Those Annoying Bubbles in Plastic Parts: Why They Happen and How to Fix Them

Title: Those Annoying Bubbles in Plastic Parts: Why They Happen and How to Fix Them

Have you ever looked at a plastic part and seen tiny bubbles inside, like air trapped in ice? You’re not alone. This is one of the most common headaches in injection molding.

But here’s the good news: bubbles are almost always fixable. The trick is to figure out what kind of bubble you're dealing with.

There are really only two main types. Think of them as Air Bubbles (trapped gas) and Vacuum Bubbles (shrinkage holes). Let’s break them down in plain English.

Type 1: Air Bubbles (The "Gas Trap")

What it looks like: Round, spherical bubbles that can be on the surface or inside the part.

How it happens: Imagine pouring syrup into a jar with no air hole. The air has nowhere to go, so it gets trapped. In molding, this happens when:

• The raw plastic pellets are wet. Moisture turns to steam inside the hot barrel.

• The injection speed is too fast, creating a "whirlpool" effect that wraps air into the part.

• The mold doesn’t have enough venting to let the air escape.

• The plastic gets too hot and starts to burn, releasing gases.

Type 2: Vacuum Bubbles (The "Shrink Hole")

What it looks like: Irregular, jagged holes, usually in the thickest part of the part.

How it happens: Picture Jell-O. The outside hardens first, but the inside is still soft. As the inside cools, it shrinks and pulls away from itself, creating an empty void. In molding, this happens when:

• Packing pressure is too low or too short—there's not enough "top-up" material.

• The gate (where plastic enters the mold) freezes off too early, stopping the flow of extra plastic.

• The wall thickness changes too dramatically (thick areas shrink more).

How to Fix Them (The Simple Cheat Sheet)

You don’t need a PhD to solve this. Just work through these steps in order:

1. First, Dry Your Material
This is step one for a reason. Nylon, PC, and PET are like sponges. If you don’t dry them properly, you’ll always get bubbles. Check your dryer settings and run a "purge shot" into the air—if it looks foamy or steamy, it's too wet.

2. Adjust Your Machine Settings

• Too many air bubbles? Slow down the injection speed. Let the air escape gently.

• Too many vacuum holes? Increase the pack pressure and hold time. Give the part enough time to "eat" more plastic as it cools.

• If all else fails, try increasing the mold temperature. This slows down the cooling, giving the plastic more time to settle evenly.

3. Check the Mold

• Add venting: Make sure there are tiny gaps (0.02–0.04mm) at the end of the flow path to let air out.

• Move the gate: Put the entry point near the thickest section so the pressure can push deeper into the part.

• Design for uniformity: If possible, make the wall thickness the same everywhere. Thick spots are bubble magnets.

A Quick Pro Tip

Here’s a simple test: If you see bubbles right after the mold opens, it’s usually a gas/air problem. If you see them after the part cools down (a few seconds later), it’s usually a shrinkage/vacuum problem.

Once you know which one it is, you already know which direction to go.

Bottom line: Bubbles are frustrating, but they're also logical. Dry your resin, slow down the fill, pack it properly, and let the air out. Do those four things, and you'll eliminate 90% of your bubble issues.

Happy molding!