Table of ContentsEdit
- Process Variants
- Design Considerations
- Relevant Calculations of Parameters
- Typical Tools
- Geometry Capabilities
- Surface Finishes & Tolerances
- Future Development Opportunities and Shortcomings
- Further Readings
Counter-gravity casting is a gravity pouring technique that utilizes a vacuum. In this type of casting, a fill pipe is lowered into the melt and a vacuum draws the melt into a cavity. The vacuum is released and the material not used in creating the casting leaves the mold. This process was patented by Hitchiner Manufacturing in 1972. Over the years, different variations of this process have been created. This casting process offers a unique array of advantages in the areas of defects reduction, detailed casting shape, and casting economics. This technology has been prevalent in both the power and automotive industries, one example being in automotive exhaust manifolds. (InTech)
In 1972, the counter-gravity casting process was invented by Hitchiner Manufacturing. This process is known by the acronym CLA, which stands for Counter-gravity Low pressure Air melt. The reason behind the creation of this casting was economics. in 1989, Greanias and Mercer developed a valve system that could increase throughput in this casting technique, by allowing the mold to be disengaged prior to complete solidification. In 2007, Li Et Al developed a multifunctional system that had capabilities of combining various systems of technology into a single piece of equipment. Although this casting process has shown to be important, the interest and knowledge in counter-gravity casting and other casting processes has not greatly expanded.
Process Variants Edit
The basic counter-gravity process is known by the acronym "CLA" for Counter-gravity Low pressure Air melt. In this process, the mold is placed in a vacuum chamber that is sealed and lowered into the melt. The vacuum created siphons the metal up into the sprue cavity. This is shown in pictures A, B, C, and D. (Hitchiner)
Over the years, the CLA process has development to creation various design adaptations for specific applications. A few of these processes are listed below:
CLV stands for counter-gravity low pressure vacuum pressure. This process uses counter gravity casting technology with reactive alloys that have to be casted in vacuum atmosphere. The metal is melted in the lower chamber; the hot mold is in a separate upper chamber. Both chambers are filled with Argon, a valve is opened, and melt is raised until the sprue snout enters the molten metal.
CLI stands for counter gravity low pressure and includes an inert gas covering the melt in order to create reactive alloy castings quickly and efficiently.
The C-cubed process uses centrifugal force to aid in mold fill out in thin sections.
SSCLA is a version of CLA that uses sand to support the mold.
SLIC stands for Several Layer Investment Casting. This is the fuel-efficeint counter gravity process introduced by Hitchiner. In this process, the shell material is reduced by 70%, mold-heating energy by 87.5% and needed floor space by 70%. This is done by reducing the number of shell layers needed to make molds.
Basic steps of the Hitchiner process:
- Wax injection- the desired casting is made with a wax imitation through injection molding.
- Assembly- each of the wax replicas are attached on a central wax piece, called the sprue. This forms the assembly.
- Shell building- the assembly is dipped in a liquid ceramic material then fine sand. This is done up to eight times to make up the shell building.
- Dewax- Once the ceramic shell hardens, the wax replicas are melted out, leaving a negative impression of the assembly.
- Casting- Using gravity pouring, the ceramic shell is filled with molten metal. Once the metal cools, the parts and gates and sprue become one solid casting.
- Knock-out- Using vibration or water blasting, the ceramic shell is broken off, leaving the solidified metal casting.
- Cut off- The parts are removed from the sprue using a high speed friction saw.
- Finished castings- After minor finishing procedures, the metal castings are complete, exactly the same as the wax models.
Design Considerations Edit
As a part of investment casting, there are certain advantages and disadvantages to using counter- gravity casting process:
- The wax mold forms intricate details and undercuts
- Smooth surface produced with no parting line
- Good dimensional accuracy
- Parts that are otherwise unmachinable can be casted to preplanned shapes
- For short runs, investment casting can replace die casting
- Better metal efficiency compared to other casting processes
- There is less turbulence, which allows gating process to be simplified
- Metal produced is free of dross and slag, which are lower density and float to the top
- Lower temperatures can be used, improving the grain structure
- Limited to smaller castings
- Holes cannot be smaller than 1/16 inch and no deeper than 1.5 times the diameter
- Production cycle times are longer compared to other casting processes
- Process is nearly infeasible for high volume manufacturing because of high cost and longer cycle times
- Most of the advantages of investment casting can be filled by other casting processes if principles of thermal design and control are applied (other casting processes can often replace this process with fewer disadvantages)
Key Decision Factors
The key factors to be reviewed before choosing counter-gravity casting are listed below:
|Decision Factor||When to Use Counter- Gravity Casting|
|Casting size and weight||Smaller castings|
|Quantity demand||Nearly infeasible for high quantities|
|Casting design||Works well for intricate design and details|
|Mechanical properties and integrity requirements||Strong grain structure|
Relevant Calculations of Parameters Edit
The characteristic of the mold filling under counter-gravity is that the molten metal is forced to fill mold cavity against gravity under the pressure difference and the casting solidifies under a determined pressure. The pressure difference is one between the pressure on the surface of the molten metal in the crucible and the pressure within the mold cavity (P2 – Pm). Obviously, there is the following hydrostatic relation:
(P2-Pm) = ρgh + L + V
where, ρ is density of the molten metal, g is gravity acceleration, and h is the height from surface level of the molten metal in the crucible to the top level of the filling front; L is Laplace force, depending on the wall thickness of the casting and the surface tension between molten metal and the molding materials; V is the viscosity force; it has something to do with the physical properties of the metal and temperature. The pressure difference (P2 – Pm) can be created by the following three methods:
1. Making the upper chamber negative pressure at the same time maintaining the lower chamber at atmosphere, which is also called vacuum absorption. For example, the famous CLA process depends on this principle).
2. Making the lower chamber positive pressure at the same time maintaining the upper chamber at atmosphere. The widely used “low pressure die casting” depends on this principle. In fact, for low-pressure casting, no upper chamber is needed.
3. Making pressures within upper and lower chambers increase simultaneously to a determined value, and then making upper pressure reduce at the same time maintaining the lower
Typical Tools EditThe basic process steps for the vacuum casting process are presented as follows. In the diagram in figure 1, a preheated investment mould with an integrated down-sprue (fill pipe) is positioned in the moulding flask. The sprue, with a conical-shaped intersection point with the rest of the mould, pokes through and sits in the conical depression of the lock-nut. The otherwise solid investment mould is made permeable by a single opening at its apex. This opening effectively connects the mould cavity with the interior space of the moulding flask, making it an extension of the moulding flask and enabling its evacuation along with the rest of the flask. The flask lid hosts the casting valve, a connecting hose to the vacuum system and lid locking mechanism. The electrical resistance furnace melts the aluminium charge, usually by a superheat of about 40 °C above the melting temperature (660 °C) of aluminium to reduce melt viscosity and ease melt up-flow into the mould. During countergravity casting, the moulding flask with the mould assembly inside, is placed on the furnace lid with the down-sprue poking through a hole in the furnace lid. The vacuum system evacuates the molding flask and the ensuing low pressure thus created causes ambient atmospheric pressure on the melt to push up the molten metal, up inside the mold. See figure 3
Geometry Capabilities Edit
Unlike many of the popular casting methods, such as investment casting and die casting, the counter-gravity system essentially eliminates shrinkage. This defect is prevented because as portions of the mold is solidifying, the sprue becomes the last to solidify due to there being a constant and maintained supply of fresh melt during the casting. Because of the near elimination of shrinkage the counter-gravity process mold filling allows for producing net-shape cast products. For aluminum metals the process can obtain thin-walled sections of about 0.5 millimeters and complex details (InTech). For higher temperature alloys, such as steels, has been reported to have thin-walled sections of 2-3 millimeters (Metal Casting Technology, Inc).
The original purpose of the counter-gravity technique was to decrease the amount of gates that needed to be re-melted. Also, due to the interior of the mold being an extension of the vacuum system, the gating system can be simplified to branches of flow channels protruding from the central sprue. The large pressure difference between the mold and the reservoir in the crucible allows the molten metal to completely spread to each cavity in the mold.
In the more conventional casting techniques there can be a higher variability in strength. It has been noted that the Cosworth Process has been successful in creating high strength structural components for air frames, gun cradles, and air tanker re-fuel ling manifolds (InTech).
Surface Finishes & Tolerances Edit
The counter-gravity process vacuum’s the molten metal from below the melt surface, which contains the metal oxides formed and aggregated. Also, the process reduces the turbulence during the mold filling unlike that of pouring techniques. Therefore, because we are using a cleaner melt for the casting we would have better surface finishes than conventional methods. The elimination of the risers and simplification of the gating system also allows for lower tolerances (InTech).
Future Development Opportunities and Shortcomings Edit
The counter-gravity process is competing with methods that have been used for years such as investment and die casting. It does not due the process any good that the cost of the mold and handling equipment can range from $50,000 and $1.25 million. This pricing was reported in 1998 so if one were to take into account inflation these prices would be much higher. These costs are all due to the high-temperature mold pre-heating ovens, the positioning units, and a complex vacuum control system, as well as the cost to set up and operate the machines.
The process is also limited to smaller sized components, mostly less than 50 kilograms. To allow for the proper orientation of the vacuum system, the molding flask can usually be small (larger flasks are more difficult to evacuate and maintain at a desired vacuum).
The mold and sprue must to heat to a decent temperature prior to the casting in order to prevent melt viscosity that causes the melt to get stuck in the sprue or even incomplete mold filling. One must also have a sophisticated vacuum control or else you can either have melt splatter or not enough melt getting to the mold before it begins solidifying (InTech).
Further Readings Edit
The article, "Timeline of Casting Technology" provides insight to the substantial role of metal casting processes throughout history.
The article can be found at: http://www.pmt.usp.br/academic/martoran/NotasFundicao/LinhaTempoFundicao.pdf
"Timeline of Casting Technology." AFS Technical Department. Web. 5 June 2015. (AFS)
Aremo, Bolaji, and Mosobalaje Adeoye. "Aluminium Countergravity Casting – Potentials and Challenges." InTech Open Science Open Mind. InTech Europe, 2011. Web. 9 June 2015. (InTech)
"Understanding Investment Casting." Hitchiner Since 1946. Hitchiner Manufacturing Company. Web. 5 June 2015. (Hitchiner)
Chandley, G. Dixon (Metal Casting Technology Inc.); Redemske, John A.; Johnson, John N.; Shah, Ramesh C.; Mikkola, Paul H. Source: SAE Technical Papers, 1997, International Congress and Exposition (MCT)