As shown in Figure 1, plastic part has a length of 109.00mm, a maximum outer diameter of φ10.20mm, a hollow inner hole length of 106.10mm (blind hole), a 6.50mm step at the top, a φ9.40mm raised matching rib in the middle, a 7.90mm long inner cover inside, and a φ5.15mm outwardly protruding matching rib at a length of 3.80mm in inner hole. There is a 12.00mm step at the other end, and a φ9.45mm raised matching rib in the middle. Plastic parts have characteristics of being small at both ends and large in the middle, with a complex matching rib structure and difficult demolding of molded plastic parts, resulting in high requirements for mold parting. Plastic parts are molded by batch injection molding, injection material is polypropylene (PP). Shrinkage rate is 1.8%~2.6% in length direction and 0.8%~1.2% in radial direction. After molding, surface of plastic parts is required to be smooth, without defects such as flash, lack of material, burning, shrinkage, etc. Matching dimensions meet requirements of drawings, tightness of matching with other parts is appropriate, and sealing is reliable.

 

In order to adapt to characteristics of batch orders in pen-making industry, mold adopts a non-standard mold frame structure with 32 cavities, feed sleeve is fed by internal point gate method to balance flow channel layout and cone head pull rod pulls material. Mold adopts method of sinking fixed mold base plate with a T-shaped gate sleeve, moving and fixed mold double guide column guidance and double locator positioning structure ensure position accuracy of mold cavity. Since there are matching ribs inside and outside plastic parts at both ends, there are problems such as complex structure, large demoulding resistance, and difficulty in ejecting after molding. In order to meet structural characteristics of matching ribs inside and outside plastic parts at both ends, ensure precision requirements of dimensions and shapes of plastic parts, mold is designed with 4-parting and 2-parting structures to solve problem of difficulty in demoulding matching ribs inside and outside plastic parts.

4-parting of mold is completed by using limit guide pins, limit retaining rings, die buckles and 3 different lengths of fixed distance pull rods; 2-parting is completed by using guide pin positioning, push rods, inserts, push plates, push tubes and lock pin components, and mold structure is shown in Figure 2.

 

1. Moving mold base plate 2. Core fixing plate 3. Moving mold support plate 4. Push tube fixing pad 5. Pad 6. Push tube fixing plate 7. Limit retaining ring 8. Unloading plate pad 9. Unloading plate 10. Cavity plate 11. Cavity plate 12. Shoulder guide sleeve 13. Shoulder guide column 14. Tie rod guide column 15. Feed sleeve fixing plate 16. Fixed mold unloading plate 17. Fixed mold base plate 18. Push tube 19. Unloading sleeve 20. Core 21. Cavity sleeve I 22. Cavity sleeve II 23. Feed sleeve 24. Cone head pull rod 25. Positioning ring 26. Fixed distance pull rod 27. Pull rod fixing plate 28. Insert 29. Runner plate 30. Cold material sleeve 31. Feed sleeve pressure plate 32. Fixed distance pull rod 33. Fixed distance pull rod 34. Precision positioning pin 35. Precision positioning sleeve 36. Die buckle 37. Die buckle 38. Screw 39. Reset rod 40. Support column 41. Small guide column 42. Small guide sleeve 43. Fixed distance pull rod 44. Water pipe screw
Figure 2 Die structure

Since there are matching ribs inside and outside head of plastic part, structure is complex, and it is difficult to demould by normal 2- or 3-parting methods. In order to ensure dimensional accuracy and shape integrity of matching ribs inside and outside during parting of plastic parts, a 4-parting structure is designed for mold fixing part, as shown in Figure 3.

 

After injection, pressure holding and cooling, mold is opened. Mold is parted for the first time by guide pins and limit guide pins under action of slider of injection molding machine. It is opened between feed sleeve fixed plate 15 and fixed mold unloading plate 16. Reverse taper of head of cone head pull rod 24 pulls condensate in runner, separates runner condensate from feed sleeve and plastic part gate, fixes it on cone head pull rod 24 and fixed mold unloading plate 16. After the first parting, it is shown in Figure 3 (a).

When mold opening distance reaches 108mm, inner side of distance rod 32 is limited by feed sleeve fixed plate 15. Since sum of clamping force between head of plastic part and inner and outer diameters of feed sleeve and friction between outer diameter of feed sleeve and inner hole of cavity sleeve is greater than pulling force of cone head pull rod 24, fixed mold unloading plate 16 is pulled back for second parting 8mm, and distance pull rod 26 is used to limit flow channel condensate from cone head pull rod 24. After second parting, it is shown in Figure 3 (b). 

After distance rod 26 is limited, inner hanging platform of distance rod 32 pulls feed sleeve fixing plate 15 to continue to open mold by 7mm, and distance rod 33 is limited, so that feed sleeve fixing plate 15 and the feed sleeve 23 move forward for third parting, and feed sleeve core pulling action is performed to make feed sleeve 23 separate from inner hole of plastic part. This parting is crucial to keep inner and outer matching ribs of plastic part head in a complete shape. Parting action and parting length must be in place. After third parting, it is shown in Figure 3 (c).

Cavity plates 10 and 11 are pulled out of control of die pull buckle by pull rod guide column 14, and precision positioning pin 34 is separated from precision positioning sleeve 35. 4th parting is carried out between cavity plates 10 and 11 and unloading plate 9. Movable and fixed molds are completely separated, and plastic parts are completely separated from fixed mold cavity plate. Plastic parts are taken out, and fixed mold is completed after 4 parting. After 4th parting, it is shown in Figure 3 (d).

Since there is another section of other end of plastic part in inner hole cavity of unloading sleeve and there is a ring-shaped matching rib, unloading sleeve cannot be directly demolded. If push tube is directly used for ejection, demolding resistance of plastic part matching rib is large, it is difficult to ensure integrity of external shape and dimensional accuracy requirements. In order to solve problem of difficulty of demolding plastic part, mold movable part is designed to eject twice, and hook-shaped lock pin assembly structure assists 2nd ejection.

As shown in Figure 4, hook-shaped lock pin assembly consists of a locking seat 49, a spring 50, a locking block 51, a hook-shaped lock pin 52 and a latch 53. Working principle of assembly is mainly to use elastic force of spring 50 to hold locking block 51 and hook-shaped lock pin 52 in a closed state, then control opening and closing of hook-shaped lock pin 52 through stroke of latch 53. Since opening and closing state of hook-shaped lock pin is controlled by latch stroke when hook-shaped lock pin assembly moves, hook-shaped lock pin assembly must be assembled according to given size and position requirements when installing. If position error is large, mold may not open and close normally.

 

44. Square locator 45. Ejector insert 46. Insert screw 47. Insert baffle 48. Ejector 49. Locking seat 50. Spring 51. Locking block 52. Hook lock pin 53. Latch
Figure 4 Lock pin assembly structure

After mold is parted, ejector pin 48 of injection molding machine is connected to ejector tube fixing pad 4 through ejector pin insert 45 to push forward. Ejector tube fixing pad 4 is connected to movable mold support plate 3, pad 5, ejector tube fixing plate 6, unloading plate pad 8, unloading plate 9 under action of hook-shaped locking pin and locking block to form a combination body guided forward by shoulder guide column 13 for the first overall ejection. Unloading sleeve 19 and ejector tube 18 simultaneously eject plastic part, so that plastic part is upwardly separated from core, providing conditions for second demolding of molded plastic part's outer shape matching rib. Ejection action is shown in Figure 5.

 

When combination of movable mold support plate 3, push tube fixed pad 4, pad 5, push tube fixed plate 6, unloading plate pad 8 and unloading plate 9 is pushed forward to 92mm position, it is limited by fixed distance pull rod, as shown in Figure 6. At the same time, latch 53 presses back locking block 51, so that hook-shaped locking pin 52 is separated from locking block 51 for second push-out. At this time, push tube fixed pad 4 and push tube fixed plate 6 are fixed into a combination by screws, separated from hook-shaped locking pin, separated from combination of movable mold support plate 3, pad 5, unloading plate pad 8 and unloading plate 9. Then, push tube fixed pad 4 drives push tube 18 to be guided forward by small guide column 41 to push plastic part out of inner hole of unloading sleeve. Push-out stroke is 20mm, as shown in Figure 5. After the first ejection, inner hole of plastic part and outer diameter of core have been pulled out to avoid air, and there is no clamping force. The second demolding by push tube has no effect on matching ribs, shape and dimensional accuracy of plastic part can be guaranteed. After demolding action is completed, spring assists injection molding machine ejector 48 to reset push tube fixing plate assembly.

 

Design of casting system should be determined based on factors such as volume, wall thickness, complexity of shape of plastic part, injection rate and runner length. Because mold is a 1-mold 32-cavity layout, a balanced runner must be used, runner length should be as short as possible. Design of all runners requires uniform distribution, cross-sectional shape of runner is trapezoidal. A cold material hole is set at the end of main runner to prevent cold material from blocking feed port, as shown in Figure 7.

 

Cooling system is divided into core internal cooling and mld plate cooling. Inner hole of core 20 is designed with cooling water channels. Eight cooling water channels are designed on movable mold base plate 1. Water pipes are connected with water pipe screws and inserted into inner hole of core to directly cool core 20 and control temperature, as shown in Figure 8. Unloading plate 9, feed sleeve fixed plate 15 and fixed mold unloading plate 16 are each designed with 2 through water channels for indirect cooling; cavity plates 10 and 11 are designed with 4 through water channels for indirect cooling due to thick mold plate thickness, as shown in Figure 9.

 

 

Exhaust grooves are processed at the end of each branch channel to reduce burning of plastic parts and trapped air caused by gas entering cavity. Exhaust groove size is 2mm long, 3mm wide and 0.02mm deep at the end of flow channel, exhaust groove size of rear section is 3mm wide and 0.15mm deep, as shown in Figure 10. Design exhaust grooves on the front of unloading plate pad 8 and back of feed sleeve fixing plate, process 8 horizontal and 4 vertical exhaust grooves with a width of 3mm and a depth of 0.5mm in the center of 32 cavity holes to avoid undesirable phenomena of burning, trapped air, and lack of material on upper and lower end faces of plastic parts, as shown in Figure 11.