Sand casting is an important process used in foundry processes to form metal castings using non-reusable sand moulds. It is a popular production method for metal components of all shapes and sizes, from a few ounces to tons of it.
Not only is it versatile in the production of metal components of all sizes, but it is also excellent at creating very complex or detailed castings. Certainly, it can be used to cast nearly any metal alloy.
Molding sand plays a vital role in the sand casting process. The molding sand used in foundry processes must be able to hold a shape well and capture very fine details of a casting. Also, it must be permeable enough to let gases escape.
A good molding sand must not crumble under the strain of having the molding pattern removed from it nor must it sink on itself. Also, it must not lose its form when it is turned upside down.
Although, Petrobond and clay dust sands share several properties in common, coal dust according to a recent study has been shown to possess some characteristics that gives it an upper hand in a casting process.
Petrobond sand is an improvement from regular moulding or green sands. Petrobond, a vital component of this sand type is a formulated bonding agent that bonds sand with oil rather than water. It is reusable and has infrequent re-mulling and re-bonding.
Generally, Petrobond sand consists basically of a mulled mixture of oil, sand, petrobonding agent and a little amount of P-1 catalyst. It was developed because of the need for a method of producing precision castings in green sand foundries.
It owes its success as a good molding sand to its use of oil as the tempering agent rather than water. Consequently, it can be used with fine sands without encountering problems associated with reduced permeability. This is because oil develops a much lower volume than water when vaporizing.
Water, on the other hand, according to Petrobond’s official manual expands up to 80 times of its original volume when vaporizing into steam. You can obtain an already mixed Petrobond sand or buy its constituent materials and mix appropriately.
With the aforementioned, it is evident why Petrobond sand is highly sought after for sand casting processes in foundry industries. However, a recent study has shown that foundry coal dust sands have an edge over Petrobond sands.
Petrobond vs Coal Dust As Facing Sand
Three moulds were prepared and filled with sand using an embossed-pattern block. The first was a mould with Petrobond sand as facing sand, and the other was with coal dust sand.
When molten cast iron was poured into the Petrobond sand mould through a hole, flames were noticed at the junction of the hole with some shrinkage. The flames are attributed to the oils in Petrobond which burn close to the part where the hot metal is poured in. There were no flames formed in the mould filled with coal dust as facing sand, since it does not consist of any flammable oils.
They were then both allowed to cool down to take the shape of the mold and the used pattern. When the Petrobond sand mould was broken, the following was discovered:
- The cast had a shiny reflective surface
- It also produced a wrinkled surface with little depressions. This is because Petrobond sand is fine and composed of oil rather than water. Consequently, the oil cools down and lifts the metal. This means that there is no time to reform the shape of the mould again; hence, the depression.
- No vents were produced at the edges.
For the mould with foundry coal dust as facing sand, the following were noticed:
- The sand came out of the mould more easily
- No expansion defects were noticed
- It produced an even surface finish
- No vents were produced at the edges of the block
- The mould was filled.
The result demonstrated that blocks prepared using coal dust low ash as the facing sand appeared better physically and had better internal features than the one made with Petrobond sand.
In the study, coal dust was also used to produce a handwheel for a lathe machine (third mould). It could be seen from the finished products that there were no blowholes, inclusions or flaws whatsoever.
Advantages Of Using Foundry Coal Dust In The Sand Casting Process
Ease of removal: It was discovered in the test carried out that using coal dust as the facing sand ensures that the mould can come out easier as opposed to Petrobond sand.
When the molten cast iron or indeed any other molten metal comes in contact with mold surfaces containing coal dust, a gaseous envelope is formed. This gaseous envelope resists the fusion of sand to metal, in this case, the surrounding mould.
No expansion defects: Using coal dust as facing sand eliminates expansion defects as seen in Petrobond sands. Although Petrobond sands are an improvement over green sands in that regard, they fall short of the results produced when clay dust is used. It is important to note that to avoid expansion defects in large surface areas, you have to ram softly.
Examples of such expansion defects are scabs and rat-tails.
Even surface finish: Using coal dust low ash in sand casting has proven to produce more even surfaces than Petrobond sand. This is because, with foundry coal dust, sand inclusion and expansion defects are eliminated. It produces whole, solid blocks such with no blow holes even when the cross-section of the cast object is analyzed.
The even surfaces produced by using coal dust as facing sand are attributed to the following:
- The heat of the molten metal poured into the mould releases carbon from the rich gases it produces. As a result, the mould is smoked long enough to provide the required render finish.
- Also, the gas formed could act as a film, layer or envelope just before going through the mould. However, this seemingly small time is just enough to let the iron glide by, without eating into the sand filling the mould.
This is in contrast to Petrobond sands which produces wrinkled surfaces because of an uneven cooling of its heated oils which do not have enough time to reform the shape of the mould before finally cooling.
Elimination of vents: Vents are an almost inevitable occurrence in most sand casting processes. Most often than not, there are usually shrinkage dimples, vents and some blowholes at the surface contact between moulding sands and the moulds used.
However, casts made using low ash coal dust as facing sand in the sand casting process do not have the characteristic vent problem.
Cost: Petrobond powder is costly and even more expensive when already mixed. Coal dust suppliers are on the other hand, a cheaper resource, easier to get and more effective at producing the required cast shape.
Increases green and dry sands porosity or permeability: When coal dust is used as facing sand in combination with dry or green sand, porosity and permeability is improved. Hence, casting defects are reduced to the barest minimum.
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