Design of components for casting

July 04, 2018 1 Comments

Why should we use casting?

Complex parts which are difficult to machine, are made by the casting process.

Almost any metal can be melted and cast. Most of the sand cast parts are made of cast iron, aluminum alloys and brass.

The size of the sand casting can be as small as 10g and as large as 200x103kg.

Sand castings have irregular and grainy surfaces and machining is required if the part is moving with respect to some other part or structure.

Cast components are stable, rigid and strong compared with machined or forged parts.

Typical examples of cast components are machine tool beds and structures, cylinder blocks of internal combustion engines, pumps and gear box housings.

Basic considerations of casting:

• Always keep the stressed areas of the parts in compression 
• Round all externalcorners

• Wherever possible, the section thickness throughout should be held as uniform as compatible with overall design considerations

• Avoid concentration of metal at the junctions 
• Avoid very thin sections

• The wall adjacent to the drilled hole should have a thickness equivalent to the 
thickness of the main body
• Oval-shaped holes are preferred with larger dimensions along the direction of 
forces

• To facilitate easy removal, the pattern must have some draft 

• Outside bosses should be omitted to facilitate a straight pattern draft 


 Always keep the stressed areas of the parts in compression


· Cast iron has more compressive strength than its tensile strength.


· The castings should be placed in such a way that they are subjected to

compressive rather than tensile stresses.

                        ·  When tensile stresses are unavoidable, a clamping device such as a tie rod or a bearing cap should be considered. 


                        ·  The clamping device relieves the cast iron components from tensile stresses. 


 Round all external corners

   It increases the endurance limit of the component and reduces the formation of 
brittle chilled edges. 


                        ·  When the metal in the corner cools faster than the metal adjacent to the corner, 
brittle chilled edges are formed. 


                        ·  Appropriate fillet radius reduces the stress concentration. 


Wherever possible, the section thickness throughout should be held as uniform as compatible with overall design considerations

Abrupt changes in the cross-section result in high stress concentration. 


 If the thickness is to be varied at all, the change should be gradual 


           Avoid concentration of metal at the junctions


     At the junction, there is a concentration of metal.
Even after the metal on the surface solidifies, the central portion still remains in the molten stage, with the result that a shrinkage cavity or blowhole may appear at the centre.

            There are two ways to avoid the concentration of metal.

            One is to provide a cored opening in webs and ribs. Alternatively, one can stagger the ribs and webs.

            Avoid very thin sections


   It depends upon the process of casting such as sand casting , permanent mold

            castingor die casting


         The wall adjacent to the drilled hole should have a thickness equivalent to the  thickness of the main body


    The inserted stud will not restore the strength of the original thickness.

     Oval-shaped holes are preferred with larger dimensions along the direction of forces 


      To facilitate easy removal, the pattern must have some draft 


· A minimum draft of 3° should be provided.

          Outside bosses should be omitted to facilitate a straight pattern draft