When molding containers with the same volume, the amount of air discharged from the blow mold is much larger than that of the injection mold. The volume of air to be excluded is equal to the volume of the cavity minus the volume after the parison has been inflated at the moment of complete mold closing. However, there is still a certain amount of air trapped between the parison and the cavity, especially for large-volume blow-molded products. In addition, the pressure inside the blow mold is very small. Therefore, the exhaust performance of blow molds is required to be high (especially molds with polished cavity). If the air trapped between the mold cavity and the parison cannot be discharged completely or as soon as possible, the parison cannot be inflated quickly, and cannot be in good contact with the mold cavity after inflation, which will cause defects such as roughness and dents on the surface of the product.

The surface text and pattern are not clear enough, which affects the appearance and external shape of the product, especially when streaks or melt fracture occur when the parison is extruded. Poor exhaust will also prolong the cooling time of the product, reduce its mechanical properties, and cause uneven wall thickness distribution. Therefore, it is necessary to try to improve the exhaust performance of the blow mold. During blow molding, the heat of the plastic melt will be continuously transferred to the mold, and the high temperature of the mold will seriously affect the productivity.

In order to keep the mold temperature in an appropriate range, in general, the mold should be equipped with a cooling device, and it is very important to design and arrange the cooling system reasonably. The general principle is: the distance between the cooling water channel and the cavity should be consistent everywhere to ensure uniform cooling and shrinkage of the product everywhere. For large molds, in order to improve the circulation of the cooling medium and improve the cooling effect, a water outlet and a water outlet should be set directly on the sealed water tank behind the blow mold. For smaller molds, a cooling water channel can be set directly on the template, the cooling water enters from the bottom of the mold, and the water outlet is located at the top of the mold, on the one hand, air bubbles can be avoided.

On the blow moulding machine other hand, the cooling water can be naturally flow in the direction of warming. In the cooling water channel with larger die surface, folds can be installed to guide the flow direction of the water, and it can also promote the effect of turbulent flow to avoid dead corners during the flow of cooling water. For some engineering plastics, such as the mold not only does not need to be cooled, but sometimes it is even required to increase the mold temperature to a certain extent to ensure the inflation of the parison and the clarity of the pattern, which can be heated in the cooling channel of the mold. The medium or point type is heated by an electric hot plate.