Compression moulds are usually made from standard tool steels with the surfaces suitably hardened and finished to impart a good surface finish to the moulding. The three main types of compression moulds are available. The flash mould is the simplest and cheapest. When the mould is fully closed the moulding pressure is borne by the flash lands, the section thickness is therefore fixed and does not vary with moulding pressure. However, pressure in the cavity is lost as soon as the lands make contact and all flow ceases in the mould. Moulds may be fixed to the platens; an arrangement favored for moulds incorporating ejection systems and moulds which house the heating system. Another method is the use of multidaylight systems, in which several moulds are stacked one above the other between multiple platens. However, mould open times are also increased significantly, and as variations in applied pressure can occur, the practice is more common in the moulding of elastomers, where the pressure requirements are lower.