How to give your parts a blemish-free complexion.

Bubbles, voids, sinks or blisters are the most significant causes of injection molded parts being rejected for cosmetic reasons. These troublesome flaws can affect part performance and be difficult to eliminate.

Bubbles are either pockets of trapped gas or vacuum voids. It is important to determine which type of bubble you have in your part in order to more quickly pinpoint the source and determine what correction to make. A simple test of warming the part area containing the bubble until it softens can be used to determine whether it is trapped air or void. If there is gas trapped in the bubble, the gas will warm up and want to expand the bubble as the part softens. Conversely, a vacuum void will collapse under atmospheric pressure as it softens. Use a hot-air gun to heat the area of the part containing bubbles or voids. The next best choice is a small lighter--or a torch, if you know what you are doing.

Free trapped air

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. It can also be due to equipment and production problems such as non-vented core pins, inadequate venting, too much decompression, or resin degradation. Bubbles may result from water vapor or volatiles in the resin, byproducts of degradation, or air trapped in ribs, threads, or non-vented projections.

Flow patterns are a major cause of bubbles. Examine the part to see if the plastic flow front is coming around on itself. Look for a "racetrack" effect or jetting that can cause air to become trapped in the polymer.

Check the flow path for backflow or trapped air in blind ribs. Examine the part to determine if the rib or support areas are covered before the part is completely filled. Run a short-shot molding sequence, changing the transfer position or shot size to make various sizes of short shots ranging from 10% to 95% of the full part. Use this to find out where and how the air entrapment occurs.

This test requires the process to be velocity controlled during injection. This cannot be done if the first-stage pressure and velocity are both reduced.

Other causes of air entrapment leading to bubbles or blisters include inadequate venting and gas traveling across the part surface during the filling or packing stage. You may need to change the gate location to avoid race-tracking or air traps.

Inadequate tool venting is a big cause of air traps. Check the number of vents as well as vent depth. Clean all parting-line and core vents. Use pressure-sensitive paper to check vent function. You might also find porous-steel inserts helpful in air removal.

If you have a hot runner, it is possible that a venturi effect can suck air between the plates into the hot runner, forming a bubble. To check for this, the tool must be disassembled and a bluing agent applied near the runner drops. Be 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.

You can also examine a purging of a normal shot to see if the bubble originates from the barrel or screw. General-purpose screws with an 18:1 L/D or lower can be the culprits for a bubble or blister because they have insufficient processing length to allow all the air to separate and vent from the feed throat. One solution is to raise the backpressure to 1000 to 1500 psi.

Another solution may be to pull a vacuum on the mold just before injection, so that air is drawn out. Moisture in the molding system or resin can also be a source of trouble--causing steam bubbles to form in the hot melt.

Excessive screw decompression can pull air into the nozzle and entrap it in the next shot. With a cold-runner system, that air is likely to move ahead of the flow front and be vented from the mold. But the higher melt inventory and residence time of a hot-runner system increase the chances that the trapped air can end up as a bubble in the part.

Avoid voids and sinks

A void occurs during cooling, usually in thick sections of the part, where there can be a significant difference in cooling rate between skin and core material. A sink is a depression in the surface of a part, caused by shrinkage. 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 try packing more plastic into the cavity to see if that helps. To prevent under-packing, make sure you have a consistent melt cushion in the injection barrel, so you are not bottoming out the screw. Try using higher pressures and longer times in the pack/hold stage to ensure packing until the gate seals.

You can also try very slow filling, higher backpressure, or gas counter-pressure in the mold. You can open up the gate diameter to allow for longer gate-seal times and more packing. Also try lowering the melt temperature because cooler melt shrinks less.

To reduce internal stresses that show up as sinks, you can try increasing the runner diameter. In addition, consider where the sink is located: Is it near the gate or farther down the flow path? If near the gate, check whether longer hold time to achieve gate-seal would help. If it is farther down the flow path, increase injection speed to decrease melt viscosity and allow more packing pressure.

Another way to eliminate voids or sinks is to thin the part wall. Thicker is not always stronger in plastic parts. Thick walls should be redesigned with ribs if strength is needed. This will save plastic and cycle time. Core out thick sections, if possible.

Change the gate location to fill thicker areas in the mold first. This may allow more polymer into the part before the gate freezes. Also try raising the mold temperature and/or ejecting the part sooner, which can avoid voids by allowing for more uniform shrinkage.

To prevent sinks in thick sections, extra cooling outside the mold may be needed: Try cooling the part in water or between aluminum sheets rather than just in air. Slower heat dissipation in air permits the still-hot core of the thick section to reheat or remelt the outside surface after ejection, allowing the surface to collapse and form a sink.

However, if the problem is vacuum voids, the answer may be to cool the part more slowly outside the mold. Keep it warm by placing it on wood or insulating foam. However, you should expect sinks with this approach.

Beware of blisters

Blisters, a thin film of plastic that bubbles up from the part surface, can ruin the aesthetics of a part. Like bubbles they can be caused by gas traveling across the surface during filling or packing or by trapped air from inadequate venting, flow patterns, or a short screw L/D. The solutions are the same as well. But blisters can also originate from process problems or degradation of the resin or additive package.

Blisters can be a symptom of surface delamination, which can result from injecting too fast. That can produce a highly oriented thin layer on the surface of the part. You can sometimes detect this by sticking adhesive tape onto the part and lifting it, which can pull this layer off.

If this occurs, try injecting more slowly. High mold temperatures near the gate can cause blisters from resin degradation. Lower the steel temperature in this area, if possible.

Plasticating screw length should be at least 20:1 L/D. If you find you need longer cycle times and higher backpressure to reduce the occurrence of blisters, then your screw design may not be well suited to the material.

Gas can be created by the degradation of the resin or additives, so be sure to stick within the material and additive suppliers' recommended melt-temperature range. Minimize residence time by making sure the machine barrel is appropriately sized for the shot. If degradation is a problem, you might avoid using regrind.

By John Bozzelli

Injection Molding Solutions

John Bozzelli has taught seminars on plastics design and processing for more than 15 years. He has extensive experience in polymer development and processing from more than 20 years with Dow Plastics. He is the founder of Injection Molding Solutions in Midland, Mich., a provider of in-plant training and consulting services. Tel: [phone] or e-mail: [e-mail]