Plastic internal screw cap is a common component of daily chemical household products. Internal and external threads are screwed together to play role of sealing bottle mouth. Its thread directly affects function and quality of daily chemical plastic packaging. Generally, plastic parts are required to have complete threads, stable tooth shapes, and accurate dimensions. When used, when tightening torque reaches standard value, it can effectively bear load without sliding teeth. Investigation found that during injection molding of internal screw cap, key molding parts of in-mold rotary demolding mechanism were easily rubbed against each other, burned and damaged, or even severely injured and bitten, which affected service life of mold and quality of molded plastic parts, restricted production efficiency of enterprise. In early stage of mold design, quality control should be done in advance for screw rotation demolding mechanism, mold structure should be optimized based on actual experience to solve problem of burnt and damaged parts, prolong service life of mold. Here mold structure and process characteristics of internal screw cap are analyzed, optimal design of injection mold structure of plastic part is explained.

 

Figure 1 Plastic structure

Structure of plastic parts is shown in Figure 1. It is used in daily chemical plastic packaging products. It is screwed and sealed with plastic bottle threads. Its external dimensions are ϕ26.0mm*16.0mm, average wall thickness is 1.2mm, and weight is 2.5g. Material is polypropylene (PP), with a shrinkage of 1.6% and a density of 0.91g / cm3. It has good moldability and fluidity, has good stress crack resistance, high bending fatigue resistance, good toughness and chemical resistance, suitable for daily chemical plastic packaging.
Main features of plastic parts are: ①Screw cap is a colored plastic part. Key to forming is continuous internal thread of M12.0mm, tooth shape is Tr trapezoidal thread, bilateral oblique angle is 10 °tooth head and tail are gradually transition, nominal diameter D = 12.0mm, tooth depth 0.75mm, tooth width is 1.2mm, lead S is 2.0mm, single-line right-hand thread, effective thread is three and a half turns, mold structure is designed to be ejected from mold by rotating it in mold; ②Protrusions inside cover are set with anti-rotation stripes, that is, 6 ribs. During relative rotary demolding motion, ribs serve to prevent rotation and fix plastic parts. Optimized mold structure is equipped with a non-rotating tooth sleeve shaft or non-rotating tooth kit; ③ Appearance of plastic parts requires uniform and bright surface, surface is smooth without shaving. Gate is designed to be injected at the center of outer top of plastic part to be molded. It ensures same shrinkage in circumferential direction of plastic part and accuracy of thread forming dimension, reduces plastic deformation, ensures uniform color dispersion and less residue at injection point.

 

Figure 2 Reaming mechanism

1. Threaded mandrel 2. Anti-rotation sleeve shaft 3. Ring-shaped sleeve shaft 4. Take-off template 5. Anti-rotation sleeve shaft fixing plate 6. Anti-rotation sleeve shaft fixing plate 7. Threaded mandrel fixing plate
Plastic parts have a small overall size, but they require a large number of parts. They adopt 48-cavity (horizontal 6 vertical 8) balanced casting system layout. Key points of mold optimization design are: first, internal mold rotation and demolding structure (that is, hinge mechanism, as shown in Figure 2); second, three-axis (threaded core shaft 1, anti-rotating sleeve shaft 2, and ring sleeve shaft 3) shaft-holding bearing combination; third, cooling cycle system of threaded mandrel 1. Difficulty is to prevent main molding parts from being burned or damaged in rotary mold release mechanism of internal thread of mold. Mold structure adopts three parting and is produced on horizontal servo injection molding machine MA2500.

Whether design of in-mold rotary demolding structure meets actual production requirements will affect normal production of mold and molding of plastic parts. Main point of design is to adopt an in-mold rotary demolding structure composed of three axes, three axes in order from inside to outside are threaded core shaft 1, anti-rotation sleeve shaft 2, and ring sleeve shaft 3. During operation, threaded mandrel 1 fixed to threaded mandrel fixing plate 7 is designed as a static shaft. When mold is opened, mandrel is fixed, non-rotating sleeve shaft 2 mounted on non-rotating sleeve shaft fixing plates 5 and 6 holds a screw cap with 6 non-rotational striped ribs, performs thread rotation under transmission of a rotary demolding gear set to complete complete demoulding action. At the same time, ring sleeve shaft 3 fixed to demoulding plate 4 cooperates to push out plastic part. This design has following functions: ① Threaded mandrel 1 with its own cooling cycle system is fixed and does not participate in thread rotation demoulding action, to avoid burns and damage caused by frictional heating between shaft sleeves due to rotation of mandrel when mandrel is spinning; ② Non-rotating toothed sleeve shaft 2 with synchronous cooling water path holds plastic parts thread for rotating and demoulding. One is to reduce clamping force of threaded mandrel 1 on threads of plastic part; the other is to reduce burn damage caused by instant frictional heating between shaft and sleeve in demolding mechanism while reducing deformation of internal thread of plastic part; ③ Ring sleeve shaft 3 on demould plate 4 is is provided with a circumferential contact surface exhaust passage in the middle of non-rotating tooth sleeve shaft 2 to reduce clamping force of plastic part and threaded mandrel 1, improve molding process conditions of plastic parts. In addition, shaft can be easily processed separately, assembly are simple.

 

Figure 3 Bearing assembly

1. Threaded mandrel 2. Slewing ring sleeve shaft 3. Sleeve ring sleeve shaft 5. Slewing ring sleeve shaft fixing plate 6. Slewing ring shaft fixing plate 8. Custom ball-type self-lubricating copper sleeve 9. Bearing 10 .Bearing 11.bearing 12.bearing
In-mold screw rotation demoulding mechanism is main stress position of main molding parts. All of them are combined with axle-bearing type bearing combination to improve coaxiality in order to eliminate frictional heating caused by uneven bearing force and rotation torque deviation. Structure is shown in Figure 3. There are a total of three groups: ① Threaded mandrel 1 that cooperates with anti-rotating sleeve shaft 2 has a maximum diameter of 28mm and a total length of 203mm. It belongs to slender force parts. The smaller diameter, the worse rigidity. Optimized design is a static shaft to avoid force deformation caused by its rotating torque, and a custom-made ball-type self-lubricating copper sleeve 8 with a sufficient length is installed at the nearest position of force-fitting section of anti-rotation sleeve shaft 2 to increase the ejection guide effect, at the same time enhance deformation resistance and overall stress; ② anti-rotating sleeve shaft 2 is designed as a rotating shaft, similarly in force contact section with ring sleeve shaft 3, a custom-made self-lubricating copper sleeve of sufficient length is also installed. Cooperating section of anti-rotating sleeve shaft fixing plate 5 and 6 is main force receiving section of gear set transmission. Symmetrical vertical pressure-resistant axial bearings 9 (unidirectional thrust ball bearings, which serve as unidirectional fixed mandrels) and concentric radial bearing 10 are respectively designed to enhance stability of rotational dynamics; ③ All gear set core shafts use force bearings, such as main drive gear set, auxiliary drive gear set and other multiple sets of power gear combinations, all adopt shaft holding radial bearings 11 (deep groove ball type ), two sets of upper and lower, Threaded mandrel rotation gear set adopts model NKX15Z bearing 12 (ball type + thrust ball combination). Different positions and stress requirements pass test, bearings and combinations with different functions are used to improve comprehensive stability of all rotating parts, extend service life of mold.

 

Figure 4 Cooling system

1. Threaded mandrel 4. Take-off template 5. Anti-rotation gear sleeve shaft fixing plate 6. Anti-rotation gear sleeve shaft fixing plate 7. Threaded mandrel fixing plate

 

Figure 5 Mold structure

1. Threaded mandrel 2. Anti-rotating gear sleeve shaft 3. Shaped ring sleeve shaft 4. Stripping template 5. Anti-rotating gear sleeve shaft fixing plate 6. Non-rotating gear sleeve shaft fixing plate 7. Threaded mandrel fixing plate 8 .Customized ball-type self-lubricating copper sleeve 9.bearing 10.bearing 11.bearing 12.bearing 13.hydraulic motor 14.support plate 15.push rod fixing plate 16.pushing plate 17.movable mold base plate 18.cooling tube 19. Spring 20. Rubber 21. Cavity plate 22. Stripper plate 23. Fixed die seat plate 24. Pull rod 25. Cavity plate insert 26. Fixed distance rod
48-cavity mold's internal screw cap molding cycle is generally 28 ~ 30s. Reasonable design of cooling water circuit not only improves appearance and dimensional accuracy of plastic parts, but also improves production efficiency. Key is to avoid burn damage caused by frictional heating expansion of threaded mandrel 1. Threaded mandrel 1 is an elongated mandrel with specifications of M12mm*203mm. A waterfall ring deep cavity cooling circulation system is designed inside mandrel. As shown in Figure 4, threaded mandrel 1 is designed as a static shaft, which can avoid difficulty of setting cooling water channel due to rotation movement, or cooling water channel is set, but risk of water leakage due to rotation action is difficult to seal. Optimization plan is: ① Design a stepped internal cooling passage. Because outer diameter of mandrel is distributed in steps, design of waterway profile steps into mandrel. Diameter of waterway is from large to small, internal depth is as close as possible to root of plastic thread. Built-in ϕ4mm copper pipe is fixed on support plate 14 (see Figure 5) to form a waterfall ring cooling cycle to ensure that mold forming parts and key parts of plastic part have good cooling efficiency. ② Threaded mandrel 1 is designed as a static shaft, that is, demolding action has been improved, and fixed shaft does not rotate. Sealing method is changed to a more reliable end face seal, which is convenient to process and simple to assemble, beneficial to daily maintenance and reduces manufacturing cost and prolongs service life of mold; ③ Number of water inlets and outlets is arranged according to 48 cavities in 6 horizontal and 8 vertical directions, design 6-in 6-out tandem cooling water channels, cooling 8 cavities in each channel, and processing inner diameter ϕ8mm water hole according to maximum diameter to maximize contact area of cooling water channel. Above optimized design not only shortens molding cycle by 3 ~ 5s, but also prolongs service life of mold and improves molding quality of plastic parts.

Based on above comprehensive analysis and optimization, injection mold structure of internal screw cap is designed. As shown in Figure 5, overall mold size is 550mm*650mm*525mm. Mold structure adopts three parting. Internal thread rotation demolding means that after internal thread cap is injection-molded, threaded mandrel 1 and anti-rotation sleeve shaft 2 of plastic part in mold perform relative rotational movements to achieve demolding. Optimally designed anti-rotation sleeve shaft 2 holds plastic part for thread rotation motion, instead of traditional threaded mandrel 1 for thread rotation demolding, then mold plate 4 with ring sleeve shaft 3 jointly push out plastic part to complete a molding cycle. Specific working process is as follows.
(1) Mold is opened, and fixed mold part is opened first, that is, parting surface II is under action of fixed distance tie rod 26, stripper plate 22 and cavity plate 21 are first opened until gate condensate is pulled off, mold continues to open parting surface I. Open 5 mm between fixed mold seat plate 23 and stripper plate 22 to ensure that flow channel condensate is separated from draw rod 24.
(2) Movable and fixed mold is opened, movable mold is partially opened, that is, main parting surface III is opened, cavity plate 21 is separated from mold release plate 4, injection molding machine fluid circuit is activated simultaneously, hydraulic motor 13 drives gear set transmission mechanism, which drives anti-rotation sleeve shaft 2 to perform thread rotation demoulding, threaded core shaft 1 is fixed on threaded core shaft fixing plate 7 because anti-rotation sleeve shaft 2 has 6 anti-rotation ribs, which rotates plastic part while rotating. At the same time, plastic part is pushed out synchronously by ring sleeve shaft 3 fixed in release plate 4 until plastic part completely disengages from threaded core shaft 1.
(3) While hydraulic motor 13 drives anti-rotation sleeve shaft 2 for demolding, parting surface Ⅴ is under action of spring 19, anti-rotation sleeve shaft fixing plate 6 and threaded mandrel fixing plate 7 are parted and opened. In order to ensure that threaded mandrel 1 and anti-rotation gear sleeve shaft 2 do not contact each other during rotation and demolding, at least a gap of more than 0.5mm is guaranteed to prevent rotation, friction, heat, burns and damage between three shafts.
(4) Parting surface IV under action of rubber 20, release plate 4 and anti-rotating sleeve shaft fixing plate 5 are separated from each other to ensure that ring sleeve shaft 3 and anti-rotating sleeve shaft 2 in push plate are demolded. No contact friction at all times to avoid heat and burn damage. The hydraulic motor 13 drives the gear set to drive the anti-rotation sleeve shaft 2 in push plate do not contact and friction during demolding to avoid heat and burn damage.  Hydraulic motor 13 drives gear set to drive anti-rotation gear sleeve shaft 2 for demolding. After plastic part is separated from threaded mandrel 1, hydraulic motor 13 stops working. After injection molding machine receives signal, ejector of injection molding machine is started to eject, acts on a pusher plate assembly composed of pusher fixing plate 15 and pusher plate 16, pusher plate component cooperates with release plate 4 to push out plastic part. Mold completes an injection cycle, and mold is closed to continue next cycle.