Design of Injection Mold for Stop Cap
Time：2023-12-25 19:36:41 / Popularity： / Source：
1 Structure and process analysis of plastic parts
Stop cover of a window lift system assembly is shown in Figure 1. The overall size is 37mm*30mm*14mm, material is PA6-GF30%, shrinkage rate is 4%. Undercut in two directions of plastic part is formed by Huff slider. Outer diameter of inner hole thread is D=φ21.6mm, and pitch of thread is P=4mm. Considering structure of plastic part, when mold is opened, mold needs to be still in closed state to complete unthreading. When unthreading, rotation stop depends on geometric shape of plastic part on fixed mold side. Movement of threaded core is controlled by thread on inner hole of tooth shaft guide sleeve. Thread parameters on inner hole of tooth shaft guide sleeve must be consistent with unscrewed thread parameters. Last thread of inner hole of plastic part formed by this thread removal mechanism will not be damaged. Rack and pinion thread removal mechanism is used, and movement of rack is controlled by Hydraulic cylinder piston rod drive. Injection mold of stop cap has a structure of 4 cavities. Because molding material contains 30% glass fiber, mold steel needs nitriding treatment to prolong service life of mold. Mold parts are made of pre-hardened steel, and heat treatment hardness is 29~33HRC.
2 Mold layout and slider design
Mold adopts split form of Huff sliders. As shown in Figure 2, undercuts in two directions of plastic part to be formed are respectively formed on the two sliders. In order to avoid flashing on claws of molded plastic part, insert tooth shaft into local structure of molded plastic part on the two sliders, and insertion angle is 6°. Plastic parts are arranged in a straight line, gate adopts side gate, section of runner 1 is U-shaped, located on fixed plate, section of runner 2 is circular, diameter is φ4mm, and it is located on sliders on both sides. Slide block is driven by inclined guide column 3, and external spring 8 is used to provide driving force for reset. In order to control preload of spring, screw 9 for fixing spring is stepped. Because slider is in the direction of sky (see Figure 2), in order to prevent mold from colliding with mold due the downward movement of slider due to its own weight, it is necessary to meet F load ≥ 2G slider; in order to ensure service life of spring up to 300,000 times, allowable compression of spring is 50%, that is, compression of spring is ≤50%, otherwise it will lead to a decrease in load and durability, or even damage. Because G slider = 12.8kgf, choose spring model SWF25-10, quantity is 2, constant of spring = 1.0kgf/mm, preload = 13mm, then F load = constant of spring * preload * number of springs = 1.0 * 13* 2=26kg, F load/G slider=26/12.8=2.03>2; spring compression=(preload+slider stroke)/spring length=(13+35)/100=48%< 50%, selected spring is qualified. In order to ensure smooth movement of slider, slider bead 13, slider guide rail 15 and wear plate 11 at the bottom of slider are made of self-lubricating materials, that is, special high-strength brass + solid lubricant. In mold, in order to prevent molded plastic parts from flashing due to lack of locking of slider, wear-resistant plate on the back of slider is made of hardened SKD61; in order to prevent misalignment of insertion parts of the two sliders and cause flashing of molded plastic parts, the two Huff sliders are positioned with notches; in order to facilitate installation of slider, for slider with a mass of more than 10kg, a lifting screw hole should be processed on the end surface.
Figure 2 Partial structure of mold
3 Design of unthreading mechanism
Thread removal mechanism is focus and difficulty of mold design. Thread removal mechanism adopts rack and gear, and rack is driven by a hydraulic cylinder piston rod. As shown in Figure 3, hydraulic cylinder 9 is fixed on hydraulic cylinder fixing block 8, and rack 6 is fixed on rack fixing block 3. When demoulding, piston rod on hydraulic cylinder 9 drives rack 6 to move downward in the direction of arrow, rack 6 drives main gear 22 to rotate counterclockwise, main gear 22 is fixed on transmission shaft 13 , drive transmission gear 11 fixed on it to rotate counterclockwise, and drive tooth shaft gear 14 meshed with it to rotate clockwise, tooth shaft gear 14 is fixed on tooth shaft 16, drive tooth shaft 16 to rotate clockwise and break away from molded plastic part. When rack fixed block 3 hits travel switch 24, signal is input to injection molding machine, and piston rod of hydraulic cylinder stops moving to complete thread demoulding. When closing mold, piston rod of hydraulic cylinder 9 drives rack 6 to move in opposite direction of arrow, rack 6 drives main gear 22 to rotate clockwise in opposite direction of arrow, and drives transmission gear 11 on transmission shaft 13 to rotate in clockwise direction, drive tooth shaft gear 14 meshed with it to rotate counterclockwise, drive tooth shaft 16 to rotate counterclockwise and reset. When rack fixed block 3 hits travel switch 2, signal is input to injection molding machine, piston rod of hydraulic cylinder stops moving to complete tooth shaft 16 reset action.
Figure 3 Unthreading mechanism
In order to ensure smooth movement of mold parts, guide sleeve 4 is made of high-nickel brass self-lubricating material. Tapered ball bearings 12 and 21 are arranged at both ends of drive shaft 13, tapered roller bearing 12 is NSK32206 (d=φ30mm; D=φ62mm; B=20mm), and tapered ball bearing 21 is NSK30302 (d=φ15mm; D=φ42mm; B =13mm). Thread inclination angle, diameter D and pitch on tooth shaft guide tooth sleeve 17 are consistent with thread of molded plastic part. Material of rack is S45C, teeth are high-frequency quenched at 45~55HRC, and one end of rack 6 is fixed. In order to prevent gears from being skewed when meshing, a deep groove ball bearing 23 is installed on opposite side of main gear 22, and choice is 6004 (d=φ20mm ; D=φ42mm; B=12mm). Tooth shaft 16 and transmission shaft 13 are made of pre-hardened steel, and heat treatment hardness is 37~43HRC. Material of gear is S45C, heat treatment hardness is 12HRC, and surface is treated with Fe3O4 protective film. Modulus determines tooth thickness of gear, and number of teeth determines size of gear. After comprehensive consideration: modulus m=2mm of main gear 22, number of teeth z=17; modulus m=2mm of rack 6; modulus m of transmission gear 11 =1.5mm, number of teeth z=84; modulus m=1.5mm of tooth shaft gear 14, number of teeth z=22.
Thread pitch P=4.02mm after shrinkage rate is added to molded plastic part, length of thread is L=14.06mm, thread core is number of turns that tooth shaft needs to rotate N=L/P+N (SNS is safety factor, in order to ensure complete rotation The margin reserved for the screw thread is generally taken as 0.25~1), then: N=3.5 (turns), for the sake of safety, to ensure that thread is completely unscrewed for smooth demoulding, tooth shaft 16 should be rotated for more than 3.5 turns. Tooth shaft gear 14 modulus m=1.5, number of teeth z=22, D=φ33mm; transmission gear 11 modulus m=1.5mm, number of teeth z=84, D=φ126mm. It can be seen from D transmission/D tooth=126/33=3.8 that transmission gear rotates once, and tooth shaft gear 14 can rotate 3.8 times>3.5, and tooth shaft 16 can be completely rotated out of molded plastic part and demoulded smoothly. Modulus of the main gear 22 is m=2mm, number of teeth z=17, D=φ34mm, and length L=34×3.14=106.76mm to be moved by rack meshed with it every time it rotates one circle. From perspective of safety, stroke of rack is designed to be 115mm, bore of hydraulic cylinder used is D=φ63mm, stroke is 140mm, stroke switch 2 is respectively set on rack fixing block 3 and bracket 5 to control starting position of rack. Gear and corresponding shaft are connected by a key, and starting position of thread on tooth shaft is consistent with direction of key groove.
4 Mold working process
Mold structure is shown in Figure 4. Working process is as follows: molten plastic flows into flow channel through nozzle of injection molding machine and enters cavity, mold is opened fixed template 2 and slider pressure plate 3 are opened first, inclined guide pillars drive the two Huff sliders to complete core pulling. Fixed-distance drawing plate 4 is fixed on fixed mold plate 2 and movable mold plate 12. After I part is separated by 180mm, it is opened at parting surface II. Fixed-distance drawing plate 4 drives movable mold late 12 together. After moving for 30 mm, under action of limit screw 6, stop moving forward to complete demoulding action, molded plastic part will automatically fall, and mold opening action is completed. When closing mold, movable mold plate 12 returns first under action of return spring 10, that is, mold closes at parting surface II, and continues to close mold. Inclined guide column enters slider to drive slider forward to complete mold closing. After mold is closed, piston rod of hydraulic cylinder drives thread removal mechanism to complete reset of tooth shaft, forming a closed cavity, and mold closing is completed, ready for next injection molding.
Figure 4 Die structure
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