. Replacement windows today are manufactured using a high tech process of glass and frame production together with high quality installation techniques.
 Glass Manufacture
Manufacturing replacement windows involves manufacturing the glazing units, the window frames and the weatherstripping.
 Float Glass
Today's replacement windows are made with float glass, a sheet of glass which is made by floating molten glass on a base of molten metal. This gives the glass a uniform thickness and a flat surface. Float glass is manufactured using sand, soda ash, limestone, dolomite and sodium sulfate. Colorants may adjust the chemical or physical properties of the glass. The raw materials are mixed in a controlled ratio together with waste glass (cullet) and then fed into a furnace. After stabilization the temperature is reduced. This molten glass is then fed into a bath of molten tin where it becomes smooth with a uniform thickness. The temperature of the bath lowers slowly and the glass sheet is pulled out of the bath and then cools gradually in a lehr kiln.
 Cutting the Glass
Glass for replacement windows can be cut to any size or shape after the manufacturing process is completed. Cutting takes place after the glass exits the lehr kiln using a steel wheel or a diamond-chip-coated cutter. The cutter is angled to enable the cutting wheel to touch the glass and the cutter is drawn across the glass, placing a moderate amount of pressure in one smooth motion. A dowel is placed under the glass, under the scored line and a sharp downward thrust on either side of the dowel will snap the glass cleanly along the score.
 Low E Coating
Low E coating reduces the solar heat gain coefficient which produces a more energy efficient glazing unit. A low-e coating can be applied by homeowners as a stick-on film but the most efficient low-E coatings are those that are applied in the factory, before the window is glazing unit is placed into the frame. There are two types of low-e coatings, soft Low-E coating and high Low-E coating.
 Hard Coat Low E Glass
Hard coat low-e glass (also known as pyrolytic glass) answers most architectural requirements. Pyrolytic coated glass can be coated on the float glass by pouring a thin layer of metallic oxide onto the float glass while the glass is still on the production line. This metallic oxide layer becomes “welded” to the surface of the glass. It is difficult to scratch or remove the coating which displays a bluish tint. CVD (Chemical Vapor Depositition) is an alternate hard coat low-e processing that is applied to a sheet of clear float glass. Both processes can be toughened after the coating is applied. They utilize passive solar heat gain and can also be combined when needed for high level performance in extreme climates. The U factor of hard coat Low E is higher (less efficient) than that of soft coat Low E coatings.
 Soft Coat Low E Coatings
Soft coat low-E glass involves glass which is manufactured using multiple layers of transparent silver which is sandwiched between layers of metal oxide. The manufacturing process takes place in a vacuum chamber, providing a nearly invisible coating. It offers high visible transmission and optimum U-values. Soft coat low-E coatings can only be used in multi glazed units and involve any needed tempering of the glass prior to the coating process. Soft coat low-E is a more expensive Low-E coating alternative than Hard Coat low-e glazing.
 Framing Material
There are a number of material options which homeowners can choose from for replacement windows framing materials. The manufacturing process of each impacts on its effectiveness as a sturdy and durable window and on its efficiency as a thermal insulator.
 Composite Window Frames
There are a number of composite replacement windows manufacturing processes including the injection molding, light resin transfer molding or compression molding.
 Injection Molding
Injection molding for replacement composite window frames and other components is accomplished by a fluid-assist injection molding process which provides configuration flexibility. Windows will have installation flanges and integral mounting posts in the sashes for handle hardware as well as encapsulated insulating air pockets.
 Light Resin Transfer Molding
Light resin transfer molding creates precision composite window frames by thoroughly saturating the materials that are built up in layers, including materials from wood, metal or other stock materials, which then form the final shape. Reinforcements (fibers or foam) are added and resins seal the materials together.
 Compression Molding
Compression molding of composite window frames involves placing preheated composite materials such as of granules, putty-like masses, or preforms in an open, heated mold cavity. When pressure is applied the molding material cures to the desired shape. Compression manufacturing creates high-strength fiberglass window frames including large intricate parts. Compression molding produces few knit lines and little fiber-length degradation in the frame.
 Fiberglass Frames
Fiberglass frames for replacement windows are manufactured using pultrusion manufacturing which involves the continuous fiber reinforcement of fiber materials. During the pultrusion process raw fiber, including carbon, glass, aramid or other fiber materials are pulled off rolls or doffs from a creel racking system. The fiber is then pulled through a bath of thermosetting resin and is led through a heated die. Excess resin is squeezed off as the composite frame exits as a fully formed frame, matching the shape of the die. A cut-off saw cuts the pultruded profiles to their desired length.
 Aluminum Window Frames
Aluminum window frames are manufactured by cutting the window framing rails to length, placing a rubber gasket around the glazing unit and attaching the rails which are screw-fastened in the corners. Newer aluminum window frames include thermal breaks which are insulating barriers placed between the outside and inside portions of the window frame as well as between the two panes of glass in a double glazed window.
 Vinyl Window Frames
Vinyl replacement window frames are manufactured using polyvinyl chloride. The vinyl compound is made from two basic substances, crude oil and salt. The petroleum is passed through high heat and the resulting pressure breaks it down into various by-products. These by-products are separated and the remainder, ethylene, is used to manufacture vinyl chloride monomer (VCM) which is then converted into vinyl resin. Vinyl resin is mixed with modifiers and additives to compound it into a flexible vinyl material which can be shaped into window frames.
 Wood Window Frames
Wood window frames can be manufactured using almost any type of hard or soft wood including Douglas fir, pine, spruce, cedar, oak and maple. Wooden window frames consists of a header, a sill and studs.
 Wood Clad Frames
Wood clad window frames are wood frames which have a vinyl or aluminum cladding that covers the exterior of the window frame. A properly constructed wood window gives the classing a good foundation to adhere securely.
Weather-stripping for replacement windows comes in many styles including felt, foam rubber or vinyl which are backed on a wood or plastic strip. Weatherstripping can be professionally installed or added to a window's perimeter with an adhesive back.
 Tips for Evaluating a Replacement Window's Construction
Industry experts and consumer affairs advocates suggest that homeowners evaluation the benefits of a window's manufacture, including its style, glazing, framing, weatherstripping and energy efficiency when choosing the best replacement window for a specific application. In addition, these observers advise homeowners to review a window's energy ratings and find a competent installation professional who can present testimonials from previous satisfied customers before placing a window order.