Analysis of Flow of Molten Metal, a foundation in the Gating System Design on Castings

Gating System Design on Castings is dictated by the flow of molten metal inside mold.

During my training in Japan in the Field of Foundry Engineering, I have learned that the flow of liquid can either be laminar or turbulent. In the laminar flow, the particles of fluid all proceed smoothly, parallel to the direction of flow. Turbulent flow is characterized by irregular movement of the particles of fluid. In the turbulent flow, air and mold sand are likely to be included and the dross inclusions does not float up.Therefore, it is of utmost importance that turbulence should be minimized if not prevented in the pursuit of attaining a sound casting free of defects. Generally, turbulent flow will occur when the velocity or direction of flow is changed, specially a sudden change of flow creates turbulence. In the principles of fluid mechanics, the flow of all fluids in ducts can be related by their Reynolds Number(N) and is equated to the fluid density(d) multiplied by the average flow Velocity(V) further multiplied by the diameter(d) of the duct and the whole divided by the viscosity(v) of the fluid (N=dVD/v).

Reynolds Number less than 2000 is laminar; 2000 to 4000 is a little bit turbulent but not harmful; greater than 4000 is turbulent. Usually Reynolds Number of about 2000 to 20000 are obtained in Foundry Practice, so nearly always turbulence appears in the classical sense. But its degree must be reduced to eliminate its harmful effect to metal quality with well-designed gating system. For light metal and bronze, the tendency of turbulence is high; for cast steel and nodular iron, the tendency is medium; and for cast iron and brass, the tendency is low.

The velocity of flow depends on the pouring rate of molten metal, the height of the ladle, and the gating system of the mold. In the gating system, the velocity of flow will change according to the law of continuity which states that for a system with impermeable walls and filled with incompressible fluid, the Rate of Flow(Q) is equal to the Cross Sectional Area(A) of the gate times the Velocity(V) of flow: Q=AV.

The effect of the height of the flow on the velocity follows the principle of the conservation of energy, known to be the Bernoulli Equation, which expresses that if no frictional losses are involved, the sum of potential energy, the kinetic energy, and the pressure energy terms for any point in the same system is equal to that for any other point.

Scientific Basis for Risering Design for Steel Castings

In one consultation with my foundry clients regarding their casting defect problems and upon evaluating their methoding practice, I have discovered that they were confused in applying density values in the risering design calculations for steel casting that is why the problem on shrinkage cavity. Wrong density choice may result to rejection due to either shrinkage or over design resulting to non-economical yield. Although mild shrinkages in steel castings might be repairable through build-up by welding, still some additional cost will be incurred, an issue on economics.

Three interesting density values of steel must be taken into mind:
1) Just above melting point for liquid low carbon steel is, 0.115 kg per cu in, as basis in calculating weight of riser in its liquid form from riser dimensions;
2) Just below the freezing point of solid steel is, 0.1195 kg per cu in, as basis in calculating casting weight from pattern dimensions; and
3) To calculate weight of casting from casting dimensions (not mold or pattern dimensions), use 0.129 kg per cu in

Erroneous density application may cause a difference in weight or volume exceeding 10%.

Shape Factor is also a major element in the design of riser but most prevalently applied in cast iron castings. It is equal to the Length plus Width of the casting which the riser must cover divided by the Main Thickness of the part where the riser is effective [(L+W)/T].

Tale of a Successful Melting Practice

I was on the process of commissioning a newly rehabilitated jobbing foundry intended to serve internal casting requirement of the steel mills I am connected with. The maiden heat that we did together with the foundrymen I had been supervising was for a non-ferrous metal, a leaded red brass type, processed in an induction furnace, acid-lined, to test the melting endurance of this recomposed facility.

The scrap material available at that time was a well segregated, cleaned and identified bronze machining from our machine shop and classified under the leaded red brass family. I made it  sure that contamination of aluminum element from other bronze machining, such as aluminum bronze and manganese bronze, was avoided because aluminum is non-miscible with lead that could certainly cause unsoundness of metal product. The selected machining charge had to undergo gas preheating to assure that hydrocarbon-mixed coolant sticking on the machining grains are burned-off leaving no trace of hydrogen that could surely dissolved into the melt during the melting operation.

There was also ready stock of alloying elements, like, zinc ingots, lead ingots, and tin ingots, of known purity, to make up for anticipated oxidation and volatilization loss during the melting process.

We prepared also the needed melt treatment materials for fluxing, degassing and deoxidizing to assure a clean and gas free melt. There are known suppliers for this type of foundry supplies, to mention in passing, FOSECO products.

Of course, a charge calculation was a must, considering no spectrometer available for instant laboratory analysis of chemical composition at that time, although, post melting wet analysis of sample was undertaken to check the resulting chemistry and we were within target.

Product made out of this heat are bushing materials for general application in the plant, liners, and entry side guides. Proof machining of these cast products indicated something of competitive quality as to surface soundness of the fabricated piece.

Cost to manufacture these castings was very low considering the base charge was already sunk being a byproduct of machining and recycled for foundry use.

How to produce quality metal castings in the foundry

To produce good quality metal castings in the foundry, the following basic guidelines must be adhered:

Proper Methoding
Rest on the engineering expertise of the Foundry Engineer to decide on the proper positioning of an item cavity in  the mold which become the basis in the design and fabrication of pattern as to the split line, the coring requirements, and other effects that a pattern should possess to enhance ease of location of risers and gates and pattern withdrawal at the mold forming stage.

Correct Design of Risers and Gating System
Refers to calculating riser and gating system dimensions and quantity to assure castings free from defects like shrinkage cavities, inclusions, misruns, and other surface imperfections.

Good Molding Practice
Covers the choice of reliable refractory materials and binders, sand preparations and setting of standardized sand and binder mixtures, mold coatings, and mold preheating prior to pouring melt into molds to avoid sand inclusions, porosity, run-out, and other related defects.

Effective Application of Chemical Metallurgy
Melting method plays a major factor in the attainment of a high quality metal castings in the foundry. First you have to be sure that the scrap charges are cleaned, classified and identified for the charge calculations to be effective in determining the alloying elements needed for a given target of metal composition. Treatment of molten metal in the melting furnace, be it, induction furnace, electric arc furnace, gas fired crucible furnace, or cupola, for fluxing, degassing and deoxidation must be strictly carried-out to completely eliminate dissolved hydrogen gas and oxygen which are the main source of gas porosities that may be present causing unsoundness in the castings. For example,  to degas and deoxidize non-ferrous melts, FOSECO products and other relevant suppliers, would be very effective material for melt treatment.

Temperature control is also a must in adjusting the correct superheat required for an equivalent section thickness of items to be poured.

Efficient Fettling and Cleaning Works
The use of shotblast machine to clean casting surfaces from sticking sand and scales, makes cleaning activity with ease and faster. For the removal of fins, risers and gates,  gas cuttiing, electric cutting, swing grinding and cut-off wheel equipment facilitates fettling and cleaning operations.


Enhancement of Mechanical Properties of Metals by  Physical Metallurgy
Mechanical properties of metals may be altered by the process called Heat Treatment. Depending on the needs, in general, metals can be softened by annealing and hardened by water or oil quenching.Normally, steel castings are annealed for ease of machining and maybe heat treated again following specifications on physical metallurgy, like normalizing, stress relieving, or hardening in accordance to customers' demand.

Brief Discussion of the Natural Sand Molding Process

As a matter of chronology, let me take a brief discussion on the Natural Sand Molding Process.

Natural Sand is still being used for green sand molding process. This is especially true for foundry without any sand treatment facilities, in which case no other molding materials can be used. As a matter of fact, given the copper base alloy and aluminum base alloy, the foundry can immediately makes use of the natural sand. The result of iron casting and steel casting using natural sand is not satisfactory to the foundrymen because of its poor quality and dimensional accuracy.. Hence, the necessity of adopting the synthetic sand. Even from the historical point of view and considering the most important technique used for the green sand mold, the adoption of the synthetic sand to replace the natural sand had greatly improved and modernized foundry.

Introduction to Molding Process

Green Sand process has a long history. However, to discuss its early state would entail research work of archaeological field. It is important for foundrymen to discuss the development of green sand process on the 20th century.

In this century, green sand process become the main molding process instead of dry sand process. This is due to various technological developments on foundry.

Green sand process was used in many foundries and contributed to the development of the foundry industry. Now a days several new sand molding process had been considered which consequently reduced the use of green sand process.

However, the foundry for the purpose of producing small and medium size castings are still using the most important molding process, green sand.

While conventional molding process using natural sand is also worth to remember, modern molding process using synthetic green sand is better to focus on.

FOUNDRY SHOP MANUFACTURING PROCESS

A foundry is an operating plant which manufactures castings of metal, both ferrous and non-ferrous. Metals are processed by melting, pouring, and casting. Iron is the most common base element processed in a modern foundry. However, other metals, such as, aluminum, copper, tin, and zinc, can be processed.

A foundry accomplishes the processing of creating molten metal by using a furnace. The furnace is one of the main parts of a contemporary foundry. Different types of Furnaces are:

Cupola : Most primitive type; ulitlizes

coke as energy for melting.

Induction Furnace: Most common melting unit;

utilizes electricity as energy

for melting; operate like a

transformer at high frequency,

where, primary is the coil,

secondary is the charge.

Arc Furnace : Another common melting unit;

utilizes electricity as energy

for melting; operate like a

welding machine, melting by

arcing; it is usually known as

steel making unit since slag

can be manipulated to reduce

impurities, such as, sulfur and

phosphorus.

More about Furnace:

Furnaces use insulated, heated vessels powered by an energy source to melt metal. Furnace design is a complex process, and the design can be optimized based on multiple factors. Furnaces in foundries can be any size, ranging from mere ounces to hundred of tons, and they are designed according to the type of metals that are to be melted. Also, furnaces are bound by the fuel available that will produce the desired temperature. For low temperature melting point alloys, such as zinc or tin, melting furnaces may reach around 600 Kelvin or 327 OC. On the high end, from steel, nickel based alloys, tungsten, all the way to other elements with higher melting points, furnaces can reach to over 3600 Kelvin or 3327OC. The fuel used to reach these high temperatures can be electricity, natural gas or propane, charcoal, coke, fuel oil, or wood. The majority of foundries specialize in particular metals and have furnaces dedicated to these metals.

For example, an iron foundry (for cast-iron) may use a cupola, similar to a small blast furnace. While a steel, bronze, or brass foundry will normally use an electrical induction furnace. However, in some cases, they may use a gas heated crucible furnace. Most aluminum foundries use either an electric resistance or gas heated crucible furnace.

Pouring Technique:

In a foundry, molten metal is poured into molds. Such molds may be made of sand, metal for permanent molding, ceramic, or refractory materials. The pouring can be accomplished with gravity, or it may be assisted with a vacuum or pressurized gas.

Casting:

When the molten metal changes states from liquid to solid, it is said to be cast into shape.

Advantages:

The finished product of a foundry can be more versatile than the product of a rolling process. Also, the process of casting molten metal, as is occurs in a foundry, can be far more automated.