Precision casting method is known as “Investment Casting”, “Ceramic Shell Casting”, as the English word means. The precision casting method is also called the “disappearing candle” and its history dates back to ancient times. The reason why the precision casting method is called “lost wax” is due to the fact that the part to be cast is first shaped from a wax model, and then covered with a heat-resistant material to form a mold, and the wax inside melts and disappears by heating the mold.
The oldest known source on the precision casting method (Schedula Diversarum) is a source by a monk who was a member of the Theopilus Church board, describing precision casting, giving information about the production processes, how it is made and applied. Benvenuto Celini produced the historical Medusa statue produced by precision casting method in Italy.
Precision casting method 19. towards the end of the century, it entered a modern industrial process and began to be used in the field of dentistry. in 1901, the American-born dentist Tagart produced the filling material by precision casting method. After filling the wax into the carved tooth cavity and making a correction by giving it its original shape, taking the wax model, embedding it in a heat-resistant filling ceramic and heating it, removing the wax inside, it was in the form of pouring metal into the remaining filling cavity using an unknown air pressure or a centrifugal-force machine. For this reason, the use of precision casting method in the field of prosthesis making and jewelry making in dentistry is also known as “centrifugal casting”.
Today, precision casting method is used in almost every industrial field where metal parts production should be. It is easily used in electronic parts, in a microwave oven, in trigger, pistol, rifle bodies that are part of a weapon, in the production of binoculars and similar parts.
The precision casting method makes it possible to cast parts with a wide variety of shapes and details of small size.
Precision Casting Method
The sensitivity of the method comes from the expendable model used. The models are produced from a plastic-based material, usually wax or similar, which has a low melting temperature and can be easily shaped.
Since the model is removed from the mold by melting, there is no separation surface in the molds, depending on the model and mold materials used, the surface quality and dimensional accuracy are high compared to traditional sand mold casting methods. Precision casting minimizes post-casting operations, as it provides a production close to the final or final shape.
Stages of Production by Precision Casting Method
Candle Model Production and Model Tree Creation
First of all, a model of the part that is desired to be cast from metal is prepared from wax or similar substances. Models that are as similar in size and weight as possible should be lined up on the same tree. Thin models and thick models should not be placed in the same model tree. After casting, the cooling and shrinkage estimate is made and the model is prepared a little large. The created wax models are attached to the body.
Refractory (Ceramic) Coating
The produced panicle models are immersed in a ceramic mud bath and the coating process is completed. In the sprinkler, the inflorescences are covered with refractory dust. The process is repeated until the appropriate shell thickness is obtained.
After the ceramic coating dries, the wax is drained and made suitable for casting. The outer hard ceramic coated part is baked and the candle model is melted and taken out. Since the ceramic is resistant to high temperatures, its shape remains intact. Due to the high temperature, the parts of the wax model that do not flow out are also evaporated. The oven temperature is increased. The ceramic shell is made resistant to metal by baking.
Molten metal is poured into the cavity of the ceramic molds left over from the wax panicles.
Breaking The Shell
After the metal has cooled and solidified, the ceramic shell is broken to reveal the piece. It is the next stage after the molten metal that fills the gap in the mold cools down and transforms back into a solid state. Solidification may vary depending on the characteristic properties of the metal. This event begins with nucleation. It continues as grain growth with the addition of liquid atoms to the formed nuclei.
Finishing Operations and Cleaning of the Part
The parts are separated by cutting from the body. Necessary deburring cleaning After surface polishing, the metal parts are separated from the runner. The necessary grinding process is performed. After the final quality test, the production is completed.
The model panicle made of wax is first immersed in refractory mud for casting metal materials that melt at high temperatures exceeding 1000 °C. After preliminary coating, the model inflorescences are molded with a refractory mixture in degrees. After the coating process, the molds are inverted and heated to a temperature of 95 to 150 °C to remove the wax that melts inside. After the metal casting process, the cooled metal is then divided into pieces and the cleaning and polishing process is performed.
Classification of Precision Casting Method
There are two known methods of precision casting. These;
1- Ceramic shell molding
It is 2-Grade shell molding
The basic processes that are also valid in each method are the preparation of the candle model, the construction of the ceramic shell, the melting of the candles in the shell, the baking of the shells and the casting of the molten metal. After casting, other changing processes can be applied optionally. (Coating, heat treatment, machining, etc.)
Precision casting method with ceramic shell is frequently preferred in the casting of carbon steels, alloy steels, stainless steels and heat treatment resistant resistant alloys. In this method, the models are immersed in a mud consisting of ceramic powder suspended in a liquid, and dry refractory grains are sprayed onto the wet ceramic-coated surface using a fluidized bed or another method and covered with adhesion. In the initial coating, which is called pre-coating, mud consisting of finely ground small grains is mostly used, and thus a smooth smooth surface is obtained. The smoother the pre-coating, the better the smoothness of the casting surface.
If we briefly summarize the graduated precision casting method, the application is made in two different ways for iron-based and non-iron-based alloys. In iron-based alloys and non-ferrous alloys with high casting temperature, the mold material must be able to withstand up to 1100 ºC without deteriorating. Gypsum, the use of which is possible for non-ferrous alloys, cannot be properly used as a binder or refractory material in iron-based ones. For alloys other than iron, no pre-coating is required in the model, while pre-coating is required for iron-based alloys. The mold is made in one stage by pouring the prepared ceramic mud into the degree where the model is placed. Although the graduated method is still used, the shell precision casting method is more popular.
Advantages of the Precision Casting Method
Advantages of Precision Casting method compared to other production methods;
1-) Special parts that are difficult or impossible to produce by traditional casting methods and machining processes can be produced by precision casting method.
2-) Special parts that are difficult or impossible to produce can be easily produced compared to traditional methods.
3-) Mold separation lines formed in parts produced by traditional casting method do not occur due to the fact that a single-piece mold is used in the precision casting method.
4-) The production of special parts is smoother than traditional methods.
5-) With precision casting technology, it is possible to produce mixed-shaped parts that are difficult or sometimes impossible to produce in short periods of time and in large quantities.
6-) While a product is produced in two parts in traditional casting methods, it can be produced in one piece as more aesthetic, stronger and lighter thanks to precision casting.
7-) It is possible to produce products requiring special production in a short time and in large quantities with precision casting technology.
8 -) Products produced with precision casting technology give lighter, stronger and more aesthetic results. Thanks to this technology, details such as cracks, holes, channels, pits are realized quite successfully. Since the prepared molds are low-cost and longer-lasting compared to other molds, the production cost is minimized.
9-) With precision casting technology, more detailed products and molds can be produced compared to traditional methods.
10-) Since the mold cost required for the precision casting method is low, the production cost is minimized.
11-) Casting of various alloys is possible in precision casting method. Materials with the requested chemical composition can be produced.