Office equipment is ubiquitous in daily work, such as printers, copiers, shredders, laminators, etc. Shredder described in this article is widely used in Europe and America, and in domestic market, it is generally only used in special office settings. This shredder's outer shell plastic part, through close coordination of appearance design and structural optimization design, integrates multiple parts from previous similar equipment, making it a one-piece body shell. Its mold structure complexity is significantly increased, but it saves production costs (materials, time, labor for finished product assembly) and mold costs (multiple plastic molds are combined into one mold), enhancing product's competitiveness. Main innovation and design points of plastic part mold design are successful solution to problem of difficult demolding of plastic parts after merging parts by adopting multi-directional slider core pulling, lifter core pulling, as well as a fixed mold tunnel-type core pulling structure, a hydraulic cylinder-driven sequential core pulling design.

Figure 1 shows three-dimensional (3D) structural diagram of plastic part of shredder's outer shell. Outer contour of plastic part is a semi-closed U-shape, with bottom area connecting to a closed elliptical shape. Maximum dimensions are 316 mm * 412 mm * 576 mm, and single part weighs approximately 1850 g, classifying it as a large plastic part based on both size and weight. Plastic part is made of high-impact polystyrene (HIPS), which has good flowability, dimensional stability, high impact strength, and high rigidity. Compared to acrylonitrile-butadiene-styrene (ABS), it has a higher cost-performance ratio and is a commonly used alternative to ABS. Average wall thickness is 3.0 mm, with a maximum thickness deviation within 0.3 mm. Product wall thickness design is very uniform, meeting requirements of injection molding. Plastic part has eight undercuts, which are demolded using three slider core-pulling mechanisms and five lifter core-pulling mechanisms. Ejection is achieved through a straight ejector block mechanism in conjunction with lifter core-pulling mechanism. Mold is large and complex.

 

Figure 1: 3D structural diagram of plastic part

Outer shell of paper shredder is an exterior part and also serves as load-bearing shell of the entire product. Average wall thickness of main body is designed to be 3.0 mm, with a maximum deviation of no more than 0.3 mm. Internal rib thickness at exterior surface is designed to be 1.5 mm, strictly adhering to standards of plastic part design specifications to prevent rib shrinkage. Maximum thickness of ribs at non-exterior surfaces is 1.85 mm, necessary for structural strength and without affecting appearance of plastic part. The overall wall thickness design is reasonable. Given shape characteristics of plastic part, a single injection point is difficult to achieve uniform injection in mold design; therefore, this mold adopts a two-point injection method. One hot nozzle is designed as an open-type nozzle, as shown in Figure 2, to maximize filling efficiency of plastic part. Placed at labeling location, it does not affect appearance after labeling during finished product manufacturing. Another nozzle is placed above bottom glue area, allowing for rapid filling of bottom glue area of plastic part. This nozzle uses a pointed tip design, combined with a recessed design, to minimize damage to surface appearance. Selection of two different types of hot nozzles considers both needs of plastic part molding production and mold costs. Arrangement of two hot nozzles fully considers requirement of balanced glue injection, ensuring that plastic part is molded under relatively low injection pressure, avoiding large internal stresses during molding and preventing deformation.

 

Figure 2: 3D schematic diagram of gating system

Figure 3 is a 3D cross-sectional view of fixed mold tunnel slider hydraulic cylinder core-pulling mechanism. (a) and (b) are cross-sectional views from two directions. Purpose of designing this slider is to release undercut of heat dissipation hole without damaging appearance of plastic part. Core-pulling mechanism consists of a hydraulic cylinder 13, a hydraulic cylinder positioning base 14, a fixed mold core-pulling block 12, a hydraulic cylinder core-pulling connecting block 38, and a fixed mold core-pulling spade base 44. Its working principle is as follows: hydraulic cylinder 13, hydraulic cylinder core-pulling connecting block 38, fixed mold core-pulling spade base 44 are connected as a whole and fixed to plate A 3 via hydraulic cylinder fixing base 14. Fixed mold core-pulling spade base 44 and fixed mold core-pulling block 12 are equipped with a trapezoidal groove mating structure. Ejection and retraction actions of hydraulic cylinder 13 are controlled by signals from injection molding machine. Trapezoidal groove mating structure drives fixed mold core-pulling block 12 to perform core-pulling and resetting actions, completing smooth demolding of plastic part. Core-pulling action and mold opening and closing actions can be controlled by injection molding machine to determine sequence of actions.

 

3-A plate; 12- Mold core-pulling block; 13- Hydraulic cylinder; 14- Hydraulic cylinder base; 19- Plastic part; 38- Hydraulic cylinder core-pulling connecting block; 44- Shovel base.
Figure 3. 3D cross-sectional view of fixed mold tunnel slider cylinder core-pulling mechanism

Figure 4 is a 3D schematic diagram of moving mold slider A core-pulling mechanism and spring pin delay structure. As shown in Figures 4(a) and (b), slider A15 is powered by inclined pin 26 for core pulling, which is synchronized with mold opening. Inclined pin 26 is made of two SUJ2 high-carbon chromium bearing steels with a diameter of Φ35 mm, ensuring strength and lifespan of inclined pin 26 during operation. On plate B4, wear-resistant blocks 22 are added to bottom of slider A15, moving mating surface during operation, and side of slider A15, locking surface that fixes slider during shovel base operation. Wear-resistant blocks 22 are made of medium carbon steel and require heat treatment to a Rockwell hardness of HRC52-56 to extend service life of slider 15. Meanwhile, due to split design of wear-resistant block, maintenance efficiency is improved and maintenance costs are reduced in later production process. Slider A15 is fixed to B plate 4 by pressure plate 20. A guide block 23 is set in slider A15 to ensure that slider A15 can move more smoothly during core pulling process. Slider A15 is completely covered with plastic parts and has many ribs, which makes clamping force on slider A15 during core pulling very large and prone to deformation. To avoid this problem, a time-delay structure of spring pin 18 is specially designed on slider A15, similar to ejection function of ejector pin. Its working principle is: when mold opens, slider A15 begins to pull outward and separate from plastic part under action of inclined pin 26. Spring pin 18 temporarily ejects plastic part without separating due to time delay stroke of 27 mm set on shovel base. When distance of mold opening exceeds time delay stroke, spring pin 18 resets and retracts under action of spring 45, separates from plastic part, as shown in Figures 4(c) and (d). When mold is fully opened, slider A15 separates from inclined pin 26 and reaches distance of 70 mm limited by limiting block 27 under its own weight, completing core-pulling action. Limiting block 27 ensures relative position of slider A15 and original mold body with shovel base when mold is closed, ensuring smooth mold closing and preventing jamming or mold collision.

 

3-A plate, 4B plate, 9-moving mold core, 15-slider A, 18-slider A spring pin, 19-plastic part, 20-pressure plate, 22-wear-resistant block, 23-guide block, 26-slanted guide post, 27-limiting block, 46-spring pin delay plane, 47-spring
Figure 4 3D structural schematic diagram of slider A core-pulling mechanism + spring pin delay mechanism

Figure 5 shows 3D structural schematic diagram of slider B core-pulling mechanism. As can be seen from Figures 5(a) and (b), basic structure of slider A core-pulling mechanism consists of slider A41, hydraulic cylinder 13, hydraulic cylinder fixing and limiting plate 37, hydraulic cylinder core-pulling connecting block 38, guide block 23, pressure plate 20, and wear-resistant block 22. Its working principle is: hydraulic cylinder 13 is fixed on plate B4 by hydraulic cylinder fixing and limiting plate 37, while limiting core-pulling stroke of slider B41 to 55 mm. Then, slider B41 and hydraulic cylinder 13 are connected using hydraulic cylinder core-pulling connecting block 38. Pressure plate 20 fixes slider B41 on plate B 4, restricting its movement in hydraulic cylinder core-pulling direction. Two guide blocks 23 are provided in the middle of slider B41, their function being to ensure more precise fit and smoother core pulling. Wear-resistant blocks 22, similar to those on slider A15, are provided on important mating surfaces of slider B41 to ensure service life of slider B41. As shown in Figures 5(c) and (d): After mold opening, slider B41 performs core-pulling action under action of hydraulic cylinder 13 and maintains core-pulling state; before mold closing, slider B41 resets before mold closing, and original spade base 43 on fixed mold plate A 3 locks slider B41. Sequence of actions is controlled by injection molding machine signal.

 

4-B plate; 9-moving mold core; 13-oil cylinder; 14-oil cylinder fixing base; 19-plastic part; 20-pressure plate; 21-water channel; 22-wear-resistant block; 23-guide block; 37-oil cylinder fixing and limiting plate; 38-oil cylinder core pulling connecting block; 41-slider B.
Figure 5 3D structural schematic diagram of slider B core pulling mechanism

Figure 6 shows 3D structural schematic diagram of inclined core pulling. Figure 6(a) is distribution diagram of inclined core pulling position. It can be seen from figure that there are 5 inclined core pulling mechanisms in this mold. Figure 6(b) and (c) are 3D schematic diagrams of state after inclined core is ejected in 2 directions. It can be seen from Figure 6 that 5 inclined core pulling mechanisms are divided into two categories, ejecting plastic part in 2 directions. Inclined core L1 (32) and inclined core L5 (40) eject in same direction, are distributed on the left and right. Inclined core L2 (33), L3 (34) and L4 (39) eject in same direction and are distributed in the middle position of plastic part. Due to excessive length of lifter, structure is designed in two sections. Upper end of connecting rod 29 connects to lifter, lower end is fixed to ejector base plate 7 via connecting seat 30 and lifter seat 31. This split design facilitates maintenance and repair during later production processes. Lifter guide block 45 and connecting seat 30 are made of special graphite copper material. Graphite copper is a high-strength copper alloy with a low coefficient of friction, allowing it to operate normally at 300 ℃ without burning out, effectively improving stability of lifter. Figure 6(d) shows distribution of straight ejector mechanism. A total of 16 straight ejector mechanisms are designed, consisting of straight ejector blocks 24 and straight ejector rods 25, working in conjunction with 5 lifter core-pulling mechanisms to ensure smooth ejection of plastic part.

 

19 - Plastic part; 24 - Ejector block; 25 - Ejector rod; 29 - Connecting rod; 30 - Connecting seat; 31 - Lifter seat; 32 - Lifter L1; 33 - Lifter 12; 34 - Lifter L3; 39 - Lifter L4; 40 - Lifter L5; 45 - Lifter guide block.
Figure 6 3D schematic diagram of distribution of lifter core-pulling mechanism + straight ejector mechanism

Structure of mold venting system is indispensable in mold design. Quality of venting system structure design directly affects final molding quality of plastic part, especially for plastic parts with high appearance requirements. Figure 7 shows a 3D schematic diagram of venting system structure of this mold. On fixed mold cavity, a two-stage venting structure is installed on planar parting surface and sliding block parting surfaces on both sides. Second-stage venting groove is blue, 10 mm wide and 0.3 mm deep; first-stage venting groove is green, 4 mm wide and 0.015 mm deep. On moving mold core, auxiliary venting is achieved by fully utilizing clearance between lifter, ejector block, ejector pin and moving mold core. This ensures that gas generated by molten material can be effectively and completely discharged from mold cavity during filling. This excellent venting structure not only improves filling efficiency of plastic part but also significantly, effectively reduces phenomenon of trapped gas and burning during filling, minimizes weld lines, provides strong protection for appearance and molding quality of plastic part.

 

Figure 7. 3D structural diagram of exhaust system

As shown in Figure 8, 3D structural diagram of mold cooling system shows that fixed mold has 8 U-shaped water channels, distributed on outer side of fixed mold surface of plastic part, basically evenly distributed according to shape of plastic part; moving mold has 7 U-shaped water channels, with 4 water wells on each of middle 5 water channels, distributed on inner side of plastic part on moving mold core; sliders A and B are each designed with their own water channels to ensure that fixed mold, moving mold, sliders are all adequately and evenly cooled. Surface temperature difference of molded plastic part is small, deformation and warpage are effectively controlled, and molding cycle is 67 seconds, exceeding initial design estimate, providing a solid foundation for mass production of plastic part.

 

Figure 8. 3D structural diagram of mold cooling system

Based on structural characteristics of plastic part, this mold has a total of 5 lifter core-pulling mechanisms, which eject in two directions to remove 5 undercut structures on plastic part. 5 undercut structures are divided into two types, as shown in Figure 1(a) and (b) 3D schematic diagrams of plastic part's snap-fit structure; 1 hydraulic cylinder-driven fixed mold tunnel slider is used to remove side heat dissipation holes; 1 top-side hydraulic cylinder-driven moving mold slider and 1 bottom-side inclined guide pillar-driven moving mold slider. Two large sliders on the top and bottom sides are used to remove side undercuts. Mold layout is 1 mold, 1 cavity. Mold base uses Longji standard size of 850 mm * 1100 mm. Maximum external dimensions of mold base (length, width, and height) are 980 mm * 1353 mm * 906 mm, and weight is approximately 5700 kg, belonging to category of large molds. The overall structure of mold is shown in Figure 9.

 

1. Panel 2. Hot runner plate 3. Plate A 4. Plate B 5. Square iron 6. Ejector plate 7. Ejector base plate 8. Base plate 9. Moving mold core 10. Wedge block 11. Parting surface positioning block 12. Fixed mold core pulling block 13. Hydraulic cylinder 14. Hydraulic cylinder fixed base 15. Slider A 16. Guide sleeve 17. Guide post 18. Slider A spring pin 19. Plastic part 20. Pressure plate 21. Water channel 22. Wear-resistant block 23. Guide block 24. Straight ejector block 25. Straight ejector rod 26. Angled guide post 27. Limiting block 28. Center support 29. Connecting rod 30. Connecting seat 31. Lifter seat 32. Lifter L1 33. Lifter L2 34. Lifter 13 35. Return spring 36. Return pin 37. Hydraulic cylinder fixing and limiting plate 38 - Hydraulic cylinder core-pulling connecting block; 39 - Lifter L4, Lifter LS; 41 - Slider B; 42 - Hot runner system; 43 - Shovel base (retained from the original); 44 - Fixed mold core-pulling shovel base; 45 - Lifter guide block.
Figure 9 Mold assembly drawing

Mold working process: (1) Mold closing: Slider B first resets, closing mold, fixed mold tunnel slider resets, and mold closing is completed; (2) Plastic part completes first molding; (3) Mold opening: Fixed mold tunnel slider first performs core pulling, mold opens, slider performs core pulling, and all mold opening actions are completed; (4) Ejecting plastic part; (5) Removing part: Reset ejection system, completing one complete injection molding cycle. Repeating above actions can implement production tasks. The biggest problem encountered during first trial molding was difficulty in removing plastic part. Analysis revealed following: As shown in Figure 10, plastic part is a concave arc. Lifters L1 and L5 have a small undercut in ejection direction. During ejection, plastic part gets stuck on lifters L1 and L5 and moves with them, causing undercuts to remain and resulting in difficulty in removing part. Improvement strategy: Ensure plastic part does not move with lifters L1 and L5 during ejection. Specifically, add a rib to straight ejector to fix position of plastic part during ejection. Lifters L1 and L5 then forcefully eject part using deformation characteristics of plastic part. After mold was modified, part removal problem was solved. Both manual and robotic removal are now possible, and mold production runs smoothly.

 

Figure 10: Actual photo of trial molding

(1) Taking full advantage of injection molding machine's ability to individually control hydraulic cylinders, considering structural characteristics of plastic part, a fixed mold hydraulic cylinder tunnel slider core-pulling mechanism and a moving mold top-side hydraulic cylinder slider core-pulling mechanism were designed to solve problem of two undercut demolding issues;
(2) To address problem of high clamping force on the bottom-side slider, which easily leads to tearing and deformation during core pulling, a solution using a slider internal delay spring pin design was successfully implemented;
(3) Due to large volume and numerous undercuts of plastic part, in addition to designed 3 slider core-pulling mechanisms, mold also employs 5 lifter core-pulling mechanisms and adds a straight ejector mechanism, successfully solving problems of plastic part ejection and demolding;
(4) Excellent cooling water channel design and venting system design ensured excellent quality and production efficiency of plastic part during molding production.