Table 1. Tolerances and Capabilities of Investment Casting Compared to Other Casting Processes Minimum


Investment


Airset/Nobake Diecasting


Green Sand—Horizontally Parted Green Sand—Vertically Parted Lost Foam


Permanent Mold


Thickness (in.) 0.04


0.1875 0.06 0.25 0.25 0.15


0.125


Typical Weight Range (lbs.)


0.1-15 0.5-150


0.2-15 lbs. 1-200 1-100 1-75


0.5-100


Typical Surface Finish Range (RMS)


50-125


150-600 90-200 250-900 250-900 125-175


250-420


Typical Draft Range (degrees)


0-1 1-2


0.25-1 2-5 1-2 0-1 1-5


Typical


Lead Time (days)


60-120 15-45


120-360 10-30 20-60


120-400 90-300


Typical


Production Rate (castings/hour)


1-1000 10


120


100-500 100-500 20-200 5-12


metal in investment casting. Alumi- num is the most common nonferrous alloy used, although magnesium and copper-base alloys also are common. Exotic alloys, such as titanium and vanadium, can be investment cast. As with any casting process, invest-


ment casting alloys can be specifi cally made for a customer, depending on the materials at the casting plant. Unlike other processes, the ceramic molds in investment casting can be brought to more than 1,500F (816C), thus increasing their strength. T is allows for high-temperature pours. Duc- tile iron and steel are poured in the 2,600-2,900F (1,427-1,593C) range, and nickel-copper alloys are poured at more than 3,100F (1,704C). T ese temperatures ensure the most effi cient pouring procedures and allow for smoother cooling methods when cast in ceramic shells. Investment castings can be pro-


duced from a fraction of an ounce to more than 1,000 lbs. (453.6 kg). Smaller components can be cast at hundreds per tree (the structure on which wax patterns are affi xed), while heavier castings are produced with an individual tree. T e weight limit of an investment casting depends on the mold handling equipment at the casting plant.


3


Maintain Consistent Wall Thickness (With Exceptions)


Almost all confi gurations can be


cast via investment casting, but not all confi gurations are economical in the process. When evaluating diff erent


metal forming processes, the compo- nent’s total cost, including machining and fi nishing, must be considered. T e investment casting process can elimi- nate some costly machining operations by incorporating more detail, but a design intended for another manufac- turing process must be redesigned for investment casting. When designing an investment cast-


ing, a constant wall thickness is desirable, but gating techniques, which determine how the metal is fed to the casting mold cavity, can yield acceptable metal integ- rity for confi gurations exhibiting both thin and heavy wall thickness, as well as isolated heavy sections.


Table 2. Normal and Premium Linear Tolerances for Investment Castings


Tolerances (+/- in.)


Dimension (in.) Up to 0.25


Up to 0.5 Up to 1 Up to 2 Up to 3 Up to 4 Up to 5 Up to 6 Up to 7 Up to 8 Up to 9 Up to 10


Normal 0.005


Premium 0.003


0.005 0.004 0.005 0.005 0.01


0.01


0.015 0.019 0.022 0.025 0.028 0.031 0.034 0.037


0.013 0.015 0.017 0.02


0.022 0.024 0.026 0.028


In investment casting, the introduc-


tion of metal is not limited by the part- ing line, the area where mold halves in other casting processes are split. Most investment cast parts can be designed without any draft allowance. T e use of water-soluble waxes in patterns and pre-formed ceramic cores for internal passageways also can produce under- cuts, reliefs and internally contoured confi gurations not attainable from many other metal forming processes.


4


Avoid Overspecifying Tolerances


T e tolerances specifi ed by custom-


ers for investment castings can have a direct bearing on both tooling and unit costs. Normal tolerances—those which can be controlled during cast- ing—should be used whenever pos- sible, and premium tolerances—those which require secondary controls and processes to comply with print specifi - cations—should by applied only when necessary (Table 2). T e dimensional repeatability of castings in production will be within 25% of the total normal tolerances from run to run. Increasing tolerances as much


as the component function permits and avoiding unusually high toler- ances will prevent delays in delivery, scrapped parts and high rejection rates. The number of close toler- ances also can cause problems. If a casting has more than one close tolerance dimension from a single surface or detail, it is impossible to alter one dimension without affect- ing all related dimensions.


Nov/Dec 2011 | METAL CASTING DESIGN & PURCHASING | 37


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