Investment Casting Process
The investment casting process is a popular metallurgical technique used to create detailed industrial parts and components through the metallic replication of wax models. Though metal workers have several processes to choose from, this particular method of production allows for the creation of near net shape products with great detail that would otherwise be difficult or impossible to achieve with other casting methods and secondary processes such as welding and soldering.
Investment casting is among the most basic techniques used and as such delivers reliable results at a reasonable cost, though it is not always the most economic of options. In short, this multi-step process involves forming a mold around a wax pattern and allowing it to harden before melting away the wax, leaving a hardened shell that can be filled with molten metal and removed once the desired form is sufficiently cooled. The process is extremely versatile and with proper knowledge of the chemical formulas and molecular composition of the materials involved, manufacturers can instill specific attributes such as durability, magnetism or weight load while diminishing or eliminating negative characteristics. Proper mechanics also allow for castings to be as small as fractions of an inch thick or as much as 1,000 pounds in weight, though most investment casting products are 15 pounds or less. Such capabilities find investment castings in the aerospace, automotive, chemical, defense, food processing, electrical, railroad, mechanical, marine, electronic, textile, engineering and several other major industries.
Investment casting products have wide-ranging variation which make their way into just about every facet of daily life. Products vary from surgical instruments, to agricultural machinery parts, to aerospace-grade airplane engine turbine components, and nearly anything else that can be produced from metal alloys. These metal alloys include stainless steel, aluminum, brass, nickel and cobalt based alloys, among others. Investment casting products are vetted for their smooth finishes making them highly sought-after commodities.
Precision investment casting is a term used to differentiate between regular investment casting processes and more intricate steps that are taken to ensure adherence to the most exact tolerances. Precision investment casting is highly involved and investment casting manufacturers are forced to comply with minute aerospace-grade tolerances. Precision casting takes the extra step to ensure the greatest levels of quality.
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Investment casting process steps stick to a uniform order. The term lost wax casting dictates the wax pattern be the initial procedural step, followed by the shell process, dewaxing of the created ceramic mold, then the pouring or casting of the molten metal, a slurry knockout and leeching phase, cutoff of the formulated parts, and then finally any post foundry finishing operations that may take place in an investment casting foundry – although these are minimal due to the casting process itself which eliminates much of this need. Casting processes initiate with already mentioned wax assembly and then move on to shell.
A more extensive look at the shell process, which comes after the wax casting process, ordains the ceramic shelling procedure take place in dip rooms that stay at a constant regulated temperature and humidity level to prevent changes in dimensions. The initial shelling step in an investment casting foundry is to place the completed wax mold in an acidic wash for a short amount of time which allows for the mold release agent, whose job it is to prevent materials from bonding to the mold, to be liquified off the surface so the ceramic shell slurry can better adhere to the mold.
The ceramic slurry itself undergoes a carefully mixed batch of powdered chemicals and distilled water (if necessary) until the correct consistency is met. Depending on the product being produced, different formulas and consistencies need to be met to accommodate the specifications required for that product.
Once the slurry is prepared the mold fixture is ready to be dipped into the tanks that contain the ceramic slurry mixture. This process can be done by hand; however, automated robotic arms are becoming the modern norm eliminating the need for live bodies in the dipping process at many manufacturing facilities. The robotic arm is programmed with appropriate parameters of depth, angle, and rotating speed ensuring the coating of the entire surface and all cavities with slurry.
On the first day of dipping the finest applications of slurry and casting sand coat the tree cluster mold. After the mold cluster has been dipped into the slurry tanks, it is then moved to either a fluidized sand bed or a rainfall station where ceramic casting sand is applied as the mold is being constantly rotated. After this procedure has been completed the hanger is hung to dry until the next application can commence. This process repeats itself multiple times until the shell is created.
Having completed the shell layers, the commencement of removing the inner wax from the ceramic mold can take place. This is done by burnout process whereby the shelled mold is flipped over upside down onto its pouring cup and placed inside of a burnout kiln, or what is called an autoclave. This heated chamber reaches high degrees in temperature hot enough to melt the wax casting cluster contained inside the shell leaving the inner cavity intact in the precise pattern it was originally formed.
This being the most involved of all the investment casting process steps and therefore the defining operating for investment casting manufacturers. The pouring casting processes may vary greatly in different facilities. In this phase of production, we come to the pouring of molten metal into the formulated ceramic mold. The alloy melting techniques can be accomplished by using a vacuum sealed melt chamber, an air melt rollover machine, or a centrifugal casting machine. The basic methodology for vacuum and air melt is the same – melt the chosen alloy inside of a crucible to thousands of degrees in readiness to pour the molten metal.
First and foremost, the ceramic shell model must be placed in a kiln and preheated for several hours to the appropriate temperature and time frame requirements (normally two or three hours). This is done so that the mold will closely match the temperature of the ready to pour molten metal to prevent the cracking of the ceramic shell.
In the vacuum seal melt process, or sometimes called vacuum-assisted casting, the different techniques as well as chamber devices have great variation from metal foundry to metal foundry. Nonetheless, the basic procedural process stays constant. The crucible and mold are placed in a melt chamber and pumps are used to suction oxygen. This process clears impurities that may interfere with the pouring of liquified metal alloy into the ceramic mold.
Conversely, in air melt rollover machines, a crucible is fitted into the casting box of a machine with 180-degree rotational abilities. Once the crucible has been compacted into the box with dry sand, a clay solution is used to place a cap over top the crucible fixing it in place. Then the appropriate height of the clamping mechanism is fixed so that the ceramic mold may be held down firmly without being crushed. As soon as the clay cap is dried and hardened, the pouring process may commence. Argon is turned on and flows through the top of the crucible eating away at any oxygen that may allow for contaminating impurities. It is at this point that the chosen metal ingot is placed inside the crucible and heated to thousands of degrees of temperature to be melted. Once the melted alloy has reached the appropriate levels, the burnout kiln is opened and the mold is retrieved and placed over the crucible opening with the pouring cup upside down and the clamp is secured over the mold bracing it in place in preparation for the 180-degree rollover. When the clamping pressure has been met the rollover may be turned over at the appropriate turnover speed per second so that the metal alloy contained within the crucible can seep into each cavity of the mold making sure to fill each point of the casting dies.
This technique in the metal casting process is often applied to manufacture cylindrical shaped components or metal piping. The most common machinery used in an industrialized setting is the horizontal centrifugal casting machine. The premade mold is placed inside the centrifugal casting machine and high-speed rotating begins. Next comes the pouring of liquified metal into the rotating apparatus and enough force builds during rotation that the metal alloy is subjected to 80 to 120 times the acceleration force of gravity. The metal hardens from the outer surface to the inner core providing a uniform and balanced coat around the mold and leftover slag is propelled to the bore for easy extraction.
Knockout and Leech
After casting, the tree cluster must be cleared of the casting shell and this is done by either hammering the mold by hand or by placing it in what is called a knockout machine where the mold is jackhammered and vibrated from the top until the biggest clumps of ceramic have been removed. Any excess slurry is then dissolved using a corrosive salt bath.
Cutoff and Post Foundry Finishing Processes
Once the completed mold has been cleared of all excess slurry it is then time to extract the desired dies from the tree cluster. This is typically done by high-speed saw cutoff wheels. If after cutoff there is leftover gate the die piece can be taken over to a grinder wheel and grinded smooth, or within a certain specified tolerance, and the casted piece may also be sand-blasted.
A finishing process that is less intrusive than machined materials often follows the cutoff process. Any blemishes to the dimensions of the now free individual part are carefully grinded off using a mini hand grinder. All imperfections are finished off and the surface is now completely smooth. All these are added luxury procedures to the precision casting process when required.
Following finishing, the near complete casting piece may undergo heat treatment. This is done to increase stability and harden or strengthen the product if desired. Samples of the completed run are also checked by quality assurance looking for distortions from the investment casting process. If all checks pass then the entire investment casting process is complete giving finality to the casting manufacturing process and the manufactured product is ready to be expedited.
An alternative to the lost wax casting method is sand casting. Whereas with wax casting the mold is made from a wax pattern, the mold in this operation is made from a cast made of sand. Typically, two halves are joined together to get the exact negative image of the part. The sand casting process is first done by placing the die in between two mold frames called flasks – the bottom flask is called the drag and the top flask and cope – and then sprinkling with powder and buried under a finely sifted sand and clay mixture to preserve the smooth dimensions of the piece. Then both flask sides are totally immersed under the remaining coarse and clumpy sand mixture. The sand casting mold is then concluded by tightly compacting the sand making sure to leave at least one opening in the cope using a pipe with an appropriate circumference for pouring that is later removed. In the finalization of the sand casting process, the cope is then taken off to remove the die and then placed back on and at this point the sand mold is concluded leaving an exact negative replica and ready for the pouring of molten metal.
Stainless Steel Casting
Cobalt and nickel based stainless steel is probably the most preferred of all casting alloys. Low levels of corrosion and abrasion resistance make it optimal for the metal casting process. Stainless steel casting is quite often preferred for its aesthetically pleasing qualities as much as anything else because of the finely polished finishes that make any die casting product stand out visually. Advantages of this alloy or its abilities to undergo several stabilizing and hardening procedures such as heat treating, solution annealing, induction hardening, and case hardening.
Investment casting with aluminum provides a stronger alloy alternative to basic steels used to manufacture products. This alloy has reflective properties and has great ability to conduct both heat and electricity, and it’s much more lightweight (about a third lighter than steel) and for that reason is often ideal for aerospace and automobile based production. Just as with stainless steel, aluminum is desired for its aesthetic features when polished and refined. Thus, making Aluminum casting widely used for medical tooling and well as kitchen cookware and weight reduction components for automobiles, airplanes, and the like. Its nonmagnetic and nontoxic properties also make it ideal for these aforementioned industries.
Iron constitutes about 35 percent of the planet and therefore is the cheapest form of metal used in the casting manufacturing process. Iron alloys are desired because of their low melting temperature points. Iron casting is highly resistant to deformation and wear which is why it is used for cylinder heads, cylinder boxes, and gearbox cases.
Brass is forged with a combination of copper and zinc at high temperatures. In addition, other alloys such as aluminum can be added to increase the corrosion resistance of brass and lead to increase its machinability. Brass casting products can be a bit more complicated to produce due to its delicate softness and other metals such as iron or tin are often added to strengthen its properties. Doorknobs, pipes and machining components are often casted from brass-based foundry manufacturing.