PET preform gate stringing and white crystallinity: causes and fixes
The gate, the small point at the bottom of the preform where the melt enters, is where most visible PET preform defects appear. The two most common are strings and white crystallinity. Both come from heat in the wrong place at the wrong time, and both can usually be fixed without changing the mould.
First: one cavity or all of them?
Lay out a full shot with cavity numbers marked. If the defect appears on only a few cavities, look at those cavities: their nozzle heater, thermocouple, valve pin or cooling circuit. If it appears on every cavity, look at the process settings and the resin first.
Gate stringing
A thin string or tail of PET is pulled from the gate when the mould opens. The melt at the gate is still too hot and fluid to break cleanly.
- Nozzle tip temperature too high: lower it in small steps.
- Valve pin closing late or not sealing fully: check the timing and look for wear on the pin or the gate seat.
- Weak cooling at the gate insert: check that water is flowing and the channels are not scaled.
- Melt temperature too high: check the barrel and manifold settings.
White crystallinity at the gate
A milky white area around the gate means the PET cooled too slowly there and crystallised instead of staying clear. It weakens the base of the bottle and can cause problems in blow moulding.
- Gate cooling: the most common cause. Check the flow and temperature of the cooling water, and clean scaled channels.
- Nozzle tip too hot, heating the gate area after injection.
- Holding time or cooling time too short for the wall thickness at the gate.
- Wet resin: moisture lowers the viscosity and makes crystallinity worse.
Do not forget drying
PET absorbs moisture from the air, and wet PET breaks down in the barrel. That causes haze, weak preforms and more gate defects. Dry the resin to your supplier's datasheet, typically several hours at around 160–180 °C with dry air, and check that the dryer's dew point is where it should be.
When the mould is the cause
If the defect stays on the same cavities after the process is corrected, the mould needs attention: a worn valve pin or gate seat, a failing heater or thermocouple, or a blocked cooling channel. A good mould maker can tell you which parts to check from photos of the shot with cavity numbers marked.
Send us photos of the defect with the cavity numbers and your main settings, and we will help you find the cause.