What is Draft Angle?
Draft angle in sand casting is considered as the taper of the vertical side of the model. (That is, the side that is perpendicular to the top surface of the mold.) If the sides of the model are perfectly vertical, the model will rub against the sand when it is removed. Inevitably, this causes some sand particles to fall out.
To prevent this, the vertical sides are slightly sloped or tapered. Now, when the model is lifted vertically, the sides break away from the sand. There is no friction, so the sand is not damaged.
In sand casting, this taper is called positive draft. It is positive because the top of the model hangs over the sand. If the draft angle is negative, the sand at the top of the model hangs over the lower area of the model. In this case, the model cannot be lifted without destroying some of the sand.
Draft angle is required for sand cast parts, mainly for the following purposes:
Easy to remove the mold: In sand casting, the model needs to be removed from the sand mold. The draft angle can make it easier to pull the model out of the sand mold when demolding, reduce the friction and adhesion between the model and the sand mold, and avoid damaging the sand mold. If there is no draft angle, the friction between the model surface and the sand mold cavity wall is large, which may cause the sand mold to partially fall off and deform during demolding, affecting the dimensional accuracy and surface quality of the casting.
Ensure the dimensional accuracy of the casting: The appropriate draft angle can ensure the smooth removal of the model, thereby ensuring the integrity and accuracy of the sand mold cavity. In this way, after pouring the molten metal, the casting can be formed according to the designed size and shape, reducing the dimensional deviation of the casting caused by damage or deformation of the sand mold.
Improve the surface quality of the casting: Since the draft angle helps the model to be smoothly pulled out of the sand mold without scratching or damaging the inner wall of the sand mold, it can make the surface of the casting smoother, reduce surface defects such as sand grain adhesion and unevenness, and improve the appearance quality and surface finish of the casting.
Why draft angles are necessary in sand casting and how to avoid them.
Patterns and Sand Casting
Sand casting involves pouring molten metal into a cavity or void created in the sand. Then, when the metal solidifies, the sand is shaken out to reveal the casting.
That cavity or void is key to getting a good part. It is formed by a pattern, which is a physical representation of the shape or part you want to cast. Traditionally, patterns are made of steel or aluminum, but other materials such as wood are sometimes used.
The pattern is placed in a sand box, and sand is filled or pushed into the void around the pattern by gravity or pressure until the sand is dense. The pattern is then removed, and the void remaining in the sand is the shape you want to cast.
A draft is designed into the pattern. Without a draft, the friction between the pattern and the sand mold increases when the pattern is removed. The pattern will pull any sand as it is removed, thus expanding the void. The final cast part will have excess metal where the sand came out. This results in a poor quality casting and waste of metal. The draft angle can avoid this problem.
How big of a draft angle is enough?
Sand casters want the draft angle to be as large as possible because it reduces the risk of producing defective castings. Most part designers want the draft angle to be as small as possible because it complicates their design. It also makes fixtures and processing more difficult.
The draft angle of sand casting parts is usually between 1° - 5°, but in actual applications it will vary depending on factors such as the material, structure, size and molding method of the casting. Here are some common situations:
Material
Gray iron castings: Due to its good fluidity, the requirements for draft angle are relatively low. Generally, the draft angle of the outer surface can be 1° - 3°, and the inner surface can be around 2° - 4° due to the difficulty of demolding.
Steel castings: The shrinkage rate of cast steel is large. In order to prevent defects such as cracks in the casting during cooling and facilitate demolding, the draft angle is usually slightly larger than that of gray iron castings. The draft angle of the outer surface is generally 1.5° - 3.5°, and the inner surface is 2.5° - 4.5°.
Aluminum alloy castings: Aluminum alloys have low density, good fluidity, and high surface quality requirements. The draft angle is generally 1° - 2.5° on the outer surface and 1.5° - 3° on the inner surface.
Structure and size
For castings with simple shapes and small heights: the draft angle can be relatively small. For example, for simple castings with a height of less than 50mm, the outer surface slope can be 1° - 1.5°, and the inner surface can be 1.5° - 2°.
For castings with complex shapes and large heights: In order to ensure smooth demolding, a larger draft angle is required. For complex castings with a height of more than 200mm, the outer surface slope may need to reach 3° - 5°, and the inner surface may be 4° - 5°.
Modeling methods
Hand molding: Due to the flexibility of manual operation, the draft angle can be relatively small, generally between 1° - 3°.
Machine molding: In order to adapt to the working characteristics of the machine and improve production efficiency, the draft angle is usually slightly larger, generally between 2° - 5°.
Usually shallower parts require more draft angle, while very deep parts require less draft angle. Advanced molding equipment can greatly reduce the size of the draft angle. Generally speaking, wooden models require a larger draft angle than metal models.
In actual production, it is necessary to determine the most appropriate draft angle through trials and experience according to the specific casting requirements and production conditions to ensure the quality and production efficiency of the casting.
How to avoid draft angle
The key is to let the part geometry provide the draft angle. For example, imagine casting a shaft. The cylindrical model is laid flat and pressed into the sand. (Or the sand is wrapped around it.) When the model is lifted, the fillet surface separates from the sand: there is no friction, so there is no damage. The part geometry eliminates any need for a draft angle.
You can use the same technique to cast a square bar. Flatten it in the sand and the sides will need a draft angle to release smoothly. But turn the model 45⁰ so that the split line goes diagonally through the bar and the model automatically has a 45⁰ draft angle.
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