【Abstract】Commonly used outer core-pulling mechanisms of injection molds mainly include oblique guide column type, spring type and spring-hook type, etc. Characteristics of three outer core-pulling mechanisms are analyzed and compared, design of bamboo storage basket mold is taken as an example. Design method of spring-hook type outer core pulling mechanism is introduced. Through actual production verification, mold structure design is simple, stable in operation, economical and convenient, and has a certain reference effect for design of same type of mold.

In application of engineering plastics, there are holes, pits, bosses and other structures perpendicular to mold opening direction on outer side of complex plastic parts, which can be seen everywhere in industrial production and daily life. In order to prevent complicated plastic parts from being hindered by parts forming them during opening of injection mold or ejection of plastic parts, movement of outer core pulling mechanism must be used to pull out structure of molded part that hinders mold opening or ejection of plastic part before ejection of mold. Therefore, design of outer core-pulling mechanism is an important part of design of injection mold.

Commonly used outer core pulling mechanisms mainly include oblique guide column type, spring type and spring-hook type, etc., which have their own typical structures and characteristics.

Figure 1 shows mold opening and closing state of a typical oblique guide column outer core pulling mechanism. Main components of mechanism include oblique guide column 3, wedge block 4, side forming core 6, fixed pin 7, sliding block 8, linkage rod 9, spring 10, limit block 11 and so on. Advantages of oblique guide column outer core pulling mechanism are stable and reliable operation, high molding precision of plastic parts, suitable for most outer core pulling mechanisms; but disadvantage is that there are more accessories, complex structure, difficult design, processing and debugging.

 

Figure 1 Schematic diagram of the inclined guide column outer core pulling mechanism
a — — Mold closing state b — — Mold opening state c — — End of core pulling
1. Cavity 2. Fixed template 3. Inclined guide column 4. Wedge block 5. Plastic part 6. Side molding core 7. Fixed pin 8. Slider 9. Linkage rod 10. Spring 11. Limit block 12. Move Template 13. Core 14. Core insert

Fig. 2 shows mold opening and closing state of a typical spring-type outer core pulling mechanism. Main components of mechanism include spring 2, inclined slider 4, sliding guide block 10, fixing screw 11, and so on. Advantage of spring-type outer core pulling mechanism is simple structure, fast operation, economical and convenient; but disadvantage is that movement of oblique slider is affected by spring. For example, mass or volume of oblique slider is large. In order to ensure sufficient mold opening force, number and load of springs need to be increased, which greatly increases difficulty of mold design and assembly. Therefore, spring-type outer core pulling mechanism is generally only suitable for small injection molds with small plastic parts and light oblique sliders.

 

 

Figure 2 Schematic diagram of spring-type outer core pulling mechanism
a — —Clamping state b — —Opening state
1. Fixed mold seat plate 2. Spring 3. Sprue sleeve 4. Inclined slider 5. Fixed template 6. Movable template 7. 11. Fixed screw 8. Plug 9. Core 10. Guide sliding limit block 12. Guide sleeve 13. Guide column 14. Plastic parts

Figure 3 shows a typical spring-draw hook type outer core pulling mechanism. Its main components include spring 11, inclined slider 12, T-shaped guide slider 5, limit block 15, draw hook 2, draw buckle 3, fixed Screws etc. Because spring-hook type outer core pulling mechanism has more hook structure than spring type, when mold is opened, inclined slider moves along inclined chute under combined action of hook and spring. Hook structure plays main role of mold opening. Quantity and load requirements are not high, which reduces difficulty of mold design and assembly, so it is also suitable for medium and large injection molds with large plastic parts and heavy weight of inclined sliding blocks.

 

Figure 3 Schematic diagram of spring-hook type outer core pulling mechanism
1, 4, 13, 14. Fixing screws 2. Pull hook 3. Pull buckle 5. T-shaped guide slider 6. Positioning pin 7. Cavity insert 8. Fixed mold seat plate 9. Gate sleeve 10. Fixed template 11 .Spring 12. Inclined slider 15. Limit block 16. Push plate 17. Core 18. Movable template
To sum up, compared with two types of outer core pulling mechanisms of oblique guide column and spring type, spring-hook type outer core pulling mechanism has advantages of simple structure, lower difficulty in design and assembly, wide application range, fast and stable operation, economical and convenient, etc., has certain practical significance and promotion significance. This article will take design of outer core-pulling mechanism of bamboo storage basket mold as an example, focus on several design points of spring-hook-type outer core-pulling mechanism for reference of mold designers.

Structure of bamboo storage basket is shown in Figure 4. Material is PP+30GF, shrinkage rate is 1%. Overall shape of plastic part is a rectangular body, maximum external dimension is 243*190*104mm, average wall thickness is 1.6mm, and demolding slope is 3°.
Demolding direction of bamboo woven storage basket mold is shown in Figure 5. Due to special appearance requirements, 4 outer sides of plastic part are made of bamboo patterns, and there are handle frame structure designs on left and right sides of plastic part. In order to ensure smooth demolding of plastic part, it is necessary to design outer core pulling mechanism on 4 sides of plastic part. Because dimensional accuracy of bamboo woven storage basket is not high, after comprehensively considering factors such as production accuracy, production cost, design and processing difficulty, etc., it is determined to adopt a spring-hook type outer core pulling mechanism.

 

Figure 4 Schematic diagram of bamboo storage basket structure

 

Figure 5 Schematic diagram of demolding of bamboo storage basket mold

Spring-hook-type outer core pulling mechanism of bamboo storage basket mold is shown in Figure 6. In view of special shape of four sides of bamboo storage basket, in order to ensure that plastic parts can be demolded smoothly, 4 sets of spring-hook type outer core pulling mechanisms are designed. These 4 sets of outer core pulling mechanisms are all composed of spring 1, T-shaped guide slider 2, wear plate 3, inclined slider 4, limit block 5, draw hook 6, draw buckle 7 and other accessories. They are different in size and position. But structure is similar, design concept is same.

 

Figure 6 Schematic diagram of core-pulling mechanism on the outside of bamboo woven storage basket mold
1. Spring 2. T-shaped guide slider 3. Wear plate 4. Inclined slider 5. Limit block 6. Pull hook 7. Pull buckle

If wedge angle of oblique slider is smaller than friction angle of steel (6.85°~8°), friction force when opening mold is relatively large, and some injection molding machines have low mold opening force, which will cause difficulty in opening mold and make a loud noise. If wedge angle of oblique slider is too large, not only size of oblique slider will increase, but oblique slider and guide slider may be self-locking. Therefore, wedge angle of inclined slider is generally 7.5° to 25°. In design of 4 sets of outer core-pulling mechanisms of bamboo storage basket mold, wedging angles of inclined sliding blocks are all set to 15°, as shown in FIG. 7.

 

Figure 7 Wedge tight angle of inclined slider

Horizontal distance between four sides of bamboo storage basket (that is, horizontal distance of inclined surface formed by inclined sliding block) is all T=9.8169mm, and wedging angle α of inclined sliding block has been determined to be 15°, as shown in Figure 8. Based on this, horizontal mold opening stroke of inclined slider can be calculated as S 1 =T+ (2~3) =12mm, of which 2~3mm is safety distance of inclined slider mold opening; diagonal total mold opening stroke of inclined slider is S=S 1 /sinα≈46mm.

 

Figure 8 Schematic diagram of mold opening stroke of inclined slider

Hook structure of spring-hook type outer core pulling mechanism is shown in Fig. 9, mainly consists of two parts, a hook and a buckle. In order to ensure that hook structure can be unhooked smoothly when mold is opened, mold is opened normally, fastening size of hook structure when mold is closed should be W=S 1 -(1~2)=10mm, as shown in Figure 9a . In order to improve rigidity of hook and buckle, reduce difficulty of mold processing, width of hook and buckle needs to be increased, which is 20 mm, as shown in Figure 9b.

 

 

Figure 9 Schematic diagram of  hook structure
a — —Size of hook and buckle b — —Engineering drawing of hook and buckle

In spring-hook-type outer core-pulling mechanism, main driving force for inclined slider to open mold is hook, spring plays an auxiliary role. At the same time, in order to reduce difficulty of mold assembly, in outer core-pulling mechanism of bamboo storage basket mold, blue springs are used as spring accessories. Spring parameter calculation formula is: compression ratio = (stroke + preload) / spring length, where stroke is S=46mm, preload value range is 5~20mm, compression ratio of blue spring is 32%~40% , spring length can be obtained by calculation =(46 + 5) /0.35≈145mm, because blue spring standard does not have 145mm, but only has two length specifications of 125mm and 150mm, so outer core pulling mechanism of bamboo storage basket mold uses a 150mm long blue standard spring.

As shown in Fig. 10, Fig. 10a shows mold clamping state of outer core pulling mechanism, and Fig. 10b shows mold opening state of outer core pulling mechanism. Four outer core-pulling mechanisms have same structure. When mold is closed, spring 21 is in a compressed state, inclined slider 15 is completely installed in fixed mold plate 8. T-shaped guide slider 17 and limit block 18 are connected to inclined slider 15 through guide slide groove, are fixed on fixed mold plate 8, draw hook 19 is fixed on inclined sliding block 15, draw buckle is fixed on movable mold plate 2, draw hook 19 and draw buckle 20 are in a buckled state. When mold is opened, movable die plate 2, core 3, and push plate 23 move backwards to separate from fixed die plate 8. Spring 21 in compressed state stretches, under joint action of hook 19 and spring 21, inclined slider 15 moves backward along inclined sliding grooves of T-shaped guide slider 17 and limit block 18. Since inclined sliding block 15 has an inclination, it can move horizontally outward while moving backward, thereby realizing side core pulling. When inclined slider 15 moves obliquely for a certain stroke, pull hook 19 and pull buckle 20 are completely separated, inclined slider 15 is positioned by spring 21 and limit block 18. After mold opening is completed, plastic part 22 is held tightly on core 3, and demolding is completed under action of push plate 23.

 

 

Figure 10 Mold opening and closing state of outer core pulling mechanism
a — —Clamping state b — —Opening state
1, 6, 9, 13. Fixed screw 2. Movable mold plate 3. Core 4. Positioning block 5. Long quick connector 7. Positioning pin 8. Fixed template 10. Fixed mold seat plate 11. Positioning ring 12. Gate sleeve 14. Cavity insert 15. Inclined slider 16. Wear plate 17. T-shaped guide slider 18. Limit block 19. Pull hook 20. Pull buckle 21. Spring 22. Plastic part 23. Push plate

This paper analyzes characteristics of three types of outer core pulling mechanisms of oblique guide column type, spring type and spring-hook type, determines that spring-hook type outside core pulling mechanism has certain practical significance and promotion significance in actual production, takes bamboo woven storage basket mold design as an example to explain some design points of spring-hook type outer core-pulling mechanism and its working process. From actual production situation, it can be known that mold structure design is simple, stable in operation, economical and convenient, and has a certain reference effect for design of similar outer core-pulling mechanisms.

 

Figure 11 Plastic parts of bamboo storage basket