In the entire process of prototype processing, after selecting the appropriate processing method (CNC processing or 3D printing), most prototypes require surface treatment. The purpose of surface treatment is to meet the corrosion resistance, wear resistance, decoration or other special functional requirements of the product. There are dozens of surface treatment techniques for prototyping. Next, we will introduce the common surface treatment techniques for prototyping.
Surface treatment process
Grinding is one of the most common surface treatment techniques. Generally, rough objects (such as sandpaper containing higher hardness particles, etc.) are used to change the physical properties of the material surface through friction and remove burrs, machining lines, and bonding marks on the surface of the workpiece. and other defects, thereby improving the flatness of the workpiece, reducing roughness, and making the surface of the workpiece smooth and fine.
Polishing uses flexible polishing tools and abrasive particles or other polishing media to modify the surface of the workpiece based on grinding. Polishing cannot improve the dimensional accuracy or geometric shape accuracy of the workpiece, but is intended to obtain a smooth surface or mirror gloss. Sometimes it is also used to eliminate gloss (matting). The surface roughness of the workpiece after polishing process can generally reach Ra0.63 ~ 0.01 micron.
Electroplating uses electrodes to pass electric current to attach metal to the surface of an object. Its purpose is to change the characteristics or size of the surface of the object. Electroplating is generally divided into two types: wet electroplating and dry electroplating. The wet method is commonly referred to as water plating; the dry method is commonly referred to as vacuum plating.
Water plating is to pass the coating metal through the electrode method to generate ion replacement and adhere to the surface of the plated part; while vacuum plating uses high voltage and large current to make the coating metal instantly vaporize into ions in a vacuum environment and then evaporate to the surface of the plated part. Water plating has good adhesion and does not require other processing in the later stage; vacuum plating has poor adhesion and generally requires PU or UV on the surface. PC cannot be electroplated, and complex mold parts cannot be water plated, but can only be vacuum plated. The color of water plating is relatively monotonous. Common water plating includes chromium plating, nickel plating, gold plating, etc., while vacuum plating can solve the problem of seven colors. Before water plating, the surface effect of the workpiece must be polished to 1500-2000 grit sandpaper, and then polished before water plating can be carried out. Therefore, water plating workpieces are generally very expensive; the polishing effect of vacuum plating can be slightly worse than 800-1000 grit sandpaper. Therefore, vacuum plating is also relatively cheap.
The oxidation treatment of metal is the interaction between the metal surface and oxygen or oxidants to form a protective oxide film to prevent metal corrosion. Oxidation is divided into chemical oxidation and electrochemical oxidation (i.e. anodic oxidation).
(1) The oxide film produced by chemical oxidation is thin, with a thickness of about 0.3-4um, porous, has good adsorption capacity, is soft and non-wear-resistant, and has good conductive properties. It is suitable for occasions with shielding requirements and can be attached to various It has a variety of colors and has good adsorption capacity. It can be painted in various colors and then painted on its surface, which can effectively improve the corrosion resistance and decoration of aluminum products.
(2) The oxide film produced by anodizing is thicker, generally between 5-20um in thickness. The thickness of the hard anodized oxide film can reach 60-2500um. It has high hardness, good wear resistance, good chemical stability, and good corrosion resistance. It has good adsorption capacity, good insulation properties, strong thermal insulation and heat resistance properties, and can be painted in various colors.
After chemical oxidation treatment of aluminum and aluminum alloys, especially anodization treatment, the oxide film formed on the surface has good protective and decorative properties. Therefore, it is widely used in aviation, electrical, electronics, machinery manufacturing and light industry. aspect.
Under certain conditions, when the potential of the metal moves in the positive direction due to an external anodic current or a local anodic current, the surface state of the originally actively dissolved metal will undergo a sudden change. The dissolution rate of metals drops rapidly. This mutation process of surface state is called passivation.
Passivation can improve the passivation performance of metal materials, promote passivation of metal materials in the use environment, improve the mechanical strength of metal, is one of the most effective ways to control corrosion, and enhances the adhesion between metal and coating films.
The blackening treatment on the surface is also called bluing. There are two commonly used methods for blackening treatment: traditional alkaline heating blackening and room temperature blackening that appears later.
The protective film obtained by blackening is black, which improves the corrosion resistance and mechanical strength of the metal surface, and can also serve as a good base layer for paint. (Stainless steel cannot be blackened, and the blackening effect of iron is best)
Phosphating is the process of treating the metal surface with an orthophosphate solution of manganese, zinc, iron and other metals to form a protective film of insoluble phosphate. The protective film generated after phosphating treatment can improve the insulation and corrosion resistance of metals, improve the protective and decorative properties of workpieces, and can also serve as a good base layer for coatings. Metal surface phosphating treatment methods are divided into cold phosphating (room temperature phosphating), hot phosphating, spray-less phosphating and electrochemical phosphating. In the automobile industry, phosphating treatment is the main pre-coating method for painted parts such as automobile panels, cabs, and trunk panels. The phosphating film is required to be fine, smooth, uniform, moderately thick, and have certain heat resistance.
It uses compressed air as power to form a high-speed jet beam to spray the spray material (copper ore, quartz sand, emery, iron sand, sea sand) at high speed onto the surface of the workpiece to be processed. 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, thereby improving the mechanical properties of the surface of the workpiece, thereby improving the fatigue resistance of the workpiece, increasing the adhesion between the workpiece and the spray paint coating, and extending the life of the paint film. Durability, also good for paint leveling and decoration.
Surface painting is one of the most widely used surface techniques. Spraying has the following advantages:
(1) Can cover surface defects of the workpiece after molding;
(2) The surface of the workpiece can be sprayed to obtain a variety of colors, different gloss levels, different visual effects and a variety of different feel;
(3) Enhance the hardness and scratch resistance of the workpiece surface;
The spraying effects include: matte, semi-matte, gloss (high gloss), various colors, various textures, etched patterns, brushed effects (metallic colors can only be brushed), leather effects, and elastic feel effects. (rubber paint) etc.
Wire drawing treatment is a surface treatment method that uses grinding products to form lines on the surface of the workpiece to achieve a decorative effect. Brushing can well reflect the texture of metal materials and can give the metal surface a non-mirror metallic luster. According to different surface effects, it can be divided into straight silk (hairline pattern) and random silk (snowflake pattern).
The quality of silk texture types is highly subjective. Each user has different requirements for surface lines and different preferences for line effects. Therefore, it is necessary to have a brushed sample to process the effect that users like and are satisfied with. Arcs (the intersection between the arc surface and the straight surface are very ugly, and the drawing is uneven) and painted surfaces (metal-colored surfaces can draw fine lines) are not suitable for drawing.
Plastic spraying (powder spraying)
Electrostatic spraying uses the phenomenon of corona discharge to adsorb powder coating to the workpiece. The process is as follows: the powder coating is fed into the spray gun by compressed air from the powder supply system, and a high voltage generated by a high-voltage electrostatic generator is added to the front end of the spray gun. Due to corona discharge, a dense charge is generated near it, and the powder is discharged from the gun nozzle. When sprayed, charged paint particles are formed. Under the action of electrostatic force, they are attracted to the workpiece with the opposite polarity. As the amount of powder sprayed increases, the charge accumulates more. When it reaches a certain thickness, due to the generation of static electricity The repulsion effect will stop the adsorption, so that the entire workpiece obtains a certain thickness of powder coating, and then the powder is melted, leveled, and solidified through heat, that is, a hard coating film is formed on the surface of the workpiece.
Advantages and disadvantages of electrostatic powder spraying: no thinner is required, no toxicity, no environmental pollution, good coating quality, very high adhesion and mechanical strength, corrosion resistance, short curing time, no need for primer, low technical requirements for workers, and powder recycling The efficiency is high; the coating is very thick, the surface effect is corrugated and not smooth, and it can only process semi-matte and glossy appearance effects.
(1) Silk screen printing: Part of the mesh of the screen printing plate can pass through the ink and leak onto the substrate; the rest of the mesh on the printing plate is blocked and cannot pass through the ink, forming a blank on the substrate. The mesh of the part of the printing plate that needs to be inked is not closed. During printing, the ink penetrates and forms ink marks on the substrate. When printing, pour ink at one end of the screen printing plate. The ink will not move by itself without external force. It leaks onto the substrate through the mesh. When the ink is scraped with a squeegee at a certain tilt angle and pressure, the ink is transferred to the substrate under the screen through the screen, thereby achieving image replication. (The printed pattern is raised)
Advantages of silk screen printing:
A. Low cost and quick results.
B. Adapt to printing on irregular substrate surfaces.
C. Strong adhesion and good inkability.
D. The ink layer is thick and the three-dimensional effect is strong.
E. It has strong rotation resistance, strong optical rotation and good color quality.
F. The printing target materials are wide and the printing format is large.
(2) Pad printing (curved surface printing): refers to using a piece of flexible rubber to print the text and patterns to be printed onto the surface of a plastic molded product with a curved or slightly uneven surface. Pad printing is to first put ink into a gravure plate with engraved text or patterns, then copy the text or pattern onto rubber, then use rubber to transfer the text or pattern to the surface of the plastic molded product, and finally use methods such as heat treatment or ultraviolet light irradiation Allow the ink to cure.
Laser engraving, also called laser engraving, is a surface treatment process that uses optical principles. For example, if you want to make a keyboard, the characters on it are blue, green, red and gray, and the key body is white. When laser engraving, first spray oil, then spray the blue characters, green characters, red characters, and gray characters with corresponding colors, so It looks like there are blue keys, green keys and other keys. Then spray a layer of white on the whole, so that it is a whole white keyboard, with all the blue and green keys covered underneath. Then we use laser technology and the film made from the button diagram submitted by the industrial designer to carve off the white oil on it, revealing the blue and green buttons.
Limitations of laser engraving
(1) Laser engraving depth: 0.1mm for aluminum, 0.2-0.3mm for plastic spray paint coating, 0.08mm for iron and other metals;
(2) The limit surface area of laser engraving is: 100X100mm. If it exceeds 100X100mm, laser engraving needs to be spliced. The joint position will not be beautiful and the joint marks will be obvious.
(3) White and close-to-white colors cannot be carved out, and characters engraved by laser on curved surfaces are prone to deformation.
This article introduces the common surface treatment processes in prototype processing. If you want to know more information about prototype processing, you can contact the sales representative of PROTO MFG.
PROTO MFG provides a wide range of manufacturing capabilities and other value-added services for all of your prototyping and production needs. Visit our website to learn more or to request a free, no-obligation quote.