What Are the Effects of Poor Blow Molding Mold Exhaust?

When molding containers with the same volume capacity, the amount of air to be exhausted in the blow molding  mold is much larger than that in the injection molding mold. The maximum amount of air to be exhausted in the injection molding mold is equal to the amount of molten material injected into the mold cavity.

1. Influence of poor exhaust of blow molding mold

For the blow mold, the air volume to be removed is equal to the cavity volume minus the volume after the parison has been inflated at the moment of complete mold closing, in which the latter accounts for a large proportion, but there is still some air trapped between the parison and the cavity, especially for large-volume blow-molded products such as barrels and fuel tanks.

For example, for a 120L blow-molded barrel, the mold cavity volume is about 145L, and the volume of the blown preform when the mold is completely closed is about 90L. In this case, about 55L of air needs to be exhausted during the final stage of blowing (about 2-38). In addition, the pressure inside the blow mold is very low compared to that in the injection mold (generally less than 1NPa). Therefore, the exhaust performance of blow molds is required to be high (especially for molds with polished cavities).

If the air trapped between the mold cavity and the preform cannot be completely or quickly exhausted, the preform cannot be blown rapidly, and after being blown, it cannot be in good contact with the mold cavity (especially at the corners), resulting in defects such as rough surfaces and dents, unclear surface texts and patterns, which affect the appearance performance and external shape of the products, especially when the preform is extruded, there will be streaks or melt fractures. Poor exhaust can also prolong the cooling time of the product, reduce its mechanical performance, and cause uneven thickness distribution of the product wall. Therefore, it is necessary to improve the exhaust performance of blow molding molds.

2. The maintenance of blow molding molds 

The Processing enterprises should first equip each mold with a resume card, which records and statistically analyzes its use, care (lubrication, cleaning, rust prevention) and damage, so as to find out which parts or components are damaged, the extent of wear, and provide information to discover and solve problems, as well as the molding process parameters and materials used for the product, to shorten the trial run time of the mold and improve production efficiency.

The processing enterprise should test various performance of the mold when the injection machine and the mold are operating normally, and measure the size of the final molded plastic parts. Through this information, the current status of the mold can be determined, the damage to the cavity, core, cooling system, and parting surface can be found, and the damage status and maintenance measures of the mold can be determined based on the information provided by the plastic parts.

The surface maintenance of blow molds should be emphasized, which directly affects the surface quality of the product, especially rust prevention. Choosing a suitable, high-quality, and professional anti-rust oil is particularly important. After the mold completes the production task, residual injection should be carefully removed according to different injection methods. Copper rods, copper wires, and professional mold cleaning agents can be used to remove residual injection and other deposits in the mold, and then air-dried. Do not use hard objects such as iron wires and steel bars to clean to avoid scratching the surface. If there are rust spots caused by corrosive injection, we should use a grinding machine to polish and spray professional anti-rust oil, and then store the mold in a dry, cool, and dust-free place.