Polystyrene:

Polystyrene plastics in the form of foam are utilized to generate some patterns for solid mould applications and are also utilized in the form of foam, for patterns utilized in the evaporative casting procedure. The evaporative procedure involves the use of expandable polystyrene beads (particles of polystyrene plastic having 5 to 8 % of a volatile liquid expanding agent). The beads are pre-expanded by vacuum, steam, or hot air to a predetermined density. The pattern dies are built of cast aluminum and machined to end dimensions. Mould cavity and core walls have small holes or vents, by which steam could enter the mould cavity. Expanded beads are blown into the mould, totally filling the cavity. Steam warm up the expanded beads, causing them to further expand, so filling the void areas among the spherical beads and fusing them together. Both of the mould and pattern are then water cooled, after that the polystyrene pattern is ejected. Now Foam pattern is ready for assembly along, sprue, pouring cup, runner, and ingate. The gating system and polystyrene pattern is assembled with contact cement. The pattern assembly is coated up by dipping it into permeable refractory mix.