The mold is the "mother" of preform molding, and its structural design directly affects the dimensional stability, cooling efficiency and release effect of the preform, and the core influencing factors include:

1. Cavity and core accuracy

Key parameters: cavity size (determines the outer diameter of the preform), core size (determines the inner diameter and wall thickness of the preform), thread accuracy (the key to fit with the cap).

If the cavity/core processing error exceeds ±0.02mm, it will lead to preform size deviation, and it is easy to have uneven wall thickness or burst during subsequent blowing.

Excessive error in thread pitch and tooth height will lead to a decrease in the tightness of the bottle cap (such as leakage and air leakage).

Optimization direction: High-precision machining equipment (such as CNC grinding, EDM) is used to ensure that the surface roughness of the cavity ≤ Ra0.4μm and the thread accuracy reaches ISO 4H/5g.

2. Cooling system design

Function: Quickly cool molten plastic, shorten the molding cycle, and reduce the internal stress of the preform (avoid deformation during subsequent blowing).

Influencing factors:

Cooling water line layout: it should be close to the surface of the cavity (distance ≤ 8mm), and the water path of each cavity should be symmetrical (ensure that the cooling speed of each cavity is the same, and the temperature difference is ≤± 2°C);

Cooling medium parameters: water temperature (usually 15-25°C), flow rate (adjusted according to the weight of the preform, the recommended flow rate for 500ml preform ≥5L/min), too high water temperature will prolong the cooling time, too low may cause condensation on the preform surface.

Common problems: Blockage or uneven distribution of water channels can lead to uneven cooling of preforms, local shrinkage (such as deformation of the mouth) or insufficient cooling (soft stickiness, easy sticking mold of preforms).

3. Mold release mechanism design

Core requirements: ensure smooth demolding of preforms without scratches, deformations or residues.

Key factors:

Demolding slope: The inner wall of the cavity should be designed with a release slope of 0.5°-1° (the higher the height of the preform, the greater the slope) to reduce the release resistance;

Ejection method: Small preforms are commonly used to "push plate ejection", and large preforms (such as 1.5L or more) need to be "ejected synchronously with multi-point ejector" to avoid uneven force causing preform bending;

Cavity surface treatment: Chrome plating (thickness 5-10μm) or nitriding treatment is used to improve surface hardness (≥HRC50) and smoothness, and reduce mold sticking.

4. Exhaust system design

Function: Discharge the air inside the cavity to avoid missing material, bubbles or weld marks in the preform.

Design Points:

Exhaust groove location: opened in the area where the melt is last filled (such as the bottom of the preform, the root of the thread);

Exhaust groove size: width 0.5-1mm, depth 0.02-0.05mm (too deep will cause flash, too shallow exhaust is insufficient).