Troubleshooting · Defect guide
Flash at the parting line
Why parts flash, and what to fix first
Flash is plastic that escapes into a gap between mould components that should not be there. This guide covers how to confirm it is flash and what its location tells you, the mechanism, four groups of causes, checks ordered from cheapest to most expensive, and an honest answer: additives have almost nothing to do with it.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
Confirm it is flash
Flash is a thin fin of plastic sticking out past the edge of the part where mould components meet. The usual places are the parting line, around ejector pins, around inserts or slides, and at the vents. Before fixing it, rule out defects that look similar.
- Flash: real excess material that you can pick off with a fingernail or trim with a knife.
- Parting line mismatch: a small step along the parting line with no extra material, caused by the two mould halves being out of alignment.
- Excess from a damaged mould: the same shape on every shot even after pressure is cut back hard, usually from a chip or worn spot on the mould face.
- Stringing at the sprue: a stretched thread of plastic at the injection point, not along the parting line.
Where the flash appears points you in a direction before you touch the machine. Flash all around the parting line usually means cavity pressure beats clamp force. Flash on one side only usually means the platens are not parallel. Flash in the middle of a plate usually means the plate is deflecting. Flash at a single spot or around a pin usually means wear or debris.
Mechanism: why plastic escapes the cavity
During injection, the melt in the cavity is under high pressure. That pressure multiplied by the projected area of the parts and runner becomes a force pushing the two mould halves apart. If that force exceeds the clamp force, the mould opens slightly. If a gap already exists from wear or debris on the parting line, molten plastic flows straight into it. Viscosity decides how narrow a gap the melt can still enter before it freezes, so easy-flowing melt flashes more readily.
The practical takeaway: flash is a gap-and-pressure problem. There are three ways to fix it: close the gap (clean, repair the mould, make sure clamp force is really there), remove pressure you do not need (speed, V/P switchover, holding pressure), or stop the melt from being too fluid (temperature, resin grade). Peak cavity pressure usually occurs just as the cavity fills and holding begins, so the V/P switchover point matters a great deal.
Four groups of causes
1. Part design
Thin walls and long flow paths need higher injection pressure to fill, so the force pushing the mould apart rises with it. A large projected area relative to machine size can leave clamp force short. A parting line placed on a curved or stepped surface is harder to seal.
2. Mould
The leading cause in practice: a worn or chipped parting line, plastic debris or residue on the parting surface, mould plates too thin that deflect in the middle under pressure, worn clearances around ejector pins or slides, and vents too deep for the viscosity of the resin being moulded.
3. Process
Switching from velocity to pressure (V/P) too late so pressure spikes as the cavity fills, holding pressure too high, injection speed too high, melt or mould temperature so high that the melt flows too easily, clamp force set below what the part needs, and on toggle machines a wrongly set mould height so the real clamp force never reaches the set value.
4. Material
A higher MFI from a new grade or lot, and an inconsistent share of regrind or recycled pellets, which often flow more easily than virgin resin. See Polymer basics for what MFI means.
Step-by-step checks, cheapest first
- Note where the flash is: all around, one side, mid-plate or a single spot, as described above. It costs nothing and rules out half the causes.
- Clean the parting line. Wipe off plastic debris, residue and dirt, then mould again. If the flash goes away, make cleaning a scheduled routine.
- Check the real clamp force on the machine, and on toggle machines the mould height setting. Compare clamp force with the projected area of parts and runner; the basic idea is in Injection molding.
- Mould with the velocity phase only, no hold. If flash appears already, the cause is speed, temperature or the mould. If it only appears once holding pressure is added, the cause is the V/P switchover or holding pressure.
- Step down holding pressure and move the V/P point earlier while watching for sink marks. If sinks appear before the flash disappears, you are beyond what the process can fix. More in Sink marks.
- Check the material. Compare the MFI on the certificate of the problem lot with a good lot, and log the regrind ratio every shift.
- Send the mould to the toolmaker. Spot the parting line with blue (bluing) to see whether it seats evenly, and check wear, pin clearances, vent depth and platen parallelism. This is the most expensive step and needs the machine down.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Debris or residue on the parting line | Flash at one spot that disappears after cleaning | Clean on a schedule; make sure ejected parts leave no fragments behind |
| Clamp force too low | Flash all around; compare real clamp force with projected area | Set enough clamp force; move to a larger machine if needed |
| Wrong mould height (toggle) | Real clamp force lower than the value on screen | Reset mould height following the machine manual |
| V/P switchover too late | Flash forms as the cavity fills; pressure spikes at switchover | Move the V/P point earlier |
| Holding pressure too high | Flash disappears when holding pressure is reduced | Lower holding pressure within the sink-free window |
| Injection speed or temperature too high | Flash already present in a velocity-only shot | Slow the end of fill; bring temperatures into the resin supplier's range |
| Worn parting line or deflecting plate | Spot with blue; flash mid-plate despite enough clamp force | Repair the parting line; add support plates or pillars |
| Worn ejector pin or slide clearance | Flash only around pins or along a slide | Repair or replace parts to restore design clearance |
| Material flowing more easily | Compare certificate MFI and regrind ratio with flashed shots | Hold the regrind ratio constant; tell the supplier when lot MFI shifts |
How much can additives help?
The honest answer is hardly at all. Flash is about the mould and pressure, and no additive can close a gap in the parting line. The only material lever is viscosity:
- Resin grade: if the part still fills, a lower-MFI grade is harder to squeeze into narrow gaps, but it needs higher injection pressure, so test both sides of that trade-off.
- Regrind and recycled pellets often flow more easily than virgin resin, and a fluctuating ratio produces flash on some shots only. Holding the ratio constant helps more than changing the formulation. If you run recycled resin routinely, see Recycled PCR / rPP compounds.
- Masterbatch with a very easy-flowing carrier at a high let-down may make the blend flow slightly more easily. If flash started after a masterbatch change, mould natural resin alone for comparison before drawing conclusions.
What additives cannot fix: a worn parting line, debris on the mould, too little clamp force, deflecting plates, and V/P or holding pressure set too high.
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
Flash questions we hear often
We increased clamp force and the flash is still there. Why?
If the mould cannot close fully because of plastic debris or residue on the parting line, a worn parting line, or a mould plate that deflects in the middle, more clamp force will not help. On a toggle machine, a wrongly set mould height means the real clamp force may never reach the value on the screen. Clamping far harder than needed also crushes the parting line and vents faster, so flash comes back worse over time. Clean and check mould contact before raising clamp force.
We raised holding pressure to fix sink marks and now get flash. What now?
Sink marks and flash sit at opposite ends of the holding-pressure range: too little gives sinks, too much gives flash. If the window with neither defect is very narrow or missing, machine settings can no longer fix it. Go to the root cause instead: thin the rib roots, enlarge or move the gate so thick sections pack at lower pressure, or repair the parting line so the mould takes more pressure without opening.
Flash started after a new resin lot or more regrind. Is that related?
It can be. A resin with a higher MFI flows more easily and squeezes into narrower gaps. PP that has been through several heat histories, such as regrind or recycled pellets, often has shorter chains and a higher MFI. If the blend ratio varies, flash appears on some shots only. Compare the MFI on each lot's certificate and hold the regrind ratio constant before adjusting the machine.
Flash only appears around ejector pins or at the vents. How do we fix it?
Flash in one fixed spot usually comes from the mould, not the process. Ejector pin or slide bores that have worn to a wider clearance need repair or new parts. Vents must be shallow enough to let air out but keep plastic in; the right depth depends on the resin's viscosity, and easy-flowing resins need shallower vents. Have the toolmaker check them against the resin supplier's guidance. Lowering pressure to hide local flash usually creates other defects such as short shots or sink marks.
Flash still there after tuning the machine?
Send photos that clearly show where the flash is, plus your machine settings, on LINE. Our technical team will help you separate mould, process and material causes.