Last updated: 2026-09-01 · 4 min read

What Causes Voids in Injection Molded Parts — and How Do You Eliminate Them?

Voids in injection molded parts are internal pockets of empty space — not surface marks, not sink marks, but hollow regions buried inside the wall. They form when the outer skin freezes faster than the core, and the shrinking core pulls away from itself because there is not enough packing pressure or hold time to feed it.

Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. See also related guide.

Ejector pin — A steel pin that pushes the molded part out of the cavity after cooling. Typical diameter: 2–10mm. Pin marks are visible on the part surface — position at non-cosmetic areas. Pin count and placement affect ejection force and part warpage. The related guide covers adjacent topics.

Why Voids Form Inside Thick Sections

The mechanism is straightforward thermal physics. When molten resin enters a thick cavity, the material touching the cold mold surface freezes within seconds, forming a rigid skin. The core — insulated by that skin — stays molten much longer. As the core finally cools and shrinks, it has no access to additional material because the gate has already frozen or the hold pressure has been released. The result is a void where the core used to be.

This is why voids correlate almost linearly with wall thickness. A 2 mm wall rarely shows voids. A 4 mm wall shows them frequently. A 6 mm boss or rib base almost guarantees them unless the process is specifically tuned to counteract the effect. The root cause is not a processing error — it is a geometry problem.

How to Tell a Void from a Bubble

The two defects look similar on a cross-section cut but have entirely different root causes and entirely different fixes.

Process Adjustments That Actually Fix Voids

Once the wall thickness is locked in, the process window for void elimination has three levers, applied in this order:

First, extend hold pressure time. The gate must remain open — meaning the resin at the gate must still be molten — for as long as the core of the thick section is shrinking. For a 4 mm wall in PC or ABS, that often means 8–12 seconds of hold pressure, which is longer than most standard cycles use. If the gate freezes before the core finishes shrinking, the core has no feed source and a void forms.

Second, increase hold pressure. Typical hold pressure is 50–80 % of injection pressure. For thick sections prone to voids, 80–95 % may be needed — but only if the gate has not frozen. Higher hold pressure without sufficient gate size just wastes energy and can flash the parting line.

Third, lower melt temperature. A cooler melt freezes faster throughout the cross-section, reducing the differential between skin and core. The trade-off is flow length — lower melt temperature shortens the maximum flow path. This lever is only useful when the part fills comfortably at the current temperature and has excess flow margin.

One failure mode we see regularly: the DFM recommends coring out a thick boss, the customer declines to keep the solid boss for “structural reasons,” and the void appears at T1. The fix at that point is either to accept the void (if it does not affect function) or to re-machine the core — which is expensive and delays the schedule. The DFM recommendation should be documented and signed by both parties before steel is cut.

Frequently Asked Questions

Can gas counter-pressure eliminate voids?

Yes. Gas counter-pressure injects nitrogen into the cavity at 10–30 bar during packing, compressing the shrinking core from the inside. It is effective for walls up to about 8 mm but adds capital equipment cost and cycle time. It is standard for automotive lens housings and thick optical parts.

Does mold temperature affect voids?

Counter-intuitively, a higher mold temperature can reduce voids in semi-crystalline resins because it slows skin formation and lets hold pressure penetrate deeper into the cross-section. For amorphous resins the effect is smaller and may not justify the cycle-time penalty.

How do I inspect for voids without cutting the part?

X-ray inspection (industrial CT scan) shows voids non-destructively and gives their exact location and volume. For a quicker check, immerse the part in a density column — a voided part will have lower density than the theoretical value for the resin grade.

Key Takeaways

1. Voids are a geometry problem, not a process problem — walls above 3 mm are the root cause.
2. The fix order is: redesign the wall first, then extend hold pressure, then adjust temperature.
3. Voids and bubbles look similar but have opposite fixes — misidentifying them wastes days of process tuning.

If the part has thick sections and voids are showing up at T1, the DFM review should have flagged the risk before steel was cut. A void that requires a process fix after T1 is a design-stage failure that landed on the machine floor — and those are always more expensive to resolve.

Sources
Celanese — knowledge center
Prospector — plastics database
MatWeb — material property data

Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet