There are several ways to piece together the vinyl extrusions of a mainframe and window sashes. Obviously, this is an extraordinarily important decision that a window manufacturer makes, because it truly affects how airtight a window is, as well as its ability to deal with moisture.

The first, and typically the least expensive, way for a manufacturer to attach the frames and sashes is what is called mechanically fastening. After the frame and sash pieces are cut, they are screwed together at the corners. Some dealers can actually show you a piece of mechanically fastened sash that they pour water into. Guess what happens to the water when it gets down to the corner? Let's just hope that the sales rep holds the mechanically fastened sash over your sink, instead of your carpet! The easiest way to think about this is to realize that every double hung window in your home has 12 corners that could be mechanically fastened together. Multiply that number by the number of windows in your home and you can begin to see the problem with mechanically fastened pieces.

The second way is some sort of a chemical or glue-like material being used to fasten the pieces of extrusion together. This is sort of an in-between process that often saves a manufacturer money, while giving the consumer the "illusion" that all those corners are sealed. The issue that sometimes arises with this method of fastening is that there have been instances of some of these connecting materials breaking down because of environmental issues, such as high humidity climates. This is a similar concept to using caulk to fill gaps in pieces of wood connecting together.

The third way to connect is called fusion welding. This option requires the most significant manufacturer investment. As we discussed earlier, vinyl shares two traits with steel; the first being the use of 90° bends for structural integrity, and the second shared characteristic is that all points at which pieces of steel are welded together are stronger than the individual pieces. Fusion welding is accomplished by placing a metal plate, heated to over 200°, between the two pieces of extrusion, and then pulling the plate out and allowing the two pieces of vinyl to fuse together, thus creating a stronger, more secure seal at the point of the weld.