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

A short shot leaves material missing: an unfilled corner, a short edge, or an incomplete feature. Unlike a cosmetic mark, it usually makes the part unusable. Your job is to decide whether the cause is a tunable process setting, a tooling limit (gate or vents), or a design that will not fill. Related defect pages live in the injection molding defects section.

If short shots, or shorts combined with flash, show up in first samples, use the short shot and flash T1 review explainer for the evidence to request before you approve a tooling change.

What Causes a Short Shot

Filling a cavity is a race against freeze-off. The melt has to reach every extremity before it cools. A short shot happens when it loses that race or gets blocked. Causes cluster like this:

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, split causes into process (pressure, speed, temperature), tooling (gate size and venting), 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. It is the cause most often left unexamined, and it has the clearest published fix.

Air 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 depths are small enough that a vent partly filled with residue stops working long before it looks different. Steve Johnson, writing in MoldMaking Technology, describes buildup of non-aqueous volatiles from resin fillers and additives etching the steel around vents. That damage cannot be wiped off; it needs diamond polishing to remove.

There is a second point in the same source: moldmakers habitually cut vents at the low end of the recommended range. A mold delivered slightly over-vented flashes and needs welding and rework, 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. Ask two cheaper questions before you live with a pressure increase: have these vents ever been optimized for our process, and when were they last measured against the baseline? See mold venting.

When It’s Design, Not Just Settings

Some short shots resolve through process work. Others come from tooling, material flow, machine capability, or part design. Raising injection pressure alone is rarely the lasting fix. Find the limiting variable and prove a stable production window.

• Thin walls over a long distance. A thin section freezes off quickly. If the melt has to travel a long, thin path, it may not make it. That is a classic wall thickness and flow interaction.
• 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 will not 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 cannot. See gate design.

A short shot that will not go away with process tuning often means the part is at the edge of what is fillable. The fix is design or gating, not settings.

What a Buyer Should Do

• Watch for thin-wall-plus-long-flow combinations in your design. Flag them in your DFM review.
• Treat material flow as part of resin selection. If you choose 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 diagnoses which lever applies instead of maxing out pressure. High pressure used to force a fill can cause flash or burn marks, so the right fix matters.

Buyer FAQs

What is a short shot in injection molding?

A short shot is incomplete cavity fill: missing material at a corner, edge, or feature. Treat it as a reject unless you have already defined an incomplete feature as non-critical in the quality plan. Ask the supplier which domain limits fill (venting, pressure/speed, gate, thin-wall flow, temperature, or resin flow) before you approve a process-only fix.

Why does a cavity fail to fill completely?

Filling is a race against freeze-off. Trapped air, low pressure or speed, a restrictive gate, thin walls over a long path, low temperatures, or a low-flow resin can each lose that race. Put the suspected cause in writing with cavity ID and process sheet before you authorize steel work or a permanent pressure increase.

Can a short shot be fixed by the molder?

Sometimes. Accept a process change only if it stays inside material, machine, tool, quality, and cycle limits, and only if the supplier shows a process window, not one fully filled sample. Persistent or pressure-sensitive shorts: require a tooling, machine-capability, material, or design review before production release.

How does wall thickness cause short shots?

A thin wall freezes fast. Over a long flow distance, melt can freeze before the end of fill. In DFM, review wall thickness and flow length together. If that geometry is fixed, gate count/location or resin flow grade become the remaining levers; document which one you are changing.

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.