In this article we will discuss about the process and characteristics of sand casting.
Process of Sand Casting:
The importance of sand casting is increasing day-by-day as the scientific research has brought about lot of applications and adaptations in the field of casting industry. This is probably the easiest and most convenient way of giving the desired shape to the metal.
Sand is the most commonly used material, because it can be easily packed to any shape, has high permeability and resistance to high temperatures. Thus, complex shapes can be easily cast by using sand moulds, which might not be possible otherwise. In order to secure the optimum cost and quality benefits, the following design details should be given due consideration.
Fig. 3.41 shows a flow diagram of sand casting process.
Automated Pouring Systems:
In order to achieve the lower material and labour costs, increased productivity, better quality and better working conditions, trend is towards automation of complete foundry. Recently, attempts have been made for automated induction pouring systems.
Automated pouring systems, act as interface between mould-making and melting. These systems hold the molten metal ready for pouring, and pour the molten metal into the mould exactly as required.
The induction pouring furnaces, using controlled stopper to pour the molten metal direct into the mould or in dosed quantities into intermediate casting ladles, are used for this purpose and these possess the following characteristics-Keep the temperature and chemical composition of the molten metal constant during holding and pouring; eliminate slag inclusions from the poured molten metal; add inoculates and alloying materials at the right time and in exactly measured quantities; adjust the pouring rate to the intake capacity of the mould; measure the weight of the poured metal exactly.
Such a pouring furnace consists of a cylindrical shell with refractory lining, pressure-sealed cover, flanged channel inductor, stopper and a pressure control system. Filling and pouring is through siphon shaped ducts, whose bottom ends are located at the base of the furnace, to ensure virtually slag free pouring.
Pressurised gas forces the molten metal into the furnace’s pouring nozzle through a stopper. The pressurized gas also keeps the level of the molten metal in the nozzle constant irrespective of varying quantity of metal in the furnace. The rate of metal pouring is controlled by the movement of stopper.
A pneumatic servo-cylinder adjusts the stopper travel continuously in accordance with the pouring program.
Since the pouring positions are not always the same, the furnace is able to move in two directions (longitudinal and transverse) relative to the moulding plant. A hydraulic tilting device allows the furnace to be completely emptied.
The angled inductor is flanged to the base of the furnace. The flange is water-cooled. Due to the tendency of the hotter metal to rise, the area around the throat is largely free of crust formation, making it accessible for mechanical cleaning. The inductor itself is easily accessible from the outside of the furnace. Because of its relatively low overall height, the furnace does not have to be placed in a pit, but can be installed on the floor of the foundry.
The filling and pouring siphons are also easy to clean mechanically. The suspended oxide particles produced in the siphon ducts, particularly when the molten metal is treated with magnesium, are deposited on the refractory lining. The upper parts of the filling and pouring siphons are therefore flanged for easy cleaning.
The induction pouring furnaces eliminate slag inclusions, ensure required pouring rates, measure the weight of the molten metal exactly and keep the temperature of the melt constant during pouring. The molten metal must be poured at a rate matched to the intake capacity of the mould.
Among the latest foundry advances is electronic monitoring of the pouring process, which can be either open- loop controlled using the teach-in principle or closed-loop controlled by regulating the mould gate level.
Productivity can be significantly increased by using intermediate ladles, operated either as tilting systems or with stopper control when special demands are made on the pouring process.
Modern, automated pouring systems allow continuous foundry operation by holding the molten metal ready for pouring at all times and ensuring that, it is poured into the mould exactly as required.
A sand casting is produced by pouring the molten metal into the mould through a port called “gate”. It is the conventional practice to locate the gate either at parting line or in the lower most portion of the casting.
The gating system (comprising of pouring basin, sprue, runner, gates etc.) achieves the following purposes:
(i) To direct the molten metal into the mould with minimum turbulence. Excessive turbulence causes aspiration of air and formation of dross.
(ii) To fill the mould system completely. (It should do so with least disturbance, thereby promoting cleanliness and reducing oxidation).
(iii) To distribute the metal with the least disturbance in order to reduce erosion of the mould material and consequent sand inclusions.
(iv) To skim or separate dross or other foreign matters i.e., as the metal flows through the gating system, loose sand, oxides and slag should be prevented from entering the cavity of mould by providing skimming action. (It is desirable that the appendages, which hinder the flow of metal should not be used. Also the thin cores or dividing walls, which might spall when subjected to hot metal be avoided in design).
Heavier sections must be fed with sufficient hot metal through heads and risers to compensate for the shrinkage allowance.
Requirements of an Ideal Gating System:
As already mentioned gating system includes pouring basin, sprue, runners, risers and gates. The gating system should promote temperature gradients favourable for directional solidification. Entering velocity of metal should be least and free of turbulence to avoid erosion of the mould and core surfaces. Gating system should be rammed as hard as or harder than the mould cavity.
Various parts of gating system should be rounded, smooth and streamlined to prevent turbulence and erosion. It should avoid the formation of oxides and other dross and kept free of loose sand and prevent their passage on to casting. A runner should be extended some distance beyond the last gate to trap any dross from the first flow of metal. The dross and slag present in the ladle should not be carried to the mould cavity.
The gating system should avoid the entrainment or absorption of air/ gases into the metal while passing through it. Metal with excessive superheat may increase the gas content of the metal and may produce more dross and adversely influence directional solidification. The gates should lead metal to the heavier sections of a casting, preferably below or through a riser. Finally gate should be practicable and economical to make.
The important points to be considered in the design of pouring system of castings are:
(i) Liquid flow should not damage (erode) the mould walls.
(ii) Liquid should not carry sand or gross into the casting.
(iii) Aspiration of gases into the stream of molten metal should be avoided.
(iv) Pouring of metal with minimum loss of temperature and establishing temperature gradient on the mould surfaces and within the metal to help the directional solidification towards the riser.
All this can be achieved by suitable design of the gating system and proper poring system.
In order that no air is absorbed by the liquid metal on its downward passage in gate, the shape of the gate should be such that the pressure of the liquid at any point in the gate passage is not below the atmospheric pressure. This is possible, when sides of the down gate are made hyperboloid in section.
Since hyperbolic shape is difficult to produce, taper section with more diameter at top and smaller at bottom can serve the purpose. In actual practice, a pouring basin is provided at top and metal is fed via tapered vertical sprue and a short horizontal gate as shown in Fig. 3.43. This arrangement minimises oxidisation and reduces the damage to the mould cavity because the force of the incoming metal is reduced.
The sprue cross-section may be circular, square or rectangular. The size of the sprue usually varies from 10 mm square for small castings (below 12 kg) to about 20 mm square for heavy castings. The size of the sprue should be so enough that it is kept full during the entire pouring operation, and the metal does not enter the mould cavity with high velocity, causing splattering and turbulence.
If the sprue is straight having sharp corners, severe aspiration resulting in turbulence in the metal occurs. Aspiration is negligible with no turbulence, if the sprue is tapered, corners rounded, sprue-well provided and dam type pouring basin made.
Pouring basin also reduces the eroding effect of the liquid metal stream coming directly from furnace and it helps in maintaining a constant pouring head. A ceramic strainer could be placed at the top of the sprue to remove dross.
A ceramic splash core could be placed at the bottom of the sprue to reduce the eroding force of the liquid metal stream. A skim bob trap placed in a horizontal gate could be provided to prevent heavier and lighter impurities from entering the mould.
Design of Gating:
Gate is defined as opening from runner (common passage way to supply metal to number of cavities) to the mould. The size and location of gate should be such as to ensure quick filling of mould, distribution of metal in the mould cavity at a proper rate, without excessive temperature loss, turbulence, minimum erosion of mould, without entrapping gases and slags, no development of cracks on cooling, and easy removal of gate without damaging casting.
To prevent loose sand and dross from entering the mould cavity and to allow the metal to fall in a small stream, a large size pouring basin is provided on the top of the sprue- cum-riser or a strainer core could be fitted in the pouring basin.
If metal is poured very slowly in a mould cavity, then solidification may start while it is not even completely filled up. If poured very fast, high velocity will erode mould surface. Thus, optimum pouring velocity is essential.
Gates, depending on their position may be top, parting and bottom type. In the case of top gating the molten metal is poured down the head or riser. Thus, erosion of mould by dropping metal should be ensured by making hard mould. In this case hot metal remains at top and thus proper temperature gradients are established for directional solidification towards the riser. Top gate may be made to serve as riser.
Top gates are usually limited to small and simple mould or larger castings made in moulds of erosion resistant material. Top gating is not advisable for light and oxidisable metals like aluminium and magnesium because of fear of entrapment due to turbulent pouring.
In the parting line gating system, the metal enters the mould cavity at same level as the mould joint or parting line. The sprue is connected to the casting through a gate in a horizontal direction. It is thus possible to provide skimbob or skim-gate to trap any slag or sand in the metal. The choke serving as a restriction controls the rate of flow.
In the bottom gating system, the molten metal flows down the bottom of the mould cavity in the drag and enters at the bottom of the casting and rises gently in the mould and around the cores. Bottom gates are best suited for large sized steel castings. Turbulence and mould erosion are least in this case. However, time taken to fill up mould is more.
Directional solidification is difficult to achieve in bottom gating because the metal continues to lose its heat into the mould cavity and when it reaches the riser, metal becomes much cooler.
In a faulty mould design, the metal velocity may be high and thus pressure may fall below atmosphere and the gases originating from baking of organic compounds may alter molten metal stream, producing porous castings.
Two cases are possible in moulds where negative pressure may be experienced. One is in the sprue design and other, where sudden change in direction of flow takes place. Referring to Fig. 3.47, it will be seen that pressure at points 1 and 3 is atmospheric.
By Bernoulli’s theorem, pressure will be negative at 2, if sprue is as shown by dotted lines. To overcome this problem sprue should be made tapered, preferably with curve as shown in firm line between 1 and 2.
Other condition is shown in Fig. 3.48, where due to change in direction of flow of metal, vena contract effect is experienced. To avoid negative pressure in this region the shape of mould should be as per vena contract profile.
Gating ratio is defined as the ratio of sprue area to total runner area to total gate area. A gating ratio of 4 : 3 : 2 results in pressurised system. In this system, the proportions of sprue, runner and gate cross- sectional area are so arranged that back pressure is maintained on the gating system by a fluid film restriction at the gates. This system is adopted for metals like steel, iron, brass etc.
The pressurised gating system is kept full of metal. The back pressure due to restriction at the gates tends to minimize danger of the metal pulling away from the mould walls with consequent air aspiration. Pressurized systems are generally smaller in volume for a given metal flow rate than unpressurised ones.
Thus, less metal is left in the gating system and casting yield is higher. However, severe turbulence may occur at junctions and corners unless careful streamlining is employed. High velocity and turbulence result in entrapment, dross formation and mould erosion.
In case of unpressurised system the primary restriction to the fluid is at or very near to the sprue. The gating ratios like 1: 3 : 3, 1: 2 : 2 will produce an unpressurised system. Such a system is adopted for light, oxidizable metals like aluminium and magnesium, where the turbulence is to be minimised by slowing down the rate of metal flow.
In the case of unpressurised systems careful design is required to ensure them, being kept filled during pouring. Drag runners and cope gates aid in maintaining a full runner, but careful streamlining is essential to eliminate the separation effects and consequent air aspiration.
As the molten metal in the mould cools down, it solidifies and contracts in volume. Since, all the parts of a casting do not cool at the, same rate due to varying sections, varying rate of heat loss to adjoining mould walls etc., voids and cavities are liable to be formed in certain regions of the casting.
In good casting design, these voids are filled up with liquid metal from the portion of the casting which is still liquid. Thus, solidification should continue progressively from the thinnest section which solidifies first towards the risers, which should be the last to solidify. This process is known as ‘Directional Solidification’, which is aimed at for producing sound castings.
Directional solidification can be ensured by designing and positioning the gating system and risers properly, increasing the thickness of certain sections of the casting by the use of padding, using exothermic materials in the risers or in the facing sand around certain portions of the casting, using chills in the moulds.
The impurities can be prevented from travelling into the casting by observing the following:
(i) Provision of pouring basin of adequate size helps in breaking down the eroding force of the stream of molten metal, as it is being poured from a ladle. A proper design of pouring basin regulates the rate of metal entry, allows the metal to flow into the sprue smoothly and prevents turbulence.
(ii) Provision of ceramic strainer in down sprue helps in preventing dross from the ladle entering the casting.
(iii) The maximum impact is felt at the bottom of vertical sprue from where, sand is likely to be eroded and enter into mould. This can be prevented by providing ceramic splash core at bottom of vertical sprue.
(iv) Sharp corners in metal flow path should be streamlined to avoid turbulence and dead pockets (Refer Fig. 3.46).
(v) Provision of skim bob helps in trapping both heavier and lighter impurities flowing towards the casting.
The minimum section thickness which can be poured for various metals is limited because of difference in solidification temperatures and fluidity. Minimum section thickness which will provide the necessary strength or weight, without requiring excessive temperatures to ensure running must be used.
The normal values of minimum thickness for castings of simple design are 3 mm for cast iron, 2.25 mm for malleable iron, 6 mm for steel, 2.25 mm for brass and bronze, and 3 mm for aluminium. If length of flow is more, then larger thickness than values given above have to be provided. The minimum value of thickness for obtaining sound castings will be high, if there are intricacies in the mould cavity.
There should be as great uniformity of metal section between bosses and lugs and the body of the casting as possible in order to permit adequate feeding of boss or lug.
Riser is a hole cut or moulded in the cope to permit the molten metal to rise above the highest point in the casting. It provides a visual check to ensure filling up of mould cavity.
It serves as a feeder to feed the molten metal into the main casting cavity to compensate for shrinkage. The design of the riser should be such that it establishes temperature gradients within the casting so that the casting solidifies directionally towards the riser. It also helps in easy ejection of the steam, gas and air from the mould cavity while filling the mould with the molten metal.
For greater soundness, in case of casting with thin sections several risers may be used. For effectiveness, the riser must be the last part of the casting to solidify.
After the mould has been filled up, metal enters the risers. Risers act as reservoir and heat gradient regulator, and provide the necessary fluid metal to compensate for liquid metal and solidification shrinkage. The risers are usually located at the uppermost part of the section being fed.
Depending on the metal being cast, their volume is kept between 25 and 55% of the casting. It is important to note that the risers are suitably located so that there is no necessity for excessive removal of metal to produce the finished contour. Risers are connected to the casting by a neck of metal called gate which enables the riser to be removed easily from the casting after solidification.
If no riser is provided during casting, the solidification will start from walls and liquid metal in the centre will be surrounded by a solidified shell and the contracting liquid will produce voids towards the centre of the casting. Further cooling of the solid in centre sets up undesirable stresses in the casting.
Provision of risers overcomes these problems as these supply molten metal for a solidifying casting. For this purpose, the risers must be large enough to remain liquid after the casting has solidified and must contain sufficient metal to provide for the contraction losses. Further these should be so positioned that they continue to supply metal throughout the solidification period.
Design and Positioning of Risers:
The most efficient shape of a riser of a certain size is that which results in a minimum of the heat loss, thus remaining hot and keeping the metal in molten state as long as possible. In other words, a riser should be designed with the minimum possible volume while maintaining a cooling rate slower than that of the casting.
The best shape for the general run of castings to achieve above objective is cylinder. The height of the riser should be tall enough so that any pipe formed in it may not penetrate the casting. The ratio of height to diameter usually varies from 1: 1 to 3 : 2.
The optimum riser diameter for a given casting can be obtained by following rules:
(a) Chvorinov’s Rule:
It states that the freezing time
(b) Caine’s Method:
This method is based on the relative freezing time of the casting and the riser. It defines the relative freezing time of the casting and the riser.
It defines the relative freezing time to complete solidification as the ratio of surface area of casting ÷ volume of the casting : surface area of riser ÷ volume of the riser.
According to Caine, (1) if the casting solidifies infinitely rapidly, the feeder (riser) volume should be equal to the solidification shrinkage of the casting, and (2) if the feeder and casting solidify at the same rate, the feeder should be infinitely large.
Fig. 3.49, shows this hyperbolic relationship between the relative freezing time and relative volume.
Further, for a casting with a low A/V ratio, as in the case of a cube and sphere, one central riser may be able to feed the entire casting. However, when A/V ratio is high, as in the case of a bar and a plate, more than one riser is necessary. Proper location of riser is essential in such a case.
For a steel plate of 100 mm thickness, one central riser is adequate if the maximum feeding distance is less than 4.5 t from the edge of riser [Refer Fig. 3.51 (a)]. If more risers are required, distance between two nearest edges of risers should be less than 4 t [Refer Fig. 3.51 (6)].
For a bar of square cross-section of 50—200 mm side (s), a central riser is good if maximum feeding distance is 30√s from the edge and distance between two risers (nearest edges) should be less than 1.2 s.
The feeding distance of the riser can be increased by using chills, which provides sharp thermal gradient and decreases feeding resistance. In the case of single riser, chill should be placed at the end and for more than one riser, it should be placed midway between two risers.
The proper placement of riser is equally important since it should be able to feed the solidifying casting effectively. If the casting is of cubical or spherical shape, (i.e. of chunky shape having low value of Ac/Vc) then a single riser is adequate to feed casting on solidification. However when value of Ac/Ac is high (as in case of bar and plate shaped castings), more than one riser may be required.
If only a single riser is used in such cases then the slushy state just prior to solidification may restrict metal flow from a single riser and cause centre-line shrinkage. As a thumb rule, it can be said that a single riser is adequate, if feeding length is less than 4.5 times the thickness of plate for 12-100 mm thick steel plates.
In the case of square bars of size (side) 50—200 mm, a central riser can be used for distances of less than 6 times V bar size. Longer feeding distances than above are possible by use of chills, which increase the cooling rate and reduce centre line feeding resistance. In the case of alloys having higher centre-line feeding resistance than steel, chills have to be used to ensure soundness of those parts of the casting requiring the greatest strength.
Exothermic materials are sometimes used in risers for producing directional solidification by creating heat. They consist of oxides of metals like iron, chromium, nickel or copper and aluminium metal in powder form.
These compounds may either be added to the surface of the molten metal in the riser just after pouring, or these may be added to the sand of riser walls. A chemical reaction takes place due to the contact with molten metal producing a large amount of heat. Thus, the metal in the riser gets superheated remaining molten for a longer time.
Provision of insulating pads and sleeves around risers helps to conserve heat. Provision of suitable chills at desired locations also helps promote directional solidification.
The design of the casting section should be such that it allows the risers to fulfil the needs of supplying hot metal and controlling directional solidification. For example, in Fig. 3.52, the molten metal will solidify inward from the metal mould interface by progressive solidification.
With proper conditions of temperature differentials, the intersection of the progressive freezing will move upward into the location of the hottest spot, which should be within the riser. This is called “directional solidification”.
If the height of any section is too much in comparison to its cross-section, then the progressive solidification rate will exceed the directional solidification, and result in fine centre-line porosity or even a larger or series of large cavities. In order to avoid such a condition, it is essential that the cross-section tapers downwards being larger at top and smaller at bottom.
If the sections can be adequately fed, the limitations of section proportioning are not so critical as the design of the junction. Contraction stresses due to widely differing temperature gradients need to be taken care of. It is ordinarily possible to produce castings without recourse to chills where sections thicknesses are not less than 80% or more than 120% of adjacent section, as regards section variations remote from risers.
Junctions and Shrinkage:
Shrinkage cavities caused by improper directional solidification are most often likely to occur in L, T, Y, X sections and where large sections are joined with small sections abruptly. What happens in these sections is that, there being a greater mass at the junction point compared to the legs, the junction point area becomes a hot spot with directional freezing progressing towards the hot spot, which in turn feeds the legs and develops the shrinkage cavity.
The hot spot can be eliminated by either making the section more uniform or using chills near the cross-section of larger mass (Refer Fig. 3.53). Although every effort must be made to prevent isolation of heavy sections which can become ‘hot spots’, sometimes it becomes difficult.
Under such circumstances, it is left to the foundry man to control the freezing by means of:
(i) Special manipulation of the position of risers in mould,
(ii) Controlling the speed of pouring,
(iii) Utilising hot metal in the risers,
(iv) Using mould materials of different thermal characteristics.
Adequate fillets at all intersections materially increase the strength soundness of castings. Size of fillets depends on the metal used, shape and thickness of wall section and the size of the casting. Fillet radius should not be more than the section thickness.
Eliminating Hot Tears:
In a casting, hot tears result from temperature gradients, establishing different rates of contraction during solidification and thereby, inducing stresses due to resistance of the sand of a magnitude sufficient to cause fracture. These can be minimised by adopting good design, i.e. avoiding abrupt changes in section, sharp angles and non-uniform webs connected to flanges.
Eliminating Gases in Castings:
Gases in castings may appear as gas holes, (big holes, few in number distributed at several places in the casting), pin holes (small holes, large in number near the top of casting), pin holes (small holes distributed throughout the casting). Proper riser design and adequate venting of permeable moulds in essential to avoid these defects.
Another source of gases is from the dissolved gases in the liquid metal at high temperature, which on cooling are given off. Vacuum melting and vacuum degassing (placing liquid metal in low pressure chamber to remove dissolved gases) can be used to reduce gas in melts.
Characteristics of Sand Castings:
1. As solidification of metal is under non-equilibrium conditions, the castings are susceptible to cooling-cracks if proper care in design is not taken. Shrinkage problems can be taken care of by promoting directional solidification by use of tapers, metal chills in mould walls, and reduction in hot spots at junctions of uniform sections.
2. Solidified metal has poor finish. Surface is affected by pattern finish, sand structure, mould dressings, mould venting and access to mould for cleaning out loose sand particles before mould is closed.
3. Sand castings are porous enough and, therefore, cannot be used for pressure tight vessels (generally used upto 10 kg/cm2).
4. Structure obtained by sand casting is loose and hence not stronger than wrought products.
5. As the grains are not close, the casting has lower density and poor strength.
6. Castings obtained by moulding process have good hardness. Internal stresses can be eliminated by avoiding sharp corners and physical restraint.
7. Sand castings have poor ductility.
8. The moulding method is suitable for moderate and particularly large castings and unsuitable for thinner sections.
9. The suitability of sand castings lies with high melting point of molten metal.
10. The sand castings are less costly because the cost of sand moulds is less.
11. Internal soundness of castings can be ensured by minimising gas evolution during solidification and avoiding turbulence while pouring. Physical restraint should be prevented as it leads to hot tearing.