Structure of plastic part is shown in Figure 1. It is 43mm high and has 4 nozzles. One of nozzles has threads on inner surface (M28mm, length 11mm). Outer diameter of nozzle where internal thread is located is φ34mm. Material of plastic part is ABS, which is relatively hard, so internal thread cannot be forcedly demoulded and needs to be demoulded by rotating thread core. Four ribs are provided on outer cylindrical surface of plastic part. Four ribs are formed in cavity to prevent plastic part from rotating together with internal thread core.

 

Since size of plastic part is small and there are 4 nozzles on plastic part, it is suitable to use a two-plate mold structure. Mold has a 2-cavity layout. Plastic parts have high requirements on appearance and cannot have obvious gate marks, so latent gates are used. Taking axis direction of internal thread as mold opening direction, internal thread core is located in movable mold, nozzle that communicates with internal thread is formed in fixed mold, and the other two nozzles are formed on two Huff sliders. In order to ensure that plastic part remains stationary when thread core rotates, parting surface is set at the step of pipe opening where internal thread is located.
In order to automatically rotate threaded core, equipment such as "hydraulic motor + worm gear + gear transmission" is installed on mold. Mold adopts a non-standard structure, eliminating push plate and push rod fixed plate. Mold structure is shown in Figure 2.

 

1. Fixed mold base plate 2. Fixed mold plate 3. Huff slider 4. Inclined guide column 5. Spring 6. Moving platen 7. Worm gear 8. Worm 9. Backing plate 10. Moving mold base plate 11. Thrust ball bearing 12. Threaded core 13. Tapered roller bearing 14. Spindle 15. Dynamic mold cavity plate insert 16, inner core 17. Side core 18. Fixed mold core 19. Gate sleeve 20. Seal ring 21. Hydraulic motor
Figure 2 Mold structure
Mold opening process: After injection is completed, when fixed mold and movable mold are separated, nozzle located in fixed mold is demoulded. Inclined guide pillar 4 and spring 5 drive two Huff sliders 3 to move to both sides, so that two transverse nozzles on plastic part are demoulded. At this time, plastic part and flow channel aggregate adhere to movable mold.
After movable mold and fixed mold are completely separated, hydraulic motor 21 starts automatically and drives threaded core 12 to rotate through worm 8 and worm gear 7. Since there are 4 ribs on plastic part, plastic part will not rotate together with threaded core. When threaded core 12 rotates, plastic part and flow channel aggregate will be screwed out of driven mold cavity. Subsequently, injection molding machine pushes movable mold and fixed mold to close. At the same time, fixed mold wedge block drives two Huff sliders to reset. After mold is reset, next injection cycle begins.

Internal thread needs to be rotated to be demoulded. However, since inner hole core with a diameter of φ10mm in Figure 1(a) fits core surface of transverse inner hole and cannot rotate, core in threaded pipe mouth must be divided into inner and outer layers, as shown in Figure 3. Outer layer is a threaded core and can be rotated; inner layer is a core with φ16mm and φ10mm inner holes and cannot be rotated. In order to facilitate rotation of internal thread core, a worm gear, a tapered roller bearing, and a thrust ball bearing are installed on internal thread core. Worm gear is connected to internal thread core through a flat key. In order to prevent inner core from rotating, bottom of inner core is designed to be square.

 

7. Worm gear 11. Thrust ball bearing 12. Threaded core 13. Tapered roller bearing 14. Spindle 16. Inner core
Figure 3 Internal thread core structure

Transverse nozzles on both sides of plastic part are demoulded using Huff slides driven by inclined guide pillars. According to structure of plastic part, side core is installed on one of sliders, and side core is not installed on the other slider, so structures of two sliders are inconsistent. Since size of the two sliders is 140mm * 126mm * 75mm, which is small, in order to ensure strength of slider and facilitate design of cooling water channels on slider, two Huff sliders adopt an integral structure, that is, cavity is designed directly on slider instead of an insert structure. In order to ensure stability of slider during mold opening process, two springs are designed on the side of slider. When slider is separated, two springs will assist inclined guide pillar to drive slider movement. In order to prevent pressing wedge from damaging slope of slider, a wear-resistant sheet is provided on slope of slider. Structure of slider is shown in Figure 4.

 

Nozzle where internal thread is located on plastic part is formed by movable mold insert, and parting surface is set at the step of nozzle, as shown in Figure 5. Four ribs formed on outer surface of internal thread pipe mouth are stuck in cavity. When internal thread core rotates, these four ribs can prevent plastic part from rotating.

 

According to structural characteristics of plastic parts, a two-plate mold with 2 cavities is used for molding. Size of plastic part is small, and only one gate can be installed in one cavity. Main channel is set in sprue sleeve, shunt is set on meeting surface of left and right sliders. Outline of shunt is in the shape of a horn, and latent gate feeds materials. In order to avoid defects on the surface of plastic part, ends of main channel and two shunt channels are appropriately extended to form a cold material cavity. Gating system is shown in Figure 6.

 

Install hydraulic motors, worm gears, bearings and other components on mold to achieve automatic demoulding of internal threads. In order to improve work efficiency, one motor drives two internal thread cores to rotate at the same time, as shown in Figure 7. In order to maintain stability of movement process of demoulding mechanism, a thrust ball bearing is installed at both ends of worm, a tapered roller bearing is installed at upper part of threaded core, and a thrust ball bearing is installed at lower part. Bottom of inner core is square and threaded to movable mold base plate, so that when outer threaded core rotates, inner core remains stationary.

 

In order to quickly cool and shape plastic parts after injection, cooling water channels must be designed on main mold parts. According to structure of plastic part, cooling water path of this mold is mainly designed on slider and dynamic mold cavity plate insert. Among them, waterway of dynamic model cavity plate insert adopts a straight-through waterway with a hole diameter of φ8mm. Four straight-through waterways are used to form a quadrilateral and surround two mold cavities, as shown in Figure 8(a). In order to control temperature of Huff slider, according to shape of plastic part, a spatial zigzag straight-through water channel is used on Huff slider, and water pipe joint is set on the side of slider, as shown in Figure 8(b).

 

Taking four-way regulating valve with internal threads as an example, for its outer surface, a demoulding mechanism of "inclined guide post + Huff slider" is designed, and for its internal thread, an internal thread automatic demoulding mechanism composed of "hydraulic motor + worm gear + gear transmission" is designed. According to structure of plastic part, a two-plate injection mold with 2 cavities is designed. Nozzle where internal thread is located is set in movable mold cavity for molding. The other two nozzles are formed in Huff slider, and slider is driven by inclined guide post. Mold has smooth demoulding action, stable molding quality of plastic parts, and reasonable mold structure.