Flash is a thin fin of plastic that escapes the cavity at the parting line, a vent, an ejector pin or a slide. It forms when pressure in the cavity is more than the clamp can hold shut, or when the melt is thin enough to run into a gap in the tool. It is usually a press or tool problem. The resin matters when its viscosity has changed.
Also described as: flashing, excess plastic at the parting line, fins, burrs.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Flash all the way around the parting line, on every cavity | Clamp force is too low for the projected area of the shot, or injection and pack pressure are too high. | Clamp force setting; transfer position and pack pressure |
| Flash in the same spot on the same cavity, every shot | A damaged or worn shutoff, or something stuck to the mold face holding the tool open. | Clean and inspect both mold faces; check the shutoff at that spot |
| Flash along one side of the tool only | The mold is not closing evenly. The platens are out of parallel, the tie bars are loaded unevenly or the mold is mounted off center. | Platen parallelism and tie bar loading; mold position on the platen |
| Flash only at vents or around ejector pins | The vents are too deep for this resin, or the pin bores are worn. | Vent depth against the resin supplier’s recommendation; pin fit |
| Flash and short shots on the same shot | The fill is unbalanced or the switch to pack comes too late, so one area is over-pressured while another is still filling. | A fill-only shot; transfer position; cavity balance |
| Flash started with a new lot, or when the regrind went up | The melt is thinner. Melt flow is higher in the new lot, or regrind has lowered the viscosity. | Melt flow on the COAs; regrind percentage |
| Flash on nylon, PBT or polycarbonate, along with splay or brittle parts | Wet resin. Moisture breaks the polymer chains in the barrel and the melt gets thinner. | Dryer temperature, time and dew point. See splay and silver streaks. |
Checks in order, cheapest first
- Clean the mold faces and confirm the clamp. A flake of flash or a smear of resin on the parting line holds the tool open and makes more flash. Then confirm the clamp force setting, and that the machine is reaching it.
- Make a fill-only shot. Turn pack pressure off or very low and see how full the part is at transfer. If the cavity is already full or flashing at transfer, the first stage is doing the packing. Move the transfer point earlier.
- Lower pack pressure and injection speed in steps. Change one at a time and watch for the point where flash stops and the part is still full.
- Check melt and mold temperature against the supplier’s range. Measure the melt, do not trust the set points. A melt at the top of the range, or a long residence time in the barrel, gives a thinner melt.
- Check the dryer. For nylon, PBT, polycarbonate and other moisture-sensitive resins, confirm temperature, time and moisture against the drying chart.
- Check regrind, then compare lots. Run a short trial without regrind. If the flash arrived with a new lot, compare melt flow on the COAs before you touch the process further.
- Look at the tool. Worn parting lines, deep vents, worn pin bores and weak support behind the cavity all let melt out. Do not raise clamp force above the rating of the mold to crush flash shut, because that damages the parting line and makes the flash permanent.
- Change the resin. If the tool is sound, the process is in range and the part still flashes, a grade with a lower melt flow is the next place to look. This is rarely the first answer.
The numbers
The resins below have thin melts and flash most easily. Running above the melt range, or molding a moisture-sensitive resin wet, makes the melt thinner still.
| Resin | Melt temperature | Mold temperature | Moisture target before molding |
|---|---|---|---|
| Nylon 6 | 482–536°F / 250–280°C | 104–212°F / 40–100°C | 0.03–0.12% |
| Nylon 66 | 527–580°F / 275–305°C | 140–212°F / 60–100°C | 0.03–0.2% |
| PBT, unfilled | 460–536°F / 238–280°C (above 536°F it decomposes) | 100–212°F / 38–100°C | 0.02% |
| PPS | 572–644°F / 300–340°C (do not exceed 644°F / 340°C) | 275–320°F / 135–160°C | 0.02% |
| Acetal copolymer | 370–446°F / 188–230°C (never above 460°F / 238°C) | 140–250°F / 60–121°C | No published maximum; dry only if exposed to moisture |
| Polypropylene homopolymer | 374–525°F / 190–274°C | 50–203°F / 10–95°C | Not moisture-sensitive (unfilled) |
| HDPE | 380–525°F / 193–275°C | 50–125°F / 10–50°C | Drying not required |
| LDPE | 320–450°F / 160–232°C | 68–150°F / 20–66°C | Drying not required |
Typical values from published processing guides. Confirm against the data sheet for your grade. The full lists are in the melt temp, mold temp and shrinkage chart and the drying chart. Overheated acetal breaks down and releases formaldehyde gas, so do not raise acetal melt temperature above the supplier’s limit.
Why parts flash
- Pressure against clamp. Cavity pressure acts on the whole projected area of the parts and runners. When that force is more than the clamp force, the tool opens a little and melt escapes.
- Thin melts find small gaps. Semi-crystalline resins such as nylon, polypropylene, polyethylene, acetal, PBT and PPS have low-viscosity melts and will flash through a gap that polycarbonate or ABS would not enter.
- Anything that thins the melt adds to it. Higher melt temperature, long residence time, wet resin, regrind and a higher melt flow lot all lower viscosity.
- Flash makes more flash. Flash caught on the parting line is crushed into the steel on the next cycles. The dent it leaves becomes a permanent gap.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A lower melt flow grade | A thicker melt is less able to enter the parting line and vents. See melt flow index explained. | Harder to fill thin walls and long flow paths, so short shots become the risk. A new grade for your customer to approve. |
| A tighter melt flow range from lot to lot | Flash that comes and goes with lots often traces back to melt flow variation. | Nothing in the part. It may narrow your choice of sources. |
| Prime resin in place of wide-spec or off-grade | Wide-spec lots vary more in melt flow. See prime vs. off-grade. | A higher price. |
| Less regrind, or a fixed regrind ratio | Regrind has a lower viscosity than virgin resin, and a ratio that wanders gives a melt that wanders. | More virgin resin used, and regrind to store or sell. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Parting line mismatch. A step you can feel across the parting line, with no thin fin, means the mold halves are out of alignment. It is not flash.
- Burn marks. Vents that are too shallow trap air and burn the end of fill. Vents cut too deep to cure that will flash. The two problems share a fix that can be overdone. See burn marks.
- Short shots. Flash and short shots can appear on the same part. Raising pressure to fill the short area makes the flash worse, so balance the fill first. See short shots.
- Gate vestige or stringing. A thread or stub at the gate is a gate or nozzle temperature issue. Flash follows a parting line, vent or pin.
Quick answers
What causes flash in injection molding?
Flash forms when pressure in the cavity is more than the clamp can hold shut, or when the melt is thin enough to run into a gap in the tool. It is usually a press or tool problem. The resin matters when its viscosity has changed, for example from a higher melt flow lot, regrind or wet resin.
How do I stop flash at the parting line?
Start by cleaning both mold faces and confirming the clamp force setting. Then make a fill-only shot, and move the transfer point earlier if the cavity is full at transfer. Lower pack pressure and injection speed in steps. Do not raise clamp force above the rating of the mold, because that damages the parting line.
Why did flash start after a resin change?
The new material most likely has a thinner melt. Melt flow may be higher in the new lot, or regrind has lowered the viscosity. Compare melt flow on the COAs and run a short trial without regrind before you touch the process further. On moisture-sensitive resins, also check the dryer, because wet resin gets thinner in the barrel.
Which resins flash most easily?
Semi-crystalline resins such as nylon, polypropylene, polyethylene, acetal, PBT and PPS flash most easily. Their melts are low in viscosity and will run through a gap that polycarbonate or ABS would not enter. Running above the melt range, or molding a moisture-sensitive resin wet, makes the melt thinner still.
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