Thinking About Draft Angles

18 7 月, 2020

Some design engineers will say you always need a draft angle on sand cast parts. The reality is, that’s not strictly correct. If you can orient the pattern to provide natural draft, there’s no need to compromise the design by adding draft angles.

Confused? Here’s an explanation of why draft angles is necessary in sand casting, and how you can avoid them.

Patterns and Sand Casting

Sand casting entails pouring molten metal into a cavity or void formed in sand. Then, when the metal has solidified the sand is shaken out to reveal the cast part.

That cavity or void is the key to getting a good part. It’s formed by a pattern which is a physical representation of the shape or part you want to cast. Traditionally, patterns were made from steel or aluminum, although other materials such as wood are sometimes used.

The pattern is put in a sand box, and the sand is filled or pushed into the void around the pattern by gravity or pressurized force until it is densely packed. Then the pattern is lifted out and the void that remains in the sand is the shape you’re about to cast.

A draft is designed onto the pattern, lacking of which will increase the friction between the pattern and sand mold when the pattern is extracted. The pattern will pull, any sand with it as it’s removed, that will enlarge the void. The final cast part will then have extra metal in places where the sand came out. It makes for a poor quality casting and it wastes metal. Draft angles avoid this problem.

Draft Angle Basics

Think of a draft angle as taper on an otherwise vertical side of the pattern. (That is, a side that’s perpendicular to the top surface of the mold.) If the pattern had completely vertical sides, when it was lifted out the pattern would rub over the sand. Inevitably, that would cause some sand particles to come free.

To prevent this the vertical sides are sloped or tapered slightly. Now when the pattern is lifted vertically the side comes clear of the sand. There’s no rubbing and so no damage to the sand.

In sand casting this taper is called positive draft. It’s positive because the top part of the pattern overhangs the sand. If the draft was negative, sand at the top of the pattern would overhang the lower region of pattern. In this scenario the pattern can’t be lifted without breaking away some of the sand.

How Much Draft Angle is Enough?

Sand casters want as much draft as possible because this reduces the risk of producing defective castings. Most part designers want the smallest draft they can get away with because it complicates their design. It makes fixturing and machining more difficult too.

The compromise is to use between 1.5⁰ and 2⁰ of draft angle. Shallower parts will have more while very deep parts can get away with less. Advanced molding equipment can greatly reduce the size of a draft angle. In general, wooden pattern requires greater draft angle than metal ones do.

How to Avoid Draft Angles

The key is to have the part geometry provide the draft angle. For example, imagine casting a shaft. The cylindrical pattern is laid flat and pressed into the sand. (Or sand is packed around it.) When the pattern is lifted away, the radiused surface separates from the sand: there’s no rubbing and so no damage. The part geometry eliminated any need for a draft angle.

You could use the same technique to cast a square bar. Push it into the sand flat and the sides need a draft angle to release smoothly. But turn the pattern 45⁰ so the split line runs diagonally through the bar and the pattern automatically has 45⁰ of draft.

Our Experience Shows

When you’re designing a part to be sand cast pattern-makers will tell you it must have draft angles on the sides perpendicular to the mold split line. This is true: add a 2⁰ positive angle and the pattern will come out of the sand without damaging it. But, if you really don’t want a draft angle on the part, think about whether the pattern could be oriented to angle the sides and so provide a natural draft.

We count a lot of experienced sand casting professionals among our ranks. They can help you with your cast parts. All you need do is Contact Us.

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