Troubleshooting · Defect guide
Short shots
Incomplete injection-molded parts, and what to fix first
A short shot happens when the melt stops before the cavity is full: not enough material, not enough pressure, a flow front that freezes too soon, or air with nowhere to go. This guide covers how to recognise it, the mechanism, four groups of causes, checks ordered from cheapest to most expensive, and an honest answer that additives only play an indirect role.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Confirm it is a short shot
A short shot is a part with material missing, usually at the point farthest from the gate or at the end of a thin feature: rib tips, thin edges and corners the melt reaches last. Before fixing anything, note two things: is it short in the same place on every shot or only on some shots, and what does the missing edge look like?
- Short shot: material is really missing. The edge where it stops is usually rounded, because the flow front halted before reaching the mould wall.
- Burn mark at the end of fill: a black or brown mark at the end of the part, caused by trapped air compressed until it gets hot. It often comes with a slight short shot and points to the vents.
- Sink mark: the part is complete but the surface dips opposite a thick section. That is a shrinkage problem, not a filling one; see Sink marks and voids.
- Soft edges (nearly full): the shape is complete but corners or raised lettering are not sharp. It is the same defect in a mild form, often fixed with holding pressure or venting.
Mechanism: why the melt stops early
As melt flows into the mould, the layer touching the steel freezes almost immediately. The farther it flows, the thicker that frozen layer becomes and the narrower the channel left for flow, so the pressure needed to push the front forward rises with flow length. A short shot happens when one of four things runs out first: material in the shot, the machine's available pressure, heat in the flow front, or an escape route for the air at the end of the cavity, which otherwise becomes a cushion that holds the melt back.
The practical takeaway: find out what runs out first, material, pressure, heat or venting, because each is fixed in a different place. Raising every setting at once usually trades the short shot for flash.
Four groups of causes
1. Part design
Walls that are thin relative to flow length, thin sections at the end of the flow path, and thin ribs branching off the main flow. Melt takes the thicker path first, so thin areas fill last and cool while they wait. Resin suppliers often publish flow data such as spiral flow length in the TDS, which is useful for comparing grades.
2. Mould
A gate or runner that is too small, long cold runners, vents that are blocked, too shallow or missing where the cavity fills last, a mould that is too cold, and hot-runner zones that run cold or are blocked by contamination.
3. Process
A shot size so small that the cushion reaches zero, low injection speed, injection pressure hitting its set limit, switching from velocity to pressure (V/P) too early, low melt temperature, and a worn check ring at the screw tip that lets melt slip back.
4. Material
Low-MFI grades flow less easily. MFI that shifts between lots, regrind or recycled-pellet ratio, and uneven feeding from the hopper all change how far the melt gets. See Polymer basics for MFI background.
Step-by-step checks, cheapest first
- Look at the cushion. If it reaches zero, the shot is too small; increase the dosing stroke before anything else. If the cushion drifts while settings stay the same, suspect a worn check ring.
- Compare peak injection pressure with the set limit. If it hits the limit every shot, the machine is pressure-limited. If it does not, more pressure will not help; the problem is elsewhere.
- Run a short-shot series. Increase shot size step by step and line the parts up. You will see the filling order; the last place to fill is where you need a vent.
- Step up injection speed. If thin areas fill better, the flow front was freezing early. Watch for flash at the same time.
- Measure real temperatures. Measure purged melt, the mould surface, and every hot-runner zone against its set point.
- Check the vents. If there is a burn mark at the end of the part, clean the vents first; additive or wax deposits can build up until they block them.
- Mould the base resin alone and check the MFI of the resin lot and the blend. If base resin fills but the blend does not, the cause is the material or the ratio.
- Review mould and design. Enlarge the gate or runner, add vents, move the gate, or change wall thickness. That is mould work and the most expensive step.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Shot too small, zero cushion | Read the cushion on the machine display | Increase dosing stroke to leave a stable cushion |
| Worn check ring | Cushion drifts with unchanged settings | Inspect and replace the check ring |
| Pressure-limited | Peak pressure equals the set limit every shot | Raise the limit within what machine and mould allow, or reduce flow resistance |
| Injection speed too low | Step speed up, watch thin areas | Use a speed that fills without flash or burn marks |
| V/P switchover too early | Mould with velocity phase only (no hold); the part should be about 95–98% full | Adjust the V/P switchover point |
| Melt or mould too cold | Measure purged melt and mould surface temperature | Bring them into the resin supplier's recommended range |
| Vents blocked or missing | Burn mark at end of fill · short-shot series | Clean vents; add vents where the cavity fills last |
| Gate, runner or hot runner blocked | Check each hot-runner tip; look for contamination in the gate | Clean; enlarge gate or runner |
| Resin MFI low or variable | Mould base resin alone; log lot and regrind ratio | Hold the blend ratio constant, check MFI when the lot changes, or choose an easier-flowing grade |
| Wall too thin for the flow length | Compare thickness and flow length with the grade's flow data in the TDS | Thicken the wall, add a gate, or change grade |
The 95–98% fill figure is a general V/P setting guideline; adjust it to the actual part.
How much can additives help?
The honest answer is only indirectly. Most short shots are fixed in the process, the mould or the choice of grade. These are the material and formulation factors that matter, including ones that have nothing to do with ACS:
- A higher-MFI grade is the most effective material change, but it is not an additive and usually trades off toughness or other mechanical properties. Confirm the part still meets its specification.
- Check the masterbatch you use. A carrier that flows less easily than the base resin, or a highly filled masterbatch dosed at a high ratio, can make the blend harder to fill. Choosing a carrier matched to the base resin genuinely helps here.
- Keep regrind and recycled pellets consistent. Drifting MFI causes intermittent short shots; see Recycled resin with drifting MFI.
- Processing aids and lubricants exist and can make flow slightly easier, but the effect is small compared with process fixes, and overdosing can build deposits in the vents and make things worse.
What additives cannot fix: a shot that is too small, a worn check ring, blocked vents, a machine without enough pressure, a cold mould, and walls too thin for their flow length.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Short shot questions we hear often
Why do we get short shots on some shots but not every shot?
Intermittent short shots usually come from something that varies, not from a wrong setting. Common causes are a worn check ring at the screw tip that lets melt slip back so the cushion drifts, uneven feeding from the hopper, especially with regrind of mixed particle size, a drifting blend ratio or resin MFI, and hot-runner or mould temperatures that have not stabilised after start-up. Log cushion and peak injection pressure on every shot and compare them with the short ones.
We raised injection pressure and the part is still short. Why?
First check whether the actual peak injection pressure reaches the set limit. If it does not, pressure is not the constraint and raising the limit changes nothing. The usual causes are then air trapped at the end of fill because vents are blocked or missing, often with a dark burn mark at the end of the part, a flow front that freezes early because injection speed is low or the melt or mould is too cold, or a shot size so small that the cushion reaches zero.
Short shots started after we added masterbatch or regrind. Is that related?
It can be. If the masterbatch carrier flows less easily than the base resin, or the masterbatch is highly filled and dosed at a high ratio, the MFI of the blend drops. Regrind and recycled pellets often have a different MFI from virgin resin and vary from lot to lot. The simplest test is to mould the base resin alone. If it fills without the additions, check the MFI of the blend and the ratio actually being dosed before adjusting the machine.
Only the tips of thin ribs are short. How do we fix that?
Melt takes the thicker path first because it offers less resistance, so the flow front entering a thin rib stalls and cools while it waits. This is called hesitation. On the machine, try higher injection speed and mould temperature, and check that the rib tip is vented so air can escape. If that is not enough, the fix is in the mould: move the gate so the rib sits closer to the main flow path, or change the rib thickness or length.
Still short after tuning the machine?
Send photos of a short-shot series, your cushion and peak pressure, and your machine settings on LINE. Our technical team will help you separate mould, process and material causes.