Features of multi-cavity molds

What are the characteristics of multi-cavity molds? Since the shrinkage rate changes due to the injection pressure, for a single-cavity mold, the cavity pressure in the cavity should be as consistent as possible. As for the multi-cavity mold, the cavity pressure difference between the cavities is very small. In the case of single cavity with multiple gates or multiple cavity with multiple gates, injection must be made with the same injection pressure to make the cavity pressure consistent. For this reason, it is necessary to ensure that the gate position is balanced. In order to make the cavity pressure consistent in the cavity, it is best to keep the pressure at the gate entrance consistent. The pressure balance at the gate is related to the flow resistance in the runner. Therefore, before the gate pressure reaches equilibrium, the runners are balanced first.

Because the melt temperature and mold temperature have an effect on the actual shrinkage rate, when designing the precision injection mold cavity, in order to facilitate the determination of the molding conditions, attention must be paid to the arrangement of the cavity. Because the molten plastic brings heat into the mold, and the temperature gradient distribution of the mold generally surrounds the cavity, which is in the shape of concentric circles with the main runner as the center.

Therefore, design measures such as runner balance, cavity arrangement and concentric arrangement centered on the sprue channel are effective in reducing the shrinkage error between the cavities, expanding the allowable range of molding conditions, and reducing costs. necessary. The cavity arrangement of the precision injection mold meets the requirements of the balance of the runner and the arrangement with the main runner as the center, and the cavity arrangement with the main runner as the line of symmetry must be adopted.

Because the mold temperature has a great influence on the molding shrinkage rate, it also directly affects the mechanical properties of the injection molded product, and also causes various molding defects such as the surface of the product. Therefore, the mold must be kept within the specified temperature range Moreover, the mold temperature must not change with time. The temperature difference between the cavities of the multi-cavity mold must not change. For this reason, temperature control measures for heating or cooling the mold must be taken in the mold design, and in order to minimize the temperature difference between the mold cavities, attention must be paid to the design of the temperature control-cooling circuit. In the cavity and core temperature control loop, there are mainly two connection modes: series cooling and parallel cooling.

From the perspective of heat exchange efficiency, the flow of cooling water is turbulent. However, in the parallel cooling circuit, the flow in the divided circuit is smaller than the flow in the series cooling circuit, which may form laminar flow, and the actual flow into each circuit is not necessarily the same. Because the temperature of the cooling water entering each circuit is the same, the temperature of each cavity is the same, but in fact, the flow rate in each circuit is different, and the cooling capacity of each circuit is also different, so that the temperature of each cavity cannot be the same. The disadvantage of using a series cooling circuit is that the flow resistance of the cooling water is large, and the temperature of the cooling water at the entrance of the front cavity is obviously different from the temperature of the cooling water at the entrance of the last cavity. The temperature difference between the inlet and outlet of the cooling water varies with the flow rate. For small precision injection molds, generally considering reducing mold costs, it is more appropriate to use a series cooling circuit.

Mold cavity and core have their own cooling water circuit system. In the design of the cooling circuit, due to the difference in the heat taken from the cavity and the core, the thermal resistance of the circuit structure is also different, and the water temperature at the entrance of the cavity and the core will have a large temperature difference. If the same system is used, the cooling circuit design is also more difficult. In addition, when taking measures to prevent warpage of injection molded products, it is also desirable to maintain a certain temperature difference between the cavity and the core. Therefore, when designing the cooling circuit of the cavity and the core, the temperature can be adjusted and controlled separately.

Maintenance of mold accuracy.

In order to maintain the accuracy of the mold under the injection pressure and clamping force, the feasibility of grinding, grinding and polishing the cavity parts must be considered when designing the mold structure. Although the processing of the cavity and core has reached the requirements of high precision, and the shrinkage rate is the same as expected, due to the center offset during molding, the relevant dimensions of the inner and outer sides of the molded product are difficult to reach the plastic Design requirements for parts. In order to maintain the dimensional accuracy of the movable and fixed mold cavity on the parting surface, in addition to setting the guide post and guide sleeve centering commonly used in conventional molds, positioning pairs such as tapered positioning pins or wedge blocks must be installed to ensure positioning Accurate and reliable.

The material for making precision injection molds should be high-quality alloy tool steel with high mechanical properties and low thermal creep. The mold material for making the cavity and runner should be selected with high hardness, good wear resistance and corrosion resistance after strict heat treatment. For materials with strong resistance to thermal deformation, the difficulty and economy of mechanical processing and electrical processing must also be considered. In order to prevent aging changes from changing the dimensional accuracy of the mold, it is necessary to specify tempering treatment or low temperature treatment to reduce the residual austenite structure of the mold material heat treatment when designing the mold.