Due to the global growing demand for PET plastic bottles, the preforms used to manufacture PET plastic bottles have also become the focus . In order to ensure the perfect forming of PET preforms, every step from temperature setting to raw materials selection needs to be strictly controlled.
PET is a hygroscopic material. If the water content is too high, hydrolysis will occur under high temperature treatment, and the PET molecular chain will be broken and thermally decomposed to produce acetaldehyde. The physical and mechanical properties are greatly reduced, and the crystallization rate is reduced. Increasing the speed, part of the preform will crystallize , which will affect preform quality. Before injection molding of the preform, the PET pellets must be dried to a moisture content of 0.06% or less, which can produce a good non-crystalline transparent preform.
Under Polarizing light, crystallization becomes more visible. Strain Viewer Polariscope is a specialized optical instrument to detect common defects in preforms. It provides 2 angles to have a thorough analysis with every preform.
For dry air, the dew point temperature must be between -30°C and -40°C. Set the temperature between 180°C and 185°C and dry for about 240 minutes.
In the first step, injection molding. The preform is formed by injecting molten glue into the cavity, and then rapidly cooling the preform to below the crystallization temperature, thereby forming a transparent preform.
The second step is to heat the preform to a temperature higher than the glass transition point, and then stretch it to blow and cool the molded body.
One-step process is to complete the injection and blow molding in the same machine, and then cool the preform to a temperature lower than the crystallization temperature, but still maintain the thermoelastic state, and finally blow into the bottle.
Two-step process is completed by two machines, an injection molding machine and a hollow molding machine. Firstly, after the injection molding machine manufactures the preform, the preform is cooled to the solidification temperature, taken out and cooled to room temperature. When the bottle is blown, it is sent to the hollow molding machine to heat the preform to the blowing temperature, and then blow molding and cold molding. In addition to the extrusion molding process, most of the injection molding process conditions for PET preform molding use injection molding to make preforms.
Injection molded preforms must be stored for more than 24 hours before they can be used. The preforms that are not used after heating must be stored for more than 48 hours to be reheated for use. The storage time of preforms cannot exceed six months.
