Shear Strength in Modern Aluminium Windows

By Bogdan Grebenyuk, Product Manager at Thermevo
on May 31, 2018
With polyamide rolling, the shear strength of aluminium can be improved on to create frames which meet modern architectural demands.
Aluminium window frames have been chosen since the 1930s for their strength to weight ratio and they remain a preferred material for many architects. Nevertheless, some high thermal insulation aluminium windows have faced design limits in the past few decades, largely due to the way in which materials have been inserted into the frames to make them perform better thermally that have caused loss of strength. Today's modern insulation materials, such as a simple ABS insulating strip or a polyamide thermal break, provide superbly lightweight solutions to the insulation issues. Therefore, the design limit of how big an aluminium frame can be, according to its shear strength, ought to simply come down to weight of the glazing it will hold. When thermal material like PA 6.6 insulation bars sit within the frames, they should have little bearing on the overall strength of the glazing product, but do they?

How Window Manufacturers Improve Insulation Without Compromising on Shear Strength

The bond between an aluminium window frame and, for example, its polyamide 6.6 insulating strip is a physical one. Therefore, it requires a mechanical process to insert the polyamide rolling into its aluminium groove. Because the aluminium required to make one section of a frame is extruded with a groove to allow for this material to be inserted, the overall shear strength of the material is lessened to some degree. The way in which top quality window makers overcome this is by knurling. Before feeding a polyamide rolling into an aluminium profile, it must be knurled, usually on a lathe or similar industrial device. This is crucial because if knurling is not carried out, then the assembled frame will often fail to meet the respective quality standards for the territory, like RAL and NF.

No knurling in aluminium ghroove
Bad knurling
Optimal knurling
Aluminium is the most common metal in the earth's crust, almost twice as abundant as iron.

Sam Kean, US writer.

The Real Function of Glue Wire in Insulating Aluminium Windows

Another factor of high-strength and insulated aluminium window frames is glue wire and how it is used in the industry. It is fair to say that many professionals who work in the window manufacturing sector are not entirely familiar with its role. Some technical know-how is required to understand fully where and how glue wire should be applied and a number of makers face problems with it after powder coating their products which can lead to wastage. This is because glue does not create a true mechanical connection between a polyamide thermal break and the window's profile. As a result, polyamide – which is absorbent - can soak up water in wet conditions. Over time, aluminium frames that have a combination of polyamide with glue wire can see a degradation in their shear strength due to the wicking effect of this material.

Squeezing the Best Shearing Strength Results Out of Polyamide Rolling

Given the noted issues with glue wire, polyamide rolling into aluminium grooves within frames represents the best approach for the continued shear strength of window products, whether they are powder coated or not. Indeed, high precision knurling is now widely regarded as the only tried and trusted method to attain the best shearing strength after the rolling and crimping of polyamide insertions. A presentation of the advantages of this approach can be found here. Other methods, such as seaming a polyamide strip into an aluminium groove by hammering it into place where it is gripped by scalloped teeth often results in a profile that bends under the weight of its load – hardly desirable in any setting.

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