1. Plastic parts structure and process characteristics

Structure of plastic part is shown in Figure 1. It is used in daily chemical plastic packaging. Upper and lower covers are closed to form an integral cover. Its overall dimensions are φ23.5mm * 30.0mm, average wall thickness is 1.1mm, and weight is (3.5 ±0.3)g. Material is polypropylene (PP), with a shrinkage rate of 1.6% and a density of 0.91g/cm3. PP material has good molding performance and fluidity, good stress cracking resistance and high bending fatigue resistance. It is light, tough and chemically resistant, and is suitable for integrated packaging of daily chemical plastic products. Main features of plastic parts: ① Upper cover is flat. When cover is facing down, the entire package can be naturally placed upside down to minimize residual liquid in the bottle and avoid waste. Upper and lower covers are connected by a bow tie to form a one-piece cover. Action of opening and closing cover should have a "three-ring" effect: it makes a sound when cover is opened (sealing needle leaves liquid outlet hole), when cover is turned over (after upper cover leaves closing angle), and when cover is closed (sealing needle enters liquid outlet hole); ② Sealing needle is 1.5mm eccentric forward on inner end of upper cover. It is closer to hand and is more conducive to receiving materials. Sealing needle has a needle diameter of φ (2.95±0.05) mm and a height of 4.2mm. It is designed as a hollow needle and requires uniform wall thickness. Head is 20° tapered and chamfered R0.2mm, which is conducive to guidance when closing cover. 4 triangular booster ribs are evenly distributed at the root to reinforce and center. Outer end of lower cover corresponds to a pyramidal high platform, with a built-in liquid outlet hole with a diameter of φ (2.85±0.05) mm. Entrance is guided by an external horn at an angle of 50°. Wall thickness is required to be uniform, contact surfaces are all naturally chamfered, and effective interference with sealing needle is 0.1mm. After cover is closed, effective matching straight line segment is ≥ 1.2mm to form a liquid-tight leak-proof seal; ③ A sealing ring protrudes from inner end of lower cover to form an inner plug shape. Outer diameter is φ14.9mm, height is 6.4mm, effective wall thickness is 0.6mm, it has a demoulding angle of 1.6° and an inlet guide chamfer of R3.5mm. It is an interference fit with inner diameter of bottle mouth and has a liquid-tight sealing function. Key to molding plastic parts: ① When opening lower cover with your fingers, a strip of tear strip is added, which is connected by 4 breakpoints to form an easy-tear function. Tear strip is a fan shape with a height of 4.8mm and a width of 110°. There is a rectangular window with a length of 6.0mm and a width of 1.1mm in the middle. At same position, upper cover is equipped with a 5.5mm*0.8mm boss flat buckle. When cover is closed, the two contact positions are slope-guided and locked. Cover can only be opened when tear tape is manually torn off, forming a lock with an anti-counterfeiting function; ② There are 5 transverse buckles evenly distributed around inner diameter of lower cover. They are strong demoulding buckles during molding, and they are all naturally chamfered. Starting reference height of buckle center is 5.0 mm, protrusion size is 3.2mm * 8.0mm * 0.8mm, and it is triangular in shape. Angle between upper corner and vertical surface is 43°, angle between lower corner and vertical surface is 22°. In conjunction with inner diameter end surface of lower cover, there are 10 vertical guide ribs, size is 12.8mm * 0.8mm * 1.5mm, effective inner diameter between symmetrical ribs is φ (18.6±0.2) mm, which forms a fastening fit with right-angled triangle buckle of bottleneck, so that it can only enter but not come out of assembly, which plays an anti-theft role; ③ Appearance of cover requires a uniform color and bright surface, surface is smooth and does not scratch hands, gate is designed at an eccentric position on outer end of lower cover, and full hot runner needle valve type point gate pouring ensures balanced pouring of bow tie and upper cover of plastic parts to be formed, avoids direction of flow marks, ensures molding dimensional accuracy, reduces plastic deformation, improves assembly stability, ensures uniform coloring, leaves no residue or scratches at pouring point.

2. Mold structure and production process

Mold structure requires upper and lower covers of plastic parts to be formed to be unfolded horizontally in cavity. Production requirements are large. It adopts a 32-cavity layout, 4 vertical (8 cavities) and 8 horizontal (4 cavities) staggered arrangements, and a balanced pouring system layout. Key points of mold structure design are: ① Automatic lid closing mechanism in mold, that is, intelligent manipulator mechanism; ② Mold component structure can be quickly replaced on machine tool, that is, insert structure; ③ Mold automated production support, that is, peripheral equipment support. Mold structure adopts one-time parting, synchronous bilateral core pulling, two-time push-out, robot closing lid in mold, auxiliary air blowing, and is produced on a horizontal hybrid injection molding machine.

2.1 Design of automatic lid closing mechanism in mold

If design of automatic lid closing mechanism in mold does not meet actual production requirements, it will affect functionality of plastic part and rationality of intelligent mechanism of mold lid closing robot. As shown in Figure 2, key point of design is to use pneumatic cylinder 11 as power mechanism. Manipulator mechanism simulates cover-closing action trajectory of artificial finger joints, combines fixation and movement frames of manipulator, cooperates with signals of injection molding machine to carry out a complete set of intelligent designs. 1. Track motion plate 2. Support stretcher 3. Ball guide bush 4. Rotation axis 5. Rotation plate 6. Front plate 7. Fixed plate 8. Joint 9. Self-contained cover roller 10. Moving template 11. Pneumatic cylinder 12. Orbital motion cylinder 13. Rear profile plate
Figure 2 Automatic lid closing mechanism in mold
One set of manipulator mechanisms are fixed on each of front and rear sides of mold. They are arranged symmetrically, consist of fixed and movable parts: ①Fixing parts: 4 pieces of aluminum alloy plates in shape of a "mouth" serve as a supporting fixed frame, 2 symmetrical upper and lower profile plates are support plates of motion track. Front end is equipped with a circumferential rotation hole, and inside of rear end is processed with a profiling closing track groove; the two front and rear profile plates are fixed support plates, and front profile plate 6 is fixed on movable mold plate 10 of mold, which is main fixed plate between robot and mold. Rear molding plate 13 is tightened with two top and bottom symmetrical profile plate screws to fix support frame; ② Movable part 1: self-rotating plate 5 connects two symmetrical upper and lower profile plates. One set of rotation axis cores 4 are respectively installed at upper and lower joints. When cover is closed and folded, rotation motion can be performed in accordance with shape. Pneumatic cylinder (including piston rod) 11 is fixed in the center of rotation plate 5, and there are 4 sets of movable guide rods and ball guide bushes 3 distributed on self-rotating plate 5; Movable part 2: robot arm fixed plate 7 is in front of rotation plate 5, front end of piston rod of pneumatic cylinder 11 is fixedly connected to center of robot arm fixed plate 7, 4 sets of movable guide rods and ball guide bushes 3 pass through rotation plate 5 and are fixed on robot arm fixed plate 7. 16 robot arms and joints 8 with their own cover rollers 9 are fixed on them. When piston rod of pneumatic cylinder 11 moves, it will drive mechanical arm and joint 8 to move together; ③Movable part three: Upper and lower ends of trajectory motion plate 1 pass through 2 sets of trajectory motion cylinders 12 and cooperate with two upper and lower symmetrical templates to perform forward and backward closing movements along track. Rear end of piston rod of pneumatic cylinder 11 is fixedly connected to trajectory motion plate 1. When piston rod of pneumatic cylinder 11 moves, it will drive trajectory motion plate 1 to move together.
Function of above design: four "mouth"-shaped template frames of fixed parts are used as fixed supports to support movement when force is exerted. As execution component of lid closing action, movable part plays role of guide and thrust, realizing linear motion of track and trajectory motion of lid closing.
During operation, piston rod of pneumatic cylinder 11 drives fixed plate 7 of mechanical arm connected to piston rod of the pneumatic cylinder 11 and trajectory motion plate 1 to move with two sets of trajectory motion cylinders 12 under guidance of 4 sets of movable guide rods and ball guide bushes 3 due to changes in air pressure. Its movement trajectory will follow profiling trajectory grooves in two symmetrical templates. Finally, self-contained lid closing roller 9 on robot arm and joint 8 contacts upper cover, completes upper lid closing action according to lid closing trajectory. At this time, lower cover remains on mold core and acts as a support body in conjunction with force of closing cover. Cover-closing profiling trajectory is shown in Figure 2. The total stroke is 225.44mm, which is divided into two sections: the first section is linear motion trajectory, which is moving distance of robot arm before approaching upper cover, which is 161.36mm; second section is simulated manual lid closing action, which is a comprehensive mechanical trajectory movement. Rationality and accuracy of this trajectory will determine effectiveness and accuracy of lid closing action.
Parts of this robot's automatic lid-closing mechanism adopt a combined reference structure. Parts can be processed independently and assembled according to reference combination and precision. It is an accessory independent device of mold. After mold is installed on injection molding machine, install them separately and fix them on the front and rear sides of mold. Injection molding machine can be selected individually according to mold specifications and requirements. It is cost-effective, simple and effective.

2.2 Structural design of replaceable components

Structural design of replaceable components is shown in Figure 3. Customized all-stainless steel mold frame adopts a framed (non-through frame) insert structure. Main molded parts of frame are: core 16 on fixed mold plate 6, and core 16 on movable mold plate 5, matching parting surface of core has 4 high-precision self-positioning corners. Core insert is assembled in mold plate frame, and is locked with screws on the front, guided and positioned by 4 φ6.0mm positioning pins on the back. Cores on main parting surfaces of movable and fixed molds are designed as front assembly structures. Movable mold plate 5 and fixed mold plate 6 are processed into an insert frame structur, and mutual matching accuracy is clearance fit. Assembly accuracy of frame and core is 0.01mm on one side, which can be installed and disassembled directly on injection molding machine to quickly deal with mold emergencies, such as mold parts replacement, emergency repair of wearing parts, normal maintenance of components, etc. One cavity uses one insert, and insert specification is 130mm*85mm. Four cavities form one group, and there are 8 groups of 32 cavities in total. Insert and mold plate should be designed with cooling water channels. Each group is a cooling channel, and four inserts are a series waterway unit. Diameter of main waterway of mold plate is φ16mm, and diameter of waterway entering insert is φ8mm. This design of mold ensures that processing accuracy of parts is controllable, it is easy to manufacture and assemble. Mechanical arms on both sides of front and rear are designed to close lid left and right, arranged 4 vertically and 8 horizontally. Each of 8 vertical cavities is staggered in height. Staggered arrangement distance is designed to be half distance between upper and lower cavities, which is located at the center point between two cavities. Distance is 85/2=42.5mm. It follows left-right symmetrical structure design, which is beneficial to closing of mold and balance of pouring. 1. Lower cover content plug core 2. Buckle insert 3. Push tube 4 Push out collar 5. Moving template 6. Fixed template 7. Upper cover core-pulling slider 8. Upper cover core-pulling slider locking block 9 .Sealing needle insert 10. Bow tie insert 11. Tear tape insert 12. Lower cover core-pulling slider 13. Lower cover core-pulling slider locking block 14. Liquid outlet insert 15. Push rod 16. Fixed model core 17. Dynamic model core
Figure 3 Replaceable component structure

2.3 Mold automatic production supporting design

Designed mold has many cavities and requires high precision. In order to meet standards of high production, high efficiency and long life, a special machine and special mold technology are adopted, and supporting production process design plan: ① Use a full needle valve type hot runner pin point balanced pouring system, minimum hot nozzle outer diameter is φ10.0mm, gate diameter is φ1.0mm. It has a built-in pneumatic valve needle switch to control automatic opening and closing of injection gate, can synchronously cool it, taking into account hot and cold balance of pouring system, as shown in Figure 4; ② Use a closed-loop video detection system in mold, equipped with high-definition cameras for movable mold and fixed mold, to monitor abnormalities on main parting surface of mold in real time 24 hours a day, with an instant shutdown alarm function to ensure safe production of mold; ③ Loss-in-weight automatic metering and mixing central feeding system is used to improve efficiency and reduce costs while improving proportion accuracy and stability of feeding and color matching. It uses servo-level dehumidifiers and ice water machines. A closed transparent dust cover is added to the top of machine to achieve a constant temperature and humidity production environment inside mold. It is also equipped with a hybrid servo injection molding machine, an electric screw and a hydraulic clamping mechanism to achieve stable production.

3. Mold working process

Structure of designed bow-knot clamshell injection mold is shown in Figure 5. Mold dimensions are 1000mm*820mm*630mm. With lid closing robot included, actual lateral width is 1830mm. Mold adopts one-time parting: after hot runner injection is completed, needle valve switch 10 is closed, and injection molding machine mold plate drives mold to open. When mold opening stroke is in place, horizontal core pulling action of upper and lower covers is completed simultaneously. Molded plastic part is pushed out in two stages: hydraulic motor 16 first performs the first pushing action, and molded plastic part is pushed out from plug core 18 in lower cover. Then manipulator performs automatic lid closing action in mold, and push tube 20 is pushed out, and lower lid is separated from buckle insert 19. Finally pneumatic device in ejection collar 21 assists in blowing molded part until molded plastic part completely falls out of mold. After that, all ejection mechanisms are reset and mold is closed again. One molding cycle is completed, and production cycle t=(15±1)s, output reaches 168,000/22h, specific working process of mold is as follows. 1. Push plate 2. Push plate 3. Push plate 4. Push plate 5. Moving template 6. Fixed template 7. Fixed plate 8. Joint 9. Roller 10 Needle valve switch 11. Upper cover core-pulling slider 12. Lower cover Core-pulling slider 13. Lower cover core-pulling slide locking block 14. Pushing mechanism 15. Reset guide spring 16. Hydraulic motor 17. Push rod 18. Lower cover inner plug core 19. Buckle insert 20. Push tube 21. Push out the collar
Figure 5 Mold structure
(1) Valve needle switch 10 on point gate of hot nozzle receives a signal, and needle valve is closed pneumatically. After cooling and maintaining pressure, mold is opened, main parting surfaces of movable and fixed molds are opened, molded plastic parts are separated from point gate aggregate. Upper cover is pushed out under action of push rod (2×φ4.0mm), leaving cavity of fixed mold plate 6 and remaining in cavity of movable mold plate 5. At the same time, lower cover core-pulling slider locking block 13 drives upper cover core-pulling slider 11 to complete core-pulling of protruding locking part of upper cover, and drives core-pulling slider 12 of lower cover to complete the entire core-pulling of lock window of lower cover and tear zone, that is, it is synchronized with mold opening to complete horizontal core-pulling (left and right) movements of upper and lower covers.
(2) Hydraulic motor 16 drives combined push plate (pieces 1 to 4) to be pushed out as a whole. Molded plastic part is pushed out together with buckle insert 19, push tube 20 and push collar 21. Push stroke is 35mm. At this time, inner diameter of lower cover is separated from inner plug core 18 of lower cover, and at the same time, upper cover is separated from cavity of movable mold plate 5. Suspended distance from main parting surface is 35mm, which is suitable for robot operation and waits for lid to be closed in mold. 
(3) Piston rod of driving cylinder links 32 extended arms and joints 8 on fixed plate 7 of mechanical arm to push in horizontal direction. Rollers 9 on mechanical arm contact top surface of upper cover, simulating flipping and closing action of artificial finger joints, so that horizontally opened lid gradually closes until it is completely closed in place. After lid is closed, cylinder receives a signal to pneumatically retreat, and acts on robot arm to automatically reset.
(4) Push rod 17 is pushed by ejector rod of injection molding machine, driving push tube 20 on combined push plate (parts 1 and 2) to perform a pushing action, so that molded plastic part is pushed out of buckle insert 19, reset guide spring 15 is immediately and automatically reset. At the same time, air path on push-out collar 21 is synchronously started to assist air blowing out, and molded plastic part is blown away from mold until it completely falls.
(5) Hydraulic motor 16 drives combined push plate (item 1 to 4) to reset as a whole, and mold is closed, ready for next injection molding.