Shrinkage Allowance In Casting
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| Shrinkage Allowance In Casting |
Shrinkage Allowance
Shrinkage allowance can be defined as allowance given to the pattern to compensate for the contraction or shrinkage in the casting during the solidification process.
Casting produced without allowance will be of undersize due to the shrinkage property of the metal during the solidification process.
To solve this problem allowance is provided on the pattern making the pattern larger in size. So, during solidification molten metal contracts and comes to the desired size on the drawing.
Shrinkage allowance is a positive allowance and is added to all linear dimensions to take care of solid shrinkage. This allowance is measured in percentage (%) and liner dimensions mm/m.
Larger casting requires higher allowance as compared to small size casting.
In this allowance, metal is added to the external dimensions and subtracted from internal dimensions when we have holes, recesses and cavities to be made in casting.
Shrinkage allowance is also called contraction allowance and is one of the allowances among the 6 types of pattern allowance in casting.
Allowance given to the cube pattern is shown below in diagram (a) below.
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| (a) Shrinkage Allowance Given To Pattern |
In the sand casting process, molten metal is poured into the mould cavity at the pouring temperature.
The temperature of metal starts dropping from the pouring temperature to the melting temperature. This will cause the metal to shrink and contract. To compensate for the liquid contraction riser is provided to feed liquid molten metal continuously to the solidifying casting.
After some time, the molten metal solidifies below the melting temperature towards room temperature. During this, the metal starts contacting or shrinking in linear dimensions.
A shrinkage allowance is provided to compensate for this contraction during the cooling or solidification process is called shrinkage allowance.
This allowance is given for solid contraction only, for liquid shrinkage and solidification shrinkage riser is provided during the solidification process.
It is important to note that contraction or shrinkage is not uniformly distributed in all sections of the casting. This causes warping defects, hot tear defects and internal stress-causing hotspot defects.
Solidification of molten metal in the mould starts from the wall of the mould moving inwards towards the centre of the mould cavity. A riser is provided to take care of liquid shrinkage and solidification shrinkage of the casting and feed molten metal to the isolated spots.
As shown below in diagram (b) shrinkage of casting with and without shrinkage/contraction allowance.
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| (b) Sand Casting With & Without Shrinkage Allowance |
Is shrinkage allowance positive or negative allowance?
Shrinkage allowance is a positive allowance as extra dimensions are added to the pattern.
"Shrinkage scale" is used to take dimensions of patterns during the pattern-making process.
This ruler is longer than regular scale, made according to the shrinkage of different metals and is called a pattern maker's ruler.
If the pattern is made from metal do we give a double shrinkage allowance?
Yes, a double shrinkage allowance is given to the pattern if we use metal as a patterned material.
What is the main difference between shrinkage allowance for metal patterns and wooden patterns?
| Metal Pattern Shrinkage Allowance | Wooden Pattern Shrinkage Allowance |
|---|---|
| Double shrinkage allowance is given to metal patterns. | The wooden pattern does not require a double shrinkage allowance. |
What is a unit of shrinkage allowance?
Shrinkage allowance unit is in `\frac{mm}{m}` and is always given in the linear direction.
Shrinkage allowance is also called contraction allowance.
Shrinkage Types
The solidification process takes place in three phases, and they are as follows:
- Liquid shrinkage. (Pouring temperature to liquidus temperature)
- Solidification shrinkage. (Liquid temperature to solidus temperature)
- Solid shrinkage. (Solidus temperature to room temperature)
Liquid Shrinkage
Liquid shrinkage can be defined as a reduction in volume when the molten metal starts cooling but remains in the liquid state. Aluminium has the highest shrinkage.
In order to compensate for liquid shrinkage, the riser is provided for a continuous supply of molten metal during the solidification process.
In liquid shrinkage molten metal drops from pouring temperature to liquidous temperature.
Liquid shrinkage is specified in "% by volume".
Solidification Shrinkage
Solidification shrinkage is where molten metal changes from a liquid state to a solid state.
Solidification takes place when the temperature of molten metal starts going below the melting temperature. Atoms in the metal start getting closer to each other and solidifying.
The solidification shrinkage phase is from liquid temperature to solidus temperature.
In order to compensate this riser is provided and shrinkage is compensated by molten metal in the riser. Solidification shrinkage is a very important phase during the solidification process that need to be assisted with the riser and chill to provide directional solidifications and reduce defect such as void and hotspots in casting.
Solid Contraction or Shrinkage
When metal is in a solid state casting starts losing its temperature and starts solidifying at the ambient temperature.
In order to take care of this shrinkage, a solid shrinkage allowance is given to the pattern. Shrinkage allowance on a pattern is provided to compensate for shrinkage when the temperature of the solid phase drops from freeze temperature to room temperature.
Solid contraction takes place when metal is at room temperature.
Solid shrinkage is not compensated by the molten metal in the riser but by shrinkage allowance or contraction allowance only.
Solid contraction or shrinkage is also called pattern makers allowance as this shrinkage is controlled largely by pattern makers' skills in giving allowance to produce desirable patterns and casting.
Any small changes in the dimensions of the final casting compared to the original casting on the drawing are checked by an inspector under CMM. (Coordinate Measurement Machine)
Shinkage Allowance Chart Of Metals
There are various types of casting metals used for making casting in the casting process. Every casting metal has a different solidification rate. Allowance is given keeping in mind the shrinkage rate of a given metal.
Shrinkage allowance for metal master pattern = Shrinkage allowance for casting metal + allowance for pattern + allowance for pattern material + required size of casting + finishing or machining allowance in casting.
The shrinkage allowance chart for various metals is given in the shrinkage metal chart below.
| Material | Shrinkage allowance in `\frac{mm}{m}` |
|---|---|
| Cast Steel | 21 |
| Aluminium | 13 |
| Grey cast iron | 7 to 10.5 |
| White cast iron | 17 to 21 |
| Copper | 16 |
| Lead | 25 |
| Magnesium | 13 |
| Aluminium bronze | 20 - 23 |
Factors To Be Considered While Providing Shrinkage Allowance
Shrinkage allowance depends on the following factors:
- Graphitisation presence in the metal. When graphitisation is more shrinkage would be less.
- Type of metal used for casting. For example, steel contacts more than aluminium.
- Linear coefficient of thermal expansion of the metal. The higher the value more will be the shrinkage allowance.
- The alloy composition of the metal.
- The complexity of pattern design and final casting.
- The material used for making mould and pattern.
- Solidification rate of molten metal.
- Rate of heat transfer between molten metal and mould.
- Desired shape and size of the casting to be manufactured.
- Shrinkage or contraction rate of the metal.
- Pouring temperature of the molten metal.
- Ramming of the sand mould, hardness and method used for moulding.
- The cooling method used in the casting process such as die casting.
- Padding provided.
- Type of mould used. In the green sand mould, casting will shrink at a faster rate compared to dry sand mould.
- The thickness of casting sections such as ribs, cavities, joints, bosses and inserts.
- Wall thickness for critical dimensions is prone to more contraction.
- Types of casting process used.
- As solid shrinkage takes place in the third phase liquid shrinkage and solidification shrinkage become important. A proper gating system should be designed to avoid defects from passing through the solid contraction.
- Moulding material and core used in sand casting.
Shrinkage allowance for casting with cavities or holes.
Casting design can have holes, recesses and cavities. When casting contracts there are restrictions on contraction because of the sand core. In such a situation, shrinkage allowance should be given on the edges to allow proper contraction of molten metal around the core.
Holes and cavities in casting are seen to be smaller after casting is taken out which will be further machined to make them larger to the required size by machining operations.
Advantages of Shrinkage Allowance
- High-quality casting is manufactured to the desired size.
- Shrinkage of casting is eliminated.
- Reducing rejection of the final casting during quality checks.
- Reducing wastage of molten metal.
- The efficiency of the casting process improves reducing the cost of casting production.
- Reduces machining operation on the casting.
- The dimensional deviation is reduced.
- Shrinkage-related defects are minimized such as voids and hotspots.
- Allowance can be strategically applied in a few sections of the casting only.
- Simulation tools and CAE tools can be used to accurately predict shrinkage and allowance.
- Cost-effective production cycles can be run in the foundry.
Shrinkage Allowance FAQ
Question: Which metal has the highest shrinkage allowance?
Answer: Lead has the highest allowance among steel, brass, aluminium and cast iron.
Question: Which metal has the lowest shrinkage allowance?
Answer: Aluminum metal and grey cast iron have the lowest allowance during shrinkage.
Grey cast iron has a negative allowance during the solidification process as grey cast iron expands..
Question: What happens when allowance is not given to the pattern?
Answer: The final casting will be a smaller size than the expected casting because molten metal goes through thermal contraction and will shrink due to its contraction properties.
Question: Is there any metal that does not shrink?
Answer: Bismuth and gallium are metals that do not shrink because they are formed by covalent bond instead of metallic bond which breaks during the solidification process. The difference between melting point and freezing point is low.
Question: How much does pattern makers' experience count in providing allowances?
Answer: Pattern makers' allowance play an important role in pattern making as pattern maker has a lot of experience from trial and error.
What is the contraction allowance for steel and nickel?
The contraction allowance for steel and nickel is 2-3%.
What is the contraction allowance for copper?
The contraction allowance for copper is 5-6%.
Shrinkage Allowance Numericals
How to calculate shrinkage allowance in casting?
Shrinkage allowance in casting can be calculated by following the steps?
Step-1 Collected data related to the shrinkage allowance of the particular metal.
Step-2 Collect data related to the dimensions of the casting.
Step-3 Apply a percentage of shrinkage allowance on the casting sides.
Step-4 Determine the new positive shrinkage allowance and add it to the dimensions of the final casting.
Step-5 Make the pattern larger in size so when casting shrinks it will be of the desired size. I have explained the numerical shrinkage or contraction allowance below.
Problems/Numericals on Shrinkage Allowance In Casting Process
Problem/Numerical 1) Calculate the shrinkage allowance of the cube made of aluminium with dimensions 100mm ✕ 100mm ✕ 100mm.
Answer: Shrinkage allowance for aluminium is 6% during the solidification process. So if we do not give allowance on the pattern the final casting will be smaller in size having dimensions of the casting as 96mm ✕ 96mm ✕ 96mm which is not desired.
So now it becomes important to add allowance to the pattern to make the casting of the required dimensions of 100mm ✕ 100mm ✕ 100mm.
This is done by adding 6% allowance on each side by making the pattern larger in size. By doing this there is an increase in the pattern dimensions as given below.
Positive allowance increase in the length of the cube = 100 ✕ 6% = 100 ✕ 6/100 = 6mm
Positive allowance increase in the height of the width of the cube = 100 ✕ 6% = 100 ✕ 6/100 = 6mm
Positive allowance increase in the width of the cube = 100 ✕ 6% = 100 ✕ 6/100 = 6mm
So we now will add 6% contraction allowance (6mm) on each side of the pattern, making the pattern larger in size.
So by adding 6% allowance on each size, the pattern will be with the length, width and height of 106mm ✕ 106mm ✕ 106mm.
After this when the pattern will be used for making a mould cavity, the mould cavity made from the larger size will be 106mm ✕ 106mm ✕ 106mm.
Now when molten metal is poured into a mould cavity with the 106mm ✕ 106mm ✕ 106mm dimensions, casting will shrink by 6% (6mm on each side) to the desired dimensions of 100mm ✕ 100mm ✕ 100mm.
Problem/Numerical 2) Calculate the shrinkage allowance of the cube made of cast iron metal with dimensions 100mm ✕ 100mm ✕ 100mm.
Answer: The requirement of the casting process is to make a casting of 100mm ✕ 100mm ✕ 100mm.
The shrinkage allowance for cast iron is 2% during the solidification process.
So if we do not give allowance on the pattern the final casting will be smaller in size having dimensions of the casting as 98mm ✕ 98mm ✕ 98mm which is not desired.
So now it becomes important to add allowance to the pattern to make the casting of the required dimensions of 100mm ✕ 100mm ✕ 100mm.
This is done by adding 2% allowance on each side of the pattern, making the pattern larger in size. By doing this there is an increase in the pattern dimensions as given below.
Positive allowance increase in the length of the cube = 100 ✕ 2% = 100 ✕ 2/100 = 2mm
Positive allowance increase in the height of the width of the cube = 100 ✕ 2% = 100 ✕ 2/100 = 2mm
Positive allowance increase in the width of the cube = 100 ✕ 2% = 100 ✕ 2/100 = 2mm
So we now will add 2% contraction allowance (6mm) for cast iron on each side of the pattern, making the pattern larger in size.
So by adding 2% allowance on each size, the pattern will be with the length, width and height of 102mm ✕ 102mm ✕ 102mm.
After this when the pattern will be used for making a mould cavity, the mould cavity made from the larger size will be 102mm ✕ 102mm ✕ 102mm.
Now when molten metal is poured into a mould cavity with the 102mm ✕ 102mm ✕ 102mm dimensions, casting will shrink by 2% (2mm on each side) to the desired dimensions of 100mm ✕ 100mm ✕ 100mm.