A short shot is the symptom of an incompletely filled cavity: melt flow has stopped before all intended part features are formed. It should not be treated as a diagnosis. The same missing end feature can result from a restricted flow path, trapped air, resin condition, insufficient delivered shot, a machine problem, or a part-specific process limit. For an overseas OEM, procurement, quality, or engineering team, the useful response is to separate what is seen from what is suspected, then request evidence that can distinguish the mechanism before approving a setting change or tool modification.
The right response depends on the approved resin grade and condition, part geometry, gate and runner layout, cavity count, mold temperature control, press capability, and the way fill and acceptance are measured. This guide does not prescribe universal temperatures, pressures, speeds, or pass/fail limits. It provides practical questions and an evidence sequence a buyer can use with a molding supplier to contain the risk, test likely causes, and verify that a correction meets the drawing and product requirements.
A fully filled trial part is not sufficient proof of correction. The supplier should show why the short shot occurred and how the revised condition meets the agreed visual, dimensional, and functional acceptance criteria.
01
1. Define the symptom and contain the affected population
Record the symptom before anyone changes the process. Identify the missing feature or end-of-fill region on the drawing or a marked photograph; note the part and tool revision, cavity identifier where available, press, resin grade and lot, color or additive condition, shift, time window, and last accepted sample. Retain both nonconforming and representative conforming parts. A short shot that repeats at one distal feature, one cavity, or one runner branch is different evidence from an intermittent loss of fill that moves between features.
Ask the supplier to establish a traceable hold boundary rather than sorting only visibly incomplete pieces. The boundary should reflect the available process, material, cavity, and inspection records. If the part has a sealing surface, a safety-related feature, a structural load path, an electrical function, or an assembly interface near the incomplete region, disposition should be decided against the controlled product requirements. A visually small loss of fill is not automatically cosmetic-only.
02
2. Separate likely causes into testable families
Flow-path and mold causes are plausible when the defect is consistently at a thin section, a long or complex flow path, a restrictive gate or runner, or an end-of-fill region with poor vent access. Hesitation can allow an earlier flow front to freeze while another route continues. A blocked, damaged, or imbalanced feed path can also make one cavity or feature behave differently. These are working hypotheses, not proof that the tool needs steel work.
Material and process causes should be considered when the event follows a resin-lot, colorant, drying, storage, recovery, or setup change. A grade may have a different flow behavior from another resin in the same family, and its processing guidance is not interchangeable. Low melt or mold temperature, an unsuitable transfer point, limited injection rate, or insufficient available shot can reduce fill, but an aggressive adjustment may introduce flash, burn marks, stress, cosmetic change, or other defects.
Machine-delivery causes are especially important when the pattern is intermittent or appears across multiple cavities. Confirm that the hopper and feed throat are supplying material, the nozzle and non-return valve are functioning as intended, the actual shot and cushion are consistent, and the selected press has adequate delivery capability for the mold and process. These checks do not assign fault; they keep a mechanical or setup issue from being misclassified as a design issue.
03
3. Request confirmation checks before authorising a remedy
A buyer can request a controlled baseline comparison against the approved setup. The record should identify the actual material and preparation condition, relevant melt and mold-temperature readings, injection and transfer information, cushion or shot indication, pressure limitation, cycle context, and cavity-coded sample results where the mold allows identification. Compare these with the last known acceptable condition instead of relying on a verbal statement that the press was set normally.
For a repeatable short shot, request a documented fill-progression study or other safe, controlled diagnostic method used by the supplier. It should show where flow stops, whether cavities fill differently, and whether an apparent end-of-fill region aligns with a restriction or venting concern. The sample is diagnostic evidence, not a final production acceptance sample. When geometry, gate placement, or runner balance is in question, ask what drawing revision, material data, and boundary conditions a simulation or tool review used.
Confirmation should be proportional to the decision. A mold inspection may need to address gate, runner, vent, shut-off, cooling, and cavity condition. A material investigation may need resin, lot, dryer, and approved regrind records. A machine check may need maintenance findings and delivery-repeatability evidence. Require the supplier to state which observation supports the proposed cause and which plausible alternatives were ruled out.
04
4. Match corrective actions to the confirmed mechanism
If evidence points to a temporary material, setup, or machine-delivery issue, the corrective action can be a controlled restoration of the approved material condition, process window, or equipment condition. Document the starting condition, the one variable or maintenance action changed, the result, and any side effects checked. Do not make a setting increase the permanent answer merely because it restores fill on a single sample.
If the investigation points to flow resistance, venting, or an unbalanced feed system, the supplier may propose cleaning, repair, vent maintenance, a gate or runner change, or a geometry adjustment. The proposal should identify the physical mechanism, affected cavities, tool-revision control, potential effects on weld lines, appearance, fiber orientation, warpage, ejection, maintenance, and cycle time, and the reason the change is feasible. Gate relocation or vent modification requires a design review; neither is a universal remedy.
If thin walls, abrupt transitions, or an excessive flow length are driving the risk, consider part and tool options together. A local wall adjustment, smoother transition, revised gate concept, or alternative approved resin grade may be evaluated, but each may affect fit, stiffness, cosmetic surfaces, shrinkage, and regulatory or customer requirements. The engineering owner should approve the relevant drawing, material, and tool changes through normal change control.
05
5. Release only with part-specific acceptance and supplier evidence
Agree the acceptance question before evaluating the corrective trial. For every affected region, identify the applicable requirement: complete formation of a feature, allowable visual condition under agreed viewing conditions, dimension from defined datums, assembly fit, leak performance, electrical continuity, load performance, or another product-specific test. Specify sample condition, inspection timing, cavity coverage, measurement method, and responsible approver. A part that looks full may still fail an assembly or functional requirement.
Request a concise corrective-action package that links the defect occurrence to its evidence. It should include containment scope and disposition, retained sample identification, the cause statement and confirmation checks, the approved corrective action, material and tool or process revision status, before-and-after results, and any ongoing control. For multi-cavity molds, pooled results can conceal a cavity-specific issue; ask for cavity-level identification when it is practical and relevant.
Close the event only when the supplier has demonstrated that the controlled condition produces parts meeting the agreed criteria and that the change has not created an unacceptable trade-off elsewhere. The amount of evidence should suit the part’s risk and production situation. For a critical component, the buyer may need more sampling, functional evidence, or formal revalidation than for a noncritical cosmetic component.
Practical takeaway
Before you move
to the next review.
- Record the exact incomplete region, part and tool revision, cavity where available, resin lot, press, shift, time window, and last accepted sample before changing settings.
- Place a traceable hold on the suspected population and preserve clearly identified good and short-shot samples for comparison.
- Ask the supplier to distinguish a flow-path or venting hypothesis from material-condition, process, and machine-delivery hypotheses with stated confirmation checks.
- Require each proposed adjustment, repair, or tool change to state the mechanism addressed, the controlled revision, expected trade-offs, and verification plan.
- Release output against agreed visual, dimensional, assembly, and functional acceptance criteria using evidence appropriate to the part and cavity risk.
FAQ
Common questions
Should we tell the supplier to increase injection pressure when a short shot appears?
Not as a default instruction. Pressure can be relevant to a controlled study, but it may not correct a restricted gate, poor venting, unstable material condition, inadequate shot delivery, or a mechanical fault. Ask what mechanism the change is intended to test, what limits apply to the actual resin, mold, and press, and what checks will confirm that other part requirements remain acceptable.
What evidence should a supplier provide for a one-cavity short shot?
Request cavity-coded samples, the tool and runner/gate inspection finding, vent observations, cavity-specific process or fill evidence where available, relevant maintenance history, and the proposed action. A common part-design change should not be approved until the supplier has considered whether the issue is local to that cavity, its feed path, or its thermal and mechanical condition.
Can part weight be used as the acceptance criterion for incomplete fill?
Part weight can be useful supporting evidence when the scale, conditioning, sampling method, and part-specific relationship to fill are defined. It is not a stand-alone universal acceptance criterion. Parts of similar weight can differ in local fill, and a completed fill may still fail dimensional, cosmetic, assembly, or functional requirements.
When is a mold-flow study worth requesting?
It is useful when a decision concerns gate location, runner balance, flow length, local wall changes, or end-of-fill vent access and the model can use the proposed CAD, resin data, and tool assumptions. Treat it as design evidence, not a replacement for tool inspection and representative molded-part verification. The supplier should explain the decision the study informs and its limitations.
References
Source material
- [1] Autodesk Moldflow, Troubleshooting Short Shot Problems
- [2] BASF, Injection Molding Problems in Engineering Thermoplastics: Causes and Solutions
- [3] Protolabs, Injection Molding Guide: Process, Design Tips and Materials
- [4] RJG, How to Prevent and Troubleshoot Short Shots
This guide is general information for early procurement and engineering discussion. It does not replace product-specific DFM, qualification, validation, regulatory assessment or legal advice.