【Abstract】Process requirements of threaded plastic parts are analyzed. By comparing design schemes of 3 kinds of thread withdrawal mechanism, withdrawal plan is determined, rifle thread withdrawal mechanism is designed, mold opening sequence and rifle movement are determined. Principle, rifle thread withdrawal mechanism, rifle thread transmission structure, mold opening distance calculation, mold structure and working process are explained, thread brace positioning method and design of precautions are explained.

Plastic parts with thread characteristics require rotation or rotation plus linear motion to complete emolding action. Mold structure of these plastic parts is complicated,manufacturing and maintenance costs are high. After long-term practice, we use working principle of rifle to design rifle thread withdrawal mechanism, design mechanism is ingenious, work is stable, production cost can be reduced, competitiveness and economic benefits of enterprise are improved.

Existing plastic cup lid shown in Figure 1 is made of ABS, with a shrinkage rate of 5‰. It requires 1 mold and 4 cavities, with high impact resistance and strong mechanical properties. There are internal threads on the side wall of inner hole of plastic part, mold needs to be designed with an unthreaded structure. Plastic part has anti-slip grooves on the exterior, and there are several anti-rotation ribs in the inner hole. Thread is a 3-thread thread, each thread is less than one turn (only 320°).

 

Figure 1 Cup lid
Solution 1: Utilize tooth structure of plastic part to unthread thread, threaded shaft rotates in situ. Plastic part cannot rotate under action of anti-rotation groove and can only move in a straight line, but anti-rotation groove is too shallow. As long as it exits a small distance, plastic part cannot stop rotating, plastic part will rotate with threaded shaft, so that tooth cannot be automatically retracted. Therefore, in this case, a push plate must be added. Under action of spring, push plate moves with plastic part, so that it has a positioning effect until plastic part completely exits threaded shaft. The push plate can also assist thread withdrawal, reduce tightening force of plastic part on threaded shaft, and avoid damaging tooth profile of plastic part.
Option 2: Usually number of design threads is small, thread can be retreated by braces, gear and rack can be used to retreat. Stroke is not large. Screw shaft rotates and exits plastic part under action of braces spiral, so that relatively speaking, anti-rotation force does not need to be too large. Mould action: After fixed mold is opened, cylinder drives rack to move linearly, and linear motion of rack drives gear to rotate to drive threaded shaft to rotate. Threaded shaft rotates and moves linearly under action of threaded sleeve to exit plastic part. If you first retract teeth and then start fixed mold, plastic part may stick to fixed mold.
Solution 3: If number of thread turns is less than one turn, thread can be unthreaded with braces, a rifle thread unthreading mechanism can be used. Screw shaft rotates, and plastic part exits under action of screw thread of brace. Structure is simple, stable, and low in manufacturing cost.
For this plastic part, anti-rotation groove of movable mold is too shallow, only 0.2mm, which cannot guarantee that anti-rotation groove will not be damaged during rotation process, so third option is chosen. ①Compared with cylinder rack structure, it shortens molding cycle, thread retreat and thread feed are synchronized with moving and fixed mold opening and closing to shorten the cycle; ②No need to connect oil cylinder to avoid oil leakage from oil cylinder, and third to save mold base space. Shape of mold is beautiful; ③Movable and fixed molds need to be designed to open mold at a fixed distance, movable and fixed molds must be separated and the entire set of molds must be removed. From perspective of processing technology, it is no longer difficult to use CNC lathes to process double lines, it is simple and convenient. In addition, in order to reduce injection force of threaded sleeve, it is necessary to insert a circle of movable mold into ring bone position, at the same time prevent R0.2mm anti-rotation groove of movable mold from not reversing, flat anti-rotation can be added to inner ring of ring bone position. (From structure of plastic part, ring-shaped inner ring of bone position is not effective, but it is beautiful) However, there will be new problems in this way. After threaded shaft exits plastic part, plastic part will remain on movable mold insert, an ejector pin needs to be added to eject plastic part.

According to plan three, design mold opening sequence, mold action: move fixed mold to open mold, while mold is opened, screw rod pulls brace to rotate, gear on brace rotates, gear on threaded shaft makes threaded shaft rotate under action of brace+ Move in a straight line to exit plastic part and push plate out of plastic part. Mold opening sequence is shown in Figure 2.

 

Figure 2 Mold opening sequence

Rifle line uses a fixed and non-rotatable screw axis linear motion to transform into a fixed rotational motion of nut axis, reverse linear motion of screw rod is converted to a nut forward and reverse rotation motion. Design of rifle line is shown in Figure 3.
Structure and working process: Lead screw is connected with nut, a section of lead screw is installed and matched with guide sleeve. Tapered roller bearings are installed at both ends of nut, and gear is installed in the middle. Power comes from injection molding machine. Screw rod performs linear reciprocating and rotational motion at the same time, drives nut to perform linear and rotational motions, then drives gear to work, thereby driving forward and backward threading work.

 

Figure 3 Rifle
1. Tapered ball bearing 2. Gear 3. Nut 4. Guide sleeve 5. Screw
Structural requirements are as follows:
(1) Diameter of screw rod should be as small as possible under condition of meeting strength. Diameter of screw rod directly affects lead of screw rod. Lead cannot be less than 3 times diameter, and at least 4 threads are required. Length of screw rod should ensure that after mold is opened, it is matched with nut with a length of 1/2 diameter of screw rod and cannot be separated. Separation may cause nut to continue to rotate under action of inertia, causing screw rod to fail to align with teeth of nut during mold clamping, and thus damage nut. Even if nut is aligned, threaded shaft cannot be reset, and mold needs to be re-matched, so a fixed-distance pull plate or fixed-distance pull rod is required to limit position.
(2) Guiding function of guide sleeve. Length of guide sleeve must be at least 1 time diameter of screw rod. If there is no place to place guide sleeve, it can be cancelled. However, it is necessary to ensure that mating length of screw rod and nut after mold is opened is at least equal to length of diameter of screw rod.
Diameter length.
(3) It is better to use a set of tapered roller bearings to fix position of nut axis. It is also possible to use a set of thrust bearings or a set of deep groove ball bearings for positioning. This will increase thickness of mold if it is too troublesome.
(4) Nut material is best to use beryllium copper, aluminum bronze, graphite, etc. Minimum length of nut is required to be 1.5 times diameter of screw rod.

According to characteristics of plastic part and mold opening sequence, design of withdrawal structure is composed of threaded sleeves, snap rings, gears, guide sleeves, inserts, threaded shafts and other parts. Specific design is shown in Figure 4.
Working process: Power comes from injection molding machine, which is transmitted to gear 4 by gear set, gear 4 drives threaded shaft to rotate and move linearly, so that threaded shaft is withdrawn from plastic part and thread is retracted.

 

Figure 4 Rifle thread withdrawal mechanism
1. Threaded sleeve 2. Snap ring 3. Gear 4. Guide sleeve 5. Insert 6. Threaded shaft 7. Plastic part
Function and value of main parts:
(1) Threaded sleeve. Injection pressure of threaded sleeve is borne by this thread. The larger projected area, the thicker thread and the longer length. Too long will increase mold thickness and increase size of molding machine. For plastic parts with a large area, try to consider middle insert to withstand injection pressure while reducing friction.
(2) Snap ring. Snap ring is used for axial positioning of gear. Snap ring is a standard part, so when designing shaft, it should be designed according to standard size of snap ring. Gear and threaded shaft are integrated. There is no need to design snap ring. You can also drill screw holes on the gear to fix gear with headless screws. According to depth of snap ring groove, it must be greater than depth of snap ring groove. Inserts can withstand injection pressure, which is convenient for water cooling.
(3) Gears. Gears act as precise transmission and force transmission. Gear modulus is determined according to tightening force, gear on threaded shaft can be made as small as possible to reduce rank and thus flow path. Generally take 1mm, 1.25mm, 1.5mm, 2mm, 2.5mm.
(4) Guide sleeve. Guide sleeve plays a role of guiding and positioning. Bearings cannot be used in this structure because shaft needs to rotate and move linearly.
(5) Inserts. Insert mainly considers shape of inner hole that needs to be formed, and the smaller size, the higher mechanical properties of material used.
(6) Threaded shaft. Threaded shaft mainly fixes position of threaded shaft in mold to prevent taper surface of threaded shaft from jamming with mold core, resulting in excessive torque of threaded shaft, generally 3~6mm.

According to shape and characteristics of plastic parts, rifle thread withdrawal mechanism is mainly composed of forming threads, plastic parts, fixed blocks, guide sleeves, gears, thread sleeves, inserts, guide sleeves, circlips, bearings, gears, bearings, and screw rods. Details are shown in Figure 5.

 

Figure 5 Rifle thread retracting drive mechanism
1. Formed thread 2. Plastic part 3. Fixed block 4. Guide sleeve 5. Gear 6. Thread sleeve 7. Insert 8. Guide sleeve 9. Circlip 10. Bearing 11. Gear 12. Bearing 13. Screw rod
Working process: Power comes from injection molding machine, which drives screw 13 to rotate and reciprocate linearly. Under movement of screw, gears are driven to mesh, gear transmits movement to gear 5 movement (4 groups) at the same time, which drives formed thread to perform thread unthreading work.
Idler gear set is shown in Figure 6. Design is particularly: gear 11 is used as a driving wheel, gear 16 is used as a driven wheel, gear 15 is a driven wheel relative to gear 14, but it is a driving wheel relative to gear 16 (4). In the entire transmission mechanism, gears 14 and 15 are idler gears, which can flexibly change transmission direction, facilitate design of left and right screw threads, can flexibly select number of gear teeth and replace for different transmission ratios according to pitch of plastic part. .

 

Figure 6 Idler group
11. Drive wheel 14. Idler wheel 15. Idler wheel 16. Driven wheel

Rifle thread withdrawal structure, screw lead and fixed-distance mold opening distance calculation: screw thread of plastic part is only 320°, rotation is less than one circle, then threaded shaft can rotate one circle, number of gear teeth can be changed to change transmission ratio and achieve precise positioning. Driving gear teeth number is 32 and driven gear teeth number is 24. Then transmission ratio is 32/24=1.333, so drive wheel can be completely retracted by just passing one turn. This mold is a large nozzle mold, and screw is installed on fixed mold plate. Mold opening distance is opening distance of screw rod. Height of plastic part is 11mm and ejection is 20mm. Then mold opening distance is 110~150mm, so lead is 140mm and fixed distance opening is 130mm, as shown in Figure 7.

 

Figure 7 Schematic diagram of mold opening distance

Braces need to be debugged and assembled, main reason is that spiral line is not as accurate as design of shape of braces during thread processing. It is difficult for lathe to align profile reference when rotating thread, and ring groove can be used for positioning, as shown in Figure 8. Annular grooves can be processed first. When there are multiple grooves, adjustment angle may be large. If annular angle is to be increased, two groups of annular grooves can also be designed. Even if flange strength is not good, ring groove positioning can be accurately positioned, but production process may be loose, and it needs to be used together with pin. When brace is worn, angle can be adjusted and used.

 

Figure 8 Positioning diagram of braces

Mold structure is shown in Figure 9. When mold is opened, force is transmitted to screw (trapezoidal thread) under action of top rod of injection molding machine. Screw rotates and moves linearly. At the same time, nut matched with screw rotates and gear rotates. Wheel transmits movement to gear set, finally drives forming thread to rotate and reaches limit device 1, spring is in a stretched state, thereby forming thread.
When mold is closed, after mold reaches limit device,ejector rod of injection molding machine does not work, spring returns to free state to drive mold to close. First, forming thread is reversed, at the same time unthreaded gear set is reversed, force is transmitted to nut to drive screw rod to reverse and reach limit position 2, avoiding over-travel to damage mold, thereby completing mold clamping process.

 

 

Figure 9 Mould structure
1. Fixed mold base plate 2. Backing plate 3. Cavity fixing plate 4. Cavity 5. Core fixing plate 6. Rifle fixing plate Ⅰ 7. Rifle fixing plate Ⅱ 8. Backing plate 9. Spring 10. Fixing Plate 11. Backing plate Ⅰ 12. Movable mold seat plate 13. Trapezoidal thread sleeve 14, 16. Screw 15, 20. Bolt 17. Limit nail 18. Bearing 19. Gear 21. Trapezoidal thread 22. Gate 23. Gate Set 24. Backing plate Ⅱ 25. Guide sleeve 26. Insert 27. Cooling water channel 28. Block 29. Screw 30. Support block 31. Fixed block Ⅰ 32. Shaft 33. Fixed seat 34. Fixed block Ⅱ 35. Threaded shaft 36. Trapezoidal nut 37. Guide post 38. Core

Following 5 points should be paid attention to in structure design of rifle thread withdrawal mechanism:
(1) Calculate number of unthreaded turns of threaded shaft, calculate transmission ratio according to number of turns and gear on threaded shaft, then calculate number of turns to be rotated by driving wheel on rifle line, determine distance of fixed-distance mold opening according to number of rotations of driving wheel. It needs to be considered comprehensively according to plastic parts of injection molding machine, height, lead of screw, and thickness of mold.
(2) Screw must use at least 4-threads, lead should be determined according to diameter and mold opening distance. HASCO standard rifle can also be used. Lead must be at least 3 times diameter of screw. The larger multiple, the greater angle, the smaller mold opening force is. If lead becomes larger, distance of fixed mold opening will be greater. Comprehensive considerations, diameter of screw rod should be as small as possible under condition of meeting strength.
(3) Mold base guide post must be installed on fixed mold, must be on same side as screw rod, and length of guide post must be at least 30mm higher than screw rod.
(4) A fixed-distance pull plate or fixed-distance pull rod should be designed between movable and fixed molds to prevent accidentally pulling movable and fixed molds from damaging threaded shaft or braces or gears. After movable mold is opened, mold must be reinstalled. Before pulling fixed mold, remove braces first, otherwise mold parts will be damaged.
(5) Check direction of thread rotation to determine whether screw is left-handed or right-handed. An idler can also be added to change direction.

Proved by practical experience and mold trials, thread-removing mechanism of plastic part thread mold is ingeniously designed, which simplifies mold structure, stabilizes production process, reduces production costs, and is easy to operate, greatly improves production efficiency, and has a certain degree of promotion value.