Figure 1 shows a plastic screw in dashboard of a car, which belongs to shaft type plastic part. Screw material is ABSPA-757, with a shrinkage rate of 0.5%. Outer circumferential surface structure of plastic part is relatively complex, mainly composed of 2 sections, large end is a rotating body, and there is a circle of grooves in the middle. Small end is a trapezoidal thread with high thread size accuracy. No parting line is allowed on the surface. Thread demoulding is a difficult point in mold design.

 

Figure 1 Plastic Screw

Mold is designed as a 4-cavity structure, using point gate injection, and feeding from the end face of plastic part to be molded. In order to ensure appearance quality of plastic part, external thread adopts an automatic thread stripping mechanism, power source is an electric motor, groove demoulding of large end of plastic part adopts 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 mold base plate 2. Push plate 3. Fixed mold insert 4. Fixed mold core 5. Pull rod 6. Gate sleeve 7. Limit pin 8. Fixed mold plate 9. Small pull rod 10. Moving mold insert 11. Moving mold plate 12. Guide nut fixing plate 13. Gear frame plate 14. Support plate 15. Reset spring 16. Reset rod 17. Push plate 18. Push rod fixing plate 19. Push rod 20. Moving mold base plate 21. Push plate guide column 22. Guide sleeve 23. Nylon plug

External thread automatic stripping mechanism is set in moving mold and driven by motor. Thread core is driven to rotate through gear transmission. Thread core rotates and retreats to separate from plastic part under guidance of guide nut, as shown in Figure 3.

 

Figure 3 External thread automatic demoulding mechanism
24. Motor 25. Motor base 26. Gear 27. Bridge gear (idler) 1 28. Transmission shaft 1 29. Bridge gear (idler) 2 30. Transmission shaft 2 31. Push rod 32. Insert gear shaft 33. Guide nut 34. Anti-rotation pin 35. Moving mold core
Design steps and design contents of gear transmission are as follows.
(1) Determine transmission ratio. Transmission ratio = number of teeth on driven gear/number of teeth on driving gear = driving gear speed/driven gear speed = driven gear pitch circle diameter/driving gear pitch circle diameter.
When determining number of gear teeth, transmission ratio must be determined first. Determination of transmission ratio is related to two factors: one is gear drive method, and the other is size of gear installation space. Gear on threaded core in injection mold is driven by an electric motor, and transmission ratio is relatively small, generally 0.25≤i≤1, which has three advantages: ① reducing impact force generated by instantaneous start of motor; ② making mold structure compact and reducing space size; ③ slowing down rotation speed of threaded core. When threaded core is driven by "hydraulic cylinder + rack + taper tooth" or rifle screw, transmission ratio should be larger due to travel limit, generally 1≤i≤4.
Mold adopts a three-stage gear reduction transmission, among which bridge gear (idler) 27 is an innovative point in mold structure design, which reduces instantaneous impact force of motor transmission and gear transmission noise, and at the same time improves smoothness of threaded core demolding.
For multi-stage reducers, distribution of each stage transmission ratio not only directly affects load-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 load capacity of each stage roughly equal; ② Make size and mass of reduction mechanism smaller; ③ Make circumferential speed of each gear smaller; ④ When using oil bath lubrication, make oil immersion depth of large gear of each gear pair smaller.
Transmission ratios of three-stage gear transmission of mold are 1, 0.5 and 0.7 respectively.
(2) Determine gear module. Module is determined according to national standard GB/T 1357-2008, size of rack, size of gear and number of plastic part threads. According to practical experience, module of transmission gear in automatic thread stripping mechanism of injection mold is usually 1, 1.5, and 2 mm. Module m of this mold is 1.5 mm.
(3) Determine number of gear teeth. Pitch circle diameter of gear = module * number of teeth. After module is determined, number of gear teeth is determined by pitch circle diameter of gear. Pitch circle diameter of gear depends on the center distance between gears determined during alignment and transmission ratio.
When center distance is constant, the more teeth there are, the smoother transmission and the lower noise. However, if number of teeth is large, modulus is small, and tooth thickness is also small, which reduces its bending strength. Therefore, under condition of meeting bending strength of gear, it is best to take a larger number of teeth and a smaller modulus. To avoid interference, number of teeth is generally 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 mold side core pulling mechanism, center distance between driving wheel and driven wheel of first-stage transmission is determined to be 60 mm, center distance between driving wheel and driven wheel of second-stage transmission is 45 mm, center distance between driving wheel and driven wheel of third-stage transmission is 25.5 mm. Starting from motor driving wheel, number of teeth of each gear is 40, 40, 20 and 14 respectively.
External thread adopts an automatic demoulding mechanism, which eliminates parting line left on the surface of external thread by conventional molding method, improves molding accuracy of thread, and dimensional accuracy of molded plastic part reaches MT3 level (GB/T 14486-2008), meeting design requirements.

Since external thread of molded plastic part needs to adopt an automatic demolding mechanism, cavity is arranged longitudinally during mold design, groove at large end of molded plastic part must be lateral core pulling. Lateral core pulling mechanism adopts "inclined guide pin + half slider" structure, that is, groove of molded plastic part is formed by two sliders in half, as shown in Figure 4. In addition to inclined guide pin 40 and half slider 37, lateral core pulling mechanism also includes a locking block 39, a positioning screw 38, a positioning spring 36 and an inclined guide pin pressure block 41.

 

Figure 4 Lateral core pulling mechanism
36. Positioning spring 37. Half slider 38. Positioning screw 39. Locking block 40. Inclined guide pin 41. Inclined guide pin pressure block 42. Fixed mold guide sleeve 43. Guide pin 44. Moving mold guide sleeve
For lateral core pulling of annular groove, core pulling distance is easy to make mistakes. It should be noted that distance depends not only on depth of groove, but also on large and small end diameters of groove. There are two methods for determining minimum distance Smin of lateral core pulling of annular groove: drawing method and calculation method.
Drawing method is shown in Figure 5. A straight line is drawn from upper generatrix of small circle along core pulling direction to intersect with large circle. 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 method is to use formula: Smin=, where R is diameter of large circle at groove, mm; r is diameter of small circle at groove, mm.
Actual core pulling distance S of lateral slider is Smin+(2~5) mm, and 5 mm is taken.

Mold adopts point gate injection, and melt enters cavity. There are 3 parting surfaces (see Figures 2 and 4). Parting surface I must be opened first. Opening distance is the total height of solidified material of casting system plus safety distance of 20~30 mm. This mold takes 90 mm; then parting surface II is opened, and opening distance is 10 mm. Finally, parting surface III is opened. Opening distance must ensure safe falling of molded plastic parts and convenience of spraying release agent. Since size of molded plastic parts is small, 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 of mold, it is necessary to design a fixed-distance parting mechanism. With built-in fixed-distance parting mechanism, under action of nylon plug 23 and pull rod 5, mold is opened from parting surface I, parting surface II and parting surface III in turn. Opening distance of parting surface I is controlled by small pull rod 9, mold opening distance of parting surface II is controlled by limit pin 7, and mold opening distance of parting surface III is controlled by injection molding machine.

After mold completes injection molding, 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. Under guidance of guide nut 33, threaded core 32 rotates and retreats. 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. At this time, casting system condensate is separated from plastic part under action of pull rod 5. After that, mold is opened from parting surface II, and push plate 2 forcibly pushes casting system condensate away from pull rod 5 and automatically falls off. Finally, mold is opened from parting surface III, and inclined guide column 40 pushes half slider 37 away to realize lateral core pulling. After lateral core pulling is completed, mold stops opening, and injection molding machine pushes push plate 17, and then pushes push rod 19 to push plastic part out of mold, completing one injection molding.
Before closing mold, threaded core is first driven by motor to reset. When closing mold, half slider 37 is pushed back by inclined guide column 40 and finally locked by locking block 39, and mold is then used for next injection molding.