Figure 1 shows structure of tail lamp housing of a certain vehicle. Material is polycarbonate (PC)/acrylonitrile-butadiene-styrene plastic (ABS), shrinkage rate is 0.5%, and the overall dimensions are 337 mm*123 mm*156 mm, average wall thickness is 2.2 mm, there should be no appearance defects such as bubbles and weld lines.

 

Figure 1 Tail light housing
Since plastic parts are structural and functional parts, there are many assembly requirements. S1 is mounting hole for lamp head, which can be demolded in a positive direction, but requires high dimensional accuracy; S2 is installation button on inner side wall of plastic part, which is inverted button in mold opening direction of mold; S3, S4, and S5 are mounting holes for fixing body sheet metal, and their demolding direction is inconsistent with mold opening direction of mold. These features increase difficulty of mold design.

Injection mold of tail lamp shell adopts a 1-mold 2-cavity layout, left and right tail lamp shells are respectively formed. Hot runner gating system is shown in Figure 2. It adopts a single-point open hot runner and side gate, and gate size is 20 mm*1.2 mm.

 

Figure 2 Tail lamp housing casting system

In order to facilitate control of molding accuracy of mounting hole of S1 lamp cap of tail lamp housing, an insert structure is designed in mold cavity, as shown in Figure 3, which is beneficial to adjustment and control of installation size of lamp cap.

 

Figure 3 Insert structure

In order to realize forming and demoulding of S2 buckle on cavity side of tail lamp shell, mold design has a sloping top demoulding mechanism, as shown in Figure 4.

 

Figure 4 Lifter demoulding mechanism
1. Guide bush 2. Guide post 3. Spring 4. Lifter seat 5. Polyurethane spring 6. Limit pin 7. Screw 8. Push rod fixing plate 9. Reset rod 10. Lifter rod
Assembly relationship of lifter demoulding mechanism on mold is shown in Figure 5. Lifter seat is fixed on push rod fixing plate, one end of lifter rod is fixed on lifter seat, and the other end is used to form S2 inverted structure. Push rod fixing plate is guided by guide post guide sleeve, polyurethane spring is installed on push rod fixing plate by screws, spring is installed in blind hole of push rod fixing plate and fixed die base plate, spring and polyurethane spring provide driving force for lifter mechanism. Limit pin is fixed on the bottom of push rod fixing plate, contacts with fixed die base plate, and supports push rod fixing plate; reset rod is fixed on push rod fixing plate by screws, and its function is to drive lifter mechanism to reset.

 

Figure 5 Assembly of lifter ejection mechanism
1. Guide post 2. Guide sleeve 3. Fixed die seat plate 4. Spring 5. Lifter seat 6. Polyurethane spring 7. Screw 8. Push rod fixing plate 9. Screw 10. Reset rod 11. Cavity plate 12. Pad piece

In order to realize forming and demoulding of body sheet metal mounting holes of tail light shells S3, S4, and S5, mold is designed with a core support plate inclined pulling and demolding mechanism, as shown in Figure 6, which is mainly composed of a core pulling rod, a guide sliding sleeve, an anti-rotation block and a sliding seat.

 

Figure 6 Core plate inclined pulling and demolding mechanism
1. Tail light housing 2. Guide sliding sleeve 3. Screw 4. Anti-rotation block 5. Core pulling rod 6. Sliding seat
Assembly relationship of core-pulling mechanism of core-pulling plate on mold is shown in Figure 7. Upper end of core-pulling rod forms mounting hole of plastic part upside down, and lower end is connected to sliding seat. Sliding seat is fixed on core supporting plate. Core supporting plate drives core pulling rod to move. Core-pulling rod passes through guide sliding sleeve, which provides lubrication and guidance for movement of core-pulling rod. Anti-rotation block is fixed on core by screws, anti-rotation block is matched with anti-rotation plane of core-pulling rod, and its function is to prevent core-pulling rod from rotating.

 

Figure 7 Assembly of pallet tilting and ejecting mechanism

Design of mold ejection system is shown in Figure 8. Ejector is used to eject plastic parts, and ejection distance is 40 mm.

 

Figure 8 Push out system

In order to avoid uneven cooling of plastic parts, which leads to long molding cycle or warping deformation of plastic parts, design features of movable and fixed mold waterways of mold are as follows.

(1). Cooling system of movable and fixed mold water channels adopts design scheme of "water channel + water well". As shown in Figure 9, cooling system as a whole is designed to follow shape of plastic part, and water well is designed to cool area where local cooling is uneven to ensure uniform cooling of plastic part.

 

Figure 9 Mold cooling system
(2) Diameter of cooling water channel is φ11 mm, diameter of water well is φ18 mm, distance between adjacent water channels is 45 mm, distance between water channel and surface of plastic part is 20-25 mm.
(3) Moving and fixed molds are connected to mold temperature machine respectively, moving and fixed mold temperatures are independently controlled.

(1). After mold is injected and filled, mold is opened, core support plate and core are first separated, core support plate core-pulling and demolding mechanism performs core-pulling movement. When core support plate is separated from core by 30 mm, it stops moving. Core support plate is inclined to pull out mold to complete core pulling and demoulding of body sheet metal mounting holes of plastic parts S3, S4 and S5, and core support plate is inclined to pull out.

 

Figure 10 shows movement termination state of ejection mechanism.
Figure 10 End state of movement of core support plate obliquely pulling mold release mechanism

(2). After movement of core support plate is completed, core and cavity begin to separate under action of die-pulling button. Push rod fixing plate of lifter demoulding mechanism moves downward under combined action of spring and polyurethane spring, push rod fixing plate drives lifter rod to move to realize demoulding of S2 upside down. When cavity plate is separated from core by 28 mm, that is, when push rod fixing plate moves downward by 28 mm, movement of lifter demoulding mechanism is terminated. As shown in Figure 11, plastic part S2 is completely demolded by upside down.

 

Figure 11 End state of movement of lifter ejection mechanism
(3) After movement of lifter demoulding mechanism is terminated, core and cavity plate continue to be separated until the entire mold opening action is completed.
(4) After separation of core and cavity plate is completed, push-out mechanism pushes out 40 mm, and then manipulator picks up pieces.
(5) After removal is completed, mold is closed and reset. Sequence of mold closing is opposite to sequence of mold opening. Core and cavity plate are reset and closed first, then lifter demoulding mechanism is reset, and finally core support plate is pulled off obliquely. Mold mechanism resets and waits for next injection molding cycle.