Bottom shell product of dynamic monitor is shown in Figure 1. Maximum outer dimension of product is 35.77 mm * 13.00 mm * 15.00 mm; average glue thickness of plastic part is 0.80 mm, plastic part material is PC + ABS, shrinkage rate is 1.004, and plastic part weight is 4.30 grams. Technical requirements for plastic parts are that there must be no defects such as peaks, underfilling, flow lines, pores, warping deformation, silver streaks, cold materials, jet lines, etc. and they must comply with ROSH environmental requirements.

 

Figure 1 Product diagram of bottom shell of dynamic monitor
As can be seen from Figure 1, the overall shape of plastic part is complex. There are 6 buckles on the back edge, and a lifter or slider molding needs to be designed. Top surface of plastic part has circular structural elements, which require design of Huff slider molding. In addition, there is a round hole on the side of big end of plastic part, and slider molding also needs to be designed. From a comprehensive analysis, it can be concluded that although plastic parts are small, their structures are complex, and it is not easy to design a reasonable mold structure. 3D drawing of mold design is shown in Figure 2.
Mold design cavity arrangement is 2 cavities, and the two plastic parts are arranged in a straight line, mainly to facilitate design of slider, as shown in Figure 3. When designing and arranging mold, attention should be paid to arranging round holes at both ends on the outside to facilitate separate design of slider core pull. Difficulty in mold design lies in analysis and arrangement of sliders, cooperation of several sliders when they intersect, friction angle between slider and front mold, etc.
Gating system design is an important part of mold design. Design of gating system includes selection of main channel, determination of cross-sectional shape and size of runner, selection of gate location, determination of gate form and gate cross-section size, and design of cold material cavity. Before designing, a comprehensive analysis should be made on material, size, geometry and possible defects of plastic part, appearance requirements of plastic part, production batch size and whether to use fully automatic injection molding production. Shape of plastic part is all appearance surface, and gate can only be designed in disk shape. After assembly here, gate will be covered and will not affect appearance of plastic part. Gate is a point gate, and glue is injected at one point. Gate design is shown in Figure 3.
During injection molding process, plastic melt enters mold cavity through gating system. Choice of type of gating system determines mold structure to a certain extent. For example, two-plate molds and three-plate molds each correspond to different gating systems. Design of gating system has a very important impact on quality and production efficiency of injection mold. If flow channel section is too large, plastic consumption will be large and cooling time will be long; if flow channel section is too small, it will cause excessive pressure drop, resulting in insufficient pressure when filling cavity. In addition, small cross-section of flow channel will cause polymer degradation due to excessive viscous heat. Therefore, cross-sectional shape and size of flow channel and length of flow channel should be selected reasonably. For multi-cavity injection molding, it is necessary to reasonably arrange layout of main runner and sub runner, cross-sectional dimensions and size of gate to ensure that each cavity is filled with same pressure at the same moment. For multi-gate single-cavity molds, different gate sizes and locations as well as different runner systems will change position of weld line, thus affecting mechanical properties of plastic part. Whether it is a single gate and single cavity, single gate and multiple cavities, multiple gates and multiple cavities, or multiple gates and single cavity layout, correct and reasonable gating system design is very important.

 

Figure 2 3D diagram of mold

 

Figure 3 Mold parting surface analysis diagram
End of slider close to glue position, cooperation between the two sides along movement direction and cavity and core need to be designed with a slope of 3゜~5゜ to make movement smooth. There are two kinds of movement angles of slider. As shown in Figure 4, A is slope X゜ on both sides of orthographic projection of slider. This angle is suitable for cooperation between slider and core. B is situation when slider is higher than parting surface and matches cavity. Slope X゜ is shown on the right view. Angle of movement direction of slider can only be one, avoid adding slopes in both horizontal and vertical directions.

 

Figure 4 Slider angle direction selection
6 buckles on the inside of plastic part are demoulded with a lifter, and its structure is a T-shaped groove driven lifter. This kind of lifter structure is widely used. Since plastic parts are small, lifter is a small lifter, and lifter base is a structure that raises lifter base. This can reduce length of lifter and extend life of lifter. Regarding design of small lifters, there are two basic structures. You can refer to previous articles in the forum. Surface of lifter needs to be CVD or PVD deposited to enhance its service life.