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, the process is more complicated than it sounds.
The first step in mold assembly is to partially fill the drag with sand. Patterns, core printing, core and gate systems are placed near the parting line. Then assemble the cope to the drag. Pour additional sand over the coping half until it covers the model, core and gating system, then vibrate or mechanically compact the sand. Excess sand is removed with a removal stick.
Now that the mold is formed, the upper mold is removed from the lower mold so that the pattern can be removed from the mold.
Pattern extraction is done carefully to avoid damaging or distorting the newly formed cavity. This can be facilitated by designing draft: a vertical taper perpendicular to the parting line. Draft taper is usually at least 1°. The rougher the surface of the pattern, the more draft is provided.
Before filling with liquid metal, a complete mold needs to be prepared; the mold cavity is usually lubricated with a mold cleaning fluid to facilitate casting removal. The core is then positioned, additional mold material such as coping cords are added to help prevent runout, and the mold halves are then closed and clamped firmly together; the upper and lower mold sections remain properly aligned with the help of pins and guides .
The mold halves must remain secure so that liquid metal does not leak from 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 upper mold 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 runner (the vertical part of the gating system), and then through the runner (the horizontal part). Accumulated gas and displaced air escape through vents. The points at which metal is introduced from the runner into the mold cavity are called gates.
After the casting has cooled, it is released from the sand mold. The process of removing sand from castings is called falling sand. Castings can be removed manually or by automated machinery. Mixing tables and rotating drums are commonly used.
Complete coping and dragging of engineering parts.
This basic process varies depending on the mode, flask type, and level of mechanization:
Benchtop molding is preferred for small jobs. The entire operation is carried out on a workbench of convenient height.
Floor Forming 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 precision and uniformity, allowing tolerances to be quickly maintained within tight limits. The main operations performed by the molding machine are sand compaction, mold tumbling, gate forming, beating and ejection.
After the sand is shaken off the intact casting, the lumps are cooled and crushed. All particles and metal particles are usually removed with the help of a magnetic field. All sand and ingredients are screened using shakers, rotary screens or shakers. The cleaned sand can then be reintroduced into the beginning of the sand production cycle.
Molding sand is prepared in a mill by mixing sand, binder and water. Aerators are used in combination to loosen the sand, making it easier to shape.
Prepared sand is usually transported by forklift or belt conveyor to the molding shop, where the molds are formed; the molds can be placed on the floor or transported by conveyor belts to the pouring station. After pouring, the castings are removed from the adhering sand at the shakeout station. The used sand is returned to the storage tank by belt conveyor or other means.
Foundry sand is usually recovered and reused through many production cycles. According to industry estimates, approximately 100 million tons of sand are used in production each year. Of that number, only four to seven million tons are discarded, and even this sand is often recycled by other industries.
Sand molding process and method
Closed sand mold ready for pouring.
wet sand mould
A typical wet sand mix consists of 89% sand, 4% water and 7% clay. Green sand is favored by the industry for its low cost and solid overall performance. The "green" in green sand refers to the amount of moisture present in the mix during the pouring process.
Green sand molds are strong enough for most sand casting applications. They also offer good foldability, permeability and reusability. The main difficulty arises with the moisture content. Excessive moisture can cause defects in castings, and moisture resistance depends on the metal being cast.
cold setting process
Sometimes non-traditional binders are used in industrial sand casting. Whereas traditional foundry adhesives require heat to cure, these alternative adhesives chemically bond at room temperature when mixed with sand – hence the name cold cure process. These relatively new sand casting processes are technologically advanced and increasingly popular. Cold setting processes are more expensive than green sand molds, but they produce castings with excellent dimensional accuracy.
Shell mold casting is a relatively new invention in mass production and finish forming technology. It was first used by Germany during World War II. The molding material is a mixture of dry fine silica sand with minimal clay content and 3-8% thermoset resin (phenolic or silicone grease). When the molding compound drips onto the heated template, a hard crust about 6mm thick forms. In order for the shell to fully cure, it must be heated to 440 to 650°F (230 to 350 tons) for a few minutes.