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°. 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 300 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.

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.

 .010"
 Over 10 thru 50 in.

 .015"
 Over 50 thru 100 in.

 .020"
 Over 100 thru 250 in.

 .025"
 Over 250 thru 500 in.

 .030"
 Over 500 in.

 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.

 .010"

 .015"
 Over 10 thru 50 in.

 .015"

 .025"
 Over 50 thru 100 in.

 .015"

 .030"
 Over 100 thru 250 in.

 .022"

 .040"
 Over 250 thru 1,000 in.

 .032"

 .060"
 Over 1,000 in.

Consult Foundry

Flatness Tolerances

 Greatest Dimension

 Tolerance
 Up thru 6 inches

 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 inches

 .045"

 .060"

 .060"
 Over 6 thru 12 inches

 .060"

 .090"

 .060"
 Over 12 thru 18 inches

 .075"

 .120"

 .090"
 Over 18 thru 24 inches

 .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.

.020"
 Over 50 thru 100 in.

.030"
 Over 100 thru 200 in.

.045"
 Over 200 in.

 Consult Foundry

Additional Information

Much of the above information was obtained from the 13th Edition of the Standards for Aluminum, Sand and Permanent Mold Castings, 1992 published by the Aluminum Association, Incorporated. The full version can be obtained from their website @ http://www.aluminum.org.

The Aluminum Association, Inc.
900 19th Street, N.W.
Washington, DC 20006
Telephone: (202) 862-5100
Fax: (202) 862-5164

Remember, this information provides general guidelines only. For part-specific information contact:

Watry Industries, Inc.
P.O. Box 131
Sheboygan, Wisconsin 53082-0131

Telephone: (920) 457-4886
Fax: (920) 457-5241

e-mail address:
wiipres@watry.com

Request For Quotation

Information that will help speed up the quotation process and enable Watry to quote an accurate price include:

Estimated part weight, or actual weight if parts already exist

Sample castings, if any exist

Alloy specification

Heat treat specification

Estimated annual quantities

Release quantities

State if design changes can, or cannot be made

Blueprints must be in English, either metric or inch are OK

State if additional operations such as painting, anodizing, machining, etc., are to be supplied by Watry. (If machining is required, we must have a machining print). Any specifications not shown on the drawing (Engineering specs., painting specs., etc.) must be sent along with the request for quotation. Faxes are sometimes hard to see clearly. However, a fax of the blueprint is OK if it is legible and all specifications are included.

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