Lost foam

Charge air distribution line
schematic representation
Disassembly
composite foam

Lost Foam ( German casting lost foam ) is a casting method in which a positive model ( Schäumling ) consists of expanded, polystyrene (styrofoam) or polymethyl methacrylate (acrylic) in a molding material , embedded, usually made of silica sand and the thus formed cavity with liquid metal poured becomes. The model is thermally decomposed by the molten metal. During casting, a corresponding positive model must therefore be provided for each cast part, which no longer exists after the casting process (lost model). A similar process is full mold casting . The main difference is that in full mold casting, sand with a binder is used, and sand with no binder is used in casting in lost foam. Both methods belong to casting with evaporating models .

The foam can be composed of several parts and thus take on complex shapes, including those with undercuts . The contours of the cast parts do not need to be reworked after casting because they are very close to their final contour.

history

In 1958, the American HF Shroyer patented the lost foam casting process, the models of a polymeric foam in chemically bonded foundry sand einformt and by the thermal energy of the inflowing melt, the model gasified .

The procedure

The lost foam process consists of 8 process steps. These are foaming and joining the model segments, subsequent model assembly and finishing, as well as molding and pouring, followed by the decomposition process and emptying.

Foam

The starting material for the production of models for aluminum casting is the expandable polystyrene raw material (EPS) mixed with the blowing agent pentane (C ₅ H ₁₂ ). The copolymer made from EPS and EPMMA (expandable polymethyl methacrylate) is used in particular for the production of cast iron and steel components. These two substances are spherical and consist of 6% polymer and pentane, the remaining 94% consist of air.

Prefoaming

Polystyrene is expandable due to the inclusion of pentane as a blowing agent. By supplying steam at around 110 ° C , the EPS material is made to expand in a foaming chamber. The resulting overpressure in the free volume between the polystyrene molecules and the softening of the polymer matrix due to the further rise in temperature cause the EPS beads to enlarge to the desired size, up to around 30 times their original volume. This process achieves the density required for further processing .

Finished foams

In this process step, the EPS beads are blown with compressed air through an injector into a metallic permanent mold, where they are welded to the desired model shape. This is done by re-steaming the surface of the mold with steam at a temperature of approx. 110 ° C, which enters the mold cavity.

Joining the model segments

After the individual model segments have been foamed and dried, they must now be put together to form the planned complex model shape. This is usually done in series with gluing machines for processing hot glue , which also fix the model parts.

Assembly of the model segments

Model grapes

In order to make the overall process more efficient, end models are connected to a sprue / sprue system where possible, creating a so-called model cluster or cluster. Different numbers of models can be attached to this cluster of models.

Finishing

Finishing process

In a dipping process, the finished model grapes are coated with a water-based, ceramic coating. The purpose of this process is to divert the liquid and gaseous components and thus control the filling of the mold. In addition, the size forms a refractory barrier. The model grapes are then dried to reduce residual moisture. Residual moisture can create porosity in the casting. This is usually done in an oven at 50 ° C, although microwave drying is also possible.

Molding

During molding, the now dried model cluster is placed in a casting container and sequentially sprinkled with a molding material . Usually quartz sand is used as molding material, but mullite can also be used. In order to shape the model or models true to detail, the model cluster is exposed to vibrations , which allows the molding material to flow into all cavities.

The pouring

The molten metal is fed through a pouring runner and a gate system into the foam model , where the thermal energy decomposes the model cluster. The melt now completely fills the vacated cavity and the exact part contour is thus formed after the metal has solidified.

Emptying

Emptying is done by tilting the casting container, whereby the molding material ( quartz sand ) can flow out of the cavities. The quartz sand can then be fed back into the sand return system. Possible residues of the size can be removed and the mechanical values can also be influenced by a water bath at 30–40 ° C. For this, however, a grape temperature of approx. 300 ° C is necessary.

advantages

Compared to other casting processes, the lost foam process offers greater freedom of design and design in the construction of cast parts. Function-integrated cast components can be represented in one piece through the layered structure of the model segments, which thus also offers a cost advantage. The process requires only minor post-processing of the casting (near net shape). The elimination of machining processes, built-in parts, screw connections, etc. also results in reduced assembly and machining costs. In addition, the individual work steps can be easily automated. This then results in a high degree of flexibility thanks to short set-up times. Long EPS mold life is also an advantage, which results in lower proportional tool costs.

disadvantage

Disadvantages of the lost foam process are, for example, the high demands on the model quality, the decomposition of the model during the casting process and the reproducibility of the mold filling behavior. The models or model clusters are sensitive, so there is a risk of deformation or damage during finishing, drying and molding. It can be disadvantageous if one does not guarantee the uniform discharge of the EPS decomposition products during the casting process. In addition, the decomposition process of the model creates hydrocarbon compounds during casting , which necessitate the use of extraction systems.

Possible components and areas of application

With the lost foam process, complex components can now be produced that cannot be produced in any other casting process. The advantages of the process allow complicated geometries to be implemented easily. Examples can be found in automotive supply, but also in many other areas where casting processes such as die casting or permanent mold casting reach their limits.

literature

Lost Foam Inside: An exchange of experiences about the lost foam casting process. 1st edition. Lost Foam Council eV, 2010, ISBN 978-3-9813947-0-2 .
Michael Hagemann: Influence of the Castyral process on selected component properties in aluminum full mold casting. Dissertation. Shaker Verlag, 2002, ISBN 3-8322-0490-3 .

Web links

Individual evidence

^ Translation for cluster from dict.cc

↑ Definition for bleed system ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. , from: Foundry Lexicon@1@ 2
↑ Product examples from different areas (PDF download; 3.3 MB), accessed on May 14, 2012.

[1] Translation for cluster from dict.cc