A hole inside a molded part is one of two things. A vacuum void is empty: the outside of the part froze and the inside shrank away from itself. A bubble has gas in it: steam from moisture, trapped air, or gas from overheated resin. They look alike in a clear part and the fixes are opposite, so find out which one you have before you change a setting.
Also described as: bubbles in the part, vacuum voids, air pockets, hollow spots, blisters.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| A hole in the thickest section, in the same place every shot. Often irregular or stretched in shape. | Vacuum void. The core shrank after the skin and the gate had frozen. | Part weight; hold pressure and hold time; cushion |
| When you warm the area until it softens, the surface over the hole pulls inward. | Vacuum void. There is no gas inside, so the softened wall collapses into it. | Packing, as above |
| When you warm the area until it softens, the surface over the hole swells outward. | Gas bubble. The gas expands as it heats and pushes the softened wall out. | The purge, for bubbles or foam; then the dryer |
| Many small round bubbles scattered through the part, in thin walls as well as thick. Often with splay on the surface. | Moisture. Water in the pellets turns to steam in the barrel. | Dryer temperature, time and dew point; moisture of pellets at the feed throat |
| Bubbles with brown or yellow streaks. Worse after a stoppage or on a long cycle. | Gas from resin breaking down in the barrel or hot runner. | Melt temperature with a probe; residence time |
| Bubbles in dry resin, often strung out along the flow path from the gate. | Air carried in with the melt: too much decompression, low back pressure, or air drawn in during screw recovery. | Decompression distance; back pressure; screw speed |
| One bubble at the end of fill, in a blind rib or where two flow fronts meet. There may be a burn mark. | Air trapped in the cavity by the fill pattern, with no vent to let it out. | Vents; a series of short shots to see how the cavity fills |
Checks in order, cheapest first
- Decide which one it is. Look at where the hole sits and what shape it is, then do the heat test: warm the area slowly with a heat gun until the plastic softens, and watch whether the surface sinks in or swells out. The part gets hot enough to burn, so handle it with gloves and work with ventilation. Do not heat PVC or acetal parts this way, because both give off hazardous fumes when overheated.
- Take an air shot and look at the purge. Bubbles, foam or crackling in the purge mean the gas is coming from the barrel: moisture, air or degraded resin. A smooth, solid purge points to the mold: a vacuum void, or air trapped in the cavity.
- If it is gas, check the dryer and measure the moisture. Confirm the air temperature at the hopper inlet, the dew point and the time the pellets spend in the hopper. Test pellets taken at the feed throat and compare with the targets below.
- Raise back pressure, reduce decompression and check melt temperature. More back pressure squeezes air out of the melt during screw recovery. Less decompression pulls less air in at the nozzle. A melt at the top of its range, or held too long, makes its own gas.
- If it is a vacuum void, pack the part. Weigh the parts, confirm there is a cushion, and run a gate freeze study: raise hold time in steps until part weight stops rising. Then raise hold pressure in steps. A warmer mold lets the skin move, which removes the void but can leave a sink mark on the surface. Which one is acceptable depends on the part.
- Run virgin resin alone and compare lots. Regrind is irregular in shape, carries more air into the screw and picks up moisture faster than pellets. If the bubbles stop without it, cut the percentage or dry it longer.
- Look at the tool. Clean and check the vents. A gate that feeds the thick section, and is large enough to stay open while it packs, removes most vacuum voids. Thick sections can often be cored out.
- Change the resin last. Voids and bubbles are usually not the resin. If drying, packing and venting are right and the holes still follow the material, look at the grade.
The numbers
Typical drying conditions for resins that most often show moisture bubbles, including the clear resins where bubbles are easiest to see. Times assume a desiccant dryer and resin from a sealed container.
| Resin | Drying temperature | Drying time | Moisture target |
|---|---|---|---|
| Polycarbonate | 248–250°F / 120–121°C | 2–4 h | ≤0.02% (one producer tightens to 0.01% for optical parts) |
| Acrylic (PMMA) | 158–208°F / 70–98°C | 2–4 h | <0.02% strict; 0.05–0.1% non-critical |
| SAN | 175–176°F / 79–80°C | 2–4 h | 0.04–0.08% |
| PSU | 275–325°F / 135–163°C | 2–4 h (shorter at higher temperature) | 0.02–0.05% |
| ABS | 176–203°F / 80–95°C | 2–4 h | 0.1% general; 0.02–0.05% for plating |
| Nylon 6, unfilled | 176°F / 80°C | 2–6 h | 0.03–0.12% |
| Nylon 66, unfilled | 175–180°F / 80–82°C | 2–4 h from a fresh bag | 0.03–0.2% |
| PBT | 250°F / 121°C typical | 2–4 h | 0.02% |
| TPU | 194–230°F / 90–110°C (aliphatic grades 104–140°F / 40–60°C) | 1–4 h | <0.02–0.05% |
Typical values from published drying guides, to confirm against the data sheet for your grade. The full list is in the resin drying chart. For vacuum voids the figure that matters is shrinkage, which is listed by resin in the sink marks guide.
Why the two look alike and behave differently
- A vacuum void is shrinkage with nowhere to go. The skin of a thick section freezes first and becomes rigid. The molten core keeps shrinking as it cools. Once the gate has frozen no more material can enter, so the core pulls apart and leaves an empty space. If the skin had been softer, the same shrinkage would have shown as a sink mark.
- A bubble is gas that arrived with the melt or was trapped by it. Steam, air and breakdown gases are compressed during injection and stay in the part when it freezes. They can appear anywhere the melt went, including thin walls where a vacuum void would not form.
- The fixes pull in opposite directions. A vacuum void needs more packing and a skin that can move. A gas bubble needs a drier, cooler, better-vented melt. More hold pressure will not remove steam, and a longer drying time will not fill a thick section.
- Clear parts show every one. In an opaque part the same holes are hidden. They are found by cutting sections, by part weight, or when the part breaks at a thick section that should have been solid.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A filled grade with lower shrinkage (vacuum voids) | Less shrinkage in the core means less volume to make up. | Not an option for clear parts. Changed stiffness, impact strength and surface finish, and with glass fiber a risk of warp. |
| A higher-flow grade of the same resin (vacuum voids) | A thinner melt carries hold pressure farther into a thick section before the path freezes. | Usually lower toughness, more tendency to flash, and a new grade for your customer to approve. |
| Less regrind, or none (bubbles) | Less trapped air, moisture and heat history going into the screw. | More virgin resin in every part, and scrap to store, sell or dispose of. |
| Resin in moisture-barrier packaging (bubbles) | It arrives drier, so your usual drying time is enough. | It still has to be dried, and the benefit is gone once the container has sat open. |
| A resin that does not absorb moisture (bubbles) | Removes absorbed water as a source of gas. | A different resin family: different shrinkage, stiffness, clarity and heat resistance. Expect tooling changes and a full part approval. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Sink marks. The same shrinkage as a vacuum void, showing on the surface. A change that removes one can produce the other. See sink marks.
- Splay. Gas that reached the surface and was smeared into silver streaks. Moisture often gives splay and bubbles together. See splay and silver streaks.
- Unmelted pellets and contamination. A solid lump or a foreign particle can look like a bubble in a clear part. Cut through it: a void or bubble is hollow. See black specks and contamination.
- Burn marks. Trapped air that got hot enough to scorch the resin leaves a dark mark at the end of fill. The cause is the same as a trapped-air bubble. See burn marks.
Quick answers
Why are there bubbles in my molded parts?
A hole inside a molded part is either a gas bubble or a vacuum void. A bubble holds gas: steam from moisture, trapped air, or gas from overheated resin. A vacuum void is empty, left when the outside of the part froze and the inside shrank away from itself.
How do I tell a void from an air bubble?
Warm the area slowly with a heat gun until the plastic softens. Over a vacuum void the surface pulls inward, and over a gas bubble it swells outward. Wear gloves and work with ventilation. Do not heat PVC or acetal parts this way, because both give off hazardous fumes when overheated.
What causes voids in thick plastic parts?
The skin of a thick section freezes first and becomes rigid while the molten core keeps shrinking. Once the gate has frozen no more material can enter, so the core pulls apart and leaves an empty space. With a softer skin, the same shrinkage would have shown as a sink mark.
How do I fix voids and bubbles in molded parts?
The fix depends on which one it is, because the two pull in opposite directions. A vacuum void needs more packing and a skin that can move, so check part weight, cushion, hold time and hold pressure. A gas bubble needs a drier, cooler, better-vented melt.
Are voids and bubbles caused by the resin?
Voids and bubbles are usually not the resin. Check drying, packing and venting first. If those are right and the holes still follow the material, look at the grade. Running virgin resin alone shows whether regrind is carrying in air and moisture.
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