【Abstract】According to structural characteristics of plastic part, parting method and optimal design of plastic part are discussed. Reasonable design and layout can effectively avoid interference between multi-sloping roof structure, inserts and ejector pins. It also introduces disassembly of inserts, ejector mechanism design, cooling water circuit layout and mold working process.

Figure 1 shows upper cover plastic part of washing machine. Size of plastic part is 397.637 * 475.789 * 42.174mm. Material is ABS, and shrinkage rate is 0.3%~0.7%. Core cavity of moving and fixed mold adopts inlay type, and material of forming parts adopts 2738 steel. Plastic parts are appearance parts, and outer surface needs to be frosted. There are many internal buckles in plastic part. There are 31 lifters in the whole mold, and local bones are deep. Considering exhaust problem, it is necessary to remove inserts, plus ejector rods, screws, and waterways, which make interior of mold intricate and complicated. How to rationalize layout is key issue to be considered in design.

 

Figure 1 3D drawing of plastic parts
After completing patch materialization, drafting, and adjusting shrinkage in sequence, slope analysis is performed. According to position shown in Figure 2, surface of area A has a positive draft angle, and surface of area B has a negative draft angle. There is a dividing line between the two, there is a height difference between B area and bottom plane on right side. To better connect them together, you need to cut B area surface for step processing.

 

Figure 2 Slope analysis
In Figure 3, buckle position requires removal of lifter, but there is a bone position on the top. When plastic part is ejected, under action of shrinking and tightening force, there may be shoveling of glue. For this reason, place is in contact with lifter. Surface needs to be treated with glue. In Figure 4, there are also high and low steps on left side. Regardless of whether parting surface is designed above or below, cutting surface needs to be processed for step differences. At the same time, better connection with right bone parting surface should be considered.

 

Figure 3 Bone position oblique glue analysis

 

Figure 4 Analyze difference type of height and drop

(1) Parting surface processing with a drop and segmentation difference at corner.
By observing enlargement at K in Figure 5, we can see that if "bottom step surface" on the right is enlarged to create parting surface sheet body I, side elevation of right area will be cut as shown in the right figure; then, after extracting equal-slope curve in +Z direction, it is stretched along 45° direction to form pierced surface of sheet body II; then sheet body III is formed by expanding surface. Sheet body I divides original surface into two molding surfaces, a movable mold and a fixed mold, with opposite draft angles, forming a step difference.

 

Figure 5 Differential processing of equal slope curve segment
(2) If there is a drop, do a parting surface treatment where rubbing surface forms a step difference at plane.
As shown in Figure 6, bottom surface of right bone position is directly flat with left side through enlarged surface. Buckle here is top of lifter. Appropriately add 0.1mm of glue to avoid phenomenon of shoveling glue when ejecting. Drop of left step is 0.352mm, which extends directly from bottom surface through enlarged surface. Position of step is designed on the right side of arc angle, and it is inclined 3° to form a rubbing surface. When vertical surface is cut, a zone A and zone B are formed. Zone A face is defined as positive draft angle of front mold face, and zone B faces on both sides are defined as negative draft angle of back mold face. Then a step difference is formed between the two areas.

 

Figure 6 Differential design of height difference and bone connection segment

As shown in Figure 7, size of plastic part after adjusting shrinkage rate is 399.625 * 478.168 * 42.385mm. Taking into account large size of plastic part itself, lifter and ejector pin occupy part of position, size of tiger's mouth, sealing surface, layout of cooling water circuit and space for screw installation must also be considered. Therefore, length and width dimensions of core cavity after parting are expanded by 50mm on the basis of plastic part.

 

Figure 7 Core cavity structure

Observing top structure design 8a, we can see that buckle here is an inclined surface. If lifter is designed along inclined surface, it will form a rubbing surface that is not easy to process and mold. Therefore, step surface of lifter is wrapped up to make a flat surface to form a collision surface. In Figure 8b, there is a groove on the right side of bone. If lifter is wrapped in and pushed out, it cannot be displaced. If side of lifter is directly attached to side of bone, side of lifter will become a bevel. Upper end of lifter is small in size and lower end is large in size, cannot be ejected. For this purpose, glue treatment is carried out to ensure that both sides of lifter are flat. At the same time, it should be noted that width of all lifters must be integers. In Figure 8c, lifter wraps around half of surrounding bones. Purpose is that bones are deeper, which is not convenient for processing and is not conducive to exhaust. Top surface of lifter in Figure 8d has a stepped surface, it is easy to shovel glue when ejecting, and adding glue effectively avoids this problem. For all lifters, angle of lifter is designed to be larger on the premise of ensuring sufficient ejection space, so that ejection distance can be shortened.

 

Figure 8 lifter structure

Analyze structural characteristics of plastic parts, split special inserts except for inserts at positions such as insufficient strength and vulnerable parts. As shown in Figure 9a, depth of bone position at this position is 17.761mm, which is relatively deep, difficult to process, and exhaust problem cannot be solved. Insert will encapsulate bone position symmetrically, takes encapsulation position as basis to ensure that thickness of insert is an integer. In Figure 9b, this position is edge of bone position. Molten plastic is also easy to be trapped and filled or burnt during filling process, so insert is also covered with glue. In Fig. 9c, there are bone positions in 3 directions around insert, all of which need to be encapsulated. In design, it is only necessary to round size of insert as much as possible.

 

Figure 9 Split structure of insert

Since there are 31 lifters in mold ejection system, and there are many other related parts, a reasonable design sequence can reduce trouble of subsequent adjustments. As shown in Figure 10, after plastic part is divided, sloping top is split first. After mold base is called, sliding seat must be designed in time, movement between sloping tops must not conflict with each other; then necessary inserts were disassembled to solve problems of air trapping and processing, it was observed whether more ejector pins were needed to assist in ejection. Considering large size of mold, a standard distribution of 7 perforations is designed for balanced ejection, then a symmetrical limit post is designed to control ejection distance of 30mm. It should be noted that limit post should be close to perforation to reduce deformation of top pole panel and bottom plate. Layout of support heads can be arranged from center of symmetry to the two sides. Distance between waterway and other parts is controlled to be more than 4mm and evenly distributed. Finally, fastening screws of cavity and core can be evenly distributed in remaining space.

 

Figure 10 Ejection system layout design

Figure 11 shows layout of cooling water circuit. Movable mold part is affected by relevant parts of ejection system, most of waterways are designed in X direction to be the most reasonable. Due to large extruding block set in mold, water nozzles of the two water channels in the middle of movable mold part are designed in same direction. Fixed mold part is affected by hot runner, extruded block, and fixed mold inserts due to waterway, and waterway cannot be reasonably arranged in X direction. Therefore, waterway can only be designed along Y direction, which brings certain troubles to installation of mold, but it can better play a cooling role.

 

Figure 11 Waterway design of cooling system

(1) Mold structure is shown in Figure 12. When mold is opened, mold starts to separate at parting surface between moving and fixed mold plates, plastic part 19 is glued to surface of movable mold core under action of shrinking and packing force. Mold is opened along with movable mold part.
(2) When movable mold moves to a certain distance, 7 ejector rods on injection molding machine press on ejector pad 25, pushing ejector pad 25 and lifter 20, ejector 2, and reset rod 21. When components are pushed out, springs 23 and 29 are also compressed by 30mm, then plastic part 19 falls off movable mold.
(3) When re-closing mold, ejector system first resets under action of spring force. When movable and fixed formwork is attached, and reset rod 21 is pushed against fixed mold base, ejector backing plate 25 is firmly attached to trash nail to ensure accuracy of resetting of ejector system.
(4) Finally, injection molding system will perform a new round of injection, pressure holding, cooling, and mold opening.

 

Figure 12 Mould structure
1. Pad iron 2. Mandrel 3. Limiting column 4. Movable template 5. Guide column 6. Extrusion block 7. Fine positioning 8. Movable mold insert 9. Guide sleeve 10. Fixed template 11. Hot runner plate 12 .Fixed mold seat plate 13. Hexagon socket screw 14. Fixed mold insert 15. Positioning ring 16. Hot runner system 17. Fixed mold insert 18. Hot nozzle 19. Plastic part 20. lifter 21. Reset rod 22. Waterway 23, 29. Return spring 24. Mandrel fixing plate 25. Mandrel pad 26. Garbage nail 27. Support head 28. Spring guide post 30. Zhongtuo 31. Movable mold seat plate

Discusses several typical characteristics of plastic parts in terms of classification, and describes in detail design method of step difference. There are a large number of lifter structures in mold, and relevant details of splitting lifter and inserts are introduced, design sequence and details of part layout of movable mold are analyzed in detail. Part of structure and characteristics of mold have good reference significance.