The insulating bar and metal profile are supposed to form a form-fit and friction-locked connection. A basic prerequisite for this to occur is that the geometries of the bar foot and the receiving chamber of the profile are compatible (Fig. 1). The first step involves both noses of the aluminium cavity being knurled. If the bar is now inserted, the pressing wheel presses the jagged noses into the plastic. As a result of the pressure, the teeth of the knurled edge ‘bite’ deeply into the thermal insulating profile on both sides (Fig. 2).
The assembly produced in this way must be able to withstand diverse stresses over a long period of time – and must be able to do so from the start. For example, withstanding the high temperature during powder coating or later, in the application, the stresses resulting from heat and cold. A flawlessly snug connection is therefore indispensable. TS EN 14024 lays down the minimum requirement relating to the shear strength of these frame assemblies.
Different factors contribute towards a shear-resistant assembly (Fig. 3):
- Through a high rolling pressure, the hammer of the metal profile is pressed against the insulating bar. BUT: this frictional connection declines, due to the material, when exposed to stresses through temperature, for example during powder coating.
- When the coating is being baked, the adhesive sheath of the Coex sealing wire melts. It thereby fills the capillary joint between the polyamide and aluminium and seals it, making it waterproof. Over and above this, this adhesive film provides an additional safeguard against displacement of the assembly. BUT: this strength alone is not sufficiently great to ensure the strength of the assembly.
- As is evident from Fig. 3, the effect brought about by sharp-toothed knurling makes a crucial contribution to a long-lasting high shear strength of the assembly.
The quality of the knurling is therefore particularly important for a shear-resistant connection. Knurling that is not cleanly executed, for example as a result of a worn knurling wheel, produces an inadequate result. Serration is ideal when a surface ratio of 4:1 of polyamide to aluminium is achieved (Fig. 4).
Combined with compatible assembly geometry, sharply toothed knurling makes it possible to create a long-lasting, robust assembly. In this way manufacturers achieve high quality standards and normative guidelines are adhered to. Last but not least, this is also good news for the building owners who can now take pleasure in their aluminium window for a long time.