Disc-shaped spiral ratchet is shown in Figure 1. It is a key part of commodity anti-theft device in supermarket. Material is polycarbonate-PC, wall thickness is relatively uniform (1~1.2mm), and 40 spiral ratchet teeth are evenly distributed at lower end. Tooth surface is a smooth spiral surface. Spiral ratchet teeth of plastic part are engaged with corresponding part tooth shape, and rotate in anti-theft device. When rotating clockwise, winding line can be tied to commodity. If there is no unlocker, it cannot be reversed, which plays an anti-theft role.

 

Figure 1 Disc spiral ratchet
Polycarbonate-PC is a colorless, transparent, amorphous thermoplastic material with good mechanical properties, but injection processability is poor and requires a higher injection pressure. Plastic parts are hollow rotating bodies with uniform wall thickness and no obvious shrinkage marks. If gate position is not properly selected or mold temperature is not appropriate, welding marks are likely to appear, affecting strength of molded plastic parts. Plastic parts have an undercut structure after injection molding and cannot be directly pushed out. Difficulty in mold design lies in what structure to use to realize plastic part demoulding and how to process spiral ratchet tooth insert. After analysis, a simple rotary demoulding structure is used to realize molding and demoulding of plastic parts, and spiral ratchet tooth insert is processed by grinding.

According to demand for plastic parts, from perspectives of economy and quality control, it is more appropriate to use a 4-cavity layout for mold, and mold cavity is arranged in a "田" shape, as shown in Figure 2. Appearance of plastic part is not allowed to have gate marks. It is possible to consider feeding material from inside of plastic part to be molded. Since mold layout is 4 cavities, runner cannot directly enter inside of plastic part. A point gate is used to pour from center of plastic part to be molded, then two latent gates are used to enter inner circle of plastic part to be molded. In this way, molded plastic part can be automatically cut off from gate condensate during demolding to achieve automated production. According to mold flow analysis and calculation, weld marks generated by two latent gates are not in stress-bearing parts of molded plastic part, and do not affect strength of molded plastic part. 40 spiral ratchet teeth at lower end of plastic part have a 15° undercut, which limits demolding of molded plastic part. If forced demolding is used, spiral ratchet teeth may be deformed; if mechanical transmission is used for demolding, mold structure is more complicated. In order to ensure molding quality of plastic parts and simplify mold structure, a simple and ingenious demolding method is developed according to characteristics of plastic parts, that is, reaction force when plastic part is pushed out is used to drive tooth-shaped insert to rotate, so that molded plastic part is ejected with a 15° undercut.

 

Figure 2 Cavity layout

Mold structure is shown in Figure 3. Stripper plate 2 and runner plate 3 are both sunk in mold plate, which can shorten pouring system as much as possible and reduce condensation of pouring system to reduce cost of plastic raw materials. It can also reduce mold exhaust burden and shorten molding cycle of plastic parts. Since PC plastic cools quickly and volume of plastic parts is small, cavity does not need to be cooled. Only a heating system is designed in mold to increase mold temperature before production. When mold is opened, PL2 parting surface has a nylon pin thermal expansion lock, so PL1 parting surface opens first, and under action of pull rod 20, point gate condensate is pulled off from cavity; when mold part moves to limit end of fixed distance pull rod 15, because clamping force of clamp is greater than force of runner to tighten pull rod 20, fixed distance pull rod 15 pulls runner plate 3 to remove runner condensate from pull rod; then runner plate 3 is limited by limit rod 16 in its movement stroke, and PL2 parting surface opens under action of fixed distance pull rod 15, completing mold opening and removal of runner condensate.

 

Figure 3 Mold structure
1. Fixed mold base plate 2. Stripper plate 3. Runner plate 4. Fixed plate mold 5. Pad 6. Moving mold plate 7. Moving mold pad 8. Push rod fixing plate 9. Push plate 10. Pad 11. Moving mold base plate 12. Push plate guide pin 13. Push plate guide sleeve 14. Locking sleeve 15. Fixed distance pull rod 16. Limit rod 17. Nylon pin 18. Stripper plate guide pin 19. Gate sleeve 20. Pull rod 21. Guide pin

After mold is opened, plastic part remains on movable mold gear ring and core. Under action of machine tool slider, push rod pushes plastic part upward for demolding. In process of pushing plastic part to demold, due to 15° undercut of ratchet teeth, forced demolding will cause molded plastic part to deform, so a ball clip rotary demolding structure is designed.
When demolding of plastic part is blocked, a lateral force will be generated at 15° angle. Lateral force acts on gear ring. If inserts are in sliding friction fit, a large lateral force is required to rotate core, which will cause plastic part to deform. Since balls are designed as supports on side walls and step surfaces of gear ring, movement between inserts becomes rolling friction (small resistance). When plastic part is pushed upward, core is fixed, and lateral force generated can push gear ring to rotate around core. When insert rotates to a certain angle, plastic part will be released from 15° undercut. Gate condensate and plastic part are automatically cut off and separated by core insert when being pushed out, realizing push-out and rotation demolding, as shown in Figures 4 and 5. During mold closing process, reset rod contacts retainer, pushing retainer and gear ring to move downward and reset to initial position.

 

Figure 4 Rotary demoulding structure

 

Figure 5 Spiral ratchet tooth insert

Spiral gear ring insert is a key part of mold. Hardness requirement is 60~64HRC, inner and outer circle size accuracy requirement is ±0.005mm, coaxiality requirement is within 0.005mm, upper end tooth surface roughness requirement is Ra0.4μm, and spiral surface requires continuity. Conventional processing technology: inner and outer circle turning to release allowance → milling side hole → heat treatment quenching/stress relief → inner and outer circle grinding machine processing → EDM tooth surface. Since disc-shaped spiral ratchet teeth are composed of a spiral scanning surface of axis, the entire part has a total of 40 teeth connected end to end, and spiral tooth surface can only be processed by EDM profiling. This processing method has high equipment requirements, high cost, long time consumption, and low precision. In order to solve problem of tooth profile processing, a grinding method was developed: semi-finished gear ring was fixed on a dividing plate (for 40 tooth divisions), dividing plate was fixed on a sine table and rotated 15° to ensure that tooth side surface was vertical, and grinding wheel was trimmed to same slope as tooth bottom surface. Each time grinding process completed a tooth profile, dividing plate was rotated 9° to process next tooth profile. In this way, 40 teeth could be ground continuously in one clamping. Ground spiral tooth surface (see Figure 5) was smooth and tooth surface shape was regular, which reduced processing cost.