The value of good knurling

Real-life examples: for a shear-resistant metal frame assembly, professionally executed knurling is absolutely essential.

Windows and glass façades bring light and quality of life to a building. Aluminium frames, which are lightweight whilst having high strength and excellent resistance to temperature and moisture fluctuations, are the first choice. The thermal insulating profiles e.g. made from fibreglass-reinforced polyamide – positioned between the metal inner and outer shells – provide a comfortable indoor climate.

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 the stresses resulting from heat and cold in the application. A flawlessly snug connection is therefore indispensable. The GB/T 5237.6-2012 lays down the minimum requirement relating to the shear strength of these frame assemblies.

Different factors contribute towards a shear-resistant assembly (Fig. 2):

  • As is evident from Fig. 2, the effect brought about by sharp-toothed knurling makes a crucial contribution to a long-lasting high shear strength of the assembly.
  • Through a sufficient rolling pressure, the hammer of the metal profile is pressed against the insulating bar.

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. 3).

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.


Figure 1: The receiving chamber of the metal profile creates the connection with the thermal insulating bar. It is essential for the geometries of the plastic bar and the cavity to match.


Figure 2:  Step by step to the shear-resistant assembly – knurling, insertion, pressing (by rolling).


Fig. 3: The knurling quality is crucial. Schematically represented on the left is unprofessionally executed knurling, in the middle sufficiently good serration, on the right the sharp-toothed knurling with an ideal ratio of polyamide to aluminium.