Improve the exhaust performance of the blow molding mold during the blow molding process

When blowing a plastic barrel to form a container, the volume of air that needs to be exhausted by the mold during operation is very large, which is the volume of the mold cavity minus the volume of the parison after it has been blown up at the moment of full mold closing. The volume after being blown up accounts for a large proportion, but there is still a certain amount of air trapped between the parison and the mold cavity, especially for large-volume blow molding products such as barrels and fuel tanks. Taking the blow molding of a 300L barrel as an example, the mold cavity volume is about 325L, and the parison has been blown up to a volume of about 210L at the moment of full mold closing. In this way, about 115L of air needs to be exhausted in the later stage of blowing.

Compared with injection molding molds, the internal pressure of blow molding molds is very small, generally less than 1MPa. Therefore, the exhaust performance requirements for blow molding molds are relatively high, especially for molds with polished cavities. If the air trapped between the mold cavity and the parison cannot be completely or quickly discharged, the parison cannot be inflated quickly, and after inflating, it cannot be in good contact with the mold cavity, especially at the corners, which will cause the surface of the product to be rough, dented and other defects, and the surface text and pattern are not clear enough, affecting the appearance performance and external shape of the product, especially when the parison is extruded, streaks or melt fracture will occur. Poor exhaust will also prolong the cooling time of the product, reduce the mechanical properties of the product, and cause uneven distribution of the wall thickness of the product. Therefore, it is necessary to improve the exhaust performance of the blow molding mold.

The exhaust methods generally used in hollow blow molding machine molds can be divided into four methods: exhaust on the mold surface, exhaust in the mold cavity, exhaust in the mold collar thread groove, and vacuum exhaust.

The hazards of poor exhaust of hollow blow molding molds in blow molding of plastic barrels:

1. Once the blow molding mold has poor exhaust during the blow molding process of the hollow blow molding machine, it will hinder the normal and rapid filling of the plastic melt, and the heat generated by gas compression may cause the plastic to burn. When the filling speed is high, the temperature is high, the material viscosity is low, the injection pressure is high, and the plastic is too thick, the gas will penetrate into the plastic part, causing defects such as pores and loose tissue.

2. The source of gas in the blow molding mold during the blow molding process of the hollow blow molding machine is mainly due to the presence of air in the mold cavity and the pouring system, and the plastic raw material contains water, which evaporates at the injection temperature and generates gas.

3. The hollow blow molding machine is prone to generate gas after the plastic raw material melts during the blow molding process of the plastic barrel. Some additives in the plastic raw material volatilize or chemically react to generate gas. The exhaust groove should be set on the parting surface as much as possible and on the side of the concave mold as much as possible to facilitate mold manufacturing and cleaning.