Plastic part is a substrate for a famous brand printer, made of ABS with a shrinkage rate of 0.5%. Detailed structure is shown in Figure 1. Characteristics of this molded part are: (1) Maximum external dimensions are 680.00×290.00×123.50 (mm), which is a large plastic part. Mold casting system is one of design difficulties; (2) Appearance quality of plastic part is very high, and defects such as weld marks, flow lines and shrinkage dents are not allowed on the surface; (3) Structure of plastic part is complex, with as many as 10 lateral concave and convex structures, including both inner and outer core pulling, both movable mold core pulling and fixed mold core pulling. Lateral core pulling mechanism of mold is second design difficulty.

 

Figure 1 Printer baseboard parts diagram

Since projected area of plastic part on parting surface is large, in order to ensure molding quality, mold adopts a hot runner multi-point injection system. Lateral concave and convex structures of plastic part are formed by fixed mold inclined slider, movable mold inclined slider, lifter and lateral core pulling mechanism of "inclined guide column + slider". Because there is a push block under movable mold slider, mold must be designed with a push rod plate reset mechanism. Detailed structure of mold is shown in Figures 2 and 3. Maximum external dimensions of mold are: 1250×750×878 (mm), with a total weight of about 6 tons, which is a large injection mold.

 

1. Angle guide pin; 2. Spring; 3. Slider; 4. Angle guide pin fixing block; 5. Wedge block; 6. Fixed mold angled slider 1; 7, 8. T-shaped buckle pull blocks; 9. Fixed mold angled slider 2; 10. Fixed mold insert; 11. Slider; 12. Wear block; 13. Push block; 14. Angle guide pin fixing block; 15. Spring; 16. Angle guide pin; 17. Push rod; 18. Moving mold weight; 19. Stop block; 20. Push rod; 21. Stop pin; 22. Support pin; 23. Push block; 24. Push rod; 25. Guide sleeve; 26. Guide pin; 27. Return spring; 28. Return lever
Figure 2. Printer base plate mold structure

Molding parts of mold adopt an inlay structure, mainly consisting of a fixed mold insert 10 and a movable mold insert 18. Molding parts and mold plate adopt a transition fit of H7/m6. Since mold is a large injection mold, in order to facilitate assembly and disassembly, two wedge blocks are designed between insert and mold plate, see part 5 in Figure 2 (b), wedge angle is 5°, and wedge block material is S136H. According to customer requirements, molding part material is NAK80, which has excellent electromachining performance and excellent polishing performance.

Molded part has 10 lateral undercuts, and mold utilizes 10 lateral core pulling mechanisms. These include two inner core pulling mechanisms in fixed mold, six outer core pulling mechanisms in movable mold, one inner core pulling mechanism in movable mold, and one diagonal inner core pulling mechanism in movable mold. Lateral core pulling mechanism is one of the most important core mechanisms in this mold. Detailed structure is shown in Figures 2 and 3.
In figure, S1, S3, S5, S6, S7, and S8 are all outer core pulling mechanisms in movable mold, using an "inclined guide pin + slider" mechanism. Core pulling distance is 37mm, positioning components are stoppers 19 plus springs 2, 15, 38, and 54.

 

29. Fixed mold fixing plate; 30. Hot runner plate; 31. Secondary hot ejection nozzle; 32. Locating ring; 33. Primary hot ejection nozzle; 34. Frame plate; 35. Slider; 36. Push block; 37. Inclined guide pin fixing block; 38. Spring; 39. Wear block; 40. Inclined guide pin; 41. Slider seat; 42. Moving mold inclined slide block; 43. T-slot pull block; 44. Push rod; 45. Square iron; 46. Push rod fixing plate; 47. Push rod base; 48. Thin cylinder; 49. Inclined ejector base; 50. Inclined ejector; 51. Inclined ejector guide block; 52. Inclined guide pin; 53. Slider; 54. Spring; 55. Inclined guide pin fixing block; 56. Fixed mold A plate
Figure 3. Gating system and movable mold lateral core pulling mechanism
S2 and S4 both use an "inclined slider + T-clip" mechanism in fixed mold, consisting of inclined sliders 6 and 9 and T-clip 7 and 8, respectively. Undercut depth at S2 and S4 is 24.6mm, and core pulling distance is 27.6mm. During core pulling process, inclined slider and T-clip do not separate, eliminating need for stoppers.
Undercut S9, located inside movable mold, presents the greatest challenge in core pulling because its pulling direction faces inward and is not perpendicular to mold opening direction, but rather at a 46° angle. Furthermore, undercut's location in the center of mold further complicates design of core pulling mechanism. To simplify mold structure, and with client's approval, S9 utilizes a "cylinder + T-clip + inclined slider" structure, consisting of a slider base 41, a movable mold inclined slider 42, and a T-slot pull block 43. Core pulling distance of S9 is 20mm. Because cylinder is installed parallel to mold opening direction, while T-slot is perpendicular to core pulling direction, cylinder piston travel distance must be calculated using a graphical method, as shown in Figure 4.

 

Figure 4 Cylinder piston travel distance
S10 is an undercut on the inside of movable mold, requiring a core pulling distance of 25.5mm. To avoid interference with S9, this lateral core pulling mechanism utilizes an lifter 50 that slides on a base 49 mounted on push rod mounting plate 46. It is important to note that due to length of ejector, a guide block 51 is required to ensure stable and reliable angular sliding of ejector. Tilt angle of ejector 50 is designed to be 12°, which is within a safe range.

Molded part has a large projected area, making it difficult to fill with plastic melt. After consultation with customer, it was decided to utilize a four-point hot runner gating system. While this approach increases mold cost, it effectively ensures required mold quality. Mold gating system consists of a primary hot runner nozzle 33, a hot runner plate 30, and four secondary hot runner nozzles 31. Hot runner plate is heated by ten heating rods. See Figures 3 and 6 for hot runner plate structure.

 

Figure 6 Printer substrate injection mold hot runner plate

Plastic parts are flat parts. Mold cavity is shallow but area is large. Design requirements of temperature control system are: first, cooling must be balanced so that temperature of mold cavity surface is roughly equal. Otherwise, molded plastic parts are prone to defects such as warping, welding marks and poor filling. Second, cooling must be sufficient. Heat of plastic melt must be transferred to mold in a timely and continuous manner, and temperature of mold must be controlled at about 60℃ to ensure labor productivity of mold. To this end, 9 straight-through cooling water pipes are used on movable mold side, and 6 straight-through water pipes + water wells are used on fixed mold side. Diameter of water pipes is 10mm, and diameter of water wells is 10mm. See Figure 2 (a) and (b) for details.

Mold mainly adopts a combined demoulding mechanism of "push rod + push block", in which push rods include 12 push rods with a diameter of 12×450mm and 23 push rods with a diameter of 6×450mm. Push block is shown as parts 13, 23 and 36 in Figures 2 and 3. Push block is mainly used to push outer edge of plastic part, which is stable and reliable. However, since push block is located just below slider, if push block has not been fully reset when slider is reset, the two may collide, causing serious safety accidents. In order to prevent safety accidents, this mold is designed with a rocker-type push block reset mechanism, as shown in Figure 7. Mechanism consists of a reset push block 64, a rocker 66 and a rocker block 67.
Special attention should be paid here: although reset rod has a reset function, it must wait until mold A and B plates are completely closed to ensure that push block is fully reset. It does not have a reset function. Reset spring has a reset function, but reset spring will fatigue and fail, and cannot guarantee 100% reliability. According to Murphy's law "if it can happen, it will happen", mold must be designed with a reset mechanism for pusher fixed plate.

 

64. Long pusher block for reset; 65. Movable mold plate B; 66. Rocker; 67. Rocker block; 68. Movable mold fixed plate
Figure 7: Pusher fixed plate reset mechanism

(1) Printer substrate has high precision and high added value. By adopting pinpoint hot runner technology, mold has achieved MT3 level dimensional accuracy, tensile test of plastic parts has increased by 25 pounds, and molding cycle has been reduced from original 40 seconds to 36 seconds, which not only meets molding quality requirements of large plastic parts substrates, but also improves labor productivity of mold.
(2) By adopting hydraulic cylinder inclined inner core pulling technology, mold not only simplifies movable mold structure, but also greatly improves stability and safety of mold's lateral core pulling action. Since mold was put into production, there has not been a single failure caused by lateral core pulling mechanism.
(3) By adopting rocker type pre-reset mechanism technology, mold effectively ensures reliability and safety of return stroke of lifter and slider lateral core pulling mechanism.
In summary, mold structure is advanced and reasonable, production has been stable since its release, and all quality indicators of molded plastic parts have met or exceeded design requirements. It is a classic example of large, complex, precise and long-life injection mold design.