Common Surface Finishing Process for Prototyping

Anodized Prototypes
Table of Contents

In the entire prototype process, after selecting the appropriate processing method (CNC processing or 3D printing), most prototypes require surface treatment.

The purpose of surface treatment is to increase the corrosion resistance, wear resistance, decoration or other special functions of the product.

There are dozens of surface treatment processes for prototype processing. Let’s introduce what are the common surface treatment processes for prototype processing.

Grinding

Sanding is one of the most common surface treatment techniques.
Generally, rough objects (such as sandpaper with higher hardness particles) are used to change the physical properties of the material surface through friction.
Remove burrs, machining lines, bonding marks and other defects on the surface of the workpiece.
Thereby, the flatness of the workpiece is improved, the roughness is reduced, and the surface of the workpiece is smooth and fine.

Polishing

Sanding & polishing

Polishing uses flexible polishing tools and abrasive particles or other polishing media to modify the surface of the workpiece on the basis of grinding.
Polishing cannot improve the dimensional accuracy or geometric accuracy of the workpiece, but aims to obtain a smooth surface or mirror gloss.
Sometimes it is also used to eliminate gloss (matting).
The surface roughness of the workpiece after the polishing process can generally reach Ra 0.63 ~ 0.01 microns.

Electroplating

Electroplating parts

Electroplating is the use of electrodes to pass current to make the metal adhere to the surface of the object. Its purpose is to change the characteristics or dimensions of the surface of the object. Electroplating is generally divided into wet electroplating and dry electroplating. Wet method is usually called water plating; dry method is usually called vacuum plating.

Water plating is to pass the plating metal through the electrode method to produce ion replacement and adhere to the surface of the plated part. The vacuum plating uses high voltage and high current to make the coating metal instantly vaporize into ions in a vacuum environment and then vaporize to the surface of the plated part.
Water plating has good adhesion, and no other treatment is needed in the later stage; vacuum plating has poor adhesion, and generally requires PU or UV on the surface.
PC cannot be electroplated, and composite parts cannot be water-plated, but can only be vacuum-plated.
The color of water plating is relatively monotonous. Common water plating includes chrome, nickel, gold, etc., while vacuum plating can solve the problem of seven colors.
The surface effect of the workpiece before water plating must be polished to 1500-2000 sandpaper, and then polished before water plating can be carried out. Therefore, water-plated workpieces are generally very expensive.
The effect of vacuum plating and polishing can be slightly worse than 800-1000 sandpaper, so vacuum plating is relatively cheap.

Oxidation

anodized parts

The oxidation treatment of metal is to form a protective oxide film on the metal surface with oxygen or oxidant to prevent metal corrosion. Oxidation is divided into chemical oxidation and electrochemical oxidation (that is, anodic oxidation).

(1) The oxide film produced by chemical oxidation is thin, about 0.3-4um in thickness, porous, has good adsorption capacity, is soft and does not wear-resistant, and has good electrical conductivity. It is suitable for occasions with shielding requirements. It can be painted in various colors and has good adsorption capacity. It can be painted in various colors and painted on its surface, which can effectively improve the resistance of aluminum products. Corrosive and decorative.

(2) The oxide film produced by anodic oxidation is relatively thick, the thickness is generally 5-20um, and the thickness of hard anodic 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 performance, strong heat insulation and heat resistance, and can be colored in various colors.

Aluminum and aluminum alloys are chemically oxidized, especially after anodizing, the oxide film formed on the surface has good protective and decorative characteristics, so it is widely used in aviation, electrical, electronic, mechanical manufacturing and light industry, etc. aspect.

Passivation

Under certain conditions, when the potential of the metal shifts to the positive direction due to the applied anode current or local anode current, the surface state of the metal that was originally active and dissolved will undergo abrupt changes. The dissolution rate of the metal drops rapidly. This sudden change of the surface state is called passivation.
Passivation can improve the passivation performance of metal materials, promote the passivation of metal materials in the use environment, and improve the mechanical strength of the metal. It is one of the most effective ways of corrosion control and enhances the adhesion of the metal to the coating film.

Blackening

The surface is blackened, which is also called bluing. There are two commonly used methods for blackening treatment: the traditional alkaline heating blackening and the late normal temperature blackening.
The protective film obtained by blackening is black, which improves the corrosion resistance and mechanical strength of the metal surface, and can also be used as a good bottom layer for coatings. (Stainless steel cannot be blackened, iron has the best blackening effect)

Phosphating

Phosphating is the process of treating the metal surface with an orthophosphate solution of manganese, zinc, iron and other metals to form an insoluble phosphate protective film.
The protective film formed after the phosphating treatment can improve the insulation and corrosion resistance of the metal, improve the protection and decoration performance of the workpiece, and can also be used as a good bottom layer of the coating.
The metal surface phosphating treatment methods are divided into cold phosphating (normal temperature phosphating), thermal phosphating, spray-less phosphating and electrochemical phosphating.
Phosphating treatment in the automotive industry is the main method of pre-coating painted parts such as automobile panels, cabs, and trunk panels. The phosphating film is required to be fine, smooth, uniform, moderately thick, and have a certain degree of heat resistance.

Sandblasting

Sandblasting

It uses compressed air as the power to form a high-speed jet beam to spray the spray material (copper ore, quartz sand, emery, iron sand, sea sand) to the surface of the workpiece to be processed at high speed.
Due to the impact and cutting action of the abrasive on the surface of the workpiece, the surface of the workpiece obtains a certain degree of cleanliness and different roughness. The mechanical properties of the surface of the workpiece are improved, so the fatigue resistance of the workpiece is improved.
It increases the adhesion between the workpiece and the spray paint coating, extends the durability of the paint film, and is also conducive to the leveling and decoration of the paint.

Spraying

Spraying

Surface painting is one of the most widely used surface processes. Spraying has the following advantages:
(1) It can cover the surface defects of the workpiece after forming;
(2) The surface of the workpiece can be sprayed to obtain a variety of colors, different gloss, different appearance and visual effects and a variety of different hand feelings;
(3) Enhance the hardness and scratch resistance of the workpiece surface;
The spraying effects are: matte, semi-matte, bright (high-gloss), various colors, various textures, etchings, drawing effects (metallic colors can only be drawn), leather effects, and elastic feel effects (Rubber paint) and so on.

Brushing

Brushing stainless steel

Brushing is a surface treatment method that forms lines on the surface of the workpiece by grinding products to achieve a decorative effect.
Brushing can well reflect the texture of metal materials, and can make the metal surface obtain a non-mirror-like metallic luster. According to different surface effects, it can be divided into straight wire (hairline pattern) and chaotic wire (snow pattern).
The quality of the silk pattern is very subjective. Each user has different requirements for the surface line pattern and different preferences for the line pattern effect.
Therefore, it is necessary to have a brushing template to process the user’s favorite and satisfactory effect. Brushing is not suitable for circular arcs (the junction of arc and straight surfaces is very ugly, and the brushing is uneven) and paint surfaces (metallic surfaces can be drawn with fine lines).

Powder coating

powder coating-1

Electrostatic powder coating uses the phenomenon of corona discharge to make the powder coating adsorb on the workpiece.
The process is as follows: the powder coating is fed into the spray gun by the powder supply system by compressed air gas, and the high voltage generated by the high-voltage electrostatic generator is added to the front of the spray gun. Due to corona discharge, dense electric charges are generated near it, and the powder is discharged from the nozzle of the gun. When sprayed, charged paint particles are formed, which are attracted to the workpiece with the opposite polarity under the action of electrostatic force. As the sprayed powder increases, the more electric charge accumulates. When it reaches a certain thickness, static electricity is generated. Repulsion does not continue to absorb, so that the entire workpiece obtains a certain thickness of powder coating, and then the powder is melted, leveled, and solidified by heat, that is, a hard coating film is formed on the surface of the workpiece.

Advantages and disadvantages of electrostatic powder coating: no thinner, non-toxic, no environmental pollution, good coating quality, very high adhesion and mechanical strength, corrosion resistance, short curing time, no primer, low technical requirements for workers, powder recycling The rate is high; the coating is very thick, the surface effect is rippled, not smooth, and it can only process the semi-matte and bright appearance effects.

Printing

Silk Screen Print

(1) Silk screen printing: part of the mesh of the screen printing plate can pass through the ink and miss printing on the substrate; the remaining part of the mesh on the printing plate is blocked and cannot penetrate the ink, forming a blank on the substrate. The mesh of the part to be inked on the printing plate is not closed. The ink penetrates during printing and forms ink marks on the substrate. When printing, pour the ink on one end of the screen printing plate. The ink will not spontaneously under the action of no external force. It leaks on the substrate through the mesh. When the ink is scraped by a squeegee at a certain inclination angle and pressure, the ink is transferred to the substrate under the screen through the screen, thereby realizing image reproduction. (The printed pattern is raised)
The advantages of silk screen printing:
A. Low cost.
B. It is suitable for printing on irregular substrate surface.
C. Strong adhesion and good ink adhesion.
D. Thick ink layer and strong three-dimensional effect.
E. Strong rotation resistance and good color forming.
F. Wide range of printing materials and large printing area.

(2) Pad printing (curved surface printing): refers to using a piece of flexible rubber to print the characters and patterns that need to be printed onto the surface of a plastic molded product with curved or slightly concave-convex surfaces. Pad printing is to first put the ink into the intaglio plate with engraved characters or patterns, then copy the characters or patterns onto rubber, and then transfer the characters or patterns to the surface of the plastic molded product with the rubber, and finally through heat treatment or ultraviolet light irradiation. The ink is cured.

Laser Engraving

Laser Engraving

Laser engraving is a process of surface treatment using optical principles. For example, to make a keyboard, the words on it are blue, green, red and gray, and the key body is white. When laser engraving, spray oil first, and spray the corresponding colors for blue, green, red, and gray. It looks like there are blue keys, green keys and other keys, and then a layer of white is sprayed on the whole, so that it is a whole white keyboard, all blue and green are wrapped under it. Then use the laser technology and the button map submitted by the industrial designer to make the film, carved out the white oil on it, and the blue and green buttons are displayed.
Limitations of Laser Engraving:
(1) Depth: 0.1mm for aluminum, 0.2-0.3mm for plastic spray paint, and 0.08mm for other metals such as iron;
(2) The limit surface area is: 100X100mm. If it exceeds 100X100mm, Laser Engraving needs to be spliced. The position of the connection is not beautiful and the traces of the connection are more obvious.
(3) White and close to white colors cannot be engraved, and the characters produced by the curved surface Laser Engraving are prone to deformation.

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