That is, as the mold opens, a long thin “string” of plastic is pulled from the nozzle attached to the sprue. When such phenomenon happens it’s not necessary related to the molded material but to the process and set up of both injection molding machine and the mold, so few points should to be checked:
- Temperature in the Screw & Nozzle – could be checked on the injection molding machine control panel, dropping the temperature might help.
- Gate size all the way with the angle, special at the connection area with the part – is it multi cavity mold or single cavity? What’s the part size and weight?
- Mold design – any cooling around the spure? It’s important in order to define if its mold design problem.
- Cooling time & Set up – you could set up different cooling times or temperature specifically to the cycle near the spure therefor allow it to “freeze” after the injection process and during the cooling time allow the gate to “break” during the injection mold open process.
- Does the injection molding machine Screw/Nozzle are in their forward position constantly or each shot the machine set up allow the nozzle go back and forward again and again, you could do both and see the results, in small parts & high Vol. different set ups could affect the cycle time therefore the product price.
- When the injection mold open try to set up a different opening speed, slow/fast open.
- How “old” is the mold? If it’s happen in a “old” mold the spure has to be checked making sure its properly maintained.
Also check to make sure the nozzle and sprue interface is correct (radius) and that there is no misalignment. Even with nozzle contact always forward, typically after injection-hold finish the carriage forward pressure will be reduced. Misaligned or poor contact can cause the nozzle tip not to freeze off or break. Make sure the front nozzle zone heater is not too far forward, every application may be different, but 10mm distance is a good reference. If you have the ability, reduce the final last zone screw position back pressure during recovery and balance suck back.
Try to “freeze” the gate after injection properly so when the injection mold opens it will break. If after all the above the stringing problem has not been solve feel free to let us know a bit more about the set up and situation and we will see how to move on.