With continuous updating of automobiles, digital products, home appliances and other products, as well as continuous improvement of product quality requirements, higher requirements have been put forward for product design and manufacturing levels. Take watch case parts as an example. In addition to high geometric accuracy requirements for mold, this part also requires high shape and position accuracy, delivery time requirements are getting shorter and shorter. At present, most companies use two-dimensional drawing systems or mixed applications of 3D/CAD and 2D /CAD system completes design tasks, cannot meet market needs. CimatronE software is used for die-casting mold design. Design of watch case die-casting mold with side bosses is introduced. Design points of gating system, cooling system, and ejection system are analyzed. Reasonable design parameters are given to realize three-dimensional mold design. Application of digital technology in die-casting mold design and manufacturing can better optimize product appearance, mold structure, forming process, etc., significantly shorten mold design and manufacturing cycle, reduce production costs, and improve product quality. Watch case mold designed using this method and castings produced after debugging are of excellent quality and have a short new product development cycle.

Figure 1 shows parts diagram and disconnection model of watch case. Volume of part shell is 5.1cm3, and material is zinc alloy. As can be seen from Figure 1, this part has two pairs of symmetrically distributed small bosses, which require a lateral core-pulling mechanism to smoothly demould. However, main body of shell is divided into upper and lower parts with middle of bosses as symmetrical plane, inward taper of upper and lower parts is 2°. This position is used as breaking surface to complete mold parting by creating a curved parting surface, which can avoid core pulling mechanism. In addition, wall thickness of casting is uneven. When designing gating system, it is necessary to ensure that all cavities can be filled effectively to obtain better forming quality. According to size of parts and actual production, a mold structure of 2 cavities is adopted. 2330 standard mold base can be loaded during design. In order to ensure that core and cavity are fully closed, gap parameters between movable and fixed mold plates are set to 0.5mm respectively. Sizes of movable and fixed mold plates are selected to be 50mm and 70mm respectively. Height of square iron is set to 80mm for ejection system, and the rest of templates use default size templates of CimatronE software.

 

Figure 1 Watch case parts and quick disconnect model

 

Figure 2 Create parting surface

 

Figure 3 Designing cavity and core parts
Dimensions of gating system vary with material, structure and specific production conditions of casting. Due to uneven thickness of casting, there will be difficulty in venting and residual scum defects. Therefore, a pouring structure on both sides is adopted to achieve sequential solidification and avoid shrinkage cavities. In order to prevent alloy liquid from washing bottom of sprue, reduce flow rate of molten metal and change flow direction, sprue uses a flow cone and a diverter cone. Calculation of gating system depends on minimum cross-sectional area, other cross-sectional areas can be determined according to proportions.

 

Figure 4 Sprue for horizontal hot chamber die casting machine
1. Diverter cone 2. Gate sleeve 3. Nozzle

 

Figure 5 Gating system
1. Gate sleeve 2. Diverter cone 3. Cross runner 4. Inner gate 5. Cold material cavity
Working temperature of alloy is 150~200℃. In order to ensure cooling effect, a water channel with a diameter of 6mm is used, distance between hole wall and wall of cavity is 15mm. According to structural characteristics of casting and characteristics of zinc alloy material, a cooling system is designed, as shown in Figure 6a. Through analysis, temperature difference is controlled at 3℃, and maximum warpage is 0.11mm, located at left and right boss positions to meet customer needs. This casting is small, diameter of ejector pin is 2mm, 10 ejector pins are evenly and symmetrically arranged in a single cavity. In order to facilitate demoulding and material cake removal, four push rods are added behind diverter cone to ensure that diverter cone can be ejected and reset. Gap is also conducive to exhaust.

 

Figure 6 Cooling system design and quality analysis