The selection of the appropriate stainless steel polishing method should be determined based on the purpose, cost, and surface quality requirements of the workpiece. For example, for food and medical equipment, electrolytic polishing can improve corrosion resistance; while for decorative items, mirror polishing can provide better visual effects.

Through scientific polishing technology, stainless steel can not only improve its aesthetics but also enhance its corrosion resistance, thereby extending its service life. If you have a demand for stainless steel products, please contact us at Huaxiao Metal for a quote and professional support!

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Introduction to Stainless Steel Polishing Methods

Stainless steel is widely used in various industries due to its excellent corrosion resistance and beautiful surface effects. However, to achieve a smooth and bright surface, stainless steel needs to be polished. The following are several common stainless steel polishing methods and their characteristics:

Mechanical Polishing

Mechanical polishing is a polishing method that relies on cutting and plastic deformation of the material surface to remove the polished convexities to obtain a smooth surface. Generally, oil stone strips, wool wheels, sandpaper, etc. are used, and special parts such as rotating bodies are mainly used. For the surface, auxiliary tools such as turntable can be used, and the method of ultra-precision polishing can be used for high surface quality requirements.

Ultra-precision polishing is the use of special abrasive tools, which are pressed tightly on the processed surface of the workpiece in a polishing fluid containing abrasives for high-speed rotation. Using this technology, the surface roughness of Ra0.008μm can be achieved, which is the highest among various polishing methods. Optical lens molds often use this method.

Brush Stainless Steel Surface, No 4 Finish Stainless Steel

Chemical Polishing

Chemical polishing is to make the surface microscopic convex part of the material in the chemical medium dissolve preferentially than the concave part, to obtain a smooth surface. The main advantage of this method is that it does not require complex equipment, can polish workpieces with complex shapes, and can polish many workpieces at the same time, with high efficiency. The core problem of chemical polishing is the preparation of polishing liquid. The surface roughness obtained by chemical polishing is generally several 10 μm.

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Electrolytic Polishing

The basic principle of electrolytic polishing is the same as that of chemical polishing, that is, it relies on the selective dissolution of small protrusions on the surface of the material to make the surface smooth. Compared with chemical polishing, the effect of the cathode reaction can be eliminated, and the effect is better.

The electrochemical polishing process is divided into two steps:

“(1) Macroscopic leveling” The dissolved product diffuses into the electrolyte, and the geometric roughness of the material surface decreases, Ra>1μm.

(2) Low-light leveling, anodic polarization, improved surface brightness, Ra<1μm.

Ultrasonic Polishing

Ultrasonic polishing puts the workpiece in the abrasive suspension and puts it together in the ultrasonic field, relying on the oscillation effect of the ultrasonic, so that the abrasive is ground and polished on the surface of the workpiece. Ultrasonic machining has a small macroscopic force and will not cause deformation of the workpiece, but it is difficult to manufacture and install tooling. Ultrasonic processing can be combined with chemical or electrochemical methods.

Based on solution corrosion and electrolysis, ultrasonic vibration is applied to stir the solution, so that the dissolved products on the surface of the workpiece are separated, and the corrosion or electrolyte near the surface is uniform; the cavitation effect of ultrasonic in the liquid can also inhibit the corrosion process and facilitate surface brightening.

Fluid Polishing

Fluid polishing relies on the high-speed flow of liquid and abrasive particles carried by scouring the surface of the workpiece to achieve the purpose of polishing. Commonly used methods are abrasive jet processing, liquid jet processing, hydrodynamic grinding, and so on. Hydrodynamic grinding is driven by hydraulic pressure to make the liquid medium carrying abrasive particles flow back and forth across the surface of the workpiece at high speed. The medium is mainly made of special compounds (polymer-like substances) with good flowability under lower pressure and mixed with abrasives. The abrasives can be made of silicon carbide powder.

Magnetic Abrasive Polishing

Magnetic Abrasive Polishing uses magnetic abrasives to form abrasive brushes under the action of a magnetic field to grind the workpiece. This method has high processing efficiency, good quality, easy control of processing conditions, and good working conditions. Using suitable abrasives, the surface roughness can reach Ra0.1μm.

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Mechanical Polishing Basic Method

The polishing mentioned in plastic mold processing is very different from the surface polishing required in other industries. Strictly speaking, the polishing of the mold should be called mirror processing. It not only has high requirements for polishing itself, but also has high standards for surface flatness, smoothness, and geometric accuracy. Surface polishing generally only requires a bright surface.

The standard of mirror surface processing is divided into four levels: AO=Ra0.008μm, A1=Ra0.016μm, A3=Ra0.032μm, A4=Ra0.063μm. It is difficult to precisely control the geometric accuracy of parts due to methods such as electrolytic polishing and fluid polishing. However, the surface quality of chemical polishing, ultrasonic polishing, magnetic abrasive polishing, and other methods is not up to the requirements, so the mirror processing of precision molds is still mainly mechanical polishing.

Mirror Polish Stainless Steel Sheet with Film

Mechanical Polishing Basic Procedure

To obtain high-quality polishing results, the most important thing is to have high-quality polishing tools and auxiliary products such as oilstone, sandpaper, and diamond lapping paste. The choice of polishing procedure depends on the surface condition after pre-processing, such as machining, EDM, grinding, and so on.

The general process of  mechanical polishing is as follows:

(1) Rough Polishing

The surface after milling, EDM, grinding, etc. can be polished by a rotating surface polishing machine or ultrasonic grinder with a speed of 35 000-40 000 rpm. The commonly used method is to use wheels with a diameter of Φ3mm and WA # 400 to remove the white spark layer. Then there is manual whetstone grinding, and the strip whetstone is added with kerosene as a lubricant or coolant. The general order of use is #180 ~ #240 ~ #320 ~ #400 ~ #600 ~ #800 ~ #1000. Many mold manufacturers choose to start with #400 to save time.

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(2) Semi-Precision Polishing

Semi-precision polishing mainly uses sandpaper and kerosene. The number of sandpaper is #400 ~ #600 ~ #800 ~ #1000 ~ #1200 ~ #1500. #1500 sandpaper only uses die steel suitable for hardening (above 52HRC), not pre-hardened steel, because it may cause burns on the surface of pre-hardened steel.

(3) Fine Polishing

Fine polishing mainly uses diamond abrasive paste. If the polishing cloth wheel is mixed with diamond abrasive powder or abrasive paste for polishing, the usual polishing order is 9μm(#1800)~ 6μm(#3000)~3μm(#8000). 9μm diamond abrasive paste and polishing cloth wheel can be used to remove the hair-like wear marks left by #1200 and #1500 sandpaper. Then use sticky felt and diamond paste for polishing, the order is 1μm (#14000) ~  1/2μm (#60000) ~ 1/4μm (#100000).

The polishing process with precision requirements above 1μm (including 1μm) can be carried out in a clean polishing room in the mold processing workshop. For more precise polishing, an absolutely clean space is necessary. Dust, smoke, dandruff, and saliva can all scrap the high-precision polished surface obtained after hours of work.

Problems to Pay Attention to in Mechanical Polishing

The following points should be paid attention to when polishing with sandpaper:

(1) Polishing with sandpaper requires the use of softwood or bamboo sticks. When polishing round or spherical surfaces, the use of cork rods can better match the curvature of the round and spherical surfaces. The harder wood strips, like cherry wood, are more suitable for polishing flat surfaces. Trim the ends of the wooden strips so that they can be consistent with the surface shape of the steel parts, to prevent the sharp angles of the wooden strips (or bamboo strips) from contacting the surface of the steel parts and causing deep scratches.

(2) When changing to a different type of sandpaper, the polishing direction should be changed from 45° to 90°, so that the streaks and shadows left by the previous type of sandpaper after polishing can be distinguished. Before changing to a different type of sandpaper, you must carefully wipe the polishing surface with 100% pure cotton moistened with a cleaning solution such as alcohol, because a small grit left on the surface will destroy the entire subsequent polishing work.

This cleaning process is equally important when changing from sandpaper polishing to diamond paste polishing. Before polishing can continue, all particles and kerosene must be completely cleaned.

(3) In order to avoid scratches and burns on the surface of the workpiece, special care must be taken when polishing with #1200 and #1500 sandpaper. Therefore, it is necessary to apply a light load and use a two-step polishing method to polish the surface. When polishing with each type of sandpaper, polishing should be done twice in two different directions, and each rotation between the two directions is 45°~90°.

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Factors Affecting Mold Polishing Quality

The effect of different hardness on the polishing process. The increase in hardness increases the difficulty of polishing, but the roughness after polishing decreases. As the hardness increases, the polishing time required to achieve a lower roughness increases accordingly. At the same time, the hardness increases, and the possibility of over-polishing is reduced accordingly.

The effect of the surface condition of the workpiece on the polishing process. During the crushing process of the cutting machine, the surface layer will be damaged due to heat, internal stress, or other factors, and improper cutting parameters will affect the polishing effect. The surface after EDM is more difficult to grind than the surface after ordinary machining or heat treatment. Therefore, before the end of EDM, it should be trimmed with precision EDM, otherwise, the surface will form a hardened thin layer.

How to Choose the Right Polishing Method for Stainless Steel?

According to Surface Quality Requirements:

  • If you are looking for a high-gloss mirror effect: choose mechanical polishing or electrolytic polishing.
  • If you have high requirements for corrosion resistance: electrolytic polishing is preferred.

According to Product Shape and Size:

  • Complex and irregular workpieces: chemical polishing or magnetic abrasive polishing is recommended.
  • Small precision workpieces: ultrasonic polishing is selected.

According to Processing Cost and Efficiency:

  • Cost-sensitive batch processing: mechanical polishing or chemical polishing is selected.
  • High-end, high-precision applications: fluid polishing or electrolytic polishing is suitable.

Different stainless steel polishing methods have their own advantages and disadvantages and should be selected according to the specific purpose, shape, cost, and surface finish requirements of the workpiece. If you have any stainless steel product polishing needs or need some guidance and suggestions from us, please contact us at Huaxiao Metal!

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