Polishing Techniques and Principles

Release Date:2023-07-12 11:33

Polishing is a process that involves using polishing wheels, polishing pastes, and fine abrasives to lightly cut and grind the surface of a part. The objective is to remove minor imperfections and achieve a smooth surface finish, improving its glossiness. Polishing is commonly applied to various metals to remove oxidation films that quickly form in the presence of air. It does not involve metal removal but rather focuses on eliminating the oxide film, accounting for approximately 5% to 20% of the weight of the coating.


Principle of Polishing:


During the polishing process, the abrasive initially removes the oxidized film on the raised areas of the surface while leaving the recessed areas untouched. Once the pure metal is exposed, it quickly oxidizes again and is subsequently removed through polishing. This repetitive process continues until a shiny surface is achieved. Polishing does not involve metal cutting but specifically targets the removal of the oxide film. The loss accounts for only a small percentage of the coating weight.


Polishing Procedure:


The polishing wheel's peripheral speed should be adjusted to 20-35 m/s. For parts with simple shapes and harder surfaces, higher speeds are suitable, while lower speeds are more appropriate for complex shapes. The selection of an appropriate polishing paste depends on the material of the part. Polishing paste consists of a binder and abrasive materials. The friction generated during polishing causes the binder in the polishing paste to melt, facilitating the polishing process. When polishing, the part is pressed against the polishing wheel at the appropriate position. The force applied polishing duration, and hand movements depend on the experience of the polisher. Polishing can be performed as a pre-processing step or as a post-plating finishing process.

 


Types of Polishing Paste:


Commonly used polishing pastes include white, yellow, green, and red polishing pastes.


1. White Polishing Paste: It consists of stearic acid, lime, grease, and paraffin. White polishing paste should not be stored for extended periods as it is prone to weathering and deterioration. It is generally used in containers during the summer.


2. Yellow Polishing Paste: It consists of stearic acid, iron oxide, feldspar powder, grease, and rosin.


3. Green Polishing Paste: It consists of stearic acid, chromium oxide, and fatty acids.


4. Red Polishing Paste: It consists of stearic acid, and fine iron oxide powder, and exhibits excellent polishing performance with minimal damage to the base material.


Batch Surface Finishing:


Manual grinding and polishing processes are labor-intensive, often conducted under poor working conditions, and have low productivity. To overcome these limitations, batch surface finishing methods are employed for smaller parts. Batch surface finishing involves placing the parts, abrasive media, water or oil, and chemical additives into specialized containers. Through vibration and rotation, friction is generated between the parts and the abrasive media, achieving tasks such as rust removal, oil removal, burr removal, and chamfering while reducing surface roughness. Depending on the processing method, batch surface finishing includes techniques such as rolling, vibratory polishing, centrifugal finishing, centrifugal disk finishing, brushing, and sandblasting.


1. Rolling: Rolling is commonly achieved using hexagonal wooden or iron drums filled with parts and water mixed with acid or alkaline solutions and surfactants to aid in oil removal, rust removal, and surface roughness reduction. This method is simple, cost-effective, and suitable for pre-treatment processes such as galvanization of steel parts. However, it is not suitable for parts with threads as the threads can be damaged. Heat-treated self-tapping screws, with their high surface hardness and lightweight, can still undergo rolling without damaging the threads.


2. Vibratory Polishing: Vibratory polishing is an efficient and versatile surface leveling process that evolved from drum rolling. It has been widely used internationally and has been adopted domestically. Vibratory polishing involves burying the parts in a large quantity of abrasive media and adding water-soluble polishing compounds. Through vibration, the abrasive media and compounds mix and continuously impact the parts' surfaces, resulting in smoothing and leveling. This process can remove burrs, and chamfer edges, and remove rust and oxide layers. It is suitable for parts of various sizes and shapes.


3. Centrifugal Finishing: Centrifugal finishing is performed by placing parts and abrasive media in a rotating turret while the container rotates in the opposite direction at high speed. The rotating turret generates a centrifugal force of up to 0.98 N, pressing the parts together. The rotation of the container causes sliding friction between the abrasive media and the parts, achieving the desired surface finish.


4. Centrifugal Disk Finishing: Centrifugal disk finishing is carried out using a bowl-shaped disk that rotates at approximately 10 m/s, installed in a fixed cylindrical container. The parts and abrasive media are placed in the container. The rotation of the bottom bowl-shaped disk causes the loaded materials to move upward along the container wall and then slide down to the center due to their weight. This repetitive motion leads to the desired surface polishing and finishing. This method is characterized by high speed similar to centrifugal finishing and superior surface finish quality comparable to vibratory finishing. It is an open process that allows operators to inspect the parts during the operation.


5. Brushing: Brushing involves mounting a brush wheel on a polishing machine and using the elastic metal wires on the brush wheel to cut off rust or dirt from the metal surface. Brushing does not change the geometric shape of the part. Brush wheels are generally made of materials such as steel wire or brass wire. For parts with harder surfaces, brushes made of harder metal wires should be selected. The rotational speed of the brush wheel ranges from 1200 to 2800 rpm. For larger-diameter brush wheels and harder material parts, higher speeds are recommended. Various types of brush wheels are available based on their production method and shape.


6. Sandblasting: Sandblasting involves using compressed air to propel sand particles onto the surface of a part to remove oxidation, rust, or dirt. Quartz sand is commonly used for sandblasting purposes. The size of the sand particles and the compressed air pressure used in sandblasting are essential factors to consider. The consumption of sand particles and compressed air is directly proportional to the nozzle diameter, compressed air pressure, and operating time.


After sandblasting, it is crucial to promptly perform surface treatment on the parts to prevent rapid oxidation and rusting of the clean surface. One method is to immerse the parts in a 50g/L sodium carbonate solution for several days.


Polishing plays a vital role in achieving smooth and glossy surfaces for various metal parts. By utilizing polishing wheels, polishing pastes, and fine abrasives, imperfections, and oxidation films can be effectively removed, resulting in improved surface quality. The polishing process involves careful adjustment of peripheral speeds, selection of suitable polishing pastes, and proper application of pressure and polishing duration. Additionally, batch surface finishing methods such as rolling, vibratory polishing, centrifugal finishing, centrifugal disk finishing, brushing, and sandblasting provide efficient and effective solutions for mass surface treatment. Each method offers unique advantages, enabling manufacturers to achieve desired surface finishes while improving productivity.

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