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Joining prepainted metal
There are many methds of joining that have been developed specificaly for prepainted metal.
The main joining techniques can be summarised as:
When mechanical fixings such as screws, bolts 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 domestic appliances. Rivets are a popular form of joining, particularly blind rivet nuts, studs and self-piercing rivets. Self-piercing rivets are particularly suited to prepainted metal, since they eliminate the possibility of surface damage during drilling operations.
Integral joining techniques
Techniques based on mechanical manipulation have been developed for joining prepainted metal. The simplest of these is the lock-formed joint (which can take many forms) where 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 the self-clinching of prepainted metal; the deformed metal forms the joint and gives a similar effect to a rivetted 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 adopted 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 a very effective technique for bonding prepainted metal joints, as it applies just the right amount of adhesive to the joint. Prepainted metal coating systems that include a thermally activated adhesive within the coating can also be used for a number of applications.
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; 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 “weldable” prepainted metal products. The coatings used on weldable prepainted metal are designed to have a suitable electrical conductivity for resistance welding. These products are often used to provide a corrosion resistant primer surface to which a topcoat is applied later in the fabrication process.
Arc welding is not generally used for prepainted metal as the temperatures involved destroy the coating. However, it can be used when the coating is mechanically removed from around the weld area 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, there is usually 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 Tata Steel