Design of Injection Mould with Side Core-pulling for Decorative Cover

Time：2021-08-26 15:47:59 / Popularity： / Source：

[Abstract] Material, structure and molding process of decorative cover are analyzed, lateral core pulling mechanism and injection mold structure are designed. Mold has a compact structure, reliable action, smooth ejection of plastic part, molded plastic part fully meets design and use requirements, which has certain reference value for design of injection molds of similar plastic parts.

1 Structural analysis of plastic parts

Figure 1 shows plastic shower decoration cover plastic parts of sanitary ware, mass-produced. Surface of parts is required to be chrome-plated, material of plastic parts should be ABS. Appearance should be smooth, surface should not have injection molding defects such as flash, shrinkage, bubbles, cracks, scratches, etc. At the same time, plastic parts are required to have good injection molding process performance, high production efficiency, and low mold manufacturing costs. Analyzing structure of plastic part, difficulty in molding is that buckle part cannot be forced to demold. In addition to multiple collisions in this part, a lateral core pulling mechanism must be used to demold.

Injection Mould with Side Core-pulling for Decorative Cover

Figure 1 Decorative plastic cover for shower head

2 Selection of side core pulling mechanism

(1) Side slider core pulling mechanism.

Side slider core-pulling is mainly used for outer core-pulling, but there are also a few applications on inner side. It is the most widely used, accounting for about 60% of all core-pulling mechanisms. It is characterized by safety and reliability, low maintenance rate and convenient maintenance.

(2) Inclined top (inclined push rod) core pulling mechanism.

Inclined top core pulling mechanism is mainly used for core pulling of concave and convex parts on inner side of plastic part, and there are also a few applications on outer side. Advantage is that inclined top has function of push rod and side pumping, it takes up less mold space. Disadvantages are small core pulling force, core pulling area and core pulling distance; high precision requirements, complicated processing, easy wear during use, high maintenance rate and troublesome maintenance. Therefore, outer sliding block can be used without inclined top, inclined top can be used without inner sliding block.

(3) Inclined slider (Huff block) core pulling mechanism.

Inclined slider core pulling mechanism is usually used in occasions where core pulling distance is small, undercut area is large, and a large core pulling force is required. It is often used for outer core pulling, and sometimes it is also used for inner core pulling.
Characteristic is to use pushing force of ejection mechanism or mold opening force of equipment (pull hook pull) to drive inclined slider according to its set inclination and ejection distance to complete vertical parting and side core pulling. Core pulling structure is relatively simple, but there is no side slider core pulling safety.

(4) T-shaped block lateral core pulling mechanism.

Working principle of T-shaped block lateral core pulling mechanism is basically same as that of inclined guide column slider core pulling mechanism, but structure is different. The former has a more compact structure, higher requirements for design and processing accuracy.
Advantage is that T-shaped block can pull core and close mold, can also compress side slider; tilt angle is large, so core pulling distance is greater than inclined guide post core pulling distance; demolding force is larger. Disadvantage is that processing accuracy is high.

(5) Oil cylinder is combined with side slider core pulling mechanism.

Advantage of core pulling mechanism is that it can obtain a larger demolding force and a longer core pulling distance, stable transmission, parting and core pulling are not restricted by mold opening time and ejection time. Disadvantage is that injection molding machine should have a hydraulic core-pulling function, otherwise core-pulling mechanism cannot be used, shape of mold will increase after oil cylinder is installed.
Based on application of above core-pulling mechanism and structural characteristics of plastic parts, core-pulling mechanism with oblique guide posts and side sliders is more suitable.

3 Selection of decorative cover parting surface

Since structure of plastic part is a small and complex plastic part, analyze structure of plastic part and select the most reasonable parting surface plan, as shown in Figure 2.

Figure 2 Parting surface selection

4 Decoration cover pouring system design

In order to reduce shape of mold and reduce cost of mold manufacturing, a two-piece mold arrangement structure is adopted. In order to make plastic fill cavity smoothly, at the same time to ensure that gating system and plastic parts are easily cut off, so as to improve productivity, this mold uses a side gate, as shown in Figure 3.

Figure 3 Gate form and cavity arrangement

5 Determine movable mold insert and core

Due to complex forming structure of slider, mold design starts with movable mold insert. From three-dimensional structure of plastic part, after initializing parting, it is determined that combined structure of movable mold insert is more reasonable than the overall structure, then size of movable mold insert is determined according to empirical method: 60*220*46mm, as shown in Figure 4. Moving mold core is shown in Figure 5.

Figure 4 Movable mold inserts

Figure 5 Moving mold core

6 Determine mold cavity insert

Fixed mold cavity insert has a simple structure and adopts an integrally embedded structure. Size of divided mold is: 60*220*30mm, as shown in Figure 6.

Figure 6 Fixed mold cavity insert

7 Determine mold and specifications of mold base

Determine size of mold base according to insert and lateral core-pulling mechanism, use Longji three-plate mold base: 2030-Cl-A60-B70-C70-250. Import mold base map, insert ranking map into dynamic mold view and fixed mold view. Improve view of the dynamic and fixed mold core pulling mechanism in follow-up design.

8 Lateral core pulling mechanism design

(1) Side slider core pulling scheme.

Both sides of lateral core pulling adopt structure of side sliders and oblique guide pillars. Oblique guide pillars are installed in fixed mold, side slides are installed in movable mold.

(2) Determination of core-pulling distance S of side slider.

Side hole is a through hole, and minimum core-pulling distance is equal to wall thickness, which is approximately equal to 2mm. Because there are more core-pulling holes on the side, for easy demolding, safety distance is 8mm, that is, sliding core-pulling distance is about 10mm.

(3) Determination of inclination angle of oblique guide column.

According to area of lateral core-pulling, height of slider is 41mm, inclination angle of oblique guide post is calculated as 13.71° by method of drawing. Since leading part of oblique guide post is hemispherical, it is an invalid length. Hole of oblique hole of slider has a chamfer of R2mm. According to experience, 5°～6° is usually added to angle obtained by drawing method. In this case, angle of inclination of inclined guide post is 19°, as shown in Figure 7.

(4) Diameter and length design of inclined guide post, as shown in Figure 7.

Figure 7 Inclination angle of oblique guide post

(5) Side slider design.

Structure of left slider is shown in Figure 8, and structure of right slider is shown in Figure 9.

Figure 8 Left slider structure

Figure 9 Right slider structure

(6) Locking block design.

Locking block is processed on fixed mold plate, and locking bevel angle is 20°, as shown in Figure 7.

9 Design of mold release mechanism

Shape of plastic part is diverging, and tightening force is small. It can be pushed out with push rods to meet requirements. Two push rods of ϕ 8mm are designed on each core. Refer to Figure 5 for moving model core diagram.

10 Mold cooling system design

(1) Fixed mold cooling.

Area of fixed-molded molded plastic parts is small, mold temperature rise is not high. Adopt water inflow from fixed mold plate (A plate), through fixed mold cavity inserts well-shaped cooling channel, diameter of channel is ϕ 6mm, as shown in Figure 6.

(2) Movable mold is cooled.

Movable mold is mainly formed by two side sliders, so side slider should be designed with a cooling system, as shown in Figure 8 and Figure 9 on the left and right sliders.

11 Design of guiding and positioning mechanism of mold

This mold adopts a standard two-plate mold base, guide post and guide sleeve are standard parts, and lateral core pulling mechanism is symmetrical. In order to increase clamping effect, it is necessary to process locking bosses on movable and fixed mold inserts to improve positioning accuracy and overall rigidity of movable and fixed molds.

12 Design of other structural parts

This mold adds two sets of push rod plate guide posts and guide sleeves to improve movement accuracy and stability of push rod plate, avoid burns of push rod. This mold adds 4 ϕ 25mm support columns to improve rigidity of movable mold plate. Add 4 push plate return springs to facilitate automated production, as shown in Figure 10.

Figure 10 Mould support column and limit column

13 Exhaust system design

Mold uses a side gate, main exhaust is on parting surface, and exhaust groove can be opened in fixed mold cavity. Because trapped air position is difficult to determine, position of exhaust groove is not drawn during design. After mold test, exhaust groove is processed according to actual situation. Depth of exhaust groove does not exceed 0.03mm and width is about 10mm.

14 Mold overall structure design

Through above analysis and design of structure of each part, the overall structure of mold is perfected and drawn, as shown in Figure 11.

Figure 11 The overall structure of the mold
1. Positioning ring 2. Fixed mold seat plate 3. Gate sleeve 4. Inclined guide column 5. Fixed template 6. Fixed mold insert 7. Movable template 8. Support column 9. Cushion block 10. Limit block 11. Push Rod fixing plate 12. Push plate 13. Limit nail 14. Movable mold seat board 15. Movable mold insert 16. Push rod 17. Movable mold core 18. Left slider 19. Pull rod 20. Right slider 21. Reset rod 22. Guide post

15 Conclusion

After completion of design and manufacture of mold, more than 200,000 plastic parts have been produced. Precision and surface quality of plastic parts meet requirements. Sliding block and core-pulling mechanism move well with few maintenance. Practice has proved that design is successful.

Gud Mould Industry Co., Ltd