Sand Casting Process & Design Consideration for Manufacturing of Sand Casting -

Design Consideration for Manufacturing of Sand Casting –

(A) Sand Casting Process –

                The most general casting process uses sand casting for the moulding & core making.

The casting process is quite complex and consists of following sub-systems.

Sand Casting Process

Now they are discussed in details as follows,

(1) Sand and sand preparation –

Natural sand (silica sand) is made available from sand mines.

Particle size – 1 to 1.5 mm

Silica sand is very hard, thermally stable and hence reusable. Such sand used in large quantam in tonnes.

Before using, the sand is cleaned, sieved for proper praportion for its grain size mix.

Grain size – 0.5 to 1.5 mm in appropriate praportion. Such sand mixed with some binders and then used for preparting moulds and cores.

Moulding Sand mixer consists of following ,

Properly cleaned and sieved silica sand, Natural clay, small amount of water, saw dust, dextrin, bentonite, graphite powder, coal dust etc

The combonation is mixed in huge size sand mixers having capacity in tonnes. This mixed appears black in color and carries good strength after compacting and even a good collapsibility.

Que – why silica sand can be reusable ?

Ans – The sand mix of silica sand can be reused but only after adding up for binders which are consumed due to the contact of the hot liquid metal.

Core Sand mixer consists of some oil like linseed oil, core oil, kerosene as binders.

The core required to be stronger and harder than the moulds. The cores are baked in oven prior to their use. Because of this, core makes more stronger and harder. Moulds are not baked.

Core sand mix cannot be reused.

Que – Why core sand need to baked before use ?

Ans – This requirement for the cores is due to relative poorer supporting system for the cores when they are at use as compared to the supporting system for moulds.

(2) Mould making and pattern system –

Mould are the cavities formed out of the moulding sand which corresponds to the outer shape of the component.This mould cavity obtained by using pattern.

Generally the pattern are divided into two parts called “Cope” and “Drag” Pattern.

By using Cope(upper mould) and Drag(Lower mould) can make outer shape of the component. The patterns are mounted on flat base plate called Pattern Plates.

Pattern plate typically for the cope pattern carries following features,

1) Sprue Pin

2) Runner Bars

3) Gates

4) Flow Off

5) Core Prints to form cavity of the component.

The drag pattern may have only core prints apart from the corresponding component shape.

 

Cope Pattern Arrangement 

The squeezing, compacting, releasing operations are performed by machine using pneumatic or hydraulic arrangements.

(3) Core Making and Core Box System –

Basically the core are used in casting to obtained desired hollowness at the components. Cores are made out of the core boxes which carry cavities to be filled by the core sand mixture.After core taken out of the core box are baked in oven to achieve mechanical properties. Baked core placed in the mould cavities at core print locations. Technically cores are smaller than the corresponding mould cavity dimensions. The gap is generated because of the size difference between core and mould size, result into wall thickness of hollow cast component. 

(4) Mould and Core Assembly System –

Mould and Core Assembly System

The cope and drag moulds produced parallely. The drag mould kept inverted on floor then the core is carefully placed at the core prints in the drag mould, now cope box is closed over the drag box. To avoid dislocation pins are used to maintained accuracy while closing. A pouring basin cavity is manually cut at the end of the sprue prior to closing the core box. Also prior to closing the cope moulds need to create narrow air vents manually at the cope mould using sharp piercing pin.

It is important that the core need to be stronger than the mould. To prevent the likely dislocation cope mould over the drag mould at the subsequent operation, both need to be calmped at the suitable location.

(5) Mould and Core Assembly System –

The raw metal is in the form of the pig iron ingots is melted in furnace. But now a days the electrical induction furnace is more in use because of its operational convenience, efficiency and output quality.

The main input is the pig iron then to improves its metallurgical properties of the liquid metal some additives like steel, ferrosilicon, ferromanganese add prior to melting. Liquid metal temperature is around 1550 to 1600 degree Celsius and density 7.5 gms/cm3.

We need to skimmed away the floating impurities manually. Now the liquid metal is slowly poured in the pouring basin at the cope box. The molten metal flow through the sprue, runners, gates and lands into the cavity which is formed by the mould and core. The mould and core are heated and release water vapour, brunt gases and smoke immediately. If the vapours, gases and smoke is available, it will be out of the air vents and flow off. The level of the liquid metal gets raised due the continuous pouring and cavity gets filled completely then the extra liquid metal comes out through the flow off. Now the poring stopped, because the cavity filled completely.

(6) Solidification, Cooling and Fettling System –

The mould is still in closed and clamped condition, do not disassembled till the liquid metal gets solidified and cooled. Its should be kept around 2 to 4 Hours. Once the metal solidified the cope box is removed, the solidified casting is removed along with the lump of mould sand and passed on to the knock-off operation, where the adjoining mould sand is knocked down. This sand is reusable, so send for recycling. The solidified casting contains solidified metal at the sprue, runner, gates, flow off etc.

The operation of removal of that extra material is called 'Fettling Operation'. If required, the casting sent for the sand blasting for its proper cleaning. Then the casting passed to the  machining operation.

• sand casting defects and common problem - read more...