Behind rapid development of automotive industry is development of automotive mold making industry. Especially in recent years, development of world automotive mold making industry is changing with each passing day, and demand for molds in the automotive industry is also growing. In the manufacturing process of a car, about 500 automotive plastic moulding are needed, including auto door mould, automotive bumper molding, auto lamp mould, glove box molds, central passage molds, positive and negative instrument panel molds, and other large auto parts mould, as well as precision auto parts mould such as various types of connectors for water tanks. It can be said that automotive plastic moulding has the largest proportion in the automobile mold making, and its importance is self-evident.

Large automotive bumper molding design

Car bumper is one of the most important appearance parts of a car. Not only it have sufficient strength and rigidity, but also can play a buffering role in a collision accident, protect car body, pursue harmony and unity with car body shape. In order to achieve these goals, current car bumpers are made of plastic, commonly known as plastic bumpers.

Shape of the front bumper of car described in this paper is similar to saddle shape. Material is PP+EPDM-T20, and shrinkage rate is 0.95%. Among them, PP is the main material of bumper. EPDM can improve elasticity of insurance cover. T20 refers to addition of 20% talcum powder to improve rigidity of bar cover.

Characteristics of automotive bumper molding are:

1, Shape is complex, size is large, wall thickness is relatively small, belonging to large thin-walled plastic parts.

2, Many K/Os, S/Os, ribs, and large resistance to melt flow.

3, Three inverted buckles on the inside of automotive bumper molding, and lateral core pulling at each place is quite difficult.

Conclusion as followed:

1, It uses internal parting technology to ensure appearance of plastic parts.

2. This automotive bumper molding adopts secondary core-pulling structure of “composite inclined roof” to solve problem of lateral core pulling of complex parts.

3. It adopts an eight-point needle valve type sequence valve hot runner casting system to solve problem of melt filling of large thin-walled plastic parts.

4. It adopts hydraulic pressure as power of demoulding system, which solves the problem that demoulding force of plastic part is large and pushing piece is difficult to reset.

Large-scale auto door mould design

Car door panel is an important component of car interior parts. It is located on the inner side of car door and has front, rear, left and right points. There are usually two or four doors depending on car system. These parts are collectively referred to door panel series.

Auto door mould described in this paper shows left and right door panel parts of car. The material is PP+EPDM. Shrinkage rate is generally 1.011. EPDM is a copolymer of ethylene, propylene and a small amount of non-conjugated diene. It is one type of propylene rubber and composed of a chemically stable saturated hydrocarbon, It has an unsaturated double bond only in the side chain. Therefore, it is excellent in ozone resistance, heat resistance, weather resistance and can improve elasticity of door panel.

Door panel is an appearance piece, and outer dimensions are: 880*670.4*105.6mm. Its structural caracteristics are as follows:

1, Outer surface requirements are high, no spots, gate marks are allowed, and defects such as shrinkage depressions, weld marks and flash edges are not allowed.

2, Door panel is a leather-like piece, appearance of mold release slope is at least 5 °.

3. Smoothness of curved surface of door panel is high, shape structure is complex, parting line is complicated with many inverted buckles. There are 11 inverted buckles on the inner and outer sides of plastic part, which makes the demoulding difficult.

Discussion:

This auto door mould adopts demoulding system introduced by “push rod + oblique push block + push block + nitrogen gas spring” and temperature control system of “straight-flow water pipe + inclined water pipe + water well”.

Temperature control system is very important in the design of auto parts mould, which has a great influence on molding cycle and molding quality of plastic parts. Reasonable temperature control has far-reaching influence on quality of plastic parts and shortening molding cycle. For design of large automotive mold making cooling systems, following two points should be observed:

1, 3 meters principle. Total length of cooling waterway should not exceed 3M. If it exceeds 3M, cooling effect will be poor. It is necessary to avoid the situation that cooling water has not been runned after mold opened. In addition, length of a single cooling channel must take into account length of drill bit, hole is too deep, and length of drill bit is not enough, or drilling will not be finished. Diameter of straight-through cooling pipe of a large-sized auto parts mould is generally 15mm. If fixed-mode cooling water passage of door-plate injection mold is designed along with shape of plastic part, it should be designed to be drilled at both ends.

2, Palm effect. Design waterway design of large-scale automotive plastic moulding, and arrange waterway to flow in one direction. The spacing is arranged like a palm, and distance between waterways is controlled between 50-60mm.

Auto parts mould temperature control system is mainly composed of following two combinations:

1, Vertical straight-through water pipe + inclined water pipe + septum type well.

2, Vertical water pipe + septum well + inclined water pipe.

Large-scale auto lamp mould design

Headlight lamp housing (also known as lamp body) is a part of headlight base and wiring harness of automobile before installation. It belongs to internal function parts, appearance parts with many assembly size, high accuracy requirement of lamp socket of lamp holder, slot size of installation harness.

Auto lamp mold described in this paper shows is a part of headlight lamp housing part, material is PP+TD20, wherein PP (polypropylene) is the base of lamp housing cover, and TD20 means that 20% of talcum powder is added to material to improve rigidity of lamp housing. This part is not a appearance part but an internal function. Average wall thickness of plastic parts is 3.00mm, and maximum size of plastic parts is 320.9*228.4*344.3mm. Characteristics of plastic parts are as follows:

1, Plastic parts are not allowed to have spots on surface, and are not allowed to have defects such as shrinkage depressions, weld marks and flash edges.

2, Plastic parts are internal functional parts, its’ lamp head hole, back cover hole have many assembly dimensions and high precision.

3, Shape of plastic parts is complex, and parting surface is complicated.

4. There are two inverted buckles on outer side of plastic part, and parting surface is complicated.

5, Left and right lamp shells are asymmetrical except for asymmetrical aperture of lamp cap.

Since auto lamp mold is a large mold and parting surface is complicated, molded part and formwork adopt an integrated structure, that is fixed mold A plate is a fixed mold forming part and moving mold B is a moving mold forming part. Advantage of this structure is that structure is compact, strength is high, rigidity is good, mold volume is small, and cumbersome processes such as opening frame, arranging frame and manufacturing beveling are avoided.

Because plastic parts are PP+TD30 and non-appearing parts, fixed and movable mold steels can be P20 or 718.

Since pushers are on the movable mold side, plastic parts must remain on the movable mold side after mold is opened. Due to complicated shape of plastic parts, plastic parts may stick to fixed mold after mold is opened. Therefore, it is necessary to prevent occurrence of mold-fixing phenomenon when designing auto lamp mold. The specific measures are that design reinforcing rib and reverse buckle pattern on the movable mold side correponds to area with strong pressing force in the plastic parts. If it is still fixed, design a buckle on the push rod to keep injection part on the moving mold side.

Designer should pay attention to design of lamp socket. Left and right of lamp hole is translation relationship, not mirror relationship. This is one of mistakes that designer of lamp mold is easy to make mistakes. Pay attention to dimensional accuracy of lamp hole. Draft angle must be within 0.5°, otherwise it will affect assembly of lamp holder. There are many ribs for plastic parts of lamp shell, and there are many BOSS columns. There are many fixed, movable mold inserts for injection molds, and push tubes. Fixed and movable mold inserts should be positioned with mushroom head to make slope. Screws are screwed from the front side as much as possible, and glued area is copper plated.

In other respects, design of molded part should pay attention to insertion angle. In the design of automobile mold, in order to improve life of mold and avoid insertion position of plastic part, all insertion positions are designed above 7 degrees.