Organic binders are mostly used for core making. Among all the above binders, the bentonite variety of clay is the most common. However, this clay alone cannot develop bonds among sand grins without the presence of moisture in molding sand and core sand. The quantity of water varies generally between 2 to 8 percent. This amount is added to the mixture of silica sand for developing bonds. This is the amount of water required to fill the pores between the particles of clay without separating them.
This amount of water is held rigidly by the clay and is mainly responsible for developing the strength in the sand. The effect of clay and water decreases permeability with increasing clay and moisture content. The green compressive strength first increases with the increase in clay content, but after a certain value, it starts decreasing. For increasing the molding sand characteristics some other additional materials besides basic constituents are added which are known as additives.
Some common used additives for enhancing the properties of molding and core sands are discussed as under. It is usually added in the molding sands for making molds for production of grey iron and malleable cast iron castings.
This allows free movement of sand grains, which finally gives rise to mould wall movement and decreases the mold expansion and hence defects in castings. Corn sand if added to molding sand and core sand improves significantly strength of the mold and core.
It belongs to starch family of carbohydrates that behaves also in a manner similar to that of the corn flour. When heated, it changes to coke which fills the pores and is unaffected by water: Because to this, the sand grains become restricted and cannot move into a dense packing pattern. Thus, sea coal reduces the mould wall movement and the permeability in mold and core sand and hence makes the mold and core surface clean and smooth.
It is distilled form of soft coal. It can be added from 0. It volatilizes when heated, thus allowing the sand grains room to expand. It also increases collapsibility of both of mold and core. It also reduces metal penetration in the walls of the molds and cores. The clay and water furnish the bond for green sand. It is fine, soft, light, and porous.
Green sand is damp, when squeezed in the hand and it retains the shape and the impression to give to it under pressure. Molds prepared by this sand are not requiring backing and hence are known as green sand molds. This sand is easily available and it possesses low cost. It is commonly employed for production of ferrous and non-ferrous castings.
It possesses more strength, rigidity and thermal stability. It is mainly suitable for larger castings. Mold prepared in this sand are known as dry sand molds. Patterns are not used for loam molding and shape is given to mold by sweeps. This is particularly employed for loam molding used for large grey iron castings. It is directly next to the surface of the pattern and it comes into contact molten metal when the mould is poured.
Initial coating around the pattern and hence for mold surface is given by this sand. This sand is subjected severest conditions and must possess, therefore, high strength refractoriness.
It is made of silica sand and clay, without the use of used sand. Different forms of carbon are used to prevent the metal burning into the sand. They are sometimes mixed with 6 to 15 times as much fine molding sand to make facings. The layer of facing sand in a mold usually ranges from mm. Used molding sand is mainly employed for this purpose.
The backing sand is sometimes called black sand because that old, repeatedly used molding sand is black in color due to addition of coal dust and burning on coming in contact with the molten metal. So-called system sand is used to fill the whole molding flask.
In mechanical sand preparation and handling units, no facing sand is used. The used sand is cleaned and re-activated by the addition of water and special additives. This is known as system sand. Since the whole mold is made of this system sand, the properties such as strength, permeability and refractoriness of the molding sand must be higher than those of backing sand.
This is clean clay-free silica sand which serves the same purpose as parting dust. This is highly rich silica sand mixed with oil binders such as core oil which composed of linseed oil, resin, light mineral oil and other bind materials. Pitch or flours and water may also be used in large cores for the sake of economy. It is a highly important characteristic of molding sands.
Refractoriness can only be increased to a limited extent. Molding sand with poor refractoriness may burn on to the casting surface and no smooth casting surface can be obtained. The degree of refractoriness depends on the SiO2 i. The higher the SiO2 content and the rougher the grain volumetric composition the higher is the refractoriness of the molding sand and core sand. Refractoriness is measured by the sinter point of the sand rather than its melting point. It is that property of sand which allows the escape of any air, gases or moisture present or generated in the mould when the molten metal is poured into it.
All these gaseous generated during pouring and solidification process must escape otherwise the casting becomes defective. Permeability is a function of grain size, grain shape, and moisture and clay contents in the molding sand. The extent of ramming of the sand directly affects the permeability of the mould. Permeability of mold can be further increased by venting using vent rods 2.
Thus, the binding capability of the molding sand gets enhanced to increase the green, dry and hot strength property of molding and core sand. It is entirely due to this property that the heavy sand mass is successfully held in the moulding flask and manipulated as desired without any danger of its falling down.
For this, the sand grains must be adhesive, i. Also, the sand grains must have the property known as cohesiveness i. By virtue of this property, the pattern can be taken out from the mould without breaking the mould and also the erosion of mould wall surfaces does not occur during the flow of molten metal.
The green strength also depends upon the grain shape and size, amount and type of clay and the moisture content. The dry strength also prevents the enlargement of mould cavity cause by the metallostatic pressure of the liquid metal.
Generally sand particles resist moving around corners or projections. In general, flowability increases with the addition of moisture and clay content and reduction of green strength and grain size.
In absence of this property the contraction of the metal is hindered by the mold and thus results in tears and cracks in the casting. This property is highly desired in cores. In order to have a good impression of the pattern in the mold, molding sand must have good plasticity. Generally, fine grained sand has better plasticity. It depends on the content of clay, which absorbs moisture, when sand is dampened. On continuous use of molding sand, the clay coating on the sand particles gets thinned out causing decrease in its strength.
Thus proper sand conditioning accomplish uniform distribution of binder around the sand grains, control moisture content, eliminate foreign particles and aerates the sands. Therefore, there is a need for sand conditioning for achieving better results.
The foreign materials, like nails, gaggers, hard sand lumps and metals from the used sand are removed. Next, the sand is screened in riddles which separate out the hard sand lumps etc.
These riddles may be manual as well as mechanical. Mechanical riddles may be either compressed air operated or electrically operated. But the electrically operated riddles are faster and can handle large quantities of sand in a short time.
The amount of fine material can be controlled to the maximum possible extent by its removal through exhaust systems under conditions of shake out. Next, the whole mixture is mulled suitably till properties are developed. After all the foreign particles are removed from and the sand is free from the hard lumps etc.
As the moisture content of the returned sand known, it is to be tested and after knowing the moisture the required amount of water is added. Now these things are mixed thoroughly in a mixing muller. Inadequate mulling makes the sand mixture weak which can only be compensated by adding more binder. Thus the adequate mulling economizes the use of binders. In the first method, first water is added to sand follow by clay, while in the other method, clay addition is followed water.
It has been suggested that the best order of adding ingredients to clay bonded sand is sand with water followed by the binders. In this way, the clay is more quickly and uniformly spread on to all the sand grains.
An additional advantage of this mixing order is that less dust is produced during the mulling operation. The muller usually consists of a cylindrical pan in which two heavy rollers; carrying two ploughs, and roll in a circular path. After the mulling is completed sand can be discharged through a door. A figure of sand muller is given in figure. Aerating can also be done by riddling the sand mixture oil on a one fourth inch mesh screen or by spraying the sand over the sand heap by flipping the shovels.
The aeration separates the sand grains and leaves each grain free to flow in the direction of ramming with less friction. In green sand mold, the molten metal is poured immediately after the same is ready, i. No baking is performed in this case. These molds are relatively weaker and softer than other types of molds. These molds are chipper and take less time but there use is limited to the production of small and medium castings only, particularly in non-ferrous metals and alloys. They are baked in an oven before being finally closed for pouring.
They are stronger and harder than green sand molds. These molds evolved less steam and gases during casting, thus requiring less permeability. Use of fine sand enables smoother surface on casting. These molds are often used for large castings and those small castings which needs high accuracy. However, on account of their higher production cost, they are preferred only when green sand molds are found to be unsuitable for the purpose.
Loam molds are first built up with bricks and often reinforced with iron plates. A loam mortar, consisting of coarse grained silica sand, saw dust, fire clay and water, is then prepared and plastered on to the backing made from brick and iron. The mold is then finished by sweeps, giving a refractory coating and finally baked to provide strength to resist the heavy flow of molten metal. Construction of these molds reduces the pattern cost, but, at the same time, their construction involves a lot of time and skill.
They are used for extremely large castings. The mixture essentially consists of silica sand, Portland cement and water, mixed thoroughly to prepare a strong band. Drying and setting of cement takes about72 hours.
Separate preparation of cope and drag is needed due to low green strength of cement in wet state. On setting, it produces a mold of high strength and hardness. Cement bonded sand requires less ramming than usual. A perfect alignment of cope and drag should be ensured.
The main advantage of these molds is the casting made in them carries very accurate and smooth surfaces. Thus minimize the need of further machining or cleaning of surfaces.
Such a mold is known as core sand mold. In these molds core oil is used as a binder. The main advantages of these molds are very high collapsibility at elevated temperatures, thereby allowing a free contraction of metals and alloys during solidification and a high degree of surface finish on casting. The carbon-dioxide gas is used only as the mold hardened.
After preparing the mold from above mixture, the gas is passed through it to obtain the desired hardness. Cores are also prepared in the same manner. These molds are prepared by heating sand and resin over the surface of the metallic pattern.
This enables the production of thin and uniform thickness which, when separate from the pattern surface, forms one part of the shell. Two such parts are joined together to form the shell mold. On seeing the metal through it, it is ensured that the mold has completely filled. They also exhaust air from the mold cavity as the molten metal flows in to the mold.
The casting processes differ from each other basically in the type of material used for the preparation of the mould and the method of pouring the molten material. The mould material is generally sand or metal and the pouring method may use gravity, vacuum, low or high pressure. Casting is most often used for making complex shapes that would be difficult or uneconomical to make by other methods.
In expendable casting, it includes sand casting, shell casting, plaster mould casting, investment casting, and evaporative-pattern casting. In non-expendable casting , it includes permanent mould casting, dies casting, semi-solid metal casting, centrifugal casting, continuous casting.
Following are the four different types of metal casting processes used for large-scale production:. It is the most extensively and widely used types of metal casting process. This is an expendable mould-permanent pattern casting process. The sand casting process involves the use of a furnace, metal, pattern, and sand mould. Hand ramming of sand around the pattern is used for simple casting. For complicated castings, the sand mixture is compacted by moulding machines.
Moulding machines not only increase the production cost but also improve the quality of casting by improving the application and distribution of forces for ramming.
It can be used for all types of metals but the surface finish and dimensional accuracy are not good compared with other casting processes. It is the most economical production process. In these processes, a mould is used repeatedly. The mould is generally on two halves and is designed for easy opening and closing. Ejector pins are provided for the removal of the solidified casting.
The refractory coating on the thinner walls of the mould not only increases the mould life but also prevents the sticking of casting on the mould walls. Preheating of the mould and controlled cooling of the mould through water circulation maintain a uniform mould temperature. Permanent with other processes are used for casting low melting point, non-ferrous materials using alloy steel moulds. The initial cost is high, so these processes are economical only when higher production volumes are required.
Die casting is a very commonly used type of permanent mould casting process. Aluminum Alloys Copper Alloys Magnesium Alloys Heat Treatment Heat Treatment of Ferrous Alloys Case Hardening Methods Heat Treatment of Nonferrous Alloys Foundry Tooling Selection of Pattern Type and Material Pattern Materials Types of patterns Pattern Design Selection of Parting Line Pattern Dimensioning Elimination of Details Gating Systems Types of Gating Systems Elements of Gating Systems and Their Functions Pouring Basin or Pouring Cup Pouring Sprue Sprue Base or Sprue Well Runner Extension Gates or Ingates Casting Yield Gating System Design Basic Principles Fluidity or Fluid Life of Molten Metal Pouring Time Pressurized vs.
Nonpressurized System Choke Area Calculation Ingate Design Slag Trap Systems Whirl Ingate Metal Filtration in Gating Systems Risering and Feeding Casting Solidiication and Shrinkage Design of Risers Riser Type and Shape Riser Dimensioning Riser Neck Dimensioning Feeding Distances Cooling Aids Feeding Aids Grouping Castings Solidiication Modeling Sand Molding and Coremaking Processes Foundry Sands Types of Sands Sand Properties Green Sand System Green Sand Preparation Testing Green Sand Properties Variables Affecting Green Sand Properties Types of Sand Molds
0コメント