Casting Design Guidelines

Some following points are to be consider when we design the mould and its pattern for the casting components.
Casting design guidelines as follows,

1. Avoiding vertical surfaces at cope and drage mould (Introduction of draft) –

During the ejection and detachment of the mould from the pattern, the separation of the horizontal surface like ‘A’ is very easy and problem free. But the vertical surface like ‘B’ is very difficult to separate. During the detachment, the mould surface goes rubbing against the vertical of the pattern and makes the sand at the to become loose and weak. Such surface is more prone to get washed away and even to collapse due to flow of liquid metal.

1. Avoiding vertical surfaces at cope and drage mould (Introduction of draft)

Instead, a surface with taper of around 2° to 5° converts the vertical surface to a draft surface. It completely avoids rubbing action during the separation and avoid sand erosion and retains the sand strength.

2. Converting a draft to a slant – 

2. Converting a draft to a slant

During the squeezing of the mould sand in vertical direction, the horizontal surface like ‘A’ get properly compacted and acquire proper strength. However, the surfaces with draft like 'B' get poorly compacted and lack at their strength. More and more slant results into better and better compaction and strength. Hence, the vertical surfaces at the component converted to have slants like 150, 300, even 450 are preferred. With such a change, sand wash. sand inclusion avoided.

3. Avoid thin wall thickness – 

3. Avoid thin wall thickness 

If the component carries thin walls, then the amount of liquid material propagating in the forward direction gets slowed down. Such material is subjected to faster cooling and can get solidified prior to its destination. Hence, depending upon the distance to be further travelled by the liquid metal, the gap between the mould and core (indirectly the wall thickness Of the component) is never allowed to be very thin. Accordingly a minimum wall thickness of around 6 to 8 mm is preferred. If the liquid material is not expected to travel by a large distance beyond certain location, the thickness can be allowed upto 4-5 mm but never less than that. Accordingly, defects like cold shut get avoided.

4. Avoid junctions with different wall thicknesses –

4. Avoid junctions with different wall thicknesses

When two or more wall thicknesses at a casting form a junction and if the wall thicknesses are considerably different, then due to different rates of solidification; the thinner walls tend to crack and the thicker walls tend to have local shrinkage. If the junctions have equal wall thicknesses then the defects like cracks or shrinkage get avoided.

5. Introducing convexity/concavity at the cope surfaces – 

When the liquid metal occupies the mould cavity and gets raised at its level. the impurities like sand particles, slag inclusions, gas bubbles keep on floating at the upper surface of the liquid metal. When the pouring comes to an end, majority of these containment's escape out through the now Off. However, if the upper surface of the cope mould (and the component) is flat and horizontal, then some of the impurities still have a chance to get trapped at the upper part of the flat surface.

 

5. Introducing convexity/concavity at the cope surfaces

Instead, if the upper side of the component (and the cope mould) is given a convex or concave shape, then due steady increase of the molten metal level, the impurities have a better chance to escape out. Even after stopping of the pouring operation and during solidification, the shape permits the escape of the impurities such provision reduces defects like sand inclusion, gas holes, blow holes etc.

6. Avoiding sharp corners –

If the cast component is required to have some sharp corners, then they are required to be provided at the moulds or cores. Such corners like A or B at the moulds or cores happen to be too weak to bear the flow of the liquid metal. Such corners break off and get washed away while pouring the liquid metal.

 

6. Avoiding sharp corners

 
But if the corners are made little smoother by providing appropriate corner radius, then they are less damaged by the flowing metal, they are capable of retaining their strength, and reduce the chances of damage the shape. sand wash. sand inclusion etc. Accordingly, a minimum corner radius of around 't' to '1.5 t' is recommended at any corner; where 't' is the wall thickness at that location.

7. Arranging far the surface to be machined at the cope –

If a component subsequently requires for any machining to be done on any surface and if such surface is arranged at the cope, then the entrapped impurities have a chance to get eliminated at the machining operation; leaving the component to be a defect free component after machining. Even at times some machining can be planned, though not called for under normal circumstances, at the upper surfaces. The defects like sand inclusions, gas bubbles, porosity can be eliminated with help of such arrangement.

7. Arranging far the surface to be machined at the cope

It must be understood that the above-mentioned corrective actions finally are concerned regarding component shape, geometry. dimensions, features, etc. These are not the only actions to overcome the defects. Some technical controlling action over the process variables like mould hardness, gas venting, permeability of sand mix, metal temperature and fluidity, addition of binders, etc. can make considerable contribution for overcoming the defects.

Do comments if any query regarding the mechanical design concepts.
I hope this information is helpful - Sand Casting Process & Design Consideration for Manufacturing of Sand Casting -

To read more ... Deciding parting line/ parting surface