3.5.3Metal substrates

Processing of HPL with metal substrates is a special field of application for specific industries, e.g. for car manufacturing, ship building, transport and the high requirements of interior building work.

The following materials are examples for metal substrates.

• Aluminium sheets (plates)
• Aluminium composite sheets (honeycomb structure)
• Steel sheets (plates)

The Sheets and plates have to be absolutely flat, without bends, bumps or any other faults. 

To avoid corrosion, different metals should not be brought into direct contact with each other.

Metal substrates have oxide or corrosion layers, are greased, oiled or additionally hardened, heat-treated, acid-treated or anodized. This influences wetting with adhesive and reliable bonding. Therefore, the surface has first to be pretreated for bonding. The time between pretreatment and bonding should generally be as short as possible to prevent renewed corrosion or soiling. The type of pretreatment depends on the type and thickness of  the metal as well  as the available equipment and the production quantity. Before bonding, metal substrates have to be fully degreased. This can be achieved by using steam, solvents or cleaning agents. After degreasing, it is important toprevent resoiling of the surface. Clean protective gloves have to be worn as even slight fingerprints can affect bonding quality.

Composite elements consisting of HPL and metal substrates combine the advantages of both materials:

• Decorative appearance: The full range of decorative designs and textures of HPL are available as a design element. In addition, the surfaces are easier to clean.
• The elements achieve a high mechanical strength compared to the weight.
• Reaction to fire: Metal sheets are categorised as non-flammable as per EN 13501. Standard HPL as per the same standard Euroclass D-s2, d0 can be supplied with flame-retardant finish. (Type F as per EN 438).
• Shaping freedom: Formed metal surfaces can be coated with HPL.
• Metal substrates can be used to achieve improved physical properties such as vapour barriers, magnetic properties, electrical properties (shielding against radiation).
   

Manufacturing these composite elements require knowledge of certain material properties which have a crucial influence on manufacturing:

• Both materials – HPL and metal sheet – are non-absorbent. This has to
  be considered during selecting and processing.
• HPL expands slightly in humid climate and shrinks as a result of dryness and extended exposure to higher temperatures. Climate conditioning before bonding, adapted to the subsequent area of application, reduces the possible dimensional changes to a minimum.
• Metal sheets, on the other hand, are unaffected by moisture, but expand significantly under higher temperatures and shrink when the temperature is reduced. This can result in a “bimetal effect” for the composite element, as e. g. the metal expands under higher temperatures. HPL on the other hand shrinks under extended heat exposure.
• The bond has to be elastic enough to compensate for the different tensions which occur. Metal sheets have to be prepared for bonding with mechanical or chemical pretreatment.
• The processing machines have to be capable of machining HPL and metal at the same time
• As a rule, these should be bonded by using the cold pressing method.
  To meet fire protection regulations, it is often necessary to use flame-retardant HPL. We recommend consulting the manufacturer of the HPL.
   

Careful steam treatment achieves good removal of grease and oil. A completely clean surface exhibits an uninterrupted water film after the steam is switched off. The cleaned sheets have to be dried with warm air before stacking.

Solvents such as acetone, xylene and MEK can be used for manually cleaning the surface. To do this, saturate a clean, lint-free cloth or a clean wad of paper with the solvent and carefully wipe the surface, changing the cloth or paper frequently. This is followed by warm water rinsing and warm air drying. In general, solvent steam degreasing is the most efficient method, though. This, however, requires special systems. It involves placing the sheets into tanks, where the lower section contains a heated solvent and the steam from this condensates on the sheets, rinsing the surface with solvents. Note: Using degreasing solvents requires special caution. General health and safety regulations and recommendations from the manufacturers have to be observed.

Mechanical roughing is used to remove the oxide, corrosion or other foreign layers, simultaneously, but also for achieving good mechanical anchoring of the adhesive. On greased or oiled metal substrates, the surfaces to be bonded also have to be degreased beforehand. Roughing can be achieved:

• with sanding paper (grain P 80 – P120 for steel sheets or grain P 320 – P
  500 for aluminium sheets)
• with wire brushes
• for sand blasting units only using sharp blasting sand such as aluminium oxide, emery or quartz (glass and metal beads are unsuitable)

We recommend always blasting on both sides due to the risk of warping. Blasting on both sides, however, is not suitable for smaller thicknesses or larger sizes, as it results in irreversible deformation. Repeated cleaning after each of these roughing processes is necessary.

Compared to mechanical pretreatment, chemical cleaning and etching is more effective, as it results in a better surface quality which is easier to control. Proceed as follows for degreasing and etching aluminium sheets and for degreasing steel sheets:

• Immerse the metal substrate in a commercially available alkaline degreasing and etching solution at approx. 50 °C. The immersion time depends on the type of metal and usually takes about five minutes.
• Remove the metal substrate from the solution and rinse it with water to remove any residue of the alkaline solution.
• Then apply a neutralisation bath in 5 % acetic acid.
• Dry with warm air before stacking.
   

Some adhesive systems require the substrate to be primed as a pretreatment for bonding. Priming and pretreatment should be conducted immediately after degreasing and directly before bonding. A primer contributes to an improved surface. The primer can be applied by machine or with hand-held rollers under normal operating conditions. Metal substrates have to be adapted to the processing temperature (room temperature) before priming to prevent condensation effects. The primer should be applied as per the manufacturer's instructions. Sufficient corrosion protection must be ensured.

When it comes to sawing and routing, HPL aluminium composite elements can be machined with tungsten carbide tipped (TCT) blades in the same way as HPL wood composite elements. The feed rate should be lower. HPL sheet steel composite elements can only be machined with metal saws, guillotine shears and nibblers. The special health and safety requirements have to be taken into account. Composite elements with a maximum HPL thickness of
1.0 mm and a maximum metal thickness of 0.8 mm can be punched cold. Preliminary tests to determine tool play have to be conducted.

It can be drilled with slow-running metal drills at a moderate feed rate, avoiding overheating. These composite elements must not be mounted rigidly, but must be provided with slack for dimensional changes by means of oversized drilled holes, washers under screw heads and a sliding film between the components.

Fasteners can be spot-welded onto the metal reverse side of the sheets (stud welding with tip ignition). For metal substrates with HPL on both sides, the HPL has to be removed in the areas where the stud is to be welded on. The minimum thickness of the metal support must be 1.5 mm. In addition to this, composite elements can also be attached with adhesive. To avoid corrosion, different metals should not come into contact.

For outdoor applications or in cases where narrow areas or surfaces of the metal substrate are exposed to moisture or corrosion, these parts must be protected.