Design of extruded aluminium surface quality

Surface quality is an aspect of aluminium profile design that receives less attention than it deserves. This question often arises after a prototype of a new profile has already been extruded. It is usually too late for anything to be done.

This happens because the design stage is the period when you can influence the surface quality of the future aluminum profile.

Design of extruded aluminium

The right alloy

Good surface quality starts with the right choice of aluminum alloy. The level of requirements for the surface quality of the aluminum profile depends on the choice of aluminum alloy. Requirements for surface quality can vary greatly depending on the purpose of the profile, Zum Beispiel, for elegant decorative products or for building structures that must look good from afar.

Zum Beispiel, for decorative items that will be anodized, the obvious choice is alloy 6060. This alloy has a relatively low silicon content, which is important for achieving a smooth surface. If the profile also has load-bearing functions, then alloy 6063 is more often chosen, which has higher mechanical properties. In such cases, alloys 6005A and 6082 are not used, since the increased silicon content in them gives a dull and streaky surface [1].

The right profile design

When aluminum profiles come out of the extrusion press, they must lie on the exit table with some of their sides. The profile at this time is still very soft. Deswegen, when it comes into contact with the table, marks may inevitably form on it, for example in the form of minor dents. That is why it is very important to coordinate with the customer and indicate on the drawing the visible (significant) surfaces of the profile.

The cross-sectional shape of a profile can also affect the quality of its surface. Large differences in wall thickness or junctions between walls and flanges can lead to a non-uniform surface structure of the metal, which gives visible streaks both during anodizing and powder coating [1, 3].

This problem is solved or mitigated, Zum Beispiel, by using slightly larger fillet radii between cross-sectional elements. This causes aluminum to flow through the matrix more evenly, which in turn reduces the heterogeneity of the metal structure on the surface of the profile.

Other measures include the use of decorative ribbing or decorative ribs to disguise such stripes. If the differences in the metal structure remain there, but we will no longer see them (Figure 1).

Abbildung 1 – Figure – Application of decorative corrugation
to prevent surface defects:
a – profile with surface defects “stripes”;
b – improved profile with decorative corrugation

Ribs on the surface can also help prevent wide surfaces of profiles from being damaged. Scratches on the wide surface of the profile always catch the eye, but the same scratch on the ribs is almost invisible.

Design of profile processings

The profile developer must not forget that the profile needs

• not only extrusin,

but also:

• Handhabung,
• packing and
• transporting.

The handling of profiles during their processing is another threat to their surface, both for the manufacturer and the customer.

The manufacturer’s profiles, which must be subjected to one or another surface treatment, may become contaminated even before this treatment. This contamination can take the form of not only oils or grease, but also ordinary fingerprints (Figure 2). Sweat from the hands is acidic and causes acid corrosion of the profile surface. These fingerprints remain visible after anodizing. The occurrence of this defect on the surface of profiles is prevented by using clean gloves during all manual work with profiles [1].

Abbildung 2 – Fingerprint corrosion

Storing profiles in piles and piles for too long can also harm the quality of their surface. This occurs as a result of natural corrosion processes.

For each profile, the method of its packaging must be determined. It is necessary to think in advance not only about the packaging method, but also about the materials that will be used. Zum Beispiel, some cardboard and paper materials have acidic properties and, when wet, can leave noticeable traces of acid corrosion on the profiles (Figure 3).

Abbildung 3 – Acid corrosion from paper packaging

Surface quality control at SAPA

The world leader in the production of extruded aluminum profiles, SAPA, has a well-thought-out surface quality control system [2].

Surface Quality Factors

When working with customers, SAPA uses six grades of surface quality for aluminum profiles. When assigning profiles to a particular class, the following factors are taken into account:

• profile assignment;
• aluminum alloy used;
• presence and location of visible surfaces;
• presence and type of protective and decorative coating.

As a result of considering all these factors, profiles are classified into one of six surface quality classes. The degree of surface quality decreases from class 1 to class 6.

For each class, except for class 6, the distance from which the surface quality is assessed for the presence of visible defects is determined:

• This distance varies from 0,6 m for class 1 zu 5 m for class 5.
• There are no requirements for surface quality for a class 6 surface.

Only 5-10 seconds

An interesting point is the time during which the inspector – manufacturer or customer – has the opportunity to detect surface defects. This time is only 5-10 seconds. This is a very important condition, since usually this time is not specified, and the inspector can “look for” surface defects for an unlimited time.

An important condition for the customer is that with an increase in the degree of surface quality, the cost of the profiles increases. This ensures that the customer places demands on the quality of the surface of the profiles, which actually correspond to their purpose.

The surface quality class of the profiles is agreed upon between the manufacturer and the customer and is entered into the order or contract documentation.

Surface quality classes

Klasse 1 – Highest requirements

• Applicable to parts of household appliances, lighting fixtures, decorative items, etc.
• Maximum delivery length: 2.4 ich, unless a different length is agreed upon.
• Klasse 1 can only be achieved for anodized profiles with special pre-treatment – chemical or mechanical. These mechanical surface treatments typically include grinding, wire brushing and polishing.
• Profiles whose surface is visible on all sides can only be processed in this class if the profile has been sanded on all sides.
• During transportation, each profile must be separately protected from damage.
• Each profile requires individual processing and visual inspection, which increases labor costs.
• Legierungen 6060, 6063 und 6463 only.
• Surface quality is assessed from a distance of 0.6 m for 5-10 seconds.

Klasse 2 – Very high requirements

• Applicable to furniture items, household appliance parts, picture frames, etc.
• Maximum delivery length: 2.4 ich, unless a different length is agreed upon.
• The highest class for brilliant anodizing, including additional mechanical processing. Bright anodizing reveals and enhances all surface defects.
• Profiles in which all sides are visible are usually not produced with a surface quality of this class.
• This class is generally applied to anodized profiles.
• During transportation, each profile must be separately protected from damage.
• Each profile requires individual processing and visual inspection, which increases labor costs.
• Legierungen 6060, 6063 und 6463 only.
• Surface quality assessment is carried out from a distance of 1 m for 5-10 seconds.

Klasse 3 – High demands

• Applicable to furniture parts, lighting fixtures, refrigerators, shower cabins, and interior decorative items.
• Typically applied to anodized profiles.
• Profiles in which all sides are visible are usually not produced with a surface quality of this class.
• Legierungen 6060, 6063 und 6463 only.
• Surface quality assessment is carried out from a distance of 2 m for 5-10 seconds.

Klasse 4 – Normal requirements

• Applicable to building systems, facades, windows, doors and handrails, as well as products for public buildings, such as furniture, exhibition stands, shower cubicles.
• Applicable to painted and anodized profiles.
• Legierungen 6060, 6063 und 6463 only.
• Surface quality assessment is carried out from a distance of 3 m for 5-10 seconds.

Klasse 5 – Low requirements

• Used for building systems, balconies, roofs, windows, doors, openings, railings, sail masts, stairs, football goals, folding partitions.
• Standard profiles, such as angles and channels.
• Legierungen 6060, 6063, 6463, 6005EIN, 6082, 6101.
• Surface quality assessment is carried out from a distance of 5 m for 5-10 seconds.

Klasse 6 – No requirements

• It is used for load-bearing structural profiles, handrails, scaffolding, scaffolding, sliding partitions.
• Standard profiles, including medium-strength alloys such as 6082. Structural profiles, z.B. for truck bodies.
• Profiles for which visible surfaces are not specified.
• Profiles made from aluminum grade 1050A can only be pressed according to this surface quality class.

Visible and semi-visible surfaces

• In addition to visible surfaces, SAPA also distinguishes semi-visible surfaces. Semi-visible surfaces are those surfaces that are not normally visible in the finished product, such as the rebates of window frames.
• Visible surfaces are indicated on the profile drawing by dashed lines, semi-visible surfaces by two solid lines.
• The profile surface quality class is established for visible surfaces. Semi-visible surfaces automatically receive a quality class one unit lower. Zum Beispiel, if visible surfaces are assigned a class of 4, then semi-visible surfaces will be assigned a class of 5.
• Profiles for which visible surfaces are not indicated belong to the lowest, sixth, class. For these profiles, only tolerances in size and shape, as well as strength properties, are controlled.

Quellen:

1. Wie Design die Oberflächenqualität beeinflussen kann, SAPA-HYDRO, 2020
2. Konstruktionshandbuch. Erfolg mit Aluminiumprofilen – SAPA, 2014.
3. Intelligentes Design kann das Risiko von Oberflächenfehlern reduzieren, WASSER, 2020