Frequently Asked Questions About Investment
1. WHAT ALLOYS CAN BE POURED AS AN
Generally all ferrous and non-ferrous materials can be investment
cast. On the ferrous side, carbon, tool and alloy steel along
with the 300, 400,15-5PH and 17-4PH stainless steels are most
commonly poured. Most aluminum, copper base, and other non-ferrous
materials can be cast.
In addition, there are the "exotic" alloys used primarily
in the aircraft engine industry to produce blades and vanes.
These alloys are primarily composed of nickel and cobalt with
a variety of secondary elements added to achieve specific
strength, corrosion and temperature resistant properties.
2. WHAT SIZE RANGE OF PARTS CAN BE PRODUCED
BY THE INVESTMENT CASTING PROCESS?
Investment castings can be produced in all alloys from a fraction
of an ounce (such as a dental brace for a tooth) to over 1000
pounds (complex aircraft engine parts). Of the approximately
three hundred investment casting foundries nationwide, most
cast parts in the ounces to 20 pound range. Presently a larger
number of foundries are increasing their capability to pour
larger parts, and pieces in the 20-120 pound range are very
common. Becker Metal Works does parts from 1/4 oz. to 80 lbs.
3. WHAT ARE THE "AS CAST" DIMENSIONAL TOLERANCES
I CAN EXPECT?
Typically, a linear tolerance of ±.005 in/in is standard
for investment casting. This varies depending on the size
and complexity of the part. Subsequent straightening or coining
procedures often enable even tighter tolerances to be held
on one or two specific dimensions.
A concerted initial effort between the customers and the foundry's
engineering staff can often result in an investment casting
drawing for a part that substantially reduces or completely
eliminates the previous machining requirements to produce
an acceptable part.
3. WHAT TYPE OF SURFACE FINISH CAN I EXPECT
FROM AN INVESTMENT CASTING?
Because the ceramic shell is built around smooth patterns
produced by injecting wax into a polished metal mold, the
resultant casting finish is excellent. A 125 microfinish is
standard and even finer finishes (63 or 32) are not uncommon
on aircraft engine castings.
Each foundry has its own standards for surface blemishes (positives,
negatives), these are discussed and agreed upon with the customer
based on the function and cosmetic requirements of the part
prior to release of the tooling order.
5. AREN'T INVESTMENT CASTINGS
EXPENSIVE? AND IF SO, HOW CAN THEY SAVE ME MONEY?
While investment castings are generally more expensive than
forged parts or those produced by other casting methods, they
make up for the higher cost through the reduction of machining
achieved by the near net shape tight tolerances that can be
held as cast. Many parts that require milling, turning, drilling,
and grinding to finish can be investment cast with only .020
- .030 finish stock. Again, it is imperative for the engineering
staff of the foundry and customer to get together and discuss
what can or cannot be cast to determine final finishing requirements
and the potential cost savings.
6. HOW MANY PIECES DO I NEED TO MAKE BUYING
AN INVESTMENT CASTING PRACTICAL?
Not as many as you think! Tooling amortization is a key factor
in determining whether or not an investment casting is practical.
The machine tool industry will often specify an investment
casting on 25 pieces of a new part. This is bought once and
after that only one or two pieces at a time for replacements.
Conversely, quantities exceeding 100,000 parts per month are
produced by some foundries largely for automotive use. The
bulk of investment castings produced fall in the 100-10,000
piece annual range.
7. WHAT TYPE OF TOOLING OR PATTERN EQUIPMENT
Typically, a split cavity metal mold is manufactured that
is the "female" mold from which the "male" wax patterns
are produced. Depending on the complexity of the casting,
various combinations of aluminum, ceramic or soluble cores
may be employed to yield the desired configuration. Most tooling
for investment castings fall in the $1,000 - $10,000 category.
8. WHAT ABOUT THE INTEGRITY OF AN INVESTMENT
CASTING? WILL I HAVE PROBLEMS WITH POROSITY AND
SHRINKAGE THAT IS USUALLY NONEXISTENT IN BAR STOCK OR FORGINGS?
Investment castings are used for many critical applications
that require the parts to be x-rayed and meet definite soundness
criteria. The integrity of an investment casting can be far
superior to parts produced by other methods.
9. WHAT ARE THE LEAD TIMES
I CAN EXPECT WHEN ORDERING AN INVESTMENT CASTING?
Nothing varies more than lead times depending on part complexity
and foundry capacity. Generally 6-12 weeks is typical for
tooling and sample castings and 6-12 weeks for production.