Casting and Moulding

I don't know the word difference.We used to describe casting for sand casting,die-casting and moulding for injection moulding.

Like you rihgtly know, in the process of sand casting you will need to prepare the mould by moulding process, whether synthetic or natural moulding(using natural sand containing sufficient clay binders).The mouldind may involve kneading prepared sand mix around a pattern, which is subsequently removed, to create a hollow which accepts the melted material(metal).

ena29,Nigeria

Dkamesh11,

The most siginifcant difference is that in "traditional" casting processes, the mold (I'm American; no "u" in our spelling) is destroyed / consumed when removing the workpiece from it. In molding, the mold is re-used multiple times. Die-casting is therefore a molding process, in spite of it's name. But sand casting, lost-wax (or lost-foam) casting, and pressure casting are all casting processes: the mold is made (from a pattern of some sort) for a single use and is destroyed in the process of unmolding.

Hope this helps.

Anna

Anna has described several process correctly, however you asked a more generic question. Originally, casting was a non-pressure gravity process. The dictionary definition for this is:

4 a : to give a shape to (a substance) by pouring in liquid or plastic form into a mold and letting harden without pressure <cast steel> b : to form by this process

while molding is usually, but not always, considered to be a pressure process, e,g, thermoplastic injection molding. Solid objects are also worked into shapes through a process called molding, which usually involves cutting or abrading material, however I suspect you are not interested in that type of molding. Earthenware objects, e.g. clay pots, etc., are "molded" by packing clay or other material into molds and then firing. Here again, the idea is that an external pressure is involved.

When talking about engineering materials such as metals, waxes, and thermoplastics, traditional casting involved pouring the liquid material into the top of a mold, by gravity only. Molds could be temporary, especially where complex shapes required the molds to be destroyed to remove the molded object, as in sand-casting, or they could be permanent, as in permanent mold steel casting, where the mold is separated to remove the part and then put back together and reused. In sand-casting new molds are created through the use of patterns which are positive copies of the object, often with extra pieces added to form vents, sprues and runners, and gates. These patterns are then placed into boxes of a mixture of sand and other materials used to hold the sand together. If the patterns are made out of wax or other meltable material (some thermoplastics are sometimes used), the mold can be pre-heated before pouring the metal and the pattern liquifies and is poured out, leaving only the mold. Sometimes the metal is poured directly into the mold without removing the pattern. The heat of the metal vaporizes or incinerates the mold as the metal is poured in. This is known as the "lost wax" method, whether or not wax is actually used as the pattern material. (Sometimes polymer-foam patterns are used and then the term "lost-foam" may be applied)

Of course, technology advances and permanent molds began to be fabricated in much the same way that stamping dies for formed-metal process were made. However, as an extension of the casting process, this process became known as "die-casting" which today includes high-pressure techniques similar to injection molding. Usually non-ferrous, lower melting point metals are produced with die-casting, including aluminum, zinc, magnesium, and even some brasses, including alloys of the same materials (aluminum-zinc and magnesium-aluminum alloys are quite common). One problem with die-casting is that the speed in which the material is injected and then cooled cause voids in the material known as porosity, sometimes due to gases being present but more commonly due to shrinkage, as the cooling metal contracts. Most diecasting machines inject the metal horizontally into the molds/dies since it is less costly to build, install, and operate horizontal machines, however vertical die-casting is gaining popularity, due primarily to inherently lower porosity.

Another casting processes is "slush-casting", where metal is poured into a mold, allowed to cool slightly, hardening on the outside of the object only, forming a shell. Then the molds are inverted to pour out the "slush" or not yet-hardened inner material. This is used to create hollow objects where the inside finish is no concern and where it is desirable to reduce weight or save material.

One process similar to slush-casting is called rotational molding. Like slush-casting, it is used to produce hollow objects, usually from thermoplastics. The raw material resin is usually in powdered form or in very small beads. These molds are permanent and reusable "Clamshells", which form the exterior. A measured amount of resin is placed inside the pre-heated molds. The molds are mounted so that they turn slowly on two axes, distrubuting the resin evenly over all the surfaces of the mold while it bakes in an oven. The heated metal melts the resin that touches it forming a skin inside the mold. As heat is transferred through the skin to the remaining resin, more of it sticks to the skin, and thickens it. Eventually, when this skin is thick enough, all or most of the resin is consumed and the mold is allowed to cool. Finding out how much resin to use is often a trial and error process, as some sections build up quicker and thicker than others.

For more information on other casting and molding processes get a good Manufacturing Engineering textbook and that will cover almost all the basic processes.