With development of automobile industry, consumers have higher and higher requirements for appearance quality of automobiles. One of important functions of automobile plastic parts is surface decoration. Therefore, surface quality requirements of automobile plastic parts are more stringent. Position of plastic part parting line is one of important factors that affect surface quality. A reasonable parting design requires that parting line can be hidden after plastic part is assembled. However, due to reasons such as automobile modeling and engineering assembly, parting line of some plastic parts has to be designed on exterior surface, which affects quality of appearance perception.
In-mold parting technology has been widely used in automobile bumper molds, but it is rarely used in injection molds for other automobile plastic parts. For decorative strip of a certain car model, an in-mold parting mechanism is designed to hide parting line on non-appearance surface of decorative strip to improve its appearance quality.

 

Decorative strip is assembled on underside of headlight. Material is ABS, surface is sprayed with high-gloss paint or chrome-plated. Due to molding and engineering requirements of plastic part, height difference of headlight decorative strip in mold opening direction is large. In traditional injection mold design, parting line at lower end of plastic part is located at the corner R of the largest outer contour, as shown in Figure 1.
Design of this outer parting line has following defects: ①Parting line of lower end area of plastic part is located on corner R of appearance, parting line is visible after installation; ②After headlight trim strip is painted or electroplated, visibility of parting line on the surface is enlarged, appearance perception quality is poor; ③Due to manufacturing accuracy of mold parts and wear in injection production process, misalignment and flashing often appear at parting line, which affects appearance quality of molded plastic parts, resulting in a low production qualification rate.

 

In order to avoid quality problems caused by outer parting line design, combined with structural characteristics of plastic part, parting line in lower end area of plastic part is designed at the bottom of round corner to form an inner parting line, as shown in Figure 2. After plastic part is assembled, parting line can be hidden, which not only avoids risk of parting line leakage, but also improves quality of surface of plastic part after painting or electroplating and pass rate of injection molding of plastic part. However, design of this internal parting line will cause rounded corners of lower side of plastic part to be inverted in mold ejection direction, which will cause plastic part to fail to be demolded normally. Rounded area at lower end of part will be damaged. Therefore, design principle of internal parting of bumper mold is used for reference, internal parting mechanism is designed for headlight decoration strip mold to realize molding and demolding of decoration strip.

 

In order to prevent plastic part C from being broken when mold is opened by upside down, plastic part needs to be elastically deformed. Working principle of internal parting mechanism is shown in Figure 3. First, large oblique push block 3 needs to make room for deformation of molded plastic part, then pull block 2 drives plastic part around fulcrum to deform inside plastic part under action of guide rail 4, pulls inverted buckle at C to inside of plastic part, so that inverted buckle at C is released in mold opening direction to realize demolding of plastic part.
Working process of internal parting mechanism: ①When mold is opened, under action of nitrogen spring and die buckle, relative position of push rod fixed plate and fixed mold cavity plate is kept unchanged, fixed mold cavity plate and large push block move synchronously, big push block drives pull block to move together; ②Using deformability of plastic part, when mold opening reaches predetermined stroke, end of plastic part is pulled inward through pull block, so that end of molded plastic part is separated from inverted buckle of fixed mold cavity; ③When mold is opened, plastic part is separated from fixed mold cavity; ④Mold continues to open, and pull block mechanism is controlled to move outward through action of big push block, pull block body and guide rail, so that plastic part returns to its original shape.

 

There is a mounting structure in tensile deformation area of plastic part, as shown in Figure 4, this mounting structure has an undercut in the direction of plastic part tensile deformation. If you don’t solve this undercut release, internal parting mechanism cannot be realized. Therefore, mold must be designed with a release mechanism to release mounting structure before plastic part is stretched and deformed. Demolding mechanism is a key factor that determines whether parting of headlight trim mold can be achieved.

 

Parting mechanism in headlight trim mold is shown in Figure 5. It mainly includes large oblique push block, large push block, small push block, small oblique push block, pull deformation guide rail, pull rod, pull block, fixed die slider and push rod fixed plate.
Guide rail, pull block and pull rod constitute a tensile deformation assembly, guide rail is fixed on core, pull rod and pull block are mutually fixed, pass through large oblique push block, plastic part is deformed and restored under guide of guide rail; large oblique push block , large push block, small push block and small oblique push block are fixed on push rod fixing plate, play role of forming plastic parts upside down and pushing out plastic parts. Large oblique pushing block also provides driving force for tensile deformation component during movement.

 

In order to escape installation undercut of deformation area before plastic part is stretched and deformed, a fixed mold slider mechanism is designed. As shown in Figure 6, slider body and slider base are installed on cavity side, inclined guide post is fixed on core. superior. During mold opening process, oblique guide post drives slider body to move on cavity side.

 

In order to assist in realization of internal parting mechanism, headlight trim mold has 4 hook mechanisms, 4 nitrogen gas springs and 4 hydraulic cylinders, as shown in Figure 7.

 

A total of 120mm has been introduced by headlamp decorative strip mold launch mechanism, which is divided into a first-level launch and a second-level launch. The first stage is 55mm, push rod fixing plate and cavity plate are driven by nitrogen spring and hook to move synchronously. At this time, hydraulic cylinder does not move, push rod fixing plate and hydraulic cylinder are in an air-evacuated state, as shown in Figure 8. The second stage is 65mm and push rod fixed plate is driven by hydraulic cylinder to move. In second stage of push out process, pulling deformation assembly mechanism is reset, oblique push block and push block complete demolding of plastic part.
The first stage push is divided into two stages: first stage pushes 35mm synchronously. Oblique guide column drives slider body to move along slider base, structure of plastic part is pulled out of deformation area, so that there is no undercut in the direction of deformation. At the same time, large oblique push block retreats, leaving room for plastic part to be stretched and deformed; in the second stage, 20mm is pushed out simultaneously, it is stretched and deformed in this stage. Big oblique push block continues to retreat, and at the same time, driving block pulls plastic part to move inward through guide rail, so as to pull out inverted buckle of plastic part in main demolding direction.
After the first stage of pushing out, cavity opens, plastic part comes out of cavity. Piston rod of hydraulic cylinder drives push rod fixing plate to perform second stage of pushing out. At this stage, 65mm is pushed out, deformation mechanism drives plastic part to reset. At the same time, oblique push block structure continues to push out other inverted buckles of plastic part in demolding direction, push block mechanism continues to push out plastic part to complete demolding of plastic part.