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  • Design Criteria

    All casting processes have their design limitations, and the permanent mold aluminum casting process is no exception. Though none of the following guidelines are etched in stone and nearly any part can be cast, following these guidelines closely will result in a part that is more easily manufactured, cost effective and trouble free.

    Draft

    Minimum draft for full permanent mold is typically 3 degrees. Less draft can be attained through creative part orientation in the mold, metal core pulls, collapsible metal cores, or sand cores. In addition to the standard methods of minimizing draft, Watry has designed and built a specialty casting machine that can produce certain types of casting with near 0 internal draft in full permanent mold. Generous draft minimizes mold maintenance and premature mold wear.

    Wall Sections

    Minimum wall section is dependent on casting alloy, metallurgical quality, complexity of design, and required surface finish. Under ideal conditions the minimum wall section can be as low as .115″-.120″. Heavy casting sections isolated from the parting line of the mold should be avoided whenever possible.

    RADII

    Sharp corners are to be avoided. Radii should be as generous as the part design will allow. Whenever possible internal and external radii should be sized to maintain uniform wall section around corners.

    Lettering

    Lettering should be raised whenever possible. Depressed letters should be avoided, as they increase initial mold cost and are subject to premature wear and damage. Minimum letter size for raised letters is 1/8″ with 1/4″ preferred. Minimum letter size for depressed letters is 3/16″ with 5/16″ preferred.

    Surface Finish

    Casting surface finish is expressed in microinches. Typical permanent mold casting finish is between 200 and 420 microinches. Again, casting design plays a large role in a foundry’s ability to maintain a predictable and consistent finish. Smoother than typical microfinishes require additional, more frequent mold prepping and increase casting cost.

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    Linear Tolerances

    Tolerances below are for fully machined mold cavities.

    Basic

    Linear Dimension Tolerance
    Up thru 1 inch +- .015″
    Additional tolerance for each additional inch +- .002″

    Across Parting Line

    Additional tolerance for dimensions across parting line.

    Projected Area Tolerance
    Up thru 10 in. 2 +- .010″
    Over 10 thru 50 in. 2 +- .015″
    Over 50 thru 100 in. 2 +- .020″
    Over 100 thru 250 in. 2 +- .025″
    Over 250 thru 500 in. 2 +- .030″
    Over 500 in. 2 Consult Foundry

    Cored Features

    Additional tolerance for cored features.

    Projected Area Metal Core
    to Perm Mold
    Sand Core
    to Perm Mold
    Up thru 10 in. 2 +- .010″ +- .010″
    Over 10 thru 50 in. 2 +- .015″ +- .015″
    Over 50 thru 100 in. 2 +- .015″ +- .020″
    Over 100 thru 250 in. 2 +- .022″ +- .025″
    Over 250 thru 1,000 in. 2 +- .032″ +- .030″
    Over 1,000 in. 2 Consult Foundry

    Flatness Tolerances

    Greatest Dimension Tolerance
    Up thru 6 in. within .020″
    Each additional 1 inch thru 24 inches .003″ per inch
    Over 24 inches Consult Foundry

    Machine Finish Allowances

    Greatest Dimension Permanent Mold Cored Areas
    Sand Shell
    Up thru 6 in. .045″ .060″ .060″
    Over 6 thru 12 in. .060″ .090″ .060″
    Over 12 thru 18 in. .075″ .120″ .090″
    Over 18 thru 24 in. .090″ .180″ .120″
    Over 24 inches Consult Foundry

    Ejector Pin Mark Tolerances

    Ejector pins are necessary to eject castings from a permanent mold. The number, location, and size of these pins vary with the size and complexity of the mold. These pins leave marks on the casting. Unless otherwise specified, the foundry determines the location of these pins. Areas of possible interference should be specified on the drawing. Marks may be specified as raised or depressed. If so specified, the permissible variations of the marks will be the total of the given tolerance band. Listed below are typical permanent mold ejector pin mark tolerances.

    Projected Area Tolerance
    Up thru 50 in. 2 +- .020″
    Over 50 thru 100 in. 2 +- .030″
    Over 100 thru 200 in. 2 +- .045″
    Over 200 in. 2 Consult Foundry