Door body of a household refrigerator is mainly composed of a door shell, an end cover, a column, a door liner, a seal, an insulation layer and a stop mechanism. Built-in door handle is a plastic part that integrates column and door handle, is connected with door shell, end cover and inner tank. For double-door refrigerators, door handles of refrigerator compartment and greenhouse are required to be beautiful and harmonious. In actual injection molding process, usually 1 mold and 2 pieces are produced as a set, which can improve production efficiency and reduce manufacturing costs. At the same time, in order to pursue a good appearance, formed plastic parts are required to have neat seams and flat joint surfaces. Design points and working principle of injection mold for streamlined refrigerator door handle 1-mold with different cavities are described.

 

Figure 1 shows door handle model of a refrigerator cold room and a changing room. Material is acrylonitrile-butadiene-styrene copolymer (ABS), which has high strength, good toughness, easy molding, dyeing and electroplating, with a shrinkage rate of 0.5% and a solid density of 1.05g/cm3. Outer dimension of refrigerator door handle is 965.6mm*80.3mm*72.0mm, the thickest part is 3.0mm, the thinnest part is 0.8mm, and average wall thickness is 1.79mm; outer dimension of variable greenhouse door handle is 777.4mm*80.3mm*72.0 mm, the thickest part is 2.3mm, the thinnest part is 0.8mm, and average wall thickness is 1.78mm. Compared with refrigerator door handle, its width and height are same, length is slightly shorter. Two door handles have uneven wall thickness and are more complicated in shape with slightly shorter curves. Two door handles have uneven wall thickness and complex shapes, including curved surfaces, grooves, reinforcing ribs, display windows, undercuts and other structures, require high surface quality and dimensional accuracy. Refrigerator door handle S1 plane and variable greenhouse door handle S2 plane are matched with their respective door shells. They are required to be smooth and flat. Process grooves with a width of 1mm and a depth of 0.5mm shown in enlarged areas of T1 and T5 are designed respectively; there is also a rectangular window for display screen on the side of T1; depth of undercut at T2 is 11mm; undercut is at T3, 5 evenly distributed side by side along length of plastic part, and need to be demolded sideways; 4 L-shaped undercuts are distributed at T4 , their demolding directions are all pointing outwards, and demolding stroke needs to be greater than 14.3mm; door handle of variable room has undercut structures at T6 and T7, with widths of 13 and 15mm respectively.
Two door handles are long thin-walled parts, appearance requires that there are no defects such as gate marks, weld marks, bubbles, etc., to ensure smooth surface, no scratches, flashing and other quality problems. Therefore, key technology of mold design has three points: ① rationally design gating system to control balance of melt flow; ② rationally design lateral demoulding mechanism; ③ rationally design cooling system to control mold temperature.

 

Mold cavity layout uses 2 cavities, single parting surface, structure is shown in Figure 2. Its design features: ①Considering that refrigerator door handle requires high appearance and dimensional accuracy, injection process is long, it adopts sequential valve hot nozzle + ordinary runner + side gate for pouring; ②Undercuts at T1~T7 are respectively adopted with 4 sets of inclined guide posts + slider outer core pulling and 5 sets of inclined push components inner core pulling and demolding, of which S1 and S2 planes of plastic part to be molded are respectively used with large sliders 25, 29 molding; plastic parts and aggregates are finally pushed out simultaneously with a round push rod, push rod + push block; ③Mold temperature control adopts water cooling, three-dimensional circulation type and partition type cooling water circuit are respectively arranged on movable and fixed mold inserts, sliding block.

 

Two door handles are thin-walled and long-flow plastic parts, need to be poured at multiple points, each side gate is designed on non-appearance surface of plastic part to be molded, as shown in Figure 3. Process ratio between refrigerator door handle and variable greenhouse door handle is about 5:4. The one-way sequence valve is used to control pouring. Combining production experience and CAE mold flow analysis results, gates for forming 2 plastic parts are evenly distributed along length, with 5 and 4 gates respectively.

 

For forming large-scale or long-process thin-walled plastic parts, use of sequence valves to control filling sequence of plastic melt can effectively eliminate weld marks, or move them to parts where appearance of plastic parts is not high, produce high-quality plastic parts that meet requirements of use. Figure 4 shows timing needle valve hot runner system, which is mainly composed of a primary thermal nozzle, a hot runner plate, a secondary thermal nozzle, a cylinder, a solenoid valve, and a junction box. Movement of valve needle in secondary thermal nozzle is controlled by cylinder and timing controller to realize opening and closing of thermal nozzle. According to structural characteristics of two plastic parts, pouring sequence is determined to be one-way, followed by G1→G2→G3→G4→G5; combined with flow front in mold flow analysis results, opening and closing time of each gate is further determined.

 

(1) Inclined guide post + slider core pulling mechanism. In order to smoothly demold side hole cores of molded door handle in various directions, four groups of inclined guide posts + slider core pulling mechanisms in different directions as shown in Figure 5 are designed. Slider A mainly forms S1 plane of refrigerator door handle and oblique side surface where rectangular window of display screen is located. Width of slider A is slightly larger than total length of plastic part. It is driven by 3 oblique guide posts and 3 springs to realize core pulling, demoulding of side holes and process groove, core-pulling stroke is 55mm; Slider B is mainly formed into S2 plane of door handle of greenhouse and process groove at T5. Core-pulling distance is short but slider width is large, so demoulding is also driven by 3 inclined guide posts and 3 springs synchronously, core pulling stroke is 10mm; 4 L-shaped undercuts at T4 and 1 undercut at T6 are formed by slider C. Width of slider is small. One inclined guide post is used to complete demolding, core-pulling stroke is 20mm; undercut structures at T2 and T7 are jointly formed by slider D, slider D is also driven by an inclined guide post to complete demolding, and the core-pulling stroke is 20mm.

 

(2) Core pulling mechanism inside oblique push assembly. There are 5 evenly distributed undercuts at T3 of refrigerator door handle, which are demoulded along length of plastic part. Core pulling distance is 5mm, 5 sets of integral oblique push assembly core pulling mechanisms are designed. As shown in Figure 6, tilt angle of oblique push assembly is 5°, head of oblique push assembly is matched with inclined surface of slider A.

 

(3) Push-out mechanism is shown in Figure 7. Two plastic parts are large in length, thin in wall thickness, many reinforcing ribs, and large in demolding resistance. Round push rod and push block are designed to be combined to push out. Inside of door handle of variable greenhouse uses 12 round push rods of φ16mm, side wall is thin, 4 sets of direct push blocks are used to increase pushing out area; structure of refrigerator door handle is more complicated. Interior adopts 6 φ16mm round push rods, side walls and rectangular window edges are pushed out with 8 sets of different sizes of direct push blocks; runner aggregate is pushed out with a φ6mm round push rod.

 

Mold temperature affects molding quality and molding cycle of plastic parts. In order to balance temperature of mold cavity, cooling water channels are opened on fixed mold inserts, movable mold inserts and each slider that have more heat accumulation, as shown in Figure 8. Fixed mold inserts are designed with 8 groups of cooling water channels, each of which is about 15mm away from wall of cavity plate, of which 3 use combination of circulating water pipes + spacer water wells to obtain uniform cooling effect; movable mold inserts are designed with 6 groups of cooling water circuits, only one of which uses 3 spacer-type water wells, and the rest are all circulating water pipes; volume of 4 sliders is larger, and 1 circulating water pipe is designed respectively. Using above cooling system, temperature of all parts of mold is balanced and cooling speed is fast.

(1) Injection molding. Under action of injection molding machine, plastic melt enters first-level hot nozzle 14, then passes through heat-preserving runner in hot runner plate 12, enters second-level hot nozzle 8, flows into ordinary runner from G1 to G5 in chronological order, and then enters mold cavity from side gate to form. After melt fills cavity, the pressure is maintained and cooled to sufficient rigidity.
(2) Open mold and pull core. When mold is opened, sliding block of injection molding machine drives movable mold seat plate 1 to retreat, eight oblique guide posts 26, 28 drive four large movable mold blocks 25, 29 to pull core laterally.
(3) Launch. After mold is opened, push plate 2 pushes oblique push rod 5, round push rod 19 and push block 18 to push out molded plastic part, pushing distance is 60mm.
(4) Reset. After plastic parts are taken out, sliding block of injection molding machine drives push plate 2, push plate 2 drives inclined push rod 5, round push rod 19 and push block 18 to reset.
(5) Clamping. When mold is closed, sliding block of injection molding machine drives movable mold seat plate 1 forward, 8 inclined guide posts 26, 28 drive 4 large movable mold sliders 25, 29 to reset, movable mold and fixed mold are closed, waiting for next injection molding.
Sequential valve heating and common composite runner system are designed to ensure appearance quality of a long process plastic part formed by a different cavity; reasonable layout of cavity, design of multi-directional core-pulling mechanisms such as sliders and oblique push components, solves problem of difficulty in multi-directional clamping and demolding; temperature of cavity is controlled by adopting combined cooling water circuit system of circulation type + spacer type; assembly effect of mold mechanisms is good, operation is stable, safe and reliable after being put into production.