In plastic parts, many industrial products and civilian products have internal and external thread designs. Solving problem of thread stripping of plastic parts is the key to this type of mold design. Generally speaking, there are two types of methods to deal with thread problem of plastic parts: one is to use method of forcible extraction when plastic parts and resin allow it; the other is to use plastic parts with deep thread, high resin strength and high thread precision requirements. parts, must adopt method of rotating out. In the rotary dethreading structure, there are two types: manual dethreading and automatic dethreading. Manual dethreading has low production efficiency, but mold structure is simple and suitable for small batch production; automatic dethreading has high efficiency and stable quality, and is suitable for mass production. In automatic rotary dethreading die structure, reliability, stability and practicability of structural design must be solved. Chain drive automatic thread removal mold structure described below, after long-term use, mold structure is stable and reliable, production efficiency is high, and product meets practical requirements. Now this mold structure is introduced as follows.

Product shown in Figure 1 is a lubricating oil bottle cap made of PP. Product features a rectangular thread, anti-counterfeiting ring and anti-rotation groove design. Product has high requirements on appearance and is in great demand. In addition, bottle cap is capped by an automatic filling line machine, so requirements for product size and thread accuracy are high. Structure of forced thread removal mold cannot meet product use requirements, and an automatic thread removal mold structure must be used.

 

According to analysis of plastic parts and user's production requirements, mold is designed as 8 cavities. Hot runner needle valve type point gate is used to feed material, and deceleration motor is used to complete automatic thread removal through chain transmission. Mold structure is shown in Figure 2.

 

Figure 2 Mold structure 1. Small sprocket 2. Chain 3. Motor 4. Motor bracket 5. Limit screw 6. Spring 7. Ejector 8. Water hole 9. Steel pipe 10. Sealing ring 11. Cooling core 12. Moving Template 13. Backing plate 14. Pad block 15. Large sprocket 16. Thrust bearing 17. Backing plate 18. Primary push plate 19. Anti-rotation ring 20. Threaded core 21. Cavity plate 22. Backing plate 23. Pad block 24. Fixed template 25. Positioning ring 26. Main channel 27. Splitter plate 28. Heat insulation pad 29. Hot nozzle 30. Inclined guide post 31. Huff block 32. Positioning ring 33. Secondary push plate 34. Pin 35. Screw 36. Tensioner wheel

 

Mold is first divided fromⅠ—Ⅰ surface, and at the same time, Huff block is pulled apart by inclined guide column to complete protruding of concave part of anti-counterfeiting ring of plastic part, and plastic part remains in movable mold part. Then motor starts to rotate, threaded core is driven to rotate through sprocket and chain. At the same time,Ⅱ-Ⅱ parting surface starts parting by spring force, relies on anti-rotation ring to prevent plastic part and threaded core from co-rotating. When screw thread is completely pulled out, ejector mechanism of injection machine moves, and second push plate is pushed by ejector rod, so that mold is divided from III-III surface, and demoulding of concave part of anti-counterfeiting ring of plastic part is completed, so that plastic part is demoulded from mold, and a production cycle is completed. It should be noted that pressure of spring should be moderate when Ⅱ-Ⅱ surface is parted. If pressure is too high, last buckle of thread will be damaged. If pressure is too small, parting surface will not be easy to open. It can be adjusted by improving manufacturing accuracy of moving parts and adjusting spring compression. 

Due to large production volume of plastic parts and high quality requirements, a hot runner needle valve point gate gating system was adopted in design. Because hot runner transports molten plastic into mold cavity by means of heating, heat insulation and temperature control, internal pressure loss of runner is small, melt fluidity is good, density is uniform, internal stress of plastic part is reduced, degree of deformation is greatly weakened, and dimensional stability is significantly improved. In addition, hot runner has no runner waste, which greatly reduces production cost, and needle valve nozzle automatically cuts off gate, which improves production efficiency. To sum up, for mass-produced plastic products, selection of hot runners is a very reasonable choice.

In order to improve production efficiency, prevent deformation of plastic parts, and prevent mold parts from being seized due to thermal expansion, cooling system of mold must be sufficiently reliable. Because threaded core is often in a rotating state and cannot be directly cooled, a cooling core is added to center of threaded core during mold design, and cooling water is introduced into core through a copper tube to form a circulation, cooling of threaded core is better completed. Cooling of fixed mold cavity adopts 8 straight-through water channels, and outside is connected with a hose to form a circulation for cooling of fixed mold.
3.4 Mold material selection. Due to large production volume of bottle cap mold, demoulding needs to be rotated, and mold has no special requirements for polishing, so mold material should be made of high wear resistance and hardenability materials. According to this, ASSAB’s CALMAX635 material was selected for key parts in mold design, and heat treatment hardness is 56-60HRC. After using it, effect is quite satisfactory.

Because there is no slippage in chain transmission movement, transmission size is relatively compact, does not require a large tension force, load acting on shaft is small, and efficiency is high, which is more suitable for transmission requirements in mold, chain, sprocket, and motor are easy to purchase in the market, do not require special processing, so chain transmission is adopted in transmission design.

Considering comprehensive factors of mold cavity size and structural design size, center distance between mold cavities should be more reasonable between 150mm. Through checking of sprocket, sprocket with chain pitch p=15.875 and number of teeth z1=23 is used in the design. Pitch circle diameter D of sprocket is: D=P/sin(180°/23)=15.875/ sin(180°/23)≈117mm, which can meet requirement of mold design center distance of 150mm. In order to increase number of meshing teeth of sprocket and make transmission more stable, two tension pulleys 36 were added during design, and use effect is good.

Thread can be removed by rotating plastic part 5 times. In order to ensure normal service life of mold, speed of thread core should not be too fast. Therefore, design is based on calculation of one rotation and demoulding every 5 seconds. Speed of sprocket should be 60r/min. Transmission ratio of sprocket is: i=n1/n2=z2/z1, if transmission ratio i≈0.5 is set, small sprocket z2≈11, n2≈120r/min, so speed of geared motor should be 120r/min about min.

 

Since bearing used in this mold not only plays role of centering, but also bears axial driving force of injection pressure on threaded core, so bearing must bear radial load and axial load at the same time when selecting bearing. Since radial thrust roller bearing can meet above conditions at the same time, this type of bearing is selected during design.

Among many thread-removing mold structures, by comparison, this design has compact structure, stable operation, and better design effects in terms of transmission and cooling. It is a typical automatic thread-removing mold structure that is worth promoting.