The polishing process we commonly use
Aluminium surface polishing is performed to improve the surface quality of aluminium parts, making them smoother and shinier, thereby enhancing their aesthetic appearance and surface corrosion resistance. The process of polishing aluminium parts can be divided into several stages, each with different requirements and methods of treatment. Below are the common processes involved in aluminium surface polishing:
Coarse polishing
Coarse polishing is the first stage in the aluminium surface polishing process, aimed at removing larger defects, the oxide layer, burrs, and irregular surface layers from the aluminium parts making the surface more even and smooth. The goal of this stage is to lay the foundation for subsequent fine polishing and mirror polishing. Therefore, it is a crucial step in the entire polishing process.
The main purpose of coarse polishing is to:
- Remove larger defects on the aluminium surface, such as scratches, dents, oxide layers, stains, and so on.
- Smooth the aluminium surface and eliminate irregular surface structures.
- Create a smooth foundation layer for subsequent fine polishing and mirror polishing.
- Improve the surface finish and reduce the workload in subsequent processes.
The table below outlines the main materials for coarse polishing and their advantages and disadvantages:
| Tool/Material | Advantages | Disadvantages |
| Sandpaper (40# – 600#) | 1. Affordable and easy to obtain. 2. Various grit sizes are available to suit different needs. | 1. Lower grinding efficiency, suitable for larger surface defects. 2. May produce excess dust during use. |
| Polishing Compound (Coarse Polishing Compound) | 1. Enhances surface gloss and removes scratches. 2. Can be used with different types of polishing wheels. | 1. Slightly corrosive, caution is needed to avoid surface damage. 2. Requires careful control of usage. |
| Polishing Wheel (Hard Polishing Wheel) | 1. Efficiently removes larger defects and oxide layers. 2. Long lifespan, durable. | 1. It needs to be used with a polishing compound for the best results. 2. High wear, suitable for coarser initial polishing. |
| Diamond Grinding Wheel | 1. Good grinding effect, quickly removes large areas of oxide layers and defects. 2. Very durable, ideal for harder materials. | 1. Higher cost. 2. Requires more specialised equipment for operation. |
| Wool Wheel | 1. Relatively soft, suitable for coarse and medium-fine polishing stages. 2. Works well with polishing compound. | 1. May shed fibres during use, affecting longevity. 2. Not suitable for very rough surfaces. |
| Nylon Wheel | 1. Highly effective at removing surface oxides and burrs. 2. Long lifespan, highly durable. | 1. Surface finish is not as glossy as wool or cloth wheels. 2. May cause slight scratches on certain materials. |
Note:
- The coarse polishing stage is primarily used to remove the oxide layer, larger defects, and scratches from the aluminium surface. Therefore, when selecting materials, the focus should be on removal efficiency and surface treatment results.
- The advantages and disadvantages of different materials determine their suitability, and the choice of material should generally be based on the condition of the aluminium surface and the specific polishing requirements.
The effect of coarse polishing.
- After coarse polishing, the aluminium surface should appear relatively even and smooth, with obvious defects and the oxide layer removed. However, the gloss is still low, and the surface remains somewhat rough. At this stage, the aluminium part is ready for the subsequent fine polishing and mirror polishing.
- The effect of coarse polishing directly impacts the results of the subsequent polishing steps. If the coarse polishing is insufficient, surface defects may be carried into the next stage by the fine polishing, affecting the final finish. Therefore, the quality of the coarse polishing stage is crucial to the final result.
Fine polishing
Fine polishing is a crucial stage in the aluminium surface polishing process, typically performed after coarse polishing. The goal is to further improve the surface finish, making it smoother and more even, thereby laying the foundation for mirror polishing or the final treatment. Fine polishing not only enhances the appearance of the aluminium parts but also improves their corrosion resistance and oxidation resistance.
The main purpose of fine polishing：
- Improve surface smoothness: The main purpose of fine polishing is to remove the fine scratches and defects left by coarse polishing, further enhancing the surface smoothness.
- Remove tiny defects: Eliminate residual scratches, oxide layers, oil stains, and other imperfections left by coarse polishing, making the surface more even.
- Enhance the finish: Through fine polishing, the aluminium surface becomes shinier, laying the foundation for mirror polishing and the final surface treatment.
- Improve corrosion resistance: After fine polishing, the surface becomes smoother, which helps enhance the corrosion resistance and oxidation resistance of the aluminium parts.
The table below outlines the main materials for fine polishing and their advantages and disadvantages.
| Tool/Material | Advantages | Disadvantages |
| Fine Grit Sandpaper (400#-800#) | 1. Suitable for most fine polishing needs. 2. Easy to control and simple to use. | 1. Prone to wear, requiring more time. 2. May leave slight scratches. |
| Wool Wheel | 1. Provides an even polishing effect. 2. Enhances surface gloss. | 1. Can be difficult to control polishing intensity. 2. Prone to wear, requiring regular replacement. |
| Cloth Wheel | 1. Softer and more even, ideal for delicate surface polishing. 2. Suitable for larger surfaces. | 1. Wears out quickly under high-intensity use. 2. May have limited applicability. |
| Polishing Compound | 1. Provides gloss and a smoother surface. 2. Enhances the corrosion resistance of aluminium parts. | 1. Requires even application. 2. May leave residue, requiring cleaning. |
| Water-based Polishing Fluid | 1. Eco-friendly and non-toxic, suitable for long-term use. 2. Does not generate excessive heat. | 1. Results are relatively mild and may require multiple applications. 2. Suitable for regular cleaning, not for heavy removal. |
| Nylon Wheel | 1. Durable and effectively removes oxide layers. 2. Suitable for finer work. | 1. May not achieve very high gloss. 2. Less effective on harder impurities. |
The effect of fine polishing
After fine polishing, the surface of the aluminium part should become smoother, more even, and exhibit a higher level of gloss. At this point, the fine scratches left by coarse polishing have been removed, greatly enhancing the smoothness and appearance. The surface of the aluminium part, now more refined, is not only visually more attractive but also smoother, which helps improve its corrosion resistance. This provides a solid foundation for mirror polishing or subsequent treatments.
Mirror polishing
Mirror polishing is an advanced stage in the aluminium surface polishing process. The goal is to achieve a mirror-like finish on the aluminium part through fine grinding and polishing, resulting in a surface that is smooth and flat, resembling a mirror. Mirror polishing not only significantly enhances the appearance and aesthetic appeal of the aluminium part but also increases its corrosion resistance and wear resistance.
The main purpose of mirror polishing:
- Enhance the finish: To make the aluminium surface highly smooth and flat, with an exceptionally high reflective gloss.
- Enhance aesthetics: Mirror polishing can significantly improve the appearance of the aluminium part, giving it a mirror-like reflective effect. It is suitable for high-end decorative applications and precision requirements.
- Improve corrosion resistance: Through mirror polishing, micro-cracks and impurities on the aluminium surface are removed, reducing the attachment of oxides and enhancing the corrosion resistance of the aluminium part.
- Reduce friction: The surface treated with mirror polishing has a lower coefficient of friction, making it suitable for certain mechanical components to reduce wear.
The table below outlines the main materials for mirror polishing and their advantages and disadvantages:
| Tool/Material | Characteristics | Advantages | Disadvantages |
| Ultra-fine Grit Sandpaper (Above 1000#) | Extremely fine grit, typically ranging from 1000# to 3000#, is used for removing finer scratches and imperfections. | 1. Removes extremely fine defects. 2. Smooth surface, yielding good results. | 1. Requires more time and detailed work. 2. May not be suitable for larger defects. |
| Wool Wheel | A tool commonly used in the mirror polishing stage, soft and capable of enhancing gloss. | 1. Provides an extremely high gloss. 2. Soft and suitable for fine polishing. | 1. Prone to excessive wear, a shorter lifespan. 2. Difficult to control, may cause uneven wear. |
| Cloth Wheel | Soft-textured wheel, suitable for detailed mirror polishing. | 1. Ideal for fine surface polishing. 2. Provides an even gloss effect. | 1. Requires consistent pressure during use. 2. May need frequent replacement. |
| Polishing Compound (e.g. White Polishing Compound) | Contains very fine abrasives, ideal for final gloss treatment. | 1. Provides ultra-high gloss. 2. Enhances surface corrosion resistance. | 2. Improper use may lead to residue. 3. Requires precise application. |
| Metal Mirror Polishing Fluid | A liquid polishing agent designed specifically for mirror finishes, often used for large-scale production. | 1. Suitable for large-area polishing. 2. Easy to control, reduces residue. | 1. Best suited for machine work, manual polishing may yield poorer results. 2. Less adaptable to complex shapes. |
The effect of mirror polishing
After mirror polishing, the surface of the aluminium part will exhibit a mirror-like smoothness and reflective effect, with almost no imperfections. It will have a high level of gloss and visual impact. This treatment not only enhances the aesthetic appeal of the aluminium part but also effectively improves its corrosion resistance and wear resistance, making it suitable for high-demand appearances and precision components.
Quality inspection after polishing
Quality inspection is a critical step in the aluminium surface polishing process, ensuring that the polished aluminium parts meet the required standards, including appearance, gloss, flatness, and the absence of defects. Through thorough quality checks, potential flaws, defects, or non-conforming parts can be identified, ensuring that the final product meets the standards and guarantees the reliability and aesthetic appeal of the product.
How Conco conducts the final product inspection:
- Visual inspection: This mainly involves visually checking the surface of the aluminium parts to ensure there are no obvious scratches, stains, bubbles, or other defects. Lighting is typically used to assess the gloss and smoothness of the surface.
- Gloss measurement: A gloss meter is used to measure the gloss of the aluminium surface, ensuring it achieves a mirror-like finish. According to standard requirements, the gloss level should reach a specific value, typically between 80-100 GU.
- Hardness and thickness measurement: A hardness tester and a thickness gauge are used to measure the hardness of the aluminium parts and the thickness of the polished surface, ensuring that the aluminium’s performance is not affected.
- Dimensional inspection: Calipers, micrometers, and other tools are used to check whether the dimensions of the aluminium parts meet the required standards. Dimensional checks are particularly important in batch production.
- Surface cleanliness inspection: Special solvents or cleaners are used to wash the surface, ensuring that no polishing compound, oil, dust, or other contaminants remain on the aluminium part.
For each step, we have a quality inspection report that is confirmed by both the quality inspector and the quality control supervisor before proceeding to the next stage. Finally, the products are packaged, stored, and then shipped.
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