Two color injection molding is one of the more popular advanced injection molding methods, which provides meaningful saving potential and new design possibility for product production and manufacturing. This paper mainly discusses the development status and trend of two-color injection molding, especially the application in automobile lamps. 1. Definition of two-color injection molding

Multi color and multi material injection molding process refers to the combination of several kinds of plastics into multi-functional parts in a manufacturing process or a production unit. This technology uses a variety of materials for injection molding production, and combines the characteristics of different materials for assembly or other suitable bonding methods in the molding process, so as to improve the functionality and aesthetics of products, and provide meaningful saving potential and new design possibility for product production and manufacturing.

2. Necessary conditions for multicolor injection molding

The matching materials of multicolor injection molding must meet two basic compatibility conditions: adhesion compatibility and processing compatibility.

Multicolor injection molding also puts forward new requirements for injection equipment. As far as the injection unit is concerned, the parallel same direction, parallel opposite direction, horizontal and vertical L-type and Y-type same direction single cylinder injection structure can be adopted. For the mixed nozzle, special nozzles such as pattern, wave, flow mark, gradual layer and interlayer can be selected. In terms of clamping die, standard type, vertical rotary table type, horizontal rotary table type, rotating shaft type, manipulator rotating type and other mechanisms can be selected.

One of the key factors of multicolor injection molding is the variability of computer control program. Because even for the same type of multi-color products, if different mold designs are adopted, different electronic control programs must be matched.

3. Multi color forming technology of lamp

With the development of national economy, automobile industry has become the pillar industry of the country. At present, the rapid development of automobile industry puts forward higher and higher requirements for automobile lighting.

The quality of automobile lighting is very important to driving safety, so the laws and regulations of all countries in the world have strict requirements for automobile lighting. The design of lamps should not only meet the safety requirements of laws and regulations, but also meet some other requirements. For example, the appearance should be integrated with the appearance of the whole vehicle, beautiful and practical, meet the requirements of aerodynamics, and make the driver and passengers feel comfortable and convenient. Therefore, the design technology of lamp changes with the development of automobile industry.

4. Material and process characteristics of lamp

The lamp is generally composed of base and lamp shell. The base is usually made of thermosetting material - BMC (mass molding plastic), and the lamp shell can be processed from PMMA, PP, ABS and other materials, usually two or three colors.

In the production process of two-color lamp, special attention should be paid to the injection part of the two-color injection molding machine, and the center distance of the two screws should correspond to the center distance of the two-color mold.

In addition, the following important points need to be paid more attention:

1. Influencing elements of lamp stability forming

Response of control unit: too long switching point, noise interference, unstable command output and unstable temperature; Noise of oil pressure system: unstable pressure, valve positioning character, hysteresis and change of oil pipe damping; Mechanical system differences: oil seal internal leakage, damping, friction difference, screw check valve positioning, check effect; Uneven plasticization: differences in plasticization properties; Mold temperature control.

2. Common problems in the design of special plastic pipe group for lamp and tail lamp

Screw idling without feeding;
The torque required by the screw is too large to rotate;
The plastic is not completely melted;
Unstable screw metering;
Uneven mixing of plastics;
Low temperature molding plastic is easy to rise in temperature;
There are bubbles in the molded product;
The formed product is easy to spit black and turn yellow;
The screw is easy to corrode, wear and short service life.

3. Main design points of screw feed pipe group

Plastic plasticizing screw tube group is the heart component of injection molding machine, which is responsible for the transportation, melting, mixing, metering and other functions of plastic raw materials. Therefore, plastic plasticizing screw is closely related to the quality of molded products.

The main purpose of improving the plasticizing screw is to:

Improve shear mixing;
Uniform mixing;
Improve plasticizing ability;
Ensure the uniformity of melt temperature.

4. Design key points of special screw for tail lamp

L / D ratio: 21 ~ 23;
The surface roughness shall be extremely smooth to avoid material accumulation;
Increase the thickness and corrosion resistance of electroplating layer;
Compression ratio: 2.3, feeding section: about 50％, compression section: about 30％, metering section: about 20％.

5. Countermeasures for poor light emission

Coke burning - reduce injection pressure, multi-stage deceleration injection and mold vacuum pumping;
Crazing - dry the plastic particles thoroughly, increase the back pressure, reduce the melt temperature and reduce the shooting speed;
Bubble - increase back pressure, reduce melt temperature, adopt dehumidification dryer, and increase gate or runner size;
Bonding line - increase melt temperature, filling speed, exhaust and mold temperature;
Burr - increase the clamping force, reduce the filling pressure and reduce the melt temperature;
Shrink - uniform thickness design, increase the holding pressure and time, and replace the check valve.