In this article we will discuss about:- 1. Definition of Sand Moulds 2. Classification of Sand Moulds 3. Feeding of Metal.
Definition of Sand Moulds:
A sand moulds may be defined as-a preformed sand container into which molten metal is poured and allowed to solidify. After casting it is removed from the sand mould, sand mould is generally destroyed. The moulds is filled by pouring the molten metal into an opening at the top of the mould and proper passages are made to allow the metal to flow to all the parts of the mould by gravity.
Small or medium sized castings are generally made in a flask—a rectangular box-shaped container, without top and bottom. The flask may be made in two or three parts, and parts are held in alignment by dowel pins. It is necessary to clamp the flask before pouring molten metal into it, in order to prevent the buoyant effect of the molten metal from lifting up the top part of flask.
Classification of Sand Moulds:
Depending upon the material used, the moulds could be classified as:
1. Green sand moulds,
2. Skin-Dry moulds,
3. Dry sand moulds,
4. Cement-Bonded moulds,
5. Metal moulds.
1. Green Sand Moulds:
Green sand moulds are those sand moulds, in which moisture is present in the sand at the time of pouring the molten metal. The grains are held together by moist clay. Moisture level has to be controlled carefully. These are used for casting, practically all ferrous alloys. Green sand is available in many kinds and is used for making small, medium and often large moulds even.
Green sand moulds are least expensive to make as the basic material for these is cheaper. Larger output can be obtained from given floor space. These do not require any backing operations or equipment but dry sand cores are to be used. These being softer than dry sand moulds, allow greater freedom in contraction, when the castings solidify and cool.
Also, the moulding is less time-consuming. However, green sand moulds have some disadvantages viz., they not being as strong as others are liable to be damaged during handling or by metal erosion. The moisture present in the sand may also cause certain defects in the casting like blow holes, gas holes etc.
These moulds cannot be stored for long time. The surface finish of the casting obtained from green sand mould is not very smooth. Sometimes additives like coal dust or organic materials are also added and then it is called loam moulding.
Following table (3.5) shows the composition of green sand for various purposes:
The three commonly used methods of green-sand moulding are:
(a) Open Sand Method:
It is simplest form in which, the entire mould is made in the foundry floor or in a bed of sand above floor level. This method is mainly employed for simple solid castings with flat tops.
After proper levelling, the pattern is pressed in the sand bed for making mould. Moulding box is not necessary and the upper surface of the mould is open to air. Pouring basin is made at one end of the mould, and the overflow channel cut at the sides of the cavity.
(b) Bedded-in Method:
In this method a cope i.e. a sand cover is necessary. It is used, when the upper surface of casting is not flat. The pattern is hammered down the sand of the foundry floor or in a drag filled partially with sand to form the mould cavity. The top of drag is smoothened and the parting sand spreaded. A cope is placed over the pattern and rammed up.
Runners and risers are cut and the cope box is lifted. The pattern is then withdrawn, the surfaces of drag and cope moulds finished and cope replaced in its correct position for completing the mould.
(c) Turn-Over Method:
This method is commonly used for solid as well as split patterns. One half of the pattern is placed with its flat side on a moulding board, a drag is rammed and rolled over. Next, the cope is placed over the other half of the pattern and is rammed and rolled over. The two pattern halves are shaken and withdrawn. Now, the cope is placed on the drag for assembling the mould.
2. Skin-Dry Moulds:
These are made of green sand with dry sand baking. In some cases, moisture is dried from the surface layer of rammed sand to a depth of 25 mm by heater or gas torches. These are more common in large moulds and can be used for casting, practically all ferrous and non-ferrous alloys.
These are less expensive to construct than dry-sand moulds but more expensive than green sand moulds of given size. It has the advantages of less equipment, cheaper materials, less time for preparation, and less floor space in comparison to dry sand moulding. However, these are not as strong as dry sand moulds and can’t be stored for long time as moisture may migrate through the dry skin.
3. Dry Sand Moulds:
These are made with that sand which doe” not require moisture to develop strength. The sand mixture for small and medium works consists of 13 parts of floor sand, 8 parts of new sand and 1 part of horse manure or saw dust. For heavy work, these proportions are 11:9:1 and for extra heavy work—10 : 10 : 1.
The mould surface is sprayed with molasses water. All parts of mould are baked in furnace at 150—300°C (until moisture is driven off) to increase the strength, resist erosion, and improve surface conditions. The dry-sand moulds may be used for many alloys but are more commonly used for steel castings.
These are used mostly in small and medium sized operations. For larger sized operations, the dry sand moulds are made in sections and assembled after baking. The dry sand moulds are stronger and can be handled more easily with less damage and also can be stored for longer time.
These resist metal erosion, and tendency for moisture related defects is eliminated. The disadvantages of these moulds are- these require more expensive moulding material, labour costs are high, and extra operation, equipment and space are needed.
4. Cement-Bonded Moulds:
In these moulds, silica sand bonded with Portland cement is used as the moulding material, which dries up in air. These moulds are most commonly used for very large ferrous work and pit moulding and in other cases, where baking is impossible. It has high strength and possesses all advantages of dry sand.
For these moulds, extra space for air drying operation has to be provided. The materials used in these moulds can’t be used again like other moulds, thus the process becomes expensive.
5. Metal Moulds:
These are used for die-casting, permanent mould and centrifugal casting processes.
Feeding of Metal in Sand Moulds:
To take care of feeding problems, particularly when casting is complex in size and shape, is the most important aspect to ensure sound castings. Problems are encountered because of slow rate of heat abstraction from a large mass of metal. This results in minimising temperature gradients and difficulty in obtaining solidification in directional manner towards the feeder.
While, it is easy to obtain directional solidification with alloys which have short freezing range, it is quite difficult with alloys having wide freezing range. Location of feeders and risers, to supply hot liquid to thicker sections, which may become isolated during solidification, and to establish temperature gradients initially, is very important. Sometimes temperature gradients can be established artificially by using chills within the moulds, insulating pads and tapered sections.
It may be noted that shrinkage cavities are likely to form in sections like Ts and crosses etc. which take longer to solidify because in such cases, volume of metal is more and surface area through which heat can escape is reduced. The solution, therefore, lies in providing shapes in which volume of metal is decreased and the surface area through, which heat can escape is increased.
In the case of crosses, best results can be obtained either by staggering the location of ribs, or introducing a cored hole at the centre of cross, or using a circular web whose walls are thinner than straight ribs. It must be ensured that last portion to solidify will be fed with metal from feeder. Design, sizing and location of feeders, gates and channels is, therefore, most important.
Usually feeding problems are best solved by experience, yet some scientific and empirical rules developed with experience will be found to serve a good guide in tackling this problem.
(i) Heat Transfer Approach (Square Root Time Effect):
(ii) Chvorinov’s Rule:
According to Chvorinov’s rule, thickness of a uniform skin,
Thus, for any shape where the same interface boundary conditions hold, according to Chvorinov’s rule, solidification time is directly proportional to the square of the ratio of volume to surface area.
In order to ensure that metal in feeder solidifies last,
For same volume, solidification time will progressively increase for the following shapes in order because their surface areas for same volumes decrease:
iv. short cylinders,
Though, theoretically sphere takes longest to solidify but is not feasible for feeders. Short cylinder is nearest approached and it is tapered to facilitate moulding.