Design of a typical automatic thread-removing injection mold

Time:2020-11-09 15:59:42 / Popularity: / Source:

1 Introduction

In plastic parts, many industrial products and civilian products have internal and external thread designs. Solving problem of thread release of plastic parts is the key to such mold design. Generally speaking, there are two kinds of methods to deal with thread problem of plastic parts: one is forced release method when plastic part and resin are allowed; the other is plastic with deep thread, high resin strength and high thread precision requirements, which must be rotated out. In rotating threading structure, there are two types: manual threading and automatic threading. Manual threading has low production efficiency, but mold structure is simple, suitable for small batch production; automatic threading has high efficiency and stable quality, which is suitable for mass production. In the structure of auto-rotating thread-removing mold, reliability, stability and practicability of structure design must be solved. Chain drive automatic threading mold structure described below, after long-term use, mold structure is stable and reliable, production efficiency is high, and product meets practical requirements. Mold structure is now introduced as follows.

2. Analysis of plastic parts

Product shown in Figure 1 is a lubricating oil bottle cap, made of PP. Product features a rectangular thread, an anti-counterfeiting ring and a rotation stop groove design. Product appearance requires high and demand is large. In addition, bottle cap is capped by an automatic filling line machine, so product size and thread accuracy requirements are high. Forced threading mold structure cannot meet product use requirements, and automatic rotating threading mold structure must be adopted.
threading mold structure 

3. Mold structure and working process

According to analysis of plastic parts and user production requirements, mold is designed with 8 cavities. It adopts hot runner needle valve type point gate feeding, and uses a geared motor to complete automatic threading through a chain drive. Mold structure is shown in Figure 2.
threading mold structure 
Figure 2 Mould structure 1. Small sprocket 2. Chain 3. Motor 4. Motor bracket 5. Limit screw 6. Spring 7. Mandrel 8. Water hole 9. Steel pipe 10. Seal ring 11. Cooling core 12. Movement Template 13. Backing plate 14. Backing block 15. Big sprocket 16. Thrust bearing 17. Backing plate 18. One-time push plate 19. Stop ring 20. Threaded core 21. Cavity plate 22. Backing plate 23. Backing block 24. Fixed template 25. Positioning ring 26. Main runner 27. Diversion plate 28. Insulation pad 29. Hot nozzle 30. Inclined guide column 31. Huff block 32. Positioning ring 33. Secondary push plate 34. Pin 35. Screw 36. Tensioner
threading mold structure 

3.1 Mold working process

Mold is firstly divided fromⅠ-Ⅰ surface, and at the same time, Huff block is pulled apart by inclined guide post to complete demould of concave part of anti-counterfeiting ring of plastic part, and leave plastic part in movable mold part. Then motor starts to rotate, sprocket and chain drive thread core to rotate. At the same time, Ⅱ-Ⅱ parting surface starts parting by spring force, relies on stop ring to prevent plastic part and thread core from rotating together. When thread is completely pulled out, ejector mechanism of injection machine moves, second push plate is pushed by ejector rod, so that mold is divided from Ⅲ-Ⅲ surface, concave part of anti-counterfeiting ring of plastic part is demolded, plastic part is removed from mold and completes a production cycle. It should be noted that spring pressure should be moderate when Ⅱ—Ⅱ surface is parted. If pressure is too large, 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 precision of moving parts and adjusting spring compression. 

3.2 Gating system design

Due to large production volume of plastic parts and high quality requirements, hot runner needle valve point gate pouring system is used 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 reduced, 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. In summary, for mass-produced plastic products, selection of hot runners is a very reasonable choice.

3.3 Mold cooling 

In order to improve production efficiency, prevent plastic parts from deforming, and prevent mold parts from seizing due to thermal expansion, mold cooling system must be sufficiently reliable. Because thread core is often in a rotating state and cannot be directly cooled, a cooling core is added to central part of thread core during mold design, and cooling water is introduced into core through copper pipe to form a circulating flow, which better completes cooling of thread core. Cooling of fixed mold cavity adopts 8 direct water channels, and external connection is connected with a hose to form a cycle for cooling fixed mold.

3.4 Mold material selection

Due to large production volume of bottle cap molds, demolding 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. Accordingly, key parts of mold design use ASSAB’s CALMAX635 material, with a heat treatment hardness of 56~60HRC. Through use, effect is relatively satisfactory.

4. Design of mold transmission part

Because there is no sliding 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. It is more suitable for transmission requirements in mold. Chains, sprockets and motors on the market are convenient to purchase, and no special processing is required, so chain transmission is used in transmission design.

4.1 Determination of mold cavity center distance and sprocket parameters

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

4.2 Motor speed and bearing selection

Because plastic part can rotate 5 times to release thread, in order to ensure normal service life of mold, speed of thread core should not be too fast, so rotation speed of large sprocket should be 60r/min according to calculation of completing a rotation demoulding every 5s. Transmission ratio of sprocket is: i=n1/n2=z2/z1. If transmission ratio is set to i≈0.5, then small sprocket is z2≈11, n2≈120r/min, so speed of geared motor should be 120r/ about min.
threading mold structure 
Because bearing used in this mold not only has function of centering, but also has to withstand axial pushing force of injection pressure on threaded core, so when bearing is selected, it must bear both radial load and axial load at the same time. Because radial thrust roller bearing can meet above conditions at the same time, this type of bearing is selected in design.

5. Conclusion

Among many de-threading mold structures, through comparison, this design has compact structure, stable operation, has achieved better design effects in transmission and cooling. It is a good typical automatic de-threading mold structure worth promoting.

Go To Top