Short shot (incomplete fill)
The cavity does not fill completely; the farthest or thinnest area of the part stays incomplete.
Symptoms
- Tip, rib or thin section of the part is missing
- Edges are rounded, irregular and dull
- Cushion is close to zero or fluctuates
Tip: Start with the no-cost settings (temperature, speed, metering), then intervene on the mold.
Order of solutions
- Check metering and cushion; the cushion must be the same every cycle
- Check whether the pressure hits the pressure limit during filling and raise the limit
- Raise the melt temperature, then the mold temperature, within the data-sheet limits
- Increase the speed; if the last-filled area has a venting problem, reduce the speed and open up the venting
- Run a short-shot study (parts filled 50/75/90 %) to see the flow balance
- Then move on to correcting the gate, runner or venting in the mold
Likely causes
Process setting
- Metering (shot volume) too small
- Injection speed too low, or the pressure limit restricts the speed
- Melt or mold temperature too low
- V/P changeover too early
- Holding pressure/time insufficient
Machine
- Check ring (non-return valve) leaking
- Nozzle or hot runner zone heater faulty
- Machine capacity (shot volume/pressure) at its limit
Mold
- Venting insufficient or blocked
- Gate or runner too small
- Hot runner nozzle partially clogged
Part design
- Flow length to wall thickness ratio too high
- Abrupt wall thinning
- Gate location far away or unbalanced
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Related articles
Other defects
Flash
Plastic escapes through the parting line or around the ejector pins.
Sink mark
A dent on the surface opposite a thick section; the surface is drawn inwards.
Void (internal vacuum void)
Internal cavity in a thick section of the part; the outer surface may look perfect.
Weld line (knit line)
A line and a weak zone where two melt fronts meet.