Automotive Headlight Two-Color Injection Molding Technology

Two-color injection molding is one of the most popular and advanced molding techniques today. It provides significant cost-saving potential and new design possibilities for product manufacturing. This article mainly explores the current status and trends of two-color injection molding, with a particular focus on its application in automotive headlights.

Definition of Two-Color Injection Molding

Multi-color/multi-material injection molding is the process of combining multiple plastics into a multifunctional part within a single manufacturing process or production unit. This technology utilizes different materials in the injection molding process, combining their characteristics through in-mold assembly or other bonding methods to enhance functionality and aesthetics while providing meaningful cost savings and innovative design opportunities.

Essential Requirements for Multi-Color Injection Molding

The paired materials for multi-color injection molding must meet two fundamental compatibility conditions: adhesion compatibility and process compatibility.

Multi-color injection molding also brings new requirements for injection equipment. For the injection unit, structures can include parallel co-directional, parallel counter-directional, transverse, and longitudinal L-type or Y-type single-cylinder injection systems. For mixing nozzles, options include special designs for patterns, waves, flow marks, gradients, and sandwich layers. Regarding clamping mechanisms, standard, vertical rotary, horizontal rotary, rotary shaft, and robot rotation types can be selected.

One of the key factors in multi-color injection molding is the variability of the computer control program. Even for the same type of multi-color product, different mold designs often require different electrical control programs.

Multi-Color Molding Technology for Headlights

The quality of automotive lighting is critical to driving safety. Therefore, countries worldwide have strict requirements for automotive lighting. Headlight design must not only comply with safety regulations but also meet other requirements such as integration with the car’s exterior design, aesthetics, practicality, aerodynamic performance, and comfort for drivers and passengers.

Materials and Process Characteristics of Headlights

Headlights are generally composed of a base and a lens. The base is typically made of thermoset material—BMC (Bulk Molding Compound), while the lens can be made from PMMA, PP, ABS, and other materials, often using two-color or three-color molding.

During the production of two-color headlights, special attention should be paid to the injection unit of the two-color injection molding machine: the screw center distance must correspond to the center distance of the two-color mold.

Factors Affecting Stable Headlight Molding

• Control Unit Response: Long switching points, noise interference, unstable command output, temperature instability.
• Hydraulic System Noise: Unstable pressure, valve positioning issues, hysteresis, changes in pipeline damping.
• Mechanical System Differences: Oil seal leakage, damping and friction differences, non-return valve positioning and effectiveness.
• Uneven Plasticizing: Poor plasticizing performance.
• Mold Temperature Control.

Common Issues in Tail Light Screw & Barrel Design

• Screw idling without feeding.
• Excessive torque required for screw rotation.
• Incomplete plastic melting.
• Unstable screw metering.
• Uneven plastic mixing.
• Low-temperature molding plastic prone to temperature rise.
• Bubbles inside molded products.
• Black specks or yellowing inside molded products.
• Screw corrosion, wear, and short service life.

Key Design Considerations for Screw & Barrel Assembly

The plasticizing screw and barrel assembly, as the core component of the injection molding machine, is responsible for transporting, melting, mixing, and metering the plastic material. Therefore, its design has a direct impact on product quality.
The main purposes of improving the plasticizing screw include:

• Enhance shear mixing effect.
• Achieve uniform mixing.
• Improve plasticizing capacity.
• Ensure uniform melt temperature.

Key Design Points for Tail Light Screws

• L/D ratio: 21–23.
• Surface roughness: Extremely smooth to avoid material build-up.
• Plating thickness: Increased to improve corrosion resistance.
• Compression ratio: 2.3; feeding section: ~50%, compression section: ~30%, metering section: ~20%.

Countermeasures for Headlight Injection Defects

• Burn marks – Reduce injection pressure, use multi-stage slow injection, apply mold vacuuming.
• Silver streaks – Thoroughly dry plastic pellets, increase back pressure, reduce melt temperature, lower injection speed.
• Bubbles – Increase back pressure, lower melt temperature, use a dehumidifying dryer, enlarge gate or runner size.
• Weld lines – Increase melt temperature, increase filling speed, improve venting and mold temperature.
• Flash – Increase clamping force, reduce filling pressure, lower melt temperature.
• Shrinkage – Use uniform wall thickness, increase holding pressure and time, replace the non-return valve.