Casting Shrinkage Calculator
Estimate pattern dimensions from finished casting size, alloy shrink rate, machining stock, draft taper, mold scale, and process allowance.
⚙Foundry Presets
📏Casting Inputs
📐Pattern Dimension Estimate
🔬Selected Material Grid
📊Material Shrink Rate Reference
| Material | Typical shrink | In per foot | Common casting use | Pattern note |
|---|---|---|---|---|
| Aluminum alloy | 1.30% | 0.156 in/ft | Housings, covers, plates | Watch thick-to-thin sections |
| Gray cast iron | 1.00% | 0.120 in/ft | Pump bodies, machine bases | Graphite expansion lowers shrink |
| Ductile iron | 1.10% | 0.132 in/ft | Brackets, arms, hubs | Confirm grade and foundry rule |
| Brass | 1.50% | 0.180 in/ft | Valves, fittings, hardware | Can vary with zinc content |
| Bronze | 1.60% | 0.192 in/ft | Bushings, art castings | Leave machining stock for bores |
| Carbon steel | 2.10% | 0.252 in/ft | Yokes, links, wear parts | Higher shrink needs riser review |
⚒Allowance Planning Table
| Allowance item | Applied to | Typical range | Calculator input | Foundry cue |
|---|---|---|---|---|
| Shrinkage | All pattern dimensions | 0.8-2.6% | Material shrink rate | Use alloy-specific data |
| Machining stock | Selected machined faces | 0.03-0.25 in | Allowance per face | Add before shrink scaling |
| Draft taper | Drawn side walls | 0.5-5 deg | Draft angle and depth | Deeper draws need more taper |
| Mold scale | Whole pattern | 0.995-1.010 | Mold scale factor | Useful for 3D printed tooling |
| Process buffer | Final pattern output | 0-2% | Process allowance | Prototype molds often use more |
📝Draft and Machining Guide
| Feature | Draft range | Machining stock | Surface risk | Pattern choice |
|---|---|---|---|---|
| External wall | 1-2 deg | 0.03-0.08 in | Low | Standard taper |
| Internal pocket | 2-5 deg | 0.06-0.12 in | Medium | Extra draft helps draw |
| Bored hole blank | 1-3 deg | 0.08-0.25 in | High | Cast undersize then machine |
| Ribbed bracket | 1.5-3 deg | 0.06-0.12 in | Medium | Blend ribs with fillets |
| Investment wax | 0-1 deg | 0.02-0.06 in | Low | Less draft may be workable |
📋Preset Dimension Reference
| Preset | Finished size | Material | Allowances | Shop note |
|---|---|---|---|---|
| Aluminum plate | 8 x 4 x 1.25 in | Aluminum | 1.3%, 0.06 in | General sand pattern |
| Gray iron cover | 12 x 9 x 1.5 in | Gray iron | 1.0%, 0.08 in | Machine gasket faces |
| Brass valve body | 6 x 3.5 x 3 in | Brass | 1.5%, 0.06 in | Allow for bore cleanup |
| Steel yoke | 10 x 4 x 2.5 in | Steel | 2.1%, 0.12 in | Higher shrink pattern |
| Metric impeller | 180 x 180 x 40 mm | Aluminum | 1.3%, 1.5 mm | Scale checked in metric |
ℹPattern Notes
Metal shrinkage are the name given to the phenomenon where the liquid metal in the mold take up more space than the solid metal that results from the cooling of the metal. The settling of the metal into a crystalline structure during the cooling process is what cause the metal to contract in volume. Thus, the final casting of the metal will be more smaller than the original pattern from which it was created.
It is essential to account for this shrinkage in the creation of the pattern; otherwise, the final metal casting will be to small to meet the requirements of an object that is to be created. The amount of shrinkage that occurs in metals can vary from alloy to alloy. For instance, steel shrink significantly more than gray iron.
Metal Shrinkage and How to Make the Pattern
As a result of these different shrinkage rates for different metals, workers will have to account for allowances for different metals when creating the patterns for metal castings. Furthermore, since gray iron contains graphite flakes that expands during the cooling of the metal, the shrinkage of gray iron is less noticeable than the shrinkage rates of other metals. A reference table for shrinkage rates for different metals can be referenced to determine the shrinkage rate for each alloy of metal in each specific casting project.
In addition to accounting for the shrinkage of metal during the solidification process, additional considerations must be made for the allowance for machining of the casting. In most cases, a pattern creator must account for an allowance for machining stock in the creation of the pattern; this allowance take into consideration the amount of metal that a cutting tool must remove from the casting of metal to create desired flat surfaces of bores in the metal part. This allowance for machining stock must be made prior to the application of the shrinkage factor to the metal part; otherwise, the part will be undersized due to the shrinkage of the machining stock itself.
In addition to the shrinkage of the metal, drafts of the metal parts must also be accounted for in their creation. A draft is an angled allowance for the vertical walls of the metal parts that allow for the removal of the pattern that was used to create the mold from the mold. If the draft is not provided for these vertical walls of the mold, the friction between the pattern and the sand mold can lead to damage of the sand mold.
An additional width must be provided for the top of the part if that part is to deepen in relation to the top of the part; deeper metal parts requires more width to provide for drafts along the vertical walls to allow for removal of the pattern. If 3D printing or CNC milling techniques is used to create the patterns of the metal parts, an additional allowance of mold scale factor may be required for those metal parts. The mold scale factor is used to account for errors that can occur during the manufacturing of the pattern; 3D printed materials, for instance, may shrink or warp during the manufacturing process.
Additionally, an allowance for process errors to account for sand mold variations caused by sand shifting during the metal pouring process may also be required. While the metal pattern dimensions can be calculated using linear mathematics, there are other considerations for metal parts that are made during the solidification of the metal. The thickness of metal parts will impact the time that it takes for the metal to solidify; thick metal parts will take longer to cool and solidify than thin metal parts.
To account for the differences in solidification times, risers and gates allow for additional liquid metal to be poured into the mold as the metal solidifies and contracts. These additional metal parts allow for the metal to remain filled during the solidification process. By accounting for machining stock allowances, draft allowances, and metal shrinkage rates for specific metals, a metal pattern can be created that is able to account for the physical properties of the metal of which the part is to be made.
