Metal Casting Zone Info Blog

Kent Kelin asked:


Brass casting, as the name suggests, involves the use of brass as the molten metal. Brass casting can be carried out by the way of sand casting only. Sand casting can be defined as a ‘cast part’ produced by formation of a mold from a mixture of sand and pouring the casting liquid (mostly molten metal) into mold. Then the air-cooling of the mold takes place. After the solidification of metal, the removal of mold takes place. The metal used here is brass. It is a known fact that brass is an alloy of copper and zinc. Hence, to be precise, the molten metal consists of two elements.

Sand molding consists of two types- ‘Green sand’ molding and ‘air set’ molding. The first one consists of a blend of moisture, clay, silica sand and other additives. The second one makes use of dry sand bonded to all the above materials except moist clay, by the way of using an adhesive, which is fast curing.

At times, there is a placing of a temporary plug (in the mold cavity) to enable the formation of a channel to pour the fluid which is to be molded. The molds of the second type, i.e. the air-set molds result in the formation of a 2-part mold. The two parts are bottom and top. The tamping-down of the sand mixture takes place as it gets added. Many a times, the final assembly of the mold is vibrated to get the sand compacted and get the unwanted voids filled. Then the molten alloy (brass) gets poured into mold. After the solidification and cooling of brass, the separation of casting from sand mold takes place. Normally, such molds are one-time usable.

Patterns: A designer or an engineer provides the design of the object to be produced. On the basis of this design, a pattern is built by an efficient pattern maker by the use of plastic, metal, or wood. Polystyrene can also be used. The casting brass would get contracted during solidification. Non-uniformity can also result out of this. Therefore, the size of the pattern should be a bit larger as compared to the final product. ‘Contraction Allowance’ is the name given to this difference. Brass enters the mold cavity through a runner system including sprue and other feeders.

Molding box: A molding box having multiple parts (also known as casting flask whose bottom and top halves are called drag and cope respectively) is constructed for receiving the pattern. There may be an addition of sand to nullify the defects introduced due to the pattern getting removed.

Chills: To have a proper control over metallurgical structure and solidification of brass, plates of brass, or any other metal can be placed in mold. A hard structure may get formed at these places. Chills can be used for promoting directional solidification as well.

Design Requirements: The thing in making and the pattern corresponding to it should be designed in such a way that every stage of process can get accommodated. One should be able to take away the pattern without causing any disturbance to molding sand.

Hobby Metal Casting

Kent Kelin asked:


There are various casting processes being implemented now days. The oldest amongst them is sand casting. Spin casting is also widely used. They can be described as follows:

Sand Casting: Sand casting involves formation of mold from a mixture of sand and to pour a casting liquid, most probably, a molten metal into mold. The metal is then allowed to solidify and the removal of mold, takes place. Sand molding consists of two types: green sand method and air set method. The first one consists of mixture of clay, moisture, silica, and many other additives. The second one consists of mixture of dry sand and other materials, not moist clay. They are mixed with the help of a quick curing adhesive. The collective use of these materials is called ‘air set’.

At times, there is temporary plug placed to pour the fluid which is to be molded. Air-set molds usually form molds consisting of two parts-the bottom and the top. The mixture of sand gets tamped down after its addition. It does not generate any by-product. After the solidification and cooling of metal, the mold gets usually destroyed. This is because its removal involves a lot of breaking and cracking. The casting accuracy depends a great deal on the sand and the process of molding used. Castings composed of green sand result into formation of rough texture on casting surface, and this characteristic makes them easily recognizable. Air-set molds produce smoother castings.

Many a times, the casting process results in losing of components of sand mixture. It is possible to reuse green sand by the way of adjusting the composition to get the lost additives and moisture replenished. The entire pattern itself is eligible to be reused for producing novel sand molds. The method of reuse can be continued for an indefinite period. In 1950, casting process got automated partially. They have been in great demand for developing production lines since then.

Spin Casting: Spin Casting is better known as Centrifugal Rubber Mold Casting (CRMC). It implies utilization of centrifugal force for producing castings out of rubber mold. As a customary practice, a mold having shape of a disc gets spun through its ‘central axis’ at a pre-decided speed. The material used for casting is usually thermoset plastic in the liquid form or a molten metal. It gets poured into the mold through the opening at its centre. Corresponding to the solidification of metal, or the setting of thermoset plastic, the spinning of the filled mold takes place.

Normally, organic rubber or vulcanized silicone is used as a mold-making substrate in spin casting. Vulcanization takes place in the middle of process of mold-making. After the successful completion of vulcanization process, venting and gating must be undergone by the mold. This implies carving of channels for ensuring proper material flow and air during the course of casting. A scalpel or knife is used to carry out the above two processes. The mold complexity is directly proportional to the time required in implementation of venting and gating.

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