Joining prepainted metal
There are many methds of joining that have been developed specificaly for prepainted metal. Some are adapted from traditional methods, while others are more innovative.
The main joining techniques can be summarised as:
Where mechanical fixings such as screws, botls or rivets are used, prepainted metal can usually be treated in the same way as uncoated metal. Screws are often used where it may be necessary to remove panels later, such as the back of appliances or DVD players. Rivets are a popular form of joining, including blind rivet nuts, studs and, increasingly popular, self-piercing rivets. The latter is particularly suited to prepainted metal, since it eliminates the possibility of surface damage during drilling operations.
Integral joining techniques
Techniques have been developed for joining prepainted metal through integral mechanical manipulation. The simplest of these is the lock-formed joint, which can take many forms, but in which sheets are joined together by rolling sections of adjacent sheets together. In a stitch-fold joint, a stitch is created between two overlapping sheets. This is usually performed on a flange folded behind the prime surface to give a smooth appearance to the exterior. Special tools such as Eckold, Tog-L-Loc and Tox have been developed for self-clinching of prepainted metal which gives a similar effect to a rivetted joint, but deforming the metal itself to form the joint.
Prepainted metal is ideally suited to adhesive bonding, since the surface is clean and provides an excellent key for various adhesives. For this reason, adhesive bonding is commonly used with prepainted metal. However, adhesive bonding works best where there are large surface areas to bond and if this joining technique is intended then the design must allow for this. Adhesive bonds also work best in tension, compression or shear loading and careful consideration of the expected loading condition is important in the initial design. Pressure-sensitive, double-sided adhesive tape is often very effective for bonding prepainted metal joints, since it applies just the right amount of adhesive. There are also prepainted metal coating systems available that include an adhesive within the coating which can be activated thermally which are rapidly finding many uses in industry.
Conventional welding cannot usually be used with prepainted metal for three reasons. Firstly, the coating is not usually conductive enough for resistance welding; secondly, the heat generated in welding usually destroys the coating in the vicinity; and thirdly the coating can weaken any weld which is produced. However, there are exceptions and some welding techniques are widely used with prepainted metal.
Resistance welding can be used with some prepainted metal products which are described as weldable. The coatings used on weldable prepainted metal are designed to have a suitable electrical conductivity for resistance welding. These are often used as corrosion resistant primers, to which a top-coat will be applied later.
Arc welding is generally not used, since the temperatures involved destroy the coating. However, this is sometimes used, with the coating usually being mechanically removed around the weld before welding. Butt welds usually work better than overlap welds, since less coating removal is required. A better alternative is laser welding in which the heat affected zone is usually only a few mm either side of the weld.
Various processes can be used to weld studs or projections to prepainted metal, including electrical discharge welding. When this is applied to the back of a sheet, very often there will be no visible affect to the coating on the front. For this reason, it is often used for welding stiffeners or bolt-heads to the back of prepainted metal parts.
All images on this page are courtesy of Corus