Design of Injection Mould for Protective Cover of Deep Cylindrical Receiver

Time:2021-07-12 11:28:05 / Popularity: / Source:

<Abstract> Forming process of protective cover of deep cylindrical receiver is introduced, and three-plate mold structure of point gate with inner slide core pulling + nozzle cooling + push plate insert is designed. Practice has proved that mold structure is reliable, molded plastic parts are pushed out smoothly, which has a certain reference effect for mold design of similar plastic parts.

1 Plastic parts analysis

Deep cylindrical receiver cover is shown in Figure 1. Size is MT3 level requirement, mass production. Raw material is ABS. ABS is a thermoplastic engineering plastic with good moldability and processability. Surface roughness of molded plastic parts is good. Average density of ABS is 1.05 g/cm³, and average shrinkage rate is 0.4%~0.7%. Commonly used shrinkage rate is 0.5%. Height of plastic part is 151 mm, bottom diameter is φ107 mm, and top diameter is φ73 mm, which is a deep cylindrical structure. Due to special use of plastic parts, maximum wall thickness of plastic parts is 5 mm, minimum wall thickness is 3 mm, and average wall thickness is about 4 mm. There is no particularly thick or thin position, and wall thickness is basically uniform, which meets requirements of plastic molding processability. Volume measured by UG NX software is 182.5 cm3 and mass is about 191.6 g. It is a small and medium-sized plastic part.
Design of Injection Mould for Protective Cover of Deep Cylindrical Receiver 
Figure 1 Cover structure

2 Parting surface design and demolding analysis

Parting surface is an important factor that determines structure of mold. It is related to the overall structure of mold, design of gating system, demoulding of plastic part and manufacturing process of mold. Therefore, selection of parting surface is a key step in design of injection mold.
Main parting surface of molded plastic part is relatively flat. As shown in Figure 2, there is a "¬"-shaped hanging feature on the outside of bottom of plastic part. In order to facilitate processing of mold parts and consider overall flatness of parting surface, a pillow position is designed to make main parting surface a large plane. There are 6 undercuts inside plastic part, and inner slide block in 3 directions shown in Figure 1 is required to complete core pulling. The overall parting surface of plastic part is better handled. Specific position of parting surface has been indicated by symbols in Figure 1. Difficulty in mold design is how to deal with structural relationship among three core-pulling mechanisms, movable mold cooling channel and plastic part ejection method. In subsequent design process, adjustments and optimizations need to be made according to actual situation of mold structure.
Design of Injection Mould for Protective Cover of Deep Cylindrical Receiver 
Figure 2 Main parting surface

3 Forming part design

Molded parts mainly include cavity plates, cores, inserts, etc. Since cavity plate is in direct contact with high temperature and high pressure plastic melt, quality of molded parts is directly related to molding quality of plastic parts, it is required to have sufficient strength, rigidity, hardness, wear resistance to withstand extrusion force of plastic melt and friction during flow force. Molded parts should have sufficient precision and appropriate surface roughness to ensure that molded plastic parts have a bright surface and are easy to demold. Generally, molded parts should be heat treated or pre-hardened to make them have a hardness of 30 HRC or more.

3.1 Structure design of cavity plate inserts

Plastic parts belong to deep cavity category, melt and cavity wall friction during injection are relatively large. In order to prolong service life of mold and facilitate processing of mold parts, cavity plate structure is designed as an integral embedded structure. Bottom diameter of plastic part is φ107 mm. Overall shape of plastic part is cone-like, outer shape is not complicated, and there is no special structure. Size of cavity plate is expanded by about 27 mm on the basis of size of plastic part. After size is rounded, cavity plate insert design is shown in Figure 3. External dimension is 160 mm*160 mm*175 mm. After being embedded in fixed mold plate, it is fixed with screws.
Design of Injection Mould for Protective Cover of Deep Cylindrical Receiver 
Figure 3 Cavity plate insert structure

3.2 Core insert structure design

In order to save materials and reduce processing volume of mold parts, core is designed as an integral embedded structure as shown in Figure 4. Bottom extends 20mm down at parting surface for core and push plate inserts. Fixed part of core insert is designed with a square structure of 145 mm*145 mm*20 mm, which is convenient for core positioning and screw fastening.
Injection Mould for Protective Cover of Deep Cylindrical Receiver 
Figure 4 Core insert structure

4 Mold design

4.1 Design of gating system

Mold has a 1-cavity structure, and gate is set at the center of concave surface of top of plastic part to be molded. According to requirements of plastic parts, it is not allowed to leave obvious gate marks on outer surface. Mold uses point gates for gating. Advantage of point gate is that gate mark is small, condensate of gating system is automatically separated from plastic part when mold is opened, and there is no need to remove condensate of gating system in later stage. Gate diameter is generally φ0.8~1.5 mm, and length is 0.8~1.2 mm. In order to facilitate better separation of gate and plastic parts, gate should be designed with a 15°~20° taper angle.
Specific structure and dimensions of gating system are shown in Figure 5. In order to ensure that condensate of gating system is automatically separated from molded plastic part when mold is opened, a 130 mm long primary runner is designed to place two pull rods so that draw rods can pull aggregates of gating system and automatically disconnect them from molded plastic parts when mold is opened. Secondary runner passes through fixed mold plate and cavity plate insert. In order to avoid misalignment caused by processing and assembly errors, runner condensate cannot be demolded, secondary runner is designed with a unilateral 0.2 mm step difference as shown in Figure 5 .
Injection Mould for Protective Cover of Deep Cylindrical Receiver 
Figure 5 Gating system

4.2 Internal core pulling structure design

There are 6 undercuts uniformly distributed in inner circumferential direction of plastic parts, and maximum distance of inside protruding undercuts is 5 mm. Generally speaking, for plastic parts with grooves or protrusions on inner side, priority is given to using inclined push structure to pull core, but small top space of plastic part will cause mutual collision and interference when inclined push structure is pushed out; in addition, concave curved surface at the top of plastic part will cause damage to molded plastic part when inclined push rod structure moves sideways, so bottom of inclined push rod seat needs to be designed with a certain slope. After analysis, plastic part is not suitable for inner core pulling by means of oblique push rod structure.
According to characteristics of plastic parts, it is designed as an integral inner slide core pulling mechanism. Inclination angle of inclined guide post is initially designed as 15°. Considering that locking angle β (wedge block angle) of inclined surface of slider should be larger than inclined guide post inclination angle α 2°~3°, angle of wedge block is designed to be 18°. Generally, core-pulling distance = lateral concave-convex depth of plastic part + safety distance = 5 mm + (2 ~ 5) mm = (7 ~ 10) mm, and height of oblique guide post hole of slider body is measured to be 40 mm. Draw a sketch in UG software, measure inclined guide column to provide a core-pulling distance of 7.8 mm>7 mm. It can be seen that angle of inclined guide column and vertical height of inclined guide-post hole of slider meet requirements of core-pulling distance.
Positioning design of inner slide block. During mold opening process, inner slide block moves for a certain distance under drive of inclined guide column. When inclined guide column leaves slide block, slider must be kept in its original position to ensure that inclined guide post can enter inclined hole of slider when mold is closed, so that it can be accurately returned to position. Sliding block is small and core pulling distance is long, inner sliding block is positioned by glass bead screws. Based on above analysis and calculation, inner slider core pulling mechanism shown in Figure 6 is designed.
Injection Mould for Protective Cover of Deep Cylindrical Receiver 
Figure 6 Inner slider core pulling mechanism
1. Oblique guide column 2. Wedge block 3. Screw 4. Integral slider

4.3 Launched structural design

Since there are 3 inner sliding blocks in the middle of molded plastic part and a nozzle-type cooling water channel, it occupies a large space and there is no place for push rod. Plastic part is of a deep cylindrical shape and has a large packing force. In order to ensure that plastic part is not deformed, demolding is stable, and there is a large enough pushing force, mold is designed as a push plate insert as shown in Figure 7. In order to reduce abrasion of push plate and core during pushing process, mating surface of push plate and core is designed as a tapered surface, inclination α of tapered surface is taken as 3°. Tapered surface can also play an auxiliary guiding role. Inner hole of push plate insert is designed to be 0.3 mm larger than single side of core forming part to prevent scratches, fraying and jamming between push plate insert and core.
Injection Mould for Protective Cover of Deep Cylindrical Receiver 
Figure 7 Push plate insert structure
1. Inner slider 2. Screw 3. Push plate insert 4. Reset rod 5. Core insert 6. Push rod 7. Return spring 8. Push rod fixing plate 9. Push plate

4.4 Design of fixed-distance parting mechanism

Structure that guarantees mold opening sequence and mold opening distance is called a fixed distance parting mechanism. There are many types of fixed distance parting mechanisms, mainly divided into two types: built-in fixed distance parting mechanism and external fixed distance parting mechanism. After measurement and analysis, internal space structure of mold is relatively large, mold design is a built-in small tie rod fixed-distance parting mechanism as shown in Figure 8. Small tie rod 1 limits mold opening distance between stripper plate and fixed template in fixed distance parting mechanism. Opening distance between stripper plate and fixed mold plate = total height of the runner aggregate + 30 mm = 50 mm + 30 mm = 80 mm ; Opening distance between stripper plate 3 and fixed mold base plate 5 is generally 6~10 mm. Its main working principle: under action of spring 4, die buckle and draw rod, mold is first opened on stripper plate 3 and fixed mold plate 2 to separate runner aggregate from molded plastic part; followed by stripper plate and fixed mold seat plate are opened, pull rod is forced out of runner aggregate by stripper plate, runner aggregate automatically falls off under action of gravity and vibration; injection molding machine slide block drives moving mold plate to continue to move backwards, die buckle is pulled out from fixed mold plate 2, mold is opened between fixed mold plate and movable mold plate, finally ejection device pushes out molded plastic part.
Injection Mould for Protective Cover of Deep Cylindrical Receiver 
Figure 8 Built-in small tie rod fixed distance parting mechanism
1. Small tie rod 2. Fixed template 3. Stripping plate 4. Spring 5. Fixed mold seat plate 6. Screw

4.5 Cooling system design

Design of cooling system should enable mold to cool quickly and evenly, facilitate processing, try to ensure thermal balance of mold to make plastic part shrink evenly. Mold belongs to deep cavity structure, core inserts are relatively high, core is designed with 3 inner sliding blocks. Space for designing cooling channel is not large, and conventional cooling channel design cannot be carried out. In order to fully cool mold, after analysis and consideration, movable mold cooling system is designed as shown in Figure 9 with 3 nozzle-type cooling channels with an inner diameter of φ6 mm. When designing nozzle type cooling, it should be noted that top of water pipe should not be too close to cavity wall, generally greater than 12 mm, so as not to affect strength of mold.
Core insert structure design 
Figure 9 Moving mold cooling system
1. Nozzle 2. Waterproof ring 3. Movable template
In order to ensure that cavity plate can be fully cooled, fixed mold is arranged with 3 layers of planar loop cooling water channels along depth of cavity. Cooling method design is shown in Figure 10. Diameter of cooling water channel is designed to be φ8 mm. Since water inlet and water outlet are on the side wall of cavity plate insert, it is inconvenient to install waterproof ring, and extended internal water connection joint is selected.
Core insert structure design 
Figure 10 Fixed mold cooling system

4.6 Exhaust system design

Plastic part is of a deep cylindrical shape, and the most easily trapped air is at the end of melt flow, that is, at the bottom of molded plastic part. 8 venting grooves are designed on push plate insert, as shown in Figure 11. A first-level exhaust groove with a depth of 0.03 mm and a length of 7 mm is designed near cavity. A second-level exhaust groove is designed after first-level exhaust groove, and exhaust groove is deepened to 0.5 mm to ensure smooth gas discharge.
Core insert structure design 
Figure 11 Exhaust slot structure

4.7 Mold structure and working process

Mold structure is shown in Figure 12. When mold is closed, convex and concave side locks 21 and 22 ensure accuracy of mold clamping. Nozzle of injection molding machine injects melt into mold cavity through sprue sleeve 18. After pressure is maintained and formed, mold is opened. Because die buckle 33 locks fixed plate 11 and movable plate 7, under action of spring 41, mold is first opened on parting surface of PL1, fixed mold plate 11 and movable mold part move backward, gate condensate is pulled off. Under action of pull rod 17, condensate of runners stays on the side of stripper 14. When fixed mold plate 11 and movable mold part move a certain distance, limit pull rod 42 fixed on fixed mold seat plate contacts counterbore of fixed mold plate 11, PL1 parting end limit lever 42 continues to drive stripper plate 14 to open at parting surface of PL2, stripper plate 14 pushes out condensate of runner and main runner. At this time, fixed mold plate and movable die part are separated from die buckle 33, opened on PL3 parting surface. Inclined guide post 27 drives integral inner slider 26 to slide toward inside of mold. After PL3 parting surface is opened for about 40 mm, 3 inner sliders complete core pulling action and reach safe distance position. Integral inner sliding block 26 is restricted by glass bead screw 25. Movable mold continues to move back. When ejector pin of injection molding machine contacts  push plate, ejection mechanism starts to work, plastic part is pushed out by push plate insert 10, plastic part and aggregate of gating system are taken out to complete an injection molding process.
Core insert structure design 
Figure 12 Mould structure
1. Movable mold seat plate 2. Cushion block 3. Push plate 4. Push rod fixed plate 5. Push rod 6. Guide sleeve 7. Movable template 8. Core insert 9. Screw 10. Push plate insert 11. Fixed Template 12. Cavity plate insert 13. Guide sleeve 14. Stripping plate 15. Straight guide sleeve 16. Guide column 17. Pull rod 18. Gate sleeve 19. Fixed mold seat plate 20. Lengthened nozzle 21. Convex Shaped side lock 22. Concave side lock 23. Limiting column 24. Screw 25. Glass bead screw 26. Integral inner slide 27. Oblique guide column 28. Screw 29. Wedge block 30. Screw 31. Nozzle 32 .Screw 33. Die buckle 34. Seal ring 35. Water outlet nozzle 36. Limit nail 37. Reset rod 38. Spring 39. Screw 40. Screw 41. Spring 42. Limit pull rod 43. Plug 44. Screw 45 .Screw

5 Concluding remarks

Combining difficult problems of plastic parts molding, a three-plate mold structure with inner slider core pulling + nozzle cooling + push plate inserts is designed. Through production practice, it is proved that mold structure is reliable, cooling is uniform, there is no deformation and shrinkage of molded plastic parts, plastic parts produced in a stable manner and mass-produced have good molding quality, which is a reference for mold design of similar plastic parts.

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