Samples are produced by numerical control equipment, 3D printing, and copying. In the rapid prototyping process, in addition to high-quality machine processing, in most cases, it may also require a lot of manual or other instruments for post-processing operations. For general appearance products, it is required that the surface treatment should be as close as possible to the finished products before the appearance effect can be tested.
Common surface treatment processes mainly include: grinding, sand blasting, polishing, spraying, powder spraying, UV spraying, printing, radium carving, electroplating, oxidation, blackening, wire drawing and phosphating. (Oxidation, blackening, wire drawing and phosphating are only applicable to metal parts)
1 Grinding: The post-treatment grinding methods can generally be divided into three types: wet grinding, mechanical grinding and dry grinding.
Mechanical grinding: during large-scale construction, mechanical grinding methods, such as electric grinding machine, can be used to improve work efficiency; Sand blasting machine, etc (Sand blasting: the compressed air is used as the power to form a high-speed jet beam to spray copper sand, quartz sand, carborundum, iron sand and sea sand to the surface of the workpiece to be treated at a high speed. Due to the impact and cutting effect of the abrasive on the surface of the workpiece, the surface of the workpiece can obtain a certain degree of cleanliness and different roughness, so that the mechanical properties of the surface of the workpiece can be improved, so the fatigue resistance of the workpiece can be improved, and the workpiece and paint can be increased The adhesion between layers extends the durability of the paint film and is also conducive to the leveling and decoration of the paint).
Dry grinding: sand paper is used for grinding. It is suitable for polishing hard and brittle paint. Its disadvantage is that a lot of dust will be produced during operation, which will affect the environmental health.
Wet grinding: sand paper dipped in water for grinding. Water grinding can reduce abrasion marks, improve the smoothness of the coating, and save sandpaper and labor. However, attention should be paid to spraying the lower layer of paint after the water grinding. First, the lower layer of paint should not be painted until the water grinding layer is completely dry, otherwise the paint layer is easy to turn white. In addition, the substrate with strong water absorption is also not suitable for water milling.
Special grinding method: the coating requirements indicate that it is “light grinding”, for example, after partial repair, fine sandpaper and skilled hands should be used at this time, otherwise, it will be counterproductive. Attention should be paid to the problem when polishing. First, rough sandpaper (such as 150 or 200 sandpaper) should be used for rough processing and polishing. Remove the coarse burrs on the surface and then polish the fine lines (such as 400 # or 600 # sandpaper). Then polish the surface to different degrees according to the requirements of the product (such as 800~1200 sandpaper)! After one-time polishing, the roughness of the product surface can be amplified by spraying a layer of primer or primer. So that the defects can be seen with the naked eye. Finally, we can focus on local correction until a product finally completes the whole grinding process.
2UV spraying: the board whose surface is protected by UV treatment UV is the abbreviation of Ultraviolet. UV paint is UV curing paint, also known as photoinitiator paint.
Advantages and disadvantages of UV:
(1) High hardness. The highest hardness can reach 5-6H;
(2) The curing speed is fast and the production efficiency is high. It can be cured in 30 minutes through ultraviolet curing;
(3) The coating has excellent performance, with very high performance indicators in hardness, wear resistance, acid and alkali resistance, salt spray resistance, gasoline resistance and other solvents; Especially, its paint film is plump and glossy;
(4) The UV curing process adopted by UV paint is pollution-free when painting, and is recognized as a green environmental protection product;
(5) UV paint is sensitive to dust, so it is strict to the construction environment;
(6) It is easy to burst during spraying;
(7) It is not resistant to sunlight and is easy to crack under sunlight;
(8) It will turn yellow after a long time, so UV glazing is generally not used when the product appearance requires pure white;
(9) The price is expensive, and the requirements for coating pretreatment are very high.
3. Polishing: It refers to the finishing processing of the workpiece surface by using flexible polishing tools, abrasive particles or other polishing media on the basis of polishing. Polishing can not improve the dimensional accuracy or geometric shape accuracy of the workpiece, but aims to obtain a smooth surface or mirror luster, sometimes used to eliminate luster (matting). The surface roughness of the workpiece after polishing can generally reach Ra0.63~0.01 μ m. PMMA transparent parts require very high polishing requirements, so the price of PMMA transparent parts is very expensive, more than 4 times that of ordinary ABS parts.
4 Surface spraying: air spraying is a widely used finishing process in the current paint coating construction. Air spraying is to use the air flow of compressed air to flow through the nozzle hole of the spray gun to form a negative pressure. The negative pressure causes the paint to be sucked in from the straw and sprayed out through the nozzle to form a paint mist. The paint mist is sprayed onto the surface of the painted parts to form a uniform paint film. Air spraying can produce uniform paint with fine and smooth coating; Concealed parts of components (such as gaps and bumps) can also be painted evenly. The coating utilization rate of this method is about 50%~60%.
After plastic parts are sprayed, the following effects can be obtained:
It can cover the surface defects of formed parts;
Because it is difficult to color the plastic itself, a variety of colors can be obtained by spraying;
The electrostatic property of the plastic is improved and the dust absorption is reduced;
The hardness and scratch resistance of plastics are enhanced;
The weatherability of plastics is improved;
The gloss of the plastic part surface can be adjusted at will;
Sand grain paint, fluff paint and other special paints can obtain better appearance and feel.
In addition, the surface state of plastic after molding has a great impact on the appearance quality. It is required that the formed surface shall be flat, smooth and uniform, free of scratches, flash, burrs, pits, spots, bubbles and obvious weld lines.
