【Abstract】According to structural characteristics of thin and deep wall of plastic part, a hydraulic secondary core pulling structure is designed to avoid damage to inner wall of plastic part when core is pulled. Fixed mold oblique slider push core is designed, which solves problem of demolding oblique hole on the top outer surface of plastic part. Bottom surface of plastic part is pulled out through core pulling of inclined guide post of movable mold; through inclined top core pulling of movable mold, demolding of inner undercut of plastic part is solved, parting surface design of plastic part and design of molded part are explained.

This plastic part is an assembly part and requires high dimensional accuracy. Material is: ABS+PC, with a shrinkage rate of 0.5%. ABS+PC has higher impact strength, hardness, better overall mechanical properties, and higher molding temperature [1], at 230℃~270℃. Maximum size of plastic part is 93*61*26mm (see Figure 1), and wall thickness is 1.9mm. There is a 20mm long undercut at the front end A, and wall thickness here is thinner than other places. Wall thickness here is only 1mm, which is difficult to demold and fill, so there must be a larger injection pressure and pressure holding time during injection molding. There are multiple undercuts at B, C and D on the outer surface of plastic part, making it difficult to demold. These undercut molds must be designed with a core-pulling mechanism before they can be demolded.

Parting surface is designed at the largest contour of plastic part, otherwise it will be difficult to peel off. Parting surface design should be conducive to mold processing and exhaust. Parting surface of this plastic part is a curved parting surface. When curved parting surface is injected, side pressure of mold is relatively large, so four corners of parting surface increase secondary positioning of convex cone surface.

 

Picture Figure 1 Plastic parts drawing
Top of bump is matched to avoid gap, so that movable mold core and fixed mold core can be tightly combined to prevent flashing. Top of bump is designed as a flat surface as a benchmark for mold maintenance and processing. Since there are slider core pullers at the front and side undercuts of plastic part, there is a step on parting surface here. Parting surface of curved surface of plastic part should extend a certain distance along direction of curved surface at the maximum contour for better sealing, and a straight flat parting surface should be added to the end to facilitate processing. Figure 2 shows parting surface of plastic parts.

 

Picture 2 Parting surface of plastic parts

Gating system includes main runner, runner and gate. This plastic part is arranged in one mold and two parts, symmetrically arranged, and glue is balanced. Due to high appearance requirements and no gate marks, plastic parts adopt submersible thimble-type submersible gates, inclined hole is processed from the end of runner. Inclined hole is connected with submersible nail 5 (see Figure 3), a part of head of submersible roof nail 5 is ground off. Secondary gate and auxiliary runner are designed on the head of submerged roof, secondary gate part is integrated with product. After being separated from movable mold, gate is manually removed. A pull rod 4 is designed at the end of sprue, head of pull rod 4 is designed as a Z-shaped inverted buckle. When mold is opened, pull rod 4 pulls sprue out of fixed mold and assists in exhaust and ejection.

Because there is a 20mm long undercut at the front end A of plastic part (see Figure 1), wall here is very thin, it is easy to damage inner wall when it is pulled out at a time, so hydraulic secondary core pulling structure is used here (see Figure 3) .

 

Figure 3 Mold assembly drawing
1. Hydraulic core pulling guide post 2. Fixed plate 3. Screw 4. Pull rod 5. Submerged top nail 6. Inclined top seat 7. Inclined top 8. Push rod 9. Pushing fixed plate 10. Backing plate 11. Moving Mould 12. Moving model core 13. Outer core 14. Pressure plate 15. Inner core 16. Tie rod connecting block 17. Tie rod 18. Hydraulic cylinder fixing plate 19. Hydraulic cylinder 20. Inner core connecting block 21. Fixed model core 22 Pressure block 23. Oblique slider 24. Oblique pin 25. Inclined slider insert 26. Fixed mold 27. Positioning ring 28. Gate sleeve 29. Positioning screw 30. Spring 31. Fixed mold seat plate 32. Guide post 33 .Guide sleeve 34. Guide sleeve 35. Press block 36. Small insert 37. Moving mold slider 38. Inclined wedge 39. Inclined guide post 40. Positioning pin 41. Spring 42. Reset rod 43. Compression block 44. Push plate
Inner core 15 is fixed in inner core connecting block 20 by steps, inner core 15 can slide in outer core 13, pressure plate 14 is connected to outer core 13 by screws, tie rod 17 is fixed in tie rod connecting block 16 through step. Tie rod connecting block 16 and inner core connecting block 20 are fixed together by screws, and are guided by guide rod 1 (see FIG. 3 for guide rod 1). Working principle: When movable mold 11 is separated from fixed mold 26, hydraulic cylinder tie rod 17 drives inner core 15 to slide in inner hole of outer core 13 by a distance of L 1 to draw core inward through tie rod connecting block 16 and inner core connecting block 20, then step of inner core 15 touches pressure plate 14, and drives outer core 13 to pull core L 2 distance, core pulling distance must satisfy following formula:
L 3 >L 1 +L 2
Where L 3 — —total core-pulling distance of hydraulic cylinder, mm
L 2 — —Inner core 15 first core pulling distance, mm
L 1 — —Outer core 13 core-pulling distance, mm

There is an oblique square hole with a width of 5mm at outer surface B of top of plastic part (see Figure 1). An undercut is formed here. Core must be pulled before movable mold 11 and fixed mold 26 are separated. Therefore, fixed mold oblique slider core is designed (see Figure 3, 4), inclined sliding block 23 is pressed by pressing block 22, can slide in groove formed by pressing block 22 and fixed mold 26. Oblique pin 24 and compression block 43 are fixed in fixed mold seat plate 31, oblique slider insert 25 is fixed in oblique slider 23 by screws. Angle between inclined surface of pressing block 43 and mold opening direction is about 2° greater than angle between inclined pin and mold opening direction, so that when mold is opened, pressing block 43 is first separated from inclined sliding block 23, then inclined sliding block 23 is driven by inclined pin 24 to slide to complete core pulling. When mold is closed, pressing block 43 presses inclined sliding block 23 to prevent loosening and realize resetting. Figure 4 is a three-dimensional view of core-pulling of fixed-mold oblique slider.

 

Figure 4 Three-dimensional drawing of core-pulling of fixed-mold oblique slider
22. Clamping block 23. Inclined slider 24. Inclined pin 43. Clamping block

There are 1.8mm undercuts at C on both sides of plastic part (see Figure 1). Since undercut is under parting surface, it is designed to pull core of movable mold oblique guide column; small insert 36 of formed undercut is embedded in movable mold slider 37, and head of small insert 36 has a 3° inclination, which is convenient for sealing and sliding. Movable mold slider 37 is pressed by pressure block 35 and can slide in guide chute formed by pressure block 35 and movable mold 11. When mold is opened, it is driven by inclined guide post 39 to realize core pulling, which is positioned by positioning pin 40, is pressed and reset by inclined wedge 38 when mold is closed.

There is a 1.7mm undercut at D inside plastic part (see Figure 1). Here, it can be designed as a movable die with an inclined top core. Inclined roof 7 is fixed in inclined roof seat 6 by screws, and bottom of inclined roof seat 6 is designed with a step. Through steps, inclined roof seat 6 can slide in push rod fixing plate 9. Angle of inclined top can be designed to be 5°, movable model core 12 is designed with an inclined hole to form a sliding fit with inclined top, and inclined hole can be processed by wire cutting, as shown in FIG. 3.

Design of cooling system is very important to molding quality and cost, cooling effect and cooling uniformity of mold must be considered when designing mold. In order to improve cooling efficiency, two sets of symmetrical circulating water channels are set up in both moving and fixed molds. Water pipe has a diameter of ϕ 8mm, enters movable mold core 12 and fixed mold core 21 through movable and fixed molds respectively. Under premise of ensuring that it does not interfere with push rod, insert, slider, and strength is sufficient, cooling water path should surround plastic part as much as possible, close to plastic part, and achieve sufficient cooling. And if necessary, set up a leak-proof rubber ring to prevent water leakage (see Figure 3 for details).

Molded parts are mold parts that determine shape and size of plastic parts, are core part of mold design and processing. Molded part of plastic part is composed of movable mold core 12, fixed mold core 21, inclined top 7, outer core 13, inner core 15, inclined slider insert 25, and small insert 36 (see Figure 3). Molded part must have good wear resistance and fatigue resistance, high mechanical strength and surface hardness. Due to high quality requirements of this plastic part, NAK80 is selected as mold material for molding part design. In order to facilitate maintenance and processing, molded parts should be made as inserts as much as possible. In order to facilitate demolding, molded parts must have a certain degree of demolding inclination. When designing parting surface of molded part, sharp corners and sharp edges should be avoided to ensure reliable sealing. Figure 5 shows core diagram of dynamic model of molded part.

 

Figure 5 Core diagram of the dynamic model of the molded part

This plastic part ejection system is composed of inclined roof 7, submersible roof nail 5, push rod 8, reset rod 42, and push plate 44. Since inner surface of plastic part is an arc surface, submersible top nail 5 and push rod 8 must be designed to prevent rotation, ejection design must be balanced; inclined top 7 and submersible top nail 5 may not be equipped with push rods near them. When mold is clamped, reset rod 42 first hits fixed mold 26 to perform a reset function. When mold is opened, pull rod 4 can pull out pouring system to movable mold, eject pouring system from movable mold during ejection (see Figure 3 for details).

After mold is closed, plastic melt enters mold cavity through pouring system, mold opens after pressure is maintained and cooled. Due to action of spring 30, mold is first opened at PL1, that is, separated from fixed mold base mold 31 and fixed mold 26. Under action of inclined pin 24, inclined slider 23 drives oblique slider insert 25 to complete core-pulling of oblique hole on upper surface of plastic part; after separating H distance, positioning screw 29 pulls fixed mold 26 and no longer moves, then separates at PL2, that is, movable mold 11 and fixed mold 26 begin to separate, inclined guide post 39 starts to drive movable mold slider 37 to complete core pulling; after fixed mold 26 no longer presses pressure plate 14, due to action of hydraulic cylinder pull rod 17, hydraulic core pulling starts; when ejecting, ejector rod of injection molding machine drives push plate 44 and push rod fixing plate 9 moves forward, then leads inclined top 7 to move diagonally to complete core pulling. At the same time, push rod 8, submerged nail 5, pull rod 4 push plastic part and gating system out of movable mold.

When inner wall of plastic part has a long and thin undercut, mold is designed with secondary hydraulic core pulling to solve problem of damage to inner wall during core pulling; Upper oblique hole on parting surface of plastic part can be designed as a fixed mold oblique slider core pull, fixed mold oblique slider core pull can be designed as a secondary parting. Fixed mold and fixed mold base plate must be separated first, first complete oblique hole core-pulling in fixed mold, then separate movable mold and fixed mold. Undercut under parting surface of plastic part can be designed to pull core of movable mold with inclined guide column and movable mold with inclined top. Structure of this plastic part is complex, core-pulling design is reasonable, and actual production runs well.