Abstract A 2-cavity two-plate mold was designed for injection molding of plastic parts of 135° elbow pipe of injection molding machine. Mold uses 4 types of demolding mechanisms for demolding of molded plastic parts, including bend pipe, straight pipe, retractable, push rod ejection and other demoulding mechanisms. Elbow core pulling mechanism uses a rack to drive sector gear, then arc slider is driven to perform rotary core pulling; straight tube core pulling mechanism uses a hydraulic cylinder to drive inner and outer wall cores to perform 2 core pulling; inner core pulling mechanism uses T-slots locking block to drive inner retractable core pulling; plastic part is finally pushed out by push rod.

135° special-shaped elbow joint of oil guide pipe of injection molding machine is used for special pipe joint position. It is a functional part that needs to be designed with special structural features. Axonometric drawing of plastic part is shown in Figure 1. Left end is a circular arc tube with ring ribs, and right end is a straight tube; on outer wall of junction of circular arc tube and straight tube, a bracket sleeve is additionally provided for sleeve connection with other plastic parts. Inner wall of bracket sleeve is provided with 3 screw posts, the lower end is provided with a straight groove. A plastic clamping groove is formed between bracket sleeve and inner wall of straight pipe. There are multiple lattice ribs on outer wall of bracket; an extension strip is also provided in bracket sleeve, a buckle is provided on extension strip. Outer wall of arc tube is provided with 4 rings of ring ribs and multiple triangular ribs. Taking into account use of plastic parts, plastic parts are made of ABS engineering plastics with a shrinkage rate of 0.5% to 0.6%.

 

Figure 1 Special-shaped pipe joints

For plastic part in Figure 1, when designing mold, setting of parting surface is mainly restricted by following factors: ①Ring ribs and multiple triangular ribs on outer wall of arc tube can only use two-part demoulding method; ②Inner wall features of stent sleeve can only be pulled out along center line of bracket sleeve and center line of three screw columns, lattice ribs on outer wall of bracket sleeve can only be pulled out in vertical direction perpendicular to surface where lattice ribs are located. Demoulding of buckle feature can only be carried out in direction perpendicular to bottom surface of buckle; ③Demoulding of inner wall of arc tube can only be demoulded by core pulling of arc rotation; ④Inner wall of straight pipe feature needs to be core-pulled and demoulded, core-pulling direction can be set in same direction as inner wall of stent sleeve.
From above analysis, it can be seen that plastic part to be molded can only be typed at PL shown in Figure 2 to facilitate demolding of molded plastic part, simplify demolding mechanism and mold structure. Under parting setting of PL surface, automatic demoulding of molded plastic part can be realized by setting only 3 slider mechanisms. Functions of corresponding three sliders are: ① Slider 1 is shaped with wall-clamping grooves, straight grooves, screw posts and formed tube features; ② Slider 2 is shaped with arc tube inner wall features; ③ Slider 3 is shaped with ribs and buckles feature. Ring rib and rest of outer wall of plastic part are formed and demolded by fixed mold insert and movable mold insert.

 

Figure 2 Parting and demolding design

In parting state shown in Figure 2, determined molded part design is shown in Figure 3. Slider 1 corresponds to a straight tube slide, slide 2 corresponds to an arc slide, and slide 3 corresponds to an inner core-pulling slide. Straight tube slide block drives core pulling through hydraulic mechanism, circular arc slide block drives rack through hydraulic mechanism, rack drives sector gear, and sector gear drives circular arc slide block through rotating shaft to implement rotating core pulling. Inner core-pulling slider integrates its driving and locking requirements, can be locked and driven by a T-slot locking block, inner core-pulling is implemented for corresponding features.

 

Figure 3 Design of molded parts
Considering production efficiency of plastic parts, layout of 2 cavities is adopted. In order to simplify mechanism and optimize mold structure, straight pipe sliders of 2 cavities in Figure 3 are driven by a common hydraulic mechanism, arc sliders in two cavities are also driven by a common rack and slider mechanism. Inner core-pulling sliders of two cavities also use common T-slot locking block for locking and driving.
In design of gating system, in view of convenience of runner processing, a φ8 mm CNC milling cutter is used to process circular runner, considering structural space setting, reducing use size of mold base, and using oblique gate for sprue bushing.

According to 2-cavity structure set on parting surface, corresponding movable and fixed mold structures are shown in Figure 4. Combining demolding mechanism and requirements of simplifying mold structure, a two-plate mold base is selected. In fixed mold setting, cavity plate adopts an eccentric arrangement to ensure that center of sprue sleeve is aligned with center of mold base. Overall cavity of mold is eccentrically poured to control injection pressure. Cavity plate insert is installed in fixed mold plate. Material is 2344 alloy steel and a single waterway is used for cooling.

 

Figure 4 Dynamic and fixed mold structure
In movable mold, 3 composite mechanisms are used to simultaneously perform core-pulling and demolding of two cavities. Corresponding mechanisms are straight tube core-pulling mechanism, elbow core-pulling mechanism and inner core-pulling mechanism. Final demolding of plastic part to be formed is carried out by pushing out with a push rod, core insert is installed on movable mold plate, material is P20, and a single waterway is also used for cooling.

Mold structure is shown in Figure 5.

 

Figure 5 Mold structure
1. Fixed mold base plate 2. Fixed template 3. Arc slider 4. Arc pressing bar 5. Limit screw 6. Rotating shaft 7. Connecting screw 8. Rotating shaft 9. Sector gear 10. Rack 11. Rack Slider 12. Spacer 13. Movable mold base 14. Pusher 15. Travel switch 16. Movable mold pad 17. Movable template 18. Limit glass beads 19. Inner core-pulling slider 20. T-shaped locking block 21. Core insert 22. Cavity plate insert 23. Diagonal sprue sleeve 24. Wall-clamping groove core 25. Straight pipe core 26. Straight pipe slider 27. Hydraulic cylinder 28. Reset rod 29. Reset rod Spring 30. Limit block 31. Support column 32. Push rod 33. Push plate guide column 34. Guide column 35. Hydraulic cylinder
(1) Mold adopts a two-plate mold base, and a movable mold backing plate 16 is added under movable mold plate 17 to facilitate installation of elbow core-pulling mechanism.
(2) In elbow core pulling mechanism assembly, core pulling assembly of a single cavity mainly includes parts 3~11. Piston rod of hydraulic cylinder 35 drives rack slider 11 to move, and rack slider 11 drives sector gear 9 to rotate around center of rotating shaft 8, rotating shaft 8 then drives arc slider 3 located in arc bead 4 on movable mold plate 17 to rotate, inner wall of circular arc tube of molded plastic part is rotated and cored. Position of circular arc slider 3 is restricted by limit screw 5.
(3) Inner core pulling mechanism. Inner core pulling mechanism components mainly include parts 18 to 20. When mold is opened, inclined T-shaped grooves on both sides of T-shaped locking block 20 simultaneously drive two inner core-pulling sliders on both sides to implement inner core-pulling. Limits of two retracting core-pulling sliders are respectively limited by their respective limit glass beads 18.
(4) Straight tube core pulling mechanism. Straight tube core pulling mechanism is a double core pulling mechanism, a delay core pulling is set between two core pullings, delay distance is 10 mm. Mechanism components include parts 24 to 27. In first core-pulling process, wall-clamping groove core 24 is first drawn out by 10 mm, then inner wall core of straight tube is drawn out. Both cores are driven by straight tube slider 26. Tube slider 26 is driven by piston rod of hydraulic cylinder 27.
(5) Launch agency. Final demolding of plastic part is realized by pushing out a plurality of push rods 32, push rod 32 is pushed by push plate 14, push plate 14 is connected and driven by ejector rod of injection molding machine through connecting rod. In order to prevent push rod 32 from interfering with slider of straight tube core pulling mechanism and arc slider of elbow core pulling mechanism when resetting, push rod 32 must be retracted to its position before resetting. Therefore, push plate 14 is equipped with a travel switch to detect its reset. Only after push plate 14 is reset, piston rods of hydraulic cylinders 27 and 35 can act to drive sliders on respective mechanisms to reset.

When mold is working, operating principle of mechanism is as follows.
(1) Piston rod of hydraulic cylinder is core-pulled first. Before mold is opened, because hydraulic cylinders 27 and 35 are in series, piston rod of hydraulic cylinder 35 moves first to complete core pulling of inner wall of elbow, then piston rod of hydraulic cylinder 27 moves to complete core pulling of wall groove and inner wall of straight pipe. .
(2) Mold has only a single parting surface. When mold is opened after injection, sliding block of injection molding machine drives movable mold to move, mold opens at parting surface.
(3) Launch. After mold is opened, movable mold continues to move, ejector rod of injection molding machine drives push plate 14, push plate 14 pushes push rod to push plastic part out of core insert 21 to realize demolding of plastic part.
(4) Reset. When resetting, ejection mechanism is reset first, reset detection is performed by travel switch 15, piston rods of hydraulic cylinders 27 and 35 are sequentially controlled to drive parts to reset.

Combined with structure of plastic part, a 2-cavity two-platen injection mold was designed. Single-point side gate was used for pouring, main runner uses inclined gate bushings to supply materials, which optimizes mold structure. In order to realize automatic demoulding of plastic parts, 4 kinds of demoulding mechanisms are used to assist demoulding, namely, bend tube core pulling mechanism, straight tube core pulling mechanism, internal retractable core pulling mechanism, and push rod ejecting demoulding mechanism. Mold structure design utilizes mold base structure space and saves material costs.