These figures are to be used with caution because most castings have several different shrinkage allowances according to their size, design, complexity, relative mass of core vs. metal, varying metal thickness, nature of metal, temperature of pouring and mold material.
Aluminum
Bismuth
Brass
Bronze
Aluminum Bronze
Manganese Bronze
Cast Iron
Cast Iron Wrought
Copper
Lead
Monel
Magnesium
Steel
Tin
Low Melting Point Alloy
Zinc
5/32
5/32
3/16
3/16
7/32
7/32
1/10-1/8
1/8
Material Shrinkage per foot
Material Shrinkage per foot
3/16
5/16
1/4
1/8-5/32
1/4
1/4
NIL
5/16
Shrink Ruler Link
Patternmaking Index
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"Kindt-Collins has been there for the patternmaker for a long time".
Shrinkage Allowance
Take for example you have an alloy or polymer with an unknown shrinkage.

Follow these steps:

1. Define the average wall thickness = D of the part you plan to cast.
2. Make a test mold W x L x D (1.000 x 12.000 x D) inches.
3. Apply mold release to test mold cavity (for polymers) or make a sand mold (for alloy).
4. Transfer the mixed polymer or melted alloy into the test mold.
5. Allow the material to set or chill (and remove sample from test mold).
6. With dial calipers measure the samples W - L - D and obtain your variable.

Example: The length (L) of my sample is 11.750 inches. 11.75 12 = .97916 = the shrink variable.

7. Multiply the shrink variable to all the nominal dimensions of the part you plan to make.
8. Proceed to layout your project.
Computation For An Unknown Shrink Variable
Note: Some epoxies and urethane elastomers need a 7 day full cure @ 72 F they may be demolded in less then an hour (according to manufactures specs) but they'll keep shrinking.