The choice of foam bead is very important to the pattern of lost foam. Its quality will directly affect the quality of the casting. The conditions to be satisfied are: high strength, low density, full combustion, less decomposition products and long storage life. At present, there are 2 kinds of beads used in Lost Foam Casting at home and abroad: EPS and STMMA. Although EPS is slightly inferior in the quality of parts to be prepared, its price is affordable and its cost performance is relatively high compared with that of common castings. In common casting materials such as pot alloy, gray cast iron, etc., EPS with low price and easy molding is generally used. We discussed
The design wall thickness of the motor shell is 6mm, and the theoretical bead size is 0.67mm. The bead size of the product involved in this paper is 0.6-0.8mm, and the content of foaming agent is required to be 5% – 8%. The storage environment of beads after purchase must be dry and avoid light. After proper maturation, the bead reduces the vacuum inside the pretreated beads and allows the pretreated beads to have good elasticity and fluidity, which can be used to produce foam patterns. However, the biggest drawback of the EPS material is that the large amount of carbon residue produced by pyrolysis at high temperature remains in the cavity after the pyrolysis of the mold, which is known as the “carburizing effect”.
HT250 material is used in this casting, which has high carbon content, so carburization has little effect on the quality of its casting. Lost foam casting is replaced by foam pattern and liquid metal. The foam pattern determines the quantity and type of decomposition products when the liquid metal is replaced, and the coating determines the amount and manner of the decomposition product. According to a large number of literature and actual production, it can be found that casting defects have a significant relationship with coating properties. The improvement of some properties of coating can reduce or eliminate many casting defects correspondingly.
The foam will continue to decompose after increasing temperature. The decomposition products are carbon atom and ammonia gas. Carbon atoms are attached to the casting to form carbon defects. Ammonia is changed into water when it meets air in the air, so the foam model will also produce water vapor after casting.
Polystyrene foam is in liquid state from 130 degrees Celsius -330 degrees Celsius. If liquid foam is quickly discharged from the cavity through the coating layer, the gasification products of foam plastics will greatly reduce. In this case, there will be few gasifiers left in the mold cavity. Even if some of the remaining gases are decomposed into carbon and ammonia, there will be little carbon produced and carbon defects can be effectively controlled. In fact, all coatings can discharge more or less part of the liquid foam. If the coating is able to discharge the liquid products more quickly and effectively, the most effective way is to instantly foam the liquid foam on the surface of the coating, including the surface of the gap. Under negative pressure, liquid foam is quickly discharged from the cavity.