Draft Allowance In Casting
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| Draft Allowance Diagram |
Draft Allowance or Taper Allowance
What is a draft allowance or taper allowance in casting and how is it provided on the pattern?Draft or taper allowance is a taper provided on all the vertical sides of the pattern so that pattern can be removed from the sand smoothly without tearing or damaging the sides of the mould cavity.
The draft allowance is also called as the taper allowance or tapering allowance in casting.
The unit of taper allowance is `\frac{mm}{m}` on a side or expressed in degree.
This allowance is always given to the sections of the pattern that are perpendicular to the parting line and parallel to the vertical side of the mould.
Draft allowance is provided for both internal and external surfaces. A smaller vertical surface of a pattern needs more draft angle than a large vertical surface.
This allowance is one of the allowances among the 6 types of allowance in casting.
As shown below in diagram (a) an ideal pattern without a draft or taper angle. This type of pattern design is prone to damage the mould cavity while removing the pattern from the mould as a pattern and moulding sand are stuck together during ramming because of the adhesive property of the sand.
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| (a) Pattern Without Draft Allowance |
When the pattern is removed or withdrawn from the mould vertical faces of the mould get damaged and tear the mould cavity as shown below in diagram (b). This will give an uneven surface to the final sand casting.
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| (b) Mould Damage By Pattern Without Draft Allowance |
In order to solve this problem taper is provided to reduce damage to the mould and produce accurate final casting in the sand casting process.
By providing an extra taper angle on the pattern, the pattern can be withdrawn from the sand mould as compared to a pattern with no taper.
The draft is a small tapered angle given to the pattern called draft angle.
Pattern with a draft angle will be tapered towards the bottom from the top. The top part of the pattern can be seen as larger as compared to the smaller bottom part as shown in diagram (c).
After providing taper to the pattern, the pattern and casting size will be larger than the original casting size.
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| (c) Positive Draft Allowance In Casting |
After allowance is given to the pattern we can see a larger mould cavity where molten metal will be poured to produce the sand casting in diagram (d).
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| (d) Mould Cavity Made By Pattern With Draft Allowance |
Shown below in diagram (e) is the difference between casting without draft and with draft angles. Casting with the draft is larger in size which is why this allowance is called positive allowance as extra metal is added to the casting than actual casting.
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| (e) Original Casting Vs Casting Produced From Draft Allowance Pattern |
Draft Angle or Taper Angle
The draft angle is different for external and internal surfaces. The inner surface requires more draft allowance details than the outer surface.
Under normal conditions draft allowances are 10-20 `\frac{mm}{m}` on the exterior side of the surface and 40 to 60 `\frac{mm}{m}` on the interior side of the surface.
Tapper allowance is generally 1-2mm/m
The most common draft angle given to the pattern is a 1° to 3° angle on the outer surface and the inner surface is given a 5° to 8° angle.
The allowance table for external and internal surfaces for wood and metal at a particular height is given below for wood, metal and plastic.
As seen below in the chart the draft angle for the metal wood is the highest as compared to metal and plastic. This is due to the fact that moulding sand tends to cling to the wood pattern because of the adhesiveness property of the sand damaging the green sand mould.
The plastic pattern has a smooth surface and tends to have an easy withdrawal of the pattern from the mould with less damage to the vertical faces of the mould cavity.
Metal pattern requires less draft than wooden pattern as moulding sand sticks less to metal pattern.
What is the draft angle on a pattern?
Draft angle is a taper given on the vertical sides of the pattern for easy withdrawal of the pattern from the sand mould to compensate for the damage or tearing to the mould cavity while pattern withdrawal.
Shows below chart of draft or taper angles for various materials used in the sand casting process.
| Pattern material | Height of the surface in mm | Draft angle (Internal surface) | Draft angle (External surface) |
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| Wood | 25 | 3 | 3 |
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25 to 50 50 to 100 |
2.50 1.50 |
1.50 1.00 |
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| Metal and Plastic | 25 | 3 | 1.50 |
| 25 to 50 | 1 | 2 |
As seen above the height of the pattern less, less is the draft angle.
Also, the draft angle is different for different materials. Wood is given a more draft angle than plastic.
It is also observed that the draft or taper angle is larger for the internal surface while the draft angle is smaller for the external surface.
Draft allowance between 3° for 25mm, 2.50° for 25mm to 50mm and 1.50° for 50mm to 100mm. For metal and plastic internal surface draft angle starts from 3° for 25mm, 1° for 25mm to 50mm of vertical height section.
Is draft allowance positive or negative allowance?
Draft allowance or taper allowance is a positive allowance given to the pattern for easy withdrawal of the pattern from the mould.
It is called positive allowance because extra metal is added to the final casting as compared to the orginal casting by making the pattern tapered.
How is the draft allowance given to circular sections?
Draft allowance is not given to the circular areas because they can be easily withdrawn from the sand mould without damaging the mould cavity. Allowance in the pattern is only given to vertical straight sections that are perpendicular to the parting line.
Factors To Consider While Selecting Taper Allowance
Draft allowance in a pattern depends on the following factors:
- The vertical height of the patterns. (For a pattern with a larger height requirement of allowance is more)
- Size of the pattern. (Smaller size patterns do not require allowance)
- The shape of the pattern. (Countered shape pattern does not require allowance for example sphere)
- Ramming of the moulding sand. (Hard rammed sand moulds required more taper allowance compared to loose rammed sand mould)
- Types of sand-casting mould. (Pattern used in green sand moulding requires more taper allowance than dry sand mould)
- Sand moulding method. (In machine moulding, mould mould-making process is standard and is not dependent on the moulder's skill resulting in the proper sand ramming, compactness and layering of sand. Thus, allowance on the pattern is low as compared to hand moulding)
- Pattern material used for making patterns. (Wood pattern material being rough tends to cling to green moulding sand as compared to the plastic pattern being rough and smoothly withdrawn from the mould without damaging the mould)
- Quality of the parting sand. (Parting sand is applied on the pattern to avoid the pattern from sticking to the mould, using high-quality parting sand reduces taper on the pattern)
- Adhesiveness property of the moulding sand. (More adhesiveness of the moulding sand will be the taper on the pattern)
- Interior surfaces require more taper as compared to exterior surfaces in the green sand moulding process.
Advantage of Taper Allowance In Casting
- Allowance permits easy withdrawal of the pattern from the sand mould.
- Less taper can be incorporated by replacing plastic or metal patterns as compared to wooden patterns. Plastic pattern and metal pattern surfaces adhere less to the green moulding sand.
- Eliminates damage or tearing of the vertical side of the mould during pattern drawing.
- Sound casting with better dimensional accuracy can be produced by providing a taper on the pattern.
- Casting with a smooth surface finish can be manufactured in green sand moulding properly as green sand tends to cling to the pattern easily damaging the mould cavity.
- The pattern used in skin dry moulds and dry sand moulds requires less allowance as compared to green sand moulds.