Lost foam casting is a solid casting method that combines a foam plastic mold using binder-free dry sand with vacuum technology. The main names in Japan are "dry sand casting" and "negative pressure casting", also known as EPC casting, and the main names overseas are the lost foam method (USA), polycast method (Italy), etc. Lost foam casting is one of the most advanced casting methods in the world, and is praised at home and abroad as a "revolution" in the history of casting, and is also called the green casting of the 21st century.
Lost foam casting (also known as full mold casting) is a full mold casting process that uses a foam plastic mold combined with binder-free dry sand and vacuum technology. It is a process of combining a foam model of similar size and shape to the object. Cast into a model cluster and brush a fireproof coating. After drying, it is buried in dry quartz sand for vibration molding, and poured under negative pressure to evaporate the model, and the liquid metal takes the position of the model, and then solidifies. And then cooled to form a new casting method.
Compared with traditional casting techniques, lost foam casting has the following characteristics: 1. Good casting quality, low cost. 2. No material restrictions, appropriate size. 3. High precision, smooth surface, less cleaning, and machine processing can be saved. 5. Internal defects are greatly reduced, and the structure is dense. 6. Large-scale and mass production is possible. The automated assembly line is environmentally friendly. 7. It can greatly improve the working environment, reduce labor intensity, and reduce energy consumption.
Manufacturing principle: This method first makes a foam model according to the process requirements, coats it with special high-temperature resistant paint, and puts it into a special sand box after drying, and fills it with dry sand according to the process. The mold is molded into the required shape, compression molded by three-dimensional vibration, and evacuated. The molten metal is poured in, and the mold evaporates and disappears. The molten metal replaces the mold, and the same casting as the foam mold is reproduced.
1. Selection of foam beads:
There are three types of foam beads that are specifically used for lost foam casting.
② Expandable methyl methacrylate-styrene copolymer resin beads (STMMA for short)
Commonly used expandable polystyrene resin beads (EPS) are used for casting non-ferrous metals, gray iron and general steel castings.
Beads characteristics: translucent beads, pre-expanding multiple 40-60, particle size 0.18-0.80 mm (6 sizes),
The particle size of the original beads usually selected is 1/9 to 1/10 or less than the minimum wall thickness of the casting.
2. Model making: There are two situations.
◎ Made of foam beads: pre-expanding - maturing - foam molding - cooling and demolding
Before EPS beads are added to the mold, they must be pre-expanded to expand the beads to a certain size. The pre-expanding process is one of the key factors that determine the density, dimensional stability and accuracy of the model. There are three suitable methods for pre-expanding EPS beads: hot water pre-expanding, steam pre-expanding, and vacuum pre-expanding. Vacuum pre-expanded beads have a high expansion rate, are dry, and are widely used.
Pre-expanded EPS beads are placed in a dry, well-ventilated silo for a certain period of time. In order to balance the external pressure in the pores of the beads, the beads are given elasticity and the ability to re-expand, and the moisture on the surface of the beads is removed. The maturation time is 8 to 48 hours.
Pre-expanded and matured EPS beads are filled into the cavity of a metal mold, heated to re-expand the beads, filling the gaps between the beads and fusing them together to form a smooth surface, that is, a model. The model must be cooled to below its softening temperature before being removed from the mold. The model can only be removed from the mold after it has hardened and taken shape. After being removed from the mold, time must be allowed for the model to dry and become dimensionally stable. There are two types of equipment: steamers and automatic molding machines.
For simple models, you can use a resistance wire cutting device to cut the foam sheet into the required model.
For complex models, you can first use a resistance wire cutting device to divide the model into several parts, and then glue them together to make a whole model.
