History
of Investment Casting
The
technique of investment casting is both one of the
oldest and most advanced of the metallurgical arts.
Investment castings are at work in the fiery combustion
chambers of jet aircraft and in the sub-zero vacuum
of space. Yet, the root of this technology, the cire
perdue or “lost wax” method dates back to at least
the fourth millennium B.C. The artists and sculptors
of ancient Egypt and Mesopotamia used the rudiments
of the investment casting process to create intricately
detailed jewelry, pectorals and idols. Remarkably,
civilizations as diverse as China’s Han Dynasty, the
Benin Kingdom in Africa and the Aztecs of pre-Columbian
Mexico employed similar techniques. The cross-cultural
adoption of this complex process implies a great degree
of commerce and communication in antiquity. In Renaissance
Europe, the Italian Sculptor, goldsmith and author,
Benvenuto Cellini, cast his bronze masterpiece “Perseus
and the head of Medusa” using the lost wax process.
The
investment casting technique was largely ignored by
modern industry until the dawn of the twentieth century,
when it was “rediscovered” by the dental profession
for producing crowns and inlays. The first authenticated
record of the use of investment casting in dentistry
appears in a paper written by Dr. D. Philbrook of
Council Bluffs, Iowa in 1897. However, the true significance
of this process was not realized until Dr. William
H. Taggart of Chicago published his research in 1907.
During World War II, with urgent military demands
overtaxing the machine tool industry, the art of investment
casting provided a shortcut for producing near-net-shape
precision parts and allowed the use of specialized
alloys which could not readily be formed by alternative
methods. The investment casting process proved practical
for many military components—and during the postwar
period it expanded into many commercial and industrial
applications where complex metal parts were needed,
however, the process was still relatively obscure.
In the decades that followed, many innovations such
as the shell process, the steam autoclave, conveyorization,
automation, and robotics have modernized and transformed
the process.
In
today’s world, investment castings touch all of our
lives. When we fly on an airplane, drive an automobile,
play golf, use a utility tool, power tool or hand
tool, we are using investment castings. Once thought
of as suitable only for low volume, high cost applications,
investment casting has evolved into a technology capable
of producing quantities of millions of pieces per
year, at costs rivaling those of less flexible and
desirable methods.
What
Prime Industries can do for you?
Do
you buy metal parts within a size range from less
than 0.5 in. (12.5mm) per side to as large as 24 in.
(610mm)?
Do the designs have wall thicknesses ranging from
0.015 in. (0.38mm) to 2.0 in. (51mm)?
Are the parts made of aluminum, lowalloy steel,
high-alloy steel, stainless steel, nickelbase alloy,
brass, bronze, titanium or cobalt-base alloy?
Are the production rates in the range of 10 to
100,000 per day?
If
the answers to some of these questions are “Yes,”
then it is worth your while to read on. See how Prime
Industries services hundreds of other customers with
similar part requirements.
Tell
us your challenges—let us participate in the solutions.
Investment
castings are dimensionally precise, have the smoothest
surface finishes of any castings for most alloys and
are available in more alloys than from any other process
for shaping metals.
Engineers
and metallurgists with experience in all forms of
metal parts manufacture are available to assist you
with design, support development and production planning.
They know if a part should be a casting or should
be made using another process and will make the best
recommendation for you, the customer.You can take
advantage of this experience by contacting Prime Industries.
Our engineers will review the part function, ambient
conditions, stress levels, needed life, needed stiffness,
type of loading (continuous or cyclic) and special
considerations such as magnetic, thermal expansion
or electrical needs to assist with alloy selection.
They can then assist you with consideration of manufacturing
approaches and test designs in those approaches to
see which process best fits the alloy and cost objectives
of the part or assembly.
The
goal is to produce the lowest-cost part which will
provide the desired performance, regardless of the
manufacturing process selected.
Achieving
this may include simplifying the design, reducing
the number of parts needed in an assembly or incorporating
new features to enhance performance while keeping
machining and assembly costs low. If investment castings
result are indicated, Prime Industries can assist
you with their design to assure the lowest cost results.
If the requirement is an assembly which requires an
investment casting, Prime Industries may submit a
proposal for doing the complete assembly. In any event,
Prime Industries will assist you in finding the best
source to refine the design and obtain pricing.
Prime
Industries knows its customer’s best interest is its
best interest and will always make the best recommendation
for the customer, regardless of the manufacturing
process needed for the part.If you decide to work
with Prime Industries, you can count on us for finite
element analysis design support, handling of CAD/CAM
software of any kind, short lead time for tooling,
installation of whatever facilities are required to
support production and production in facilities which
are qualified to the most stringent international
and major company quality standards.
SURFACE FINISH
As-cast finishes will vary with alloy specified but
generally fall within the range quoted. For a ground
finish an allowance of 0.010 in. (0.25mm) should be
made.
Metal |
C.I.A.
value |
Micro-inches |
As-cast |
Machined |
Stainless
Steels |
90-126
60-125 |
Cobalt
Chrome Alloys |
80-100
50-100 |
Carbon
Steels |
90-125 60-125 |
GENERAL TOLERANCES
Shape and alloy selected will influence accuracies
achieved but values given are normally accepted through
out the investment casting industry.
Dimension |
Tolerance |
Upto 25 mm |
±0.25mm |
Above 25 mm |
±0.25mm/25mm |
Closer tolerance may be held on selected
dimensions following design evaluation.
STRAIGHTNESS
There are practical limits to straightness which can
be achieved but mechanical straightning can reduce
variations.
Casting
Length |
As-cost |
Corrected |
25 mm |
±0.50
mm |
±0.25
mm |
50-100
mm |
±0.75
mm |
±0.50
mm |
100-150
mm |
±1.00
mm |
±0.50
mm |
150
mm |
±1.50
mm |
±0.62
mm |
FLATNESS
Flatness is also affected by dimensions of casting.
Addition of ribs will minimize bowing, twisting and
distortion and mechanical straightning will reduce
variations where necessary.
Casting
Length |
As-Cast |
Corrected |
25 mm |
±0.20
mm |
±0.10
mm |
50 mm |
±0.37
mm |
±0.15
mm |
75 mm |
±0.50
mm |
±0.20
mm |
100
mm |
±0.62
mm |
±0.25mm |
CAST HOLES
Depth (D) and Diameter(d) of hole must allow for adequate
penetration of investment material. Values given show
usual limits for cast holes but pre-formed ceramic
cores allow this restrictions tobe exceeded, although
usually at increased cost.
Through
holes |
Diameter
(d) mm |
Maximum
Depth (D) mm |
3.17-6.35
D
upto 1.5 d |
6.35-12.70 D
upto 3.0 d |
12.70 D
upto 5.0 d |
Blind
Holes |
Diameter
(d) mm |
Maximum
Depth (D) mm |
4.76-12.70 D
upto 1.5d |
12.70 D
upto 2.0 d |
CONCENTRICITY
The larger the outside diameter becomes, the closer
to concentric an inside diameter can be cast.
Mechanical correction can normally be made where wall
thickness is thin enough to allow plastic deformation.
O.D.(mm) |
I.D.
(mm) |
Eccentricity (mm) |
As-cast |
Corrected |
18.75 |
6.25 |
±0.100
±0.100 |
25.00 |
12.50 |
±0.125 ±0.125 |
37.50 |
18.45 |
±0.200 ±0.200 |
50.00 |
25.00 |
±0.250 ±0.200 |
ROUNDNESS
For solid bars roundness is affected by solidification
stresses and tolerance required increases nearly in
proportion to diameter, inline with usual 'Lost Wax'
tolerances.
Values shown are for as-cost roundness on inside and
outside diameters of tubes of varying sizes. Wall
thickness is important and tighten tolerance can be
obtained by mechanical correction if desired.
O.D.
(mm) |
Tolerance
(mm) |
I.D.
(mm) |
Tolerance
(mm) |
12.50 |
±0.25 |
upto
6.25 |
±0.30 |
25.00 |
±0.50 |
6.25-12.50 |
±0.40 |
37.50 |
±0.60 |
12.50-25.00 |
±0.50 |
50.00 |
±0.75 |
above
25.00 |
±0.50
(per 25 mm) |
MINIMUM SECTION THICKNESS
Wall thickness will depend upon area of casting and
alloy selected. The following values are of guide
from general experience.
Material |
Minimun
wall thickness obtainable |
18/8
stainless steels |
1.62
mm |
25/12
stainless steels |
1.50
mm |
Carbon
steels |
2.25
mm |
Cobalt-Chrome
alloy |
1.12
mm |
PARALLEL SECTIONS
Parallelism can be maintain by adding fire-bars to
minimize distorsions. Values shown are typical tolerances
for various gap-widths between parallel sections per
inch cost.
Gap
(mm) |
Tolerance(mm)
As-cast |
Corrected |
6.25 |
±0.075 |
±0.075 |
12.50 |
±0.125 |
±0.100 |
18.65 |
±0.150 |
±0.100 |
25.00 |
±0.175 |
±0.125 |
OTHER FEATURES :
Angles : Angular tolerance to ±1/2° (Closer
by mechanical corrections).
Symbols : Latters, Numbers, etc., can be reproduce
in relief or inset.
Serrations, Splines and Gear Teeth may be cast in
certain instances subject to prior agreement.
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