Aluminum Casting Standards: 356 and 357 Properties, Tolerances, and Draft
Protocast works to the Aluminum Association’s Standards for Aluminum Sand and Permanent Mold Castings. Most of what it pours is 356 and 357. These are the values engineers ask for most often on those two alloys: chemistry, minimum mechanical properties, and the tolerances each process holds.
Chemistry
356 and 357 are the same aluminum and silicon family. 357 carries more magnesium, which gives it the higher strength after heat treat, and much less iron.
| Element | 356.0 | 357.0 |
|---|---|---|
| Silicon | 6.5 to 7.5 percent | 6.5 to 7.5 percent |
| Magnesium | 0.20 to 0.45 percent | 0.45 to 0.60 percent |
| Iron, maximum | 0.6 percent | 0.15 percent |
Per Aluminum Association, Standards for Aluminum Sand and Permanent Mold Castings. Percent by weight. The full standard lists the limits for every other element.
Protocast verifies chemistry on its in-house spectrometer.
Minimum mechanical properties
Permanent mold carries higher minimums than sand for the same alloy. The steel die cools the metal faster, and the finer grain is stronger. Typical test results run above these minimums in both processes.
| Alloy and temper | Process | Tensile (ksi) | Yield (ksi) | Elongation |
|---|---|---|---|---|
| 356.0-F | Sand | 19.0 | Not specified | 2.0% |
| 356.0-T51 | Sand | 23.0 | 16.0 | Not specified |
| 356.0-T6 | Sand | 30.0 | 20.0 | 3.0% |
| 356.0-F | Permanent mold | 21.0 | Not specified | 3.0% |
| 356.0-T51 | Permanent mold | 25.0 | Not specified | Not specified |
| 356.0-T6 | Permanent mold | 33.0 | 22.0 | 3.0% |
| 357.0-T6 | Permanent mold | 45.0 | Not specified | 3.0% |
Per Aluminum Association, Standards for Aluminum Sand and Permanent Mold Castings. “Not specified” means the standard sets no minimum for that property. 357.0 is specified in permanent mold only.
Protocast heat treats to T6 in house. The 356 vs. 357 guide covers when the extra strength of 357 is worth specifying.
Tolerances
| Process | Linear tolerance as cast |
|---|---|
| Permanent mold | Plus or minus 0.015 inch for the first inch, plus 0.002 inch for each inch after that |
| Sand | Starts at plus or minus 0.030 inch for the first inch |
Per Aluminum Association, Standards for Aluminum Sand and Permanent Mold Castings.
Dimensions across the parting line hold looser than dimensions on one side of it. Features that need tighter control are machined, and Protocast machines castings in house on CNC equipment. Put the tolerances you need on the drawing, and the quote will say what holds as cast and what gets machined.
Draft
Permanent mold usually needs 1 to 3 degrees of draft on faces parallel to the pull. Outside walls sit at the low end. Inside walls and deep pockets need more. Designing for Permanent Mold covers draft, walls, and radii in detail.
The full standard
The Aluminum Association publishes the complete standard, with every alloy, temper, and tolerance table. Get the standard from the Aluminum Association. Engineers working on a Protocast quote can also ask their representative for the values that apply to their part.
Send the model. Get a quote with engineering feedback.
Have a drawing with a tolerance or property callout? Send it with the annual volume, and the quote will say what holds as cast and what gets machined.
- Engineering review with every quote
- Flow simulation before tooling is cut
- Cast, heat treated, and machined in one building
- Permanent mold tooling cut in house whenever possible
