Invest casting is a famous manufacturing technique that uses a wax pattern which is coated with refractory ceramic materials. The technique is based on the ancient process of lost-wax-casting. The ceramic material that is poured which then hardens and takes the shape of the wax pattern. The wax is subsequently melted and extracted. The ceramic pattern is now ready for the molten metal which is poured to replace the wax. Once the metal solidifies, the ceramic casting is broken, and the shaped metallic part is extracted.
An Overview of the Process
- Preparing the Pattern- The first step is preparing the wax pattern which determines the overall shape of the final product. Since each wax pattern is melted out to create the final product, it needs to be created every time for each casting. The wax patterns are made in a master die whose dimensions are defined precisely. It is important to maintain this mold routinely and look for changes in dimension as it decides the quality and shape of the final product. While designing this master die, various shrinkages of wax and ceramic must be taken into consideration. This master mold can have complex geometries since it needs to be made only once.
- Coating the Pattern with Ceramic- After the wax pattern is ready, a cup or funnel is attached at the top of it. The mold is then dipped in a sludge composed of fine-grained silica, water and a variety of binders. The mold is subsequently dipped several times more to form a thick layer of ceramic over the wax surface. Once the coating has achieved the desired thickness, it is allowed to dry out.
- Extracting the Wax Pattern- Once the ceramic coating has dried, the entire structure is turned upside-down so that the cup is at the bottom. The pattern is then heated to temperatures of around 200F-375F. The wax trapped inside the ceramic coating melts and easily flows out through the cup. The area previously occupied by the wax pattern is left vacant for the molten metal.
- Pouring the Molten Metal- The ceramic mold which has been freed from the wax is further heated to around 1000F-2000F in order to enhance the material strength and durability. This also eliminates any leftover water residue or wax deposits from the interior which might cause errors in shape and dimensions. The molten metal is poured into the mold while it’s still hot. This practice not only ensures better flow of metal through all the complex pathways but also ensures dimensional accuracy as the metal and mold cool and shrinks together. Once the liquid metal settles in, the mold is set aside for the metal to solidify completely.
Breaking the Ceramic and Quality Checks- The final step involves breaking the ceramic mold and retrieving the metal component which has taken shape. After removing all forms of residue, the component is inspected for any damage, irregularities or changes in dimension. After the necessary quality checks, the process is confirmed to be co