The causes of shrinkage at the bottle neck during PET preform injection molding can be summarized as follows:

Main Causes of Shrinkage:

1. Injection Molding Machine Issues:

   • Short injection time (packing pressure ends before the gate solidifies).

   • Low packing pressure.

   • Insufficient material dosing.

   • Premature switchover from injection to packing pressure.

   • Low injection pressure.

   • Slow injection speed.

   • Short cooling time.

   • High material temperature.

   • Worn or damaged check valve.

   • Nozzle orifice deformation (causing pressure loss) or material leakage.

2. Mold-Related Problems:

   • High mold temperature.

   • Non-uniform mold cooling (localized overheating).

   • Small gate size.

   • Poor mold structural design.

   • Improper ejector pin placement or design.

3. Material Properties:

   • High inherent shrinkage rate of the raw material.

Solutions and Preventive Measures:

1. Optimize Mold Design:

   • Avoid unnecessary thickness and ensure uniform wall thickness in the product.

   • Design gates and cooling channels to promote even solidification.

2. Adjust Process Parameters:

   • Reduce molding and mold temperatures to minimize shrinkage (note: this may require increasing pressure).

   • Increase injection pressure, packing pressure, and packing time.

   • Optimize injection speed and cooling time.

   • Ensure proper material dosing and avoid excessive material temperature.

3. Equipment Maintenance:

   • Regularly inspect and replace worn components (e.g., check valve, nozzle).

4. Material Selection:

   • Use raw materials with lower shrinkage rates where possible.