Generally, plastic molds only have an ejection mechanism in moving mold. For products with special structures, in addition to an ejection mechanism in moving mold direction, an ejection mechanism is also needed in fixed mold. Taking indoor unit base of a certain brand of split air conditioner as an example, because product has a large cavity in fixed mold direction, a vacuum is easily formed in fixed mold direction during demolding, causing plastic part to crack. In addition, product has a long, high, and thin-walled rib in fixed mold direction. In order to solve problem of vacuum formation during mold opening and to prevent insufficient glue in fixed mold rib during injection molding, making demolding difficult, an ejection mechanism is set in fixed mold. This ejection mechanism uses movement of slider as motion source during mold opening, without need for other power sources, and has good reliability. Design of this fixed mold ejection mechanism is ingenious and worth promoting.

Product structure is shown in Figure 1, with dimensions of 1075 mm * 322 mm * 292 mm. Middle part is a cavity facing fixed mold, with a length of 741 mm and a cross-section of R58 mm arc, forming a large cavity. During mold opening, a vacuum can easily form in fixed mold direction, leading to product cracking. Product has several ribs and stud holes at both ends. There is a square hole on the side of product for venting cold air; this square hole contains two ribs, each with a circular hole of ϕ9 mm diameter. When product is hung on a wall, inner and outer ribs form inner and outer water channels with product body, respectively, to collect condensate during air conditioning operation and discharge it through drain pipes on both sides. Bottom of product has a bottom wall, which helps maintain product's balance when hung on a wall. Inner rib shown in Figure 1(b), located in fixed mold, has a length of 741 mm, a height of 37 mm, and a top wall thickness of 1.2 mm. It is prone to insufficient material during injection molding and difficult to demold upon mold opening. Design challenge of this mold is to ensure that rib in fixed mold is full during injection molding, smoothly demolds upon mold opening, and overcomes vacuum formed in fixed mold during mold opening.

 

Figure 1 Product Image
After comprehensive review, plane passing through center of semicircle at both ends of product was selected as parting surface, as shown in left and right views of Figure 1(a). A demolding mechanism was designed under this structure.
Due to large size of mold and numerous ribs on product, to facilitate processing, enhance rigidity and strength of mold parts, moving mold and fixed mold core are each divided into 5 inserts, as shown in Figure 2. Since slider shown in Figure 4 is located below moving mold insert A, bottom of insert A needs to allow space for slider's movement and is not attached to bottom surface of moving mold base. To enhance rigidity and strength of insert A, two support pillars are installed at the bottom of insert A. Two square holes are opened on the side of insert D, and three square holes are opened on inclined surface to serve as grooves for other sliders.

 

Image 2 Design of moving and fixed mold inserts

 

Image 3 Demolding mechanism for upper and lower holes of vent

 

Image 4 Demolding mechanism for bottom wall and outer rib
Vent on the side of product is formed by collision of moving mold insert D and fixed mold insert ③. There are three square holes below the vent, and a V-shaped groove above vent with four cylindrical holes (ϕ3 mm * 21 mm), as shown in enlarged Figure II. Since demolding direction of both is at an angle of 32° to parting surface, same slider is used for demolding, and inclination angle of groove is set to 32°, as shown in Figure 3. Since core of three square holes below vent needs to pass through insert D in Figure 3(a), length of square hole core extending from side of slider is relatively long. Three square holes are provided on inclined surface of insert D, serving as grooves for three square hole cores. For ease of machining, slider seat is designed as a small insert, fixed to left and right sides of slider with bolts. For easy mold repair, four cylindrical hole cores in V-groove are designed as inserts. For ribs in V-groove, to prevent incomplete injection molding, inserts are placed in fixed mold plate, using a structure where fixed mold insert collides with slider to form ribs in V-groove.
Two external ribs shown in Figure 1(c) share a slider driven by an inclined guide post for demolding with bottom wall of product. To prevent leakage at these two external ribs, slider consists of two large inserts, with a small insert placed between two large inserts along contour of external rib, as shown in Figure 4(a). Two flat holes are provided on this slider to allow space between two support pillars of insert A in Figure 2(a), as shown in Figure 4(b). To enhance mold-closing accuracy of slider, a clamping wedge insert is added to surface of fixed mold plate. For ease of machining, slider seat is configured as two small inserts, fixed to lower left and right corners of slider with bolts. Wear-resistant plates are installed on slider surface to increase its service life.
There are two transverse demolding latches on the bottom wall, using a slider mechanism for demolding, including a slider seat, slider, wedge, locking block, etc., as shown in Figure 5. Two square holes are provided on the side of insert D in Figure 2(a) as grooves for slider. Slider seat is relatively long; for easy mold repair and modification, slider seat and slider are connected by a cylindrical pin. Head of wedge has a double-sloped structure; when mold is opened, it drives slider to demold; when mold is closed, it clamps slider.

 

1 - Slider 2 - Cylindrical pin 3 - Slider seat 4 - Guide block 5 - Locking block 6 - Wedge
Figure 5 Bottom wall latch demolding mechanism
Due to large size of product, a vacuum can easily form between product and fixed mold during mold opening, leading to product cracking. Additionally, product has a rib on fixed mold side, measuring 741 mm in length, 37 mm in height, and with a minimum wall thickness of 1.2 mm. This rib is prone to short-filling during injection molding and makes demolding difficult. To overcome these issues, a fixed mold ejection mechanism is implemented, consisting of a hook, pull rod, push plate, ejector pin, and spring, as shown in Figure 6. Ejector pin is positioned at the top of rib, push plate is positioned between hot runner plate and fixed mold insert, hook is fixed to slider, and pull rod is fixed to push plate. Guide pillars are installed on push plate to ensure smooth push plate movement. When mold is reset, distance between push plate and fixed mold insert is 21 mm, engagement length between hook and pull rod is 6 mm. When mold opens, slider hooks pull rod, pulling ejection mechanism downwards, thus ejecting product from fixed mold core. After slider moves 6 mm laterally, hook disengages from pull rod, ejection mechanism stops moving. This ejection mechanism uses movement of slider during mold opening as its power source, ensuring high reliability.

 

1-Slider; 2-Fixed mold plate; 3-Hook; 4-Pull rod; 5-Hot runner plate; 6-Ejector plate post; 8-Spring; 9-Fixed mold insert; 10-Ejector pin slider
Figure 6. Ejection Structure of Fixed Mold and Slider Mechanism at Both Ends of Product
For snap-fit positions at both ends of product, a "slanted guide post + slider" structure is used for demolding, as shown in Figure 6(a). During injection molding, since one slider is located above mold and the other is located below mold, structures of these two sliders are slightly different: After long-term use, due to wear and tear, a stop is installed on the side of mold base to prevent lower slider from falling to ground during mold opening; to prevent upper slider from automatically resetting under gravity after mold opening, a spring is installed on the side of slider seat, which prevents slider from approaching mold.
For cylindrical hole (ϕ3 mm * 19 mm) in upper right corner of Figure 1(b), angle between its axis and parting surface is 32°, as shown in enlarged view I. Due to small hole diameter and fact that it is a through hole, burrs are prone to appear at opening of cylindrical hole, and its core pulling is prone to breakage and wear. To facilitate processing and mold repair, a "top cylinder assembly + slider" mechanism is used for demolding, as shown in Figure 7. Core-pulling mechanism is designed as an ejector assembly, and a T-slot perpendicular to axis of cylindrical hole is created on the side of slider seat. When slider performs its demolding motion, guide block's 32° inclined groove will also drive slider to move within T-slot.

 

1. Guide block; 2. Slider; 3. Top cylinder assembly; 4. Inclined guide post; 5. Slider seat; 6. Slide groove.
Figure 7 shows cylindrical hole demolding mechanism
Each end of product has a downward-sloping water outlet pipe (ϕ13 mm * 32 mm, inclination angle 62°), as shown in enlarged Figure III. Due to large inclination angle, a "wedge + connecting rod + slider" demolding mechanism is used. Connecting rod is placed at the bottom of moving mold plate and connected to slider seat through inclined T-slot, as shown in Figure 8. To prevent incomplete injection molding, core-pulling mechanism for water outlet pipe is designed as an insert. To ensure smooth movement of demolding mechanism, a shim and support column are placed at the bottom of connecting rod. To enhance rigidity and strength of wedge, a locking block is placed at the bottom of wedge. Wedge has a double-sloping surface structure; when mold opens, it drives slider to demold; when mold closes, wedge drives slider to return to its original position.

 

1 - Support column 2 - Heating element 3 - Round tube core puller 4 - Slider 5 - Slider seat 6 - Connecting rod 7 - Wedge seat 8 - Wedge 9 - Locking block
Figure 8. Demolding Mechanism for Water Outlet Pipe
There are two ribs in the middle of ventilation opening, each with a small hole (ϕ9 mm). A lifter mechanism is used for demolding, consisting of a lifter block, lifter rod, lifter seat, and sliding plate, as shown in Figure 9. Lifter rod is fixed to lifter seat with bolts, lifter seat and sliding plate are connected by cylindrical pins, which helps eliminate machining errors between lifter seat and sliding plate. A groove is opened between ejector base plate and ejector plate to provide space for horizontal movement of sliding plate. When lifter performs its ejection movement, sliding plate will also move horizontally simultaneously.

 

Figure 9. Lifter Demolding Mechanism

This mold uses an 8-gate hot runner gating system. Gates are divided into two rows, arranged on both sides of product. To ensure that injection volume of each gate is basically same, the shorter row has 3 gates, and the longer row has 5 gates, as shown in Figure 10(a). Since this product is not an appearance part, gate mark does not affect appearance, and injection is performed directly on product. Because gate area has a thin wall, only 3mm thick, a protrusion is added to the side of gate area to eliminate flow marks and other defects in injection molding process. To facilitate removal of gate material, structure of gate area is slightly modified, making it slightly lower than surrounding area, as shown in Figure 10(b).

 

Figure 10. Gating System Design
Generally, fixed and moving mold plates do not have separate cooling water channels. To maintain uniform mold temperature and prevent product deformation, cooling water channels are designed on fixed and moving mold plates of this mold, using straight-through unidirectional water channels. Fixed mold plate has 7 cooling water channels, as shown in Figure 11(a). Rubber surfaces of sliders at both ends of product are uneven; water well channels are designed at protruding positions, straight-through conformal water channels are set at flatter areas, as shown in Figure 11(b). Fixed mold insert is divided into many parts, each insert has an independent cooling water channel. Cooling water channel of fixed mold insert ① is divided into two independent water wells + a straight-through conformal water channel, as shown in Figure 11(c). Moving mold insert is composed of 5 inserts joined together. Different inserts use different water channels. For example, moving mold insert E uses a straight-through conformal water channel, as shown in Figure 11(d).

 

Figure 11 Cooling System Design
A two-plate mold with hot runner gating is used. Different ejection mechanisms are employed for different snap-fit positions. Mold cross-sectional view is shown in Figure 12.

 

1. Moving mold base plate; 2. Push plate; 3. Ejector pin fixing plate; 4. Pad block; 5. Return spring; 6. Moving mold plate; 7. Moving mold ejector pin; 8. Insert D; 9. Slide groove; 10. Fixed mold insert; 11. Hot runner insert; 12. Part 3; 13. Hot runner pipe; 14. Hot runner plate; 15. Fixed mold base plate; 16. Fixed mold plate; 17. Part 5; 18. Ejector block; 19. Ejector pin; 20. Insert A; 21. Slider; 22. Support column; 23. Angled guide column; 24. Limit block; 25. Slider seat; 26. Slider; 27. Spring; 28. Angled wedge; 29. Round hole core; 30. Square concave core; 31. Slider; 32. Slider seat; 33. Slider; 34. Round tube core puller; 35. Angled guide column; 36. Connecting rod; 37. Angled wedge; 38. Support column; 39. Locking block. 40-Stop; 41-Angled guide post; 42-Slider; 43-Pull hook; 44-Pull rod; 45-Fixed mold ejector pin; 46-Fixed mold ejector plate; 47-Guide post; 48-Spring; 49-Positioning ring; 50-Ejector pin + ejector cylinder; 51-Slider; 52-Slider seat; 53-Angled guide post
Figure 12 Cross-sectional View of Mold
Mold Working Process: After injection molding, moving mold separates from fixed mold. All sliders are simultaneously ejected under drive of inclined guide pillars. At the same time, under action of spring 48, hook 43, and pull rod 44, fixed mold ejector 45 pushes product away from fixed mold core. When mold is separated by about 20 mm, sliders 42 and hooks 43 at both ends of product disengage from pull rod 44. At this time, fixed mold ejection mechanism stops moving. When moving mold and fixed mold are completely separated, all sliders have completed demolding. Then, ejection mechanism of moving mold (including all moving mold ejector pins 7 and lifter mechanism (not shown in figure) ejects plastic part from moving mold core. After product is removed, ejection mechanism begins to reset under action of return spring, then injection molding machine pushes moving mold of mold to start closing mold. During mold closing process, all sliders reset simultaneously. After mold closes, next production can begin.

(1) Based on structure of indoor unit base of split air conditioner, a mold structure with one cavity was designed, and a hot runner gating system was adopted. For long and deep ribs in fixed mold direction, which are prone to insufficient glue during injection molding and difficult to demold during mold opening, and large cavity in fixed mold direction, which easily forms a vacuum during mold opening, leading to product cracking, a fixed mold ejection mechanism was set up. Sliders at both ends of product hook fixed mold ejection mechanism, ejecting product from fixed mold core during mold opening. When sliders move to both ends and disengage from fixed mold ejection mechanism, fixed mold ejection mechanism stops moving.
(2) Different demolding mechanisms were designed for different product structures. Some demolding mechanisms are quite special, such as use of clearance holes in sliders on outer ribs and bottom wall to allow space for support pillars of moving mold insert above; sliders on the bottom wall need to pass through moving mold insert, so sliders are extended and driven by double-sloping wedges; V-shaped groove above vent and three latches below vent share same demolding mechanism; small cylindrical hole in upper right corner and water outlets on left and right sides have large inclination angles, so inclined slider mechanisms are used for demolding; to overcome defects of cold runners, hot runners are used for casting, and gate is set in a cylindrical shape. To ensure uniform mold temperature, various cooling water channels of different shapes were designed. Production verification has shown that demolding action of this mold is smooth, quality of produced plastic parts is stable, and it is currently in mass production.