Title : Casting and Moulding Procedures
Objective : To practice CO2 casting process and moulding procedure.
Apparatus : Silica sand, sodium silicate, CO2 gas, sand mixer machine, weighing, scope, flash, pattern and core, aluminium ingot, furnace, hammer, core sticker.
Safety Procedure and PPE
Follow general safety guideline/procedure in workshop.
Safety show, apron, goggle, leather shoes, cotton glove, mask, safety helmet, face shield.
Various casting processes used to produce manufactured items. The metal most frequently cast are iron, steel, brass, bronze, magnesium, certain zinc alloys and nickel based superalloys. Of these, cast iron and aluminium are dominant, primarily because of their low cost, good fluidity, adaptability to a various processes and the wide range of possible properties.
Sand casting is by far the most common and possibly the most versatile of the casting processes. Granular refractory material (such as silica, or zircon) is mixed with small amounts of other materials, such as clay and water and is then packed around that has the shape of the desired casting. Because the grains can pack into thin sections and can be economically used in large quantities, products spanning a wide range of sizes and detail can be made by this method. If the pattern must be removed before pouring, the mold is usually made in two or more pieces. An opening called a sprue hole is cut from the top of the mold through the sand and connected to a system of channels called runner. The molten metal is poured down the sprue hole, flows through the runners, and enters the mold cavity though one or more openings, called gates. Gravity flow is the most common means of introducing the metal into the mold.
After modification, the mold is broken and the finished casting is removed. Because the mold is destroyed, a new mold must be made for each casting.
In CO2 casting, molding are prepared by mixing silica sand liquid sodium silicate in a muller and packed into flash. The mixture remains soft and moldable until it is exposed to a flow of CO2 gas. The harden sands, however have poor collapsibility, making shakeout and core removal difficult. Additive that will burn out during the pour are often used to enhance the collapsibility of sodium silicate molds.
CO2 MOLD PREPARATION:
1. 20kg silica sand and 0.8kg sodium silicate mixed together by using mixer machine.
2. Place the mixture in both top and flash with desired pattern and core and before the pouring cup and air vent prepared.
3. For better flash removal, slowly knock the flash.
4. For a few minutes flow the CO2 gas in the mold till the mold became harden.
5. The flash, pattern, core and pouring cup core had been removed.
6. Finally, by using core sticker joint the top and bottom mold with carefully.
1. Melt aluminium ingot in furnace till reached 700˚C.
2. Pour molten aluminium in sand mold.
3. Leave for about 15 minutes.
4. Using hammer, broke the mold to gain the product.
.POROSITY OR BLOW HOLES
Gas porosity or blow holes are caused by accumulated gas or air which is trapped by the metal. These discontinuities are usually smooth-walled rounded cavities of a spherical, elongated or flattened shape. If the sprue is not high enough to provide the necessary heat transfer needed to force the gas or air out of the mold, the gas or air will be trapped as the molten metal begins to solidify. Blows can also be caused by sand that is too fine, too wet, or by sand that has a low permeability so that gas cannot escape. Too high a moisture content in the sand makes it difficult to carry the excessive volumes of water vapor away from the casting. Another cause of blows can be attributed to using green ladles, rusty or damp chills and chaplets.
• Blowholes and pinholes are produced because of gas entrapped in the metal during the course of solidification:
• Excessive gas content in metal bath (charge materials, melting method, atmosphere, etc.); Dissolved gases are released during solidification;
• In the case of steel and cast irons: formation of carbon monoxide by the reaction of carbon and oxygen, presents as a gas or in oxide form. Blowholes from carbon monoxide may increase in size by diffusion of hydrogen or, less often, nitrogen;
• Excessive moisture in molds or cores;
• Insufficient evacuation of air and gas from the mold cavity; -insufficient mold and core permeability;
• Make adequate provision for evacuation of air and gas from the mold cavity;
• Increase permeability of mold and cores;
• Avoid improper gating systems;
• Assure adequate baking of dry sand molds;
• Control moisture levels in green sand molding;
• Increase static pressure by enlarging runner height.
A Radiographic figure of the blow holes
Shrinkage is a form of discontinuity that appears as dark spots on the radiograph. Shrinkage assumes various forms, but in all cases it occurs because molten metal shrinks as it solidifies, in all portions of the final casting. Shrinkage is avoided by making sure that the volume of the casting is adequately fed by risers which sacrificially retain the shrinkage. Shrinkage in its various forms can be recognized by a number of characteristics on radiographs. There are at least four types of shrinkage: (1) cavity; (2) dendritic; (3) filamentary; and (4) sponge types. But the shrinkage that happen to sample is cavity shrinkage.
CAVITY SHRINKAGECavity shrinkage appears as areas with distinct jagged boundaries. It may be produced when metal solidifies between two original streams of melt coming from opposite directions to join a common front. Cavity shrinkage usually occurs at a time when the melt has almost reached solidification temperature and there is no source of supplementary liquid to feed possible cavi
Hot cracking. A crack often scarcely visible because the casting in general has not separated into fragments. The fracture surfaces may be discolored because of oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.
Can cause damage to the casting while hot temperature due to rough handling or excessive temperature at shakeout.
1. Care in shakeout and in handling the casting while it is still hot;
2. Sufficient cooling of the casting in the mold;
3. For metallic molds; delay knockout, assure mold alignment, use ejector pins.
CARBON DIOXIDE MOULDING
Carbon dioxide molding is a sand casting process that employs a molding mixture of sand and liquid silicate binder. The molding mixture is then hardened by blowing carbon dioxide gas through it.
Figure above showing some defects on the sample.
Sand casting is by far the most common and possibly the most versatile of the casting processes. Granular refractory material (such as silica, or zircon) is mixed with small amounts of other materials, such as clay and water and is then packed around that has the shape of the desired casting.
. Figure showing blowholes most probably caused by inefficient evacuation of air during pouring process.
CARBON DIOXIDE MOULDING ADVANTAGES
1. Provides good dimensional tolerances through strong core and mold
2. Provides excellent casting surface finishes
3. Generally used for high-production runs
4. Accommodates a wide range of core and mold sizes.
5. When used for making cores, the CO2 process can be automated for long durations & speedy production runs.
1. Ideal for casting applications where speed and flexibility is paramount.
1. When the silica sand and sodium silicate mixed, CO2 gas was pumped into it. The gas will react with the sodium silicate where it hardened the mold.
2. To get a beautiful finish we have to make sure that the ratio of the mixture (silica sand, sodium silicate and water) is suitable for the work.
3. Knock and remove the mixture that had pumped with CO¬¬2 properly from the mould to get a good finish.
From the casting and moulding processes that we have done, we are able to understand the procedures and precautions for both dye casting and CO2 moulding. These make the process met its objective which are to expose and practically hand on cast and mould processes thus conclude that the objective has been achieved.