Riser In Casting

Riser

Riser In Casting Process

What is the riser in the casting process?

A riser in the casting process is utilized to compensate for the liquid shrinkage and solidification shrinkage during the solidification process.

The riser promotes directional solidification, provides a thermal gradient and reduces shrinkage defects in the final casting during the sand casting process.

The riser or feed head is a reservoir of molten metal that feeds the casting until it solidifies as shown below in diagram (a).

(a) Riser In Casting

Riser supplies molten metal to heavy/thicker sections of casting in the mould cavity from the liquid stage to the solid state during the solidification process.

The riser keeps feeding molten metal to the hot spots in solidifying casting. Hot spots are spots which are more prone to voids.

Molten metal in the mould cavity goes through volumetric shrinkage, risers are provided to compensate for this shrinkage reducing voids and increasing the production of sound casting.

When more than one riser is used casting is divided separately and riser design and calculation are done separately for each part.

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Three Stages Of Shrinkage During Solidification Process

What are the three stages of shrinkage during the solidification process?

Three stages of shrinkage during the solidification process are as follows:

Liquid shrinkage.
Solidification shrinkage.
Solid shrinkage.

Liquid shrinkage: Liquis shrinkage takes place at the temperature of molten metal to the temperature at which metal starts solidifying. The riser is provided for liquid shrinkage.

Solidification shrinkage: Solidification shrinkage takes place when a metal loses its latent heat and metal changes from a liquid state to a solid state. A riser is provided for solidification shrinkage.

Solid shrinkage: Solid shrinkage takes place when metal goes from a solid state to room temperature. In order to compensate for this shrinkage allowance or pattern maker's contraction allowance is given in the casting process.

Directional Solidification In Casting Process

When molten metal is poured into the mould cavity, the molten metal starts cooling around the mould wall forming a shell.

Molten metal in the centre is in liquid form while at the mould wall, it is in solid form. The molten metal in the middle is in a "mushy state' or 'semi-solid state'.

Solidification that takes place from the mould wall towards the centre of the mould uniformly is called lateral solidification or progressive solidification.

Solidification that takes place uniformly reducing voids, cracks and porosity is called directional solidification.

For directional solidification to take place molten metal should solidify from a thinner section to a thicker section uniformly without missing any spot or section in the casting process eliminating shrinkage and voids.

Shown below in diagram (b) voids in casting formed due to non-uniform solidification of casting in the mould cavity.

(b) Void

Different materials have different shrinkage and depending upon the material riser type, size and shape are chosen.

The design, position, and location of the riser depend upon the material of casting, casting type, temperature gradient, complexity of casting and directional solidification rate of casting.

More than one riser can be used in the casting process for large-size casting to reduce shrinkage.

A top riser is also called an open riser, cold riser or dead riser in the casting process because they cool faster than the final casting.

The riser located between the runner and casting is called a live riser or top riser.

If deeper moulds are used in the casting process then the riser is called the end riser as the riser is till the bottom end.

When molten metal reaches the mould cavity riser allows the escape of steam, air and gases before getting filled with molten metal.

Risers freeze at last in the casting process and are cut, separated from the final casting.

The riser in casting is a part of the gating system in casting.

Requirement Of Riser In Casting Process

To maintain temperature gradient in casting to promote directional solidification of casting.
To reduce shrinkage, hotspots and voids in the casting.
Feed molten metal to the mould cavity and keep the mould filled to produce the casting.
To maintain the fluidity of molten metal so the metal can feed the contacting cavity.
To remove gases produced when molten metal comes in contact with mould cavity.
Feed the isolated, remote and thicker section of casting.
To promote uniform solidification in the casting.
Should be designed to have the highest casting modulus.

Riser Feeding Range, Shape And Size

The riser in casting should provide a maximum volume-to-surface area ratio.

This volume-to-surface area ratio is highest for the sphere which is an ideal shape for a riser.

Even after the sphere provides a better volume-to-surface area ratio it is not used as it becomes difficult to make in mould.

The closest shape to the sphere having an appropriate volume-to-surface area ratio is a cylinder.

Height to diameter ratio of the cylinder riser is 1:1 to 1.5:1 in general.

Feeding range in casting is the distance from where molten metal is fed into the casting.

Feed range for plate casting is 4.5 times the thickness of the plate (casting thickness).

Feed range for bar casting is 2 to 2.5 times the bar thickness (casting thickness).

The riser kept in the centre of the casting has a better feeding range than the riser placed at the end of the casting.

For steel casting riser height of 4.5 times the thickness of casting is best suited.

Riser Sleeve

In order to reduce the heat transfer riser sleeves are employed for insulation throughout the wall of the riser improving riser feed efficiency.

Riser feeds are made of sawdust and fire clay and are fired to turn sawdust. Sawdust and fire clay are used for ferrous metal for non-ferrous metal plaster of paris is used.

After using the riser sleeve solidification time of molten metal in the riser increases.

Molten metal is fed to the casting during the solidification process of the riser needs to be maintained by reducing the modulus of the riser.

The modulus of the riser is given by the following formula:

`M_{r}=\frac{1.2}{f}M_{c}`

Exothermic materials or insulators are used at the top of the riser to avoid the solidification of molten metal.

Exothermic material when comes in contact with heat generates heat heating all sides of the riser. These materials are best for top/open gates where the heat loss of molten metal is greater as the riser is open to the atmosphere.

The advantage of using exothermic material is that the size of the riser can now be reduced as there is no need to have excess molten metal in the riser to maintain a temperature gradient.

Exothermic materials used for risers are as follows:

Thermit mixture.
Charcoal powder.
Wood/sawdust.
Rice hulls.
Graphite powder.

Chills in casting process are another directional solidification technique used between risers to achieve better temperature gradients and uniform solidification.

Riser comes with a cost of making the riser, removing it and resulting in low casting yield which is replaced by using padding in casting process.

Riser Requisites In Casting Process

The riser used in the casting process should have the following requisites:

Molten metal in the riser should solidify last.
Molten metal in the riser should feed the mould cavity slowly and steadily.
The volume of molten metal in the riser should be sufficient to compensate for the liquid shrinkage.
The riser should have an appropriate metastatic head to maintain steady positive pressure during the solidification process.
Proper directional solidification should be achieved with a riser in the foundry.

Riser Consideration In Casting Process

Hot spots and shrinkage are to be handled during the casting process.
Easy removal of the riser from the mould after solidification of casting.
Number of risers and feeding distance between casting and riser.
Size, shape and location of the riser.
Solidification characteristics of the metal.

Types Of Riser In Casting

How many types of riser are there in casting process?

There are two types of risers in the casting process namely top riser and blind riser and they are explained in detail below.

Top Riser or Open Riser
Blind Riser or Internal Riser

Top Riser or Open Riser In Casting

The top riser is at the topmost position and is open to the atmosphere during the casting process as shown above in diagram (c).

The top riser is the most conventional type of riser advised for light metals.

The biggest disadvantage of the top riser is that it loses heat due to radiation and convection.

This can cause molten metal to solidify earlier than the molten metal in the mould cavity.

To reduce heat loss riser needs to be provided insulation of plaster of paris.

To overcome this disadvantage blind riser is made which has slow heat dissipation throughout the casting process.

It is observed that the top riser is more efficient than the side riser in the casting process.

In order to maintain the temperature of molten metal in the top riser following arrangements are made in the casting process.

Heat the riser externally to maintain the proper temperature gradient of molten metal in the riser.
Pour molten metal into the riser at last after the mould cavity is filled.
Use extotehrmic material in the riser. When molten metal comes in contact with exothermic material, exothermic material releases heat keeping the top riser hot.
Insulation of riser with padding.
Radiation shield to be used on top of the riser.

Advantage Of Top Riser In Casting

Easy to mould and most common riser.
No vacuum is created in casting.
Slag, dross, impurities and inclusions are floated on the top of the riser.
Top risers are more efficient than blind risers.
Riser can be easily fettled from the final casting.
Gases in a molten metal can exhaust easily.
Hot spots are easy to excess from the top.

Disadvantage Of Top Riser In Casting

Heat loss as molten metal in the riser is exposed to the atmosphere.
Chances of dense foreign metal entering into the mould cavity as the top part of the riser is exposed to the atmosphere.
When top risers are used in the casting process, the casting yield is reduced.
The position of the riser is only in the cope part of the mould.
Padding is required in the riser neck as the riser neck tends to solidify first.

Blind Riser or Internal Riser

(d) Blind Riser

The top riser or open riser is exposed to the atmosphere while the internal riser is exactly the opposite of the top riser.

This riser is best to eliminate hot spots below the parting line for complex casting sections with different heights.

An internal or blind riser is made enclosed internally in the mould as shown below in diagram (d).

A blind riser is most preferred for casting which is of cylindrical shape and hollow in nature.

Blind riser has slow heat dissipation and less heat loss as it is an internal part of the mould.

The heat from the mould maintains the temperature gradient of the riser and the molten metal in it. The mould should be properly vented to remove gases from the mould.

Advantages of Blind or Internal Riser

The molten metal cools slowly as compared to the top or open riser.
The proper thermal gradient is maintained in the riser as the riser is hidden in the mould.
The blind riser does not require the use of chills and padding to extend the cooling of metal in the riser.
The riser can be made smaller for casting with the same size as compared to the top riser.
Hot spots below the parting line can be easily eliminated.
Best for complex castings and products such as valves and couplings.
The size and height required for a blind riser are small as compared to a blind riser. This also reduces the time for making riser elements.
The riser neck does not solidify and padding near the riser neck is not required as the required thermal gradient is maintained in the blind riser.

Disadvantages of Blind or Internal Riser

Not visible to the operator as the riser is enclosed in the mould.
Slag, dross, oxides, impurities and dirt cannot be removed from the riser as the riser is enclosed.
The blind riser is difficult to mould as compared to the top riser.
Mould gases developed in the mould cannot be exhausted from the mould. These gases are developed when the molten metal comes in contact with the moulding sand and moisture during the casting process.
Proper ventilation of the mould should be provided if the blind riser is used for gases to exhaust.
The felting cost to separate the riser from casting is high.

Riser In Casting Design (Riser Size)

Naval Research Laboratory Method

The naval research method becomes important when calculating volume to the surface area becomes difficult.

The naval research laboratory method is the simplest and easiest way to calculate riser size.

Therefore riser is designed by calculating the shape factor, with only the length, width and thickness of the casting.

The shape factor formula is as follows:

Shape Factor = Length+ Width/Thickness

For circular sections, the diameter should be considered as length, width and thickness.

For height to diameter (h/d) value should be 1 and for top riser height to diameter ratio (h/d) should be 0.5.

Modules Method

Wlodawer documented the modulus method for calculating riser size.

A riser with a height-to-diameter (h/d) ratio of 1 is chosen.

It is proved that if the modulus or riser `M_{c}` is 1.2 times that of the modulus of casting `M_{c}` it is seen that solidification will be satisfactory.

Modulus of casting `M_{c}` can be calculated by following formula:

`M_{c}`= `\frac{aH}{2(a+H)-c}`

a = Thickness
b = Width

Modulus of riser `M_{r}` can be calculated by following formula:

`M_{r}` =0.2D

`M_{r}`=1.2`M_{c}`

D=6 `M_{c}`

D= Diameter of the cylinder.

Modulus Of Casting `M_{c}` For Various Shapes

Modulus of casting `M_{c}` for Plate = 0.5t
Modulus of casting `M_{c}` for Long bar = `\frac{ab}{2(a+b)}`
Modulus of casting `M_{c}` for Cube = D/6
Modulus of casting `M_{c}` for Sphere = D/6
Modulus of casting `M_{c}` for Hollow cylinder = `\frac{rH}{2(r+H)}`

Canine Method

The size of the riser can be determined by the relation between the volume of casting, riser and relative freezing time of casting.

The freezing ratio or freezing time formula is as follows:

`X=\frac{(\frac{SA}{V})_{C}}{(\frac{SA}{V})_{R}}`

`Y=\frac{Volume of R}{Volume of C}`

R=Riser.

C=Casting.

The canine formula equation is as follows:

`X=\frac{a}{Y-b}+c`

a = Freezing characteristics constant.
b = Liquid solid solidification constant.
c = Relative freezing rate of riser and casting.

The value of a,b and c can be taken from below according to metal.

Aluminimum a = 0.12, b = 0.05 and c = 1.00.
Aluminimum Bronze a = 0.25, b = 0.0017 and c = 1.00.
Steel a = 0.12, b = 0.05 and c = 1.00.
Gray Cast Iron a = 0.33, b = 0.03 and c = 1.00.

The canine hyperbolic curve is used to determine if casting produced by using the above formula is going to be sound casting or defective casting.

If the value of X and Y meets above the hyperbolic curve casting is sound without any defects.

If the value of X and Y meets below the hyperbolic curve final casting is with defects.

Chovrinov's Rule

Solidification time or freezing time can be calculated by using chovrinov's rule.

By calculating the ratio of surface area to volume riser characteristics can be calculated.

If the ratio is higher casting cools faster and vice versa.

According to chovrinov's rule solidification time (t) is proportional to the square of the ratio between the volume to the surface area of casting.

Constant (C) used in chovrinov's formula is a constant of proportionality which depends upon the following factors:

Pouring temperature.
Thermal mould charatsrestics.
Casting

Solidification time (t) is calculated by the following formula:

`t=C(\frac{V}{SA})^{2}`

V= Volume of casting.

SA = Surface area of casting.

C= Constant proportionately.