Injection Molding Underfill (Short Shot): Causes, Troubleshooting and Fixes

What is an Underfill/Short Shot?

An underfill, also called a short shot, occurs when the molten plastic does not completely fill the mold cavity, resulting in an incomplete part. This is one of the most common injection molding defects, accounting for approximately 10-15% of all molding rejects.

Root Causes

1. Insufficient Material Injection

• Inadequate shot size (too small charge volume)
• Low injection pressure or speed
• Nozzle too small or partially blocked
• Screw back pressure too high, reducing effective shot

2. Material Flow Issues

• Low melt temperature (material too viscous)
• Low mold temperature (premature freezing)
• Moisture in material (reduces flow)
• Poor flow characteristics of material grade

3. Mold Design Problems

• Gates too small or incorrectly positioned
• Runner system too restrictive
• Inadequate venting (trapped air creates back-pressure)
• Wall thickness too thin for material flow length

Troubleshooting Sequence

1. Check material availability: Is the hopper full? Is the material flowing?
2. Review shot size: Increase by 2-5% and observe fill pattern
3. Increase injection speed by 10-15%
4. Increase injection pressure by 10-20%
5. Verify melt temperature is at mid-to-high spec range
6. Check mold temperature at the short shot location (5-10°C increase)

Mold Modifications for Persistent Underfill

Increase Gate Size

For edge gates: increase depth by 0.1-0.2mm. For pin gates: increase diameter by 0.1-0.2mm. Larger gates reduce flow restriction and delay freeze-off.

Runner Optimization

Full-round runners flow 25% better than trapezoidal. Increasing runner diameter by 1mm reduces pressure drop by 20-30%.

Add Venting

At the short shot location, trapped air may be creating back-pressure. Add 0.02-0.04mm deep vents (depending on material).

Material-Specific Solutions