design and processing with a passion you will remember. Take some of your valuable time to learn practical molding techniques that improve your profits tomorrow while eliminating the state of ”fire fighting“ currently found in many molding facilities. Let us keep plastic manufacturing strong in North America.|
The views, opinions and technical analyses presented here are those of the author, and are not necessarily those of UL, ULProspector.com or Knowledge.ULProspector.com. While the editors of this site make every effort to verify the accuracy of its content, we assume no responsibility for errors made by the author, editorial staff or any other contributor. All content is subject to copyright and may not be reproduced without prior authorization from Prospector.