Determination of number of injection molding cavity is an important part of structural design of injection moulding die. Determination of number of mold cavity is constrained by many factors. Reasonable determination of number of mold cavity is a prerequisite to ensure quality of plastic parts, reduce production costs, and give full play to equipment's production potential.

 

Theoretical injection volume of an injection moulding machine refers to maximum injection volume that can be achieved by an injection device when injection screw or plunger makes a maximum injection stroke under condition of air injection. There is a certain difference between actual rated injection volume of injection moulding machine and theoretical injection volume. Actual rated injection volume for injection moulding mainly depends on theoretical injection volume, and also has a certain relationship with plastic density, compression ratio, and expansion rate. Actual rated injection volume of injection moulding machine must be greater than total injection volume required for injection molding plastic parts, otherwise it will cause defects such as incomplete shape of product, loose internal organization, or decreased product strength; but it should not be too large, otherwise utilization of injection moulding machine is low, waste electricity, and plastic is in a viscous state for a long time, which can lead to decomposition and deterioration of plastic.

Melting process of a plastic from solid particles to a molten state is called plasticization of plastic. Plasticizing capacity of an injection molding machine refers to amount of plastic that injection molding machine can plasticize in a unit of time, so it is also called plasticizing rate.
When designing mold, plasticizing capacity of selected equipment is generally about 20% -25% greater than actual need to ensure that injection molding machine has sufficient time to complete plasticization of rated amount of raw materials in next cycle after completing injection moulding process, and ensure that mold cavity is full.

Clamping force refers to maximum clamping force exerted on mold by clamping device of injection moulding machine. When melt fills cavity, force generated by injection pressure in cavity will cause mold to expand along parting plane. Therefore, clamping force of injection molding machine must be greater than force of melt in cavity on moving mold to avoid flashover and mold expansion.

When mold is installed on injection molding machine, its maximum external dimensions must be compatible with installation space of injection molding machine. Number and layout of mold cavity directly affect length and width of mold. In order that mold can be fixed on movable and fixed mold fixing plate through tie rod space during installation, length or width of mold should be smaller than distance between rods of injection molding machine.

 

When there are a large number of cavities, due to manufacturing errors of runners and gates, it is difficult to adjust injection moulding process parameters of each cavity to optimal value at the same time, even if runners are balanced. So it is impossible to ensure shrinkage rate of each cavity plastic is uniform. One or several cavities are not full, or even if it is full, there are defects such as poor welding or loose internal tissues. To increase injection pressure, it is easy to cause flashing of other plastic parts. For plastic parts with high accuracy requirements, their interchangeability will be seriously affected.
Practice has proved that dimensional accuracy of plastic parts is reduced by about 4% for each additional cavity (experiments abroad have shown that dimensional accuracy of molded products decreases by 5% for each additional cavity). When molding high-precision plastic parts, cavity should not be too many, usually not more than 4 cavities, and way of distributing runners must be balanced. For plastic parts that are generally required, some people in Japan have suggested that it should not exceed 16 cavities. According to experience, even if products in each cavity are same, size is small, and molding is easy, if one mold has more than 24 cavities, it must be carefully considered. Because multi-cavity is difficult to make molding conditions of cavity consistent.

For large or thin-walled plastic parts, melt may not be able to fill entire mold cavity due to too long a flow distance or too much flow resistance. For this reason, a mold-cavity is usually used. If a multi-cavity mold is used, length of runner system is increased. In order to ensure that each mold cavity is full, flow distance ratio (flow ratio refers to sum of ratio of length of each section of flow channel and cavity of each section to thickness of corresponding section when melt flows through the longest distance in mold) during injection moulding should be checked to ensure that flow distance ratio during injection moulding is less than maximum flow ratio allowed for production raw materials.

Structure of plastic part has a certain correspondence to gate form. Large-scale deep cavity parts generally use direct gates, ring and rotary parts generally use disc gates, spokes or claw gates. These gates are used in molds of ordinary casting systems, number of mold cavities can only be one mold and one cavity. For some plastic parts with different structures, when production accuracy is low and same raw materials are used, mold cavities can be arranged in same sets to achieve multi-cavity mold.

Different types of plastics have different requirements on the form of gate due to their melt viscosity, fluidity, heat sensitivity, filling characteristics. Table 1 lists gate types that some plastics are suitable for. Different gate forms have different effects on quality of molding and performance of plastic parts. As mentioned above, if only direct gates, disc gates can be used, corresponding mold can only be one mold and one cavity, other types of gates can use one mold and multiple cavities.

 

Benefits are lifeblood of enterprise development, plastic parts production also pursues the highest benefits and the lowest costs. From an economic point of view, number of mold cavities is not only related to annual output, but also has a certain functional relationship with production batch, molding cycle of plastic part, and mold processing cost.
In the era of rapid economic development, products are rapidly updated and development cycles are short. Customers often specify order quantity and delivery date. To some extent, order quantity and delivery date also determine number of cavities when mold is designed. Single-cavity molds are usually used for small batch production; multi-cavity mold should be used for large-scale production. The greater number of cavities, the higher mold maintenance requirements and the longer molding process cycle. In actual production, determination of number of mold cavities should also consider working capacity of existing callable equipment.

When mold structure requires symmetry and force balance, it can be satisfied by changing number of mold cavities and their layout. Number of cavities is also related to factors such as injection mold casting system, temperature control system, and demolding structure, which should be considered in mold design process.
Accuracy of mold manufacturing directly affects accuracy of plastic parts. If mold company has a strong mold manufacturing capability and high mold accuracy, a large value can be selected to increase number of mold cavities.
In addition, do not blindly pursue number of cavities in mold design process. The larger number of mold cavities, the higher manufacturing cost, and the more difficult it is to manufacture. It is difficult to meet requirements of balanced feeding and solidification, and quality of mold is difficult to guarantee. The larger mold size, the larger size of injection moulding machine. Large injection molding machine is expensive, operating costs are high, and they operate slowly, which may not be advantageous for multi-cavity injection molds. The larger number of mold cavities, the higher failure rate. If one of cavities has a problem, it must be repaired immediately, otherwise injection molding machine and mold will be damaged, frequent shutdown of mold repair will inevitably affect production efficiency.