Hoop Stress 1

[dezignit]

Can anyone give me a quick method of calculating the steel wall thickness required in an injection mould which is round (bucket mold)in shape. The internal injection pressure is 75 Mpa, the steel is P20, and I require minimum deflection of the outer wall.

Thanks in advance,

Mirko

 

[CRG]

You should be able to look this up in “Roark’s Formulas for Stress & Strain”

 

[ShaunE]

hoop stress (in Pa) = PD/2T

P=Pressure (Pa)= 75*10^6 Pa
D = nominal diameter of hoop / cylinder in metres
T = thickness of cylinder wall in metres

This works for thin walled cylinders, diameter to wall thickness ratio 20:1(ish)(for thick walled cylinder you want a formula called Lame's formula.)

to calculate deflection

Steel stiffness is 210 GPa = stress/strain

and strain = change in length /original length (length of circumference)

The stress comes from the hoop formula above.

so strain = stress/210*10^9

Finally: change in circumference = strain * original circumference

The result will be conservative because extra stiffness will be provided by any end caps.