Last updated: 2026-09-04 · 5 min read
Short Shot Diagnosis — How Do You Identify the Root Cause of Incomplete Mold Filling, and What Are the Corrective Actions for Each Cause?
A short shot is an incomplete fill — the melt front freezes before reaching the cavity extremity. The root cause is always one of 6 categories: material (low MFI, moisture), machine (pressure limit), mold (thin section, venting), process (temperature too low), part design or gate/runner design. Applying the wrong fix wastes time and may introduce other defects. The diagnostic sequence: check material → check process sheet → check mold temperature → check venting.
Key Facts
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Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.
What Are the Six Root Causes of Short Shots — and How Do You Identify Each One?
Six root causes, each with a distinct diagnostic sign and corrective action:
| Root Cause | Diagnostic Sign | Corrective Action |
|---|---|---|
| Material: low MFI | Short shot at farthest point from gate; consistent across shots | Switch to higher MFI grade or increase melt temperature 10–20°C |
| Material: moisture | Short shot with splay marks (silver streaks) near gate | Dry material below 0.02% moisture; check dryer temp and dwell time |
| Machine: pressure limit | Short shot with injection pressure at machine limit (>90% max) | Increase pressure (if below limit) or move to larger machine |
| Mold: thin section | Short shot at or after thin section (below 0.8mm) | Increase wall thickness or add flow leader |
| Mold: inadequate venting | Short shot at last fill area; burn marks nearby | Add vent channels (0.02–0.05mm deep) at last fill area |
| Process: temperature too low | Short shot improves when melt or mold temperature increases | Increase melt temp 10–20°C or mold temp 10–20°C |
The diagnostic approach: first, check the process sheet — is the injection pressure at the machine limit? If yes, the cause is machine capacity or flow restriction. If no, the cause is material, temperature or venting. Second, check the short shot location — farthest from gate (flow length) or at thin section (restriction). Third, check for associated defects — splay marks indicate moisture; burn marks indicate trapped gas.
What Is the Step-by-Step Diagnostic Sequence?
Four steps in order:
- Check the material. Verify resin grade (MFI), drying condition (moisture below 0.02%), and material lot. If MFI is below the supplier’s recommended range for the part weight and wall thickness, switch to a higher MFI grade. If moisture exceeds 0.02%, extend drying time or increase dryer temperature.
- Check the process parameters. Review the process sheet — is injection pressure above 90% of machine maximum? If yes, reduce injection speed (to reduce pressure drop) or increase melt temperature (to reduce viscosity). If below the limit, increase pressure by 100–200 bar and observe whether the short shot fills.
- Check the mold temperature. Measure cavity surface temperature near the short shot area. If below the resin supplier’s recommended range, increase by 10–20°C. A cold mold freezes the melt front before it reaches the flow path end — most common cause in semi-crystalline resins (PP, PA, PBT, POM).
- Check the venting. Inspect parting line vents, ejector pin vents and core vent channels near the short shot area. If vent depth is below 0.02mm or vents are blocked by debris, clean or deepen them. Trapped gas compresses and blocks the melt front — diagnostic sign: burn marks (diesel effect) at the short shot location.
After these four steps, the root cause is identified in most cases. If the short shot persists, check the check ring (non-return valve) — a worn check ring allows melt to flow back past the screw, reducing effective injection pressure. The sign is inconsistent shot size with no pattern. Replace the check ring ($200–800, 2–4 hours downtime).
What Are the Long-Term Fixes — and When Do They Require Mold Modification?
Five fix categories, from least to most invasive:
| Fix | Mold Change? | Cost | When to Use |
|---|---|---|---|
| Process adjustment (increase melt/mold temp, pressure, speed) | No | $0 | Marginal processing conditions; may introduce flash, warpage |
| Material change (higher MFI grade) | No | $0 | Insufficient flow; may reduce impact strength |
| Gate/runner enlargement | Yes | $500–2,000 | Flow restriction at gate; permanent fix |
| Wall thickness increase | Yes (part design change) | $1,000–5,000 | Thin section restriction; requires customer approval |
| Additional venting | Yes | $200–800 | Trapped gas; only fix for venting-caused short shots |
Process adjustments are the fastest fix but may introduce other defects if pushed too far. Material change (higher MFI) reduces mechanical properties — higher MFI means shorter polymer chains and lower impact strength. Mold modifications (gate enlargement, venting, wall increase) are permanent fixes but require downtime and cost.
Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.
The diagnostic sequence matters more than the fix. Applying a short shot fix (higher pressure, larger gate) to a sink mark problem makes the sink mark worse. Identify the root cause first — then apply the corrective action.
Frequently Asked Questions
How do you distinguish a short shot from a sink mark?
A short shot is a missing section — the plastic did not that area. A sink mark is a depression on the surface — the plastic reached the area but shrunk inward because the local wall was too thick or packing pressure was insufficient.
The corrective actions are opposite: short shot requires more flow (higher pressure, higher temperature, larger gate); sink mark requires less flow in the thick section (reduced wall, lower melt temperature, longer packing time). Applying a short shot fix to a sink mark makes it worse.
Can short shots be caused by a worn check ring?
Yes — a worn check ring (non-return valve) allows melt to flow back past the screw during injection, reducing effective injection pressure. The diagnostic sign is inconsistent shot size — some shots fill completely, others are short, with no pattern.
The fix is to replace the check ring ($200–800, 2–4 hours downtime). Check ring wear accelerates with filled resins (GF, mineral) — replace every 500,000 shots for unfilled resins and every 200,000 shots for GF30+ resins.
How do you calculate the maximum flow length before a short shot occurs?
The maximum flow length is proportional to wall thickness. For unfilled resins: 150–250× the wall thickness. For GF30 resins: 100–150× (glass fibers increase viscosity). For a 2mm wall in unfilled PP: max flow length = 2mm × 200 = 400mm.
If the actual flow length exceeds this value, the melt front freezes before reaching the cavity end. The fix: increase wall thickness, add a second gate, or switch to a higher MFI grade.
Key Takeaways
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Permanent fixes: enlarge gate, add venting — require mold modification.
If your mold is producing short shots, send the process sheet, part drawing and photos of the short shot samples. The review identifies the root cause, recommends the corrective action and estimates the cost and timeline for the fix.
Sources: Kazmer — Injection Molding Process Troubleshooting · Omnexus Selection Guide — MFI and Flow Length · Colex — Injection Molding Defects Reference · Thermo Fisher — Injection Molding Troubleshooting · Plastics Technology — Troubleshooting Short Shots