Whether you are coating, bonding, welding, or assembling metal parts, surface preparation is essential to ensure the high quality of your product.
Failure to do proper surface preparation can lead to quality issues. For example, structural integrity may be compromised, or coatings may peel off.
Surface preparation is the process of treating the surface of a material before applying coatings or adhesives, or before welding or assembly. It involves techniques like mechanical abrasives, chemical treatments, and laser technology. These methods of surface preparation ensure optimal adhesion and enhance material properties.
In the following sections, you'll understand how different steps to prepare surfaces help coatings, adhesives, welds, and assemblies stand the test of time.
- Surface Preparation Steps
- Refurbishing Old Parts
- Methods of Surface Preparation
- Examples of Surface Preparation
Surface preparation can be divided into the following 5 steps.
The presence of contaminants on a surface can compromise the quality and effectiveness of various processes, including coatings, adhesives, and welding. To ensure successful outcomes and strong connections, it is essential to remove contaminants such as dirt, dust, rust, grease, and oil before any of these applications.
Methods like laser cleaning, abrasive blasting, and chemical cleaning can be used to remove contaminants and obtain a clean surface.
Removing contaminants for coatings and adhesives: When contaminants are present on a surface, they create a barrier between the surface and the coating or adhesive. This results in inadequate bonding, leading to coatings that peel off or adhesives that don't hold as intended. Removing contaminants is crucial to ensure a strong and lasting bond.
Removing contaminants for welding: Contaminants and coatings on surfaces being welded end up inside the welds and can have multiple negative effects:
- Poor Fusion: Fusion is the process of melting metal surfaces at the joint to create a solid, continuous connection. If contaminants are present, they can prevent the metals from fusing together effectively, resulting in weak joints.
- High Porosity: Contaminants can lead to the formation of small voids (or pores) within the weld. This is known as porosity, and it affects the structural integrity of the weld, leading to cracks and failures.
- Low Strength: Welds compromised by contaminants are weaker, leading to potential failure in the joined components under stress or load.
Sanding, laser texturing, and chemical etching can be used to roughen surfaces. While increasing roughness is not always necessary, it helps coatings and adhesives adhere better to surfaces. This is especially true for metals and plastics. Surface roughening is used to prepare surfaces for various processes. Examples include thermal spray coating, adhesive bonding, and painting.
The chemical composition of a surface determines its interaction with other materials. By changing it, you can obtain better bonding capabilities.
Methods like laser texturing and chemical treatments, for example, can introduce oxides that improve bonding. It does so by increasing the surface free energy (i.e., the number of bonding sites for coatings and adhesives).
When abrasives such as sandpaper, sandblasting, and grinding wheels are used to prepare surfaces, they generate dust and loose particles that remain on the prepared surface. These contaminants must be removed so they don’t affect the quality of the bond. This can be done using methods such as air blowing, brushing, and solvent cleaning.
One of our clients, for example, had problems with dust originating from the blast media when preparing metal surfaces. They got rid of this problem when they replaced grit blasting with laser texturing:
We used aluminum oxide for grit blasting. If you eliminate aluminum oxide, that’s one less contaminant, and less waste to deal with. In our case, some of the oxide [dust] ended up inside the parts, which isn't desirable either. Using laser texturing eliminates this problem.”
Chemicals used for surface preparation often play a crucial role in altering the surface's chemical composition. But chemical residues (such as solvents or detergents) are contaminants that can interfere with the subsequent coating and bonding processes. Properly removing these residues is essential.
For this reason, additional rinsing steps are often required when using chemicals. The resulting wastewater needs to be treated and disposed of properly.
Refurbishing old parts involves a specialized approach to surface preparation. Over time, surfaces can accumulate layers of corrosion, coatings, and wear. These require careful restoration to ensure the optimal performance of new coatings.
Here are surface preparation steps you can follow when refurbishing old parts:
- Assess the surface: Old parts need to be inspected for signs of corrosion, degradation, or existing coatings. You can also evaluate the extent of wear and damage to determine the appropriate refurbishing approach.
- Remove oil and grease: Oil and grease residues can significantly affect coating adhesion and performance. Degreasers and solvent cleaners can effectively remove these contaminants.
- Strip old coatings: If the old part has existing coatings that need replacement, stripping them is crucial. Methods like abrasive blasting or chemical stripping can be used to remove old coatings.
- Remove corrosion: Old parts may have corrosion that needs to be removed before refurbishing. Techniques like abrasive blasting, chemical treatments, or mechanical abrasion can be used to remove corrosion layers.
- Smooth the surface: After removing old coatings and corrosion, the surface might be uneven or pitted. Sanding, grinding, or chemical treatments can be used to achieve a smoother and more uniform surface.
- Restore the surface profile: For certain applications, restoring the original surface profile is important. Abrasive blasting can help create the desired surface texture for optimal performance.
- Condition the surface: Before applying new coatings or treatments, the refurbished surface may require conditioning. Methods like chemical treatments or mechanical abrasion can enhance the surface's readiness for subsequent processes.
Here are the benefits and limitations of the most common methods of surface preparation.
Mechanical abrasives include methods like abrasive blast cleaning, grinding wheels, and wire brushes. These methods are cheap, can remove tough and thick contaminants, and can process large surfaces quickly.
However, they require high maintenance, are imprecise, generate lots of dust, and can damage the substrate. The equipment and maintenance costs can also add up.
Chemical treatments often involve specific chemicals tailored to the material and the desired surface properties. They can be applied by immersion, spray, or manual application. Examples include chromate conversion coating and phosphoric acid anodizing. Chemicals offer precise control and can create uniform surfaces.
However, they are hazardous for the environment and require strict safety measures. In addition, they may slow down the manufacturing process. This is due to longer processing times and the need for thorough rinsing to remove residues.
Laser surface treatments include laser cleaning and laser texturing. As non-contact processes, these methods generate no mechanical wear and hence little maintenance. With their high precision, they can be used to prepare specific areas without affecting other areas. Lasers can help manufacturers scale up their production by processing parts faster.
However, they require a high-initial investment and tend to only provide a good ROI in high-volume production lines. In addition, they are not ideal to remove thick coatings, clear coats, mill scale, or to process large surfaces.
Surface preparation takes many forms and is used to prepare parts for processes like welding, bonding, coating, and assembly. Examples include epoxy removal, rust removal, oxide removal, e-coating removal, and surface texturing.
Epoxy can penetrate and contaminate welds if it is not removed. In EV stator manufacturing for example, epoxy is removed from connector tabs and hairpins before welding. This contributes to the overall quality and reliability of the finished stator.
Rust is a result of a chemical reaction called oxidation, where iron atoms react with oxygen in the presence of water to create iron oxide, commonly known as rust. Removing rust from steel surfaces is essential when preparing surfaces for coating and welding.
Oxides need to be removed from metal surfaces before operations like welding, painting, coating, anodizing, and assembly. They also need to be removed after welding to prevent coatings from failing near the joints.
Some oxides are relatively easy to remove. These include aluminum oxides and black oxides formed after welding stainless steel. With these materials, manufacturers can remove oxides using less aggressive techniques like laser cleaning.
Mill scale is a type of oxide that is harder to remove. It is formed during steel production as a result of the oxidation of the iron present in the steel at high temperatures. It is best removed with mechanical abrasives such as wire brushes, sanding discs, or abrasive blast cleaning. These techniques are ideal for larger surfaces or when more aggressive removal is needed.
E-coat reduces direct metal-to-metal contact between parts. Since parts typically adhere better to one another when the bare metal surfaces are in direct contact, it is best to remove the e-coat from surfaces intended for assembly. This improves mechanical interlocking and increases the stability of the assembled components.
Coatings and adhesives bond better to rough surfaces than to smooth ones. By increasing roughness, surfaces gain “surface free energy”, creating more sites for bonding.
This results in stronger and more durable bonds and is essential for applications like adhesive bonding and thermal spray coating.
Laser Expert Guidance for Effective Surface Preparation
If you are exploring surface preparation methods, our experts can help you conduct thorough testing and understand the costs and benefits of implementing laser technology.