1. Characteristics of plastics used in automobiles
Because the use of plastic parts instead of metal parts can reduce the weight of the car, save fuel when driving, and the plastic parts have good corrosion resistance, low density, and the mechanical properties of engineering plastics are not worse than metal parts, so the use of plastic parts in cars has been increasing in recent years. , The proportion of mass in the vehicle is about 6% to 8%.
The plastic parts used in automobiles include exterior panels, bumpers, general partitions, wheel covers, exterior mirror seats, spoilers, tail lights, etc. These plastics are generally processed by reaction injection molding, compression molding or injection molding. Due to different processing methods, the materials and surface characteristics of various plastic products are very different, so their coating processes are also very different. The difficulty of plastic coating is that most thermoplastics are not resistant to high temperature, and they will be deformed when they exceed 100 ℃. They cannot be coated with the coating process of metal workpieces. Another problem is that the adhesion between plastics and coatings is poor, so plastic coatings The equipment should use special coatings for plastics and its special coating process.
2. Coating of automotive plastic parts
Due to the smooth surface, low polarity and poor lubricity of plastic, the formed coating film has poor adhesion to it. Generally, the surface should be pretreated before coating. See Chapter 2 of this book for related content. There are also differences in the plastic coating process of different manufacturing methods. For example, injection-molded thermoplastics, including ABS (polystyrene-acrylonitrile-butadiene copolymer), PP (polypropylene), etc., are hard and semi-rigid materials. The adhesion of the coating film to them is poor, so when painting, it is necessary to choose a suitable pre-painting treatment process and use a variety of paints with good adaptability to obtain a good coating effect. Coating is generally divided into two steps of primer and topcoat. At present, for polypropylene (PP) and nylon substrates, special primers have been developed to improve the bonding force between the coating film and the substrate. After surface treatment, spray this primer, then dry it for 3-5 minutes, and then spray the topcoat according to the normal spraying method to obtain a better coating.
The molded plastic parts are mainly glass fiber reinforced unsaturated polyester (ERP or sMc), which has good heat resistance and high strength, but because the substrate has holes, it should be noted that it is prone to bubbles when coating.
The plastic parts of reaction injection molding are soft materials, so special attention should be paid to keep the flexibility of the coating consistent with the plastic when coating, so as to avoid cracking and detachment of the coating film.
Due to the poor heat resistance of plastics, high-performance baking varnishes cannot be used, and only baking varnishes that can be forced to dry at low temperatures below 80°C can be used, so the choice of paints is limited. At present, the coatings used for the coating of automotive plastic parts are mainly one-component paints and two-component paints, such as two-component polyurethane paints, thermoplastic acrylic paints, and unsaturated polyester paints. Among them, the primer generally adopts epoxy, acrylic, polyurethane, unsaturated polyester and other coatings. Among them, the unsaturated polyester cyan paint has better hole filling performance and can form a hard and wear-resistant coating. The top coat mainly uses polyurethane paint, acrylic paint and unsaturated polyester paint, and nitro paint, acid-catalyzed amino paint and alkyd resin paint can also be used.
3. Application of spraying in the coating of automobile plastic parts
(1) General application Plastic parts generally use the air spray method. Due to the poor conductivity of the plastic surface, when electrostatic spraying is used, after the charged paint mist particles are adsorbed to the surface of the workpiece, the charge cannot be dissipated quickly, resulting in the accumulation of charge on the plastic surface. As a result, the plastic parts cannot be in a well-grounded zero-potential state, which reduces the static voltage at the nozzle of the electric Spray Gun, causing the coating to be difficult to charge and the atomization performance to deteriorate; and with the increase of the residual charge on the plastic surface, the subsequent spraying The anti-electric field force generated by the paint mist on the workpiece increases, the deposition rate of the sprayed paint mist will further decrease, and the unevenness of the residual charge distribution on the plastic surface will cause the uniformity of the coating film thickness to deteriorate. Moreover, the existence of residual charge makes the plastic surface easier to absorb dust, resulting in more surface particles in the coating film, which affects the appearance and quality of the coating film. Therefore, it is necessary to improve the conductivity of the plastic surface and speed up the dissipation of surface charge. If Using electrostatic spraying, it is necessary to coat the plastic parts with conductive primer or 1%~2% (mass fraction) isopropanol solution of surfactant for wetting treatment, so as to reduce the resistivity and form a good grounding of the workpiece. . In order to prevent the accumulation of charge on the plastic surface, the time for the paint mist to reach the plastic surface from the Spray Gun should be as short as possible, such as 0.ls when using rotary cup electrostatic spraying, and 0.0ls when using air-assisted electrostatic spraying. Because plastic is very easy to charge and absorb dust, special ionization fan should be used to remove dust before spraying to neutralize the charge on the surface of the plastic and make the adsorbed dust fall off.
If powder coatings are used to coat plastic parts, in order to prevent thermal deformation of plastic parts during the curing and heating process, UV-cured polyester thermosetting powder coatings should be used and cured by ultraviolet light.
(2) Automobile plastic parts spraying process ABS is a commonly used plastic in automobiles. For example, the reflector lampshade of the automobile headlight and the plastic cover plate on the outside of the wheel are all made of ABS. Its spraying process is: pre-painting treatment (Degreasing, surface adjustment)→Electrostatic dust removal→Spray wetting agent or conductive primer→Air spraying or electrostatic spraying primer→Drying→Drying→Cooling→Fine grinding and decals→Spray varnish→Drying→Drying → cooling → grinding and polishing.
At present, a coating production line with a high degree of automation has been built in the automobile production plant. For example, a production line is composed of a 5-chamber cleaning machine, 3 air-supply and down-draft rheumatic spraying rooms, 3 hot-air drying rooms, surface preparation rooms, drying and leveling rooms, and grinding rooms. The speed of the workpiece conveying chain is generally about 3m/min, and the speed can be adjusted as needed. The light sliding structure conveyor system is used to run through the whole line. The spraying operation adopts robotic spraying and manual supplementary spraying to complete the spraying of primer and topcoat. When electrostatic spraying is used, the conductive primer should be sprayed before the primer.
