Short Shots in Injection Molding: Causes and Fixes

A short shot is a part where the melt failed to completely fill the mold cavity, leaving it visibly incomplete. The most common causes are insufficient injection pressure or speed, low melt or mold temperature, trapped air from poor venting, or a runner system/machine that's undersized for the part. Fix it by checking those four causes in order, changing one parameter at a time.

TL;DR — check these first:

• Increase injection speed in stages, then pressure only if speed alone doesn't resolve it
• Raise melt or mold temperature within the resin's recommended processing window
• Check and clean mold vents (typical depth 0.02-0.04 mm for most thermoplastics)
• Compare shot volume (part + runners) against the machine's rated capacity
• Rule out moisture in a hygroscopic resin, degraded material, or a worn check ring

Is injection pressure or speed the cause?

Insufficient pressure or speed means the melt front slows down or partially freezes before filling the cavity, especially in thin sections.

How to confirm: Run a short-shot study, filling to 95-98% of cavity volume with no hold/pack phase, and observe the fill pattern. If fill stops uniformly before reaching thin sections or the end of flow, pressure or speed is a likely cause.

Fix: Increase the injection speed profile in stages, and only raise the pressure limit if speed alone doesn't resolve it — watching for flash as the ceiling.

Is low melt or mold temperature the cause?

Colder melt has higher viscosity and freezes prematurely, particularly in thin ribs and areas far from the gate.

How to confirm: Compare the actual barrel temperature profile against the material data sheet; measure actual mold surface temperature with a probe, not just the controller's setpoint.

Fix: Raise barrel zone temperatures incrementally (5-10°C steps), and raise mold temperature within the range the resin manufacturer recommends — many engineering resins need mold temperatures well above default factory settings.

Is poor mold venting the cause?

Trapped air creates back-pressure that resists melt flow, especially at the last-fill area or in thin ribs.

How to confirm: Look for burn marks (dieseling) at the last-fill point — dark, scorch-like marks indicate compressed, superheated air, a telltale sign of a venting problem rather than a plain short shot.

Fix: Clean existing vents, add or deepen venting at the last-fill location, and confirm ejector pins and the parting line aren't clogged with residue.

Is the machine or runner system undersized for the part?

Shot volume close to or exceeding the barrel's rated capacity gives an inconsistent cushion and pressure loss cycle to cycle.

How to confirm: Compare the part-plus-runner volume against the machine's maximum shot capacity — for consistent quality, the injected volume should be roughly 20-80% of the barrel's rated shot capacity.

Fix: Move to a higher-capacity machine, or reduce runner volume and reconsider gate location.

Cause overview

Perotec Systems supplies Japan Steel Works (JSW) full-electric injection molding machines from 30 to 3000 tons for the regional market. Browse the full injection molding machines range or contact us with your process data — we help systematically identify the short-shot cause for your specific tool and machine.