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What should I pay attention to in PET preform making?
1. When designing extruded plastic bottles, if the material is high-density polyethylene or polypropylene, the cross-section of the plastic bottle should be rectangular or elliptical. For low-density polyethylene or other flexible plastic bottles, the cross section It is better to use a circle. This facilitates the extrusion of the contents from the plastic bottle. The plastic parts used in conjunction with the plastic bottle mouth are mainly caps and seals. The design of the plastic bottle mouth should be focused on; considering how to make the plastic bottle mouth better fit the bottom of the plastic bottle with the cover and the sealer, it is the weak part of the mechanical properties of the plastic bottle. Therefore, the bottom of the plastic bottle is generally designed to be concave; the corners of the plastic bottle and the concave portion are excessively curved. In order to facilitate the stacking of plastic bottles, the stacking stability of the plastic bottles is increased, and the bottom of the plastic bottles should be designed with inner grooves.
2. When the surface of the plastic preform is marked, the surface of the label should be flat. A “sash” can be designed on the surface of the plastic bottle to make the label accurately positioned without moving. At the time of blow molding, the portion where the parison inflates first contacts always tends to be hardened first. Therefore, the wall thickness of this part is also larger. The edge and the corner portion are the portions where the parison inflates the last contact, and the wall thickness of the portion is small. Therefore, the edges and horns of the plastic bottle should be designed to be rounded. Changing the surface shape of the plastic bottle, such as the middle of the plastic bottle is relatively thin, increasing the circumferential groove or rib of the surface of the plastic bottle, which can improve the rigidity and bending resistance of the plastic bottle. Longitudinal grooves or ribs eliminate the offset, sagging or deformation of plastic bottles under long-term load.
3. The printing surface of the plastic bottle, the plastic preform is the most concentrated part of the consumer's attention. The printing surface should be flat and continuous; if the plastic bottle contains handles, grooves, ribs and other structures, care should be taken not to inconvenience the printing operation. Elliptical plastic bottles have higher stiffness, but the manufacturing cost of the mold is higher. Therefore, in order to ensure the rigidity of the plastic bottle, in addition to selecting a material with high rigidity, the shape of the plastic bottle is also designed to enhance the rigidity and load resistance of the plastic bottle.
4. Since most plastics have notch sensitivity, plastic bottles are prone to cracks and cracks at sharp corners, roots of the mouth threads, necks, etc., so these parts should be designed to be rounded. For the transfer of rectangular plastic bottles, it is necessary to support most of the load of the plastic bottle, so locally increasing the wall thickness at this place is also beneficial to increase the rigidity and load resistance of the plastic bottle.
5. When preparing plastic preforms, the PET chips are first injection-molded into preforms, which requires that the proportion of secondary regrind should not be too high (less than 5%), the number of recovery should not exceed two, and the molecular weight and viscosity should not be too low (molecular weight 31000- 50000, intrinsic viscosity 0.78-0.85 cm3/g). Injection molded preforms must be stored for 48 hours to be used for 3 months.
6. The quality of plastic preforms depends largely on the quality of PET materials. Materials that are easy to inflate and easy to shape should be selected, and a reasonable preform molding process should be established. Experiments show that the preforms of PET materials with the same viscosity are easier to blow molding than the domestic materials; while the same batch of preforms has different production dates, the blow molding process may also have a big difference. The advantages and disadvantages of the preform determine the difficulty of the blow molding process. The requirements for the preform are pure, transparent, free of impurities, no discoloration, suitable length of the spot and surrounding vignetting.
7. The heating of the preform is done by a heating oven, the temperature of which is manually set and automatically adjusted. In the oven, far infrared rays are emitted from the far-infrared tube to radiate heat to the preform, and the fan at the bottom of the oven performs thermal circulation to make the temperature in the oven uniform. The preform rotates while moving forward in the oven, so that the wall of the preform is heated evenly.