Expendable pattern casting

In expendable pattern casting, the pattern is not removed from the mould before pouring. The pattern, which is made of expanded material, is a single-use one, which is destroyed when poured. These expandable patterns may be embedded either in chemically-bonded sands or in binderless sand, hardened by vibration. 

This process, commonly called Lost Foam casting , was developed 30 years ago and its commercial growth was initially rather slow. However, during the last 10 to 15 years, it is has become used more often, primarily for the mass production of automotive parts or similar products, in spite of significant set-up difficulties. 

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Expendable pattern casting (Lost Foam/full mould casting) 3.9.6.1 Unbonded sand - Lost Foam... 

A variation of investment casting is lost-foam (or expendable pattern) casting. Here, the expendable pattern is a foam that can be formed by compressing polystyrene beads into... 

This process is used to produce intricate, thin-section parts with great dimensional accuracy, fine detail, and very smooth surfaces. All ferrous and nonferrous alloys can be cast in investment molds. Investment casting begins with expendable wax patterns that are assembled into clusters, then coated with a series of successively coarser ceramic slurries. The assembly is then fired in a furnace to dry and harden the ceramic shell and to melt out the wax, leaving a cavity into which molten metal is poured to form the casting. 

Lost-foam sand casting Sand (expendable but recyclable) pattern is consumed Ferrous and nonferrous As above plus no need for parting line better surface finish than conventional sand casting increased dimensional accuracy Increased cost, added process steps As above... 

Figure 2.37 Expendable polystyrene pattern (r.) for the Lost Foam process and an example of a casting produced by the Lost Foam method (1.)...

The process steps are depicted in Figure 2.38. They begin with the manufacture of expendable wax patterns, by injecting molten wax into an aluminium or epoxy die to form a pattern that is virtually an exact replica of the desired casting. The wax may contain fillers. For smaller castings, several wax patterns are attached to a wax gating system. Water soluble die release agents are used to facilitate the wax model take-out. 

The process starts with the production of an expendable wax pattern of the shape required which is then coated with a refractory material to produce the mould, which is allowed to dry. The mould is heated, melting the wax, which is allowed to run out and so produce a cavity. Further heat is applied to fire the mould before pouring the casting metal to fill the cavity left by the melted wax. When the molten metal has solidified, the refractory shell is broken away to release the casting. The casting is cut away from any runners and is dressed or fettled and finished as required. 

The expendable wax pattern, exactly the shape and size of the required casting, with allowances for pattern contraction and contraction... 

Investment casting (lost-wax process) n. A metals-casting method in which patterns made from wax or other expendable material are mounted on sprues, then invested , i.e., covered with a ceramic slurry that sets at room temperature. The set slurry is then heated to melt away the pattern, leaving a mold into which metal is poured. 

The expendable wax pattern, exactly the shape and size of the required casting, with allowances for pattern contraction and contraction of the casting during solidification, is produced in a split pattern die. Fig. 18.1. The wax is injected into the pattern die in a plastic state, under pressure, and solidifies quickly. It is then ejected from the pattern die. Fig. 18.2. Preformed ceramic or water-soluble cores can be introduced into the wax pattern for the production of castings which... 

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