Three-dimensional structure of a certain elbow fitting in mold is shown in Figure 1. Main part consists of an outer wall and an inner straight tube, with a hollow inner groove between them. Extension of inner straight tube at other end of main part is an elbow with external threads. An elastic buckle is positioned on the ridge of outer wall. Buckle base is equipped with arranged ribs and bosses. To ensure buckle elasticity, a gap is provided at base. Plastic part material is PC-modified ABS with a shrinkage rate of 0.52% to 0.55%. The overall dimensions of elbow fitting are 127.4 mm * 46 mm * 58 mm, with a maximum wall thickness of 3.2 mm, a minimum wall thickness of 1.5 mm, and an average wall thickness of 2.4 mm. Inner straight tube has an inner diameter of φ35 mm and a wall thickness of 2 mm. External thread is a trapezoidal cross-section thread with a pitch of 4 mm.

 

Figure 1 Plastic part shape and main parting surface

After analysis, main difficulties in molding plastic parts are following three points.
(1) Demolding of external threads on outer wall of plastic parts. Demolding of external thread features can only be done by half-side core pulling, or by designing cavity plate insert and core insert as separate demolding, but this will make demolding of other features difficult.
(2) Demolding of bent tubes of plastic parts. Since straight tube and bent tube in plastic part are connected, demolding of bent tube can only be done by rotating core in one direction, which limits placement and design space of demolding mechanism.
(3) Demolding of buckle features. This feature can only be demolded by side core pulling. When cavity of plastic part to be molded is arranged as shown in Figure 1, design space of side core pulling mechanism of two plastic parts is limited, and side core pulling mechanism is not easy to design.

Main parting surface is configured as shown in Figure 1. This ensures that external thread is demolded through main parting surface. Internal groove and straight tube are demolded via side core pulling with a straight slider. Curved tube within external thread requires a separate elbow core pulling mechanism for demolding. Ribs and gap openings are demolded via side core pulling with a side slider mechanism. Buckle position uses a separate side core pulling slider for both molding and demolding.
With this demolding arrangement, mold's demolding mechanism is located on mold opening surface, simplifying mold structure, reducing difficulty of molded parts, improving mechanism reliability and ease of maintenance. Considering space required for mechanism design and number of parts to be molded, a two-cavity mold is selected. Gating system uses a side gate, located on ridgeline of outer wall of part to be molded (see Figure 1). Gate is rectangular.

Based on aforementioned layout of parting and demolding mechanisms and difficulties in mold design, cavity structure on movable mold side of 1-mold, 2-cavity layout is shown in Figure 2. Side core pulling mechanism is primarily located on movable mold side. External thread is formed and demolded by core insert and cavity plate insert. Internal groove and straight tube are formed and demolded by side core. Ribs, gaps, and buckle positions are formed and demolded by inner core. Curved tube within external thread is formed and demolded by arc slider.

 

Figure 2 Demolding mechanism design
Straight tube sections of two plastic parts are optimized to use a hydraulic cylinder to implement side core pulling. Curved tube section core is optimized to a combination of rotary demolding and curved core pulling. Each section uses an arc slider mechanism, driven by a rack-type arc core pulling mechanism. Side slider of this mechanism drives rack, which drives sector-shaped drive block, which in turn drives arc slider on top of it around axis to perform rotary core pulling.

As shown in Figure 3, mold frame is a two-platen structure, employing two types of guides to guide mold's moving parts. One guide element guides movement of movable platen 3 and fixed platen 2 during closing. This is achieved by combination of guide pins 22 and guide sleeves 27. Four pairs of guide pins and sleeves are provided, guide pins at mold's reference angles must be offset to prevent misalignment of movable and fixed molds. The other guide element guides movement of mold's ejector plate. These elements consist of ejector plate guide pins 18 mated with corresponding ejector plate guide sleeves. Four pairs of these guides are also installed at four corners of ejector plate.

 

1. Fixed mold base plate 2. Fixed mold plate 3. Moving mold plate 4. Hydraulic cylinder 5. Hydraulic cylinder base plate 6. Moving mold backing plate 7. Spacer 8. Moving mold base plate 9. Reset lever 10. Slider pressure strip 11. Bend lower slide 12. Sector drive block 13. Push rod 14. Rotating shaft 15. Rack 16. Stopper 17. Support column 18. Push plate guide 19. Push plate 20. Push rod fixing plate 21. Core insert 22. Guide column 23. Hydraulic cylinder base plate 24. Hydraulic cylinder 25. Wear plate 26. Locking block 27. Guide bushing 28. Straight tube slide 29. Clamping block 30. Inner groove core 31. Retaining ring 32. Sprue bushing 33. Cavity plate insert 34. Arc slider 35. Pressure strip 36. Right slider 37. Left slider
Figure 3 Mold structure
Additional support pins 17 enhance support strength of movable platen 6 on mold frame, preventing deformation of movable platen 3 caused by injection pressure transmitted from core insert 21. Cavity plate insert 33 and core insert 21 of molded part are mounted in grooves in fixed and movable platens 2 and 3, respectively, and secured with screws. Cavity plate insert 33 and core insert 21 are based on lower right corner datum, same as mold base. This minimizes mold system errors caused by inconsistencies in design, processing, and assembly datums. Material selection: S136 steel, heat-treated to a hardness of 47-50 HRC, for cavity plate insert 33. Type 45 steel, heat-treated to a hardness of 38-42 HRC, is used for core insert 21.
Molded components of inner wall are assembled using a straight tube core, an inner groove core 30, and arc-shaped sliders 34. Left and right sliders 37 and 36, respectively, form sidewall buckle. These sliders are made of imported S136 alloy steel, known for its high hardness and excellent processing properties.
All molded components are cooled by 8mm φ cooling pipes. Two water channels on fixed platen 2 provide cooling for cavity plate insert 33, while three water channels on movable platen 3 provide cooling for core insert 21. Independent mold temperature control channels are provided in core insert 21 and cavity plate insert 33 of main molding components to ensure a cooling water temperature difference ΔT of ≤5℃ between inlet and outlet channels. To ensure leak-proof sealing of water channel between molding insert and mold plate, a φ2.5mm sealing ring (see Figure 4(a)) is used. Cavity is cast using a side gate, as shown in Figure 4(b). Gate is a contraction-type flat side gate, and runner uses a φ6mm circular runner. Molded part is ultimately ejected by push rod. Ejection system is composed of a push plate 19 and a push rod fixing plate 20. Ejection of molded part is achieved by ejector rod of injection molding machine, pushing push plate 19. Four reset springs are provided on reset rod 9 to ensure push plate's balanced return. Reset rod 9 ensures that push plate returns to its original position when mold is closed. Figure 5 shows demolding mechanisms. Three demolding mechanisms are used for this part: a straight tube secondary delayed side core pulling mechanism, a bent tube core pulling mechanism, and a double-sided inward core pulling mechanism. Straight tube secondary delayed side core pulling mechanism is used to stage demolding of straight tube wall and inner groove of straight tube section of part. Bent tube core pulling mechanism forms and demolds inner wall of bent tube section of part. A double-sided inward core pulling mechanism is used to demold buckle feature.

 

Figure 4 Sealing and gate design

 

3. Moving platen 11. Bend lower slide 12. Sector drive block 13. Push rod 14. Rotating shaft 15. Rack 21. Core insert 28. Straight tube slide 29. Clamping block 30. Inner groove core 32. Sprue bushing 34. Arc slide 35. Hold-down strip 38. Straight tube core
Figure 5 Demolding mechanism

(1) Components of straight tube secondary delay side core pulling mechanism include parts 23 to 30. As shown in Figure 5 (a), in molded parts of straight tube section of plastic part, inner groove core 30 is coaxially sleeved on outside of straight tube core 38. Inner groove core 30 is pressed by clamping block 29 and installed on straight tube slider 28. Shoulder at tail end of straight tube core 38 and top of corresponding pipe sleeve of inner groove core 30 are provided with a delay distance of 10mm to ensure that inner groove core 30 is first pulled out 10mm before straight tube core 38 is pulled out from inner wall of straight tube of molded plastic part. Working principle of mechanism: Hydraulic cylinder 24 drives locking block 26 through piston rod to drive straight tube core 38 to pull core. When straight tube slider 28 moves outward, it first drives inner groove core 30 to be pulled out 10mm, then drives straight tube core 38 to be pulled out from inner wall of straight tube of plastic part, so as to reduce single core pulling force and prevent plastic part from being deformed during demolding.

(2) Rack 15 of bending core pulling mechanism shown in Figure 5 (b) is installed on lower slider 11 of bending tube, and the two are fastened with screws. Hydraulic cylinder 4 is used to drive rack 15. Hydraulic cylinder 4 is installed under movable plate 3 as a power component. Rack 15 is used to drive fan-shaped driving block 12 to rotate. Fan-shaped driving block 12 is mounted on rotating shaft 14 and connected to rotating shaft 14 through a key. Rotating shaft 14 is installed in corresponding shaft hole on movable plate 3 to ensure that fan-shaped driving block 12 can be driven by rack 15 to rotate around rotating shaft 14. Fan-shaped driving block 12 is provided with a push rod 13. Upper end of push rod 13 is inserted into central hole of straight tube core 38. When fan-shaped driving block 12 rotates, it can synchronously drive arc slider 34 to rotate, driving bending tube core to rotate and pull core. Arc slider 34 is guided by pressure strip 35, and arc stroke is limited by limit screw. Design of this mechanism is an optimized curved track core pulling mechanism. 3D printing is used to manufacture special-shaped guide rails (bending radius R ≥ 50mm). In combination with linear bearings and needle roller guide rails, friction coefficient is reduced to 0.08. After mold is applied, core pulling speed is increased by 40%. A segmented hydraulic drive design is used to design a three-stage telescopic hydraulic cylinder (stroke ratio is 1:2.5), and a pressure sensor (adjustable from 0 to 20MPa) is configured to achieve precise control. H13 steel is used with nano-coating to improve wear resistance of slider, and a hydraulic synchronization system is used to ensure consistency of multi-slide movement.
(3) Double-sided inward core pulling mechanism consists of parts 36 and 37. Inclined slot sliders of the two are fixed to fixed mold plate 2 by screws, right slider 36 and left slider 37 are driven synchronously to move toward middle to perform inward core pulling through inclined T-shaped slots on both sides. Core pulling stroke of two sliders is limited by corresponding limit glass beads. Through above design scheme, demoulding qualification rate can be increased to more than 99.5%, service life of mold can be extended to 500,000 times, and production cycle can be shortened by 25% to 35%. In the first batch of mold trials, synchronization of each mechanism is monitored, hydraulic pressure and ejection displacement curve are monitored in real time through data acquisition system to ensure that optimization effect is stable and reliable.

(1) Preparation. Injection mold is installed on injection molding machine and is ready for production after debugging. After mold is closed, injection molding machine nozzle injects molten plastic into mold cavity. After filling, pressure holding, and cooling, it is ready to open mold.
(2) Mold opening. Mold movable mold retreats and opens at parting surface. Right slider 36 and left slider 37 complete core pulling, straight tube slider 28 and straight tube core 38 are unlocked.
(3) Straight tube section core pulling. Piston rod of hydraulic cylinder 24 drives straight tube slider 28 and straight tube core 38 to complete core pulling.
(4) Bend tube rotation core pulling. Driven by piston rod of hydraulic cylinder 4, rack 15 completes rotation and core pulling of plastic part's bent pipe section.
(5) Plastic part is ejected. Movable mold continues to retreat, push rod 13 pushes plastic part out of core insert 21 and demolds it.
(6) Reset. Reset process is opposite of mold opening process.