Stringing
Fine threads sticking to the part at the gate area. Common with PP, PE and materials with a narrow processing window (PA, PBT, POM).
Stringing is melt left at the nozzle tip being drawn out like a fine wire as the mold opens. The broken string falls onto the mold surface in the next cycle and leaves a mark on the part, or prevents the mold from closing and strains the parting surface. Its source is temperature and pressure management in the nozzle area.
How to Recognize It
- A short cone/string at the hot runner gate, or a string that stretches in the machine nozzle area as the mold opens.
- If the string stays in the mold, it also spoils the quality of the NEXT shot.
Physical Causes
- Because of continuous heat input at the gate area, the outer layer solidifies very thinly; early mold opening/unit retraction tears this layer and the fluid material is drawn out as a string.
Solution Steps at the Machine
Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.
- Change the nozzle temperature (usually lower it); in a hot runner, lower the tip temperature.
- Increase the holding time and cooling time (so the gate freezes completely).
- Reduce the melt temperature; lower the back pressure and screw speed.
- Retract the unit immediately after metering or with a delay (optimize by trial).
Mold, Design and Material Solutions
- In a hot runner, check the position of the nozzle tip relative to the gate bushing (if the tip goes in too deep the heat input increases; if it is pulled back the cross-section increases).
- Check the gate cross-section (if it is too large, there is no clean break).
- Optimize the cooling of the gate area; verify the temperature sensor; replace a blocked nozzle tip.
Which defect is it confused with?
A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.
| Confused with | How to tell them apart |
|---|
| Cold Slug | A cold slug is a piece of melt that has cooled in the nozzle. With stringing the melt is too fluid — the two arise from opposite causes and their solutions are opposite too. |
| Flash | Flash is melt leaking from the parting surface. A string comes from the nozzle tip and sticks to the part afterward. |
Frequently asked questions
I lowered the nozzle temperature, and now cold slugs have started. What should I do?
The band between the two defects is narrow and hard to manage with temperature alone. The permanent solution is to use a shut-off nozzle; because it cuts the flow mechanically, it eliminates both defects at once.
Does the suck-back (decompression) setting help?
Yes. Retracting the screw slightly after injection lowers the pressure in the nozzle and reduces string formation. Too much, however, leads to air being drawn in; increase it gradually and watch the result.
Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice