In ordinary runner injection molds, materials in gate and runner are all formed aggregates, which are called head or aggregate in production practice. After injection is completed, condensate must be separated from molded plastic part. The key to separation of condensate is separation of gate condensate from plastic part, followed by separation of runner condensate from runner. There are two types of gates in commonly used ordinary runner injection molds: one is side gate with a relatively large cross-sectional area; the other is a point gate with a relatively small cross-sectional area where gate and molded plastic part are combined. Common side gates include rectangular, fan-shaped, etc., and point gates include vertical point gates, latent gates, etc. Commonly used in injection molds are rectangular side gates and vertical point gates, and corresponding mold structures are two-plate molds or three-plate molds. In two-plate mold, demoulding method of side gate condensate is that after mold is opened, movable mold retreats. Through pulling of core and pull rod, plastic part and runner condensate are pulled out of mold cavity and remain in movable mold. Movable mold continues to retreat, core is separated from plastic part. After separation, condensate is combined with plastic part, and finally condensate is manually cut off from plastic part. Vertical point gate of three-plate mold is opened three times. When it is opened for the first time, condensate and plastic part break at point gate position, plastic part and condensate are separated. When it is opened for the second time, stripper plate will release condensate from pull rod on fixed platen, allowing condensate to fall off automatically from mold; when it is opened for the third time, plastic part is separated from core. Separation of above two types of aggregates and plastic parts cannot be separated from manual assistance, and automated production of plastic parts cannot be fully realized. To sum up, existing technology for detaching injection mold aggregate has problems of relying on manual assistance, requiring a lot of labor, and having low efficiency. This problem needs to be solved urgently in actual production. Based on characteristics of engine hook attachment, an automatic decondensation mechanism has been designed.
Hook attachment is shown in Figure 1. Difficulties in molding: ① Outer dimensions of plastic part are large, with a total length of 291mm, a maximum height of 43mm, and a width of 62mm. Multi-point pouring is required to ensure molding quality of plastic part; ② There are multiple side holes (blind holes) on the side of plastic part, and they are located on appearance surface. It is necessary to use a side core pulling mechanism for core pulling and demoulding; ③ The lower the overall manufacturing cost of mold, the better. It is necessary to optimize design plan when designing mold.

 

For engine hook accessories, point gate automatic injection molding is required. Generally, a three-plate mold base is used. There are three types of three-plate mold bases: simplified type, standard type, and additional type. Basic structure of the three types is same, but simplified type is reduced. There is a pair of guide posts and bushings on cavity side, while additional type adds a push plate and a backing plate on movable mold side from number of mold mold plates. Typical simplified three-plate mold structure is shown in Figure 2. Compared with two-plate mold, it can achieve automated production of molded plastic parts and runner aggregate separation, but its manufacturing cost and manufacturing difficulty have increased, mainly reflected in following aspects .

 

Figure 2 Simplified three-plate mold structure
1. Gate sleeve 2. Cavity plate 3. Slider 4. Return rod 5. Return spring 6. Push rod fixed plate 7. Push plate 8. Limiting nail 9. Movable mold seat plate 10. Pad 11. Limit Position column 12. Push rod 13. Insert lock 14. Moving mold plate 15. Guide sleeve 16. Guide column 17. Fixed distance tie rod 18. Cavity plate fixed plate 19. Fixed mold base plate
(1) Mold structure is more complex, number of templates increases, opening and closing parting surfaces of mold plates increase, additional template opening and closing control mechanisms need to be added.
(2) In order to ensure accuracy of reset of each mold plate after opening and closing, guide elements of mold are increased.
(3) Runner of pouring system becomes longer, special gate flanges and runner condensation control parts need to be designed separately.
(4) In order to ensure repeated positioning accuracy of molded parts, it is necessary to add repeated positioning components, such as precision positioning blocks, etc. Based on actual production, it was found that compared with two-plate mold, three-plate mold mainly adds function of automatic de-runner condensation. Mold structure needs to add a runner plate to realize this function, causing mold structure to become more complex and relatively more difficult to manufacture, resulting in increased mold manufacturing costs. Therefore, whether two-plate mold structure can be used to realize function of automatically separating runner condensate and plastic part that three-plate mold has, satisfy point gate pouring function, and ensure a good appearance of pouring point on molded plastic part is an area that needs to be improved in structural design of injection mold.
Combined with actual production needs of point gate pouring method for molded plastic parts automatic separation of runner condensate and plastic parts, structure of two-plate mold was improved, involving point gate automatic decondensation mechanism of two-plate mold. Specifically, a three-plate mold automatic decondensation mechanism is used in two-plate mold structure to realize automatic separation of gate condensate and plastic part, as shown in Figure 3. Horizontal runner of aggregate in mechanism is opened on runner plate 1, and vertical runner is opened on cavity plate 6, which is connected to cavity through point gate at its bottom end. Pulling block 2 of mechanism is installed in fixed mold plate 13. It is guided by pulling guide rod 3 and its downward stroke is limited. Pulling guide rod 3 slides with guide sleeve installed on pulling block 2.

 

Figure 3 Mechanism installation
1. Runner plate 2. Pulling block 3. Pulling guide rod 4. Sprue sleeve 5. Fixed mold base plate 6. Cavity plate 7. Guide pillar 8. Pad 9 Moving mold base plate 10. Core 11 .Moving mold plate 12. Oblique pin 13.Fixed mold plate

Decomposed structure of two-plate mold point gate automatic decondensation mechanism is shown in Figure 4. As can be seen from Figure 4, vertical runner part of aggregate 2 is opened in four holes corresponding to cavity plate 6. Pulling block 1 is designed to have a certain tapered shape, is installed in groove of cavity plate 6, moves up and down through guide sleeve on it and drawing guide rod 5 in sliding fit. Upper surface of pulling block 1 can push condensate 2 to move upward, forcing lower end of condensate to separate from molded plastic part in cavity. After separation, plastic part remains in mold cavity, while condensate can fall and be demoulded.

 

Figure 4 Decomposition of organizational components
1. Pulling block 2. Aggregate 3. Gate sleeve 4. Runner plate 5. Pulling guide 6. Cavity plate

Improved two-plate mold realizes point gate function of three-plate mold, combined with application of improved automatic stripping mechanism. Its working principle is shown in Figure 5. Mold is opened in two parts. When it is opened for the first time, since plastic part in cavity is connected to aggregate through point gate, aggregate can be pulled out from runner plate, as shown in Figure 5(a). After movable mold continues to retreat for a certain distance, material block is pulled by material guide rod. With the help of mold opening power, aggregate and plastic part are forced to break at point gate, aggregate is pulled out of cavity plate by pulling block and automatically falls off, as shown in Figure 5(b). Movable mold continues to retreat, and second parting is opened. Plastic part comes out of mold cavity and remains on core. Finally, it is pushed out by ejection mechanism to realize automatic demoulding.