Recommendations on the Polystyrene Material used for Patterns Manufacturing for the Lost Foam Process



It is neccesary to mention that the base to obtain quality castings with the lost foam process is conditioned by the quality of the pattern itself. In this way, Fundiciones Fumbarri deems it fundamental to improve the exchange of knowledge and relationship between: Designers, Patterns Makers, Testers and Smelters. To this end, Fumbarri wants to provide its experience, which with this series of recommendations, contribute to improving the quality of the castings.


The most important aspects of the polystyrene material that is used for patterns manufacturing are presented below:


It is FUNDAMENTAL that the polystyrene’s density be as normally used in patterns manufacturing, that is, 20 Kg/m3. It is essential because the amount of iron to be poured into the mould is calculated on the basis of ratio between the polystyrene and iron density.

a) If the density is less than standard, the casting is carried out with less iron, with the risk that the mould is not filled, losing both the pattern and the casting.

b) If the density is greater, the casting will be carried out with an excessive amount of iron, and there may be several tonnes of excess in the ladle, with nowhere to pour it.

Additionally, low-density polystyrene is easily ruined and broken, with inferior surface quality after machining, which is transmitted to the casting.


Regarding polystyrene drying status is important to know that if the polystyrene is too fresh when the pattern is made, it loses its moisture, and a contraction occurs when the pattern is finished. This contraction will be added to the iron’s solidification. Therefore, for large grade elevations, plane differences may be problematic. In order to confirm this fact, a study was performed on polystyrene contraction. The drying process in the furnaces was monitored, both gross plates (factory origin) and patterns built with polystyrene. This study verified that the plates decreased by up 0.30%, according to the freshness of the material, and the polystyrene patterns contracted at a magnitude of 0.20%.

To avoid possible problems with the casting, where repair is costly or impossible, it is highly recommended taking the quality of the polystyrene into account IN TWO ASPECTS:

 1. In order to be able to use polystyrene plates, they must be completely contracted. That is, enough time should pass from their manufacture to their use in patterns, so their dimensions are not affected later on.

 2. The density of the polystyrene that is cured in this fashion, should be 20 Kg/m3 to prevent false calculations.


To obtain a good surface finish for the castings, it is fundamental that the polystyrene’s quality is good and for the surface finish to be thin. In this regard, it is recommended:

– The polystyrene plates must be manufactured with pearls (balls) with small diameters (Ø1.5 to Ø2.5 mm).
– Polystyrene machining conditions must be adequate, such that the polystyrene is not ripped or burned from friction.


– Using a cutting tool that is in good shape and quality is crucial to obtain thin surface finishes. It is recommended to use a specific tool for polystyrene machining.

– The surface quality of casting is conditioning by the thinness of the pattern. If the pattern’s finish is thin, a thin casting surface shall be obtained.