The early aluminium frames had no thermal barrier, these would condensate easily in the winter months and also required a timber subframe for strength.
The second-generation aluminium showed a marked improvement in the insulation by the insertion of a ‘resin’ thermal break. Imagine a window cut in two down through itself so that half fell inside and half outside, now bring those halves back together and bond them with a resin that stops the outer aluminium touching the inner; that is how the thermal barrier works.
The mainframes have been increased in depth so that windows and doors can now be fitted direct to the brickwork opening without a timber subframe.
We are now into the third generation aluminium. The thermal efficiency has been increased by using a polyamide barrier. The method of fitting the polyamide strips allow for two pre-painted sections to be joined by machine and is cost-effective for dual colour products.
Early finishes incorporated anodizing which could be coloured or clear which left the aluminium looking natural. Bronze and black were common options but the colour tone could not be guaranteed between batches. We now seem to paint everything. Powder coating leaves a finished depth three times that of anodising and gives the same colour on a regular basis. Paints are colour coded to the German RAL system and matt, semi-gloss or full-gloss finishes are available in most.
As with plastic windows, foils can be added to the profile face to give a woodgrain effect. The picture shows a grained stencil on a colour.