Product of end cover of diagnostic instrument is shown in Figure 1. Maximum external dimension of product is 142.85 mm * 42.30 mm * 34.30 mm, average thickness of plastic part is 2.50 mm, plastic part material is PC, shrinkage rate is 1.006, and plastic part weight is 43.27 grams. Technical requirements for plastic parts are that there shall be no defects such as peaks, underfilling, flow lines, pores, warpage deformation, silver lines, cold materials, and spray lines.

 

Figure 1 Product diagram of end cover of diagnostic instrument
As can be seen from Figure 1, structure of plastic part is a retracted shell with a large top surface and a small bottom surface, closed on three sides, edge of internal glue position is inclined, and bottom has three screw posts. According to structural analysis of plastic parts, difficulty of mold design lies in need to design large inclined sliders on inner side, and sliders also need to be designed on both sides of slider. Mold structure is structure form of composite slider in which slider drives slider.
Structure of plastic part needs to put all glue out of rear mold, and slider is also slider of rear mold. According to size and shape of plastic parts, mold design cavity is 2, forces of cavity and two sliders can also be balanced. Standard mold base CH4545, mold design layout is shown in Figure 2. Gating system is designed as a submerged gate, and molten plastic enters parting surface from vertical main channel and directly enters submerged gate through a short runner. For PC materials, effective means to prevent injection defects during injection molding is to have short and thick runners to obtain better product quality.
Structure diagram of rear mold slider is shown in Figure 3. Plastic parts inside plastic parts are closed on three sides. If core is pulled directly at one time, plastic parts will be pulled and deformed. Therefore, when opening mold, it is necessary to separate core several times to decompose wrapping force of plastic part on slider. First, small red slider in the middle of plastic part pulls core first, and its movement is guided by T-shaped groove at the bottom. After red small slider is removed, small sliders on both sides (see Figure 4) will move to center along T-slot at the bottom under action of two small springs, so that glue positions on two sides of plastic part are demolded, movement guide of the two small sliders is also T-slot at the bottom. Finally, large slider drives the entire slider system to complete core pulling.
Due to inclined large sliders on both sides of rear mold, strength of B plate and large sliders needs to be paid attention to. On the other hand, two sides of rear mold core are open structures, and rear mold core needs to be positioned reliably. Therefore, positioning blocks are required on the top and bottom of rear mold core to prevent mold core from moving under action of injection molding lateral force.
Drive of slider should be driven by inclined guide column as much as possible. Inclined guide column drive has characteristics of simple structure, easy manufacture and reliable action, has been widely used in export molds at home and abroad. In addition, wear plates are installed on the slope and bottom of slider for easy adjustment and maintenance. When slider is running, oil groove of wear plate is also easy to store oil and dirt.

 

Figure 2 Mold Design Ranking Diagram

 

Figure 3 Structure diagram of rear die slider

 

Figure 4 Structural diagram of side small slider
Figure 5 Large slider of rear mold
When moulding, it is necessary to pay attention to processing technology of mould parts. Since glue position of plastic parts is often irregular, mould parts must be designed with two vertical planes as reference plane in non glue position part. Machining datum of small sliders on both sides is shown in Figure 7. In mold design, if machining benchmark of mold parts is ignored, it will be difficult to process mold parts. For example CNC machining, wire EDM and EDM, all require machining benchmarks.

 

Figure 6 B-board structure diagram

 

Figure 7 Machining benchmark of small slider