Structure of refrigerator drawer is complex. As shown in Figure 1, upper part of front side is an arc-shaped surface, and there is a buckle in the center of front side. Buckle is used to facilitate pulling drawer out of refrigerator. There is a buckle on the left and right sides of drawer, and there is a skirt along mouth of refrigerator drawer body. Width of skirt is about 8mm. Outer dimensions of refrigerator drawer are 446mm*432mm*150mm, average wall thickness is 3mm, material is PS, volume is large, weight is about 1.2kg, and customer requires an injection cycle of about 70s. For buckle on the front side of plastic part, a fixed mold slider structure is used for demolding. For buckle on inner surface of front side of plastic part, an oblique push demolding mechanism is used to demold along buckle direction.

 

In order to demould buckle position on the front side of refrigerator drawer, it is necessary to design a slider structure driven by an inclined guide column to demould along inclination direction of buckle position, as shown in Figure 2. Inclined surface of slide groove is consistent with inclination direction of buckle position. Since contour of slider is not allowed to appear on the front side of refrigerator drawer, the entire front side glue surface is formed by slider. In order to ensure strength of slider, slider adopts an integral structure. In order to facilitate processing of slider, slide groove is set on slider seat.

 

There is a buckle position on each side of refrigerator drawer, and both adopt an inclined guide column + slider structure for demoulding, as shown in Figure 3. In order to facilitate processing, slider and slider seat are processed separately, and slider is fixed to slider seat with screws. Due to small size of slider, it is driven by 1 inclined guide column.

 

Handle of refrigerator drawer is on inner surface, and corresponding structure is a buckle position. In order to demould it, an oblique push demoulding mechanism is adopted, as shown in Figure 4. Oblique push rod and oblique push seat are connected by a T-shaped groove. Due to large size of inclined push block, in order to make movement of inclined push block balanced, inclined push block is supported by two inclined push rods. In order to make inclined push mechanism perform demoulding movement along inclination direction of buckle position, T-shaped groove of inclined push seat is designed with an inclined surface along inclination direction of buckle position. When push plate pushes inclined push seat to move, inclined push mechanism simultaneously moves obliquely downward along inclined surface of inclined push seat to demould buckle position on inner surface of handle.

 

Refrigerator drawer is a transparent plastic part. In order to prevent existence of push rod marks on plastic part, a push plate is used to push it out during demoulding, and skirt of refrigerator drawer is formed on push plate. In order to simplify mold structure, push plate adopts an inlay structure and is embedded in moving mold plate. In order to prevent push plate from getting stuck when it is pushed out, side of push plate is designed to be an inclined surface, which is conducive to push plate to maintain balance when it is pushed out. In order to avoid slider and inclined push mechanism set on the front side of plastic part to be molded, push plate adopts a U-shaped structure, that is, a three-sided ring structure and an open structure on one side, as shown in Figure 5. Push plate is made of three pieces of material, which can ensure its strength and rigidity. A push rod is set at the bottom of push plate, and push rod pushes push plate to perform demolding. In order to strengthen correlation of three push plates, push plate in the middle and push plates on both sides share a flat push rod. There are two screws on flat push rod, one fixed on middle push plate and other fixed on push plate next to it.

 

Refrigerator drawer is a box structure, which occupies a large space. When mold is opened, a vacuum is easily formed between plastic part and cavity, causing plastic part to crack. In order to overcome vacuum phenomenon, a blowing device is set on both moving and fixed mold inserts, as shown in Figure 6. When mold is opened, blower blows air into cavity, so that outside air quickly enters cavity to prevent formation of a vacuum. Volume of drawer cavity is large. In order to quickly fill the entire cavity with air when drawer is demolded, two sets of blowing devices are designed on moving and fixed mold inserts. Working order of blowing devices on movable and fixed mold inserts is inconsistent. When movable and fixed molds of mold are opened, fixed mold blowing device starts blowing while movable mold blowing device remains stationary. When mold is completely separated, ejection device of movable mold starts to move, and movable mold blowing device also starts blowing.

 

Refrigerator drawer is large in size. In order to eliminate defects of ordinary runner casting on molded plastic parts, hot runner is used for casting. According to design experience of similar molds, selection of one hot runner for casting can meet injection requirements of cavity. Hot runner adopts Mold-Master needle valve hot nozzle. Casting position is shown in Figure 7 (a). Casting position has been analyzed and verified by MoldFlow software. In order to eliminate weld marks on the surface of molded plastic parts and facilitate removal of gate condensate, surface where gate is located is designed to be concave, as shown in Figure 7 (b).

 

Due to large volume of cavity, in order to strengthen strength of fixed mold plate, fixed mold plate adopts an integral structure, and moving mold adopts an insert structure for easy processing, as shown in Figure 8. In addition to using guide pins and guide sleeves for guidance, mold also has a pit for positioning on fixed mold plate and a boss for positioning on moving mold plate. Sides of pit and boss are 10° inclined surfaces.

 

During molding, in order to cool and shape plastic parts as quickly as possible and reduce their deformation, cooling system should be designed according to plastic part structure and mold structure. Main parts of mold, such as fixed mold plate, moving mold inserts and inclined push components, must be equipped with cooling water channels. Fixed mold plate adopts a straight-through water channel, water channels are set on the bottom and three sides of cavity plate. Each straight-through water channel is independent, as shown in Figure 9 (a). Movable mold insert also adopts a straight-through conformal cooling water channel, which is divided into two independent water channels, which can shorten length of cooling water channel and prevent cooling water from bringing heat from one area to another during circulation process, which is conducive to maintaining uniform mold temperature, as shown in Figure 9 (b). There are two inclined push rods on inclined push assembly, and water channel is introduced from each inclined push rod. A zigzag straight-through water channel is set in inclined push block, as shown in Figure 9 (c), which is conducive to uniform mold temperature.

 

Refrigerator drawer structure is a two-plate mold, with a needle valve hot runner pipe for pouring. For buckle position on the front side, slider is selected for demolding, and buckle position on inner surface of front side is selected for demolding. Ejection mechanism selects ejection plate for ejection. Mold structure is shown in Figure 10. Mold working process: After injection is completed, ejector of injection molding machine pulls movable mold of mold back, mold is separated between movable mold plate 6 and fixed mold plate 15. At the same time, inclined guide column 13 drives slider 11, and inclined guide column 27 drives side slider 26 to do demolding movement. When movable mold plate 6 is completely separated from fixed mold plate 15, ejector rod of injection molding machine drives ejection mechanism of mold (push plate 2, push rod fixing plate 3, inclined push seat 4, inclined push rod 8, inclined push block 12, push rod 23 and push plate 21) to move. After plastic parts are taken out, ejector rod of injection molding machine drives movable mold of mold to start resetting. Resetting process is opposite to mold opening process. When mold is completely reset, next cycle of injection can be started.

 

1. Moving mold base plate 2. Push plate 3. Push rod fixing plate 4. Oblique push seat 5. Cushion block 6. Moving mold plate 7. Guide block 8. Oblique push rod 9. Slider seat 10. Wear plate 11. Slider 12. Oblique push block 13. Oblique guide column 14. Fixed mold base plate 15. Fixed mold plate 16. Hot runner pipe 17. Positioning ring 18. Fixed mold blowing mechanism 19. Moving mold insert 20. Moving mold blowing mechanism 21. Push plate 22. Positioning bump 23. Push rod 24. Push plate guide column 25. Side slider seat 26. Side slider 27. Oblique guide column 28. Stopper
Figure 10 Mold structure

Fixed mold plate material is 2711 pre-hardened steel, movable mold insert material is 2311, slider, inclined push assembly and other molding parts are made of 2711 pre-hardened steel, of which nitriding depth of inclined push assembly is about 0.15mm, heat treatment hardness is 50~52HRC, mold frame material is medium carbon steel 1050, mold frame standard parts (such as guide pins and guide sleeves, etc.) are made of HASC0 and other specifications.
Since refrigerator drawers have high requirements for surface quality, in order to prevent obvious steps between different parts of moving mold (such as core, inclined push assembly and slider), some parts are assembled and then final finishing is performed. In view of fact that assembly position of moving mold push plate is easy to overcut, arc transitions should be added to corners to avoid sudden changes in processing direction; for side of push plate contacting glue position, processing allowance should be appropriately increased to protect it to prevent overcutting. Moving mold push plates should be electro-etched together after assembly to prevent obvious mold lines between push plates. When electro-etching moving mold, it should be electro-etched one by one in one direction. In order to ensure matching accuracy between different moving mold inserts, wire cutting is performed after NC processing, and auxiliary reference provided by NC processing is used for alignment during wire cutting.