Knowledge Base

How to Remove and Prevent Bubbles in Parts Made from Injection Molds

Dec 6, 2022, 12:57 PM by The Stoner Molding Solutions Team
Producing plastic parts on injection molds has many benefits. It’s a cost-effective way to produce high volumes of the same part quickly. You can work with a part designer to create a unique mold tool that can be used for custom plastic injection molding.

Dark grey injection molded parts on a table.Producing plastic parts on injection molds has many benefits. It’s a cost-effective way to produce high volumes of the same part quickly. You can work with a part designer to create a unique mold tool that can be used for custom plastic injection molding. And if a few parts have minor defects, most of the thermoplastic resin can be remelted and used again for a new part.

But when defects happen too often, it can negatively impact the productivity of your injection molding service. A common defect found in plastic production molding is bubbles. Bubbles can appear as trapped air inside the part, sinks or voids on the product's surface, or as blisters on the surface finish. These defects can often affect the part's structural integrity, making it a top priority for manufacturers to remedy the situation.

Unfortunately, there isn’t one answer for removing air bubbles in your plastic material. It depends on your process, what type of bubble it is, and if you’re using the correct kind of molding products on your tools. High-quality mold cleaners, sealers, and releases will help prevent defects and allow you to mold more parts in less time. Learn more about how to prevent bubbles in your injection molded parts, then speak with one of our experts for recommendations on the right molding products for you.

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What Causes Bubbles in Injection-Molded Parts?

Bubbles are pockets of trapped gas or vacuum voids. To understand what is causing bubbles or other defects that cause a part to be rejected, such as sinks, voids, and blisters, you need to determine what type of bubble it is. To determine the source, gently warm the part with the bubble using a heat gun. If the bubble expands as the part softens, your problem is trapped gas. If it instead collapses when heated, it is a void.

In the case of trapped air, bubbles are caused by flow-front issues, like converging fronts or jetting, poor venting, too much decompression, or degradation of your resin. These bubbles can also be formed by water vapor, contaminants in the resin, or the melt flow pattern. If you determine your bubble is a void, it is caused by insufficient plastic in the mold cavity or too low pressure.

How Do You Reduce Air Traps?

Because several issues can cause air traps in molded products, you need to examine your process every step of the way to ensure the injection molding process is working as it should. Ask yourself these questions:

  • Are you using a high-quality mold sealer to fill in the microporosity of the mold and provide a base coat for mold release?
  • Is the mold release you’re using specifically designed for the manufacturing process you’re performing?
  • Are you cleaning your mold regularly with the best products that prevent build-up?
  • Are the ribs or support area around where the trapped air is being covered before the machine injects the rest of the molten plastic to fill it?
  • Is the venturi effect sucking air between the plates and into the hot runner, pulling air into the plastic material and causing a bubble?

Once you take the time to determine what is causing the trapped air, you can take steps to remedy it. Depending on the cause, you may need to raise the backpressure, pull a vacuum on the mold just before injection to pull out excess air, change the gate location to avoid race tracking, trapping air, and promote uniform filling.

How Can I Avoid Sinks and Voids?

Sink marks or sinks are a defect that many injection molding services face. A sink appears as a localized depression on the surface of the product. These are caused by the holding time and pressure being too low, too short of cooling time, excessive materials, or too great wall thickness. Voids can appear as empty spaces in the product and look like air pockets. These defects are caused by insufficient resin in the cavity and are more likely to occur in thicker areas.

If you find voids and sinks in your finished plastic part, ensure you are not bottoming out the screw. You should maintain a consistent cushion on the press. You can also try increasing the pressure in the hold stage and lengthening the time it’s in that stage of the injection mold. Other solutions may include slower fill rates, gas counter-pressure, or increased back pressure.

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What Can You Do to Prevent Blisters?

A blister can look like a bubble on parts made from thermoplastic injection molds. However, unliked trapped air bubbles under the surface of the part, a blister is a raised or layered bubble on the surface of the plastic. Blistering is often caused by tools or melted resin that is too hot and does not have adequate time to cool.

If you want to remove blisters from your part, you can try slowing down the injection rate on your injection molding machine. If you are remelting defective parts from previous molding runs, try using virgin material or a new lot of the additive being used. Also, be sure you are using the right barrel size and melt temperature, as recommended by the resin manufacturer.

Looking for Better Results from Your Injection Molds?

Trying some of the suggestions above will help you reduce the number of bubbles, sinks, voids, and blisters on your plastic molded parts. But if you aren’t using the right mold cleaners, sealers, and releases, you will likely face these issues and more. Use our products on your mold tools, so you don’t have to worry.

We specifically formulate our molding products for the needs of the thermoplastic industry. With a wide variety of mold releases, rust preventatives, lubricants, and more, you are sure to find the exact kind you need to improve your productivity. If we don’t have the right one for your unique injection molds, we can custom formulate one for you! Get in touch with our experts today to get started!

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