Figure 1 shows a plastic screw in a car dashboard, which is a shaft-type plastic part. Screw material is ABSPA-757 with a shrinkage rate of 0.5% [4]. Outer circumferential surface of plastic part has a complex structure, mainly composed of 2 sections, big end is a revolving body, and there is a groove in the middle. Small end is a trapezoidal thread, which has high dimensional accuracy and no parting line on the surface. Thread demolding is a difficulty in mold design.

 

Figure 1 Plastic Screws

Mold is designed as a 4-cavity structure, using point gate injection, and feeding from end of plastic part to be formed. In order to ensure appearance quality of plastic parts, external thread adopts an automatic de-threading mechanism, and power source is motor. Large end of plastic part is demolded by a lateral core-pulling mechanism, and finally plastic part is pushed out by push rod. Mold structure is shown in Figure 2.

 

Figure 2 Mold structure
1. Fixed die base plate 2. Pusher plate 3. Fixed die insert 4. Fixed die core 5. Pull rod 6. Sprue sleeve 7. Limit pin 8. Fixed template 9. Small tie rod 10. Moving die insert Parts 11. Moving template 12. Guide nut fixing plate 13. Gear frame plate 14. Support plate 15. Return spring 16. Reset rod 17. Push plate 18. Push rod fixing plate 19. Push rod 20. Moving mold seat plate 21. Push plate guide post 22. Guide sleeve 23. Nylon plug

External thread automatic demolding mechanism is set in movable mold, driven by a motor, and drives threaded core to rotate through gear transmission.

 

Figure 3 External thread automatic demolding mechanism
24. Motor 25. Motor base 26. Gear 27. Bridge gear (idler) 1 28. Drive shaft 1 29. Bridge gear (idler) 2 30. Drive shaft 2 31. Push rod 32. Insert gear shaft 33. Guide nut 34. Anti-rotation pin 35. Moving model core
Design steps and design contents of gear transmission are as follows.
(1) Determine transmission ratio. Transmission ratio = number of teeth of driven wheel/number of teeth of driving wheel = speed of driving wheel/speed of driven wheel = diameter of indexing circle of driven wheel/diameter of indexing circle of driving wheel.
When determining number of gear teeth, transmission ratio must be determined first. Determination of transmission ratio is related to two factors, one is driving mode of gear, and the other is installation space of gear [6]. Gear on threaded core in injection mold is driven by a motor, and transmission is relatively small. Generally, 0.25≤i≤1 is used. There are three advantages: ①Reduce impact force generated by instant start of motor; ②Make mold structure compact and reduce space size; ③ Slow down rotation speed of thread core. When using "hydraulic cylinder + rack + taper teeth" or screw thread to drive threaded core, due to limitation of transmission stroke, transmission ratio should be larger, generally 1≤i≤4[7].
Mold adopts three-stage gear reduction transmission, among which bridge gear (idler gear) 27 is an innovative point in design of mold structure, which reduces instantaneous impact force of motor drive and noise of gear drive, and at the same time improves smoothness of threaded core demoulding.
For multi-stage reducers, distribution of transmission ratios at all levels not only directly affects bearing capacity and service life of reduction mechanism, but also affects its volume, quality and lubrication. Multi-stage transmission ratio is generally allocated according to following principles: ① Make transmission carrying capacity of each level roughly equal; ② Make size and mass of reduction mechanism smaller; ③ Make peripheral speed of gears at each level smaller; ④ When oil bath lubrication is used, make each level smaller. Difference in oil immersion depth of large gear of stage gear pair is small.
Transmission ratio of the three-stage gear transmission of mold is 1, 0.5 and 0.7 respectively.
(2) Determine gear module. Modulus is determined according to national standard GB/T 1357-2008, size of rack, size of gear and number of thread turns of plastic part. According to practical experience, modulus of transmission gear in automatic de-threading mechanism of injection mold is usually 1, 1.5, and 2 mm, and modulus m of mold is 1.5 mm.
(3) Determine number of gear teeth. Diameter of gear index circle = module * number of teeth, after module is determined, number of teeth of gear is determined by diameter of gear index circle. Diameter of gear index circle in turn depends on center distance and transmission ratio between gears determined during ranking.
When center distance is constant, the more teeth, the smoother transmission and the lower noise. However, if number of teeth is large, modulus is small, and thickness of teeth is also small, which reduces bending strength. In order to avoid interference, number of teeth is generally taken as Z ≥ 17, and number of teeth of thread core is as small as possible, but minimum is not less than 14 teeth.
In summary, considering size of molded plastic part and lateral core-pulling mechanism of mold, center distance between driving wheel and passive wheel of primary transmission is determined to be 60 mm, distance between driving wheel and passive wheel of secondary transmission is determined to be 60 mm. Center distance of three-stage transmission is 45 mm, center distance between driving wheel and driven wheel of three-stage transmission is 25.5 mm. Number of teeth of each gear from motor drive wheel is 40, 40, 20 and 14 respectively.
External thread adopts an automatic demoulding mechanism, which eliminates parting line left by conventional molding method on the surface of external thread, improves forming accuracy of thread, dimensional accuracy of molded plastic parts reaches MT3 level (GB/T 14486-2008), which meets design requirements.

Since external thread of molded plastic part needs to adopt an automatic demoulding mechanism, cavity is arranged longitudinally when mold is designed, and groove of big end of molded plastic part must be core-pulled sideways. Lateral core-pulling mechanism adopts structure of “inclined guide post + Hough slider”, that is, groove of plastic part to be formed is formed by two sliders in half, as shown in Figure 4. In addition to oblique guide column 40 and Hough slider 37 , lateral core pulling mechanism also includes a locking block 39 , a positioning screw 38 , a positioning spring 36 and an oblique guide column pressing block 41 .

 

Figure 4 Lateral core pulling mechanism
36. Positioning spring 37. Huff slider 38. Positioning screw 39. Locking block 40. Inclined guide post 41. Inclined guide post pressure block 42. Fixed die guide sleeve 43. Guide post 44. Moving die guide sleeve
For lateral core pulling of annular groove, core pulling distance is prone to errors. It should be noted that distance depends not only on depth of groove, but also on diameter of large and small ends of groove. Methods for determining minimum distance Smin of lateral core-pulling of annular groove include drawing method and calculation method.
Drawing method is shown in Figure 5. A straight line is drawn from upper busbar of small circle along core-pulling direction to intersect large circle, and length Smin of line segment is minimum distance of lateral core-pulling.

 

Figure 5. Drawing method to determine minimum distance of lateral core pulling
Calculation rule is to use formula: Smin=picture, where R is diameter of large circle at groove, mm; r is diameter of small circle at groove, mm.
Actual core-pulling distance of lateral slider S=Smin+(2～5) mm, take 5 mm.

Mold adopts point gate injection, melt enters cavity, there are 3 parting surfaces (see Figure 2 and Figure 4), parting surface I must be opened first, opening distance is the total height of gating system condensate plus safety distance 20-30 mm, this mold takes 90 mm; then parting surface II is opened, opening distance is 10 mm. Finally, parting surface III is opened, opening distance must ensure that molded plastic parts fall off safely. At the same time, it is also necessary to ensure convenience of spraying mold release agent. Due to small size of molded plastic parts, the latter is mainly considered, opening distance of parting surface III is 100 mm. In order to ensure mold opening sequence and mold opening distance, it is necessary to design a fixed-distance parting mechanism [11]. Using built-in fixed-distance parting mechanism, under action of nylon plug 23 and pulling rod 5, mold is opened successively from parting surface I, parting surface II and parting surface III, opening distance of parting surface I is controlled by small pull rod 9, opening distance of parting surface II is controlled by limit pin 7, and opening distance of parting surface III is controlled by limit pin 7, and mold opening distance of parting surface III is controlled by injection molding machine.

After injection molding of mold is completed, motor 24 drives gear 26, gear 26 drives bridge gear 27, bridge gear 27 drives bridge gear 29, and bridge gear 29 drives threaded core 32 to rotate. Threaded core 32 is rotated while retreating. When threaded core 32 is separated from plastic part, injection molding machine drives movable mold to open mold. Under action of fixed-distance parting mechanism, mold is first opened from parting surface I. System condensate is separated from plastic parts under action of pulling rod 5 . After that, mold is opened again from parting surface II, and pushing plate 2 forcibly pushes pouring system condensate away from pulling rod 5, and automatically falls off. Finally, mold is opened from parting surface III, oblique guide column 40 pushes Huff slider 37 away to realize lateral core pulling. Then push push rod 19 to push plastic part out of mold to complete one injection molding.
Before mold is closed, threaded core is driven to reset by motor. When mold is closed, Huff slider 37 is pushed back by inclined guide column 40 and finally locked by locking block 39, mold is then injected for next time.