Plastic part is a HM04.0-shaped sensor for interior of a certain car. Material is PA66+FD30. Three-dimensional structure is shown in Figure 1, and two-dimensional structure is shown in Figure 2. Plastic part has a complex structure, many inserts, and difficult molding. Through analysis, it can be seen that difficulty of plastic part molding is: during injection process, insert needs to be placed in mold and molded together with injection melt, and L-shaped insert needs to be placed in moving mold to cooperate with slider. Parallelism deviation of L-shaped insert relative to plastic part is required to be less than 0.02 mm, and periphery of molded plastic part is frosted. Focus needs to be on solving molding and positioning problems of insert in mold and injection melt.

 

Figure 1 Three-dimensional structure of plastic parts

 

Figure 2 Two-dimensional structure of plastic parts

Before designing mold, parting surface of plastic part to be molded must be determined to ensure that mold can be demolded smoothly after manufacturing. Parting surface is shown in Figure 3, which is separated from middle of plastic part to be molded.

 

Figure 3 Parting surface

Structure of molded plastic part is complex, core and cavity plate are difficult to process. Therefore, core and cavity plate adopt an insert-type combined structure. In order to facilitate processing of mold parts, multiple inserts are removed from core and cavity plate. After insert processing is completed, they are assembled, as shown in Figures 4 and 5.

 

Figure 4 Core insert installation
1. Cavity plate 2. Insert 3. Insert 4. Insert 5. Insert

 

Figure 5 Cavity plate insert installation
1. Core 2. Insert 3. Insert 4. Insert 5. Insert 6. Insert 7. Insert 8. Insert

Analyze core pulling position required for plastic part. Core pulling position can be pulled by a slider parallel to parting surface.

Side core pulling of molded plastic part needs to cooperate with L-shaped insert. Because L-shaped insert is slender and easy to deform, if insert is placed on core first, then slider is driven by traditional inclined guide column to close mold, there will be a risk of interference between side core pulling and insert, causing molded plastic part to be scrapped. Mold adopts a split movable core-pulling structure. Before closing mold, insert is manually placed in split movable slider insert, then placed in core before closing mold.
Slider side core-pulling split first reset mechanism is shown in Figure 6. Slider and side pull core are split and connected by spring buckles. Side pull core can be reset first and insert is matched, slider is reset later and connected to side pull core. In actual production, multiple side core-pulling mechanisms should be prepared for replacement, insert should be installed outside mold in advance to improve production efficiency.

 

Figure 6 Slider side core split type reset mechanism
1. Side core 2. Insert 3. Core 4. Slider 5. Push rod 6. Spring block mechanism

Side core split structure is shown in Figure 7, which consists of 5 inserts. Insert 2 is rotated 180°, assembled with insert 3 and placed in square groove of side core 1. Then assembled side core assembly is placed in side core 5 and limited by cylindrical pin 4. Side core assembly structure is shown in Figure 8.

 

Figure 7 Side core split structure
1. Side core 2. Insert 3. Insert 4. Cylindrical pin 5. Side core

 

Figure 8 Side core assembly structure
1. Insert 2. Slider assembly

Slider assembly is shown in Figure 9. Spring is placed in slider body, a spring is placed behind it and fixed with a baffle. Baffle has a 15° dovetail to prevent spring from ejecting baffle, there is a step behind buckle to prevent buckle position from deviating and causing side-pull core to be unable to connect normally.

 

Figure 9 Slider assembly
1. Slider body 2. Spring block 3. Spring 4. Baffle

After mold is opened, ejector under side-pull core will push side-pull core upward while plastic part is pushed out, then side-pull core with previously installed insert is replaced, as shown in Figure 10.

 

Figure 10 Replacement of side-pull core
A limit mechanism is set inside side-pull core to prevent insert from being too deep or too shallow when installing insert. Then side-pull core assembly with insert is installed on core to ensure that insert is smoothly inserted into core positioning square hole, as shown in Figure 11.

 

Figure 11 Side-pull core installation
After side-pull core is installed, slider is driven to reset by inclined guide column, rear inclined surface of side-pull core collides and squeezes with inclined surface of buckle, spring is compressed, and buckle is squeezed backward. After side-pull core is completely reset, spring rebounds to complete connection, as shown in Figure 12.

 

Figure 12 Slider reset

According to parting surface of plastic part and position of the best gate in mold flow analysis, a multi-point gate method is adopted, as shown in Figure 13. Casting system includes a common runner casting system and a hot runner casting system. In order to reduce production costs and design problems, common runner casting system is used to mold plastic part.

 

Figure 13 Casting system
Main runner diameter of casting system should be φ0.5~φ1 mm larger than nozzle hole diameter of injection molding machine, main runner taper angle is 3°, small end diameter d=φ1 mm, and inner surface roughness value is Ra0.4 μm. When designing runner, although circular cross-section has a higher thermal efficiency, in application of point gate pouring system, considering problem of automatic falling of pouring system condensate, runner cross-section is selected as "U" shape, with a width of 6 mm and a depth of 4 mm. In order to facilitate smooth demolding of gate condensate, demolding slope of runner is 5° on one side.

According to characteristics of plastic part, pouring method and actual output, mold adopts a 1-cavity structure, as shown in Figure 14. Mold is opened after injection, pressure holding and cooling. Under friction force of pull die buckle 21 and elastic force of spring 13, parting surface I opens first, runner condensate and plastic part are separated under pulling force of pull rod 19. Mold continues to open. Under limit of pull rod assembly 11, parting surface II opens, fixed mold base plate 1 and runner plate 2 open 5 mm, and runner condensate is peeled off from pull rod 19. Mold continues to open, movable and fixed molds open at parting surface III, inclined guide column 15 drives slider 37 to move outward, under action of spring 35 and elastic block 36, slider assembly 29-36 is pulled out 18 mm from molded plastic part to complete core pulling, and slider is positioned by positioning steel ball 8. After mold is opened, under action of injection molding machine ejector, push plate 10 pushes push rod 27 to push plastic part and slider assembly 29-36 out together, completing demolding of molded plastic part. When closing mold, reset rod 23 first drives ejection mechanism to reset under action of spring, insert is manually assembled into slider assembly 29~36 in advance, then insert and slider assembly are placed in moving mold core. Mold is closed, and injection molding machine pushes moving mold to move closer to fixed mold. Driven by inclined guide column 15, slider part moves inward. During movement, inclined surface of tail of core puller presses spring block 36 outward, slider assembly 29~36 is stuck in slider 37 to complete slider reset, and locked by inclined wedge 14. At the same time, moving and fixed molds are locked under locking force of injection molding machine slider. Mold is closed and enters next cycle of injection.

 

Figure 14 Mold structure
(a) Main view structure (b) Left view structure (c) Exploded view;
1. Fixed mold base plate 2. Runner plate 3. Guide pillar 4. Fixed mold plate 5. Moving mold plate 6. Pad 7. Moving mold base plate 8. Positioning steel ball 9. Push rod fixing plate 10. Push plate 11. Pull rod assembly 12. Cylindrical pin 13. Spring 14. Wedge 15. Inclined guide pillar 16. Insert 17. Insert 18. Gate sleeve 19. Pull rod 20. Cavity plate 21. Pull die buckle 22. Core 23. Reset rod 24. Insert 25. Insert 26. Insert 27. Push rod 28. Insert 29. Cylindrical pin 30. Core pull base 31. Side core 32. Core pull insert 33. Core pull insert 34. Baffle 35. Spring 36. Spring block 37. Slider