In its most basic form, single-nozzle structural foam molded parts will have a visible swirl on the outer surface. This swirling is also known as silvering or streaking. Swirling is usually not an issue for parts that do not require a high cosmetic finish, for example on reservoir or tank units that will be buried underground. When a product’s specifications call for a more attractive outer surface, techniques such as gas-assisted molding reduces and even eliminates swirling.