The production of preforms mainly relies on the injection molding process, the core is to inject molten plastic raw materials into the mold, and after cooling, it forms a tubular semi-finished product, which can be divided into 5 key steps:
Raw material preparation and drying
Raw materials are mostly granular (such as PET slices), and the water needs to be removed through the dryer first (especially PET, the moisture content needs to be controlled below 0.02%, otherwise it will be hydrolyzed at high temperatures, resulting in bubbles and embrittlement of preforms);
If color adjustment or modification is required, add masterbatch, antioxidants, barrier agents and other additives after drying and mix evenly.
Injection molding machine melting plasticization The
mixed raw materials enter the barrel of the injection molding machine, and melt the raw materials into a viscous "melt" through the heating ring outside the barrel (the temperature is usually 180-280°C, adjusted according to the material).
The screw in the barrel rotates continuously, pushing the melt forward while further mixing and plasticization are completed.
Mold injection molding and pressure retention The
molten plastic melt is quickly injected into the pre-designed "preform mold" (there are multiple cavities corresponding to the shape of the preform in the mold) under the high pressure of the screw;
After the melt fills the mold cavity, the screw continues to maintain a certain pressure (i.e., "packing") to replenish the voids caused by the melt shrinkage during mold cooling, ensuring uniform density and dimensional integrity of the preform.
Cooling and shaping Cooling water is passed into the
mold, and the heat is taken away through the mold wall, so that the molten plastic melt can quickly cool and solidify, forming a solid preform shape.
The cooling time needs to be precisely controlled (usually a few seconds to ten seconds): insufficient cooling will cause preform deformation and mold sticking; Excessive cooling will affect the tensile properties of subsequent blow molding.
Demolding and dressing After
cooling is completed, the mold is opened and the preform is pushed out of the cavity by an ejection mechanism (i.e., "demolding");
The demolded preform usually has a "gate" (residue at the entrance of the melt into the cavity), which needs to be removed by cutting equipment to obtain a qualified preform semi-finished product.