In the injection molding process of beverage bottle preforms, the design of the exhaust system of the mold plays an extremely critical role, which is directly related to the quality of the preform and whether molding defects occur.
First of all, the main purpose of the exhaust system is to remove the air in the mold cavity and the volatile gas generated by the plastic during the melting and injection process. If the exhaust is not smooth, the gas in the cavity will be compressed under the push of the high-pressure plastic melt to form a high-pressure air pocket. This will cause defects such as surface burning, gas marks, and material shortages in the preform. For example, when the gas in the cavity cannot be discharged in time, during the melt filling process, the gas is compressed to generate high temperature, causing the plastic to overheat and burn locally, seriously affecting the appearance and performance of the preform.
Secondly, the design points of the exhaust system of beverage bottle preform mold include the location, size and number of exhaust channels. The exhaust channel should be set at the part where the melt is last filled, usually at the parting surface of the mold, the gap between the inserts, and around the ejector hole. The size of the exhaust channel needs to be determined according to factors such as the characteristics of the plastic raw material, the injection speed and the volume of the mold cavity. If the exhaust channel is too small, the gas exhaust speed is slow, which will still cause poor exhaust; if it is too large, it may cause the plastic melt to overflow and form flash. For example, for PET plastic with good fluidity, the exhaust channel diameter can be between 0.03-0.05 mm, while for some high-viscosity plastics, the channel diameter may need to be appropriately increased. The number of exhaust channels should also be reasonably distributed to ensure that the gas in the entire cavity can be effectively exhausted.
Furthermore, the exhaust system has various forms. Common ones include gap exhaust, exhaust groove exhaust and exhaust plug exhaust. Gap exhaust is to exhaust the air using the tiny gap between mold parts, such as the gap between the parting surfaces. This method is simple and effective, but the exhaust volume is limited. Exhaust groove exhaust is to specially open an exhaust groove of a certain width and depth on the mold, which can provide a larger exhaust area and is suitable for large molds or situations with high exhaust requirements. Exhaust plug exhaust can be used in areas where it is difficult to exhaust locally. It exhausts the gas through the internal porous structure. Reasonable combination of these exhaust methods can better prevent preform molding defects.
Finally, when designing the exhaust system of the beverage bottle preform mold, the overall structure and processing technology of the mold must also be considered. The exhaust system should be coordinated with the cooling system and pouring system of the mold, and should not affect the strength and other properties of the mold. At the same time, during the mold processing, the processing accuracy and surface finish of the exhaust channel must be ensured to prevent poor exhaust or blockage due to poor processing. By carefully designing and optimizing the exhaust system, the molding defects of beverage bottle preforms can be effectively prevented, the quality and production efficiency of beverage bottle preform molds can be improved, the production cost can be reduced, and the large demand for high-quality preforms in the beverage packaging industry can be met.
