Design points of injection mold positioning system
1. Small mold positioning
The problem of guiding and positioning is the repeated staggered actions when the injection mold is closed and opened. When closing the mold, the guide pin and guide sleeve are the first to guide, to ensure that the mold parts have the correct initial movement trajectory. After the orientation is over, the positioning comes into play. Due to the different structure of plastic parts, each plastic part will have lateral force more or less during injection molding. Deep cavity plastic parts and asymmetric plastic parts have the largest lateral force. There is a gap between the guide posts and guide sleeves, and its working state is actually line contact. Therefore, in the injection mold, the huge lateral force during injection cannot be withstood only by the guide post and the guide sleeve. Therefore, the positioning system must be designed to withstand the lateral force. The structure of the injection mold to ensure that the moving parts move according to the established trajectory is called the guiding system; the structure to ensure the relative position accuracy between the front and rear molds and between the movable parts is called the positioning system. As an important part in the mold production process, the guide post is used for guiding, supporting and positioning. Support means that the guide post needs to bear the weight of the template.
Small molds have low lateral force, and the positioning problem can be solved by using the four corners of the mold core to design the tiger's mouth. Small molds can also take advantage of the structural characteristics of plastic parts to design corresponding front and rear mold Rende locking systems. The small mold is locked by the parting surface as shown in Figure 1.
When the four corners of the front and rear mold cores are designed for the positioning of the tiger's mouth, the inclined angle of the tiger's mouth should be smaller, generally 5゜～10゜. A small angle can effectively protect the penetration surface of the mold from damage. The disadvantage of the positioning of the die core four-corner tiger mouth is that it has high requirements for machining. The tiger mouth generally needs CNC machine tool processing, and the machining accuracy is easily limited by the accuracy of the machine tool and the tool, and it often fails to meet the accuracy requirements. For example, the fit of the inclined plane must be above 80% for reliable precise positioning.
Figure 2 shows the two-way interlocking of mold cores. 5゜ inclined plane A and 5゜ inclined plane A'cooperate with each other; 5゜ inclined plane B and 5゜ inclined plane B'cooperate with each other. All mating surfaces can be ground by a grinder, and the positioning accuracy is significantly higher than that of the four-cornered tiger's mouth. When the plastic part is in a small and medium-sized mold where the cavity and the core glue position are roughly symmetrical, the design of the two-way interlocking of the mold core can effectively improve the clamping accuracy of the dynamic and fixed molds. Therefore, this positioning method is widely used in European molds.
Figure 3 shows the slope positioning block showing the edge of the mold blank. When designing the slope positioning block, it is necessary to pay attention to the middle of the edge of the mold base, so as to resist thermal expansion caused by thermal expansion and contraction.
The positioning of the mold base positioning block is usually called the secondary positioning. The secondary positioning is the precise positioning of the front and rear mold blanks. The positioning accuracy of the zero-degree positioning block is much higher than that of the guide post and guide sleeve. The matching groove of the positioning block on the mold base should be processed in the mold base factory as far as possible to ensure the matching accuracy of the front and rear mold bases. The positioning block material adopts alloy steel heat treatment or copper and graphite material, and there should be a lubricating oil groove on the friction surface of the positioning block. The zero-degree positioning block has two functions of guiding and positioning, which can effectively protect the weak penetrating surface on the mold core. The mold base positioning block is very important for precision molds. Therefore, the zero-degree positioning block is a must in North American molds.
The mating height of the two sides of the zero-degree positioning block determines the length of the positioning effect. After the zero-degree positioning block is designed, the movable and fixed molds produce a certain distance of precise guidance and positioning before the mold is closed. Therefore, in the mold with more insertion positions, the zero-degree positioning block can effectively protect the inserts of the insertion position. Pieces are not damaged. Figure 4 shows the HASCO zero-degree positioning block Z085.
