Two-color injection molding is one of the more popular advanced injection molding methods currently, providing meaningful savings potential and new design possibilities for product manufacturing. This article mainly discusses development status and trends of two-color injection molding, with a special focus on application of automotive lights.

 

Multi-color and multi-material injection molding process is process of compounding several plastics into multi-functional components in a single manufacturing process or production unit. This technology uses a variety of materials for injection molding production, combines characteristics of different materials for assembly or other suitable bonding methods during molding process, in order to improve product functionality and aesthetics, bring meaningful savings potential and new design possibilities to product manufacturing.

 

Paired materials for multi-color injection molding must meet two basic compatibility conditions, namely bonding compatibility and processing process compatibility.
Multi-color injection molding also brings new demands to injection equipment. In terms of injection unit, parallel and same-direction, parallel-opposed, transverse and longitudinal L-shaped, Y-shaped single-cylinder injection structures can be used. In terms of mixing nozzles, you can choose special nozzles with patterns, waves, flow marks, gradients and interlayers. As for clamping mold, you can choose standard type, vertical turntable type, horizontal turntable type, rotating shaft type, robot rotating type and other mechanisms.
One of key factors in multi-color injection molding is variability of computer control programs. Because even for same type of multi-color products, if different mold designs are used, different electronic control programs must be used.

 

With development of national economy, automobile industry has become a pillar industry of country. Rapid development of current automobile industry has put forward increasingly higher requirements for automobile lighting.
Quality of automotive lighting is crucial to driving safety, so regulations in various countries around the world have imposed stringent requirements on automotive lighting. Design of lamp must not only meet safety requirements of regulations, but also meet some other requirements. For example, appearance must be integrated with appearance of the entire car, be beautiful and practical, this utility model must meet aerodynamic requirements, bring comfort and convenience to driver and passengers. Therefore, design technology of car lights is also changing with each passing day as automobile industry continues to develop.

 

Car lights generally consist of a base and a lamp shell. Material of base is usually a thermosetting material - BMC (bulk molding compound). Lamp shell can be made of PMMA, PP, ABS and other materials, usually two or three colors. In production process of two-color car lights, special attention should be paid to injection part of two-color injection molding machine. Center distance of two screws corresponds to center distance of two-color mold. In addition, there are several important aspects that should attract our attention:

(1) Control unit response: too long switching point, noise interference, unstable command output, unstable temperature;
(2) Noise from hydraulic system: pressure instability, valve positioning quality, hysteresis, and oil pipe damping changes;
(3) Differences in mechanical systems: oil seal internal leakage, damping, friction differences, screw check valve positioning, and anti-return effect;
(4) Uneven plasticization: differences in plasticization properties;
(5) Mold temperature control.

 

(1) Screw is idling and no material is fed;
(2) Screw requires too much torque to rotate;
(3) Plastic is not completely melted;
(4) Screw metering is unstable;
(5) Uneven plastic mixing;
(6) Low-temperature molded plastics are prone to temperature rise;
(7) There are bubbles in molded product;
(8) Inside of molded product is easy to turn black and turn yellow;
(9) Screw is prone to corrosion, wear and tear, and has a short service life.

As heart component of injection molding machine, plastic plasticizing screw and tube group is responsible for transportation, melting, mixing and metering of plastic raw materials. Therefore, plastic plasticizing screw is related to quality of molded product.

 

Main purpose of improving plasticizing screw is to:
(1) Improve shear mixing effect;
(2) Mix evenly;
(3) Improve plasticizing ability;
(4) Ensure uniformity of melt temperature.

(1)L/D ratio: 21~23;
(2) Surface roughness needs to be extremely smooth to avoid material accumulation;
(3) Increase corrosion resistance of electroplating layer thickness;
(4) Compression ratio: 2.3, feeding section: about 50%, compression section: about 30%, metering section: about 20%.

(1) Scorching - reduce injection pressure, slow down injection in multiple stages, and evacuate mold;
(2) Silver streak—thoroughly dry plastic pellets, increase back pressure, lower melt temperature, and reduce firing rate;
(3) Bubbles - increase back pressure, decrease melt temperature, use a dehumidifying dryer, and increase size of gate or runner;
(4) Bonding line—increase melt temperature, increase filling speed, increase exhaust gas and mold temperature;
(5) Burrs—increase clamping force, reduce filling pressure, and reduce melt temperature;
(6) Shrinkage - Uniform thickness design, increase holding pressure and time, and replace check valve.