When a vinyl frame is extruded, Milgard's vinyl compound material is forced through a series of long steel dies and calibrators under specific heating and cooling stages to make a frame profile. Every profile or part of a window requires a different series of dies and calibrators.
It's important to understand that any excessive stresses in the material during this processing can cause problems, given the right circumstances. Given some time or heat, for example, and a window with heavy processing stress can deform. This tendency to revert to an original shape is known as "memory."
Memory isn't exclusive to vinyl products. When you bend a tree branch -- applying stress -- and then let go, it tends to return to its original shape. It relieves the stress.
Milgard's equipment, processing parameters and profile designs are engineered to control the stresses in their final window products. They're engineered to actually make any stresses cancel each other out. So a frame profile's final shape is the shape it will remain. Even through the hottest summers, coldest winters and years of use.