Short shot: causes and the fastest solution order

A short shot means the melt doesn't completely fill the cavity. The part's farthest point is left incomplete. Since there are many possible causes, going in order rather than changing settings at random saves both time and scrap.

1. First, "is there actually enough material?"

• Dosage and cushion: Is the cushion the same every cycle? If the cushion drops to zero, dosage is insufficient. If the cushion varies cycle to cycle, the check ring (non-return valve) may be leaking.
• If the shot volume is close to the machine's capacity (e.g. above 80%), dosage precision drops; a machine with a smaller screw, or higher capacity, may be needed.

2. Is pressure limiting it, or speed?

In speed-controlled filling, the machine produces whatever pressure is needed to maintain your set speed. If it hits the pressure limit, speed quietly drops and the cavity doesn't fill.

• If the hydraulic pressure gauge is pinned at the limit during filling, raise the limit (up to what the machine allows).
• If the limit is already at maximum, the problem isn't machine power, it's flow resistance: thin walls, a small gate, a long flow path.

3. Temperatures

As melt and mold temperature drop, viscosity rises, the frozen layer thickens, and flow stops. In order:

1. Raise the melt temperature within the range on the material's data sheet (don't approach the upper limit, due to degradation risk).
2. Raise the mold temperature; its effect is large in thin-walled parts.
3. Check hot runner zone temperatures; a single faulty heater alone can cause the problem.

4. Geometry: flow length / wall thickness ratio

As the ratio of flow length to wall thickness (L/T) increases, filling becomes harder. The limit varies with material: high for free-flowing materials, low for viscous and filled materials. If the ratio is at the limit, the solution isn't in the process but in the design: increasing wall thickness, enlarging the gate, or adding a second gate.

5. Venting

If air can't escape, it gets trapped and stops the melt. Short shots and burn marks are usually seen together at the part's last-filling point. In this case, paradoxically, lowering speed can reduce the problem. Opening a vent channel in the mold is the permanent fix.

Short-shot study

Don't try to solve the part in one shot: reduce the dosage and fill the part to 50%, 75%, 90%, photographing each one. This shows how the melt front advances, where it slows down, and which corner fills last. This way you find whether the problem comes from the gate, flow balance or venting by observation rather than guessing.

Note: This article is a general field-tested check order. Your material's and mold's data sheets and the mold maker's recommendations always take priority.

Our calculation tool checks the L/T ratio, required pressure and machine limit for the part and machine you enter. If filling looks insufficient, it gives a warning on the results page: open the calculation tool.