For answers to frequently asked questions about aluminum sand castings select from the topics in this list:

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In answering these questions we have set guidelines that will result in an economical casting. In terms of foundry cost, "more is better" so greater tolerances, draft, wall thickness, etc. will generally result in a more economical casting. On the other hand tighter tolerances, less draft, thinner walls, etc. can often be achieved, however, at a somewhat greater cost. In these instances it is important to consult with Rainier Cast Parts. Our answers to these questions are based on experience at Rainier Cast Parts and may differ for other foundries.

Tolerances

A general tolerance of +/-.030" is obtainable on sand castings up to 12" in length. For castings larger than 12" the tolerance should be increased an additional +/-.002" per inch.

Wherever a casting design permits, allowing a greater tolerance (even if only on specific dimensions) will result in a more economical design.

Surfaces that will be machined should be allowed double the suggested tolerance in the plus direction.

For example: a casting with a general tolerance of +/-.030 should have a tolerance of +.060/-.030 on surfaces that will be machined.

This will minimize the work required in finishing the casting.

It is possible to hold tolerances as close as one half of those described above on certain features. This is possible by controlling tool design, molding methods and secondary finishing operations. However, you should consult with our foundry engineering staff before finalizing your design.

What about the added weight due to increased tolerances?

Increasing tolerances can potentially increase the weight of a dimensionally acceptable casting. However, if weight is critical, a better way of controlling it is to specify a maximum weight. This is best accomplished by calculating a maximum design weight and then refining it, if necessary, during initial casting production. Controlling weight in this manner allows the foundry greater flexibility and results in a more economical casting.

Draft

A draft angle of one degree is adequate for vertical surfaces between 1 and 4 inches in height. For surfaces under 1" in height, .010" to .015" is required regardless of height. For surfaces over 4" in height, .060" every 8" to 10" is required.

How should draft be shown on a drawing?

Draft should not be shown in the field of the drawing but should be specified in a drawing note such as "Draft angle: 1 degree max.". This will allow the foundry to use minimum draft where required, up to a maximum of 1 degree, on those surfaces that are vertical to the mold parting line or, in the case of cores, vertical to the line of draw. On those surfaces under 1" in height the draft will actually exceed 1 degree but this excess will be within the general drawing tolerances and is not considered a violation of the general draft note.

What if draft will interfere with the casting function?

If a draft angle of 1 degree on certain surfaces will interfere with casting function you should identify these surfaces on the drawing and specify how much draft, if any, may be allowed.

Wall Thickness

The thinnest nominal wall that can economically be cast depends on several factors:

Total area of the wall;
Orientation of the wall in the mold;
Alloy to be cast;
Distance of the wall from the gating system; and
Casting soundness and strength requirements.

Nominal walls of .12" can be cast up to 3" in height and 50 square inches in area. For each additional 1.5" in height or 50 square inches in area the nominal wall thickness should be increased .03".

Producing localized thin walls in castings that are predominantly thick walled can often create problems for the foundry, especially when the casting soundness and strength requirements are high. Consult with Rainier Cast Parts foundry in these instances.

Surface Finish

Surface finish on castings is affected by several factors:

Molding methods and materials;
Weight and surface area of the casting;
Finishing methods;
Wall thickness; and
Casting soundness and strength requirements.

Machine Stock

Machine stock requirements vary with casting size:

Up to 10" in length allow .09" per surface;
Over 10" and up to 25" in length allow .13" per surface;
Over 25" and up to 40" in length allow .16" per surface;
Over 40" and up to 55" in length allow .19" per surface.

Casting Drawings

Obviously some sort of drawing, either a hard copy or CAD file, is necessary to describe the part geometry. However, a casting drawing describing only the casting requirements is not necessary.

A finished part drawing that describes both the casting and machining requirements is often superior to a casting drawing. Since it defines the extent of machining the foundry can take this into consideration when specifying the tooling, gating system and foundry processes. This allows the foundry to restrict, where possible, gating contact points to surfaces that will be machined. Also, casting requirements such as surface finish will not be applied to surfaces that will be machined away.

Porosity

This is a general term referring to voids within a casting. There are three types of porosity:

Gas Porosity:

This term refers to hydrogen gas within a casting. Molten aluminum has such an affinity for hydrogen that it will disassociate it from other molecules such as water and form a solution with it. As with most solutions, as the temperature drops the hydrogen becomes less soluble and precipitates as hydrogen gas. The greater the amount of hydrogen in the molten aluminum and the slower it solidifies the greater the hydrogen voids will be. These voids are generally smooth, round or slightly elongated and may be somewhat localized to the areas of the casting that solidify last. This type of porosity is generally undetectable visually since the surface of the casting solidifies quickest preventing the hydrogen from forming holes large enough to be visible on the surface except by using fluorescent penetrant inspection.

Gas Holes:

These are generally large and more localized voids than gas porosity but they retain the smooth, round or slightly elongated shape. They are usually caused by reaction in the mold media producing gas which bubbles through the molten metal.

Shrinkage porosity:

This type of porosity has a rough irregular shape. It is caused by a lack of adequate feed metal during solidification.

There will always be some porosity in aluminum castings. However, through the use of proper foundry techniques (and sometimes slight modification of casting geometry) it can be reduced to acceptable levels.

A key to assisting the foundry is to establish clear requirements for porosity and to identify those areas where porosity levels are most critical. It also helps if the foundry knows which surfaces will be machined.

How are porosity limits specified?

The most common way for aircraft and aerospace castings is by specifying a radiographic standard such as MIL-STD-2175. This specification establishes four grade levels for porosity and other internal defects that can be found in castings. Each grade refers to various standard plates in ASTM E155.

Commercial castings may also use MIL-STD-2175 or other specifications such as ASTM B26 or the Aluminum Association's AA-CS-M Series. These specifications also establish grade levels referring to ASTM E155.

All of the specifications listed above provide for various levels of inspection from foundry control (radiographic inspection of initial sample casting(s) only) to production sample plans (radiographic inspection of samples taken from each lot of castings) and 100% inspection of every casting.

ASTM E155 is a set of radiographic plates for classifying defect levels in aluminum castings. Plate 1 levels are barely detectable in x-rays. However, achieving Plate 1 levels can be quite costly. Most aircraft castings require Plate 2 or 3 levels while commercial castings are generally at Plate 4, 5 or 6 levels.

Can porosity levels be specified without radiography?

If the only concern is porosity exposed on the surface of the casting, limits can be set for the maximum allowable size. However, if internal porosity is of concern, radiography is the only practical detection method.

Casting Prices

Because of the many factors that determine casting prices it is impossible to answer this question in other than general terms.

At Rainier Cast Parts our aluminum castings generally range from $4.50 to $12 per pound and zinc castings from $2.50 to $5 per pound.

Note added 5-23-06:
I've updated the above costs. However, in the past four months aluminum prices have increased nearly 50% and zinc 250% and it looks like they will continue to rise in the coming months.

Copper prices are also rising but because of the great number of alloys and their wide variation in prices, I can't give a reasonable range for pricing.

We establish casting prices on an individual basis: each casting and its requirements is reviewed to determine tool design and manufacturing methods. From this the material, labor and outside process requirements can be estimated and firm pricing established.

Factors that influence casting prices include:

Casting Factors:

Casting weight, size and geometry;
Tolerances;
Alloy;
Surface finish;
Soundness and strength requirements;
Machining requirements; and
Finishing requirements such as painting, etc.

Production Factors:

Total quantity required; and
Production rate.

In order for us to quote a firm price for your casting you should specify all of these requirements in your inquiry.