Taking design of injection mold for caster of medical bed as an example, characteristics of European two-color mold and Japanese two-color mold are introduced. Analyzed design method of hot runner two-color mold, designed a novel structure of splitter plate, introduced structure, applicable occasions and design points of two-color mold ejected from middle plate. It shows how to integrate hot runner technology and two-color mold technology in same mold. Provide reference for personnel engaged in mold design to reasonably determine design scheme of hot runner two-color mold.

Medical bed casters are moving parts of medical beds, usually made of plastic. Figure 1 shows plastic part of caster of medical bed. Hard plastic part of plastic part is PP and soft plastic part is TPE. Plastic part adopts structure of soft rubber and hard rubber to make appearance of plastic part beautiful. At the same time, soft rubber has elasticity and can reduce vibration. Shape of caster plastic part of medical bed is round, multiple bones are evenly distributed on outer side of inner central column and inner side of edge. Soft rubber surrounds hard rubber in a circle. Diameter of inner hole of inner hollow column is ϕ6.25mm, and inner wall has a small undercut, which requires forced demolding. Technical requirements of plastic parts are not to have various defects such as peaking, under-injection, flow lines, pores, warpage deformation, silver streaks, cold materials, jet lines, etc. Soft glue and hard glue are tightly and reliably combined, edges of joints are neat and beautiful.

 

Figure 1 Plastic parts of medical bed casters

Medical bed casters are wheels for light-load mobile devices. They are standard parts and are in large demand. Production batch of molds belongs to large batches. Molds are arranged for soft and hard rubber with 8 cavities respectively, glue feeding method is two-color hot runner injection. Multi-cavity hot runner molds must be designed with a manifold, and a runner must be designed on the manifold to deliver molten plastic to each cavity. For two-color molds, design of hot runner system can be divided into two options. The first is to design a separate splitter plate for each material, and the two splitter plates are installed side by side in fixed mold plate; the second is to design runners of the two materials in same splitter plate in layers. Runners are staggered with each other. The second solution can make mold structure compact, suitable for multi-cavity molds of small plastic parts. This sub-mold chooses second option.
Both hard rubber and soft rubber are used to inject molten plastic on runner with two hot nozzles 45 and 47 respectively, and then divided into 4 channels into 4 cavities. Gates are all latent gates.

Two-color mold structure can be divided into two categories according to mold design style and injection molding machine. Two-color injection molding machines in Japan and China represent Asian style, both have two parallel injection systems; while two-color injection molding machines in Europe have one barrel horizontal and one vertical, and there is only one two-color mold. Principle of two-color molds of the two styles is basically similar. Shape of upper and lower rows of fixed mold is different, movable mold is rotated 180° to form the first and second fixed molds respectively. Mold structure of two-color injection molding machine can be an integral mold structure, or it can be designed separately into a structure of two mold bases. Medical bed caster is a European-style two-color mold. Mold structure is a mold structure with a mold base, a middle plate ejected and rotated. Runners of the two materials are designed in same shunt plate. Center of fixed mold base plate 2 is primary sprue sleeve 4, secondary sprue sleeve 1 is designed on the side of mold. Both the first injection and second injection are latent gates and glue is injected into mold.

Molds are arranged in soft and hard rubber with 8 cavities respectively, and mold structure design is shown in Figure 2. Mold base is a non-standard mold base 4045. 8-cavity hard rubber and 8-cavity soft rubber are located on both sides. 4 sides of mold base are designed with a slope positioning block 11 to provide precise positioning for mold. Middle plate is ejected by center rotating shaft 27. Top end of central rotating shaft 27 is mounted on the bottom of core 10 and guided by two sets of plane bearings 23 and 25. A cooling water transport spacer is designed in the middle of central rotating shaft, and cooling water transport is connected with core 10 to form a water transport circuit. A gear 22 is mounted on the central rotating shaft, and gear 22 meshes with rack 16. Rack 16 is fixed on rack fixing seat 48, and oil cylinder 49 drives rack 16 to move. Rack 16 is guided by two sets of guide bearings 34. Guide bearing 34 is fixed on ejector plate 18 through a bearing fixing seat 35.
Bottom of central rotating shaft 27 is fixed on ejector plate 18 through guide sleeve 24. Ejector plate 18 is ejected by two oil cylinders 41. Runner push rod 37 and push rod 39 are all fixed on push plate 20. Ejection of push plate 20 and ejection plate 18 is guided by guide post 36, so that ejection is stable.

 

Figure 2 Two-color mold for casters of medical bed
1. Secondary sprue sleeve 2. Fixed mold seat plate 3. Diversion plate 4. Primary sprue sleeve 5. Positioning ring 6. Center pad 7. Pressure bearing pad 8. Support pad 9. Cavity 10. Core 11 .Inclination fixed module 12. Sealing ring 13. Bushing 14. Movable template 15. Limiting column 16. Rack 17. Support block 18. Ejector plate 19. Push rod fixed plate 20. Push plate 21. Movable mold bottom plate 22. Gear 23. Plane bearing 24. Guide sleeve 25. Plane bearing 26. Bearing pressure plate 27. Center shaft 28. Flat key 29. Movable mold insert 30. Center insert needle 31. Core compression block 32. Insert fixed Plate 33. Ball guide sleeve 34. Guide bearing 35. Bearing holder 36. Guide post 37. Runner push rod 38. Reset rod 39. Push rod 40. Spring 41. Cylinder 42. Guide sleeve 43. Guide post 44. Positioning Block 45. Primary hot nozzle 46. Support head 47. Secondary hot nozzle 48. Rack fixing seat 49. Cylinder

After one injection is completed, mold is opened, runner push rod 37 pushes hard glue nozzle out and automatically falls. Oil cylinder 41 drives ejector plate 18 to eject central rotating shaft 27. Central rotating shaft 27 pushes out core 10 and insert fixing plate 32 together, movable mold insert 29 drives plastic part to separate from center insert 30, inner hole of center of plastic part is reversed to achieve strong release. Oil cylinder 49 drives gear 22 through rack 16 to realize rotation of middle plate mechanism. After rotating 180, mold is closed again, after injection molding and cooling, finished plastic part is ejected out of mold under action of push rod 39.

Hot runner two-color mold designed by this sub-mold concentrates two materials on same manifold. Mold structure is novel and reliable. Usage conditions since it was put into production show that mold structure design is advanced, various indicators meet requirements of mold style book, which has won praise from foreign customers.