Cosmetic plastic packaging has requirements for screwing orientation of bottle and cap. By controlling direction and size of screw of bottle and cap, under premise of sealing function, when capping torque reaches standard value, accurate positioning of bottle and cap can be achieved . During injection production of threaded caps, threaded rotary demolding gear combination needs to wait for mold opening and closing strokes to be in place before starting threaded rotary demolding and thread reversal reset actions, which prolongs injection production molding cycle. Now we will explain positioning of threaded cap and its mold structure, introduce optimized design and working process of injection mold structure for forming product.

 

Figure 1 Product structure

Structure of product is shown in Figure 1. It is a wide-mouth internal threaded cap, which is screwed and sealed with plastic bottle mouth thread and oriented. Its external dimension is ϕ66.3mm*18.5mm, average wall thickness t is 1.7mm, and weight is ( 10.5±1) g. Product material is made of polypropylene (PP) with a shrinkage rate of 1.6% and a density of 0.91g/cm3. This material has good fluidity and molding properties, good impact strength and chemical resistance.
Main features of product: ①Appearance of screw cap is uniformly colored and has no weld marks. It is required that there is no trace of gate point and no flashing. After cap and bottle thread are screwed in place, logo pattern on the top of cap is required to be consistent with front of bottle, and angle of each opening is same, which is positioning and orientation standard; ②Anti-rotation tooth position is set to hidden type, thickness of inner side wall of cover is used to design anti-rotation position of 1.0mm*1.1mm, and anti-rotation strength meets requirements; ③Thread is Tr trapezoidal, average included angle with horizontal is 25°, starting position of thread requires positioning and orientation, starting height of positioning datum is 2.6mm, orientation datum is consistent with direction of logo centerline, nominal thread diameter D is ϕ62.7mm, thread depth h is 1.1 mm, tooth width b is 1.8mm, lead S is 6.0mm, double thread right-hand thread, effective thread number n is 1.7 turns.
Mold is designed as an in-mold rotary ejector push-out structure, gate is reversely designed at the center of concave end of product to be molded. It is a full-hot runner pin-point gating, so that periphery of molded product can have same shrinkage rate, ensuring dimensional accuracy of thread molding, reducing internal stress after injection molding.

 

Figure 2 Layout of products to be formed

Product is a wide-mouth internal threaded cap, and demand is large. It adopts a balanced injection gating system with 16 cavities (4*4 layout), as shown in Figure 2. Mold thread rotation demolding and ejection mechanism consists of two parts: one is 16-cavity main thread rotary demolding and ejection mechanism (large rifle), that is, two sets of symmetrically distributed large rivers are set up, and main threaded rotary demolding gear is driven by mechanical power. Group; second is auxiliary screw thread rotary demoulding and ejection mechanism (small rifle wire), that is, 4 sets of small rifle wires are set, and they are evenly layout to form an auxiliary rotary ejection mechanism, which is also driven by mechanical power. Key points of mold structure design are: First, add a gear positioning mechanism with adjustable starting thread to gear combination of threaded rotation demoulding ejection mechanism to realize modular design of mold parts; Second, use a double-line combination of large and small reciprocating lines, use a mechanical transmission structure to realize design of rapid screw rotation and quick reset. Mold adopts 4 sub-types, which belongs to composite mold back injection hot runner structure. At the same time, cavity plate and core are provided with a surrounding cooling water channel and an imitated exhaust channel to improve cooling effect and production efficiency. It is also equipped with a mechanical arm automatic pick-up and automatic feeding system to increase automatic production rate and produce on horizontal servo injection molding machine MA3800II/2250.

 

Figure 3 Initial thread positioning adjustable mechanism

1. Small lead screw 2. Template 3. Positioning gear 4. Gear positioning piece
Whether optimization of adjustable mechanism for initial position of internal thread of mold is reasonable and practical will affect positioning quality of complete product assembly and production efficiency of mold. Initial thread position real-time adjustable mechanism is designed as a modular structure. Design point is to add 4 sets of gear positioning mechanisms. Each set of gear positioning mechanism is composed of a positioning gear 3 and a gear positioning piece 4 to form a sleeve, in which positioning gears 3 is equipped with a set of axial bearings on upper and lower shoulders to maintain concentric rotation accuracy, assemble them on small lead screw 1 in sequence through keyway between hole and shaft, as shown in Figure 3. When working, you need to loosen fastening screw between gear positioning piece 4 and positioning gear 3 fixed on mold plate 2 first, remove gear positioning piece 4 so that pieces 3 and 4 are no longer meshed and are in a loose state. At this time, manually adjust left and right angular position of positioning gear 3, small lead screw 1 and small lead nut coaxially fixed with it will also rotate in same direction and offset, synchronously driving coaxial threaded rotating demolding gear set fixed on small-lead screw 1, thread is rotated and demolded. Thread rotation demolding gear set synchronously drives thread forming insert on shaft to rotate left and right, then changes starting direction and position of cap thread on thread forming insert, gradually adjust in order until start rotation direction and position of cap thread of 16 cavities are consistent, consistency of thread starting distance is obtained. Linkage effective transmission chain transmission realizes directional adjustment function of initial angle positioning of inner thread of cover.
Main functions of above design are: ①Mold is based on original threaded rotation demoulding structure, through addition of a modular gear positioning mechanism, to achieve design requirement of instant adjustment of thread starting position, realize optimization of modular design of mold structure; ②Additional gear positioning mechanism is designed on the side of mold, all angle adjustments can be operated on mold of injection molding machine at any time, realizing convenience of disassembly, assembly and real-time adjustment at any time. Operability of mold is optimized through design of this structure.

 

Figure 4 Two-line rifle thread rotary demoulding mechanism

1. Small lead screw 2. Template 5. Template 6. Template 7. Push plate 8. Push plate fixing plate 9. Cavity plate fixing plate 10. Movable mold seat plate 11. Large lead screw 12. Nut 13. Nut
Threaded rotary demoulding mechanism of mold is mainly composed of large and small tortoise lines to form a double-line linkage mechanism, which can determine sequence of actions, use mold opening process of injection molding machine mold plate to complete main related actions of mold, as shown in Figure 4. Each set of large rifle wire is assembled by large lead screw 11 and nut 12 respectively, as main thread rotation demolding and ejection mechanism, responsible for transmission of 16 thread core gear sets to achieve thread rotation demolding, 2 sets of large rifle wire are installed symmetrically in mold plate before and after. Large lead screw 11 is fixedly installed in transmission gear hole on mold plate 5 and 6, center hole of bearing through keyway between hole and shaft. Nut 12 penetrates parts 7, 8, 9, 10, and fixed on movable mold base plate 10. Specifications of each set of customized big rifle wire are: steel heat-treated large lead screw 11, outer diameter ϕ45mm, left-handed, pitch P=80mm, equipped with wear-resistant material nut 12, effective mold opening stroke is 200mm, that is, sum of mold opening strokes of three parting surfaces of PL2, PL3, PL4; small rifle line is assembled by small lead screw 1 and nut 13, as a secondary thread rotating ejection mechanism, 4 small rifle lines are layout according to 4*4 cavity specifications. Each set of small rifle wires is equipped with 4 threaded core gear sets linkage. Small lead screw 1 is fixedly installed in the center hole of positioning gear 3 on mold plate 2 through keyway between hole and shaft, nut 13 is installed in the center hole of transmission gear on mold plate 5 and 6. Specifications of each set of customized small rifle wires are: steel heat-treated small lead screw 1 outer diameter ϕ22mm, right-handed, screw pitch P=80mm, equipped with wear-resistant material nut 13, effective screw stroke 15mm, which is mold opening stroke of PL2 parting surface.
Mechanism design principle: ①Screw rotation demoulding and ejection mechanism of mold adopts large and small reciprocating lines to cooperate with simultaneous use, uses its stable screw thread force, mechanical kinetic energy of mold opening is directly converted into power of thread rotation demolding and mold plate ejection; ②Symmetrical structure of large and small reciprocating lines are directly installed in formwork of mold base, sequence of actions can be determined, force of each unit is balanced, purely mechanical transmission, action and stroke are accurate; ③Using a double-rifle thread rotation demoulding mechanism, mold opening process is thread rotation demolding process, which saves time of waiting for thread rotation demoulding after opening stroke is in place; ④Mold clamping process is thread reversal reset process, which saves waiting for thread reset.
Design and optimization of integrated mold structure shortened product molding cycle by 4-6s. Optimization was within 30s, which prolonged service life of mold, improved molding process of product, maximized productivity and benefits.

 

Figure 5 Mould structure

1. Small lead screw 2. Template 3. Positioning gear 4. Gear positioning part 5. Template 6. Template 7. Push plate 8. Push plate fixing plate 9. Cavity plate fixing plate 10. Movable mold seat plate 11. Large Lead screw 12. Nut 13. Nut 14. Cavity plate insert 15. Push-out insert 16. Anti-rotation tooth forming insert 17. Thread forming insert 18. Hot nozzle insert 19. Forming insert 20. Limit Screw 21. Spring 22. Limit screw 23. Limit screw 24. Fixed distance rod 25. Fixed distance rod 26. Fixed distance rod 27. Hot runner template 28. Fixed mold base plate
Based on above analysis and comprehensive optimization, an injection mold with adjustable thread starting position positioning is designed. Mold structure is shown in Figure 5. Overall mold size is 700mm*650mm*571mm, which requires 4 parting. It adopts injection in the center of product to be molded, full hot runner needle point reverse injection, which is different from conventional injection method, so that movable and fixed molds of mold structure are opposite, main cavity is in movable mold, threaded rotary demoulding mechanism spans moving and fixed molds, double-line rifle mechanism is adopted to realize threaded rotary demolding action by mechanical opening and closing of mold. Key point of mold design is to adjust position of initial thread of product to be molded in 16 cavities through additional gear positioning mechanism before molded product, ensuring accurate thread orientation during mass production of product, simple mold manufacturing, convenient production debugging. Specific working process is as follows.
(1) Before mold on injection molding machine forms product, according to orientation requirements of screw cap to be formed, starting position of each screw cap to be formed must be adjusted first, that is, first part to confirm and consistency of thread starting position of cap, first take out gear positioning piece 4, manually fine-tune circumferential direction of positioning gear 3, adjust compensation value of angle difference according to thread angle deviation range, finally confirm that angular positioning of 4 sets of positioning gears is accurate and consistent, to ensure that thread starting position of threaded cap formed by16 cavities is correct.
(2) Mold is opened, movable mold part is divided first. Under action of fixed distance rod 25 and spring 21, parting surface of PL1 is opened first. Under action of limit screw 20, parting surface of PL1 is opened by 10mm, so that hot nozzle insert 18 and molding insert 19 are separated from product, condensate at gate point is synchronously disconnected. At the same time, small rifle line of secondary screw rotation ejection mechanism starts to move. Small lead screw 1 and nut 13 are in injection molding machine. Under action of mold opening force, thread self-rotating push-out action is started. Small-lead screw 1 pushes push plate 7, PL2 parting surface opens. Large rifle line of main thread rotation ejection mechanism also moves synchronously. Large-lead screw 11 and the nut 12 are simultaneously under action of opening force of injection molding machine, and threaded rotating demolding gear set fixed on it cooperates with threaded rotating demolding action. Due to action of fixed distance rod 24, parting surfaces of PL2 and PL3 are opened at the same time, due to effect of anti-rotation tooth forming insert 16 on tightness of product, thread forming insert 17 starts and continues to perform thread rotation and demolding until common stroke reaches 15mm. Opening distance of PL2 parting surface is stopped by limit screw 22, while PL3 main parting surface continues to open.
(3) Continue to open mold. At this time, under action of distance rod 26, PL4 parting surface is divided. Distance rod 26 pulls push plate fixing plate 8 with ejection insert 15 to jointly eject product from mold. Due to action of limit screw 23, parting surface of PL4 stops after 10mm opening, parting surface of PL3 stops after opening 200mm, and mold opening stroke ends. Robot arm enters main parting surface PL3 of mold, takes out molded product, and leaves mold together. After removal is completed, mold clamping starts. Large and small reciprocating lines synchronously drive thread forming insert 17 to complete all actions such as thread reversal and reset, until thread starting position is completely consistent, mold completes a molding cycle, and mold is closed to continue next cycle of molding.
(1) A gear positioning mechanism is added to threaded rotation demoulding gear assembly, which realizes real-time adjustment of initial thread position on injection molding machine.
(2) Using large and small reciprocating lines to cooperate with thread rotation action, using mechanical mold opening power to realize thread rapid rotation demoulding and resetting.
(3) Through optimized design of mold structure, actual production proved that mold work is stable, injection production process is reasonable, 50 million qualified products have been continuously produced, and quality indicators have reached the standard.