Breast shields are composed of complex curved surfaces. Outer surface requirements are high, there must be a smooth transition between different curved surfaces. There are flat cover, horn cover, and circular cover structures. These three structures are connected through middle circular tubular structure and have different opening directions, as shown in Figure 1. Core of trumpet cover and core of central round tube of plastic part are integrated into one piece, side surfaces of flat cover are connected to central round tube of plastic part, and cores of the two are connected by insertion. Bottom of circular cover is connected to circular pipe in the middle of plastic part through a section of circular pipe. There are 4 buckles evenly distributed on inner wall of circular cover. These 4 buckles need to be demoulded using an inclined push mechanism.

 

In addition to creating entities, UG can also replace AutoCAD to design engineering drawings. Specific method is to convert 3D drawings into 2D engineering drawings, generate main views, side views, section views, etc., and mark dimensions, as shown in Figure 2. Operation process is convenient and quick.

 

In order to ensure normal demoulding of plastic parts, prevent shrinkage and other phenomena, before designing mold, plastic parts should be analyzed according to molding process requirements of mold, 3D drawing should be checked to see whether there is a draft angle, whether wall thickness is uniform, etc. Plastic part material is ABS, wall thickness is 1.5mm, and draft angle is 1°~1.5°. Use "Check Minimum Wall Thickness" and "Check Maximum Extension Angle" of "Additive Manufacturing Design Verification" command in UG analysis menu bar to analyze wall thickness and demoulding angle of plastic part. For position with specified value of wall thickness or demoulding angle, UG will display red, which is helpful for design engineers to check defects of plastic part. Wall thickness analysis result of this plastic part is that wall thickness at buckle position is thin, and wall thickness at other locations is normal, as shown in Figure 3.

 

According to structure of plastic part, a two-plate mold structure is adopted and ordinary runners are used for pouring. Flat cover, horn cover and round cover structures are all demoulded using a side slider mechanism, in which side sliders of flat cover and horn cover are driven by inclined guide pillars. Since there are 4 buckles on inner surface of circular cover, combined slider mechanism driven by "inclined guide column + hydraulic cylinder" combination is used to demould. 4 buckling positions inside circular cover are driven by inclined guide pillars to demould slider. After four buckles are demoulded, hydraulic cylinder piston rod drives the entire slider to be demoulded. Mold structure is shown in Figure 4. Mold has a 1-mold, 1-cavity layout, uses 1 side gate to feed material, and uses a circular runner with a diameter of φ6mm. Gate sleeve is fixed on fixed mold plate with screws, then pressed tightly with positioning ring of fixed mold base plate. Since slope height of ejection mechanism of horn cover and flat cover is relatively large, in order to better compress slider, two compression blocks are installed on the surface of fixed mold plate.

 

Parting surface design affects complexity of mold structure and demoulding mechanism. Parting surface should have a smooth transition and should not have sharp corners, thin walls or other structures that would affect processing and service life of mold parts. After placing plastic part to be formed sideways, parting surface design methods are different at different locations. Parting surface in the middle is a horizontal plane, located at maximum contour of plastic part to be formed; parting surface of circular cover is a rotating curved surface, flat cover is a tensile curved surface, and parting surface of horn cover is a vertical surface, parallel to glue surface of bell mouth, as shown in Figure 5. Due to symmetry of plastic part structure, parting surface divides it into two parts along center of plastic part. In order to prevent wear of slider, parting surfaces of circular cover and flat cover are each designed with a 5° slope in movement direction of slider.

 

In order to facilitate processing, mold cavity plate is designed as an insert structure, convex and concave positioning blocks are designed at four corners of movable and fixed mold cavity plates to prevent misalignment during installation. Slope of side of positioning block is set to 5 °. Design 2 hanging die threaded holes on diagonal corners of movable and fixed model cavity plates. When disassembly and assembly are required, screw hanging mold screws into threaded holes of hanging mold, lift movable and fixed mold cavity plates before disassembling and assembling them to facilitate fitter's operation. Runner is designed to be circular and is opened on parting surface, pouring from side of plastic part to be formed. In UG injection mold wizard tool command bar, use parting surface function to separate movable and fixed mold cavity plates, then separate three core-pulling slide blocks from driven model cavity plate. Designed moving and fixed mold cavity plates are shown in Figure 6.

 

Three nozzles of plastic part have different structures, and designed slider structure is also different. Slider core of speaker cover and the slider seat are a whole. Slider core is located on the center line of slider and has a large volume. Slider movement is guided by two inclined guide pillars, as shown in Figure 7(a); slider core and slider seat of flat cover are also a whole, with a small volume. An inclined guide pillar guides movement of slider. Since inclined guide post needs to avoid cooling water path in slider, core and inclined guide post hole of slider are located on both sides of centerline of slider, as shown in Figure 7(b); in order to facilitate processing, circular cover slider core adopts an insert structure, and slider core is fixed on slider seat with screws, as shown in Figure 7(c). In order to demould four buckle positions on inner surface of circular cover, a combined slider structure is adopted, and dovetail groove block drives movement of four buckle sliders, as shown in Figure 7(d) (core of slider seat and circular cover is hidden, and only demoulding mechanism with 4 buckles is shown). According to material characteristics of plastic parts and requirements for stable molding quality, mold slider material is 718, heat treated to about 45HRC, and nitrided.

 

1. Buckle slider 2. Round cover slider 3. Combined slider seat 4. Small slider seat 5. Spring 6. Dovetail groove block 7. Buckle slider 8. Buckle slider 9. Buckle slider 10. Buckle slider

Structure of circular cover combined slider is shown in Figure 8. Demoulding distance of four buckle sliders is 1.85mm. Slope of dovetail groove block is 8°. Slope of inclined guide column is 74°. The slope of the inclined guide column is 74°, and limited height of inclined guide post is 71mm. When opening mold, demoulding distance of four buckle sliders driven by inclined guide pillar is 71*tan16°*tan8°=2.86mm, which is greater than demoulding distance of four buckle sliders (1.85mm), and mold can be demolded normally.

 

2. Round cover slider 3. Combined slider seat 4. Small slider seat 6. Dovetail groove block
Movement form of circular cover combination slider: there are 4 buckle positions on inner wall of circular cover, buckle sliders 7, 8, 9, and 10 are used to release mold. Dovetail block 6 is fixed on small slider seat with screws, and four inclined dovetail grooves are opened on dovetail block 6, which are respectively connected to four buckle slide blocks (Part 7 to Part 10). At initial stage of mold opening, combined slider seat 3 remains stationary, inclined guide column drives small slider seat 4 and dovetail groove block 6 to move in V direction. Under blocking action of circular cover slider 2, four buckle sliders move toward center. Among them, buckle slider 7 moves in V1 direction, buckle slider 8 moves in V2 direction, buckle slider 9 moves in V3 direction, and buckle slider 10 moves in V4 direction, realizing demoulding of 4 buckle positions. When four buckle slide blocks are completely demoulded, hydraulic cylinder piston rod drives combined slide block seat 3 to move in V direction, driving the entire slide block to be demoulded. Direction of movement is exactly opposite when closing mold.

A good cooling system plays an important role in mold, which can quickly cool plastic parts and prevent deformation of molded plastic parts. In order to achieve a good cooling effect, different water paths should be designed according to structure of plastic part and mold part. Movable and fixed mold inserts adopt a straight-through cooling water path, as shown in Figure 9(a); since sliders of horn cover and flat cover adopt an integral structure, cooling water path adopts a structure that combines spacer-type water wells with a straight-through structure. Among them, slider seat adopts a straight-through waterway. Cores of the two have a longer protruding position on slider seat, and a spacer-type water well waterway is used, as shown in Figure 9 (b) and (c).

 

Traditional mold design idea is to first design assembly drawing and mold parts drawing, then design 3D drawing of mold. Opposite idea of using UG to design mold is to first use UG to design 3D drawing of mold (including assembly drawing and mold parts drawing) . After checking that there is no interference between parts, 3D drawing is converted into a 2D drawing. Conversion process is simple. 2D drawing of UG design is shown in Figure 10.

 

1. Moving mold base plate 2. Push plate 3. Push rod fixed plate 4. Pad 5. Return spring 6. Moving mold plate 7. Push rod 8. Speaker cover inclined wedge 9. Horn cover inclined guide post 10. Speaker cover slider 11. Fixed mold plate 12. Fixed mold base plate 13. Fixed mold insert 14. Flat cover inclined guide post 15. Flat cover slider 16. Moving mold insert 17. Positioning ring 18. Gate sleeve 19, circular cover inclined guide column 20. Circular cover slider 21. Dovetail groove block 22. Buckle slider 23. Hydraulic cylinder 24. Hydraulic cylinder bracket, 25. Combined slide block seat 26. Pulling rod
Mold working process: After injection is completed, movable and fixed molds are opened, and three sliders each begin to demould under drive of inclined guide pillar. When mold parting distance is 36mm, buckle slider 22 inside circular cover completes demoulding; when mold parting distance reaches 62mm, flat cover slider 15 completes demoulding; when mold parting distance reaches 75mm, horn cover slider 10 completes demolding.
When movable mold and fixed mold are completely separated, piston rod of hydraulic cylinder 23 drives circular cover slider 20 to complete demoulding, then ejector rod of injection molding machine pushes push plate 2, push rod fixed plate 3 and push rod 7 to perform demoulding movement to push out molded plastic part; after taking out plastic part, ejector pin of injection molding machine pushes, fixes and closes mold. Mold closing process is opposite of mold opening process. When mold is completely closed, next plastic part can be produced.