Structure of mobile phone shell is relatively complex, including a large number of undercuts, appearance surface must not have defects such as weld marks, cold material marks, bubbles, surface must be clean and beautiful. Main difficulty in design of injection mould for mobile phone shell lies in treatment of undercut and design of gating point. Push-out system and cooling system can adopt traditional structural design. To solve problem of undercut molding and demolding, side core pulling can be adopted. Side core pulling is divided into outer core pulling and inner core pulling. When designing multiple undercut ejection structures, it is necessary to pay attention to no interference between mold parts. Now take complex mobile phone shell as an example, analyze structural characteristics of plastic parts, design mold structure, and optimize structure of side core and lifter block according to characteristics of undercut.

 

(A) Dimensions

 

(B) Wall thickness analysis

Figure 1 Plastic part structure

Appearance quality of plastic parts is relatively high, defects such as shrinkage marks, weld marks, lack of material, flashing and deformation that affect appearance are not allowed. External dimensions of mobile phone shell are 105mm*46mm*11.75mm. As shown in Figure 1, average wall thickness is 0.71mm, the thinnest part is 0.5mm, and the thickest part is 2.04mm. Wall thickness analysis is shown in Figure 1(b). Plastic part material chooses ABS (acrylonitrile-butadiene-styrene), which has good moldability, low creep, excellent dimensional stability and high impact strength. Surface forming quality of plastic parts can be improved by increasing mold temperature. Shrinkage rate of ABS is 0.5%, melting temperature is 195~240℃, molding temperature is 38~93℃, and molding pressure is 120~140MPa.

 

(A) Side hole formed by side core pulling

 

(B) Undercut formed by lifter block

Figure 2 Side hole and undercut analysis

Structure of plastic part is complex, with multiple side holes and undercuts. As shown in Figure 2, M1~M4 are side holes on outer surface of plastic part, and M5~M7 are inner side holes of plastic part. These side holes are formed by side core-pulling sliders; M8~M19 are internal inverted buckles of plastic part, and these inverted buckles are formed by lifter blocks. Difficulty of mold design lies in design of side core pulling mechanism and demoulding mechanism. Considering complexity of plastic part structure and aesthetics of appearance, mold adopts a 1-cavity structure, and multi-point injection molding is performed by multiple point gates.

 

(A) Fixed mold insert

 

(B) Movable mold inserts

Figure 3 Insert structure

In order to ensure that molded plastic part can be demolded smoothly without affecting surface quality, maximum contour line of plastic part to be molded is used as parting surface, and NX10 is used for 3D parting. Movable and fixed mold insert structures are shown in Figure 3. Fixed mold insert adopts an integral embedded structure, and material is 8407 mold steel, which has good workability, polishing and heat treatment stability. Movable mold insert adopts an integral combined structure. Mold base material is FDAC mold steel, which has good processability, wear resistance and toughness; core is made of S136 mold steel, which has good polishing and wear resistance and dimensional stability.

Plastic part has 4 side holes M1～M4 in 2 directions, and it needs to be pulled outwards for smooth demoulding. Surface quality of plastic part is required to be high and there must be no weld marks. Outer core pulling mechanism cannot use moving mold slider core pulling (Structure is simple but it will cause splicing marks on the surface of molded plastic part), and only fixed mold slider can be used to pull core (structure is relatively complex but surface quality of molded plastic part is good).

 

Figure 4 Outer core pulling structure

1. Fixed mold seat plate 2. Pusher plate 3. Fixed mold plate 4. Glass bead spring 5. Movable mold plate 6. Screw 7. Press plate 8. Movable mold insert 9. Outer slider 10. Fixed mold insert 11. Wedge Tight block 12. Screw
Distance between M1~M3 side holes is relatively short, and demoulding can be completed in 1 set of core-pulling mechanism; M4 is demolded by a single set of side core-pulling mechanism. Core pulling structure of outer fixed mold slider is shown in Fig. 4, mechanism is composed of a wedge block 11, an outer slider 9 and a glass bead spring 4. Wedge block 11 is fixed to fixed mold base plate 1 under action of two M6mm screws 12, outer slider passes through fixed mold insert 10 and is connected to fixed mold plate 3 through pressure plate 7. Function of wedge block 11 is to wedge outer slider when mold is closed to prevent slider from retreating under action of injection pressure, and angle of inclined surface matched with slider is 5°.
When mold is opened, mold is opened at PL1, and outer slider 9 moves outward under drive of wedge block 11. When outer slider moves 4mm and is blocked by glass bead spring, slider separates from side holes M1~M4 of plastic part.
There are 3 side holes M5~M7 inside fixed mold side of plastic part, side core pulling is needed to demould smoothly, using fixed mold inner slide block or fixed mold inclined top block to pull core. Since outside core of mold has adopted a fixed mold slider mechanism, in order to make mold structure simple and reasonable, fixed mold slider is used for demolding.

 

Figure 5 Internal core pulling structure

1. Fixed mold base plate 2. Pusher plate 3. Fixed template 4. Fixed mold insert 5. Movable mold insert 6. Inner slider 7. Spring glass bead 8. Wedge block 9. Screw
Inner side holes M5~M6 have same corners in same direction and relatively close distance. Core is pulled in same inner core pulling mechanism, and M7 is demolded by a single inner core pulling mechanism. Core pulling structure of inner fixed mold slider is shown in Fig. 5, which is composed of wedge block 8, inner slider 6 and glass bead spring 7. Wedge block 8 is fixed to fixed mold seat plate 1 under action of M6mm screw 9, inner sliding block 6 is positioned on fixed mold insert 4 through a step and is connected with wedge block 8 through a T-shaped groove. Inclined plane angle of wedge block 8 and inner sliding block 6 is 8°.
When mold is opened, mold is opened at PL1, and inner sliding block 6 moves backward under drive of wedge block 8. When inner sliding block 6 moves 2.1mm and is blocked by glass bead spring, sliding block is separated from inner hole M5 of plastic part~ M7.
There are 12 undercuts M8~M19 inside plastic part, which need to be pulled out of mold smoothly. Lifter block side core pulling mechanism can be used for processing. M18 and M19 inverted buttons are in same direction and can be formed by one lifter mechanism; other inverted buttons are separately formed with lifter block. There are 11 sets of lifter block mechanisms in mold.

Sprue size of gating system: 45mm in length, ϕ2.5mm in small end and 4.2mm in big end; cross section of sub runner is 5mm*5mm, and inclination is trapezoid with 5°. Through analysis of structure of plastic part, point gate is used for inject, and gate is opened on non-appearance surface of plastic part to be molded, and three points are injected at the same time, as shown in Figure 6 (a).

 

(A) Gating system

 

(B) Cooling system

Figure 6 Gating system and cooling system

According to structural characteristics of plastic parts and mold structure, cooling water channels are used to cool fixed and moving molds respectively to ensure uniform cooling, improve molding quality of molded plastic parts, reduce injection defects and increase production efficiency. Cooling water channel is shown in Figure 6(b). Temperature difference between inlet and outlet of cooling water is less than 2℃.

 

Figure 7 Push rod layout

Complex structure of plastic part will affect balance of force during demolding. Mold adopts a push-out mechanism that combines a round push rod and a flat push rod, and a total of 42 push rods are designed. There are 2 round push rods with ϕ1.5mm, 17 with ϕ2mm, and 19 with ϕ2.5mm; there are 4 flat push rods, and layout of push rods is shown in Figure 7.

 

Figure 8 Mould structure

1. Fixed mold seat plate 2. Wedge block 3. Pull rod 4. Glass bead spring 5. Slider 6. Inclined top block 7. Limiting column 8. Inclined top seat 9. Movable mold insert 10. Movable mold Frame 11. Die buckle 12. Fixed mold insert 13. Sprue sleeve 14. Positioning ring 15. Wedge block 16. Wedge block 17. Glass bead spring 18. Glass bead spring 19. Outer slider 20. Inner slide Block 21. Reset rod 22. Reset spring 23. Distance rod 24. Wedge block 25. Inner slider 26. Glass bead spring 27. Spring
Mold structure is shown in Figure 8, which is 1-cavity structure. Mold working process is as follows.
(1) PL1 parting. After injection is completed, mold is opened, and mold is first divided in PL1 under action of fixed distance tie rod and die button. Runner aggregate is disconnected from plastic part at gate under action of pull rod; at the same time, 4 fixed mold sliders are driven by wedge block to perform side core pulling to complete demoulding with M1~M7 characteristics, opening distance of PL1 parting position is 100mm.
(2) PL2 parting. Mold continues to open and opens at PL2 parting surface. Runner aggregate is pushed out from pull rod by pusher plate. Opening distance of PL2 parting position is 10mm.
(3) PL3 parting. Mold continues to open, and mold opening force overcomes suction force of die buckle. Mold opens at PL3 parting surface, and plastic part is separated from fixed mold cavity. Mold opening distance at PL3 parting position is 150mm.
(4) Launch. After mold opening is completed, ejector pin of injection molding machine pushes mold push plate to push 11 inclined ejector blocks for side core pulling to complete demolding of M8~M19 characteristics; at the same time, push push rod to push plastic part out of cavity to complete ejection of plastic part. Ejection distance is 20mm.
(5) Reset. Ejection mechanism is first reset under action of reset rod and reset spring, and then mold parts are closed at each parting surface in turn, while wedge block pushes fixed mold slider back to original position, and mold is completely closed.