Figure 1 shows an automobile air-conditioning damper. Plastic part is a plate-shaped structure with hinges and bevel gears. The overall size is 74 mm*45 mm*13 mm, and material is ABS (acrylonitrile-butadiene-styrene). After molding, exterior of plastic part is covered with TPU (thermoplastic polyurethane elastomer), ABS has medium fluidity, and molding shrinkage rate is 0.5%.

 

Figure 1 Plastic structure
Plane wall thickness of plastic part is 1.5 mm, and maximum wall thickness is 3 mm. Size, precision, surface quality and structure meet injection molding requirements. Difficulties in mold design for molding plastic parts: ① Customer requires mold to adopt side gates, so that molded plastic parts and runner aggregate can be automatically separated; ② Core pulling of protruding shaft at hinge and through hole;

According to output and external dimensions of plastic parts, mold adopts a molding plan with a structure of 4 cavities. Parting surface design is shown in Figure 2. Plastic part is a plate-shaped structure with hinges and bevel gears. The largest contour of plastic part is taken as parting surface, and parting surface is a stepped surface. In order to keep molded plastic part on the side of movable mold after mold is opened, cavity is designed on the side of fixed mold, and core of small hole of molded plastic part is designed on the side of movable mold. Since hinge and bevel gear of plastic part hinder demoulding, two core-pulling mechanisms on the side of inclined guide column must be designed at hinge, and a core-pulling mechanism on the side of inclined push rod is designed at bevel gear because of small space.

 

Figure 2 parting surface design

In order to facilitate manufacture of mold parts, movable and fixed mold inserts are divided into 2 pieces each, and each piece is formed into 2 plastic parts. In order to ensure accuracy of mold clamping, a tiger mouth structure for positioning is designed on movable and fixed mold inserts.
Fixed mold insert is shown in Figure 3. It is cavity plate of mold and adopts an integral structure. Considering exhaust requirements when plastic part is formed, an exhaust groove is designed on parting surface around fixed mold insert. Exhaust groove is composed of a primary exhaust groove and a secondary exhaust groove. Close to cavity is primary exhaust groove with a depth of 0.01 mm, away from cavity is secondary exhaust groove with a depth of 0.2 mm and a width of 6 mm.

 

Figure 3 fixed mold insert
Moving mold insert is shown in Figure 4. Since protrusion height of plastic part is small, there is no need for exhaust, and moving mold insert also adopts an integral structure. Considering that formed plastic part remains on the side of movable mold after mold is opened, and core of small hole of formed plastic part is also designed on the side of movable mold. For convenience of processing, core of formed small hole adopts an insert structure.

 

Figure 4 Moving mold insert
After plastic parts are molded, outside should be covered with rubber, and appearance requirements are not high. Material of fixed mold insert is domestic S136, and heat treatment hardness is 50~52 HRC; movable mold insert material is H13, and heat treatment hardness is 48~50 HRC.

Gating system is shown in Figure 5. Mold adopts a side gate gating system with 4 cavities. Advantage of this design is that it can not only meet quality requirements of plastic parts molding, but also simplify mold structure and reduce mold production cost. Diameter of primary runner is φ6 mm, diameter of secondary runner is φ4 mm, cross-sectional shape of gate is rectangular, and size is 0.75 m×1.4 mm.

 

Figure 5 Gating system

Figure 6 shows core-pulling mechanism. Protruding shaft and through hole at hinge of plastic part and bevel gear hinder demoulding, so core-pulling mechanism must be designed. Length of protruding shaft at hinge is 2.3 mm, and length of through hole is 15 mm. Core-pulling mechanism on the side of inclined guide post can meet requirements. Due to narrow space of bevel gear, it is not suitable to design slider, and inclined push rod side core-pulling mechanism is adopted. Considering that plastic part is required to stay on the side of moving mold after mold is opened and mold structure of slider is designed on the side of moving mold is relatively simple, so core-pulling slider and inclined push rod are designed on the side of moving mold.

 

Figure 6 Core Pulling Mechanism

Push-out mechanism is shown in Figure 7. Since surface of plastic part is a non-appearance surface, mold adopts a push rod ejection mechanism with simple processing, and push rod is composed of a push rod + a flat push rod + an oblique push rod. Depth of mold cavity is shallow, and demoulding force of plastic part is small. According to principle that push rod should be designed at part with the largest demoulding resistance, push rod is distributed along contour of plastic part, but it should be noted that runner aggregate should also be designed to push out push rod, and mold is designed with 3 push rods to push out runner aggregate .

 

Figure 7 launch mechanism

Cooling system is shown in Figure 8. Plastic part has a flat structure, cooling channels can be designed on upper and lower surfaces of cavity to achieve cooling. Mold adopts a straight-through cooling water channel, and cooling water channel is respectively designed in fixed platen on upper surface of molded plastic part and movable platen on lower surface. Due to small volume of core-pulling slider, no cooling water channel is designed.

 

Figure 8 cooling system

Characteristic of mold is that a mechanism for automatically cutting off gate aggregate is designed, as shown in Figure 9, mechanism is composed of a shear ring and a flat push rod, purpose of cutting gate aggregate is achieved by using relative movement of shearing ring and flat push rod. Shearing ring is fixed on movable mold plate, and flat push rod is fixed on push rod fixing plate. Shear ring does not move when plastic part is demolded, flat push rod is pushed out by push plate. Flat push rod pushes plastic part out of mold and cuts off gate part connecting plastic part and sprue aggregate. Mechanism design points: ①Cross-sectional shape of gate is designed as a rectangle, and upper surface is processed into a large slope, which is equivalent to cutting edge. The larger slope, the sharper cutting edge. ②A flat push rod is designed next to shear ring. Flat push rod has a large action area and a large push force, so that there is enough force to cut off gate aggregate; ③Effect of using this mechanism for brittle plastics is better, tough plastics should use a larger bevel angle, and mechanism cannot be used for plastic parts formed by soft rubber.

 

Figure 9 Automatic gate cut-off mechanism

Mold structure is shown in Figure 10. When injection is completed and mold is opened, inclined guide column 5 fixed on fixed mold side is separated from slider 4 assembled on movable mold side, under action of inclined guide post 5, slider 4 moves outward to complete core pulling at hinge. When demoulding, plastic part is released from mold under push of push rod, flat push rod 11 and inclined push rod 15. At the same time, inclined push rod 15 completes core pulling at bevel gear, flat push rod 11 pushes plastic part through shear ring 10 to cut off side gate condensate.

 

Figure 10 Mold structure
1. Fixed mold seat plate 2. Fixed mold plate 3. Inclined wedge 4. Slider 5. Inclined guide column 6. Limit screw 7. Moving mold plate 8. Fixed mold insert 9. Moving mold insert 10. Shearing ring 11 .Flat push rod 12. Push rod fixing plate 13. Push plate 14. Guide block 15. Inclined push rod
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