Injection Molding Defects: Bubbles – What Causes Them and Fixes

In injection molding, the formation of bubbles within a part is a common and problematic issue. These internal flaws can weaken the component, create cosmetic imperfections, and lead to part failure in demanding applications.

Understanding the origin of these bubbles is fundamental to producing consistent, high-quality parts. By identifying the root causes, manufacturers can implement targeted solutions to eliminate them, thereby enhancing part integrity, reducing scrap rates, and upholding stringent quality standards. This article will examine what bubbles are, explore their underlying causes, and outline the effective methods for their prevention.

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What Exactly Are Bubbles?

Bubbles in injection molding, also referred to as gas pockets, are trapped pockets of gas that appear within the body of a finished plastic part. They are distinct from "sinks," which are surface depressions, though the two defects can sometimes share common causes. Bubbles are typically visible as small, often spherical, hollow areas when looking at a transparent part or through a cross-section of an opaque one.

The presence of these bubbles indicates that gas was unable to escape the mold cavity during the injection process or that vacuum pockets formed due to material shrinkage. Depending on their size and location, bubbles can severely compromise the structural integrity, mechanical strength, and visual appeal of a molded component, making them unacceptable for nearly any application.

What Are the Root Causes of Bubbles?

The formation of bubbles is generally traced back to one of two primary sources: trapped gases within the mold cavity or excessive material shrinkage during cooling. Identifying which type of bubble is occurring is the first step toward finding an effective solution.

Trapped Gases

Gases can become trapped in the mold for several reasons. One of the most common is inadequate venting in the mold tool. Vents are tiny channels that allow the air already inside the cavity to escape as the molten plastic flows in. If these vents are too small, poorly placed, or clogged, the air has no way out and becomes compressed by the incoming plastic, forming a bubble.

Another source of gas is the plastic material itself. If plastic resin is not dried properly before molding, the moisture it contains will turn into steam (gas) when heated in the injection barrel. This steam then gets carried into the mold with the plastic, leading to bubbles. Similarly, if the melt temperature is too high, it can cause the plastic material itself to degrade and release gases, which then become trapped.

Shrinkage Voids

Bubbles can also form due to the natural tendency of plastic to shrink as it cools. This is particularly common in parts with thick cross-sections. As the outer surface of a thick section cools and solidifies, the molten material in the center begins to cool and contract. If there isn't enough holding pressure or material to pack out this shrinkage, a vacuum void, or bubble, will form in the core.

Insufficient holding pressure or holding time is a primary processing cause. The holding phase is meant to push more material into the cavity to compensate for shrinkage as the part cools. If the pressure is too low or the time is too short, it won't be enough to prevent voids from forming in thicker areas. A gate that freezes off too early will also prevent proper packing of the part, leading to shrinkage voids. You can read more about voids here.

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How Can Bubbles Be Prevented and Eliminated?

Resolving issues with bubbles requires a systematic approach that addresses both mold design and processing parameters. The solution will depend on whether the bubbles are caused by trapped gas or material shrinkage.

Improving Mold Design and Maintenance

If trapped gas is the culprit, the first place to look is the mold's venting system. Adding or enlarging vents in the areas where bubbles form can provide an escape path for trapped air. Regular tool maintenance is also vital to keep existing vents clean and clear of any residue or buildup.

This is where expert plastic injection mold design services come into play. Partnering with a design and manufacturing company experienced in injection molding tooling can help optimize part geometry, gating, and venting for bubble-free production.

For bubbles caused by moisture, the solution is straightforward: implement a proper material drying procedure. Every plastic material has a recommended drying time and temperature that must be followed to remove excess moisture before processing.

Optimizing Processing Parameters for a Solid Part

When shrinkage is the cause, adjusting the process parameters is the most effective solution. The following steps are typically taken:

• Increase Holding Pressure and Time: Raising the holding pressure or extending the holding time forces more material into the mold to compensate for shrinkage as the part cools. This is often the most effective way to eliminate shrinkage voids.
• Increase Shot Size: A larger shot size can help pack the mold more densely, reducing the chance of voids.
• Decrease Melt Temperature: Lowering the temperature of the molten plastic reduces the overall amount of shrinkage, though this must be balanced to avoid causing other defects like short shots.
• Check Gate Size: If the gate is too small, it may freeze off before the part is fully packed out. A larger gate may be required for parts with thick sections.

These refinements can be achieved with support from a design for manufacturing services provider. Incorporating Design for Injection Molding (DfIM) practices and DFM for Medical Devices ensures that both design and tooling are tuned for precision and performance.

Why Is Eliminating Bubbles a Priority in Manufacturing?

Bubbles are a critical defect that directly impacts the performance and reliability of a finished product. A part with bubbles is a weak part, susceptible to failure under stress. Addressing the root causes of bubbles, be it through improved tool venting, proper material handling, or optimized process control, is a fundamental aspect of ISO-Certified Manufacturing and injection molding quality control. The production of bubble-free parts is a testament to a well-designed tool and a precisely controlled process, yielding components that are strong, solid, and fit for their intended purpose.

If bubbles are impacting the quality of your parts, don’t hesitate to reach out to Aprios. We have the expertise to tweak your process and get rid of those troublesome bubbles.