Casting is made by pouring molten metal into a mold so as it then solidifies into a geometric design or shape. In other examples, molten metal is filled into a die with a cavity in the needed shape of that part. Casting is generally the fastest way to make either a small portions of prototypes or extremely large production quantities. Casting process is used by many and major industry because all metals can be casted. Frequent mill forms like plate and bar are created from cast metal. The lineament of a casting is all about its uniform grain form.
On contrary to wrought metals with directional grain formation, the cast structure is isotropic. Meanwhile cast material thickness is not as much as wrought metals, isostatic pressing may enhance mechanical properties. In the same sequence, surface porosity can be removed by sealing using either a metal coating or resin. Various casting processes are using patterns that create the cavity of the mold and could be created from wood, plastic, metal or plastic.
Expandable Patterns which are consumed by mold processing or by pouring metal within casting) are made of plastic, or wax, or polystyrene foam. Patterns are shaped in different sizes to fit for metal shrinkage during solidification. In addition, they will also have taper to make pattern release easier and to make facilitation for metal transfer and finishing via the mold. Cores are urgently needed to make recesses, hollows, and undercuts are required in the part.
There are three main casting Steps:
1. expendable mold
2. expendable pattern
3. permanent mold
Types of Casting
Below is a brief explanation to two types of mold casting only, however, the list of types are longer than these two.
Expendable Mold Casting:
Expendable mold casting is a generic categorizing which includes sand, plastic, shell, plaster, and lost-wax techniques molding processes. This type of mold casting include using temporary (non-reusable molds).
Sand Casting:
Sand casting is considered one of the well-used and most popular as well as simplest types of casting that existed for years and centuries. Sand casting let smaller batches to be created in comparison to permanent mold casting at a low cost. This method does not allow manufacturers to create products at a reasonable cost only, but there are various benefits coming from sand casting, like very minute (small) size operations. It starts from castings which fit in your hands to train beds. It is known that one mold is sufficient to make full rail beds. It can be done using sand casting. The process of sand casting lets most metals to be cast as per the type of sand used during molding. Sand casting needs a lead time of days to perform production at high output rates (around 1–20 pieces/hr.-mold). It does not surpass in large-part production. Green sand has nearly no part-weight limit, but dry sand has a specific part mass limit (2,300–2,700 kg). Least part-weight varies from (0.075–0.1 kg). The sand is joined altogether using clays, polymerized oils or chemical binders (like motors oil). Sand can be reusable as many times as possible in most operations but it requires some maintenance.
Pattern Material:
The material from which the pattern is made is depend on the kind of metal and mold casting process, the dimensional accuracy required, the casting's geometry and size, and the number of metal castings to be produced with the use of the pattern. Patterns can be made from wood (softwood), mahogany (hardwood), different types of plastics, or even metal like aluminum, steel or cast iron. In most cases of manufacturing operations, patterns usually get coated with a parting agent to facilitate the removal stage from the mold.
Process Selection:
Selecting the right process depends on 4 elements. These elements are metal and mold casting, dimensional accuracy required, casting geometry and size, and the number of metal casting to be manufactured. Considerations to be taken care of when selecting a production metal casting process should include the following:
1. Cost: Die cast parts are always the cheapest options available. Nevertheless, when the cost of the tooling is included in, it might take extra number of parts to accomplish the die casting to break even with the available processes.
2. Timing and Speed: it is how are the parts needed? Machined parts can be ready within 1 to 5 days. Sand casting and plaster casting can yield parts in 1 to 2 weeks often. Die cast parts usually take around 8 to 12 weeks and sometimes more than 12 weeks.
3. The size and geometry of the part.
4. The quantities required in according to timeframe. It also is relying on how many parts do you want to manufacture and in what time. 12,000 parts in one batch is extremely unlike 500 parts/month in a period of two years. Due to up-front funds and project budgets, it may be more needed to pay more parts, yet less up-front tooling
5. The finishing styles on the part. Some parts need to be very smoothly finished.
6. The strength needed per part. Metal parts are strong by nature. Hence, stronger is not better all the time. Every process requires parts with various strength, internal stress, elongation, and other physical criteria. The selected process should be decided according to how it meets the requirements.
7. Thermal features required per part. Every casting process has a set of alloys that fit the process perfectly. These alloys have several thermal features.
There is no always-right answer for each casting selection process. If there’s one overriding need, it would tip the selection in one route or another. Even if a process does not score highest in any one group, the whole combination of features might be the correct selection in the end.
Casting Economic Consideration (Cost):
The costs of each unit of casting part depends on various elements that comes in regard to materials, equipment, labor and tooling. Preparing to cast a product also include the molds production and dies which are rely on raw materials, effort and time which influence the production cost mainly. There are main costs intended in making patters ready for casting. Costs go in melting and pouring the molten metal into shapes and molds in heat treatment, cleaning and inspecting the casting as well. It is known that heat treatment is crucial part in the production process of different alloys sets. It might be needed to manufacture improved mechanical properties. The cost of equipment should decrease when the parts cast increase. If the demand is small, the cost is usually extremely increase because then it becomes more economically sufficient to produce the parts using sand casting.
