[Abstract] Improvement process of structural design of sliding block in fixed mold of injection mold of telephone receiver shell is analyzed, a method of improving quality of product and prolonging service life of mold is introduced.

Quality, performance and service life of mold are determined by multiple factors such as quality of mold processing, mold material and its heat treatment, rationality of mold structure design. Amongm, whether mold structure design is reasonable will have a significant impact on performance and service life of mold. Unreasonable mold structure will cause premature wear and deformation of mold, until it fails, or even cannot be used at all. In this paper, improvement of structural design of sliding block in fixed mold of injection mold of telephone receiver shell is taken as an example to discuss rationality of mold structure design.

Plastic part shown in Figure 1 is outer shell of a telephone handset. Material is polyethylene (PE) plastic. Fractal surface is an arc-shaped fractal surface. Two ends of handset are listening end and speaking end. Among them, listening end is provided with 12 listening through holes, and through holes are perpendicular to plane where listening end is located, and are 20° with opening direction of mold (structural diagrams of other parts of plastic part are omitted for convenience of description). For this reason, mold has designed a locking block and an internally inclined sliding block collision structure.

 

Figure 1 Telephone handset shell
Original mold structure is shown in Figure 2. In actual production and use process of mold, it is found that resistance is great when mold is closed. Joint surface of locking block and inner sliding block, inner sliding block and right side of inclined slide are very frictional and abraded. (See Figure 3), which seriously affects performance and service life of mold.

 

Figure 2 Original mold structure
1. Movable mold seat plate 2. Cushion block 3. Supporting plate 4. Movable mold fixed plate 5. Fixed mold fixed plate 6. Spring 7. Fixed mold seat plate 8. Sliding block in fixed mold 9. Positioning ring 10. Gate Sleeve 11. Core 12. Guide sleeve 13. Guide column 14. Top rod 15. Reset rod 16. Push rod fixed plate 17. Push plate
In order to find reason, according to mold structure, force status of inner sliding block was analyzed, and force is shown in Figure 4. Clamping force N 1 is all transmitted to inner sliding block through joint surface of locking block and inner sliding block. A large friction force (F=μN 1) is generated on joint surface of locking block and inner sliding block, which causes serious friction and wear between locking block and inner sliding block, which leads to premature failure of mold. Then, clamping force N 1 passes through inner sliding block, decomposes into component force along axis of inclined slide N 2 =N 1 cos20° and component force perpendicular to right side of inclined slide N 3 =N 1 sin20°, where component force N 2 is useful force for using inner sliding block to move along inclined slide; component force N 3 generates a positive pressure on right side of inclined slide, causing a lot of friction between inner slide and right side of inclined slide,  a great resistance to mold clamping, resulting in severe deformation and wear between inner sliding block and right side of inclined slide, which eventually leads to failure and shortens life of mold.

 

Figure 3 structure of original inner slider

 

Figure 4 Force analysis of inner sliding block before improvement

In order to improve stress condition of inner sliding block, structure of locking block and inner sliding block is improved as shown in FIG. 5. Fig. 6 shows force condition of inner sliding block after improvement.

 

Figure 5 Improved inner sliding block structure

 

Figure 6 Force analysis of improved inner sliding block
As can be seen from Figure 6, because locking block and inner slide block are smoothly constrained, when clamping force N 1 passes through joint surface of locking block and inner slide block, a positive pressure N 2 =N 1 cos20° is generated for inner sliding block along normal direction of joint surface (that is, along axis of inclined slideway), that is, positive pressure on inner slide block at this time N 2 =N 1 cos20°<N 1, which is significantly lower than positive pressure on inner slide block shown in Fig. 4 (N 2 =N 1), friction between locking block and inner sliding block (F=μN 2 =μN 1 cos20°) is also significantly reduced (Note: in case of Figure 4, this place friction force is F=μN 1), so friction and wear between two are reduced. In addition, positive pressure between inner slide and right side of inclined slide is gone (note: in Figure 4, it is N 3 =N 1 sin20°), so friction and wear between inner slide and right side of inclined slide are greatly reduced, mold clamping process becomes very smooth. Improved mold structure is shown in Figure 7.

 

Figure 7 Improved mold structure

1. Movable mold seat plate 2. Cushion block 3. Supporting plate 4. Movable mold fixed plate 5. Fixed mold fixing plate 6. Spring 7. Fixed mold seat plate 8. Fixed mold inner slide block 9. Positioning ring 10. Gate Sleeve 11. Core 12. Guide sleeve 13. Guide column 14. Top rod 15. Reset rod 16. Push rod fixed plate 17. Push plate

After above structural improvements, force condition of mold is more reasonable, use performance of mold is significantly improved, friction and wear of related mold parts are reduced, service life of mold is prolonged, cost of mold is reduced, and economic benefit is improved.