Casting is a fundamental technique in the metalworking process and is at the heart of many industries and products throughout the world. Not to be confused with other techniques such as forging or moulding, a casting is achieved by pouring hot molten metal into a mould that contains a hollow cavity of the desired shape. As the molten liquid settles into the hollow cavity, the metal slowly solidifies and takes on the shape of the hollow cavity. Once the metal is completely set it is then removed from the mould to complete the casting process.
Although this is how castings work, there are a number of different types of processes, all of which are unique in their own right. The four main types of casting are: sand, investment, plaster and die. Each one of these serve a particular purpose so it is important understand why and when each should be used so that the right choice can always be made.
Casting sand usually consists of finely ground, spherical grains that can be tightly packed into a smooth moulding surface. They are designed primarily for eliminating the risk of tearing, cracking and other common issues and this is achieved by allowing a reasonable amount of flexibility and shrinkage during the cooling phase of the process. For added strength, the sand can be combined with clay which helps the particles bond more closely. Sand casting is often used for automotive products such as engine blocks and housing.
Investment casting involves the use a disposable wax pattern which is either injected directly into a mould or pre-coated beforehand with a liquid obstinate material. The wax pattern is then set into shape by molten forming material which is poured into the mould and allowed to harden. Once the component is removed, the wax pattern is melted out of the casting for re-use. Similar to sand casting, investment casting is used for automotive parts and also used regularly for aerospace and military appliances.
Using a mixture of gypsum, strengthening compound and water, the plaster casting process is very similar to that of sand casting. Typically coated with an anti-adhesive compound in order to prevent it sticking, the plaster is able to fill any gaps in the mould. Once it is removed to cast a part, it quite often cracks or forms defects which requires it be replaced with fresh material. As a result of this, the plaster casting processing tends to be more expensive than alternative methods. Despite this, plaster casting is a more effective and cost-efficient process when the quality of surface finish is requirement.
Die casting is the process of moulding non-ferrous metals and alloys such as zinc, tin and copper under high pressure. Again, the mould is coated with lubricant to help with the components release as well as to regulate the die’s temperature. After this, molten metal is continuously injected into the die under high pressure until the piece solidifies. The high-pressure insertion of molten metal is done quickly In order to prevent the material from hardening before being cast. Once the process is complete, the component is taken out of the die and any remaining scrap material is removed.
Moulding v Castings
It is easy to see why moulding v castings causes confusion with this castings article mentioning moulds a number of times already, so what is the difference?
The easiest way to remember is that a casting can be only carried out once a mould of an object is available. Moulding on the other hand is the process of taking an object and creating an impression of it into a form of material. Castings create exact replicas of objects that have been moulded, thus the process of moulding material is what comes first.
Industry leader Impregnation Services Limited is a casting recovery service provider specialising in impregnation of porous castings and other ferrous and non-ferrous materials.
Based in Ashton-under-Lyne, just outside Manchester, Impregnation Services Limited has been established for 50 years. Throughout this time ISL has been providing high quality impregnation services to a wide range of industries and are able to enhance quality and reduce scrap in cast metal parts.
Our recently refurbished plant has the capacity to work on single parts to full production runs, offering services on 1000s of parts and specific applications. Our team strives to offer our customers the best in vacuum impregnation services.
If you would like to find out more about our porosity sealing services, speak to a member of our team on 0161 344 1004.