(Abstract) Taking oblique core-pulling mechanism in injection mold as research object, for larger oblique core-pulling angles, by optimizing slider structure, guiding T-slots and force-driven conversion of composite angles, structure of core-pulling mechanism is simplified and force driving is reasonable, working process of mold is introduced. This slider structure makes mold design more reasonable, structure is more compact, action is stable and reliable, quality and production efficiency ofmolded plastic parts are improved.

With rapid development of consumer electronic products, plastic parts are increasingly used in various products. Injection molds are main equipment for forming plastic parts, which determine quality and performance of plastic parts. When connecting and fixing plastic parts, in addition to traditional screw fixation, buckle is a commonly used functional structure feature, demolding of buckle needs to be completed by a core pulling mechanism. Core pulling mechanism is often used for demolding of plastic part to be molded, such as side convex buckle, side groove or 0° area functional surface, etc. Common mechanisms include sliders and oblique push rods. For example, there is a 45° oblique undercut groove on inner side of plastic part, which is functional area for installing hinge. If direct demolding is used here, mold must be driven by movable mold parting once, resulting in a complex mold structure, large size, and large lateral force which makes it difficult to drive core pulling mechanism. For this reason, slider core pulling mechanism is optimized, T-shaped groove and compound angle force driving conversion are adopted to simplify core pulling mechanism, stabilize core pulling action.

Plastic part is back cover of a remote control, as shown in Figure 1. Plastic part material is PC/ABS CX7240, its physical properties and process parameters are shown in Table 1. Plastic part is both an appearance part and a functional part, which requires certain strength, abrasion resistance, dimensional stability and smooth outer surface. Appearance surface of plastic parts shall be mirror polished A3, colors are black, cool gray, white and brown, external dimensions are about 43 mm*57 mm*7 mm.

 

Figure 1 Plastic parts

 

Table 1 Physical properties and process parameters of PC/ABS CX7240

Through comprehensive evaluation of plastic part structure and demolding analysis, first determine opening direction of plastic part, that is, determine corresponding cavity and core of molded plastic part. Simulation software is used to analyze demolding angle. As shown in Figure 2, visible area of plastic part is set in positive direction of demolding. Since visible area is appearance surface of plastic part and needs to be mirror polished, it is not allowed to set push rods and gates. Surface of positive angle area is designed as cavity surface. Mold opening method and direction are shown in Figure 3. Appearance surface of plastic part is designed in cavity and inner surface area of plastic part is designed in core forming.

 

Figure 2 Analysis of demoulding angle of plastic parts

 

Figure 3 Mold opening direction
There is a 45° chute buckle on the inside of plastic part, as shown in Figure 4, this is functional area of plastic part, and a hinge will be installed inside to realize rotating flip. In order to ensure that back cover will not fall when opened, 45° inverted buckle is designed. Following describes mold structure for forming plastic part.

 

Figure 4 45° chute buckle

Gating system is an important part of injection molding [1], it is key to forming high-quality plastic parts, also an important factor affecting mold structure and manufacturing cost. Its type, size, feeding method and location play a decisive role in molding quality of plastic parts and choice of injection molding machine.
According to annual demand of plastic parts and drawing requirements, consider need to change colors on one mold to produce plastic parts of different colors to meet customer's needs for four colors of plastic parts. Mold design is a common runner with 2 cavities. Two-platen mold structure is adopted, and injection molding machine pressure is 750 kN.
After mold flow analysis, circular runner and latent point gate are comprehensively considered. Gating system is shown in Figure 5(a). After main runner feeds material, it enters circular runner, then enters latent point gate, and finally fills cavity. Main runner size length L is 60 mm, entrance diameter d is φ3.5 mm, large end diameter D is φ4.5 mm, main runner taper angle is α=1°, main runner structure size is shown in Figure 5(b) . Runner diameter is φ4.0 mm, length is 30.0 mm, and latent point gate is φ1.5 mm.

 

Figure 5 Gating system

Core-pulling mechanism of injection molds is commonly used to realize demolding of molded plastic part [2]. There is a 45° chute on inner side of plastic part that requires core-pulling mechanism to realize demolding. If dynamic mold side floating is used to directly pull core, undercut slope is 45°, dynamic mold floating is mold opening direction. Force analysis is shown in Figure 6, and various forces satisfy following formula [3].
Fw=Fc×sinß (1)
Fk=Fw×tanß (2)
In formula: ß——angle of inclined core pulling, (°); Fw——lateral force of inclined core pulling, N; Fc——axial force of inclined core pulling, N; Fk——mould opening force of diagonal core pulling, N.

 

Figure 6 Force analysis of 45° core-pulling
From equations (1) and (2), it can be seen that as ß increases, Fw and Fk increase, lateral force on diagonal core-pulling also increases, which affects strength and rigidity of diagonal core-pulling; on the contrary, if ß decreases, Fw and Fk decrease, lateral force experienced by diagonal core pulling is also reduced, which is beneficial to core pulling movement. At present, lateral force angle is 45°, which is not suitable for oblique core pulling, which is easy to cause jamming and self-locking phenomenon [4]. It is necessary to change mechanism to realize 45° chute buckle core pulling.
After analysis, core pulling of 45° chute buckle adopts a sliding block structure, opening and closing movement of inclined core pulling is driven by sliding forward and backward movement. Key is that 45° core pulling insert and slider seat are connected through T-shaped groove to realize conversion drive, as shown in Figure 7, force is decomposed so that normal direction of core-pulling of 45° chute buckle is 0° core-pulling, and lateral force is not on core-pulling insert of 45° chute buckle, generated lateral force is converted to slider seat.

 

Figure 7 T-slot
Slider and core-pulling parts are precision processed with a grinder, accuracy is controlled at 0.005 mm, oblique hole on core is precision processed by slow wire cutting, accuracy is controlled at 0.005 mm, slide groove on mold plate is processed by CNC precision milling, and accuracy is controlled at 0.01 mm. When ensuring machining accuracy of a single part, mutual fit clearance and mutual grinding of parts are also very important. Mating sliding area adopts grinding fit to ensure sliding clearance and smooth sliding.
Core-pulling parts of 45° chute buckle are required to have high hardness, good toughness and thermal stability. Therefore, ASSAB's Viking steel is selected. Reference value of strength test of material after heat treatment is shown in Table 2. Sample is a φ35 mm round bar. Sample is oil-quenched and tempered at (1 010±10) ℃ to different hardnesses. Finally, core-pulling parts have a heat treatment hardness of 57~58 HRC.

 

Table 2 Reference value of strength test after Viking heat treatment

Whether selection of mold parts materials is reasonable will directly affect service life of mold, and it is key to choose matching steel when mold structure is reasonable [5]. In order to meet stability and smoothness of core-pulling mechanism, design of oil groove must be considered. Appearance surface of plastic parts is mirror-finished and highly polished, fixed mold plate should be made of steel with good polishability and good corrosion resistance in order to meet requirements of drawings and production requirements. After analysis, mold parts materials are shown in Table 3.

 

Table 3 Mould parts materials

Figure 8 shows structure of precision injection mold for oblique core-pulling slider.

 

Figure 8 Injection mold structure
1. Fixed mold base plate 2. Fixed template 3. Fixed mold core 4. Wedge block 5. Slider insert 6. Slider seat 7. Slider limit screw 8. Slider wear plate 9. Core 10 Gate insert 11. Straight push rod 12. Movable template 13. Limit block 14. Push rod fixed plate 15. Push plate 16. Movable mold seat plate 17. Push plate guide column 18. Support column 19. Push block 20 . Screw 21. Screw 22. Guide post 23. Screw 24. Positioning ring 25. Sprue sleeve 26. Precision positioning block
(1) Fixed mold part. Fixed mold core 3 and sprue sleeve 25 are installed in fixed mold plate 2. Fixed mold seat plate 1 presses sprue sleeve 25 to fix it. Sprue sleeve 25 is positioned and fitted in fixed mold core 3, wedge block 4 is fixed in fixed mold plate 2. Wedge block 4 is not only an oblique block for driving sliding block to move, but also a locking block, which functions to lock sliding block so that it is closed in place, facilitating injection molding. Head of wedge block cooperates with movable mold plate 12 to form an interlocking structure to form a support, guide post 22 is fixed on fixed mold plate 2 to ensure guidance, positioning of mold opening and closing actions.
(2) Movable mold part. Core 9 is installed and fixed in movable mold plate 12, straight push rod 11 is installed on push rod fixing plate 14, push plate guide column 17 is fixed on movable mold base plate 16, push plate guide column 17 cooperates with push plate guide sleeve to guide, push rod fixing plate 14 and push plate 15 reciprocate to realize ejection of plastic part. Slider insert 5 is installed on slider base 6. Slider insert 5 and slider base 6 are connected by a T-shaped groove. When slider base 6 moves horizontally, slider insert 5 is driven by T-shaped groove to perform a 45° core-pulling motion. Slider base 8 is designed under slider base 6 to ensure smooth movement of slider and service life of slider base. Precision positioning block 26 is installed between fixed mold plate 2 and movable mold plate 12. After mold is closed, relative positions of fixed mold and the movable mold are accurately positioned to ensure that mold has same accuracy every time mold is closed, so that mold is stable and continues to form qualified plastic parts.

After injection and holding pressure is over, injection molding machine nozzle stops pouring, plastic part cools and solidifies. After plastic part cools, mold opens, movable mold part moves backward. First, main parting surface is opened, plastic part leaves cavity and stays on core, slider seat 6 and wedge block 4 rigidly touch, slider seat 6 is driven outward by lateral force generated by inclined surface and starts to draw. At the same time, because slider insert 5 and slider seat 6 are connected, driven by T-shaped groove, when slider is retracted horizontally, T-slot will decompose force so that slider insert 5 will be pulled in core 9 at an angle of 45°, slider base 6 will stop moving when it touches slider limit screw 7, completes core pulling action to realize demolding of plastic part. Ejector rod of injection molding machine pushes push block 19 to make push plate 15 and push rod fixed plate 14 push out, drive straight push rod 11 to complete ejection of plastic parts and runner aggregates. When limit block 13 on push rod fixing plate 14 touches bottom surface of movable mold plate 12, pushing action stops, manipulator grabs plastic part and places it on conveyor belt. Finally, pallets are loaded into box, and runner aggregates fall freely after pushing out.
When mold is closed, ejector rod of injection molding machine must be pulled back to push rod fixing plate 14 and push plate 15 through push block 19 to reset ejection system of mold, then close mold after reset action is completed to ensure that push rod does not collide with fixed mold core 3. Continue to close mold. When movable mold moves to a certain position, slider base 6 touches wedge block 4. Driven by inclined plane, slider base 6 is closed inward, at the same time, force is converted by T-slot, so that slider insert 5 rises 45° to close, finally mold is clamped in place. At this time, end face of head of wedge block 4 is attached to movable mold plate 12 to form an interlock, slider is reset to mold clamping state and finally positioned.

Injection mold adopts a T-shaped groove structure for force decomposition and conversion, so that 45° oblique undercut can be demolded smoothly. Through early analysis of plastic part structure and material, filling analysis is carried out with aid of simulation software, gating system is optimized to meet injection requirements at one time, avoiding adjustment of mold structure due to unreasonable design of gating system. Machining accuracy of mold is controlled at ±0.01 mm, parts of 45° oblique core-pulling mechanism are processed by a grinder to ensure that accuracy is about 0.005 mm. Production practice proved that 45° core-pulling slider injection mold has reliable structure, stable movement, color-changing production meets requirements, precision and appearance of plastic parts meet requirements of drawings, which is suitable for mass production.