Typically, bumper mould use hot runner systems rather than cold runner systems. Hot runner systems are preferred for molding bumpers and other large automotive parts. Due to the size and requirements of bumper mould, hot runner system can achieve efficient and high-quality result.
Hot runner bumper mould can be divided into open hot runner molds and needle valve hot runner molds.
Because the open hot runner can only be injected into the mold when the injection machine screw moves forward and breaks through the condensation gate, on the contrary, the glue outlet can only be closed when the screw moves backward in the injection machine and the gate condenses. Therefore, the selection of gate size is usually very important, otherwise, due to the temperature of the gate area, either the condensation blocks the hot nozzle and the glue cannot be ejected, or there is wire drawing and drooling at the hot nozzle gate. Also, if the gate cannot condense at the same time when the screw moves backward, it will form a reverse suction force on the mold cavity, which is not conducive to cavity pressure maintenance. Commonly used are point gate hot runners, straight gate hot runners, and sub-gate hot runners with diverter channels. Because the open hot runner has a simple structure, simple mold processing and low maintenance cost, and the requirements for the appearance of the injection product are not too high or the purchase cost is considered, there is no sprue head or very little sprue head after use, so there are still many users who use this kind of hot runner. Commonly used hot runners include point gate hot runners, straight gate hot runners, and sub-gate hot runners with diverter channels.
For needle valve hot runners, when multiple hot nozzles are injected into a mold, the hot nozzle gate moves forward with the injection machine screw to inject glue. The valve needle can be adjusted in sequence by air (liquid) pressure to open the gate and inject glue, which can adjust the water line, shrinkage and warping of the product appearance. On the contrary, when the screw in the injection machine retreats, the valve needle can be adjusted in sequence by air (liquid) pressure to close the gate without injecting glue. This sequential control can also compensate for the defect of uneven filling of the cavity when multiple hot nozzles are injected at the same time. In addition, the gate of the needle valve hot runner is slightly larger to reduce the shear heat of the glue outlet. By instantly opening and closing the glue outlet, the cavity is filled quickly, the pressure is better, and there is no drawing and drooling. Only needle valve hot runners can be used for laminated cavity molds and multi-cavity family product injection molding. The needle valve hot runner has a relatively complex structure and requires relatively high mold processing accuracy and maintenance. Although users consider the purchase cost, it has many advantages and is also adopted by most users.
Needle valve hot runners are divided into three types: cylinder needle valve hot runners, oil cylinder needle valve hot runners and spring needle valve hot runners. The cylinder needle valve hot runner uses air pressure to push the valve needle to open or close the gate to complete the injection mode. This method is convenient to use because there is ready-made compressed air in the injection workshop, and the air pipe is directly input into the hot runner cylinder, but the air pressure is limited, and it is also used by a large number of users. The oil cylinder needle valve hot runner uses hydraulic pressure to push the valve needle to open or close the gate to complete the injection mode. It needs to connect the oil pressure pipe to the hot runner cylinder. This method is often used for injection molding of plastic parts with large surface area and thick wall due to the high hydraulic pressure. The spring-loaded needle valve hot runner uses a locking spring compression method to push the valve needle to open or close the gate to complete the injection mode. The pressure adjustment is relatively small, but it is easy to use and has fewer users.
