Green sand molding for casting
This molding technique is known as green sand since it contains some moisture, in addition to silica, it has other necessary components for the casting process.
Green sand components for gray iron casting
– Silica sand: SiO2, and is used in foundries for its refractory properties, expressed as the ability to resist high temperatures, without melting or degrading (Groover, 1997). This sand is made up of a disorderly set of grains of different shapes and mineral composition, their sizes are between 0.02 and 5mm (Crespo, 2003).
– Bentonite: Bentonite is a clay formed essentially by minerals from the smectite group, regardless of their occurrence or origin. It is classified into sodium and calcium, based on the predominant cation between the layers and the ability to expand.
– Charcoal: it is obtained from the controlled combustion of small tree branches or shrubs (willow, vine and walnut the most used) being half-burned charcoal particles)
– Water: H2O
Type of sands used in the process
It is important to note that for the green sand molding process, there are 2 types (references) of sand:
– Contact sand: This sand is passed through a sieve (mesh) to obtain a finer sand, this sand is the one that comes into direct contact with the part to be produced in casting, for example the wheels for pulleys, wedges for cableways and other castings produced in the company.
Filling sand: It is a coarser sand than contact sand, prepared with the same elements described above. It is located in the layers after the contact sand.
Sand casting molding techniques
It can be manual or automatic, the pressure on the sand to form the cavity is done with an electric, pneumatic or hydraulic machine.
When the piece exceeds the dimensions of the available presses, or due to its handling or required quantity, it is not possible to carry out the process in the press, the entire process is carried out manually on the floor.
To carry out the molding process, in addition to preparing the sand, a model must be available, this is the “pattern” to repeat or copy in the sand, the model can be:
- Hand mold
- Plate for press, in terms of plate models there are 2 versions:
- Integral: When the model as such is part of the plate
- Registered: You have 2 halves, installed on a plate (fixing).
It is important to clarify that not any piece can be used as a model for casting, the models must meet special characteristics such as draft angles, machining increases, core signals, design of drains and compensators, among others.
Once the model is available, the sands have been prepared and the technique has been chosen, we proceed to manufacture the temporary mold in sand, in which the casting process of the molten material will be carried out, in this case, gray iron.
Regardless of the technique, a sand mold is composed of “2 halves“, the pressing process consists of agglomerating the sand against the molds, on both sides, then removing the model and closing the casting box again, in this way , a “void” is formed in the sand. It is this space without sand, which the molten material will occupy and thus a new casting will be formed.
In order to generate special shapes that would not be achieved with machining, or to save time later in the final finish, cores are used, these are pieces in sand that prevent the formed material from lodging and thus the desired cavities or shapes are formed. .
Once both parts of the mold are manufactured, the lumps are placed in the space designated for this purpose, awaiting the casting process of the molten material.