Multiple different plastic parts formed in same pair of molds are called family molds. Advantages are that it can improve utilization rate of molds, reduce number of molds, and reduce mold manufacturing costs. Points to pay attention to when designing family molds: ①When using one pair of molds for injection molding of plastic parts with large differences in volume, if a common runner gating system is used, a runner balance design needs to be carried out; ②Design of demolding mechanism of molded plastic parts should be set by the overall method to save space of mold base; ③Cavity plate and core should be closed and positioned to ensure accuracy of mold closed positioning. This article introduces same mold injection molding of a complete set of plastic parts for a self-generating fitness bicycle fork seat, which can provide a reference for molding of similar complete sets of plastic parts.

Complete set of plastic parts for fork buckle seat consists of four plastic parts: base, turning cover, bomb cover, and limit button. According to customer's request, a family mold needs to be used for one-time injection molding. Plastic parts are made of ABS plastic, and shrinkage rate is 1.052~1.058, as shown in Figure 1.

 

Four plastic parts are injection molded with one pair of molds, and layout of cavity of each plastic part to be molded in mold is shown in Figure 2(a). Layout of parting surface should use plane parting as much as possible to facilitate processing of mold parts. When it comes to cavity of each plastic part to be formed, a parting surface in a special position should be set according to sealing requirements of cavity. Under this premise, design of final parting surface is shown in Figure 2(b). Combined with needs of surrounding sealing of specific molded plastic parts, 4 partial profiles are set, which are I~IV. For each cavity, after designing flow channel balance, selected flow channels are R1~R4 respectively, and diameters of each flow channel are R1=φ8mm, R2=φ4mm, R3=φ6.5mm, R4=φ3.5mm. Cavity plate and core must be closed and positioned with 4 tabs at 4 corners to ensure accuracy of mold closing. Cavity plate material is NAK80, and core material is NAK55. NAK80 die steel has following characteristics: no need for heat treatment, good polishability, good machinability and corrosion resistance; pure steel after vacuum degassing and refining treatment; spheroidizing annealing softening treatment, good cutting performance; excellent wear resistance. NAK55 is pre-hardened steel, pre-hardened to 370~400HB, suitable for uneven core processing.

 

3 of 4 molded plastic parts need to be equipped with a special demoulding mechanism to ensure their automatic injection production. These mechanisms include fixed mold oblique push-out mechanism, movable mold oblique push-out mechanism, fixed mold core-pulling slider mechanism, push rod mechanism.

(1) There are 7 special demoulding mechanisms to be set up for molding of base, as shown in Figure 3, which are 2 anti-buckle oblique push mechanisms on fixed mold side, A-segment ring groove slider mechanism, B segment ring groove slider mechanism (side hole slider), C segment ring groove slider mechanism, 2 side counterbore oblique push mechanism, 1 inverted buckle oblique push mechanism. Combined with plastic part shown in Figure 1(a), because there are 3 reverse buckles on inner wall of upper surface of seat, it is necessary to set up a reverse buckled oblique pushing mechanism to form it and side-pull it for demoulding. Since outer wall of base is provided with 3 sections of outer ring grooves, each section of ring groove needs to be equipped with corresponding side core-pulling sliders for forming and side core-pulling demoulding, and 3 ring-groove segmented forming sliders. B-section ring-groove slider integrates forming of 2 side holes and side core-pulling and demoulding. There are 2 side countersunk holes and 1 inverted button on lower wall of base, 2 side counterbore oblique push mechanisms and 1 inverted button tilt mechanism are required to be used for molding of these 3 features and side core-pulling demoulding. For inclined push mechanism and slider molding, material is SKD11, and heat treatment hardness is 48~52HRC.

 

(2) Seven oblique pushing mechanisms should be set up for molding and demoulding for turning cover molding, distribution of mechanisms is shown in Figure 4. Revolving cover has 2 side countersunk holes, 2 inverted buttons, 2 pin holes, and 1 rib inverted button. These features need to be equipped with a corresponding oblique push mechanism for forming and demoulding.
(3) As shown in Figure 1(d), limit button has a side surface with high requirements, and no parting marks can appear on side surface. For molding of this appearance surface, molding is performed by means of C-section molding of outer ring groove of seat, and molding setting is shown in FIG. 5 .

 

(4) There is no need to set up a demoulding mechanism for bomb cover, it is directly formed by movable and fixed molds, and is pushed out by push rod for demoulding.
(5) Installation of each mechanism component in mold is shown in Figure 6. Among 7 special demoulding mechanisms of base, oblique pushing mechanism of reversed side is pushing mechanism of fixed mold side push plate. Upper cover plate 12 and upper push plate 11 are connected by screws to form a combined push plate, which is driven by push spring 6. Upper mold plate 10 is fixed on fixed mold plate for fixing push-out spring 6, so that push-out spring 6 can drive combined push plate to push down, upper reset rod 13 and pull rod 5 are both installed in upper cover plate 12. Upper reset rod 13 is used for moving mold plate 22 to push combined push plate back to its position against pulling force of push spring 6, lower end of pull rod 5 is provided with a T-shaped rod head, through which T-shaped rod head is movably connected with T-shaped groove at upper end of oblique push block 4, which is used to drive oblique push block 4 to push molded plastic part out of cavity, and at the same time, side core pulling and demoulding of reverse buckle side of molded plastic part is performed.

 

1. C-section ring groove slider 2, T-shaped guide block 3, curved pin 4, reverse buckle side oblique push block 5, pull rod 6, push-out spring 7, B-section ring groove slider 8, curved pin 9, curved pin 10 , Upper template 11, upper push block 12, upper cover plate 13, upper reset rod 14, A-segment ring groove slider 15, side counterbore inclined push block 16, inverted buckle inclined push block 17, side counterbore side pusher 18 , upside down inclined push block 19, rib upside down inclined push block 20, Side counterbore inclined push mechanism 21. Pin hole inclined push block 22. Moving mold plate 23. Pin hole inclined push block 24. Inverted buckle inclined push block 25. Side counterbore inclined push mechanism 26. Core
Drive of ring groove slider mechanism of A section, ring groove slider mechanism of B section (side hole slider), and ring groove slider mechanism of C section all use curved pins, which are curved pin 9, curved pin 8, and curved pin 3 respectively. Structure of 3 bending pins is same, upper end is installed on fixed die base plate, and lower end is provided with a T-shaped guide block 2 to drive C-section ring groove slider 1 to perform side core pulling. These 3 sliders are all fixed die first. Core-pulling slider mechanism must complete core-pulling action before mold is opened.
Two side countersink oblique push mechanisms and one undercut oblique push mechanism set on lower wall of base are installed on one side of mold moving die, corresponding to parts 15 to 17 in FIG. 6 .
Seven oblique push mechanisms provided for rotating cover are also installed on one side of movable mold, corresponding to parts 18 to 24 in FIG. 6 . In mold, parts 15~24 all use universal inclined push mechanism to ensure that inclined push mechanism has enough design space, and at the same time, it can effectively reduce size of mold base, improve space utilization rate of mold base, and reduce cost of mold manufacturing.

Mold structure is shown in Figure 7. Its working principle: mold is opened in two times, first mold opening is at I position, and when I parting surface is opened, three curved pins (taking curved pins 3 as an example) drive corresponding sliders to complete first core pulling and demoulding on fixed mold side. When mold is opened for second time at II position, when II parting surface is opened, push-out spring 6 pushes reverse side oblique push block 4 to push down, so that molded plastic part comes out of cavity plate 39 . After II parting surface is opened, and movable mold continues to run for a certain distance, ejector rod of injection molding machine presses against push plate 32 to drive 10 oblique push blocks and push rods out, so as to realize complete ejection and demoulding of 4 molded plastic parts.

 

1. C-section ring groove slider 3, curved pin 4, reversed-edge oblique push block 6, push-out spring 10, upper mold plate 11, upper push block 13, upper reset rod 18, inverted oblique push block 22, moving mold plate 26, core 27, guide post 28, fixed mold plate guide sleeve 29, movable mold plate guide sleeve 30, inclined push block seat 31, movable mold base plate 32, push plate 33, push rod fixing plate 34, spacer 35, long screw 36 , reset rod 37, fixed mold plate 38, fixed mold seat plate 39, cavity plate