Last updated: 2026-09-05 · 5 min read
Short Shots in Injection Molding — Why Does the Melt Stop Before Filling the Cavity, What Are the Root Causes and How Do You Fix Each One?
A short shot is an incomplete part — the melt freezes before reaching the farthest cavity point. The cause: flow resistance exceeds available injection pressure. Root causes fall into four categories: low melt temperature (viscous melt), low injection speed or pressure, inadequate venting (trapped gas blocks flow), and geometry issues (thin walls or long flow paths). Fix by raising melt temperature 10–20°C, increasing speed, adding vents or enlarging the gate.
Key Facts
- Short shots appear at the flow end — the last area to fill. If the part is short at the same location every shot, the cause is geometry or gate (flow path too long, gate too small). If the short location shifts between shots, the cause is process variation (melt temperature drifting, inconsistent back pressure). Check the short area: is it a thin wall, a long flow path from the gate, or a region with no vent?
- Melt temperature is the most common fix — raising it by 10–20°C reduces viscosity by 20–40%, allowing the melt to flow further. For ABS: typical melt 220–250°C; if shorting at 220°C, raise to 240°C. For PA66 GF30: typical 280–310°C; if shorting at 280°C, raise to 300°C. Do not exceed the resin manufacturer’s maximum — above that, the resin degrades (ABS chars, nylon loses molecular weight).
- Inadequate venting is the second most common cause — trapped gas compresses to 100–300 bar and stops the melt. Vents: 0.02–0.05mm deep (depending on resin), 5–15mm wide, placed at the flow end and weld lines. If shorting in a rib or boss, add a vent at the rib tip (0.03mm for ABS, 0.02mm for glass-filled). If vents are present but the short persists, they are clogged with mold release or degraded resin — clean them.
How Do You Diagnose Which Cause Is Responsible for the Short Shot?
| Symptom | Likely Cause | First Check | Fix |
|---|---|---|---|
| Short at same location, every shot | Geometry or gate | Flow length-to-wall ratio | Enlarge gate, add runner, raise melt temp |
| Short location shifts between shots | Process drift | Melt temp stability, back pressure | Stabilize process, check heater bands |
| Short in ribs or thin features only | Venting | Vent depth at flow end | Add or clean vents at rib tips |
| Short after 10k+ shots (was OK before) | Vent clogging or wear | Vent condition, check for deposits | Clean vents, check for mold release buildup |
| Short with new resin lot | MFI variation | Resin MFI vs. spec | Adjust melt temp for lower-MFI lot |
The fastest diagnostic is a short shot study — gradually reduce the injection volume and observe where the melt stops. The fill pattern reveals the problem: if the melt flows evenly then suddenly stops, the issue is venting (gas lock). If the melt slows gradually and never reaches the end, the issue is flow resistance (melt temperature or gate size).
Short shot study — a diagnostic technique where injection volume is gradually reduced shot by shot to map the fill pattern. The melt front position at each shot reveals flow path issues: even flow then sudden stop = gas lock (venting problem); gradual slowdown = flow resistance (temperature or gate). The study takes 5–10 shots and identifies whether the fix is process (temperature, speed) or mold (vent, gate). The weld line strength guide shows how fill pattern affects weld line position.
What Three Fixes Address the Most Common Short Shot Causes?
- Raise the melt temperature — this is the fastest process fix. Increase the barrel temperature in 10°C increments, starting from the rear zone and moving forward to the nozzle. Each 10°C increase reduces viscosity by 10–20% — the melt flows further. For semi-crystalline resins (PA, POM, PBT), also raise mold temperature by 10–20°C — these resins freeze at a sharp melting point, and a hotter mold delays freeze-off. For amorphous resins (ABS, PC, PMMA), mold temperature has less effect — focus on melt temperature. The weld line guide explains how melt temperature also affects weld line strength.
- Increase injection speed and pressure — this pushes the melt further before it freezes. Switch from velocity control to pressure control at a later point — this extends the fill phase. Increase the injection pressure limit from 800 bar to 1,000 bar (if press and mold allow). Increase injection speed from 50mm/s to 80mm/s — faster flow generates shear heating, reducing viscosity at the flow front. Watch for flash — if the part flashes at the parting line when you increase speed, the clamp force is insufficient.
- Add or enlarge vents at the flow end — this releases trapped gas that blocks the melt. Place vents at the last point of fill (identified by the short shot study or by burn marks). Vent depth: 0.02mm for glass-filled, 0.03mm for unfilled, 0.05mm for PP and PE. Vent width: 5–15mm. If vents are clogged: clean with a brass wire brush or ultrasonic cleaner. For production molds, add vent inserts (sintered metal or vent pins) at hard-to-vent areas — these maintain function longer than machined vents.
Shear heating (in injection molding) — the temperature rise in the melt caused by viscous friction as it flows through thin sections or small gates. At high injection speed (80–120mm/s), shear heating raises the melt temperature at the flow front by 10–30°C above the barrel setting — reducing viscosity locally and helping the melt flow further. This is why increasing injection speed can fix a short shot even without changing barrel temperature. The gate vestige guide covers how gate size affects shear at the flow front.
Short shot: melt freezes before filling the cavity. Cause = flow resistance > injection pressure. Fix: raise melt temp +10–20°C, increase speed/pressure, add/clean vents. Short at same location = geometry. Shifting location = process drift. Short in ribs = venting. Short after 10k shots = vent clogging.
- Short location consistent or shifting — same location = geometry/gate; shifting = process drift.
- Melt temperature raised in 10°C increments — each 10°C reduces viscosity 10–20%; do not exceed max.
- Vents at flow end and rib tips — 0.02mm (GF), 0.03mm (unfilled), 0.05mm (PP/PE).
- Burn marks present or absent — burn marks = gas trap (add vents); smooth edge = flow resistance.
- MFI checked on new resin lot — ±20% lot variation can cause shorting; raise temp 10–15°C for low-MFI lot.
Frequently Asked Questions
Why does my part short only in the thin ribs while the main body fills completely?
The ribs have a smaller cross-section — melt freezes faster in thin sections because the surface-area-to-volume ratio is higher. A 1mm rib freezes in 0.5–1.0 seconds; a 3mm wall in 3–5 seconds. If the rib is at the flow end, the melt arriving is already cooler and more viscous — it may not fill before freezing. Fix: add a vent at the rib tip (0.03mm), raise melt temperature by 10°C, or widen the rib from 1mm to 1.2mm.
If the rib is deeper than 3× its thickness (e.g., a 1mm rib that is 4mm deep), the flow resistance is extreme — the melt cannot fill the rib regardless of process settings. Redesign the rib to a maximum depth of 2× its thickness, or add a secondary gate near the rib base.
Can short shots be caused by the resin itself, not the process or mold?
Yes — if the new resin lot has a lower MFI (melt flow index) than the previous lot, the melt is more viscous and flows less. A lot-to-lot MFI variation of ±20% is common — if your process was tuned to the high-MFI end, a low-MFI lot will short. Check the MFI certificate. If MFI is 20% lower, raise melt temperature by 10–15°C. For critical molds, specify a narrow MFI range in the purchase order (e.g., MFI 12–14 instead of 10–16).
How do I know if the short shot is from trapped gas or from flow resistance?
Look for burn marks (diesel effect). Black or brown burn marks at the short area mean trapped gas — the gas compresses, heats up and burns the melt. Fix: add vents. No burn marks and a smooth short edge mean flow resistance — the melt ran out of pressure. Fix: raise melt temperature, increase speed, or enlarge the gate. A short shot study confirms — gas lock shows as a sudden stop; flow resistance shows as gradual slowdown.
Key Takeaways
- Short shot = flow resistance > injection pressure. Always.
- Same location = geometry/gate. Shifting = process drift. Ribs = venting.
- Fix order: raise melt temp, increase speed, add/clean vents, enlarge gate.
- Burn marks = gas trap. Smooth edge = flow resistance.
Send the part drawing with the short shot location, the resin and the current process parameters — the troubleshooting review identifies the root cause and specifies the fix (vent, gate or process change) needed to fill the cavity completely.
Sources: Plastics Technology — Short Shot Diagnosis and Fix Methods · Plastics Technology Online — Venting Design for Injection Molds · Society of Plastics Engineers (SPE) — Short Shot and Flow Analysis Technical Papers · BASF — Processing Guidelines for Engineering Resins · DuPont — Troubleshooting Guide for Injection Molding Defects
— Arthur, Tooling Engineer at aCatMould