When designing an injection mold, demolding method affects mold structure. Main manifestation is that selection of parting method needs to consider convenient demoulding of plastic part, simple mechanism and convenient processing of mold parts. When plastic part has multiple characteristics and demolding direction is different, if conventional sliding block, inclined push rod and other mechanisms cannot be demolded, demoulding can be achieved by multiple partitions and sequential demolding. Therefore, mold structure and demoulding mechanism of specific parts need to be innovatively designed.

 

Deflector structure of a curtain bead automatic machine is shown in Figure 1. It is a multi-curved surface composite thin-shell plastic part. Its main feature is a curved sheet, on which a diversion groove is set. After diversion groove is combined with curved sheet, an undercut groove is formed between flange on upper end of curved sheet and deep groove inside diversion groove. Characteristic of undercut groove and hole ③ is difficulty of demolding molded plastic part, which affects mold structure design. According to characteristics of undercut groove and hole ③, a special demolding mechanism needs to be designed to realize automatic demolding of two characteristics. PC material has good strength, but there is notch effect, while ABC material has good formability but not as strong as PC, so plastic part material uses modified alloy plastic ABS+PC to combine advantages of ABS and PC, material shrinkage rate is 0.55%, molded plastic parts have good surface smoothness and wear resistance.

 

In view of difficulty of demolding of undercut groove and hole ③, parting setting is shown in Figure 2. After plastic part to be formed is placed in position shown in Figure 2, maximum outer contour of plastic part to be formed is used as parting line. Parts of cavity molded plastic part are composed of cavity plate inserts, core inserts, floating cores, and side top cores. After using this parting setting, demolding steps of plastic part are as follows: ①Open PL surface, cavity plate insert is separated first; ②Plastic part is lifted by floating core in Z+ direction, and core insert is separated from plastic part; ③Side top core ejects plastic part from upper side of floating core, and plastic part remains on the side top core; ④High-pressure air gun blows plastic part from side top core to realize demolding of plastic part.

Mold uses a 1-cavity layout, and gating system uses a common runner + side gate.

 

Shape of molded part is shown in Fig. 3. Floating core 1 is a movable molding insert, which is connected to it by a push rod on push plate and fixed together for linkage. Considering stability of floating core and other molded parts after closing, upper and lower parts need to be designed with inverted cones to ensure its stability by locking tapered surface of mold plate. Core insert 2 is fastened to movable mold plate by screws, and upper end of core insert 2 is provided with an edge frustum for matching with cavity plate insert when it is closed.
Side top core 3 is installed in floating core 1, pushed out by hydraulic cylinder 5 and ejector rod 4 for side ejection. Materials of cavity plate insert, core insert 2, floating core 1 and side top core 3 are alloy steel 2738H (Cr-Ni-Mo alloy steel). Steel is added with about 1% Ni, pre-hardness is good, and hardenability is improved. Hardness of large-section thick plate is evenly distributed. It is mainly used in large-scale plastic molds, such as automobile bumpers, TV shells and other molds.

 

1. Core insert 2. Cavity plate insert 3. Floating core 4. Spring material pin 5. Side top core 6. Side mandrel 7. Upper taper block 8. Hydraulic cylinder 9. Lower taper block 10. Hydraulic cylinder taper top block 11. Hydraulic cylinder top rod 12. Floating core top rod 13. Side top rod sleeve 14. Support column 15. Movable mold seat plate 16. Push plate 17. Side top core positioning stopper 18. Moving mold plate 19. Fine positioning block 20. Fixed mold plate 21 Fixed mold base
Mold structure is shown in Figure 4, which adopts a two-plate mold structure and a layout of one cavity. Runner and gate are directly processed on fixed mold plate 20, and runner condensate is ejected by fixed mold elastic pin 4 when runner is demolded. When designing cooling system, cooling water channels are set for cavity plate insert 2, core insert 1, floating core 3 and side top core 5 to ensure uniform cooling of plastic parts. Cavity is vented through fit gap between molded parts, and fit gap is ≤0.015mm. When fixed mold plate 20 and movable mold plate 18 are closed, four precision positioning blocks 19 distributed at four corners are used to enhance positioning accuracy of closing.

Plastic part demoulding mechanism is a composite structure, including 2 sub-mechanisms: ① ejection mechanism for floating core 3; ②side ejection mechanism for side top core 5 installed on floating core 3.
Ejection mechanism components used for floating core 3 mainly include: a hydraulic cylinder taper block 10, a hydraulic cylinder mandrel 11, three floating core mandrels 12 and a floating core 1. Floating core mandrel 12 and hydraulic cylinder mandrel 11 are both installed on push plate fixing plate and driven by push plate 16. Hydraulic cylinder mandrel 11 is combined with hydraulic cylinder taper block 10 through a screw, which is used to simultaneously eject hydraulic cylinder 8 and floating core 3, relies on its taper for accurate resetting when resetting. Similarly, floating core 3 is also fastened and combined with floating core ejector pin 12 by screws at the end of ejector rod. Upper and lower parts of floating core 3 are set in the shape of a peripheral cone, an upper taper block 7 and a lower taper block 9 are respectively added to fixed mold plate 20 and movable template 18 to ensure accuracy of resetting.
Side ejection mechanism components include: side top core 5, side top rod 6, hydraulic cylinder 8, side top rod sleeve 13 and side top core positioning stopper 17. Mechanism is installed in floating core 3, side top core 5 is used to eject plastic part from side of floating core 3.

After injection is completed, movable and fixed molds are opened at PL, and partial area of plastic part is ejected from cavity plate insert 2. When movable mold moves down in -Z direction, after opening a certain distance, ejector pin of injection molding machine will hold push plate 16 and keep it still, while movable mold plate 18 continues to descend. Floating core ejector pin 12 drives floating core 3 and side ejection mechanism above it to move, local area of plastic part is ejected from core insert 1. After core 3 is ejected for a certain distance, hydraulic cylinder 8 drives side top core 5 to move in -Y direction, thereby pushing out part of plastic part from floating core 3, leaving plastic part on the side top core 5. After moving for a certain distance, plastic part is blown off from side top core 5 through external air gun of injection molding machine to realize demolding of plastic part.
Combining shape and characteristics of plastic parts, a two-platen injection mold with one cavity was designed. In view of difficulty of demolding plastic parts, a composite demolding mechanism is designed to realize automatic demoulding of plastic parts. Composite demolding mechanism is composed of a floating core ejection mechanism and a side ejection mechanism. When demolding mechanism is working, first ejector of injection molding machine pushes push plate to drive floating core and side ejection mechanism to realize demolding of part of plastic part, then side core is driven by hydraulic cylinder to push plastic part from floating core in side direction. And finally plastic part is blown off from side core by external blowing system to ensure automatic production of plastic part. Mold structure and demolding mechanism are simple and practical, action reliability is good, which can effectively reduce difficulty of mold processing and mold manufacturing costs.