The matching of injection molding process parameters is the "core variable" of preform molding, which mainly includes three categories: temperature, pressure, and time:

1. Temperature parameters

Barrel temperature: PET is a crystalline plastic and requires precise control of the melting temperature (to avoid degradation or insufficient melting).

Typical temperature range: feed section 240-260°C, melting section 260-280°C, nozzle section 270-290°C;

Impact: Low temperature→ poor fluidity of raw materials, lack of preforms, and obvious welding marks; If the temperature is too high→ PET degrades (acetaldehyde is produced, affecting the taste of the beverage), the preform turns yellow, and the mechanical properties decrease.

Mold temperature: usually controlled at 10-40°C (adjusted according to preform thickness).

Thin-walled preforms (such as less than 300ml): Mold temperature 10-20°C for accelerated cooling;

Thick-walled preforms (such as more than 1L): Mold temperature 20-40°C to reduce internal stress (avoid surface cracking caused by too fast cooling).

2. Pressure parameters

Injection Pressure: The key to propelling the molten PET to fill the cavity, typically 80-120MPa.

too low→ insufficient filling, lack of preform material, small size;

If the mold is too high→ it is easy to wear out due to excessive force, and the internal stress of the preform increases (subsequent blowing is easy to burst).

Holding pressure: After filling is completed, the pressure is supplemented to compensate for the cooling shrinkage of the plastic, usually 50%-70% of the injection molding pressure.

Insufficient pressure retention→ shrinkage marks and uneven wall thickness on the surface of the preform;

Too high holding pressure → deformation of the bottle mouth (such as bulging at the thread).

Back pressure: The counter pressure (usually 5-15MPa) when the screw rotates, which is used to compact the raw material, remove air, and improve the melt uniformity.

The back pressure is too low→ the molten material contains bubbles, and the preform has pinholes.

If the back pressure is too high→ the screw load will increase, and the risk of raw material degradation will increase.

3. Time parameters

Injection Molding Time: The time it takes for the melt to fill the cavity (typically 1-3 seconds, adjusted based on preform weight).

If the filling speed is too short→ the filling speed is too fast, the cavity exhaust is not enough, and bubbles are generated;

Too long → production efficiency decreases, and the melt stays in the barrel for too long and is easy to degrade.

Holding time: Typically 2-5 seconds, ensure that the preform is cooled to release hardness (to avoid ejection deformation).

Cooling time: 60%-80% of the molding cycle (typically 5-15 seconds), set according to the thickness of the preform (thick-walled preforms extend the cooling time).