Figure 1 Carburetor

Plastic cylinder shown in Figure 1 is a certain type of car carburetor, material is PA, shrinkage rate is 1.005%, and outer dimension is 79.7mm*78.8mm*52.6mm. Molded plastic parts are required to have no weld marks, bubbles, shrinkage and flow marks in appearance, high strength, good dimensional stability. Plastic parts are also required to have characteristics of high hardness, high temperature resistance, impact resistance, abrasion resistance, chemical corrosion resistance, and easy molding.

The thickest wall thickness of carburetor is 2.7mm, average wall thickness is 2.5mm. Structure is complex and injection molding is difficult.
(1) Due to opposite directions and different positions of two internal threads, two sets of thread demolding mechanisms can be designed, and demolding mechanisms are driven by two cylinders respectively.
(2) There is a stepped hole at hole I. Outside is a hexagonal prism hole, and inside is a round hole. Hole is 15mm deep. It is located on the upper part of carburetor. It must be designed in a fixed mold for molding, cylinder drives core.
(3) Holes II and V are deep holes, with opposite directions and different positions. Two sets of core-pulling mechanisms must be designed, two cylinders driving core-pulling.
(4) There are curved surfaces, shallow holes, irregular grooves, chamfers, and rounded corners on the front, rear, left and right sides. 4 sets of slider inserts can be designed for molding and demolding.
(5) There are holes and curved surfaces on the upper and lower surfaces of plastic part. All upper features can be designed in fixed molds, and all lower features can be designed in moving molds.

 

Figure 2 Gating system
Mold adopts a structure of 1 cavity. MoldFlow is used to analyze matching of gate position, and the best gate position of plastic part to be molded is obtained. According to results of analysis and matching, considering appearance requirements of molded plastic parts and processing factors of mold parts, designed runner is a hot runner, gate diameter is ϕ4mm, and designed gating system is shown in Figure 2.

 

Figure 3 Core

 

Figure 4 Cavity plate
Molding parts of mold include core, cavity plate and inserts. In order to ensure close fit of core, cavity plate and inserts, matching openings are set for these molding parts to ensure automatic alignment during mold clamping and avoid collision accidents. Due to large size of slider, inlaid structure is adopted. Molding part can be made of die steel with a heat treatment hardness of 45~50HRC. Slider seat is made of S136 steel and does not require heat treatment. In order to prolong service life of slider, wear-resistant blocks are laid on contact surface of wedge of slider and bottom of slider. Material of wear-resistant block is H13 steel, and heat treatment hardness is 51~53HRC. Core and cavity plate are shown in Figure 3 and Figure 4 respectively.

 

Figure 5 Slider assembly

1. Left slider assembly 2. Front slider assembly 3. Rear slider assembly 4 Right slider assembly

 

Figure 6 Left slider assembly
1. Slider 2. Screw 3. Wear block 4. Guide post 5. Slider 6. Slider insert
Since molded plastic part has holes, steps, etc. around it, shape is complicated, and slider assembly needs to be used for molding. There are 4 sets of sliders. As shown in Figure 5, slider components 1 and 4 are distributed on left and right sides. Movable and fixed mold opening forces are used to drive slider insert to rotate and demold. Two slider components have same working principle and similar structure. Slider components 2 and 3 are used for forming front and back shapes of plastic parts to be molded, core-pulling irregular inner holes and square holes. Basic principles and structures of two slider components are basically same. Because four groups of slider components are distributed in different directions, design principles are basically same. Take design of left slider component as an example, and structure is shown in Figure 6.
Angle of inclined surface of slider is 20°, main function is to facilitate locking block to press slider; matching angle between locking block and inclined surface of slider is 22°, which mainly drives movement of slider. There is a wear-resistant block on the back of slider seat to prevent friction between locking block and slider from reducing accuracy, ensure normal operation of mold. Even if wear-resistant block is worn, there is no need to dismantle and replace mold, just replace wear block. Slider insert and slider seat are fixed by a T-shaped insert for limit, screw locking, processing method is simple, insert is easy to wear and replace, and it can be completed by only disassembling T-shaped insert. Slider seat adopts a wire-cut processing groove, angle of boss and side wall slope is 5°, which can solve positioning problem of insert; lower end of slider seat is equipped with glass beads to limit displacement of slider, which limits displacement to 12.16mm .

 

Figure 7 Unthreading mechanism
1. Air cylinder 2. Rack and pinion 3. Gear 4. Gear 5. Travel switch contact 6. Travel switch limit device 7. Pressure plate 8. Gear 9. Gear 10. Nut
Threading mechanism is a difficult point in design of mold. Plastic part has two internal threads, upper and lower positions are different, axis is different, but direction is opposite. Thread-removing mechanism is shown in Figure 7. Rack and pinion modules are required to be equal, otherwise transmission cannot be engaged. Transmission ratio is determined according to three conditions: pitch of formed thread, module of rack and gear, and number of teeth, to ensure precise release distance of formed thread.
Working process of mechanism: cylinder drives rack 2 to move, rack 2 drives gear 3 to rotate, gear 3 drives gear 4 to rotate, and gear 4 drives gear 8 to rotate. Gear 8 is coaxial with forming thread to complete demolding of forming thread.

 

Figure 8 Launch agency
Design of push-out mechanism is shown in Figure 8. Position of push-out mechanism is designed at the place where resistance of plastic part to be molded is the largest. Push-out force is uniform and is beneficial to exhaust of cavity. It is designed with 9 round push rods, 4 push rods for molding parts, which cannot be rotated, top is rounded off, 2 push tubes form 2 holes on plastic part and push out plastic part.

 

Figure 9 Fixed mold cooling water circuit

 

Figure 10 Movable mold cooling water circuit
Shape of plastic part is more complicated. In order to shorten cooling time, improve molding efficiency, and improve molding quality of plastic part, mold is equipped with 6 sets of cooling channels, 1 set of fixed mold, 4 sets of movable mold, and 1 set of sliders, as shown in Figure 9 and Figure 10.

 

Figure 11 Mould structure
1. Hydraulic cylinder 2. Hydraulic cylinder seat 3. Connecting rod 4. Wear block 5. Slider seat 6. Slider 7. Guide sleeve 8. Threaded shaft 9. Nut 10. Guide sleeve 11. Slider insert 12. Positioning ring 13. Heating ring 14. Slider insert 15. Slider seat 16. Slider insert 17. Slider 18. Slider seat 19. Slider 20. Wear block 21. Gear 22. Bearing 23. Shaft 24 Hydraulic cylinder 25. Gear 26. Shaft 27. Bearing 28. Support column 29. Hydraulic cylinder 30. Rack 31. Slide block insert 32. Hydraulic cylinder 33. Rack 34. Bearing 35. Shaft 36. Gear 37. Slide Block insert 38. Heat insulation plate 39. Fixed mold seat plate 40. Fixed template 41. Slider insert 42. Movable template 43. Support block 44. Push rod fixed plate 45. Push plate 46. Movable mold seat plate 47. Core 48. Lengthened faucet 49. Wear block 50. Slider 51. Wear block 52. Slider 53. Inclined guide column 54. Cavity plate
Mold structure is shown in Figure 11. Mold is fixed on injection molding machine for injection. After pressure is maintained and cooled, mold is opened after melt is stable and formed.
(1) Start hydraulic cylinder 24, piston rod of hydraulic cylinder 24 drives slider mechanism to move, drives slider 17 and slider seat 15 to drive slider insert 14 for core pulling.
(2) Slider of injection molding machine drives moving and fixed mold to open mold. During mold opening and closing movement, under action of inclined guide post, front and rear slider components are driven to move laterally at the same time. Slider 50 drives insert to move, two sets of slider assemblies complete core pulling from both sides.
(3) Piston rod of hydraulic cylinder 1 drives gear and rack, then drives unthreading mechanism to drive formed thread to rotate for linear motion. Transmission of force source in this process is completed by rack 33, bearing 34, shaft 35, gear 36, etc. Finally, forming thread is driven to rotate, and thread of plastic part to be formed is core-pulled.
(4) Continue to start hydraulic cylinder to drive left and right slider components to move, at the same time drive slider inserts 11, 31, 37 to pull cores of side holes and irregular grooves on both sides.
(5) Use push rod of injection molding machine to push push plate to push out molded plastic part, at the same time push tube to complete formation of local small holes of plastic part to be molded.
(6) All parts of mold are reset for next injection.
Designed mold structure adopts 4 sets of slider components and 2 sets of threaded demoulding mechanisms. Test shows that design achieves purpose of simplifying mold structure, stabilizing production, reducing manufacturing costs, simple operation, and improving production efficiency.