- Slides: 39
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 Ø To get uniform density and compaction. Ø To make intricate shapes. Ø To get objects with great dimensional tolerance. Ø To get homogeneous structure. v. Figure 1 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: • • 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 : v. Cold isostatic pressing tooling is composed of two parts—the elastomeric mold, or bag, and a mandrel. v. Elastomeric molds are made of a variety of materials; some are flexible, while others are fairly rigid. v. An outside fixture to hold the loaded form is required if the mold is exceptionally flexible. v. A number of factors must be considered in the selection of a mold material. v. Primarily, the mold material must not interact with either the powder or the pressure medium.
v. Materials have a range of durabilities. v Depending on the size of the production run and the abrasiveness of the powder, a material with the appropriate wear resistance should be selected. v. 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.
Ø If internal shapes or cavities are desired in the compacted part, a rigid mandrel can be used in the tooling. Ø 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
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