The design of cavity (determining the outer diameter and appearance of the preform) and core (determining the inner diameter and wall thickness of the preform) should be combined with the characteristics of PET material and the rules of the molding process to avoid precision deviations in subsequent processing and use due to design defects.

1. Dimensional design: reserve the "material shrinkage compensation" (key core)

PET material will shrink due to cooling after injection molding (shrinkage rate is about 1.5%-2.5%, fluctuating with molding temperature and cooling speed), if the preform size is directly used in the design, it will lead to the actual size after molding, and the shrinkage compensation amount needs to be accurately calculated.

Compensation amount calculation logic:

Mold cavity / core design size = preform finished product size × (1 + shrinkage)

Example: If the outer diameter of the preform is 28mm and the shrinkage rate is 2%, the inner diameter of the mold cavity is designed to be 28mm × 1.02 = 28.56mm;

Special part adjustment: for key mating parts such as thread spacing and bottle mouth sealing surface, the shrinkage rate should be taken at the upper limit (2.2%-2.5%) to avoid mating gaps; The shrinkage rate of the preform wall thickness is taken as the lower limit (1.5%-1.8%) to avoid the wall thickness being too thin.

Software assistance: Use CAD/CAE software (e.g., SolidWorks, Moldflow) for shrinkage simulation analysis, and set "differentiated shrinkage compensation" (if the bottom of the bottle cools slowly, the shrinkage rate can be increased to 2.3%) according to the difference in cooling speed of different parts (mouth, bottle body, bottle bottom).

2. Structural design: reduce the risk of machining and forming errors

Avoid accuracy deviations caused by complex structures:

The inner wall of the cavity adopts a "smooth transition" design to avoid sharp edges and corners (such as the radius of the fillet at the bottom of the preform ≥1mm) and reduce tool wear during machining (the edges and corners are prone to insufficient machining accuracy);

If the core surface needs to be designed with stiffeners (such as the inner wall support structure of the preform), the width of the ribs should be ≥ 1/3 of the wall thickness (such as wall thickness 3mm, rib width ≥1mm) to avoid dimensional deviation caused by too thin structure during processing.

Positioning and orientation design:

Set a "high-precision locating pin" (diameter error ≤± 0.005mm) on the mating surface of the mold cavity and core to ensure the coaxiality of the cavity and the core (deviation ≤ 0.01mm) during mold closing, and avoid uneven preform wall thickness;

The root of the core is equipped with an "anti-rotation structure" (such as flat position and keyway) to prevent the core from rotating due to melt impact during injection molding, resulting in thread misalignment.