Pattern Allowances in Casting are very important if it is neglected during the casting process. Then the parts we produce may be smaller than the required size. This can result in huge financial losses. So choosing suitable allowances in casting pattern can greatly improve efficiency and avoid waste. So how to use Pattern Allowances in casting process. What are the forms in Pattern Allowances in Casting?
The usual allowances considered for Pattern and Casting are as follows.
- Shrinkage allowance in casting
- Machining allowance in casting
- Draft allowance in casting
- Knock or shake allowance
- Camber allowance or distortion allowance in casting
Shrinkage allowance in casting
Shrinkage allowance is the reduction in the size of the casting during the cooling or solidification process. It is a characteristic that exists in all materials. Their shrinkage is influenced by the material and shape of the casting as well as the temperature. The size of shrinkage varies from material to material. So we will end up with a smaller casting than the mold.
Shrinkage is divided into solid shrinkage and liquid shrinkage.
Liquid shrinkage: The volume reduction of the metal as it changes from liquid to solid at solid line temperature.
Solution: Set up a riser in the mold.
Solid shrinkage: Volume reduction caused by the loss of temperature of the metal in the solid state.
Solution: Shrinkage allowance in casting is provided in the allowances in casting pattern
Generally, shrinkage exists in all materials unless it is limited in some special way. And the percentage of shrinkage is certain for different materials, temperatures, thicknesses, etc. So an experienced manufacturer can provide pre-shrinkage data before production.
So let’s understand the shrinkage allowance for other casting metals.
| Type of metal | Amount of shrinkage (in %) |
|---|---|
| Grey cast iron | 0.55 % – 1.00 % |
| White cast iron | 2.10% |
| Malleable cast iron | 1.00% |
| Steels | 2.00% |
| Manganese steel | 2.60% |
| Manganese | 1.80% |
| Zinc | 2.60% |
| Brass | 1.30 % – 1.55 % |
| Bronze | 1.05 % – 2.10 % |
| Aluminum | 1.65% |
| Aluminum alloys | 1.30 % – 1.60 % |
| Tin | 2.00% |
Machining allowance in casting
As we all know, the casting process is the cheapest and most direct way to cast metal. Therefore, there are some defects in the common allowances in sand casting process in dimensional accuracy and surface finish. The machining allowance is to solve the surface finish of the casting. The surface of the castings just after production is rough and needs to be manually polished or ground.
So we have to ensure a certain thickness for surface machining, which is the machining allowance in casting. Depending on the size and material of the part, we can set aside a machining allowance of 2 – 15 mm. If the surface is more polished, we should allow more machining allowance accordingly.
| Types of metals and alloys | Machining allowance | |
|---|---|---|
| Cast irons | Large size castings (>1000 mm) | 10.0 mm |
| Medium size castings (< 150 mm) | 3.0 mm | |
| Cast steels | Large size castings (>1000 mm) | 12.0 mm |
| Medium size castings (< 150 mm) | 4.3 mm | |
| Non-ferrous materials | Large size castings (>1000 mm) | 5.0 mm |
| Medium size castings (< 150 mm) | 1.5 mm | |
Draft allowance in casting
The surface parallel to the pattern removal direction is slightly damaged when the pattern is removed from the mold. A tapered somewhat surface is generated on the pattern’s vertical side. These parallel surfaces on the pattern are slightly tapered to compensate for these alterations (close to 1 – 2 degrees). This enables the design to be easily removed from the mold without affecting the casting. Draft margins are variations in the pattern surface that protect it from harm. The draft depth is normally between 5 and 30 mm per meter on the vertical side of the pattern. The draft in precision casting is around 3 to 6 mm per meter.
The amount of draft required for a pattern depends on the following factors.
- Shape of the casting
- Molding material
- Casting depth
- Molding method
Rapping or shaking allowance
These small changes in pattern size during the casting process are called Rapping or shaking allowance. after we finish casting, the pattern is removed from the mold, and there is a slight shaking. This process causes the mold to become more prominent. So we usually make the size of the mold smaller than the actual size of the casting.
Camber allowance or distortion allowance in casting
May deform the metal during cooling due to thinness in some locations. Alternatively, uneven heating during the casting process may cause a gradual deformation or camber during cooling. To avoid such bending or deformation during casting, the curvature is adjusted in the opposite direction, causing the casting to straighten when bending occurs owing to unequal metal thickness. Turning allowance is a change in pattern design that compensates for bending during casting.
These are a few common allowances in the casting pattern. We must take various precautions before casting so as to ensure that the casting meets the requirements at the very beginning.