How design can influence surface quality
Surface quality is an aspect that tends to be given less attention than it deserves. It is a subject that often arises only after the design of the aluminium profile has been completed. That can be too late.
That’s because the design stage is precisely the time when you can impact the desired surface quality. In this article, I will explain to you how you can influence surface quality at an early stage of the design process so that the end-product meets all your surface requirements.
Choosing the right alloy
Good surface quality starts with the right choice of alloys, and this should be related to your surface requirements. It makes a big difference whether the product has a highly decorative function, or whether it simply has to look good from a distance.
If the application is decorative and the product has to be anodized, then the obvious choice is a 6060 aluminium alloy. This alloy has relatively low silicon (Si) content, which is important to obtaining a smooth surface. If the profile also has a structural or weight-bearing function, people will most likely opt for a 6063 alloy, for its higher mechanical values.
Alloys such as 6005, 6005A and 6082 are not preferred because their higher silicon content causes a dull, stripy surface.
With powder coatings there are more options, because only the structure of the surface is important, and tangible markings will become immediately visible.
Consider your design
When aluminium profiles come out of the extrusion press, they have to be placed on the run-out table on a certain side. Since the profile is still soft when it comes into contact with the table, marks could be made in the profile. My suggestion is that you let your extruder know how the profile will be used in the end-product, and what the primary visible surface will be.
The shape of the profile also has an impact on surface quality. A big difference in wall thickness, for instance, could lead to visual anomalies with anodizing, and with powder coating, tangible marks could result.
There are ways of solving this problem without adversely affecting the profile’s function. As an example, designing a slightly larger radius in the profile will make the aluminium flow smoothly during the extrusion process. This will result in fewer structural differences in the walls of the profile.
Other options include using decorative ridges or grooves to hide the marks. The marks will still be there, but you can’t see them.
Surface ridges can also help prevent the broad surfaces in the profile from being damaged. A scratch on a broad surface always stands out like a sore thumb. But scratches in the surface of the ridges are barely noticeable.
Packing and handling with care
Handling is another challenge, because products are handled both by the manufacturer and by the customer.
In the first case, profiles that are given subsequent surface treatment might already have been contaminated before the surface treatment is given. This could take the form of oil or grease, but also fingerprints, which etch the profile with natural acids. These fingerprints remain permanently visible after anodizing. This can be prevented by using clean gloves, for example, when packing the profiles.
Keeping the products in stock for lengthy periods of time may also affect surface quality after anodizing. This is a consequence of the natural corrosion process. Pre-treatments can help solve this problem.
And while you think about how you handle the products while packing, you need to consider the method of packing, too. Think hard about the materials you use for packing, and condition of these materials. Damp cardboard, for example, will leave marks on the material.
The pictures above show (clockwise, from top left) Si marks that have been caused by insufficient cooling, fingerprints, a billet transition that had not been sawn out, and the effects of damp cardboard.
So even if you do everything right, and take surface quality aspects into account in the design process, things can always go wrong. Let me know if you see surface irregularities in your products, and we can try to figure out why.
