What is Polishing
Polishing is a surface finishing process that uses physical, chemical or electrochemical means to refine the surface of a material. Polishing can not only make the surface of an object beautiful, smooth, flat, bright or even mirror-like, but also improve the corrosion resistance of the surface of the object.
The purpose of polishing
The core purpose of polishing is to eliminate the microscopic roughness of the surface, reduce scratches or oxide layers, and make the surface smooth, flat, bright or even mirror-like. Polishing can not only enhance the aesthetics of the material, but also optimize its functionality, such as reducing friction, enhancing corrosion resistance, and improving optical properties. Just like polished car parts are less likely to rust, polished mobile phone screen glass can make the touch more sensitive, and so on.
The process of polishing
1. Rough polishing (removal of scratches and macro defects)
To quickly remove deep scratches, oxidized layer or machining marks on the surface of the workpiece, and initially flatten the surface.
Operation points:
Adopt hard polishing wheel (such as brown corundum grinding wheel) or vibration grinder (batch small pieces);
Path planning: cross-polishing along the direction of the original machining marks to avoid localized over-cutting;
Real-time detection: measuring surface roughness every 5 minutes interval (Ra value control at 1.6~3.2μm)
2. Medium polishing (uniform surface transition)
Eliminate the microscopic scratches left by rough polishing, make the surface texture uniform, and lay the foundation for fine polishing.
Operation points:
Replace the flexible media (such as non-woven wheel, nylon brush), with semi-solid polishing agent;
Adopt “figure of 8” or spiral trajectory polishing, to ensure that the coverage rate > 95%;
Cleaning check: after wiping with acetone, observe under 10 times magnification without visible continuous scratches.
3. Fine polishing (mirror effect achieved)
Goal: Achieve ultra-smooth surface with Ra <0.1μm to meet the demand of optical or sealing function.
Operation points:
Use microfiber cloth wheels or polyurethane polishing pads, with precise pH control of the polishing solution;
Operate in a constant temperature and humidity workshop (recommended temperature 22±1℃, humidity 50%±5%);
Real-time monitoring: white light interferometer or atomic force microscope (AFM) online feedback of surface topography.
4. Cleaning (removal of residual contaminants)
Thoroughly remove polish, metal debris and grease to ensure surface purity.
Types of Polishing
Mechanical Polishing
The most common method, using abrasives (sandpaper, grinding wheels) to physically grind surfaces. Ideal for metals and ceramics, it achieves precise roughness control (Ra 0.01–1.6 μm) but risks embedding abrasive particles.
Chemical Polishing
Submerges materials in acidic/alkaline solutions (e.g., HNO3 for aluminum) to dissolve microscopic peaks. Perfect for complex geometries, though requires strict temperature and concentration control.
Electrolytic Polishing (Electropolishing)
An electrochemical process where metal ions dissolve in a charged bath (e.g., stainless steel in H3PO4). Produces ultra-smooth, corrosion-resistant surfaces for medical implants and aerospace components.
Vibratory Polishing
Tumbles small parts in a vibrating container filled with abrasive media. Efficient for mass-produced items like jewelry or screws, but limited to small, durable components.
Buffing
Uses soft cloth wheels with polishing compounds (e.g., rouge for gold) to create decorative shines. Common in automotive trim and tableware, though less precise than mechanical methods.
Barrel Polishing
Rotates parts in a drum with abrasive pellets—a low-cost option for deburring hardware, but slower and prone to part-on-part damage.
Materials that can be used for polishing
1. Stainless steel
Applications: medical equipment, kitchenware, architectural decoration, automobile exhaust pipes.
- Electropolishing is commonly used to remove the surface oxide layer and enhance corrosion resistance.
- Mirror polishing requires the use of diamond paste or alumina wheels.
2. Aluminum and aluminum alloys
Applications: automobile wheels, electronic housings, aerospace components.
- Chemical polishing (alkaline solution) can quickly remove oxide scale, but it is easy to produce “white fog”.
- High-purity aluminum requires ultra-precision polishing (such as hard disk substrates).
3.Copper and copper alloys
Applications: jewelry, musical instruments, electrical components.
Easy to oxidize, need to be polished with rust inhibitor.
Manual polishing often uses wool wheels and red wax.
4.Titanium and titanium alloys
Applications: medical implants (artificial joints), aerospace fasteners.
High hardness, diamond grinding paste or laser polishing is required.
Electrolytic polishing can reduce bacterial adhesion on biological implants.
5.Precious metals (gold, silver, platinum)
Applications: jewelry, high-end electronic contacts.
Soft metals, avoid excessive polishing (easy to deform).
Ammonia + calcium carbonate polishing paste is often used to remove sulfide black spots on silverware.
6.Carbon steel and alloy steel
Application: molds, tools, mechanical parts.
Grinding wheel (60~120 mesh) for rough polishing, oilstone + chromium oxide paste for fine polishing.
After polishing, anti-rust oil should be applied to prevent oxidation.
7.Magnesium alloy
Application: 3C product shell, lightweight automotive parts.
Flammable, wet polishing (water-based coolant) is required.
Chemical polishing uses a nitric acid-phosphoric acid mixture (to control the reaction speed).
What Are the Differences Between Lapping and Polishing?
While both lapping and polishing refine surfaces, their objectives, methods, and outcomes differ significantly:
Aspect | Lapping | Polishing |
Primary Objective | Achieve ultra-flat surfaces (planarity ±0.001mm) and dimensional accuracy, correcting geometric defects (e.g., warping). | Improve surface smoothness (low roughness) or create decorative finishes (e.g., mirror gloss), prioritizing aesthetics or functionality. |
Material Removal | Rigid lapping plate (cast iron/ceramic) + abrasive slurry (diamond/aluminum oxide). Workpiece and plate rotate oppositely. | Soft tools (cloth/foam) + fine abrasives (cerium oxide/silica). Minimal material removal via shear/chemical action. |
Surface Characteristics | Matte finish (Ra 0.01–0.1 μm) Extremely flat (e.g., optical flats <0.1 μm/m²) but non-reflective. | Mirror-like finish (Ra <0.01 μm) May retain slight waviness; gloss prioritized over absolute flatness. |
Typical Applications | Semiconductor wafers, mechanical seals, gauge blocks, bearing races. | Jewelry, medical implants, telescope mirrors, automotive chrome trim. |
Equipment & Cost | Heavy-duty precision machines (rigid setup) Slow per-unit processing, suited for high-precision batches. | Flexible setups (manual to robotic) Faster for decorative/functional finishes. |
What Are the Pros and Cons of Polishing?
Pros
Aesthetic Boost: Polishing turns dull surfaces into eye-catching mirrors (think jewelry or car rims). No more scratches or oxidation stains!
Functionality Upgrade: Smoother surfaces resist corrosion, reduce friction (good for engine parts), and even improve hygiene (medical tools).
Versatility: Works on metals, plastics, glass—you name it. Hand-polish a vintage watch or automate it for mass-produced gadgets.
Cons
Time & Cost: Mirror finishes demand multiple stages. Fancy methods like electrolytic polishing need pricey equipment and skilled labor.
Material Loss: Over-polishing thins components (bad for precision parts like aerospace fittings).
How to Maintain Polished Surfaces?
- Gentle Cleaning: Use microfiber cloths and pH-neutral cleaners (dish soap + water works). Avoid abrasive scrubs!
- Protective Coatings: Apply car wax (for metals) or nano-sealants (for countertops) to block scratches and oxidation.
- Dry Immediately: Water spots stain polished brass/copper—wipe dry after contact.
- Storage Matters: Wrap jewelry in anti-tarnish bags; keep tools in lined cases to prevent friction marks.
- Regular Touch-ups: Buff lightly with polishing paste every 3–6 months to revive dull spots.
Polishing applications in industrial manufacturing: machined parts and castings
Polishing applications for machined parts
1. Removing machining marks
CNC milling/turning parts: Microscopic burrs or knife marks left by the tool path are eliminated by vibration polishing or magnetorheological polishing to avoid assembly interference.
Precision molds: The surface of the injection mold cavity is polished to Ra 0.1μm or less to reduce the demoulding resistance of plastic products and improve the surface finish.
2.Functional optimization
Inner wall of hydraulic cylinder barrel: A combination of honing and electrolytic polishing is used to achieve a mirror effect and reduce piston seal wear.
Gear meshing surface: Superfine polishing reduces tooth surface roughness, transmission noise and heat generation.
Polishing application of castings
1. Surface defect repair
Sand castings: After sandblasting pretreatment, use fiber wheels and alumina paste to remove oxide scale and sticky sand.
Die-cast aluminum alloy: Chemical polishing (NaOH-based solution) quickly dissolves surface pores and improves electroplating adhesion.
2. Flow channel and inner cavity finishing
Engine cylinder: The inner wall is centrifugally polished, and stainless steel abrasive balls are scoured at high speed to clean the residual sand and burrs of the casting.
Complex thin-walled parts (such as pump housings): Electrochemical polishing selectively dissolves protrusions to avoid deformation caused by mechanical polishing.
3. Corrosion resistance enhancement
Stainless steel castings: Electrolytic polishing (phosphoric acid-sulfuric acid solution) removes the surface chromium-poor area.
Cast iron valves: Nickel-based alloy coating and mechanical polishing block pitting caused by exposed graphite phase.
Conclusion
Polishing can significantly enhance the corrosion resistance, wear resistance and aesthetics of products by eliminating microscopic defects and improving surface finish. Whether you need a high-gloss mirror, matte texture, or functional ultra-smooth surface, we can provide the best solution.
Frequently Asked Questions (FAQ)
Q: What exactly is polishing in manufacturing?
A: Polishing is a surface refinement process that removes microscopic imperfections (scratches, oxidation) using mechanical, chemical, or electrochemical methods to achieve smoothness, shine, or functional performance.
Q: How does electropolishing differ from mechanical polishing?
A: Electropolishing dissolves surface layers via an electrochemical reaction, ideal for complex shapes and corrosion resistance. Mechanical polishing relies on abrasives for visual shine but may leave tool marks.
Q: What industries rely most on polishing?
A: Key sectors include aerospace (turbine blades), medical (implants), automotive (chrome parts), electronics (semiconductors), and luxury goods (jewelry).
Q: Is polishing always necessary for metal parts?
A: Not always—it depends on function. Structural components may skip polishing, while surfaces exposed to friction, aesthetics, or hygiene-critical environments require it.
