For tapered pipe plastic parts, common mold structure is to place plastic part to be formed in vertical direction. Core on inner surface of molded plastic part is positioned on movable mold plate and remains stationary. Plastic parts are pushed out from core by pusher plate and ejected from mold. For some plastic parts of inclined tapered pipes with special structures, above mold structure cannot be used for molding. For example, end cover is composed of 10 inclined tapered tubes in a circle. Since axis of inclined tapered tubes is tilted toward middle to form a buckle, formed plastic parts cannot be directly pushed out with a push rod, and a special mold structure needs to be designed.

Structure of plastic part is shown in Figure 1. Material is ABS. ABS has excellent comprehensive physical and mechanical properties, good low-temperature impact resistance, dimensional stability, electrical properties, wear resistance, chemical resistance, dyeability, etc., but thermal distortion temperature is low and heat resistance is poor. Melting temperature is 217~237℃, and thermal decomposition temperature is above 250℃.

 

Plastic part is surrounded by 10 inclined tapered tubes in a circle. Maximum peripheral diameter is φ147mm and height is 88mm. Large end diameter of tapered tapered tube is φ30mm and taper is 20°. Inclination angle of center line of tapered tapered tube is 20°. Large end of each oblique tapered tube has a straight section parallel to axis direction, and height of straight section is 11mm. There is a ring-shaped structure at the waist of the plastic part. There is a buckle at the top of each inclined tapered tube. There are 3 buckles between bottom of plastic part and ring structure. These buckles must be formed and demoulded with a slider. Distance in horizontal direction between small end and inner surface of big end of plastic part's inclined tapered tube is 11mm. When demoulding, molded plastic part cannot be pushed out directly with push rod. After calculation, formed inclined tapered tube needs to be moved at least 34mm along axis direction before push rod can be used to push out molded plastic parts.

For buckle position on the side of each inclined tapered tube, a mechanism combining inclined guide posts and slide blocks is used to demould. Since there are buckles between adjacent beveled tapered tubes, in order to prevent processed slider from having sharp corners and other structures that affect service life of mold, adjacent halves of two adjacent inclined tapered tube buckles are formed by same slider, as shown in Figure 2(a). In order to prevent leakage of molten plastic from small holes in side buckle positions, core of small holes in side buckle positions adopts an insert structure, as shown in Figure 2(b).

 

Each oblique tapered tube corresponds to one slider, and 10 sliders surround plastic part to be formed. Therefore, projected area of molded plastic part on parting surface is small, and area occupied by slider is larger. For this reason, a non-standard mold base was designed. One pad was used to replace two pads on both sides of mold frame, and a square hole was opened in the middle of pad, push plate and push rod fixing plate of mold were set in square hole, as shown in Figure 3.

 

Since each inclined tapered tube tilts at an angle of 20° toward the center of plastic part, it cannot be pushed out directly with a push rod. Instead, it needs to be demoulded diagonally along axis of tapered tube, using an inclined core structure, as shown in Figure 4 ( a). In order to reduce wear between mold parts, rolling elements and shafts are installed under pressure block. When pressure block drives inclined core to move downward, rolling elements will move horizontally along bottom surface of pressure block, causing inclined core to move along axis direction. In order to make obliquely drawn core move along axis direction, obliquely drawn core seat fixed plate, backing plate and movable mold seat plate are arranged in sequence at the lower part of movable mold plate. As shown in Figure 4(b), pressing block is fixed on the back of obliquely drawn core base fixing plate. A movable mold plate is installed on upper surface of fixed plate of inclined core base. When fixed plate of inclined core base is separated from movable mold plate, inclined core can be driven to move along axis direction.

 

In order to facilitate processing of mold parts, movable mold core adopts an insert structure. There are 10 arc-shaped depressions distributed around interior of movable mold insert, as shown in Figure 5(a). Since most of outer surface of plastic part is formed by slider, only a small area is formed by fixed mold core, and area of fixed mold insert is small. In order to save mold manufacturing costs and facilitate processing of mold parts, fixed mold insert molding plastic part is assembled from 10 parts. In addition, in order to prevent under-injection on plastic parts to be formed, if holes on plastic parts are formed in fixed mold part, core adopts an insert structure, as shown in Figure 5(b).

 

After inclined tapered tube moves at least 34mm along axis, molded plastic part can be pushed out with a push rod. There are 6 cylindrical holes (diameter φ2.8mm, depth 23mm) on the back of plastic part. Cores for molding these cylindrical holes are easy to break. In order to facilitate mold maintenance and prevent under-filling, cores of these six cylindrical holes all adopt a push tube structure, which also plays the role of pushing out molded plastic parts, as shown in Figure 6.

 

In order to overcome injection defects on the surface of plastic parts to be molded, latent gates are used for pouring. In order to achieve same injection pressure for forming each inclined tapered tube, each inclined tapered tube has a gate, as shown in Figure 7.

 

In order to keep mold temperature uniform, cooling water channels are set up for main mold parts. Since shape of movable mold insert is decagonal, movable mold insert adopts a polygonal straight water path and is divided into 2 independent water paths, which can effectively prevent cooling water from bringing heat from other areas during circulation process. Structure of movable mold cooling waterway is shown in Figure 8(a). In fixed mold core structure, due to small size of 10 fixed mold inserts, it is not suitable to open cooling water channels, and only a straight-through cooling water channel can be opened on fixed mold plate, as shown in Figure 8(b). Due to large size of slider, in order to control temperature of slider, a straight-through cooling water path is opened in slider. In order to facilitate operation of employees, a longer water pipe joint is used, as shown in Figure 8(c). Inclined core uses a water well waterway, and a water pipe is set up in water well. There is a ring at one end of water pipe, as shown in Figure 8(d).

 

Projected area of plastic part on parting surface is small, and plastic part is lightweight, so it is suitable for molding with a small injection molding machine. Since there are 10 sliders around plastic part to be molded and mold dimensions are 615mm * 620mm, in order to use an injection molding machine with a smaller workbench as much as possible, a straight mold base is selected for mold; in order to make first opening of mold between movable mold plate and inclined core fixing plate, an auxiliary mold opening mechanism is installed on the side of mold base; in order to prevent mold from opening during turnover process, a clamping strip is installed on the side of mold base; in order to enhance positioning accuracy between moving and fixed mold plates, a lock module is installed on parting surface; mold feet are installed on mold base, and mold feet are usually in contact with ground, which facilitates forklift forks to be inserted into bottom of mold; a hanging module is installed opposite mold foot. When mold is lifted by a crane, center of gravity of mold passes through hanging module, which can maintain effective balance of mold. Appearance structure of mold is shown in Figure 9.

 

Since projected area of plastic part on parting surface is small, size of mold is larger than plastic part, and ejection mechanism of mold is also relatively small, it is suitable to use a non-standard mold base. After obliquely drawn core of molded plastic part is demoulded along axial direction, molded plastic part is pushed out with a push rod. Mold is opened twice, with an oblique core fixing plate installed under movable mold plate, the first (PL1) mold opening is placed between movable mold plate and oblique core seat fixed plate. In order to enhance positioning accuracy of mold, a total of 8 sets of guide posts and bushings, 4 sets each of fixed mold and movable mold, are installed. Two-dimensional structure of mold is shown in Figure 10.

 

1. Moving mold base plate 2. Backing plate 3. Inclined core base fixed plate 4. Rotating shaft 5. Inclined core base 6. Water pipe joint 7. Water riser 8. Moving mold insert 9. Inclined Core 10. Movable mold insert 11. Fixed mold base plate 12. Fixed mold insert 13. Positioning ring 14. Gate sleeve 15. Fixed mold plate 16. Inclined guide post 17. Fixed mold insert 18. Fixed mold insert 19 .Sliding block 20, push tube 21. Moving mold plate 22. Pulling rod 23. Push plate guide column 24. Push rod fixed plate 25. Push plate 26. Moving mold guide column 27. Moving mold guide sleeve 28. Auxiliary mold opening mechanism 29. Fixed mold guide bush 30. Fixed mold guide post

Figure 10 Mould structure
Mold working process: After injection is completed, mold is opened for the first time between movable mold plate 21 and inclined core base fixed plate 3. At this time, inclined-drawing core seat fixing plate 3 drives inclined-drawing core 9 to move along axis direction of inclined tapered tube. After the first mold opening is completed, second mold opening (PL2) is performed, that is, movable mold plate 21 and fixed mold plate 15 are separated. At this time, inclined guide post 16 drives slider 19 to demould. When mold is fully opened, push plate 25 and push rod fixing plate 24 of mold push push tube 20, pull rod 22, etc. to push out plastic part and gate aggregate. After taking out plastic part, movable mold and fixed mold begin to close. Mold closing process is opposite to mold opening process. When mold is closed, next plastic part is formed.

In view of structural characteristics of annular end cover composed of 10 inclined tapered tubes, two pads on both sides of mold were merged into one, a push plate and a push rod fixing plate were set in the middle of pad. In order to make inclined core move along axial direction, a fixed plate for inclined core is set under movable mold plate, the first mold opening is set between movable mold plate and inclined core fixed plate. Relative movement between movable mold plate and fixed plate of inclined core is used to guide ejection of inclined core. After fixed mold and movable mold are separated, molded plastic part is pushed out by push tube. Mold uses a non-standard mold base with a special structure, which can provide a reference for molding of similar plastic parts.