【Abstract】Introduces design and working process of an injection mold for an electrical base. Mold adopts two-way ejection and fixed mold core pulling, fixed mold ejection structure adopts joint form of nylon mold clamp and spring, so that it can separate plastic parts from fixed mold cavity first during mold opening process to avoid occurrence of plastic parts sticking to fixed mold, fixed mold is ejected smoothly, processing is simple, mold structure is compact, and work is reliable.

Figure 1 shows base of appliance, material is PA6. Material has excellent thermal stability, high heat resistance, dimensional stability and other characteristics, shrinkage rate is 0.6%, maximum size of outer contour of plastic part is 78.8*54.0*51.7mm, wall thickness is 1.6mm, and there is a 11.6*2.2mm square hole at side D of plastic part.
Main points of plastic part process: surrounding of plastic part and A direction (pointed by arrow) are appearance surface, and there should be no appearance defects, such as flash or burr, ejector traces, etc. Depth of step at plastic part E is 39.9mm, which requires a short injection cycle, can meet production requirements of medium and high output.

 

Mold adopts a latent gate two-plate mold structure, molding cavity and core adopt an inlaid structure. Mold structure is shown in Figure 2. Following is a brief description of design concept of plastic parts.

 

 

Figure 2 Mold structure
1. Fixed die base plate 2. Fixed platen 3. Pull spring 4. Nylon die clamp 5. Support guide post 6. Support guide post gasket 7. Moving formwork 8. Pad iron 9. Ejector pin fixing plate 10. Top Rod pad 11. Moving die seat plate 12. Support column 13. Top rod 14. Moving die insert 15. Side core 16. Slider 17. Guide rail 18. Lock template 19. Fixed model core 20. Fixed die insert 21. Locking oblique pin 22. Cooling water pipe 23. Positioning ring
(1) Periphery of plastic part and A direction are appearance surfaces. All appearance molding designs are in fixed mold. Step size at E of plastic part is 39.9mm, which has risk of wrapping middle core. Plastic part adopts a two-way ejection structure and is designed according to conventional fixed-die ejector method. There will be a phenomenon that ejector pin is difficult to eject due to excessive clamping force, and there is also the risk of ejection. Fixed mold ejection is in the form of a nylon mold clamp and a spring in fixed mold direction, so that plastic part is released from intermediate fixed mold core. Here we focus on design of fixed die ejection and working process of die. Considering output requirements, die design is two cavities.
(2) There is a 11.6*2.2mm square hole on the side D of mold, which is completed by using a diagonal pin to drive side core. This diagonal pin has a locking function. When mold is opened for the first time, slider pulls side core away from plastic part under action of locking oblique pin.
(3) Fixed mold direction of mold is designed with a plane demoulding structure, forming step at E adopts a mosaic structure and is fixed on the bottom plate of fixed mold. Core and fixed mold insert have a clearance fit. Size of gap on one side is 0.01mm, fit is smooth and there will be no burrs. Taper is designed on the back of fixed mold insert, which makes mold installation and demolding convenient, and achieves a smooth ejection effect. Fixed mold bottom plate and fixed mold plate are designed with spring steps, nylon mold clamp is designed on movable mold plate to realize the first mold opening, fixed mold bottom plate and fixed mold plate are connected with support guide column to realize second mold opening.
(4) Runner of plastic part is designed as a semi-circular runner, which has characteristics of small cross-sectional area and flow velocity block. A latent gate is set on the side of plastic part (this surface is attached to other plastic parts, and gate mark does not affect appearance), moving die inserts are designed with an inverted conical pulling trough, which is convenient for pulling down fixed die gate. In order to reduce waste of material head, injection nozzle is used to press into mold plate structure design.
(5) Design of cooling system takes into account principles of cooling efficiency and cooling uniformity, and cooling water channels are arranged on mold plate, fixed mold core and insert. Core at E is higher, only cooling water holes are opened on insert and mold plate, which will cause uneven cooling, which will affect dimensional stability of plastic parts. Therefore, a threaded cooling water pipe is designed behind core at E, back is sealed with a sealing ring. As shown in Figure 3.

 

Figure 3 Cooling system
1. Cooling water pipe of core at e 2. Cooling water pipe of fixed mold base plate 3. Cooling water pipe of fixed mold plate 4. Cooling water pipe of fixed mold block 5. Cooling water pipe of movable mold block 6. Cooling water pipe of moving mold block

After injection molding is completed, injection molding machine moves, movable mold plate 7 opens mold with injection molding machine, nylon mold clamp 4 tightens fixed mold plate 2, and at the same time, under force of pulling spring 3, fixed mold core 19 does not move, fixed mold plate, fixed mold inserts, other fixed mold cores, and plastic parts move back together with moving mold plate, plastic parts are released from middle fixed mold core 19. At the same time, side core 15 and slider 16 are separated from plastic part under action of locking oblique pin 21, and the first mold opening process is completed; when mold opening distance reaches 15mm, support guide column and support backing plate tighten fixed mold plate, fixed mold plate does not move, plastic part wraps moving mold cavity, moving mold system continues to open. As moving mold system continues to open mold, latent gate automatically cuts off plastic parts under action of inclined shear force of moving mold pulling trough, gate runner and plastic parts are automatically separated, ejector stick of injection molding machine moves forward against ejector backing plate 10, ejector pin fixing plate 9 and all ejector pins 13, pushes plastic part out of mold cavity, and plastic part falls out under action of gravity, completing second mold opening process, as shown in Figure 4. When mold is closed, moving mold plate presses fixed mold plate , slider 16 and side core 15 are reset under action of locking oblique pin 21, the entire mold is closed, and next cycle is entered.

 

Figure 4 Fixed mold in open mold state
1. Locking oblique pin 2. Slider 3. Plastic parts are released from cavity 4. Support guide post

Two-way ejection mechanism is stable, mold structure is compact, processing is easy, and economy is good. After being used in production, quality of plastic parts is stable, appearance is beautiful, and technical requirements of users are met.