【Abstract】According to structural characteristics of plastic parts of automobile inner door handles, an integral injection mold structure with one mold and two cavities was designed. Mold structure optimization design of plastic part's inverted buckle is carried out through secondary core pulling mechanism of inclined guide column slider, inclined core pulling and fixed mold lifter. With design of circulating waterway and ejector rod, hot runner turns to latent glue feeding method to avoid manual cutting of gate and realize efficient automatic injection molding of plastic parts. Production practice proves that mold structure design is reasonable, work is stable, and quality of produced plastic parts meets requirements of use.

Modern automobile manufacturing has a large number of plastic parts as important parts, so mold design and injection molding technology of automobile plastic parts are very important. Automotive plastic parts generally have a relatively complex structure, including complex appearance and a large number of undercuts. It is necessary to ensure that performance of plastic parts and surface are clean and beautiful, and there is no looseness after assembly. Contours of matching parts should be consistent, excessive should be smooth. In design of injection molds for automotive plastic parts, design of cooling system is more complicated due to large size and irregular shape of plastic parts, difficulty of mold structure is mainly reflected in processing method of undercut. This article takes plastic part of automobile inner door handle as an example, analyzes structural characteristics of plastic part, carries out the overall design of mold structure, optimizes design of side extraction system and inclined roof system according to characteristics of inverted buckle.

Material of plastic parts is PA6+GF40% (polyamide 6+40% glass fiber). This material has good processability, good mechanical properties, heat resistance, creep resistance and fatigue resistance, low moisture absorption, but lacks abrasion resistance, is very suitable for production of automotive interior plastic parts. Shrinkage rate is 0.5%, melting temperature is 250℃ to 280℃, molding temperature is 80℃ to 90℃, and injection pressure is 140 to 180MPa. Appearance quality of plastic parts is required to be high, no welding marks, shrinkage marks, flashing, lack of material, deformation and other process defects that affect appearance are not allowed. Outer dimension of inner door handle is about 256.18×84.60×59.63mm, as shown in Figure 1a, plastic part volume is 96.5cm3 measured by UG8.5, weight is 140g, average wall thickness is 2.58mm, and thickest part is 5.77mm. Thickness analysis is shown in Figure 1b.

 

Figure 1 Structural analysis of plastic parts
a-overall structure size b-wall thickness analysis
Structure of plastic part is relatively complicated, and there are multiple undercut structures. As shown in Figure 2, K1～K3 are undercut grooves on the outside of plastic part, which are processed in the form of core pulling on slider side; K4～K5 are oblique undercut holes of plastic parts, and these undercuts are processed in the form of oblique core pulling; K6 is an inverted button groove in plastic part, and these inverted buttons are processed with a fixed mold lifter structure. Difficulty in design of mold lies in design of fixed-mold lifter mechanism and inclined core-pulling mechanism. Considering demand and cost of plastic parts, mold adopts a two-cavity structure. In order to improve production efficiency and simplify mold structure, multi-point injection molding is performed by using hot runner to latent injection molding.

 

Figure 2 Analysis of plastic parts undercut

In order not to affect appearance quality of plastic part and to ensure smooth demolding of plastic part, contour line of the largest projection surface of plastic part is used as parting line, and parting surface is designed based on this. Parting surface is shown in Figure 3. Cavity structure of 3D parting dynamic model is shown in Figure 4. Cavity and core adopt integral embedded structure, and material is 8407 mold steel. This mold steel has good mirror polishing, easy processing and heat treatment dimensional stability, is a commonly used steel for precision molds.

 

Figure 3 Parting surface design

 

Figure 4 Cavity core design
a——moving mold cavity b——fixed model core

There are 6 undercuts in plastic part, and different side core pulling mechanisms are used to deal with different situations of undercuts. Core pulling mechanism on each side will now be introduced in detail.

Sside K1 is an inverted buckle with an oblique downward angle of 45°. For its special structure, a secondary core pulling mechanism with an oblique guide post and slider is adopted, as shown in Figure 5. Oblique guide column 2 is fixed to fixed mold through wedge block 1, the first-level sliding block 3 and second-level sliding block 4 are matched with dovetail groove, then connected to movable mold through sliding block pressing bar 6 and sliding block pressing bar 9. When mold is opened, the first-stage slider 3 is driven by inclined guide column 2 to move back by 29 mm, movement distance is determined by wave ball spring 7. At the same time, under action of dovetail groove, secondary slider 4 moves obliquely downwards along slider bar 6 by 17.98mm and leaves undercut position to complete side core pulling action.

 

Figure 5 Secondary core pulling mechanism of inclined guide post and slider at K1
1. Wedge block 2. Inclined guide post 3. Primary slide 4. Secondary slide 5. Guide slide 6. Slider bead Ⅰ 7. Wave ball spring 8. Wear-resistant block 9. Slider bead Ⅱ
Undercuts at sides K2 and K3 are diagonally upward by 30°. For its special structure, a secondary core-pulling mechanism with an inclined guide post and slider is also used, as shown in Figure 6. Inclined guide post 3 is fixed to fixed mold through wedge block 2, primary slider 1 and secondary slider 4 are matched with dovetail groove, then connected to movable die through slider pressing bar 6 and slider pressing bar 7. When mold is opened, the first-stage slider 1 is driven by inclined guide post 3 to move back 29mm, and movement distance is determined by wave ball spring 8. At the same time, under action of dovetail groove and slider spring 5, secondary slider 4 moves diagonally upwards along guide slider 6 by 25.11mm from undercut position to complete side core pulling action.

 

Figure 6 Secondary core pulling mechanism diagram of inclined guide post slider at K2 and K3
1. Primary slide block 2. Wedge block 3. Inclined guide post 4. Secondary slide block 5. Slider spring 6. Slider pressure strip Ⅰ 7. Slider pressure strip Ⅱ 8. Ball spring

Plastic part K6 has an inverted buckle on inside of plastic part with a depth of 4.57mm. Fixed mold lifter core pulling mechanism is used for demolding. Complete set of fixed mold lifter mechanism is installed in inner groove of fixed mold bearing plate and connected with fixed mold side, as shown in Figure 7. When mold is opened, under action of spring 11, fixed mold lifter mechanism springs 40mm, lifter lateral core pulls 5.02mm, and it leaves undercut position to complete side core pulling action.

 

Figure 7 K6 fixed mold lifter core pulling mechanism diagram
1. Heat insulation board 2. Fixed mold fixing plate 3. Fixed mold bearing plate 4. lifter and bottom plate 5. lifter panel 6. lifter seat 7. Fixed mold plate 8. Fixed mold cavity 9. lifter 10. Spring sleeve 11. Spring 12. Spring pressure plate

K4 and K5 of plastic part are oblique holes with a depth of 11.31mm and an inclination angle of 28°. Oblique drawing mechanism is used for demolding, as shown in Figure 8. After mold is opened and before being pushed out, hydraulic cylinder 7 pulls oblique pumping plate 6 back by 25mm, and drives oblique pumping seat 4 to move together. Under action of T-slot, oblique pumping insert 3 moves obliquely downwards by 17.58mm, then exits oblique hole to complete core pulling action.

 

Figure 8 K4, K5 oblique core pulling mechanism diagram
1. Movable mold plate 2. Movable mold cavity 3. Diagonal drawing insert 4. Diagonal drawing seat 5. Guide post and guide sleeve 6. Inclined pumping plate 7. Hydraulic cylinder

Gating system adopts hot runner to cold runner and latent gate glue feeding method. Gating system is shown in Figure 9. In order to shorten molding cycle and save raw materials, sprue adopts a large nozzle hot nozzle, diameter of hot nozzle is 35mm, nozzle spherical radius is SR40mm, and diameter of sprue gate is ϕ2mm. Runner adopts a U-shaped cross-section runner, runner width is 10mm, depth is 8mm, and demoulding angle is 5°. Gate adopts 3-point latent injection, and gate diameter is ϕ2mm.

 

Figure 9 Design drawing of gating system
Structure and shape of plastic parts are more complicated. In order to shorten production cycle and improve production efficiency, cooling water channels are set up in fixed mold, movable mold and slider to ensure uniform cooling. Cooling system is shown in Figure 10, temperature difference between inlet and outlet of cooling water is less than 2℃.

 

Figure 10 Cooling system design drawing
Inclination of plastic part is relatively large and demolding is not difficult, so mold can be smoothly pushed out by using only 4 dome rods. There are two dome rods with ϕ12mm and two with ϕ6mm. Distribution of top rods is shown in Figure 11.

 

Figure 11 Launch design
Whether exhaust system is designed reasonably or not will have a greater impact on quality of plastic parts. Cavitation analysis of plastic parts is carried out by MoldFlow software, and analysis results are shown in Figure 12. It can be seen from Figure 12a that there is air accumulation around plastic part and some corners. In order to solve cavitation problem, an exhaust groove is set around plastic part, movable mold insert and ejector pin are used for exhaust, as shown in Figure 12b.

 

Figure 12 Exhaust system design drawing
a——cavitation analysis b——exhaust system

Mold structure is shown in Figure 13, and mold base adopts non-standard mold base CT7550-A145-B185-C135. Mold cavity is arranged as 2 cavities. Mold working process is as follows:
(1) After injection molding is completed, mold is opened, and fixed mold lifter mechanism pulls core under action of lifter spring to complete demolding at K6. At the same time, slider seat moves sideways under action of inclined guide post, T-slot is used to drive slider to complete diagonal core pulling and demolding at K1, K2, and K3.
(2) After mold opening is completed, hydraulic cylinder provides power to drive oblique pumping plate and oblique pumping seat to complete oblique core-pulling demoulding at K4 and K5.
(3) After diagonal drawing is completed, ejector pin of injection molding machine pushes ejector plate of mold to push ejector pin to push plastic part out of cavity to complete demolding of plastic part.
(4) Reset. Push-out mechanism is first reset under action of reset rod and reset spring, then hydraulic cylinder is used to complete oblique pumping reset. Fixed mold reset rod resets lifter mechanism, and at the same time, wedge block pushes slider mechanism back to original position, and mold is completely closed.

 

Figure 13 Mould structure diagram
1. Press block 2. Positioning ring 3. Fixed mold positioning pin 4. Large nozzle hot nozzle 5. Inclined guide column I 6. Slide block seat I 7. Wear block I 8. Side lock 9. Slide block I 10. Sliding Block spring 11. Reset lever 12. Oblique pumping 13. Return spring 14. Oblique pumping seat 15. Oblique pumping guide column 16. Oblique pumping push plate 17. Zhongtuo 18. Hydraulic cylinder 19. lifter and lower push plate 20. Oblique Top push plate 21. lifter 22. Fixed model cavity 23. Slider Ⅱ 24. Wedge block 25. Inclined guide column Ⅱ 26. Wear-resistant block Ⅱ 27. Slider seat Ⅱ 28. Long water nozzle 29. Wear-resistant Block Ⅲ 30. Movable model core 31. Support column 32. Fixed mold heat insulation plate 33. Fixed mold seat plate 34. Fixed mold backing plate 35. Fixed mold center support 36. Fixed template 37. Guide column 38. Moving template 39 .Square iron 40. Ejector fixed plate 41. Ejector backing plate 42. Spring compression block 43 lifter spring 44. Spring sleeve 45. Fixed mold reset rod 46. Balance block 47. Ejector 48. Movable mold seat plate 49. Movable mold insulation board

Aiming at structure of car's inner door handle, side hole core pulling mechanism of plastic part is designed as a secondary core pulling mechanism of inclined guide column and slider. Buckle is demolded by a fixed-mold slider mechanism. It also uses hot runner to cold runner and latent gate glue feeding method to realize automated production of mold. Mold is reliable in operation,  molded plastic parts meet requirements of use, which can provide a reference for mold design of similar structural plastic parts.