How to remove bubbles in plastic injection molding

One of the main reasons for Custom Injection Molding Services to fail when molding parts is bubbles. This troublesome part defect not only causes cosmetic problems, but also impairs physical properties. Bubbles are common and often difficult to resolve.
When troubleshooting bubbles, many moldmakers will incorrectly guess what the bubble is and immediately start adjusting process parameters to eliminate the bubble. I urge you to resist the temptation to start adjusting and start by defining what the bubble actually is.

There are only two possibilities:
1. Trapped gas, including air, water vapor, volatiles in the resin, or decomposition gases in the polymer or additives.
2. Vacuum failure.

It is important to determine what type of bubble your part has and what the root cause may be. Determining the bubble type will enable you to pinpoint the source and determine your next actions to eliminate the problem. How do you test to determine if it is gas or vacuum? Many people claim (as I once did) that you can tell by the shape, location, or some other feature of one or more bubbles. But you can easily be fooled by this method. You should use a simple test instead. It takes less than 15 minutes, but requires a little patience to perform.
It is important to determine what type of bubbles your part has, and what the root cause might be.
Test your part by gently heating the area of ​​the part that contains one or more bubbles until it softens. I emphasize this mildly because some operators tend to pick up the nearest flashlight and point it at the part. Plastic does not transfer heat quickly through nominal walls, so our flashlight friend might catch the part on fire.

Instead, use a heat gun or something similar. Then, as you gently heat the area of ​​the part where the bubble is, the bubble should change shape. If it is a bubble, the gas will heat up and expand, lifting the surface, and will usually pop as the surface of the part softens. If there is no air in the bubble, but a vacuum, the bubble will pop due to atmospheric pressure pushing against the softened surface of the part. Now you know more about what the problem is.
This test requires some conditions to work. Ideally, find a bubble that is at least 3 mm (about 0.125 inches) in diameter or larger, and make sure the part is no older than 4 hours. The bubble may start out as a void, but over time, the air will migrate through the plastic and the void will become a bubble. On a quick inspection, you might think this bubble is trapped gas. Let’s begin our troubleshooting discussion assuming your testing proves that it is indeed a bubble – that is, when the bubble collapses, it expands or even pops. Bubbles can stem from flow front problems such as converging fronts, jetting, or mold/machine problems such as unvented core pins, poor venting (try vacuum venting), excessive decompression, or resin degradation due to overheating or long residence time. Gases can come from water vapor, volatiles, or decomposition byproducts of the resin. As the part fills or packs, air trapped in non-vented protrusions in the ribs or nominal walls will be pushed out, leaving a trail of bubbles. In most cases, determining the source of the gas is more important than knowing what the gas is made of, and there is no one simple test to find out.

The first step in the procedure is to remove the hold or second stage by adjusting the hold pressure to a very low number and see if the bubble is still there. If so, at least you don’t have to worry about the parameters involved in the second stage. Assuming you still see bubbles, the next check is to understand the fill pattern to determine if the gas is trapped by air when filling the part.
Turn off the second stage and make a 99% complete part by volume for a short-term study. That is, reduce the shot size from 99% full to 5% full in 10% increments. Don’t start with a short shot and increase the shot size because you may get a different flow pattern. Also, this test requires that the speed of the first stage shot is controlled. If the first stage is limited by pressure, you may not get the consistency you need for accurate results.

Where and when do bubbles appear? Check the flow pattern for each part to see if the plastic flow front appears on its own or if there is hesitation in the flow front as it fills the thin section of the part. Is the bubble always in the same area? If so, that means the bubble is coming from a fixed place. Look out for racetracking effects or jetting that could cause air to be trapped in the polymer.
Check for ribs or any projections on the nominal wall. If they are short, that means there is air in that area and it is pushed out to form a bubble as the rib is filled. Sometimes you can actually see a bubble trail from that projection. Do the bubbles only appear after the part is 85% full? If so, it could be a venting issue. Check the vents.

There are several possibilities: ribs, ejector pins, poor fit of the nozzle tip to the sprue bushing, misaligned nozzles, and separated plates in the hot runner. These are harder to detect, but must be checked on the tool when you rule out other sources. Apply bluing agent near the hot runner drip and on the plate mating surface, being careful not to get anything into the flow path. If bluing agent appears at startup, you have found the source of the problem. Another common source of bubbles is excessive decompression, especially in hot runner molds.

Another source is the screw, more specifically the back zone or feed section. A general purpose screw with an L/D of 18:1 or less may be the culprit. Try using a lower back zone temperature and/or higher back pressure. Another solution may be to pull a vacuum on the mold before injection.

Voids
Voids occur when a part is inside or outside the mold, usually in thicker sections, during cooling. In thicker sections of the part, the center cools slowly and the polymer contracts more, pulling away from itself to form bubbles. If you run the mold hotter and the bubbles go away, but you end up with a sink, that indicates your bubbles are empty. Voids and sinks are signs of internal stress and are warning signs that the part may not perform as expected.
Not enough plastic is the primary cause of sinks or voids, so packing the cavity with more material is recommended. Make sure you have a consistent cushion and are not bottoming out the screws so that you can pack the part properly. Higher packing pressures or longer hold times can help, but many times the gate freezes before you can adequately crowd the center of the nominal wall.

Thin the nominal wall. Core the thicker part if possible.
To resolve voids or sinks, try reducing the fill rate, using gas back pressure, or increasing back pressure. Make sure the runner or gate is not freezing prematurely, and longer hold times will allow for more packing in the second stage. If the gate is freezing prematurely, just open the gate slightly, as a small change in diameter will result in a longer time for the gate to seal. Also try reducing the melt temperature if possible.

Other ways to eliminate voids or sink marks are to thin the nominal walls. Thicker plastic parts are not always stronger. Thicker nominal walls should be redesigned to be thinner and with reinforcing ribs. This will save plastic and cycle time.
Core the thicker part if possible. Changing the gate location to fill thicker areas of the mold first may allow more polymer to enter the part before the gate freezes. You can also try increasing the mold temperature and/or ejecting the part as quickly as possible, which can prevent voids by allowing the outer walls to collapse during cooling, although this may cause sink marks.