Sand casting is a process that uses non-reusable sand molds to form metal castings. On the one hand, casting is a deceptively simple manufacturing process: anyone who has ever built a castle on the beach knows that sand can be used to create elaborate shapes. However, when dealing with the heat of molten metal in a foundry, many factors must be considered in order to be successful. Casting is used to make metal parts of various sizes, from a few ounces to several tons. Sand molds can be formed to make castings with fine exterior details, cores and other shapes. Almost any metal alloy can be sand cast. The hollow is made from wet sand, filled with molten metal, and then cooled.
How to Make Sand Castings
Sand casting involves four basic steps: assembling the sand mold, pouring liquid metal into the mold, allowing the metal to cool, then removing the sand and removing the casting. Of course, this process is more complicated than it sounds.
The first step in mold assembly is to partially fill the resistance with sand. Patterns, cores, cores and gating systems are placed near the parting line. Then assemble the top to the bottom. Additional sand is poured over the top half until the model, core and gating system are covered, then the sand is compacted by vibration or mechanical means. Remove excess sand with a tap stick.
Now that the mold is formed, the upper mold is removed from the lower mold so the pattern can be extracted from the mold.
Pattern extraction is performed carefully to avoid damaging or distorting the newly formed mold cavity. This can be achieved by design draft: a vertical taper perpendicular to the parting line. Draft taper is usually at least 1°. The rougher the surface of the pattern, the greater the amount of draft that needs to be provided.
Before filling with liquid metal, the complete mold needs to be prepared; the cavity is often lubricated with a mold cleaner to facilitate removal of the casting. The core is then placed, additional mold material is added (such as a cap cord) to help prevent runout, and the mold halves are closed and clamped securely together; the cope and drag sections are held in proper alignment with the help of pins and guides.
The mold halves must remain secure so that liquid metal cannot leak through the parting line. Before pouring a boxless mold, a wood or metal pouring sleeve is usually placed around the mold and a weight is placed on top to prevent the lid from lifting.
The molten metal enters the mold cavity through the gating system: the molten metal is poured into the mold through the pouring cup, continues down the sprue (the vertical part of the gating system) and then through the runner (the horizontal part). Accumulated gas and displaced air escape through the vents. The points at which metal enters the mold cavity from the runners are called gates.
After the casting has cooled, it is removed from the sand mold. The process of removing grit from castings is called shakeout. Castings can be removed manually or by automated machinery. Typically a stirring table and rotating drums are used.
This basic process varies depending on the mode, flask type, and level of mechanization:
Benchtop molding is the first choice for small jobs. The entire operation takes place on a bench at a convenient height.
Floor molding For medium and large jobs. As the name suggests, the mold is placed on the floor before pouring.
Machine forming for mass production. Machine forming saves labor and provides excellent accuracy and uniformity, allowing fast tolerances to be held within tight limits. The main operations performed by the molding machine are tamping the sand, rolling it through the mold, forming the gate, striking the pattern and its ejection.
recycled sand
After the sand is shaken from a full casting, the block is cooled and crushed. All particles and metal particles are usually removed with the help of a magnetic field. All sand and ingredients are screened with shakers, rotary sieves or shakers. The cleaned sand can then be reintroduced at the beginning of the molding sand production cycle.
Molding sand is prepared in a grinder by mixing sand, binder and water. Aerators are used in combination to loosen the sand, making it easier to shape.
The prepared sand is usually transported by forklift or belt conveyor to the molding shop, where the mold is formed; the mold can be placed on the floor or transported by a conveyor belt to the pouring station. After pouring, the casting is removed from the adhering sand at the shakeout station. The used sand is returned to the storage bin by belt conveyor or other means.
Foundry sand is usually recycled and reused in many production cycles. According to industry estimates, approximately 100 million tons of sand are used in production each year. Of that number, only four or seven million tons are discarded, and even that sand is often recycled by other industries.
