The Design Standards for Stack Injection Molds
A stack mold is two or more molds with the same dimensions and superficial area stacked next to each other. This system allows greater productivity of parts for cycles with the same machine. For example, a two-level stack mold doubles the production of parts without requiring a higher clamp force of the injection machine. However, the machine will need a higher injection capacity.
The difference from a conventional mold is that the cavity of the injection mold is distributed on two or more levels and is arranged in an overlapping manner, which is equivalent to stacking and combining multiple pairs of plastic injection molds.
Compared with a conventional mold, the two-layer mold has a nearly 100% improvement in production efficiency compared to the standard plastic injection mold, and the mold clamping pressure is only about 10% greater than the standard plastic injection mold.
Double-layer plastic injection mold technology can also produce two different products at the same time. The products can even be made from different materials and have different colors.
Along with increased productivity, another key benefit of stack molding is the decrease in the required number of machines to produce large quantities of parts. This improves machine efficiency, decreases the cycle time compared to single molding processes, saves manpower, and can lower costs if exacting design standards are maintained.
There are several design standards for stack injection molding to ensure superior production, including the following:
Mold injection pressure. This assesses whether the molding injection pressure can meet molding needs. The stacked mold uses hot runner technology, due to the increase in process flow and projected area, the required plastic molding injection pressure is larger than that of a single-layer cold runner injection mold.
Plastic flow into mold. The pressure check should be combined with the computer simulation flow analysis to determine the molding injection pressure of the plastic molded parts, and then compared with the rated plastic molding injection pressure of the injection molding machine.
Mold clamping force. The cavity of each layer of the injection mold is set back-to-back and theoretically the mold clamping force does not increase. However, because the center main nozzle of the injection mold and the splitter plate increase the flow channel, the plastic molded part area and the pouring system increase, and the flow channel is extended due to the lamination, resulting in increased cavity pressure. Therefore, the mold clamping force is increased.
Injection mold opening stroke. The injection mold is divided into two levels to open the mold and eject the plastic molded parts. A synchronous injection mold-opening mechanism, such as a toggle or a gear rack, connecting rod injection mold opening device with the same transmission ratio must be used.
Main nozzle length. Since the center’s main nozzle moves with the middle part of the injection mold during parting, it should not be too long or short so that when the injection mold is closed. The center’s main nozzle will not exceed the maximum distance that the plastic injection molding machine nozzle retreats or advances on the base. Checking will prevent the overflow of the center’s main nozzle head from dripping into the fixed model cavity wall.
Gating system. The injection mold hot runner pouring system can better transfer the plastic molding injection pressure, which is beneficial to improve the injection molding quality of plastic parts.
Mold temperature. Mold temperature is one of the important factors that affect the injection molding quality of plastic molded parts. The design of injection molds should ensure that the temperature conditions of each layer cavity are controlled uniformly.
Plastic injection mold opening mechanism. To make the plastic molded parts shrink uniformly, the cooling time in each cavity should be equal. This way the injection mold will ensure that the parting surfaces of the cavity of each layer are opened at the same time.
Demolding mechanism. According to the same requirements of cooling time, the injection mold should eject the plastic molded parts in the cavity of each layer at the same time.
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