【Abstract】Taking control panel of a certain inverter washing machine as an example, a pair of 1-cavity latent injection point gate mold was designed. Design difficulties are: inner side of panel is undercut, there are many deep pillars, they are all installation structures, and there are three rectangular areas that require mirror finish; appearance of panel is required to be high (filming on subsequent surface), defects such as pouring marks, dents, and gas lines are not allowed. After actual production verification, mold runs stably, quality of plastic parts meets customer requirements, which can provide a reference for design of plastic parts with similar structures.

With continuous improvement of people's quality of life, high-performance electrical products have become a necessity for thousands of households. As a representative of smart home appliances, inverter washing machines have become mainstream products in this field due to their energy saving, ultra-low noise, variable water flow, and high spin speed. Control panel is an important exterior part of washing machine, which integrates practicability, safety, and aesthetics.
In this paper, in view of problems in molding process of a certain brand of washing machine control panel, mold optimization design was carried out, and good results were finally achieved.

Figure 1 shows parts drawing of washing machine control panel. Material is transparent ABS, which is characterized by low molding shrinkage, high surface finish, excellent electrical and mechanical properties, and a shrinkage rate of 5‰. Structural characteristics of plastic parts are as follows: ①Maximum external dimension of plastic part is 500*92*46mm, average wall thickness is 1.82mm; ②There is no undercut on outer surface of plastic part, but there are many undercuts on inner side. There are 9 places L 1 to L 9 all of which are installation structures and require high precision; ③There are 11 deep columns on inner side of plastic part, three rectangular frames A, B, and C require mirror finish; ④Appearance quality of plastic part is high, and no casting is allowed. For defects such as marks, dents, weld marks, gas lines and flashes, surface of subsequent appearance needs to be filmed.

 

Figure 1 Parts diagram of washing machine control panel

Control panel of washing machine is large in size and upper surface is required to be smooth and free of air lines. After analysis, ranking adopts a 1-cavity structure, glue feeding method adopts a point gate to a 3-point submerged gate, a submerged gate injects glue, plastic part and gate condensate are automatically separated during demolding. According to previous design experience, second runner is designed to be tapered, with a length of about 78mm, a taper of 1°, a small end diameter of ϕ 2.5mm, and a large end diameter of ϕ 8mm. Final gating system design is shown in Figure 2. Plastic part was imported into CAE software and filling simulation was carried out. As shown in Figure 3, time for plastic melt to fill cavity was 2.417s, filling effect was good, and there was no short shot.

 

Figure 2 Gating system design

 

Figure 3 Filling simulation results

When designing water circuit, the overall structure of plastic part is regular and wall thickness is relatively uniform. Therefore, cooling water circuit arrangement is consistent with shape of plastic part.
As shown in Figure 4, fixed mold position: Arrange two circular water channels with a diameter of ϕ 10mm, distance from surface of plastic part is about 15mm, cooling water pipe spacing is 30mm; movable mold position: In order to avoid movable mold mechanisms such as inserts, inclined tops, push rods, and barrels, 7 straight waterways with a diameter of ϕ 10mm are arranged at movable mold position.

 

Figure 4 Cooling system
a — Fixed mold cooling system b — Moving mold cooling system c — Overall cooling system arrangement

Function of exhaust system is to smoothly discharge original gas in cavity or gating system out of mold, so as to avoid high temperature generated by compressed gas, which will locally scorch plastic parts or cause short shots, gas lines, weld marks and other molding defects. As shown in Figure 5, mold cavity has a relatively large volume. In order to avoid above-mentioned defects, 25 exhaust slots are opened at parting surface of fixed mold cavity (exhaust devices are also designed at inclined roof and push rod). Width of exhaust groove is designed to be 8mm, and depth is 0.02mm.

 

Figure 5 Exhaust system

There is no buckle on the sides of control panel, and no side demolding core pulling mechanism is required. There are 9 undercuts on inner side and 11 deep column positions. Buckle position is demoulded by inclined top method. Angle is 12°. Material is SKD61. It is heat-treated and high-frequency quenched. Deep column uses cylinder ejection method. Material is SKD61. Heat treatment hardness is 50~54HRC. In particular, it should be noted that there are 3 rectangular areas on inner side that require mirror-like cross-section. Push rods cannot be designed here. Considering ejection balance, push rods are distributed as evenly as possible, and discharge state is good after actual production verification. In summary, mold demolding adopts a combination structure of "push rod + cylinder + inclined top", as shown in Figure 6.

 

Figure 6 Design of demoulding mechanism

UG (NX) software has always been mainstream 3D design software in domestic mold industry due to its strong advantages in the fields of layer, extraction surface, split body, extended surface, synchronous modeling, etc. Through above analysis, UG (NX) modeling is used, as shown in Figure 7.

 

Figure 7 Three-dimensional structure of control panel mold
a — —moving mold b — —fixing mold c — —mold overall structure

Mold base is a point gate mold base, size is 750*350*526mm, two-dimensional structure of mold is shown in Figure 8. Mold working process: During injection, plastic melt enters from main runner through injection molding machine, then enters cavity through runner and 3 submerged gates at the same time. Molded control panel is cooled by holding pressure and then mold is opened. After mold is opened, inclined top device is first pulled out laterally, inner side is reversed and demolded. Subsequently, movable mold side continues to make mold opening movement, ejector pin of injection molding machine pushes push rod fixed plate, push rod fixed plate drives push rod to move plastic part out of movable mold cavity with an ejection distance of 150mm. After mold is closed, ejection mechanism is reset by reset rod and reset spring. After closing mold, prepare for next injection molding.

 

Figure 8 Two-dimensional drawing of control panel mold
1. Movable mold seat plate 2. Push plate 3. Push rod fixed plate 4. Square iron 5. Inclined ejector rod 6. Movable mold plate 7. Press block 8. Fixed mold plate 9. Backing plate 10. Fixed mold seat plate 11. Positioning Ring 12. Pump 13. Waterway 14. Throat 15. Rubber ring 16. Fixed model cavity 17. Moving model core 18. Insert 19. Division tube 20. Long screw 21. Runner 22. Discharge rod 23. Push Rod 24. Spring 25. Guide sleeve 26. Balance block 27. Guide column 28. Reset lever 29. Reset spring 30. Side lock 31. Zhongtuo 32. Support head 33. Limiting column

Outer side of washing machine control panel requires a smooth surface, inner side has multiple buckles and deep column features. Final mold adopts a 1-cavity layout design. Gating system selects a point gate to a 3-point submerged glue port, and a submerged gate for glue feeding. Waterway layout in cooling system is basically same as shape of plastic part. Taking into account local characteristics of washing machine control panel, in order to ensure smooth demolding of parts, mold is equipped with 9 inclined tops, 11 cylinders, and 6 push rods to ensure balance of force when plastic parts are ejected. Through actual production verification, mold runs smoothly and product quality meets customer requirements, which can provide a reference for design of plastic molds with similar structures.