【Abstract】According to structural characteristics of juicer shell and technical requirements that outer surface does not allow contour marks of slider, a simplified three-plate mold base was selected, a gating system of "circular runner + sector gate", a fixed mold slider + an inclined top core pulling mechanism was designed on mold. Choosing a circular runner + sector gate feeding system solves defects such as weld marks and bubbles on plastic parts; choosing a simplified three-plate mold base eliminates pusher plate on mold base, fixed mold seat plate drives wedge movement; inclined surfaces are designed on both sides of inclined wedge to solve problem of demoulding of sliding block driven by inclined wedge; inclined roof is designed into a structure of "inclined ejector rod + "T" slot + bolt", which solves problem of difficult disassembly and assembly of inclined roof; reset block of ejection mechanism is installed on lower surface of movable mold base plate to replace return spring, which solves problem of reset of ejection mechanism.

Degree of difficulty of plastic mold manufacturing is closely related to shape of plastic part, is also related to structure, position, and number of core-pulling mechanism on plastic mold [1~3]. Structure of plastic part determines parting surface of plastic mold, also determines mold opening direction of mold. When mold opening direction has been set, some special core-pulling mechanisms on plastic mold, because movement direction is different from mold opening direction, it is necessary to design a mechanism that converts power in mold opening direction into power to move these special core-pulling mechanisms, so that these core-pulling mechanisms can be demolded [4~7]. In mold structure design, rational design of these special core-pulling mechanisms is an important and difficult point in mold structure design. Especially when designing these core-pulling mechanisms, needs of plastic part forming should be combined. On basis of ensuring complete forming of plastic parts, not only should core-pulling mechanism operate smoothly and with high reliability, but also structure of core-pulling mechanism should be simplified as much as possible to reduce failure rate of mold, reduces maintenance rate of mold, enables stable production of mold [8~11]. Taking a certain brand of juicer shell plastic parts as an example, in view of complexity of its structural features and problem that outer surface cannot be traced by slider, a fixed mold slider core pulling mechanism and a movable mold inclined top mechanism were designed for reference by mold design colleagues.

Plastic shell structure of a certain brand of juicer is shown in Figure 1. Shape is cylindrical, maximum size of outline is ϕ 246.7*109.7mm, bottom has a through hole with a diameter of ϕ 147.3mm, material is ABS, and wall thickness is 2.0mm.

 

Figure 1 Shell structure diagram
Three-dimensional shape of plastic part is shown in Figure 2. There are 4 screw holes and 10 small grooves on the bottom of plastic part. Bottom surface of small groove is glue position, and there are through holes on both sides. Role of side hole of small groove is to drain water in plastic part. There is a buckle in each of 4 different directions on inner surface of plastic part. Among 4 different directions on outer surface of plastic part, there are 3 recessed grooves and a square hole, and there are several countersunk through holes in the middle of 3 recessed grooves. Two elliptical grooves on outer surface are at opposite positions, used to install handle, one round groove is used to install electric control switch, and one square hole is used to push power socket. These features of inner and outer surfaces cannot be demolded normally, and core pulling mechanism is needed to demold. Reasonable design of core pulling mechanism with these characteristics is an important reason for difficulty in designing injection mold.

 

Figure 2 3D modeling diagram of plastic parts
a — —View Ⅰ b — —View Ⅱ
1. Buckle Ⅱ 2. Buckle Ⅰ 3. Buckle Ⅳ 4. Oval slot 5, 11, 13. Countersunk hole 6. Square hole 7. Round slot 8. Buckle Ⅲ 9. Round hole 10. Oval slot 12. Side through hole 14. Square hole 15. Slot

Reasonable selection of parting surface is very important to success of mold. During injection molding, whether plastic part can be demolded smoothly, structure of core-pulling mechanism, difficulty of mold parts processing depend on parting surface of these characteristics. In order to realize automatic injection molding of plastic parts, it is necessary to carefully select a reasonable parting surface for features identified in Figure 2. Under premise of giving priority to easy demolding of mold, outer surface of plastic part is selected as fixed mold, inner surface is movable mold. For plastic part studied in this paper, main parting surface is shown in Figure 3. Under premise of determining main parting surface, demoulding mechanism of plastic part is set according to following types [12]: ①For 4 stud holes at the bottom of plastic part, a fixed mold insert structure is adopted; ②For side holes on 10 grooves at the bottom of plastic part, fixed and movable mold insertion structure is adopted, glue position at the bottom of groove is on movable mold core; ③For 3 grooves and one square hole around plastic part, set 4 fixed mold slider mechanisms to implement demolding, glue position is on fixed mold slider; ④For 4 buckles on inner surface of plastic part, 4 inclined top mechanisms are used to implement demolding; ⑤For through hole on the side of plastic part, a structure in which fixed mold slider and movable mold slanted top collide is adopted.

 

Figure 3 Design of main parting surface
1. Fixed mold insert 2. 3. Fixed mold slider 4. Insertion 5. Inclined top and sliding block collision position 6. Insertion position 7. Moving and fixed mold collision 8. Inclined top demoulding

Layout of plastic parts adopts 1 cavity layout. Three side gates are used for gating, side gates are located on the side wall of inner hole at the bottom of plastic part. Sprue is located in sprue sleeve, cross-section of three runners is circular, and gate is fan-shaped. This is because when cross-sectional area is equal, circumference of circle is the shortest, so circular flow channel has the least influence on flow channel properties of melt. Thickness of fan-shaped gate is very thin, which has the least impact on plastic parts, which is conducive to manual trimming of gate material. Setting of gating system is shown in Figure 4.

 

Figure 4 Gating system

1. Main runner 2. Dividing runner 3. Cold slug well 4. Fan gate 

3 recessed grooves and a square hole on outer surface of plastic part cannot be demolded normally, and it must be demolded in the form of a slider structure. If you use a movable mold slider to demold, it will leave a slider contour mark on outer surface of plastic part, which is not allowed by appearance of plastic part. Therefore, characteristics of 3 recessed grooves and a square hole on the outer surface of plastic part select fixed mold slider to demold. For 4 buckles on inner surface of plastic part, it is necessary to demold with a movable mold inclined top structure. In injection mold of this plastic part, a total of 4 fixed mold sliders and 4 movable mold slanted tops are required, as shown in Figure 5. Hole position around plastic part needs to be realized in the form of inclined top and sliding block collide. In order to ensure that there is no flare on outer surface of plastic part, these holes are pierced at movable mold. 4 fixed mold sliders and 4 movable mold sloping tops in mold directly affect difficulty of structural design and manufacturing of mold. Oblique wedge of fixed mold slide block slides up and down in assembly hole of fixed mold plate. Assembly hole of fixed mold strengthens rigidity of oblique wedge, and can prevent oblique wedge from being twisted when mold is closed. Sliding block and inclined top mechanism have a simple structure, high reliability, and easy processing at the same time, which is very practical.

A common slider structure is a movable mold slider, which is composed of a slider, an oblique guide post and an oblique wedge. Oblique guide post and oblique wedge are installed in fixed mold, slider slides in sliding groove of movable mold. However, slider mechanism in this mold is a fixed mold slider, there is no oblique guide column, only consists of a slider and an oblique wedge. A chute is opened on fixed mold plate, slider slides in the chute of fixed mold. Oblique wedge has an inclination at the front and back, oblique guide post is used to guide movement of slider [9], as shown in Figure 6. When mold is opened, side A of diagonal wedge drives slider to move away from plastic part. When mold is closed, side B of diagonal wedge drives slider to move closer to plastic part. The largest contour line of slider is edge of groove. Core of round hole is located on slider. Oblique wedge is connected to fixed mold base plate by bolts. Fixed mold base plate drives oblique wedge movement of slider, thereby driving slider movement.

 

Figure 5 Design of core pulling mechanism
a — —moving mold+slider b — —fixed mold

1. Inclined wedge II 2. Inclined top I 3. Inclined wedge I 4. Fixed mold slider I 5. Press plate 6. Movable template 7. Cushion block 8. Movable mold seat plate 9. Fixed mold slider IV 10. Inclined top Ⅳ 11. Oblique wedge Ⅳ 12. Fixed mold slider Ⅲ 13. Oblique wedge Ⅲ 14. Inclined top Ⅲ 15. Inclined top Ⅱ 16. "T-shaped block" 17. Cavity 18. Fixed mold oblique wedge

 

Figure 6 Slider structure
1. Oblique wedge 2. Slider

There are 4 buckles on the inner surface of plastic part, which must be ejected by inclined top mechanism to be demolded. Inclined top mechanism is composed of inclined top rods, guide sliders, "T" blocks and bolts. Surface of through holes around plastic parts is located on inclined top. Inclined top structure is shown in Figure 7. Inclined top "T"-shaped block is sleeved in push plate fixing plate, and push plate is fixed with bolts. Guide block is inlaid on the back of movable mold plate. When inclined top moves up and down, guide block can also guide inclined top to move horizontally and guide inclined top to demold.
This inclined roof structure has two advantages: the first advantage is that inclined top head and inclined top rod are integrated, which is reliable, easy to process, and can be directly assembled, which is very convenient; second advantage is that when inclined top needs to be repaired, you can directly loosen bolts of inclined top rod from bottom of movable mold base plate, take out "T"-shaped block from push rod fixing plate, then inclined top can be taken out of core, without need to disassemble the entire movable mold part, the entire disassembly and assembly process is very convenient.

 

Figure 7 Inclined roof structure
1. Inclined top 2. Slide block 3. "T" shaped block 4. Bolt 5. Ejector fixing plate 6. Push plate

Injection molding material is ABS, melt filling time is set to 3s, melt temperature is set to 250℃, mold temperature is set to 70℃, maximum injection pressure is set to 80MPa. Mold flow analysis results are shown in Figure 8. It can be seen from Fig. 8 that injection time of end of plastic part is about 2.5 seconds, as shown in Fig. 8a. Required injection pressure for cavity is 48MPa. When injection molding is finished, pressure at the edge of plastic part is about 4.8MPa. As shown in Figure 8b, injection pressure and injection time are sufficient, small injection molding machine can meet required injection pressure; There are several weld marks on plastic part, weld marks are shorter in length, as shown by black lines in Fig. 8c; there are several bubbles on plastic part, but bubbles are smaller and scattered, as shown by black dots in Fig. 8d. Clamping force-time curve reflects 4 stages of injection molding. As shown in Figure 8e, 0~0.2s is high-speed filling stage, clamping force is 0kN; 0.2s~2.1s is low-speed filling stage, clamping force is 0~900kN, gradually increase; 2.1s~2.35s is pressure holding stage, used to compact melt, 900~1,100kN, the clamping force is further increased, slope of this section of curve is larger; 2.35s~ 2.6s is cooling stage, clamping force gradually decreases; main flow channel pressure-time curve is shown in Figure 8f, which is basically consistent with clamping force-time curve. During 0~0.2s, pressure of main flow continues to rise, at 0.2 ~0.8s, pressure of main runner will fall behind, then continue to rise, 0.8~2.3s is pressure-holding phase, pressure of main runner remains constant, and 2.3~2.6s is cooling phase, pressure of main runner drops.
For defects such as weld marks and bubbles on plastic parts shown in mold flow analysis, solution is to set ejector pin at position where weld marks and bubbles may occur on mold. During injection molding, gas in the cavity can be discharged from gap around ejector pin, thereby avoiding weld marks and bubbles on plastic part. Therefore, it can be considered that change from main runner to 3 sub-runners and use of 3-point pouring meets requirements of injection mold for this plastic part.

 

Figure 8 Mold flow analysis diagram
a — — Melt filling time b — — Injection pressure c — — Weld line d — — Air bubbles e — — Clamping force — time curve f — — Main runner pressure — time curve

In this mold, there is no return spring. Instead, ejection mechanism reset fixed block is installed on push plate, then set on ejector pin of injection molding machine. When ejector pin of injection molding machine is ejected, ejector plate can be pushed to perform an ejection movement. when ejector rod of injection molding machine is reset, it can drive push plate and ejector rod fixing plate to reset, structure of ejection mechanism reset fixed block is shown in FIG. 9.

 

Figure 9 Ejection and reset mechanism
1. Reset block of ejector mechanism 2. Movable mold base 3. Pusher 4. Push rod fixed board 5. Spacer 6. Movable template

Different from common mold structure, in this mold, there is no return spring, but ejector mechanism reset fixed block is installed on push plate, push plate and ejector fixed plate are driven to reset when ejector rod of injection molding machine is reset. Using a simplified fine nozzle mold base, guide sleeve is installed on fixed mold seat plate, guide post is relatively long and extends into cushion block. Opening of mold is divided into two times. The first time is separation of fixed mold base plate and fixed mold plate, fixed mold base plate drives oblique wedge movement, the second time is to separate fixed mold plate from moving mold plate. Ejection of plastic part is carried out by pushing ejector plate of mold by ejector rod of injection molding machine. Ejection distance of mold is 50mm, and mold structure is shown in Figure 10.
Mold working process: When injection molding is completed, injection molding machine drives movable mold and fixed mold to separate. Mold is first separated at I-I, fixed mold seat plate drives inclined wedge to separate from fixed mold plate. Under action of wedge A surface, sliding block slides away from glue position. When sliding block is completely demolded, under action of limit bolt, fixed mold seat plate stops and fixed mold plate is further separated. Then movable mold plate is separated from fixed mold plate, that is, mold is separated at II-II. When movable mold moves to limit position, movable mold stops. Under action of ejector rod of injection molding machine, push plate drives push rod and inclined top to perform ejection motion. Under  action of guide slider, inclined top moves sideways (ie demoulding movement) while performing ejection movement. When ejection distance is 50mm, plastic part will be separated from core and inclined top will be completely separated from mold. When operator takes out plastic part, under combined action of ejector rod of injection molding machine and reset fixed block of ejector mechanism, push plate is driven to reset, push plate drives ejector fixed plate, ejector rod, and inclined top to reset. When ejector system is completely reset, under action of injection molding machine, movable mold and fixed mold start to close. Mold clamping process is opposite of parting process, that is, fixed mold plate and movable template are first closed, that is, at II-II Clamping. Then fixed mold base plate and movable mold plate are clamped, that is, mold is clamped at I-I. When mold is clamped at I-I, under action of side B of inclined wedge, slider slides in the direction close to glue position. When mold is completely closed, next injection cycle begins.

 

Figure 10 Mould structure
1. Fixed mold seat plate 2. Inclined wedge 3. Fixed mold insert 4. Fixed template 5. Core 6. Slider 7. Inclined top 8. Movable template 9. Spacer 10. Ejection mechanism reset block 11. Top Rod fixing plate 12. Push plate 13. Movable mold seat plate 14. Positioning ring 15. Guide column 16. Guide sleeve 17. Push plate guide column 18. Limit column 19. Support column 20. Pull rod 21. Mandrel 21 .Ejector mechanism reset fixed block

There are many kinds of plastic injection mold structures, different mold structures are suitable for plastic parts with different structures. In this mold, three-point fan gate glue feeding, movable mold inclined top and fixed mold slider are designed. Simplified point gate mold base is adopted. Guide post is installed on fixed mold base plate. When mold is opened, fixed mold base plate and fixed mold plate are first separated, fixed mold base plate drives inclined wedge, and inclined wedge is driven by fixed mold base plate, and oblique wedge drives slider to perform demolding movement.
Production practice proves that mold can be produced normally, structure of fixed mold slide block and movable mold inclined top can work normally, action is reliable, surface quality of plastic part meets requirements, and it has good engineering design reference significance.