Application of Vacuum Aluminizing Film in Automotive Lighting

【China Aluminum Industry Network】1. Application of vacuum aluminum coating on automotive lamps

1.1 basic classification of plastic aluminum film lighting

1.1.1 Aluminium lighting

Automotive lamps are mainly divided into headlights, fog lights, taillights and other decorative lights, as shown in Figure 1. The headlights of automobiles serve as lighting and vehicle visibility for roads at night. The parts that need to be plated are mainly mirrors and their reflective circles and decorative eyes. Fog lights are turned on in low visibility weather conditions such as rain, fog, and snow to warn vehicles before and after. The aluminum-plated parts are mainly mirrors; the taillights of vehicles include turn signals, brake lights, rear fog lights, reversing lights, etc., and they play a role in warning the rear driving signals. The aluminized parts are mainly light bodies.

1.1.2 aluminum plated substrate

Lamp aluminum plated substrates are PC (polycarbonate), ABS (acrylonitrile-butadiene-styrene), modified PP (polypropylene), ABS+PC and PBT (polybutylene terephthalate) ) And so on. The reflectors, vision circles, decorative mirrors, etc. in the headlights are subject to high light temperatures. PCs are usually used, and their long-term use temperatures can reach 130°C. The heat source of the existing lamp application, the heat generated in the working state can generate a high temperature of nearly 200° C. in a narrow space, and radiates heat and conduction through the heat convection. Therefore, the PC plastic part must be able to withstand 160 °C long-term high temperature test. Lamps need to use high-temperature PC materials with heat-resistant temperatures up to 185°C. Bayer MaterialScience Co., Ltd.'s copolymerized polycarbonate Apec(PC-HT) is a high heat-resistant stable material based on bisphenol A (forming a continuous phase of polycarbonate) and bisphenol TMC (trimethylcyclohexane) Polycarbonate produced by bisphenol) has a heat-resistant temperature of 165-185°C, and individual models have a heat-resistant temperature of approximately 200°C. In the lamp, PC-HT materials such as fog lamp mirror (average temperature lower than 175°C) are used for parts such as light distribution mirrors and eyelets near the light source. Fog reflectors are also made of materials such as PBT or BMC (Unsaturated Polyester Moulded Plastic). The average temperature endured by car taillights is below 80°C. There is no high headlights, and the lamp structure is relatively simple. Most taillights use aluminum plating on the lamp body. The materials involved are ABS, modified PP, and ABS+PC alloys.

Vacuum aluminum plating requires good heat resistance and low volatile matter content of the substrate. On the one hand, during the aluminum plating process, the substrate is exposed to the radiant heat of the evaporation source and the condensation heat of the evaporating material. The substrate is heated and heated. If the heat resistance is poor, there will be thermal deformation and wrinkles or shrinkage of the coating. On the other hand, when the temperature is raised, small molecules of volatile substances in the substrate are volatile, affecting the quality of the aluminum plating layer. The substrate must have a certain strength and surface smoothness, the surface of the aluminized substrate should be free from oil traces, and silver wires, weld marks, shrinkage, and scratches are required. Since bottom plating and bottomless plating cannot cover these defects. In addition, the moisture content of the vacuum aluminum plating substrate should generally be less than 0.1%. When the moisture content is high, the aluminum plating film will be fogged. Therefore, the hygroscopic substrate should be dried before aluminum plating.

1.1.3 aluminum plating method

The thickness of aluminum plating film for automotive lamps is 0.4-1.2μm. The surface is smooth and has a high gloss. The aluminizing method is divided into two kinds: non-bottom plating and bottom plating. The choice of aluminum plating method is related to the nature of the substrate.

Polyesters (such as PET, PBT, PC) are polar polymers with high surface free energy and surface wet tension above 40dyn/cm (1dyn/cm=1mN/m), which is very good with aluminum plating , can be used for bottomless plating. That is, a direct aluminum plated protective film is applied to a substrate (such as a PC polar polymer). Aluminized protective film has the function of improving the mechanical strength of aluminum film, blocking the harmful gas (such as atmospheric O2) or the erosion of the material on the aluminum film [6], etc., so that the coating has a bright metallic luster, excellent gas and light barrier properties. , Good moisture-proof, heat-resistant and puncture-proof performance. The articles will not be contaminated and corroded during transportation, storage and use, and will remain bright and clean. Since the PC backbone contains ester groups, the substrate has a high water absorption rate (0.15%-0.19%) at a certain humidity at room temperature. It is better to carry out aluminum plating within 2 hours from the injection molding die, and the storage time is longer. There will be haze after the aluminum plating caused by the water absorption of the substrate.

Polyolefin materials such as PE and PP, and ABS are non-polar polymers, which have small surface free energy, low surface wet tension (typically 30 dyn/cm), relatively rough, and poor adhesion to the aluminum plating layer. The pre-coating can obtain a smooth and smooth coating with a mirror effect, which can cover the substrate and prevent the volatile impurities in the plastic substrate from escaping during the vacuum coating and affect the quality of the coating. Therefore, this kind of material generally adopts the method of pre-coating the primer and plating aluminum to improve the adhesion between the coating and the substrate. At present, more domestic use of acrylic primer, and adjust the primer formula according to the type of substrate, mainly for the heat curing primer and ultraviolet (UV) curing primer, good applicability. Most of the plastic parts of lamps and lanterns are pre-coated with aluminum and then coated with a protective film. The protective film is essentially silicon dioxide. The SiO2 film is very protective and allows the aluminum film to be intact in a 10% alkaline solution. In order to reduce the number of pre-primed processes and reduce production costs, PBT has developed primer-free materials and made new progress. Regardless of the method used, all aluminum parts should be tested for adhesion and only qualified for use.

1.2 Performance Test Standards for Automotive Lamp Aluminum Coatings

The performance test standards of aluminum alloy film for automotive lamps mainly include GB/T10485-2007 "Environmental Durability of Exterior Lighting and Light Signaling Devices for Road Vehicles" and GB/T28786-2012 "Adhesion Method for Measurement of Bonding Strength of Vacuum Coatings for Vacuum Technology". Adhesion testing is a basic project. Use a small knife to sculpt 100 small squares with a spacing of 1mm on the test surface. Use 3M adhesive tape to stick the surface tightly and quickly tear the tape from the vertical direction to observe whether there is any peeling metal on the tape. membrane. When no visual observation was possible, the defects such as the absence of metal plating peeling were observed with a 10x microscope. The heat resistance is required for the aluminum fittings to adapt to the high temperature environment inside the lamp. The test is conducted in the high and low temperature test chambers to test the drying requirements of the spray primer. The time is (60±3) min.

The main purpose of the tests for acid-resistant (1% sulfuric acid solution), alkaline (1% caustic potash aqueous solution), and salt (3% NaCl solution) tests for the aluminum plating fittings of the lamp is to test the protective effect of the protective film on aluminum plating. One of them can be selected as required. . The specific operation is: at room temperature, the sample is soaked in the test solution, or the test droplets on the surface of the sample, 10 minutes after the bottom is not qualified. The above test can determine the bonding force between the vacuum aluminized film layers and the performance and quality of the spray primer and the protective film.

2·Application examples

Product Name: ABS reversing lamp body, weighing 170g, mass production.

2.1 Basic performance requirements of aluminum plating

(1) Appearance: The surface is smooth, no apparent defects, and the color and brightness are the same as electroplating chromium.

(2) Cut adhesion: 100/100.

(3) Alkali-resistant titration performance: A 1% NaOH solution was dropped on the surface of the sample and was not exposed for 10 minutes.

(4) Heat-resisting performance: After (60±3) minutes at (90.0±2.5)°C, the product is free from coloring, fogging, and deformation.

2.2 Operating sequence

2.2.1 Workpiece Preprocessing

Eliminates oil contamination, residual mold release agents, static electricity, and dust on the surface of the workpiece.

2.2.2 Coating Coating

U-342 (Toyo Kogyo Co., Ltd.) heat-cured coating evenly sprayed on the surface of the workpiece (with a clamp), the operator must control the movement to be uniform, after curing the coating thickness of 15 - 20μm.

2.2.3 Leveling

After coating, the workpiece was placed on a rack and heated at 50-55° C. for infrared heating for (5±1) minutes.

2.2.4 Heat curing

The shelf was used to advance the coated product into the oven and baked at (85 ± 5) °C for (60 ± 5) min.

2.2.5 Extension

Check the workpiece and install the qualified workpiece into the vacuum coating chamber.

2.2.6 vacuum aluminum plating

Aluminizing the film with vacuum coating equipment, the basic process flow is: pre-vacuum - ion cleaning - aluminum plating - ion bombardment - plating protective film - deflation. Prevacuum evacuated the coating chamber to 5×10 −2 Pa, filled with 430 mL of argon gas, and used the ion bombardment generated by two appropriate low-voltage glow discharges between the electrodes to achieve cleaning and auxiliary preheating of the coating substrate. Bombardment time It is 150s; The aluminum plating evaporator quickly rises to a high temperature, aluminum wire from the preheating, melting, evaporation, aluminum plating to complete the aluminum plating time is more than 30s; secondary ion bombardment, remove the film surface granular particles, so that the film layer Densification, bombardment time of 80s; plating protective film time (200 ± 50) s, vacuum degree of 3 × 10? 2Pa, a small amount of argon gas flow from the filled silicone oil flow to control the degree of vacuum, silicone oil flow control by the valve, Until the deflation is over.

2.2.7 Inspection and packaging Since the aluminum-plated film is easily scratched or bumped, the aluminum-plated part should be effectively protected after the aluminum-plated aluminum film is tested for its basic performance requirements. The friction between the products should be avoided when packing. Bruised. Two-layer plastic bags are used for packaging. The inner layer must be thin (usually 0.005mm thick) and soft and close to the plastic parts. The outer layer is packed in bubble plastic pockets. Each layer is separated by cardboard to reduce mutual impact. Probability.

2.3 Precautions

Coating coating is an important process before aluminum plating, and it is related to the quality of subsequent aluminum plating. The coating environment (including drying equipment) is dust-free and can reduce the adhesion of coating surface particles and dust fibers to the surface. Most of the coating is based on manual operation. The operation technology of the sprayer is very important. The uneven flow and coating often cause the aluminum-plated products to be scrapped. The coated parts must have sufficient drying temperature and time. If the drying is not good, the high temperature test may cause fog or wrinkles on the aluminum-plated surface. In the case of aluminized films, improper mounting of the workpiece can cause displacement of the aluminum plating area and discard; if the purity of the aluminum wire is insufficient or evaporation is controlled improperly, impurities in the evaporation will escape to form aluminum traces and remain on the surface of the plastic parts; evaporation temperature High or low temperature and unstable surface temperature of the evaporator will leave small particles of white spots on the surface of the aluminized film (observe the light); poor vacuum results in unacceptable coating adhesion; poor control of the coating flow rate also leads to Coating color and so on. Therefore, aluminum plating is an integrated manufacturing process that integrates the working environment, processing equipment, personnel technology, material targets, and process control.

3. Conclusion

The performance of aluminum plating depends on the quality of plastic parts and coating layers. It is dubbed the “game of the rich,” indicating that the coating quality requirements are quite high. The key to coating quality is the quality of the primer layer. Although there is no coating at the bottom, the quality requirements and cost of the mold are high, there are problems such as insufficient reflection brightness of the aluminum plating and defects of the plastic parts, which easily leads to the high scrap rate of the aluminum plating products. In general, the rate of scrapping without primer is around 10%, with primers around 20% or even higher. In order to solve this confusion, domestic manufacturers have used more than 10 years of hard work, using a primer in the furnace and curing to produce a bright surface (no pollution residue), high-pressure ion cleaning pretreatment, vacuum evaporation aluminum plating And coating protective film, so that the product pass rate of 98% or more, each year can save a lot of spray paint, paint energy consumption and labor costs. However, the cost of new equipment is much higher than that of conventional equipment of the same capacity, which makes it difficult for small and medium-sized manufacturers to accept it, and promotion has certain difficulties. Small and medium manufacturers choose to rely on human control to maintain production, and the scrap rate remains high. From the perspective of the development of aluminum plating coatings for lamps and lanterns, using polar polymer materials or primer-free materials, under the premise of improving the quality and brightness of the mold, the bottomless coating is the trend of future development and is conducive to the needs of the manufacture of automotive lamps. At present, vacuum evaporation is more often used for resistance evaporation evaporation method, but electron beam evaporation source evaporation method, high-frequency induction evaporation source evaporation method, and laser beam evaporation source evaporation method have emerged, in which electron beam evaporation source evaporation method is used. Will be an important heating method and development direction in vacuum aluminum plating technology. In a word, the vacuum aluminum coating is an indispensable processing technology for the surface processing of the plastic parts of the lamps and lanterns, and will have a broader space for development in the field of lamp manufacturing.