Lost Foam Casting Process
Production of Highly Complex Components Using the Lost Foam Casting Process
The Lost Foam casting process is based on the American full-mold casting process, which has been known since the 1950s. This approach made it possible to produce complex components suitable for mass production in one processing step.
In the lost foam casting process (LFCP), polymer foam models are molded into binder-free sand and cast directly. The hot melt then decomposes the model. This casting technique using lost molds and models offers the greatest possible degree of freedom in the design of multifunctional castings, even without the use of sand cores.
In addition to the high component complexity, the advantages of the process include the possibility to integrate secondary functions and to reduce the weight of the casting compared to other processes as well as the reduced effort involved in machining the raw parts and the fact that there is no misalignment or burr.
The versatility of the lost foam casting process opens up countless new possibilities for the production of castings and complements other casting processes perfectly. The process enables both the lucrative production of prototypes and small series as well as the production of large series. The LFCP is basically divided into two processing steps: model production and actual casting.
For the production of aluminum castings, the expandable raw material polystyrene (EPS) is mixed with the blowing agent pentane, while a copolymer of EPS and EPMMA (expandable polymethyl methacrylate) is used to produce models for iron and steel casting. Models for LFCP components are produced with conventional foaming technology for larger series or milled from a block for prototypes and small series.
In the foaming chamber, steam is added to make the EPS material soft and to cause the pentane to expand. The EPS beads expand to about 30 times their original volume.
For the production of the model segments, compressed air blows the pre-expanded EPS beads through an injector into a metallic permanent mold (foaming tool), where they are welded to the desired model geometry. This is done by re-vaporizing the entire surface of the mold with water vapor, which is inserted into the cavity of the mold via slot nozzles or holes.
The models, which have been broken down into undercut-free segments for production reasons, now have to be reassembled into a complex model. In series applications, this is done with the aid of machines that process hot melt adhesive, fix the model segments and complete the joining process. In individual cases, the model segments are also joined by other adhesive systems or joined by welding, clipping or plugging. In order to increase the efficiency of the overall process, several models are combined with a sprue system wherever possible.
The assembled (joined) clusters are dipped in a water-based ceramic coating. The purpose of this coating is to dissipate the liquid and gaseous components of the model material, which arise during the casting process because of the decomposition of the model, into the molding material and thus to control the mold filling. In addition, the coating forms a supporting and refractory barrier that prevents mold material from penetrating into the cavity created by the decomposition of the model material. After finishing, the clusters are dried to prevent porosity in the casting caused by residual moisture.
The Casting Process
The Lost Foam casting process can be divided into three different stages:
- 1. Manufacturing of a polymer foam model
- 2. Coating of the model
- 3. Casting process to finish the component
The coated cluster is placed in a flask and sequentially sprinkled by a precisely measured quantity of unbound molding materials (usually quartz sand). The uniform shaping of the cluster is produced by vibration. This causes the molding material to begin to flow and fill all the cavities of the model. In order to achieve uniform compaction of the molding material, the frequency, direction and amplitude of the vibration must be set according to the molding material height, model geometry and flask.
The molten metal is poured into a funnel placed over the sprue and fed into the model by means of the previously applied casting runs and gating systems. The thermal energy of the melt decomposes the model, the cavity that is released is completely filled by the metal and, after solidification, exactly reproduces the model geometry. The decomposition products, mainly gases, are discharged via the coating into the binder-free molding material.
After the melt has solidified, the flasks are emptied by tilting. The molding material flows out of the cavities of the castings and the cluster is demolded. In order to remove any coating that may still adhere and to influence the mechanical properties of the alloy, the entire casting dust can be dipped (quenched) into a water bath of 30 °C to 40 °C immediately after the residual casting material has been emptied at a temperature of approx. 300 °C. Then the parts are forwarded to further processing steps and quality assurance.
Compared to other casting processes, the lost foam process offers greater freedom of design as well as the possibility of weight reduction due to the segmented structure of the positive models. Draft angles can often be completely omitted. In addition, the process offers the possibility of realizing components in one piece that were previously cast in several parts. The pre-casting of bore holes, the elimination of mold divisions and the resulting elimination of misalignment of the molded parts, significantly reduce post-processing efforts of the raw part. The reduction or even complete elimination of machining and assembly processes can significantly reduce processing costs. Another cost advantage results from the relatively inexpensive tools, which hardly show any signs of wear. Moreover, the process can be carried out by a small number of operators. Since the individual process steps can be fully automated, the process can be used for both the production of prototypes and small-series production as well as for large-series production.
In addition, it offers the following advantages:
- Omission or reduction of venting
- Omission or reduction of feeders
- Easy demolding
- Flexible integration of secondary functions
Furthermore, there are some positive ecological and health aspects:
- Low physical strain due to light models and low odor development
- Low cleaning effort, low dust and noise emissions
- Molding material can be reused
- No binding agents required
On November 7th and 8th 2018, an international conference on the lost foam process took take place in Bremen, which offers an excellent platform for exchanging views on this innovative casting process with experts from all parts of the world.
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