Figure 1 shows shell of optical measuring instrument level, with an outer dimension of 82 mm*104 mm*173.4 mm. Since its use environment is mostly in outdoor construction sites, it is inevitable to bump and squeeze. At the same time, in order to avoid excessive weight, material is aluminum alloy (YL113), with a shrinkage rate of 3‰~9‰ (different due to changes in shrinkage conditions), and a density of liquid aluminum alloy of 2.4 g/cm³. Casting is formed by a die casting mold, which can not only ensure molding consistency but also reduce production costs. Inner and outer draft angles of casting are 0.5°~1° (due to different draft depths), which meets demolding requirements. Surface of casting is mostly curved, appearance is a drum-shaped cylinder on all four sides, and base is composed of several cylinders. Internal structure of casting is complex, and multiple directions need to cooperate with corresponding parts. As a part of optical instrument, casting has high requirements for geometric tolerance. Due to deep hole spacing at objective lens assembly, a hydraulic core pulling mechanism is used for demolding; eyepiece mating part uses an inclined guide column slider core pulling mechanism for demolding; fine-tuning mechanism on both sides is relatively small and has high size requirements. Core is inlaid to facilitate subsequent installation and maintenance of mold; base and upper cover use an inclined guide column slider core pulling mechanism for demolding.

 

Figure 1 Shell structure
Difficulty of casting molding can be obtained through experience and theoretical analysis, so mold structure is improved at the beginning of design.
(1) At size of 36.5 mm and φ27 mm, due to draft angle and narrow space, size of 36.5 mm will be too small and there is no processing allowance. Adding 0.1~0.3 mm of processing allowance to size of 36.5 mm improves casting qualification rate and saves production costs.
(2) Due to thermal expansion and contraction, molded parts at φ8.6 mm hole are prone to wear, resulting in a smaller size. Molded parts at this location use an insert structure to facilitate subsequent replacement and maintenance.
(3) Since coaxiality tolerance of holes on both sides of objective lens and eyepiece is φ0.05 mm, sufficient mold repair margin should be considered during processing and assembly of sliders on both sides during design.
(4) End face of base is 51 mm away from center, and parallelism tolerance with X reference plane must be 0.05 mm. Since circular ring at the end of base is thin and narrow, it encounters greater resistance during feeding, resulting in insufficient machining margin for end face size of 51 mm, and 51 mm size is increased by 0.3 mm.
(5) Surface roughness of φ43.4 mm hole must be lower than Ra0.1 μm, and boring processing cannot be used. Therefore, a calendering knife is used for processing to ensure roughness requirements.
(6) Two threaded holes in main view are on arc surface and need to be processed separately using a four-axis machining center. When designing die-casting mold, machining allowance must be set at these two locations.

Parting design is carried out according to structural characteristics of casting. Initially, convex and concave molds are designed according to actual placement of casting. However, during analysis, it was found that there were parting marks on multiple parts of outer surface of casting, which not only increased workload of polishing, but also made it difficult to ensure surface quality of casting, reducing production qualification rate. For parting of four core pulling mechanisms, under principle of ensuring uniformity of surface, parting surfaces of each slider are shown in Figure 2.

 

Figure 2 Parting surface design
Mold molding part is shown in Figure 3. Mold molding part is composed of an upper mold, a lower mold and four sliders. For slender holes, a mosaic structure is used to mold to ensure service life of mold. In order to ensure mold quality and surface roughness of casting, 8433 mold steel is used for convex mold, concave mold and four sliders. Due to complex structure of punch and die, multiple copper electrodes must be used for processing, high-speed CNC processing technology and wire cutting technology are also required.

 

Figure 3 Molded parts
Objective lens connection adopts a hydraulic core-pulling mechanism, and rest of base, eyepiece, and upper cover adopt traditional inclined guide column slider mechanism for core-pulling. Slider core-pulling mechanism is shown in Figure 4, of which 3 are inclined guide column slider mechanisms and 1 is a hydraulic core-pulling mechanism.

 

Figure 4 Slider core-pulling mechanism
Since objective lens hole is a deep hole, core-pulling distance is long, requiring a large core-pulling force, and there is also a coaxial shallow hole in slider II. Considering smoothness of transmission and deformation of casting, slider IV is designed with a hydraulic core-pulling mechanism. At the same time, a wedge block integrated with upper mold is designed to provide locking force, saving installation time of mold. Due to mutual movement between slider and wedge block, wear is large, so a wear-resistant block is designed and installed on slider. If wear-resistant block is not used, the entire upper mold or slider needs to be disassembled when replaced, resulting in extended installation time and reduced production efficiency.
Since shallow hole and deep hole are coaxial, coaxiality requirements should be paid attention to during processing. Different from design of deep hole core-pulling slider, slider II and slider III adopt design of separating forming part from guiding part. Forming parts of slider II and slider III are made of high-temperature resistant materials, and guiding parts are made of steel with good comprehensive performance. This can not only ensure quality of casting molding, but also control production cost.
Slider I forms base of level gauge. Since base needs to cooperate with dial, there are cylindrical requirements at base matching point, there are verticality requirements between center threaded hole and base.
Slider I requires that end face of base is 51 mm away from center, and parallelism tolerance with X reference plane must be 0.05 mm. At the same time, in order to ensure machining allowance, 51 mm size is increased by 0.3 mm. Secondly, precision requirements of die-casting mold are guaranteed by cutting processing. If machining allowance is too small, yield rate will be reduced, so sufficient machining allowance needs to be guaranteed in actual production.

Casting size is not large, but structure is complex. According to company's press and other related equipment, mold is equipped with 4 core-pulling mechanisms, adopting a 1-cavity layout. After analysis and verification, size of single-cavity mold frame is 600 mm*550 mm, as shown in Figure 5. Core material is low-carbon, low-chromium, high-molybdenum, high-tungsten mold steel, and service life is 80,000 molds.

 

Figure 5 Layout of lower model cavity

Through analysis of casting shape, diverter cone adopts a ring gate to feeding position, which can be arranged at left and right ends in theory. According to arrangement of slider oblique guide column, it is selected at left end of objective lens shown in Figure 5; if side gate is selected to feed from surface of shell of casting to be formed, a gate mark will be left at parting of outer surface, affecting appearance quality, and making it more difficult to remove condensate.
In order to ensure appearance quality of shell, slag bag treatment is used at the end of melt pouring to remove gas and cold dirty metal liquid in cavity, stabilize flow state, reduce eddy currents, and remove gas on both sides of gate. When molten metal fills cavity and flows into slag bag, all slag bags are connected by slag belts and finally enter slag buffer belt, as shown in Figure 6 (a), which ensures both melt filling speed and filling quality. These slag bags and slag belts will be pushed out with casting later, and then separated manually. After separation, they need to be polished for subsequent processing. Structure of pouring system is shown in Figure 6. Thickness of slag bag overflow port at slider position was originally 1.3 mm, but when sawing slag bag, waste material is easy to collapse into casting. Now thickness of slag bag overflow port at slider position is reduced to 0.8 mm, as shown in arrow of Figure 6 (b). Through empirical analysis, it can be seen that wall thickness of shell casting is thin and weight is light. Gap between molding part and slag bag groove can be used for exhaust, and there is no need to add an exhaust groove.

 

Figure 6 Casting system structure

Analysis of molded casting found that demolding and tightening force of casting is mainly borne by slider, upper and lower molds are easy to demold due to cooling and shrinkage of material. In order to ensure good appearance of casting, no push rod is set on outer surface of casting, but a push rod is used at slag bag position, as shown in Figure 7. In order to ensure balanced push and sufficient push force, number of push rods should be increased in parts with large clamping force, and number of push rods can be appropriately reduced in parts with small clamping force. Parts with sufficient space are pushed out with 7 φ8 mm round push rods, and parts with narrow space are pushed out with 3 φ6 mm round push rods. Push mechanism has been proven feasible through on-site production practice and has been applied in similar molds.

 

Figure 7 Push mechanism

Sliders are small in size, narrow in space, and wall thickness of casting is thin, so no cooling water channel is designed in slider. Gates of mold are concentrated and evenly distributed, so ordinary linear cooling water channels are used. Diameter of cooling water channel at core is φ12.0 mm, and diameter of cooling water channel at mold plate is φ22.0 mm, as shown in Figure 8.

 

Figure 8 Cooling water channel
1. Upper mold core water channel 2. Lower mold core water channel 3. Casting

Mold structure is shown in Figure 9. After casting is formed by pressure maintenance and cooling, die-casting machine template drives mold lower mold sleeve 6 to move downward to open mold. Mold is first split along PL, inclined guide pillars 2 and 16 push slider inserts 4 and 17, so that sliders 5 and 19 have relative movement and move outward along inclined guide pillars 2 and 16 to complete side core pulling action of shallow hole. Similarly, inclined guide pillars on symmetrical surface of slider 19 will also push slider III (see Figure 4) to move outward to complete core pulling action at the end cover.

 

Figure 9 Mould structure
1. Upper mould sleeve 2. Inclined guide column 3. Wear block 4. Slider insert 5. Slider II 6. Lower mould sleeve 7. Pad 8. Limit pin 9. Push plate guide column 10. Screw 11. Screw 12. Slider IV 13. Core-pulling slider 14. Wear block 15. Screw 16. Inclined guide column 17. Slider insert 18. Wear block 19. Slider I 20. Push rod fixing plate 21. Push plate 22. Push plate guide column 23. Push rod 24. Reset rod 25. Lower mould plate 26. Forming block 27. Forming block 28. Gate sleeve 29. Screw 30. Fixed ring 31. Upper mould plate 32. Casting
Three sliders complete core-pulling action when mould is opened and reach maximum position limited by limit block. Then hydraulic core-pulling mechanism will drive slider 12 to perform core-pulling movement. After mold is opened, casting 32 and its slag bag will remain in lower mold.
After all sliders are pulled out, ejector device of die-casting machine pushes push plate 21, driving push rod fixing plate 20 and push rod 23 to move. After casting is completely pushed out by push rod, it falls to discharge position of die-casting machine with anti-fall measures in advance. Check whether there is any residue in melt port and molding part of die-casting mold. If not, mold can be closed and die-casting can be continued. When closing mold, first reset hydraulic core-pulling mechanism, and reset rod 24 will drive push rod fixing plate 20, push plate 21, and push rod 23 to reset.
During mold closing process, slider 12 is first reset by hydraulic mechanism, and three inclined guide column slider core-pulling mechanisms are reset by inclined guide column with remaining three sliders. Finally, upper and lower molds are closed at parting surface. When pressure of die-casting machine reaches threshold of casting, next die-casting cycle can be carried out. Casting is polished and machined as shown in Figure 10.

 

Figure 10 Actual parts