What’s Polishing? Tips and Techniques for a Flawless Surface

Polishing is an art as well as a science. This metal surface finishing method is one of the best means of creating parts with aesthetic appeal. Not only do these parts look great, but they also function better and last longer with a polishing finish. Metal surfaces often tend to tarnish over time. This is usually due to contact with degrading elements such as oxygen in the air. Contact with high temperature and wear levels also tend to compromise the surfaces of these metals.

However, a polished metal part is prevented from contamination and oxidation. It also creates a reflective surface while deterring corrosion to keep the surface in excellent condition. Polishing works great with a wide range of metals used in custom manufacturing. In this article, you will learn about the various polishing methods in manufacturing. We will cover the different benefits of these processes and their multiple applications. Also, you will learn some essential tips needed for an adequate polishing process.

What’s Polishing Finish and How It Works

Generally, polishing involves using an abrasive material to finish a metal surface. It aims to remove scratches and nicks from finished machined parts while improving their shine and luster. As a result of buffing, the polished surface achieves a mirror-bright finish.

As mentioned earlier, a polished finish is effective on a wide range of metal materials. The choice of method depends on the type of metal. For instance, automatic or robotic techniques are better suited for hard metals, while softer ones may require manual methods combined with buffing.

Typically, the finishing process in manufacturing includes the following steps:

Step #1: Roughing

A CNC machining process may have included some welding and joining, leaving some scratches and defects on the metal surface. Therefore, there is a need to roughen up the surface in a bid to eliminate such defects. This process can be selectively done on certain parts of the component. A roto-orbital sander can also help roughen the entire part surface. Grinding small areas may require abrasives of about 32mm in diameter with grains between 1500 to 3000. On the other hand, abrasive disks of 150mm in diameter and 1200 or 1500 grain are more suitable for larger surfaces.

Step #2: Polishing

Once the roughing step is complete, a quick-cut abrasive past helps brush up the affected area or the entire surface. Wool berets and polishing sponges are used to distribute the paste evenly, with varying degrees of hardness to achieve the desired surface quality for prototypes and custom parts. After polishing, a microfiber cloth can help effectively clean the surface.

Step #3: Brightening

The polishing step often leaves a nuanced surface after the process. Thus, it is vital to brighten the surface to recover its initial, or even better, appearance. The brightening process also offers uniformity to the finish of the whole component.

Step #4: Protecting

The goal of most finishing processes is to create a durable and resilient component. To preserve the final finish after brightening, a coat of protective wax is often applied. This additional layer enhances the durability of the polished surface.

Different Types of Polishing Techniques

There are several methods of imparting and polished finish on the surfaces of custom parts. Each of these methods has its benefits and drawbacks. Consequently, your choice of method will depend on several factors, including the product’s intended application, total cost, and many others. We shall cover this in the latter sections of this guide.

1. Mechanical Polishing

This method relies on plastic deformation or cutting of the metal surface to achieve a smooth finish by removing raised areas. Common materials used include sandpaper, flat wheels, and abrasive sticks, which are mostly suited for manual operations. However, auxiliary tools like ultra-precision techniques and turntables are employed for surfaces requiring high-quality finishes.

Ultra-precision polishing uses specialized abrasive tools pressed against the surface of a workpiece. The process occurs at high-speed rotation in a polishing fluid containing abrasives. This technology achieves a surface roughness of approximately 0.008μm, making it ideal for producing optical lens molds for injection molding.

Pros

• High brightness
• Improved surface finish
• Better surface cleanability
• Higher aesthetic appeal
• Reduced product adhesion

Cons

• High labor intensity
• Complicated parts can’t be treated
• Gloss may be consistent and may not last a long time
• Could be vulnerable to corrosion

2. Chemical Polishing 

This type of surface finish involves the selective dissolving of the protruding parts of a material surface in a chemical medium to achieve a smooth surface. During the process, the chemical solution and galvanic couples interact with the metal surface, leading to the formation of passivation layers.

Typically, chemical polishing effectively smooths out micro-roughness and causes the parallel dissolution of the upper layer of the material.

Pros

• Workpieces with complex shapes can be polished
• Can work on several parts at the same time
• Less equipment investment
• High efficiency
• Good corrosion resistance

Cons

• It leads to inhomogeneous brightness
• Difficult to heat
• The process may release harmful materials
• It is difficult to adjust and regenerate polishing solutions

3. Electrolytic Polishing (Electropolishing)

This method, also known as electrochemical polishing or anodic polishing, follows a principle similar to chemical polishing. It uses chemical solutions to dissolve protrusions on metal surfaces, resulting in a smoother surface finish.

Unlike chemical polishing, electrolytic polishing does not involve a cathode reaction, making its effect more pronounced. This process efficiently removes excess material from the metal component, leveling out valleys and micro-peaks, which reduces surface roughness and enhances the overall surface finish.

Pros

• Luster lasts longer
• Internal and external colors are consistent
• It can help with an extensive range of mat
• Low cost, short cycle
• Encourages minimal pollution
• High corrosion resistance

Cons

• High investment in equipment
• Complex pre-polishing process
• Tooling and auxiliary electrodes needed for complex parts

Mechanical Polishing vs. Electropolishing

Electropolishing or electrolytic polishing delivers a smooth, bright, highly corrosion and wear-resistant finish on custom parts. Some key benefits of electropolishing include:

• Reducing the surface area to a microscopic level, minimizes product adhesion and contamination buildup, leading to easier cleaning of the finished part.
• Quick removal of burrs due to higher current densities at raised points, resulting in a clean, even surface.
• Enhancing the appearance of the part provides a lustrous finish that aids in visual inspection by highlighting any remaining imperfections.

However, mechanical methods are often the better choice in specific scenarios:

• For parts with intricate shapes that challenge uniform current distribution.
• When only certain areas need refinement.
• Some materials may not react well to the chemicals used in electropolishing, making mechanical methods more suitable.
• For projects with budget constraints, mechanical finishing might be more economical, especially in low-volume production.
• If a particular surface texture or a more matte finish is desired, mechanical processes can be adjusted to meet these specific needs.

Choosing between electropolishing and mechanical finishing should be guided by the specific requirements of the project, including material type, desired finish, complexity, and budget.

Top Tips for Achieving an Optimal Polishing Finish

The following expert tips will help ensure the polishing process is effective:

• Abrasive discs with polyester films are ideal for roughing, as they produce consistent roughness across the surface.
• Avoid polishing in a single direction. Varying the directions helps achieve uniformity, especially on materials with uneven shapes and sizes.
• Use the polishing compound sparingly. Applying too much can lead to grease marks on the workpiece.
• Ensure the abrasive material is in good condition—neither too hard nor too soft.
• A low working speed of around 900 rpm is generally recommended for polishers.
• Prevent overheating by using fast-cutting pastes.
• Remove any grease marks from the component using a dry microfiber cloth.
• Apply a small amount of polishing compound frequently rather than a large amount all at once.
• If possible, polish across scratches or marks rather than along them.
• Remove any lacquer from the surface of the metal component before starting polishing.
• Always wear appropriate safety gear, as polishing can be hazardous if you’re not protected.
• Use a separate buffing wheel for each color of the polishing compound to avoid mixing different compounds.
• If using different polishing compounds with one buffing wheel, clean the residue by running a wire brush against the rotating wheel.
• Fixing your drill in a vice will make polishing smaller parts more efficient.· Protect an already polished surface with a good wax polish.

Pros and Cons of Polishing Finish

Polishing has its fair share of advantages and drawbacks. Here are some of them:

Applications of Metal Polishing

A key advantage of a highly polished surface is its smoothness, which prevents the buildup of dirt or microbes and makes the part easier to clean, even to a sterile level. This cleanliness is maintained over time, ensuring the part stays free from contaminants.

Additionally, polished metal surfaces reflect light effectively, making them suitable for various applications. As a result, industries such as medical, aerospace, and electrical sectors frequently use polished surfaces on metal components. The applications of these polished parts include, but are not limited to, the following:

• Medical equipment manufacturing
• Reflectors and mirrors (e.g., solar energy concentrators, safety mirrors, etc.)
• Fire truck bumpers
• Fasteners for marine applications· Springs and wire cracks

Ensuring Quality with Premium Surface Finishes at RapidDirect

The ultimate aim of every prototyping or low-volume production process is to ensure that the end products meet quality demands. Surface finish is one of the most vital post-production processes to ensure that the products are of the required standards. We understand this at RapidDirect, and we are ready to bring your prototyping desire to reality.

At RapidDirect, we not only have the best technicians and manufacturing techniques, but we also boast top-of-the-class finishes, including the polishing surface finish. We work with you to help you make the right choice for your machined part. The best news is that all our services come at competitive pricing. Upload your design file today to get an instant quote.

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Conclusion

The polishing surface finish brings out the shine in your products’ surface. It improves the quality and performance of such products in many different ways. In this article, we discussed the various applications of the polishing finish as well as its benefits. The tips provided will also help you get the best out of the polishing process in manufacturing. At RapidDirect, we are always ready to offer you the finest polishing services for your custom parts. Our expertise will get your products to the exact specifications. Contact us today, and let’s get to work.