Dennis discusses the benefits of welded corners in Double Glazed Windows and Doors - how the use of a solid welded corner makes the frame much more sturdy and enhances the overall security.

All windows and doors have joints in the uPVC profile.

A joint is where the divisions within the window or door (uprights which are called mullions, horizontal pieces are called transoms, and the corners), and sometimes the ends of those sections are milled to the shape of the outer frame of the profile so that the sections slot together better.

Most manufacturers then screw the sections together through screw ports into the reinforcement.

However, a welded joint is obviously more perfectly sealed and 'moulded', it won't be leaky or draughty and it doesn't allow any moisture into the screws or the reinforcing - thus preventing long term build up of moisture and condensation inside the frame.

A welded joint is far better!

It is also much stronger because you've got a solid connection with the additional benefit of the solid welded corner blocks that enhance not on the build quality, but also the security.