Bubbles commonly appear at the final weld line (joint) of plastic parts or in thicker sections of the product. During molding, the outer surface of the part cools first. Once the surface has solidified, the inner core remains molten. As the core subsequently cools and shrinks, the already-solidified outer layer prevents outward contraction—resulting in internal voids or bubbles.

Notably, many thick-walled parts may appear bubble-free upon ejection from the mold, yet develop internal bubbles within tens of seconds afterward—especially in the central region.

Bubble defects can manifest in various forms, each with distinct causes. Below is a detailed breakdown:

Q: Thick parts exhibit surface swelling (bulging), and cross-sections reveal internal bubbles. Possible causes include:

• Excessive injection speed, which traps air inside the cavity before it can be vented, leading to bubble formation.
• Incorrect shot volume (either too much or too little) or excessively high injection pressure, both of which can induce such defects; adjusting the shot volume often resolves the issue.
• Insufficient cooling time, particularly critical for thick-walled parts. A practical remedy is external cooling using cold water or a cold mold after ejection.
• Thermal degradation (black streaks), caused by excessive barrel or melt temperatures. If minor, lowering the temperature may suffice; if severe decomposition occurs throughout the barrel, disassembly and cleaning are required.
• Small gate size and poor mold venting, which hinder air escape and promote bubble formation.

Q: Solid parts contain dark-colored voids (cavities)

In solid-part molding, low injection pressure¹ and slow injection speed are essential. High pressure combined with fast injection causes entrapped air to compress, heat up through friction, and carbonize—resulting in blackened cavities trapped within the part.

To resolve this:

• Reduce injection pressure and speed.
• Verify and adjust shot volume—if excessive, reducing it helps eliminate voids.

Q: Small bubbles in thicker sections, or large black bubbles in exceptionally thick areas

When multiple parts of varying geometries are molded simultaneously, ensure that gate sizes² are proportionally matched to part weight/volume. Otherwise, thicker parts are prone to bubble formation.

Also note:

• Parts that remain on the cavity side (front mold) during ejection tend to have poorer venting, increasing the risk of such defects.

Solutions:

• Modify the mold design (e.g., improve venting, optimize gate location/size).
• Increase packing (hold) time and/or cooling time to compensate for insufficient pressure in thick sections.
• Most importantly, during product design, avoid excessively thick sections whenever possible—uniform wall thickness³ significantly reduces bubble-related defects.