Pouring metal into the mold
After that, as metal, for example, cast aluminum alloy, melted and heated to a casting temperature, it is ready for introduction into the mold. The key issue of high-quality production of metal castings is to design a good gating system. This is even more important, if the casting is produced by gravity, and not using injection pressure, low or high.
pouring molten metal into a mold to be performed carefully and neatly. Otherwise, the resulting casting after solidification of casting defects will be different, the reason for which was precisely the incorrect pouring of molten metal:
- too rapid flow of liquid metal can cause damage to the mold,
- strongly turbulent air flow can capture foreign matter and high, a
- too slow filling of the mold may lead to cold plugs.
Good gating system
Properly designed as runner system provides proper control of liquid metal flow when filling the mold.
An optimal gating system design can:
- reduce the turbulence of the molten metal flow;
- to minimize the content of gases and inclusions in the casting;
- reduce the amount of slag.
Incorrect gating system inevitably leads to violations of smoothness and continuity of the metal flow. This will result in poor quality castings. This is especially true for aluminum and its alloys casting, which are very sensitive to disturbances smooth flow of the molten aluminum alloy because of increased formation of slag and oxides.
Aluminum alloys are very actively react with oxygen to form aluminum oxide. When the flow of molten aluminum takes place smoothly, These oxides are formed on the surface of the melt and remain there. However, if the melt flow is turbulent, these oxides enter into the melt and bring to gases and inclusion. therefore, to avoid discontinuities of the molten aluminum runner system design flow thus, to avoid problems with air entrapment. This is achieved by preventing the formation of areas with low pressure, which could lead to the air sucked into the mold.
Elements of gating system
The figure below shows a cross section of a typical runner system with sand casting. This mold illustrates the basic principles of the molten metal pouring process, including, casting aluminum alloys.
Figure - Main elements of a typical sand mold.
Flask - a wooden box, wherein the molding sand is a mixture of.
Lower mold - is the lower part of the mold.
Outer mold - the upper part of the mold.
Gating system - a network of canals, which are used to supply the molten metal from entering the mold cavity in its.
Rod - is the element of sand, which is inserted into the mold, to carry out the internal parts of the casting.
Chaplet - anchorage rod.
Sprue cup - it's part of the gating system, which receives the molten metal from the ladle. The sprue cup controls the flow of metal into the mold. From sprue metal bowls should be down on the runner riser - the vertical part of the gating system, and then goes on horizontal channels - the runner moves - and, finally, controlled through inputs - feeders or gates - the cavity of the mold.
Profit - a reservoir for molten metal, which delivers the metal to the elements of the mold to prevent shrinkage during solidification.
Physical principles of gating system
To get a good design of the gating system should follow some basic principles. Molten metal is behaving in accordance with the fundamental principles of hydraulics. The conclusions of these principles can be very useful in understanding the operation of any of the gating system.
The process flow of molten metal through the runner system into the mold is controlled by the principles and concepts of continuum mechanics, such as:
- Burnley's theorem;
- the principle of continuity;
- including Reynolydsa.
Bernoulli's theorem for the flow of melt
Bernoulli's theorem - is a consequence of the law of conservation of energy for a steady flow of an incompressible fluid. Bernoulli's theorem for the flow of molten metal is, that the sum of the potential and kinetic energy at any point of such a flow is a constant. Potential energy flow determined height with respect to a reference plane. The kinetic energy depends on the flow rate.
If we neglect the friction losses and consider, that the entire gating system is exposed to atmospheric pressure, some of Bernoulli's theorem, that the rate v the flow of molten aluminum at the bottom of the feeder of the mold depends on the height h , on which the sprue bowl is located according to the formula:
v = (2gh)1/2
From this formula it follows, for example, the higher is the gate bowl, the greater the speed of runner at the entrance to the mold.
The principle of continuity of the melt flow
continuity principle is, that, for an incompressible liquid - molten metal - in a tight walls runner system the volumetric flow rate Q is kept constant. It means, that for any two points of the runner system 1 и 2:
Q = A1v1 = A2v2
A - cross-sectional area of the gating system;
v - melt flow rate of the runner system.
this implies, that for acceleration of the liquid metal flow cross-sectional area along the channel gating system flow must decrease.
Melt Flow Characteristics
When designing the gating system is very important to consider the characteristics of the flow of molten metal, on which the, be it for a laminar, Turbulent or mixed.
Laminar flow melt
In laminar flow fluid moves layers, which do not intersect. Thus, laminar flow is not necessarily straightforward. In laminar flow the current goes along the curved surfaces, and goes smoothly, layers. Moreover, fluid layers can slide relative to each other without any exchange of fluid between the layers.
Turbulent flow melt
In the turbulent flow in the main current superimposed secondary random movement. In this type of flow occurs already liquid exchange between adjacent layers of fluid. Moreover, in such a flow, energy is exchanged between slow and fast particles of a liquid: slow particles are accelerated, fast - slow down.
The Reynolds number for the metal melt
Type of flow - laminar or turbulent - is determined by the ratio of internal inertial forces in the fluid in an internal viscous forces. This ratio is expressed as a dimensionless Reynolds number (Re), which can be simplified as follows:
Re = (inertial forces) / (viscous forces)
Viscous forces arise due to internal friction in the liquid. It depends on the dynamic viscosity of the liquid. Decrease with increasing temperature.
Inertial forces are accelerating fluid resistance. Increases with increasing fluid density and flow velocity.
During the low Reynolds number inertial forces are negligibly small compared with the viscous forces, whereas at high Reynolds numbers viscous forces are small compared to inertial forces. For low Reynolds numbers characteristic of laminar flow, and for large - turbulent.
Artist: Vukota Boljanovic, Metal Shaping Processes, 2010
More about aluminum casting: