Riser Design | Manufacturing Technology

by / Thursday, 15 June 2017 / Published in Blog


Riser Design

1. When a molten metal is paused into the cavity its temperature then is , much higher than its melting point.

2. Say, if its melting point is 500°C , then its pausing temperature would be around 700°C.

3. This difference between pausing and melting point is known as “Degree of superheat”.

4. This ensures the fidelity in the metal.

5. The molten metals while cooling undergo various changes. Its volume reduces due to densification as it solidifier.

6. The molten metal firstly cooler down to its freezing temperature in liquid phase only by loosing sensible heat to the atmosphere around it.

7. Then a phase changes occur as it further looser latent heat .In both the above mentioned step contraction happens that is known as “Liquid Contraction”.

8. Further the metal now in solid state cools down to room temperature. During this provers the contraction happens which is known as “Solid Contraction”.

9.“Liquid Contractions” are compensated by using “Risers” whereas “Solid Contractions” are handled using “allowances”.

10. Riser supply hot molten metal when the casting solidifier. That means, it has to contain metal in liquid phase even when the casting has started to solidify, in the same mould.

11. This could be achieved by designing the riser, such that it has the least surface area to exchange heat with the atmosphere.

12. One such shape is a sphere. Sphere has the least surface area for a given volume. But

13. the problem with spherical risers is that when the molten metal solidifier it starts from the surface and thus blocks the outlet for molten metal.

14. The next best is cylinder. Now there are 2 ways in which are can place a cylindrical riser.

1. Top Cylindrical riser
2. Side Cylindrical riser

 

  • Top cylindrical Riser

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Thus, in order to minimize the surface area diameter should be twice of the height.

 

  • Side cylindrical Riser

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So, in order to minimize the surface area diameter should be equal to its height. There are 4 conditions that are considered before designing a riser.

Condition-1 (Necessary condition)

Vr≥3Vsc

The volume of the riser should be thrice the shrinkage volume. Shrinkage volume is the volume reduction occurring during the liquid contraction.

Condition-3

Location of the riser should be in such a manner that it should be able to supply molten easily to the part where it is needed the most.

Condition-4

Shape of the riser, it should be selected for least surface area for a given volume.

 

Method of Riser Design

  1. Caine’s Method
  2. Modulus Method

 

  • Caine’s Method

 

It is used to check the adequacy of the riser size for a steel casting. It is based on assumption that the cooling rate is linearly proportional to the ratio of the surface area/volume.

A typical risering curve is shown in figure. The ordinate of a point on the curve shows the volume ratio abscissa the freezing ratio. For a given casting-riser combination if the point falls to the right of the curve, the adequacy of the riser is ensured. The equation for a risering curve is of the form.

x=ay-b+c

Where, a is freezing constant

b is contraction ratio from liquid to solid

and c is the constant depending upon different media around the riser and casting. The value of ‘c’ is unity if the mould material around the casting and riser is same.

Condition-1 (Necessary condition)

Vr≥3Vsc

The volume of the riser should be thrice the shrinkage volume. Shrinkage volume is the volume reduction occurring during the liquid contraction.

 

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  • Modulus Method

 

It is based on Chvorinov’s Rule, which is

τ=Cm(VA)2

It says that the solidification time for a casting will be higher for modulus with a higher value. Modulus is the ratio between volume to surface area.

M=VolumeSurface area

If (M)riser>(M)casting

Then (τ)riser>(τ)casting

From, the experiment it has been found that if (M)riser=1.2(M)casting, then the riser can be used very conveniently.

 

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