dokumen.tips isostatic-pressing-cold-isostatic-pressing-cip-is-a-materials-processing

ISOSTATIC PRESSING
ISOSTATIC PRESSING
• COLD ISOSTATIC PRESSING (CIP) is a materials processing
technique in which high pressure is applied to metal powder in a
sealed elastomer container shaped for the application.
• The powder is converted from a loose aggregate into a partially dense
compact that has sufficient green strength to permit careful handling
and transfer to the following process operation.
• Compacting pressures range from 207 to 414 MPa (30 to 60 ksi),
although pressures as high as 758 MPa (110 ksi) have been used.
• Compaction is performed at ambient temperature.
• The density of the loose powder poured into the elastomer mold is
increased from 55 to 65% of theoretical, a nominal range for
uncompacted powder, to 75 to 85% of the 100% theoretical density
value of the metal being processed.
ISOSTATIC PRESSING
• The metal powder is compacted uniformly in all
directions so that the compact becomes an accurate
scale down of the mould.------- uniform density ------ a homogeneous microstructure
• For this purpose the powder is sealed in a flexible
envelope and the assembly (mould-powder) is
immersed in a fluid which is pressurized.
• There are virtually no residual stresses in the
compacted material, because there is no die wall
friction.
Why Isostatic Pressing
To get uniform density and compaction.
To make intricate shapes.
To get objects with great dimensional tolerance.
To get homogeneous structure.
Figure1 shows the use of formers and use of
containers with holes for support purposes.
Fig. 1: Examples of use of (a) container with holes and
(b) (c) formers for producing shaped components by
isostatic pressing.
Process Characteristics:
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Unique aspects of CIP, as compared to die compaction, for
P/M parts include:
Hydrostatic application of pressure over all surfaces of the
mold produces uniform powder density for simple and
complicated shapes.
Die-wall friction is not a factor in the densification process
because of the elastic behavior of the mold.
Organic binder or lubricant additions to the metal powders
are not required to achieve useful green strength. In fact,
these materials are detrimental because of the adverse
influence on metal chemistry and related mechanical
properties for many of the reactive metals commonly
processed by CIP if all these materials were present.
Shapes with high ratios (greater than 10) of length to
characteristic diameter, can be densified and handled.
• Parts with reentrant and three-dimensional curved
geometries can be made.
• Elastomer tooling can be combined with metal
tooling inserts to exactly control certain part
dimensions such as cylinder bores or outside
diameters.
• Thin-walled sections and parts can be pressed.
• Elastomer tooling costs are low.
Recognized limitations of the process include:
• Dimensional control is generally less precise than
with metal die compaction because of the absence
of exactly dimensioned reference surfaces and
variability of the local poured powder density
within the mold.
• Surface finish of cold isostatically pressed parts is
rough compared to die-compacted parts except for
areas in contact with hard tool inserts in hybrid
molds.
• Production rates are low compared to metal die
compaction.
• Elastomer molds have a relatively short life
because of abrasive wear.
• Leakage of a mold in the CIP vessel due to mold
tearing or poor closure and sealing practice results
in loss of material due to contamination by the
working fluid.
• With reactive fine powders, such as aluminum, in
large molds water leakage into the mold can
produce a hazardous exothermic reaction.
•
Figure: Complex cold isostatically pressed P/M
part made using internal round and square hardtooling inserts.
Two types of processes:
i) Cold Isostatic Pressing
ii) Hot Isostatic Pressing
* Cold Isostaic Process ----- two methods are used :
(a) free mould or "wet bag" process is suited for
(i) batch scale production;
(ii) the mold is filled and sealed outside the pressure vessel.
(iii) After the mold is introduced in to the pressure vessel, it is
completely immersed in the pressure medium, usually
water containing lubricating and corrosion-preventive
additives;
(iv) complex parts;
(v) research and prototype work;
(vi) several moulds in one run-even with differing shape, i.e.
parts of different sizes and shapes that require the same
process parameters can be pressed in the same cycle.
a) Powder fill by weight or volume.
b) Filling of the mould from the top.
c) Top lid put on the mould container.
d) Mould is placed inside the pressure vessel.
e) Top lid put on the pressure vessel after necessary
evacuation and filling with the pressurizing medium.
f) Top lid removed after required isostatic compaction for
removal of the mould.
(b) fixed mould or "dry bag" process (Figure 3) which is
characterized by
(i) envelope is permanently fixed into the pressure vessel,
(ii) After the elastomeric mold is filled with powder, pressure
is applied by introducing pressurized oil between the
fixed mold and the vessel wall,
(iii) only one compact at a time is used,
(iv) more simple shapes are made and
(v) more suited for mass production and faster production
rates.
Fig.3: Schematic of equipment for dry-bag isostatic pressing.
Pressure Generators
• Pressure is generated in the pressure medium through
the use of air-driven and hydraulically driven pumps
and pressure intensifiers.
• The pressure medium typically is oil for dry bag
processes; water containing additives (water-soluble
oil or rust inhibitors) is used for wet bag processes. A
filtering system should be included with all systems to
protect the pressure-generating equipment from
particulate contamination.
•Depressurization Systems. Depressurization can be
accomplished with a single metering valve.
Tooling for Isostatic Compacting :
Cold isostatic pressing tooling is composed of two
parts—the elastomeric mold, or bag, and a mandrel.
Elastomeric molds are made of a variety of
materials; some are flexible, while others are fairly
rigid.
An outside fixture to hold the loaded form is
required if the mold is exceptionally flexible.
A number of factors must be considered in the
selection of a mold material.
Primarily, the mold material must not interact with
either the powder or the pressure medium.
Materials have a range of durabilities.
 Depending on the size of the production run and the
abrasiveness of the powder, a material with the
appropriate wear resistance should be selected.
Mold materials used include natural rubber,
neoprene, urethane, polyvinyl chloride, butyl, nitrile,
and silicone.
(A) suggestions for mould are:
(1) Disposable and reusable mould materials are
recommended.
Disposable mould materials are polystyrene- polyethylene
which provide thin wall envelopes and prevent breakage
of the given compact, but the mould becomes expensive.
Reusable materials are: urethane, silicone rubber,
neoprene, natural rubber and provide (a) greater wall
thickness of the envelope (b) good shape control but
complex parts may break upon pressure release.
(2) The mould material must be sufficiently flexible (high
elastic deformation) with low permanent set to
accommodate the volume changes during compaction.
(3) It must have a sufficient hardness and high
resistance to abrasive wear. High hardness also
means no penetration of the material between
powder particles and hence a good surface finish of
the compacted part. But in case of too high a
hardness, compressibility is poor and intricate
shapes are difficult to fabricate. A hardness between
65 and 95 shore A is used depending on the powder
material and part shape.
(4) Flexible thin walled envelopes may deform when
filled with powder and support must be provided to
control the compact shape e.g. container with holes
is used or use of fasteners is made. Fig. 1.
(5) Auxiliary tooling elements are:
(a) a vacuum fan behind the support to pull the elastic
mould exactly in place during filling. For mixed
mould processing the same action can be carried
out by a reversed pumping action on the pressure
medium.
(b) a vacuum fan as reversed pumping can also be
used to open up the mould for stripping compacted
parts.
(c) a vacuum connection to the filled area to de-air the
filled mould before the compacting operation.
(d) sealing support and clamping elements are also
used.
If internal shapes or cavities are desired in the
compacted part, a rigid mandrel can be used in the
tooling.
The mandrel usually is made of hardenable steel. It
should have a very smooth surface finish to
encourage part release. Complex shapes with
reasonable tolerances can be achieved with the use
of a mandrel.
B) Suggestions for Equipment are as under;
1) Direct and indirect compression devices are used.
2) Pressure vessel material recommended is forged
medium carbon steel, alloyed and heat treated.
3) Pressure vessels which have cover plates both on
top and on the bottom of the mould may be used so
that the powder can be introduced at the top and the
compact ejected at the bottom.
4) Specialized equipment - the ROTOPRESS - 10 to
15 pieces per minute production units presently are
being designed with 150 cm dia and maximum
pressure 2000-3000 atmospheres or 60 cm dia and
maximum pressure 10,000 - 25,000 atmospheres.
5) Through the use of automated equipment, it is
possible to achieve
(a) careful processing (including particle size control),
(b) avoid contamination of the powders,
(c) maintain uniform temperature of the pressurizing
liquid and of the powders,
(d) control of humidity of powders, pressure and
compacting time and
(e) ultimate excellent dimensional control.
Applications of Cold Isostatic Compaction:
These include
(i) Titanium e.g. hydraulic aircraft fittings using Ti powder
and Al-V master alloy powder;
(ii) High-speed tool steels (e.g. cutting tools);
(iii) Compacts with interior threads;
(iv) Long hollow cylindrical filters (stainless steel and
titanium powder);
(v) Large shapes from tungsten powder such as rocket nozzle;
(vi) Tungsten and molybdenum blooms or slabs for further
forging and/or rolling;
Cross-section of Molding Bag
Body Armor
Bag and Molding Core Pin
Bag and Pressing Mandrel
A Sample of Bags
Cross-section of Bag
cold isostatic press for ceramic
Tubes Made by Isostatic Pressing
􀁺Isopressing Mold and Mandrel
hot isostatic press