Short Shots in Injection Molding: Troubleshooting Guide

A short shot is a part that did not fill: the flow front stopped or froze before reaching the end of the cavity. Troubleshoot it in this order: confirm there was enough melt (shot size and cushion), confirm the fill was not pressure-limited, check venting at the last-to-fill area, check whether the gate or a thin section froze early, then look at melt temperature and the material lot. Those checks are quick at the press and avoid the mistake of raising pressure and temperature until the part fills while the real cause, such as a blocked vent or an empty cushion, stays in place.

What Is Racing in a Short Shot

The melt enters the mold above its freezing range while the mold steel is far colder, so a frozen layer grows along the walls as the front advances. The cavity fills if the front reaches the end before the flow channel narrows too far. Four things decide that race:

• Flow length relative to wall thickness: longer and thinner is harder.
• Viscosity: colder melt or a lower-flow lot fills less far at the same pressure.
• Pressure and speed available: a fill that hits the pressure limit slows down and loses heat.
• Air: air ahead of the front must escape through vents; if it cannot, it compresses and stops the front.

Step 1: Enough Melt

• Cushion. If the screw bottoms out (zero cushion), the shot ran out of material. Increase shot size so a stable cushion remains.
• Check ring. A worn non-return valve lets melt leak back during injection; cushion varies or drifts to zero, and part weight wanders.
• Shot capacity. The shot (parts plus runner) has to fit the barrel sensibly, as covered in matching clamp force to part requirements.
• Feed problems. Bridging in the hopper or an empty loader gives intermittent short shots.

Step 2: Pressure-Limited Fill

Compare the peak injection pressure with the pressure limit. If the press reaches the limit before the end of fill, the screw slows down and the fill is pressure-limited. The causes are restriction (small gates, long or narrow runners, a partly blocked hot runner tip), cold melt, or a high-viscosity lot. Raising the limit can fill the part but adds pressure that may cause flash; reducing the restriction is the durable fix. The difference between fill and hold control is explained in injection pressure versus hold pressure.

Step 3: Venting

A short shot in the same place every time, especially at the end of flow, around a rib top, or in a blind pocket, points to trapped air. Signs:

• burn marks or a dark, glossy edge at the short area (air compressed and heated);
• the short spot does not move when speed or temperature changes;
• vents are plugged with residue or were damaged by over-clamping.

Clean or add vents at the last-to-fill location (cleaning at the press is done under the plant’s safety procedure for working in the mold area), which a short-shot sequence (parts molded at increasing shot sizes) identifies directly.

Step 4: Early Freezing

• Gate freeze. An undersized gate can freeze before fill completes on thicker parts or with slow fill.
• Hesitation. Melt can stall in a thin section while it keeps flowing through an adjacent thick one, and the stalled front freezes. The fix is in design (gating or wall changes) or a faster fill.
• Thin walls after transfer. On thin-walled parts, a fill-only setting that stops well short of full may never fill out under hold pressure because the front freezes when velocity control ends, as RJG notes in its guidance on establishing a fill-only part (RJG). These parts may need to transfer later.

Step 5: Temperatures and Material

• Melt temperature. Measure the actual melt temperature rather than trusting setpoints, using the press maker’s and the plant’s procedure for purging and air shots (guarding in place, face and hand protection against hot melt, trained personnel only); a failed heater band or thermocouple shows as a low reading.
• Mold temperature. A cold mold freezes the front sooner; measure the steel, not only the TCU setpoint, and state the unit (°C or °F) on every setup sheet.
• Material lot. A lower melt flow rate than the previous lot fills less far; compare certificates or test incoming melt flow.
• Moisture and regrind. Both can change viscosity and lot behavior.

Multi-Cavity Molds

If one cavity is short and the others are full, the cause is local: that cavity’s gate, a blocked hot runner tip, its vents, or runner imbalance. If all cavities are short, the cause is common: shot size, pressure limit, melt temperature, or material.

Quick Reference

Short shots and flash are two sides of the same balance between gaps and pressure, covered in flash causes and elimination. The durable fix is the one that addresses why the melt ran out, stalled, or was blocked, not the one that adds pressure until it gets through.