There are different types of polishing for different materials. The polishing types include mechanical polishing, chemical polishing, electropolishing, and nano polishing. These polishing methods refine metal surfaces and achieve lower surface roughness values.
The basic difference between these polishing types is the polishing process. For example, mechanical polishing involves the use of abrasives for material removal, while chemical polishing uses a chemical solution to achieve surface smoothness on the micro scale.
Each polishing method is suitable for different surface smoothness and finish. Read below to understand how to pick the correct type of polishing method for your project.
What Is Polishing Surface Finish?
Polished surface finish or polishing surface finish refers to the final texture and reflectivity achieved on a metal surface post-polishing methods. The result is often reduced surface imperfections, removal of light rust, and achieving a mirror-like finish.
In polished surface finishes, the surface topography is altered at a microscopic level to even out bumps and improve surface integrity and roughness average.
Surface roughness is measured by the Ra value (roughness average), and polishing reduces the surface roughness, which improves corrosion resistance. For stainless steel, polishing lowers Ra below 0.1 micrometers, resulting in a mirror-like finish.
Using polishing to achieve a consistent surface finish ensures that metal parts are functional and achieve surface integrity.
Steps of Polishing Process
The polishing process refines metal surfaces through a staged surface, ensuring minimal surface damage for critical components in foot processing plants and aerospace parts.
The process of polishing involves four sequential process stages:
Roughing
Roughing is the first step in the polishing process, which removes large bumps and surface imperfections from a rough surface. In roughing, coarse-grit abrasive (60 – 80 grit) is mainly used on mechanical tools such as grinders.
Light rust, paint, and any sticky particles are removed at this stage. For alloy steels like carbon steel, the surface roughness is brought down to 1 – 2 Ra as a preparatory step for further polishing.
Roughing
The polishing stage uses medium abrasives (120 – 400 grit) on polishing pads to further lower the surface roughness. In the polishing stage, material removal removes any micron surface defects, achieving an Ra 0.4 – 0.8 micrometers.
Manual polishing, rotary buffing, and vibratory finishing are some of the polishing techniques to achieve the desired smoothness.
Brightening
Brightening uses fine polishing compounds to create a shiny surface. Brightening is a low-pressure step that refines stainless steel to Ra 0.1 – 0.2 micrometers. Such surface roughness is widely used in stainless steel parts for surgical instruments.
Protection
Protection is a passivating step. It is a final treatment or coating to improve corrosion resistance. Coatings preserve the surface finish of metal parts to make the polished finish durable and long-lasting.
Protection uses passivators like acids to form a passive film that offers increased protection to the polished surface.
Types Of Polishing
Mechanical Polishing
Protection is a passivating step. It is a final treatment or coating to improve corrosion resistance. Coatings preserve the surface finish of metal parts to make the polished finish durable and long-lasting.
Protection uses passivators like acids to form a passive film that offers increased protection to the polished surface.
Chemical Polishing
In chemical polishing, the material removal is done using a chemical solution. The chemical solution reacts selectively with high points on the surface of the metal, and this smooths the irregularities.
Some of the chemicals used include phosphoric acid, sulfuric acid, and nitric acid. Chemical polishing is suitable for complex shapes and geometries. Parts with internal features and undercuts are generally refined through chemical polishing.
Electropolishing
Electropolishing is an electrochemical polishing method that uses an electric current to dissolve a thin layer from the metal surface.
Electropolishing works by connecting the positive terminal with the workpiece while a cathode serves as the negative terminal in a chemical bath. Electropolishing first dissolves the metal from the surface peaks through current. This results in a polished, bright surface.
Electropolishing is expensive and is mostly used for parts where achieving a smooth, mirror-like surface is essential.
Nano Polishing
Nano polishing uses ultra-fine abrasives and precise control over the contact surface to achieve finishes at the microscopic level.
Nanopolishing is mostly used in electronics, silicon semiconductor wafers, and chip-making applications.
When To Use Mechanical Polishing VS Chemical Polishing
The choice between mechanical and chemical polishing mostly depends on the material and geometry. Mechanical polishing suits flat metal parts where surface smoothness is required on certain zones and parts. For instance, a cylindrical part with an external tip that requires a certain surface finish.
It is ideal for manual polishing and low-volume parts. Chemical polishing is ideal for complex geometries with unreachable sections. Both types of polishing enhance corrosion resistance in stainless steel.
| Criteria | Mechanical Polishing | Chemical Polishing |
| Geometry | Suitable for flat or simple surfaces | Suitable for complex geometries and internal features |
| Material Removal | Controlled via grit size and mechanical tools | Driven via chemical bath and nitric acid |
| Surface Roughness | Achieves Ra 0.1–1 µm, adjustable | Achieves Ra |
| Corrosion Resistance | Moderate improvement | High resistance; forms a passive film |
| Applications | Tooling and automotive metal parts | Surgical instruments and food processing |
| Labor/Cost | Labor-intensive; involves manual polishing | Automated, efficient management costs |
| Surface Defects | Removes scratches and burrs | Etches and highlights surface imperfectio/td> |
In some parts, it is possible to use a hybrid chemical-mechanical polishing for precision applications, optimizing surface finish.
Types of Polishing Tools
Polishing tools vary by polishing method
Mechanical tools: Grinders, lathes, orbital sanders
Polishing pads: Foam, wool, microfiber
Electropolishing equipment: Power supplies, electrolyte tanks
Manual polishing kits: For small-scale or delicate work
Types of Polishing Pads
Polishing pads are tools used in the polishing process to refine the surface finish of different materials.
Foam pads: Versatile, used for moderate surface finish
Wool pads: Ideal for heavy-duty metal polishing. Wool pads can cut surface imperfections on carbon steel, but high speed and the use of force can produce marks.
Microfiber pads: – Excellent for removing swirl marks and fine surface defects.
Hybrid pads: Combine foam and fiber for specialized tasks
Ceramic Pads: Ceramic pads are durable for high-heat production processes and support mechanical polishing of hard surface metals such as titanium.
Best Methods To Get a Perfect Polished Surface Finish on Metals
To achieve a perfect polished surface finish on metals, you can follow these engineering practices.
Prepare the surface: It is important to remove impurities that can interfere with the polishing process. These include oils, grease, and chips. Degrease metal surfaces using solvents to ensure a clean surface.
Mechanical Polishing: It is important to start with sequential abrasives and step up to higher grits. The starting grit depends on the material; for most steels, 200 grit is a good starting point to reduce surface roughness.
Chemical Polishing: Chemical solutions with an acidic formula are used to etch the surfaces of complex geometries and smooth surface imperfections.
Protection: Apply citric acid or nitric acid to form a passive film
Some common process tips include:
Use of compatible polishing pads and polishing compounds
Monitoring surface area, temperature, and pressure
Conclusion
Types of polishing include four polishing methods: mechanical polishing, chemical polishing, electropolishing, and nano polishing for different Ra values, materials, and applications. The main objective of all polishing types is to achieve a finer and improved surface finish on metal parts.
FAQs
Q: What are the three types of polishing?
A: Mechanical polishing, chemical polishing, and electropolishing are the main polishing types. Nano polishing is an additional polishing method, but it is largely used in the semiconductor industry.
Q: What are the three stages of polishing?
A: Roughing, polishing, and brightening reduce surface roughness and produce a shiny, polished surface. At the end, a protective coat is applied to make the polish durable.
Q: What is polishing vs. buffing?
A: Polishing reduces surface roughness with abrasives; buffing (or brightening) uses fine polishing compounds to create a mirror-like finish.
Q: Are there different types of polish?
A: There are different types of polishing: mechanical polishing, chemical polishing, electropolishing, and nano polishing.
Q: What is the difference between mechanical polishing and electropolishing?
A: Mechanical polishing uses physical abrasives to remove material for surface smoothening, while electropolishing uses electrical current and chemical solutions in a bath to etch away protruding imperfections from a surface.