Short Shot in Injection Molding: Why Parts Don't Fully Fill

A short shot occurs when the cavity fails to fill completely, resulting in a part with missing material—often an unfilled corner or a short edge. Unlike superficial cosmetic defects, a short shot typically renders the part structurally or functionally unusable. Identifying why the cavity failed to fill helps buyers determine whether the root cause is a tunable process parameter, a tooling constraint, or a restrictive part design. This guide is part of the injection molding defects section.

If short shots—or shorts combined with flash—appear in first samples, the short shot and flash T1 review explainer details the evidence buyers should request before approving a tooling change.

What Causes a Short Shot

Filling a cavity completely is a race against time: the plastic has to reach every extremity before it cools and freezes off. A short shot happens when it loses that race or gets blocked. The causes cluster into a few groups:

Source	What’s happening
Trapped air / poor venting	Air with nowhere to escape blocks the plastic from filling
Insufficient injection pressure or speed	Not enough push to fill before the melt freezes
Gate too small or poorly placed	Restricts or misdirects flow so far areas don’t fill
Thin walls / long flow length	The melt freezes off before reaching the end of a long, thin path
Low melt or mold temperature	Colder plastic freezes sooner and flows less far
Material flow / shortage	A low-flow resin, or simply running short of material, leaves the cavity unfilled

For a buyer, the key split again is between process (pressure, speed, temperature, tunable), tooling (gate size and venting, built into the mold), and design (thin walls and long flow paths, decided before tooling).

The one on that list a buyer can actually check

Venting deserves separating out, because it is the cause most often left unexamined and the one with the clearest published fix.

Air in the cavity has to leave through channels measured in thousandths of an inch — around 0.0015 in for PP, 0.002 in for ABS, 0.0005–0.0007 in for nylon, per figures reported by FimmTech. Those numbers are small enough that a vent partly filled with residue stops working long before it looks any different. Steve Johnson, writing in MoldMaking Technology, describes buildup of non-aqueous volatiles from resin fillers and additives etching the steel around vents — damage that cannot be wiped off and needs diamond polishing to remove.

There is a second, more surprising point in the same source: moldmakers habitually cut vents at the low end of the recommended range, because a mold delivered slightly over-vented flashes and needs welding and rework to correct, and they cannot know how you will run the tool. The expectation is that the molder finishes the job at startup, stepping the depth up 0.0002–0.0003 in at a time until whisker flash appears, then backing off.

So a part that will not fill in one corner, on a tool nobody has adjusted, may be short of air rather than short of pressure. Two questions follow: have these vents ever been optimised for our process, and when were they last measured against the baseline? Both are cheaper to ask than a pressure increase is to live with. See mold venting.

When It’s Design, Not Just Settings

While some short shots resolve through validated process adjustments, others stem from fundamental limitations in tooling, material flow, machine capability, or part design. Merely increasing injection pressure is rarely the correct long-term fix; the objective is to identify the limiting variable and establish a stable production window.

• Thin walls over a long distance. A thin section acts like a narrowing pipe that freezes off quickly. If the melt has to travel a long, thin path, it may not make it, a classic interaction between wall thickness and flow.
• Flow length beyond what the resin supports. Each resin has a practical flow distance for a given wall thickness. Push past it and the part won’t fill regardless of process.
• Gate location and count. Where and how the part is gated decides how far the melt has to travel. Adding or relocating gates can solve a fill problem that pressure alone can’t, see gate design.

This is why a short shot that won’t go away with process tuning is often telling you the part is at the edge of what’s fillable, and the fix is design or gating rather than settings.

What a Buyer Should Do

• Watch for thin-wall-plus-long-flow combinations in your design. These are the geometries most prone to filling trouble. Flag them in your DFM review.
• Treat material flow as part of resin selection. If you’re choosing a stiff, low-flow, or filled grade, fill behavior is part of that decision; discuss it with your supplier.
• Ask about gating and venting if filling is marginal. These tooling factors often matter more than raw pressure.
• Resolve at sampling with evidence. Request fill-only or short-shot progression samples where useful, cavity-specific results on multi-cavity tools, the process conditions used, and the supplier’s verification plan.

A capable supplier will diagnose which of these applies rather than simply maxing out pressure, and high pressure used to force a fill can itself cause other defects like flash or burn marks, so the right fix matters.

Buyer FAQs

What is a short shot in injection molding?

A short shot is a part in which the cavity didn’t fully fill, leaving material missing, an unfilled corner, a short edge, or an incomplete feature. It happens when the molten plastic fails to reach every part of the cavity before it cools and freezes off, whether because of trapped air, insufficient pressure, a restrictive gate, thin walls over a long flow path, low temperature, or low-flow material.

Why does a cavity fail to fill completely?

Filling is a race against the melt freezing. The plastic loses that race when air is trapped with no venting, when injection pressure or speed is too low, when the gate is too small or poorly placed, when walls are too thin over too long a distance, when temperatures are too low, or when the resin doesn’t flow far enough. The cause can be process, tooling, or design, often a combination.

Can a short shot be fixed by the molder?

Sometimes. A process change may resolve a short shot if the original process was not appropriate, but the correction must remain inside the material, machine, tool, quality, and cycle requirements. Persistent or pressure-sensitive short shots may require a tooling, machine-capability, material, or design review. Ask for a demonstrated process window rather than approval based on one fully filled sample.

How does wall thickness cause short shots?

A thin wall acts like a narrow channel that the melt fills and freezes in quickly. If a thin section has to carry flow over a long distance, the plastic can freeze off before reaching the end, leaving a short shot. This is why thin walls combined with long flow paths are a frequent filling problem, and why wall thickness and flow length are reviewed together in design.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.