Short shots (also known as underfills) occur when the molten plastic fails to completely fill the mold cavity, resulting in an incomplete part. This defect typically appears in thin-walled sections or areas farthest from the gate.

Pro Tip: For internal features that do not affect aesthetics or functionality, minor short shots may be acceptable. However, attempting to fix these by over-packing can lead to flash (excess material leaking out), so always evaluate the necessity of the adjustment first.

Short shots are caused by a variety of factors ranging from machine settings to mold design. Below are the 12 most effective strategies to diagnose and resolve this common defect.

1. Insufficient Shot Size or Cushion

If the amount of material injected is less than what the cavity requires, the part will not fill.

• Solution: Gradually increase the shot size or cushion value until the part fills completely without causing flash.

2. Barrel Temperature Too Low

Low melt temperatures increase the viscosity of the plastic, creating high resistance during flow.

• Solution: Appropriately raise the barrel temperature zones. This reduces viscosity and enhances the flowability of the melt, allowing it to reach remote areas of the mold.

3. Injection Pressure or Speed Too Low

The melt needs sufficient driving force to flow through the entire cavity before it cools.

• Solution: Increase the injection pressure and speed. This ensures the melt receives adequate pressure and material (supplement) before it solidifies.

4. Insufficient Injection Time

Filling a specific weight of material requires a specific duration. If the timer cuts off too early, the shot is incomplete.

• Solution: Extend the injection time until the mold cavity is fully packed. Monitor the transfer position to ensure consistency.

5. Improper Holding Pressure¹ (Switch-over Point)

A common cause is switching from injection to holding pressure too early. If the switch-over point is set too far forward, too much material remains to be packed by holding pressure, which often isn't enough to finish filling.

• Solution: Re-adjust the V/P (Velocity to Pressure) switch-over point to the optimal position. The goal is to fill 95-98% of the cavity by speed, then pack the rest by pressure.

6. Mold Temperature Too Low²

When part geometry varies significantly in thickness, a cold mold can cause the melt to freeze prematurely, consuming excessive injection pressure.

• Solution: Increase the mold temperature or reconfigure the cooling water channels to ensure uniform heating, especially in thin sections.

7. Poor Nozzle-to-Sprue Bushing Alignment³

If the machine nozzle does not align perfectly with the mold sprue bushing, material leaks (drools) during injection, reducing the actual shot volume.

• Solution: Re-align the mold or adjust the nozzle position to ensure a tight seal between the nozzle and the sprue bushing.

8. Damaged or Partially Blocked Nozzle⁴

In operations where the carriage retracts (suck-back), repeated impact between the nozzle and the mold can deform the nozzle tip, narrowing the flow channel. This increases shear heat and can cause cold slugs to block the orifice.

• Solution:
  • Remove and repair or clean the nozzle.
  • Reset the nozzle forward limit position to reduce impact force to a reasonable level.

9. Worn Check Ring⁵ (Non-Return Valve)

Wear on the check ring (stop ring) and thrust ring on the screw tip creates a gap. During injection, molten plastic flows backward (leakage) instead of moving forward, resulting in a loss of shot volume.

How to Verify Check Ring Wear:

1. Complete an injection cycle and switch to Manual Mode.
2. Set injection pressure and speed to low values.
3. Perform a dosing (recovery) stroke.
4. Manually execute injection while observing the screw position indicator.
   • Observation: If the screw moves forward easily with little resistance, the leakage is severe.
   • Solution: If wear is significant, replace the check ring immediately. Continuing production with a worn ring guarantees inconsistent part quality.

10. Poor Mold Venting

Trapped air in the cavity acts like a compressed gas spring, blocking the melt from reaching the end of the flow path. This often happens if air cannot escape through parting lines or ejector pins quickly enough.

• Solution:
  • Machine appropriate venting channels at the air-trap locations on the parting line.
  • If traps are not on the parting line, modify existing sleeves or ejector pins to act as internal vents.
  • Consider relocating the gate to change the flow front and push air toward existing vents.

11. Ribs Too Thin or Too Deep

Deep, thin ribs are classic air traps and flow restrictions. They are difficult to fill because the material cools rapidly in the narrow channel.

• Solution:
  • Increase the thickness of the rib (typically 50-60% of the main wall thickness).
  • Add generous radii at the base of the rib to improve flow.
  • Install dedicated venting measures at the end of the rib.

12. Unbalanced Runner or Gate System

While single-cavity molds are usually well-designed, multi-cavity molds often suffer from flow imbalance. Tiny differences in runner dimensions or gate sizes can cause some cavities to fill while others short shot.

• Solution: Analyze the flow balance. You may need to manually restrict gates on the "easy-to-fill" cavities (using gate land adjustments) or machine the runners to ensure equal flow distribution to all cavities.

Fazit

Solving short shots requires a systematic approach. Start by checking the simplest machine parameters (shot size, temperature, pressure) before moving to mechanical inspections (check rings, nozzle) and finally mold modifications (venting, runner balance). By applying these 12 proven techniques, you can significantly reduce scrap rates and improve your injection molding efficiency.