Cold isostatic pressing
General principle of cold isostatic pressing (CIP)
The cold isostatic pressing (CIP) is based on the effect that a pressure applied to a liquid spreads evenly in all directions (so-called Pascal's principle). Inside the liquid, this leads to a homogeneous pressure distribution, which can be used for shaping ceramic and metallic powders if the powder is encapsulated in an elastic die. Elastic polymers (elastomers) such as polyurethane (PU), polyvinyl chloride (PVC), synthetic or natural rubber have proven successful as materials for the elastic die. The main components of a cold isostatic press are the recipient filled with the pressure transmission medium, the high-pressure pump, the locking system, the overpressure protection and the system control. The recipient is a pressure cylinder, which consists of either a high-strength, thick-walled steel cylinder or a thin-walled steel vessel with a pre-tensioned wire winding. Figure 1 shows a cold isostatic press in lab scale.
The pressure transmission media used are preferably water with the addition of an anti-corrosion agent, oil-water emulsions or glycerine. A hydraulic unit like an axial piston pump is used to build up pressure. This unit is connected to the recipient via a fluid transport system with appropriate valves and throttles. The pressure cylinder is sealed, for example, by a polymer O-ring that is integrated into the cover plate and is pressed into a groove when the pressure is built up. In order to absorb the large forces in the axial direction, the cover plate of the recipient is usually supported by a massive yoke. A rupture disc is used to protect against overpressure, which fails when the critical pressure is exceeded and thus enables a regulated pressure release before irreversible damage to the recipient occurs. Cold isostatic pressing is carried out in most cases at room temperature. In specially designed presses, moderate heating of the pressure medium up to a maximum temperature of 300 °C is possible.
In technical practice, a distinction is made between two basic principles of cold isostatic pressing, the wet bag process and the dry bag process. In addition, the wet bag process can also be used for the post-compaction of components uniaxial pre-pressed in a rigid form. The three variants are explained in more detail below:
i.) Wet bag process: In the wet bag process, the die is a self-supporting, elastic hollow mold that is filled with the powder outside the cold isostatic press. Optionally, a metallic mandrel can be integrated into the hollow mold for the production of pipes. After filling, the die is closed with an elastic cover plate. In order to protect the interior of the die against the ingress of liquid, the die can also be enveloped with an elastic foil, which is then evacuated and sealed in a liquid-tight manner. The wet bag process is often used in research and development due to its great flexibility with regard to the system parameters (including pressures up to approx. 400 MPa), the component geometries and the number of components per cycle. It is also used in industry for the production of large-volume components (e.g. filter candles) and semi-finished products (e.g. for manufacturing of ceramic insulators) with dimensions up to the meter range. Disadvantages of the wet bag process are the limited dimensional accuracy due to the elastic die and the comparatively long cycle times of several minutes.
ii.) Dry bag process: In dry bag pressing, the elastic die is firmly fixed to the recipient. Correspondingly, the mold filling takes place inside the cold isostatic press. The main advantages compared to wet bag pressing are the possibility of full automation, fast cycle times in the range of 10 s - 100 s depending on the component size (increase in cycle time with component size) and better dimensional accuracy. However, it should be noted that in the area of fixing the elastic die to the recipient there are restrictions with regard to the homogeneity of the pressure distribution, which, however, are less relevant for simpler geometries such as plates, cylinders or tubes. Dry die pressing is preferred in industrial production. The best-known application example are spark plug insulators, which are manufactured in a fully automated manner in quantities of millions.
iii.) Post-compaction of uniaxially pressed components: The shaping of ceramic and metallic powders via the uniaxial pressing in rigid molds is restricted by the wall friction, which leads to increasing density gradients in the component with increasing height/diameter ratio. The result is an inhomogeneous shrinkage during the subsequent sintering. In order to use the advantages of uniaxial pressing in terms of dimensional accuracy and at the same time to achieve a homogeneous density distribution in the component, it is possible to pre-press components uniaxially in a rigid die at moderate pressures, e.g. in the range of 50 - 100 MPa, and then post-compact them using cold isostatic pressing, which can be done e.g. at pressures in the range of 300 - 400 MPa. For the pressure transfer, the component must be liquid-tight welded into an elastic foil. When welding the foil, it must made sure that the foil is not damaged by sharp edges or particles.
Figure 1: Cold isostatic press on a laboratory scale operated at the institute IEK-1, Forschungszentrum Jülich (manufacturer EPSI, type CIP 400-125 * 300Y). Usable volume: Diameter 125 mm, height = 300 mm.
Advantages and limitations of cold isostatic pressing
i.) Advantages: The main advantage of cold isostatic pressing is the elimination of friction-related variation in density, such as occurring in the case of uniaxial pressing in rigid molds. Therefore, higher green densities, higher green strengths and more homogeneous structures can be achieved with cold isostatic pressing. For example, press-related textures in the structure can be avoided to a large extent. Furthermore, components with a height / diameter ratio greater than 3: 1 (usually specified as limit for uniaxial pressing) can be produced relatively easily. Because there is no wall friction, it is possible to omit dispense with lubricants during pressing and the required proportion of other pressing additives can be significantly reduced. Furthermore, ceramic feedstocks can be compacted that contain abrasive solid particles that would lead to high tool wear in the case of uniaxial pressing.
ii.) Limits: Cold isostatic pressing is only suitable for relatively simple component geometries and is therefore restricted with respect to near-net-shape production. Due to the elastic properties of the die, there are limitations with regard to dimensional accuracy even with simpler geometries. Furthermore, the surface roughness is usually also relatively high, since the die wall yields elastically during pressing. Furthermore, sharp-edged particles can be pressed into the die wall, whereby the die is damaged in the worst case and demolding is aggravated. In order to increase the dimensional accuracy, mechanical post-processing of the cold isostatically pressed semi-finished products (so-called green machining) is often carried out. The relatively high green strength of cold isostatically pressed semi-finished products has a positive effect here.
How to conduct a cold isostatic pressing cycle
In the following, it is explained in detail which processing steps are carried out in cold isostatic pressing and what must be considered in order to obtain crack-free samples and avoid damage to the system. The processing steps shown were carried out in a cold isostatic laboratory press of the following type:
Type: CIP 400-125*300Y
Manufacturer: EPSI, Temse, Belgium
Material of recipient: SA 723 Class 3
Recipient inner diameter: 125 mm
Recipient inner height: 300 mm
Maximum sample size: Limited by recipient volume
Maximum filling height: 295 mm
Recipient volume: 3.7 Liter
Nominal operating pressure: 4000 bar (400 MPa)
Maximum operating pressure: 4400 bar (440 MPa)
Safety factor: 1,1 (related to 4840 bar, 484 MPa)
i.) Encapsulation of the samples: Even with cold isostatic pressing, a uniform filling of the mold and a high filling density are basic requirements for successful compaction of the powder. In order to estimate the flowability of the powder, it is recommended to measure the bulk and tap density in accordance with DIN ISO 3923 and DIN ISO 3953. The flowability of a powder can be improved by granulating in a fluidizing bed or by spray drying. During granulation, spherical, easily flowable agglomerates are formed, which have a relatively low stability and are destroyed during the cold isostatic pressing. Some powder manufacturers offer powder directly in a granulated state. In order to increase the bulk density in the mold, the elastic die can optionally be vibrated or subjected to ultrasonic treatment. As an example, Figure 2 shows a simple elastic mold made of rubber for cold isostatic pressing of round bars.
Figure 2: Elastic mold (rubber tube with cover plug) for cold isostatic pressing of round bars.
As mentioned above, there is also the possibility of post-compaction of uniaxially pressed semi-finished parts using cold isostatic pressing. Figure 3 shows how the welding of a pre-pressed part into an elastic foil can be carried out. For this purpose, at the IEK-1 institute, a vacuum sealer is used, which is usually applied in the household for sealing and freezing food. Elastic foils, which are established for this purpose, are suitable for encapsulation of pre-pressed parts in cold isostatic pressing as well. The sample is placed in the prepared bag, then interior is evacuated and the bag is finally liquid-tight welded. A more primitive alternative is to put the sample in a latex glove, which is evacuated, for example, by a water jet pump and sealed with a string.
Figure 3: Sealing of a uniaxially pre-pressed sample in an elastic foil using a household vacuum sealer a.) Insertion of the uniaxially pre-pressed sample b.) Evacuation c.) Liquid-tight sealed sample.
ii.) Opening the recipient: Figure 4 shows the individual steps for opening the recipient of the cold isostatic press of the type CIP 400-125 * 300Y (EPSI, Belgium). The press is equipped with a hydraulic lifting device. A screw is attached to this, via which the cover plate is connected to the hydraulic lifting device. The screw is protected from damage by a cage. After the screw has been attached, the cover is lifted using the lifting device. The sealing ring can be seen on the lower edge of the cover plate. This ring is pressed into the groove when the pressure is built up.
iii.) Introducing the sample into the recipient: For safety reasons it is important that the welded sample is not positioned directly in the oil-water emulsion. In the worst case, the sample or parts of the elastic foil can get into the liquid transport system of the cold isostatic press and block it, so that a pressure reduction is no longer possible and a service technician has to be called in. To safely avoid this risk, the sample must be placed in the oil-water emulsion in a specifically designed metal cage shown in Figure 4f. Spacers are welded onto the underpart of the metal cage, which prevent the metal mesh from resting directly on the bottom of the recipient. In this way, an unhindered pressure build-up and release is ensured. The metal cage also makes it easier to remove the sample after the cycle from the oil-water emulsion.
Figure 4: Preparation of the system for the CIP cycle a.) Closed system b.) Attaching the screw to the cover plate c.) Starting the opening process d.) Open system e.) View on the oil-water emulsion f.) Metal cage for placing the encapsulated sample in the CIP device.
iv.) Closing the recipient: After installing the metal cage including the sample in the recipient, the cover plate is closed via the hydraulic lifting system and the screw for fixing the cover plate is removed. The yoke, which absorbs the pressure in the axial direction, is then positioned over the cover plate. A specially designed locking system guarantees the exact position of the yoke in relation to the recipient. The locking bar is manually locked in the pressing position with a lever. Figure 5 shows the individual steps for closing the recipient.
Figure 5: a., b.) Closing the recipient using the hydraulic lifting device c.) Locking the yoke before starting the pressing cycle.
v.) Pressure build-up and dwell time, protection against overpressure: As mentioned at the beginning, the pressure build-up takes place by a high-pressure pump which is connected to the interior of the recipient via the liquid transport system. Figure 6 shows the system components relevant for pressure build-up. The desired pressure and the dwell time at this pressure are programmed via the system control display. The pressure is applied at the speed as defined in the program. The run of the pressure curve and the actual pressure can be read directly on the display and on an analog manometer. The cold isostatic press operated on the IEK-1 is designed for a maximum pressure of 4400 bar (440 MPa). To be on the safe side, the pressure in standard operation of the system should not exceed 4000 bar (400 MPa). The system is equipped with a rupture disc as overpressure protection. The rupture disc protects the system from uncontrolled failure and is designed with a safety factor of 1.1 based on the maximum pressure.
Figure 6: a.) Programming of the pressing process via the display b.) Indication of the pressure via the manometer c.) Rupture disc as protection against overpressure.
vi.) Decompression: Decompression is the most critical step in cold isostatic pressing. If there is still air in the sample after the sample preparation, it is strongly compressed during the CIP cycle. The high compression of the powder compact is associated with a strong reduction in the residual porosity. Accordingly, after pressing, there is a high flow resistance for the compressed air to escape and the pressure release takes a certain amount of time (up to a few minutes, depending on the pressing density achieved). If the pressure is released suddenly, this can lead to failure of the compact by severe cracking. It should be noted that a powder compact is also deformed elastically in addition to plastic deformation. When the pressure is released, there is therefore an elastic rebound of the pressed part ("spring-back" effect), which can be between 0.5 - 2%. Since the compression takes place in an elastic mold, the spring-back is viewed as less critical, but in the worst case it can also lead to damage to the pressed part, e.g. if it sticks to the die wall.
The decompression at the laboratory facility of the IEK-1 institute (type CIP 400-125 * 300Y, EPSI, Belgium) is initiated by opening a pneumatic high-pressure valve. The pressure release can then be regulated either manually via a valve or via an adjustable throttle valve. The throttle valve consists of a high pressure tube in which a needle is inserted. The decompression speed is controlled by the inner diameter of the tube and the position and thickness of the needle. Figure 7 shows the display for initiating decompression and the throttle valve, which is located on the rear of the cold isostatic press.
Figure 7: a.) Display for selecting the decompression mode b.) Throttle valve.
After the pressure has been completely released, the system can be opened as already described under i.). Afterwards, the metal cage together with the sample can be removed. After removing the elastic foil, the sample is ready for use.
Applications of cold isostatic pressing
Cold isostatic pressing is widely used in industry as well as in research and development. One advantage is the great flexibility of the method and the comparatively low costs for the die production. Cold isostatic pressing can be used universally for the compression of oxides, nitrides, borides, graphite, hard metals and other composite materials, as well as for metallic powders. The starting powder should have good compressibility, since the use of high proportions of binder is rather unusual with this method. In general, this method is used to manufacture semi-finished products as well as structural and functional components with moderate geometrical complexity. Specific application examples are spark plug insulators made of alumina, cap insulators, components for lambda sensors, semi-finished products for ceramic insulators and semi-finished products for ceramic balls. Metallic filter cartridges with a length of up to 1.8 m and a diameter of up to 300 mm are also manufactured using this process.