【Abstract】Take design of an injection mold for end cover of a water purifier as an example, analyze structural characteristics of end cover of water purifier, increase angle of main runner, design an S-shaped runner and side gate, adopt side core pulling mechanism to solve demoulding problem of multiple slots on the side of plastic part, adopt supporting needle and inserted needle ejection mechanism to better push out plastic part, adopt circulating cooling water way to ensure that mold temperature is in appropriate range, so as to realize mass production of plastic parts. By optimizing casting system, push-out system and cooling system of mold, it provides experience reference for similar plastic parts.

In the early stage of design of injection mold for end cover of water purifier, feasibility analysis of each characteristic element of plastic part is first carried out, molding material, direction of mold opening, and parting surface are determined. Secondly, carry out structural design of injection mould gating system, push-out system and cooling system. Large end face of plastic part will shrink, glue can be reduced; increase sprue angle and side gate to obtain good fluidity, avoid weld marks and cold material; in order to better push out plastic parts, ejector pins below 2mm are all made into supporting pins, fixed mold is designed to insert pins, movable mold side is designed to move; cooling system is designed as a symmetrical circulating water circuit to ensure mold temperature. By optimizing design of these three systems, mass production of plastic parts is realized, quality of plastic parts is improved, user's beauty of product is enhanced, needs of customers are met.

End cover of water purifier is shown in Figure 1. Material is ABS+PC, maximum dimension is 51.98*51.83*25mm, minimum wall thickness is 0.91mm, draft angle is 1°, plastic part has 3 stepped holes on the surface. There are 3 square grooves and a round hole on the side, 6 stiffeners on reverse side. Therefore, fixed mold can design insert pin, movable mold design position. Parting surface is selected at the largest contour of plastic part. In order to facilitate demolding, leave plastic part on the side of movable mold to facilitate design of ejection mechanism. According to structural characteristics of end cover of water purifier, model of injection molding machine and other factors, a 2-cavity mold design is adopted to ensure quality of plastic parts. As shown in Figure 2, there is shrinkage on dark surface, which requires glue reduction.

 

Figure 1 Water purifier end cover
a — —Three-dimensional drawing of end cover of water purifier b — —Two-dimensional drawing of end cover of water purifier

 

Figure 2 Shrinkage analysis of plastic parts

 

In order to obtain good pressure transmission and uniform filling, molten plastic can quickly flow through runners and evenly fill cavity. S-shaped path and U-shaped cross-section runners are adopted, H 1 =1.25R 1, R 1 =0.5B.

In order to make molten plastic fill cavity evenly after flowing into cavity and distribute more evenly in lateral direction, reduce phenomenon of welding marks and other surface defects of plastic part, fan-shaped side gate is adopted. Based on above analysis, combined with MoldFlow mold flow analysis software, sprue angle is determined to be 5° and gate position. Gating system design of end cover of water purifier is shown in Figure 3.

 

Figure 3 Design drawing of gating system

According to material of plastic part is ABS+PC, mold temperature during injection molding is 50℃~100℃, melting temperature is 230℃~300℃. Therefore, cooling water circuit of mold core and cavity is designed as a symmetrical circulating water path, with 4 water paths each for fixed and movable molds, cross-sectional diameter of water path is ϕ 8mm. Put a waterproof sealing rubber ring on waterway of fixed and movable mold plate, waterway of fixed and movable mold core and cavity, use a copper plug to block waterway on the side of core and cavity to prevent water leakage. Quick connectors of water channels are designed on the left and right sides of mold to facilitate mold installation. Design of cooling system ensures uniform cooling of plastic parts, as shown in Figure 4. During injection molding process, gas will be generated in cavity. In order to ensure quality of plastic part and smooth closing of mold, an exhaust groove is designed on parting surface of movable mold core. Edge of exhaust groove is 4~8mm away from edge of plastic part, groove width is 4mm, depth is about 0.2mm.

 

Figure 4 Cooling water circuit of fixed and moving mold core and cavity

Push-out mechanism system mainly consists of ejector rod, ejector rod fixing plate, push plate, reset rod and side core pulling mechanism. This system is mainly aimed at design of side core pulling mechanism and ejector pin.

(1), Design of side core pulling mechanism. Because there are square grooves and round holes on the side of plastic part, a lateral core pulling mechanism is required to complete injection molding of mold and obtain a qualified plastic part. Mechanism is mainly composed of sliders, molding inserts, oblique guide posts, oblique wedges, wear-resistant blocks, springs and positioning screws. As shown in Figure 5, main parameter principles of slider design are: ①Length of slider body C≥0.6*height H of slider or 0.6*length L of slider (at maximum value); ②Angle of inclined wedge F=angle of oblique guide column E+2°; ③Effective height of inclined wedge h≥2/3*H or height of glue position of slider (according to maximum value); ④Height of guide rail B≥2/3*height of slider H, depth of guide rail A≥1/3*height of slider H; ⑤Angle of oblique guide pillar E<25°, chamfer of oblique guide pillar e=angle of oblique guide pillar E; ⑥Safety factor of at least 3.0mm is reserved for stroke T of slider.

 

Figure 5 Slider design parameters
According to principle of slider parameter design, slider seat and slider insert are designed. Two-dimensional diagram is shown in Fig. 6, side core pulling mechanism is shown in Fig. 7.

(2). Mandrel design. Ejector rod mainly includes 9 dome rods of ϕ 5mm, 16 double-section dome rods of ϕ 2mm, one insert for each of ϕ 7.5mm and ϕ 8.3mm. Top of dome rod is matched with core surface, its end surface is about 0.1mm higher than core surface, so that plastic part will not form a boss, which ensures beautiful appearance of plastic part. Ejector pin and ejector pin hole of movable mold plate are matched with H8/f7. Ejector pin fixed end and ejector pin fixed plate adopt a unilateral 0.5mm gap, which is convenient for processing and can prevent jamming. Position coordinate distribution of ejector pin is shown in Figure 8.

 

 

Figure 6 Two-dimensional drawing of slider seat and slider insert
a — — Slider seat b — — Slider insert

 

Figure 7 Schematic diagram of side core pulling mechanism

 

Figure 8 Coordinate diagram of ejector pin position

According to shape of plastic part, size of core and cavity, LKM mold base is selected as CI3525A60B80C70, UG12 is used for the overall assembly design of plastic part, as shown in Figure 9.

 

Figure 9 3D assembly structure of the mold
a — fixed mold b — movable mold c — mold 3D assembly structure

Two-dimensional structure of mold is shown in Figure 10. Working process of mold: during injection, high-temperature melt flows into sprue and shunt through nozzle, enters cavity from gate, molded water purifier end cover is cooled for a certain period of time after pressure is maintained and mold is opened. Due to mold opening force of injection molding machine, inclined guide post 11 on fixed mold side follows movement, under force of spring, slider base 14 and slider insert 15 are respectively driven to preferentially move backward to positioning screw. Under action of ejector rod of injection molding machine, push plate 7, ejector fixed plate 8, and reset rod are pushed forward to push out plastic part, pulling rod 18 hooks off aggregate head, and plastic part automatically falls off. When mold is closed, spring 10 on reset rod 9 first resets push plate 7 and drives ejector rods 18, 19, 20 to return to initial state. Under guiding action of guide post, guide sleeve and oblique guide post, side core pulling mechanism preferentially moves forward to complete mold clamping

 

Figure 10 Two-dimensional structure of mold
1. Positioning ring 2. Fixed mold seat plate 3. Fixed mold plate 4. Movable mold plate 5. Pad iron 6. Bottom plate 7. Push plate 8. Ejector fixed plate 9. Reset rod 10. Spring 11. Inclined guide column 12. Oblique Wedge 13. Wear-resistant block 14. Slider seat 15. Slider insert 16. Mandrel plate guide column 17. Support column 18. Pull rod 19. Dome rod 20. Double dome rod 21. Insert needle 22 .Moving model core 23. Moving model core waterway 24. Fixed model cavity 25. Fixed model cavity waterway 26. Insert needle 27. Sprue sleeve

There are multiple stepped holes on the surface of water purifier end cover. There are multiple thin-walled features on the back, square grooves and round holes on the side. Glue is reduced, main runner angle is increased, fan-shaped side gate is designed, exhaust groove is added to prevent plastic parts from shrinking, welding marks, bubbles and other undesirable phenomena. In order to ensure smooth demolding of plastic part, according to structural characteristics of plastic part, two side core-pulling structures are designed. ϕ 2mm ejector rod is changed to a double-section dome rod to increase strength and ensure force balance when plastic part is pushed out. At the same time, a support column and garbage nails are designed to ensure force balance during injection and prevent plastic parts from being deformed. By optimizing design of gating system, cooling system and launching system, actual production has verified that mold production runs well, dimensional accuracy and appearance of plastic parts meet customer requirements, which can provide experience for design of plastic injection mold for structure type of water purifier.