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What are the preparations for the injection molding machine before injection molding?
1. Pretreatment of raw materials
According to the characteristics of the plastic and the supply of materials, the general injection molding machine should test the appearance and process performance of the raw materials before injection molding. If the plastic used is in powder form, such as polyvinyl chloride (PVC), it should also be batched and dry blended; if the plastic part has color requirements, you can add an appropriate amount of colorant or color masterbatch; the supplied pellets often contain different The degree of moisture, solvents and other volatile low-molecular-weight substances, especially some plastics with a tendency to absorb moisture, always exceed the allowable limit for processing. Therefore, it must be dried before processing and the water content must be measured. The moisture content of polycarbonate (PC), which is sensitive to water at high temperatures, is required to be below 0.2%, even 0.03%-0.05%, so vacuum drying ovens are often used for drying. The dried plastic must be properly sealed and stored to prevent the plastic from absorbing moisture from the air and losing the drying effect. For this reason, the use of a dry hopper can continuously provide dry hot materials for the injection molding machine, which is useful for simplifying operations, maintaining cleanliness, and improving quality. It is beneficial to increase the injection rate. The filling amount of the drying hopper is generally 2.5 times the hourly consumption of the injection molding machine.
2. Preheating of the insert
In order to meet the requirements of assembly and strength of injection molded products, metal inserts need to be embedded in the plastic parts. During injection molding, when the cold metal insert placed in the cavity of the injection molding machine and the hot plastic melt are cooled together, due to the significant difference in shrinkage of metal and plastic, large internal stresses are often generated around the insert (especially like PS and other rigid chain polymers are more prominent). The existence of this kind of internal stress will cause cracks around the insert, which will greatly reduce the performance of the plastic part. This can be achieved by selecting metals with large thermal expansion coefficients (aluminum, steel, etc.) as inserts, and preheating the inserts (especially large metal inserts). At the same time, when designing plastic parts, adopt measures such as larger wall thickness around the insert.
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