Coffee machine control panel is in an arc shape, as shown in Figure 1. There are two countersunk holes at one end of plastic part, and a slider structure is used for demolding. There are three buckles on inner surface of the other end of plastic part, and buckle opening direction faces outward. There are four buckles on inner surface of middle position of plastic part. There are two stud holes on platform on inner surface of plastic part. In order to prevent shrinkage during molding process of plastic part, platform at the root of stud hole is hollowed out to form buckles. These buckles are demolded by an oblique push structure. Since outer surface of plastic part is covered with a layer of PC film with a thickness of 0.18mm, a pair of in-mold film injection molds need to be developed to mold this plastic part.

 

Since mold is an in-mold film injection mold, a layer of PC film is placed on cavity surface before injection. To ensure smooth positioning of PC film after it is placed in cavity, parting surface of in-mold film injection mold is designed as follows: curve of mouth of plastic part is used to create a stretching surface with a height of about 6mm along mold opening direction, then a strip surface is created along curve of edge of stretching surface, as shown in Figure 2. In order to enhance mold closing accuracy of fixed mold and movable mold, a positioning is created at each of four corners of parting surface, and there are three protrusions at one end of plastic part that are inclined parting surfaces.

 

During injection, a layer of film will be placed on the surface of fixed mold cavity, so material cannot be fed from outer surface and side of plastic part to be molded, but must be fed from inner surface, and runner and gate need to be opened at one end of plastic part to be molded. If a two-plate mold is selected, positioning ring and gate sleeve will be located on one side of mold instead of in the center of mold. Since mold needs to be installed on one side of injection molding machine installation panel during injection production in order to align gate sleeve with injection molding machine nozzle, a larger injection molding machine is required to produce plastic part. In order to match appropriate injection molding machine, a three-plate mold is selected, mold positioning ring and gate sleeve are set in the center of mold, and a runner is opened on stripper plate to introduce melt into one end of cavity for injection. A smaller injection molding machine can be selected for production.

A circular runner (φ6mm) is opened on parting surface, and point gate is aligned with circular runner on parting surface. A bull horn gate is selected to feed from inner surface of plastic part to be molded. Structure of bull horn gate is shown in Figure 3 (a). Since height of side wall at one end of plastic part is 19mm, it is not suitable to set a bull horn gate. Material must be fed from the end without side wall on plastic part.

 

In order to avoid inclined push rod in the middle of plastic part, a gate must be set next to inclined push rod. Two gates are set to balance pouring. Their positions are shown in Figure 3 (b).

According to structure of plastic part, both fixed mold and movable mold are in the form of inserts, which is convenient for manufacturing and processing, as shown in Figure 4. There is a positioning at each of four corners of movable and fixed mold inserts to enhance mold clamping accuracy of movable and fixed molds. An oblique push assembly position is set at one end of movable and fixed mold inserts, a slide groove of slider is opened at the other end, and slide groove is designed to be stepped to prevent affecting strength of movable mold insert.

 

There are two φ5mm countersunk holes on the side of plastic part, and a slider is designed for each countersunk hole. Slider structure is driven by an oblique guide column to demould. When designing slider, an integral structure is adopted, that is, working part of slider and slider seat are an integral whole. It is necessary to open a slide groove on movable mold insert, and part extending from slider seat is designed to be stepped, which does not affect strength of movable mold insert and ensures strength of slider. Size close to slider seat is large, and size close to plastic part is small, as shown in Figure 5.

 

A film pressing slider is designed on each side of plastic part, as shown in Figure 6 (a). Its function is to press film after it is placed in cavity to prevent it from falling during mold closing process.
Structure of film pressing slider is different from that of ordinary slider. It consists of a slider and an inclined wedge, in which slider is set in fixed mold and inclined wedge is set in movable mold, as shown in Figure 6 (b). After film is placed in cavity, operator manually moves film pressing slider toward center of mold to press film. When movable and fixed molds are closed, film is pressed, and inclined wedge drives film pressing slider to move to both sides, so that film pressing slider is away from molded plastic part, so that ejection mechanism can smoothly push plastic part out when mold is opened.

 

Plastic part has 7 buckle positions and 4 hole positions, and an oblique push structure is required for demolding. It consists of an oblique push rod, an oblique push seat, a guide block and a screw, in which oblique push rod and oblique push seat are connected by a T-slot. Since buckle size on plastic part is small, size of inclined push rod is also small. In order to strengthen strength of inclined push rod, inclined push part and inclined push rod are designed as an integral structure, as shown in Figure 7. Slider seat is fixed to push plate by screws at the bottom. When inclined push rod needs to be repaired, it only needs to loosen screws from bottom to remove inclined push rod from movable mold, and demolding workload is small.

 

There are several reinforcing ribs on inner surface of plastic part. In order to push plastic part out smoothly, 11 φ8mm, 4 φ6mm, 10 φ5mm and 2 φ3mm push rods are set in movable mold, a total of 27 push rods, which are mainly set at reinforcing ribs of plastic part, as shown in Figure 8.

 

If coated plastic part shrinks during injection process, its outer surface is more obvious than outer surface of uncoated plastic part. A set of cooling system that can effectively overcome shrinkage must be designed according to mold structure and shape of plastic part, as shown in Figure 9. Since 27 push rods and 11 inclined push rods are set in the area of projection of plastic part on movable mold insert, there is not enough space to set up cooling water channels. A square straight water channel is set in the area outside projection; plastic part is arched, 4 straight water channels are set on fixed mold insert according to shape of plastic part. In order to make temperature of fixed mold insert uniform, 4 straight water channels are divided into 2 groups of independent water channels to shorten length of straight water channels and prevent cooling water from bringing heat from one area to another during circulation process. Diameter of straight water channel is φ10mm. A larger water channel diameter can accommodate more cooling water, which is conducive to maintaining uniform mold temperature.

 

Before closing mold, operator needs to place film in fixed mold cavity in advance. Specific process is to first separate two film pressing slides to both sides, put film steadily into cavity, then press film to prevent it from moving, and move film pressing slide closer to plastic part to be molded until film is pressed firmly. In order to prevent dust from entering cavity surface and keep mold cavity clean, try to perform injection in a dust-free workshop. If conditions of a dust-free workshop cannot be met, after every 4 to 5 injection moldings, filtered air must be blown to cavity surface with an air pipe, or cavity must be wiped with a cloth with alcohol to prevent dust in the air from being adsorbed on cavity surface, causing foreign matter on the surface of plastic part and affecting appearance quality.

In order to enhance stability of mold, 8 φ40mm support columns are set, as shown in A1~A8 in Figure 10. 4 pairs of push plate guide pillars and guide sleeves are designed to keep push plate and push rod fixed plate moving smoothly, as shown in B1~B4 in Figure 10. After ejection mechanism is reset, a chip space must be maintained between push plate and movable mold base plate to prevent dust in the air from entering between the two, causing ejection mechanism to be unable to be completely reset. A chip column is set between push plate and movable mold base plate, as shown in C1~C8 in Figure 10.

 

Three ejector holes are set on movable mold base plate, as shown in D1~D3 in Figure 10, so that ejector of injection molding machine can be evenly pushed on push plate. Designed push plate reset switch can accurately reset ejector, and a prying mold groove is set between adjacent templates to prevent damage to mold when disassembling mold.

A three-plate mold is used, melt enters flow channel opened on parting surface from point gate, then bullhorn gate is used to feed from inner surface of plastic part to be molded. Mold cross-section structure is shown in Figure 11, and actual plastic part produced is shown in Figure 12.

 

1. Moving mold base plate 2. Push plate 3. Push rod fixing plate 4. Pad 5. Moving mold plate 6. Push rod 7. Nylon pull hook 8. Moving mold insert 9. Fixed mold insert 10. Fixed mold plate 11. Stripper plate 12. Fixed mold base plate 13. Gate sleeve 14. Guide sleeve 15. Oblique push rod 16. Horn-shaped gate insert 17. Pull rod 18. Guide sleeve 19. Support column 20. Oblique push seat 21. Chip column 22. Push plate guide column 23. Reset rod 24. Limit screw 25- Film pressing slider
Figure 11 Mold structure

 

Figure 12 Actual plastic part