The process Milgard uses to create their fiberglass window and patio door frame lineals is called pultrusion. Glass rovings and mats are pulled at tremendous forces through a resin bath and a series of dies. These materials combine and catalyze to provide tensile and torsion strength. Milgard regularly tests its fiberglass lineals for color uniformity, gloss, film thickness and chemical resistance for quality and long-lasting performance.