Injection molding defects hub
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Injection molding defects
Short shot
When molten plastic stops before the last detail fills, the part is short. This guide explains the fill-versus-freeze race, how to troubleshoot without guessing, and when design or tooling must change.
Defect troubleshooting
What is a short shot in injection molding?
A short shot is an incomplete fill: ribs, edges, or whole regions never receive melt before the flow front stalls or the gate freezes.
The underlying mechanism is a race between plastic flow and solidification. Melt must reach the last point of the impression before viscosity rises, vents block, or the gate seals. When resistance wins, the shot stops short and the part may look thin, missing detail, or partially formed.
Short shots are often confused with voids, sink, or gas traps. Confirm the unfilled region is open to the flow path and not a collapsed thick section. Document cushion, fill-only behavior, and whether the defect moves when you change injection speed or melt temperature before assuming the mold is worn.
For new programs, catching short-fill risk early in CAD review reduces trial shots on the floor. NetProto returns DFM feedback with instant quotes so teams can adjust geometry or gate strategy before a hardened steel tool is released, depending on part size and complexity.
Root causes
Short shot causes mapped to floor symptoms
Use this table to narrow whether the next change belongs on the machine, in material prep, or in the mold.
| Likely driver | What you see | First lever to try |
|---|---|---|
| Insufficient shot or zero cushion | Same unfilled region every cycle; screw bottoms out | Increase shot size; verify non-return valve seals shot to shot |
| Low injection pressure or speed at fill | Short fill on thin or distant features only | Raise fill pressure or speed in small steps; watch for flash or burn marks |
| Cold melt or mold | Short fill worsens on fast cycles or thin areas | Increase melt and mold heat within material limits; confirm heater bands and coolant flow |
| Trapped air or blocked vents | Short fill at last-fill points; possible burn at same spot | Clean vents; add venting at end of fill; confirm parting line seal |
| Restricted flow path | Short fill tracks to thin ribs, long flow length, or sharp transitions | Relieve geometry in CAD; open runner or gate only after fill-only study confirms restriction |
| Material not ready | Intermittent shorts after material change or humid lot | Confirm drying and lot match setup; compare viscosity to known-good run |
Troubleshooting
Four-step short shot troubleshooting order
Industry practice starts with fill-only shots so pack and hold do not mask an incomplete first stage.
- 1
Secure a fill-only sample
Isolate first stage
Turn off pack and hold, then produce a fill-only part. The flow pattern shows whether melt reaches each region before freeze-off. Mark unfilled areas on the sample and compare to vent locations and gate direction.
- 2
Verify machine and material setup
Baseline the process
Match settings to the approved setup sheet. Confirm shot size, cushion, back pressure, and melt temperature are stable. Inspect the non-return valve and nozzle for drool or blockage that steals material from the cavity.
- 3
Adjust one fill variable at a time
Controlled experiments
If cushion reads low, increase shot size first. If cushion is healthy but corners stay short, address venting at last fill, then raise fill speed or pressure incrementally. Re-run a few cycles after each single change so you know which lever completed the fill.
- 4
Inspect steel and balance
When process hits a ceiling
If fill-only parts still stop at the same steel boundary, review gate location, runner balance, and cooling uniformity. Uneven cooling can make one region freeze early. Escalate to mold maintenance or CAD revision when the defect is locked to geometry.
Prevention
Design and process checklist to prevent short shots
Most recurring shorts trace back to geometry, venting, or setup drift rather than a single bad cycle.
Model fill length and thin features in CAD; avoid abrupt wall transitions that freeze before distant ribs fill.
Place gates so the longest flow path fills before the gate seals; review with mold-flow simulation when geometry is complex.
Specify vent paths at last-fill points and deep ribs so air escapes ahead of the melt front.
Dry hygroscopic resins per supplier guidance and keep lot records tied to each production run.
Maintain vents, runners, and shutoffs on a schedule so gradual blockage does not appear as mystery shorts.
Upload CAD early for DFM feedback so fill risk is flagged before tooling release on qualified programs.
FAQ
Short shot injection molding FAQ
Is a short shot the same as a void or sink mark?
No. Voids are internal pockets, often in thick sections. Sink shows as surface collapse after pack. A short shot is open unfilled volume where melt never arrived. Fill-only samples and part section views tell them apart quickly.
Can trapped air alone cause a short shot without low pressure?
Yes. If air cannot escape at last fill, back pressure from compressed gas can stall the melt front. Adding or cleaning vents at the stall point often completes the fill without raising peak pressure.
Should I increase pack and hold to fix a short shot?
Pack and hold cannot fill volume the first stage never reached. Fix first-stage fill, cushion, and venting first. Adding pack to an incomplete fill may hide sink elsewhere but leaves the part structurally incomplete.
When is a short shot a design problem versus a machine problem?
When fill-only parts consistently stop at the same thin rib or flow length on multiple machines and lots, geometry or gate placement is the limiter. When the same mold runs full on one press and short on another, focus on setup, valve wear, and material prep.
How does NetProto help before production starts?
Upload CAD for an instant quote and DFM feedback. Engineers can flag long flow paths, thin sections, and gate strategy issues while the hardened steel tool is still in planning, depending on part size and complexity. There is no minimum order quantity once the tool is running.
Ready to quote
Qualify fill before you scale production
Bring short-shot photos and CAD into an instant quote so DFM review and tooling plans align with your fill targets. The same hardened steel tool is built for the life of the part and can typically achieve 100,000+ parts over its service life when the process window is proven.