Polishing Process Explained, learn how to Make Your Parts Shine

Polishing is a processing method that uses mechanical, chemical or electrochemical action to reduce the surface roughness of a workpiece in order to obtain a bright, flat surface

Overview of Polishing

Mirror polishing

Polishing is a processing method that uses mechanical, chemical or electrochemical action to reduce the surface roughness of a workpiece to obtain a bright, flat surface. In general, it is a finishing process on the surface of a workpiece using polishing tools and abrasive particles or other polishing media, usually followed by a very thorough cleaning of the surface prior to the process. The surface of the polished part is smooth and slightly reflective. The result of polishing is an improved gloss and luster of the surface. A mirror-like shiny surface can also be obtained with good polishing.

How does Polishing Work?

Polishing is the act of removing a very thin layer from the surface being polished using a light abrasive product. Polishing removes a very thin layer, which makes the surface of the part shiny and flat. If the surface defect is deeper than what can be removed by polishing, the surface defect will still be visible, although the partial removal of the defect will make it less visible. For example, if a surface defect is 5 microns thick and only 3 microns can be removed by polishing, there will still be 2 microns left. Although the defect is 3 microns less deep and may be less visible, it may still be visible.

Benefits of Polishing

• Ability to seal high pressure gases and liquids
• Cosmetic use
• Ability to use optical flatness measuring instruments
• Reduces the amount of surface and sub-surface damage
• Provides better uniformity for surfaces requiring epitaxial processes or deposited materials
• Produces sharper edges on cutting tools

Choosing the Right Abrasive Material

The success of any polishing project depends heavily on selecting the appropriate abrasive material. The correct choice ensures the efficient removal of surface imperfections and a smooth, polished finish.

To make an informed decision, consider the following factors such as material hardness, grit size, abrasive type, and compatibility, which are discussed below:

Material Hardness

The abrasive material should be harder than the surface being polished. The hardness of the material significantly impacts the abrasive selection. For example, titanium metal polish requires diamonds and other hard abrasives. Meanwhile, steel can be polished with relatively softer abrasives like aluminum oxides. This ensures that the abrasive effectively removes imperfections without being worn down too quickly.

Aluminum oxide: Suitable for polishing metals, wood, and some plastics
Silicon carbide: Ideal for polishing hard materials, such as glass, ceramics, and stone
Diamond: The hardest abrasive, used for polishing extremely hard materials, such as tungsten carbide, ceramics, and gemstones

Grit Size

The size of the abrasive particles, or grit, plays a critical role in determining the smoothness of the final finish. Coarser grits are used for the initial polishing stages to remove significant imperfections, while finer grits are employed for final polishing to achieve a smooth and glossy surface. Grit size is typically indicated by a number, with lower numbers representing coarser grits and higher numbers denoting finer grits. Some examples include:

60-80 grit: Coarse grit for aggressive material removal and leveling deep scratches or uneven surfaces
180-220 grit: Medium grit for smoothing surfaces and removing shallow scratches or marks
600-1000 grit: Fine grit for final polishing stages and achieving a smooth, reflective finish

Abrasive Type

Abrasive materials can be categorized into two main types: bonded and coated abrasives. The choice between these types depends on the polishing technique and the specific requirements of your project:

Bonded Abrasives: These abrasives are formed by bonding abrasive grains together with a resin, ceramic, or other bonding agent. Examples include grinding wheels, cutting discs, and sharpening stones. Bonded abrasives are ideal for heavy material removal and precision grinding.
Coated Abrasives: In this type, abrasive grains are attached to a flexible backing, such as paper, cloth, or plastic film. Examples include sandpaper, abrasive belts, and polishing pads. Coated abrasives are suitable for surface preparation, smoothing, and finishing.

Compatibility with Polishing Equipment

Consider the compatibility of the abrasive material with your polishing equipment. For example, some abrasive materials may be specifically designed for use with certain polishing machines or tools. Be sure to consult the manufacturer’s recommendations and guidelines to ensure optimal performance and prevent damage to your equipment.

By taking into account the factors mentioned above, you can make an informed decision and select the most appropriate abrasive material for your polishing project, ensuring a successful and satisfying result.

Types of Polishing

 Mechanical polishing

This polishing method is based on plastic deformation or cutting of the material surface to obtain a smooth surface by removing the polished convex surface. Mechanical polishing generally uses abrasive rods, felt wheels, and sandpaper, and is mainly manual. Rotating body and other special parts can use auxiliary tools such as turntables, and ultra-precision polishing can be used for high surface quality requirements.

Ultra-precision polishing is to use special abrasives to rotate the processed surface of the workpiece at high speed by pressing it in the polishing solution containing abrasives. A surface roughness of 0.008μm can be achieved using this technique, which is the best among various polishing methods. This method is often used for optical lens molds.

Advantages

1. High brightness
2. Better surface cleanability
3. Higher aesthetic appeal
4. Reduced product adhesion
5. Better surface finish

Disadvantages

1. High labor cost
2. Cannot handle complex part structures
3. Shine may be consistent and may not last long
4. May be susceptible to corrosion

Chemical polishing

Chemical mechanical polishing

This type of polishing uses the principle that the protruding parts of the material surface are preferentially dissolved in the chemical medium, thus making the surface of the workpiece smooth after the chemical reaction is completed. The core of chemical polishing is the preparation of the polishing solution, which can achieve surface roughness of several 10 μm, but the direct result of chemical polishing is the smoothing and polishing of micro-rough parts. It also leads to parallel dissolution of the upper layer of the part.

Advantages of chemical polishing

Possibility to polish complex shapes as no direct manual involvement is required

High efficiency

Possibility to polish several parts at the same time

Reduced investment in equipment

Good corrosion resistance, allowing the formation of a passivation layer on the surface of the part

Disadvantages of chemical polishing

Uneven brightness

Difficult to perform heat treatment

Gas easily spills

Not environmentally friendly, may release harmful gases

Difficult adjustment and regeneration of polishing solution

Electrolytic polishing

Stainless steel electrolytic polishing

The basic principle of electrolytic polishing is the same as chemical polishing, both use the dissolving solution to dissolve the small protrusions on the surface and get a smooth surface. However, compared with chemical polishing, the effect of cathodic reaction can be eliminated and the polishing effect is better. Electropolishing removes material from metal workpieces, reduces surface roughness and improves surface finish by smoothing out micro peaks and valleys. The process of electrochemical polishing can be divided into two steps, firstly, macro polishing, where the dissolution products diffuse into the electrolyte, making the material surface roughness decrease, rather>1μm, and then anodic polarization, making the surface brightness increase. Ra<1μm.

Advantages

1. Longer lasting luster
2. Consistent color inside and out
3. A wide range of materials can be treated
4. Low cost and short cycle time
5. Encourages less contamination
6. High corrosion resistance

Disadvantages

1. High fixed investment
2. Complex pre-polishing process
3. Tools and auxiliary electrodes required for complex parts
4. Poor versatility of electrolyte

Polishing is usually the last process in manufacturing and is one of the keys to ensure that prototypes or mass production meet the standards. It is extremely important to our customers that the surface of the part is bright and flat through precise and high-quality polishing. You can check out our surface treatment services for more information.Prolean Tech’s surface finishing services offer standard and popular finishes for parts. Our CNC machines and other surface finishing technologies are capable of achieving tight tolerances and high quality, uniform surfaces for all types of parts. Simply upload your CAD file for a quick, free quote and consultation on related services.