Metal parts are used for a variety of applications and often require some level of surface improvement prior to being used. Surface finishing, or polishing, is essential because it ensures the metal parts are adequately prepared for their intended application. By eliminating imperfections found on the surface of the parts, corrosion is minimized, coating materials are able to adhere properly and the overall appearance of the part is enhanced.
The initial surface roughness is determined by the type of material (stainless, titanium, plastic) as well as the means used to manufacture the parts (machined, injection molded, cast). Heat treatment, welding, blasting and other various secondary operations will also have an effect on the surface prior to finishing. Understanding these factors helps to tailor the surfacing finishing process and prepare metal parts for surface coating.
While surface finishing is typically used to reduce overall surface roughness there are situations where an ultra-smooth finish is not ideal. Surface finishing can be used to create varying surface textures such as a stippled or matte finish to improve the adhesion of certain coatings.
The Surface Finishing Process
As noted above, preparing metal parts for surface finishing can vary greatly. The type of surface coating being applied and the final intended use of the product will dictate the level of surface finishing required. Regardless of the coating, metal surfaces must be free of contaminants such as grease, oil and corrosion the parts may encounter during manufacturing or shipping.
To achieve a clean surface, metal parts would likely go through the following process.
- Pre-Polishing & Polishing
- Surface Coating Application
Removing heat treat or casting scale is the first step when preparing parts for surface coating. It allows for the complete visual inspection of the surface to uncover any manufacturing defects or voids. For example, descaling can expose casting imperfections, such as those on cast magnesium parts for the automotive industry. Parts identified with imperfections can be reworked or scrapped, rather than being send thru the entire finishing and coating process.
Next, the interior and exterior of the parts undergo deburring using either vibratory finishing or centrifugal barrel finishing. The deburring process is meant to remove a build-up of machining burrs and other imperfections from a finished surface and is a necessary prerequisite to surface coating. Deburring also improves product quality, increases the lifespan of a part, and enhances aesthetic appeal.
Once the deburring operation is complete, both the interior and exterior surfaces of the parts go through a pre-polishing and final polishing phase. Polishing is essential as it continues to improve the surface with incrementally less coarse media, resulting a uniform, smooth and clean surface. In some instances where there are intricate interior areas not accessible by the tumbling media, electropolishing or other chemical processes may be used. Surface coatings adhere more completely to the refined surface, increasing the lifespan and corrosion resistance of metal parts.
Surface Coating Application
With the proper surface roughness achieved, a surface coating can be applied. The type of surface coating applied is often determined by its application. Common types of surface coatings for metal parts include powder coating, chrome plating, titanium nitriding and DLC. ISO Finishing also works closely with Mustang Vacuum Systems to prepare metal parts for the application of their innovative SP4 coating. SP4 is used for a range of applications including firearms, performance automotive, and medical devices.