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This simple equation tells you that if you apply a given force to half the area, Read more about pressure, temperature, and how molecules behave Photo: Scientific autoclaving: US Navy engineers load an autoclave with a. GSBME Autoclave Training Program. This presentation Describe the mechanisms of autoclave sterilisation Temperatures of °C at set pressure for. PHARMACEUTICAL ENGINEERING NOVEMBER/DECEMBER facilities and equipment steam autoclaves. utes and a sterilization temperature of °C (°F) is used. A typical relationship values are shown in saturated steam tables A typical cycle pressure in an autoclave is limited by the specifications .
The purchase price of steam coil heating is roughly comparable to that of electric heating, but the operating cost is dramatically lower.
If high pressure steam is not available, consider a small dedicated boiler for the autoclave. The cost can be surprisingly low, making this alternative nearly as economical as direct gas-firing of an internal heat exchanger. It also enables you to run your autoclave on natural gas, propane, butane, or fuel oil, sometimes interchangeably if the boiler is set up for dual-fuel operation. Where gas supplies are susceptible to interruption, using a small high-pressure steam boiler to run the autoclave and ovens can be a life-saver when dual-fuel firing is incorporated.
A small vertical boiler requires little floor space. If local laws require the licensing of high-pressure boiler operators, this can often be a simple matter of training existing plant personnel and having them licensed for single-boiler operation. Equally economical to operate is an autoclave with a gas-fired heat exchanger built into the pressure vessel.
Although this presents some design limitations, it is simpler than using synthetic heat transfer fluidsand of somewhat lower cost. The gas burner assembly is fitted to the far end or the side of the vessel and fires into a heat exchanger inside the air duct. The hot end of the replaceable tube is covered with turbulators for better heat transfer.
This recovers the greatest part of the energy of the flue gas. It is simple and reliable, using ordinary natural gasbutanepropaneor other industrial fuel gas. There are alternative configurations, including a secondary circulating loop which ducts a portion of the primary air flow through an external pressurized heat exchanger.
This bypass flow can also be utilized for cool-down using an air-over heat exchanger. While gas firing does not readily lend itself to small machines, it can be fitted to autoclaves of three to four foot diameter and up. The longer the machine, the longer the heat exchanger tube and thus the more efficient it will be. This heating option is less costly than hot oil and more costly than electric or steam assuming an existing boiler to purchase, but the extra expense is paid back very quickly.
Over its full service life, the electrically heated autoclave will cost enough to have paid for another four or five comparable autoclaves. For any but the smallest lab machines, gas firing and steam heating are, to put it plainly, the best alternatives to consider. In some circumstances, when steam is available in the plant, considerable money may be saved by using live steam injection.
In this approach, the entire interior of the autoclave is filled with live steam at the appropriate pressure. Commonly used in the rubber products industries, this can be adapted to use in curing composites.
It requires different vacuum bagging materials but has the advantage of eliminating heaters, ducts, and the circulation fan. With external insulation, there is more room available for workloads, for a given size of pressure vessel. Naturally, this approach presupposes the availability of an appropriately rated boiler. In certain applications, a low-pressure steam autoclave can replace an ordinary curing oven.
The combination of vacuum consolidation, which is equivalent to approximately ten to fourteen psi external pressure, and steam at about the same gauge pressure, will give better results and faster heat-up than the oven would. This approach would be less suitable for materials that have to be brought to curing temperature slowly, since steam transfers its heat fairly quickly compared to even a turbulent circulating air flow.
Furthermore, since the interior of the vessel is repeatedly exposed to steam and then air, over and over again, an allowance must be made for corrosion of the vessel walls. This has both advantages - gas or oil can be used as a fuel without much concern for the space occupied inside the autoclave working volume - and disadvantages - the cost is very high, and it can be trickier to maintain properly.
Additionally, it can serve to heat and cool the autoclave by routing the heat transfer fluid through either the heater or the cooling coil, as required by the process. Taking all things into account, the most cost-effective heating options, over the full service life of the autoclave, will be either a high-pressure steam boiler or gas-firing using an internal or external heat exchanger. Cooling[ edit ] Cool-down at the end of the process cycle requires a means of extracting heat from the autoclave.
The necessity of controlled cool-down will itself depend upon the work being processed. With some composite materials in thick lay-ups, slow cooling prevents internal microcracking of the resin matrix resulting from thermally induced stresses. The cooling method used will depend upon the highest temperature reached before cool-down and the degree of precision that must be maintained as the chamber temperature ramps down.
For low temperatures and cool-down rates that can be allowed to vary significantly or simply cool-down at any rate results from a fixed flow of coolantwater circulated through a coil in the airstream will be effective and inexpensive. A serpentine finned coil placed at the circulating fan input or adjacent to the heater array serves this purpose using plant water as a coolant.
If once-through cooling water and a cooling tower are not available or are not acceptable, then a simple closed loop cooler can be built into the autoclave. Precisely controlled cool-down rates may or may not be easily attainable with this arrangement. In an autoclave operated at high temperatures, special precautions must be taken in cooling. It also makes it difficult to control the cool-down rate. Disposal of the steam and the hot water may prove to be difficult, and the service life of the cooling system may be short.
This can be alleviated to some extent by first precooling the coil with a flow of compressed air followed by a mist of water and compressed air. This is only marginally better than cold water by itself, and it does little to eliminate flash steam. Passing the hot oil through an air heating coil with controlled air flow will enable the system to modulate heat flow out of the autoclave precisely enough to maintain a specified cool-down ramp rate.
This heated air can be dumped wherever it will result in the most good or the least harm.
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The drawback to the use of fluid heat transfer is initial cost. It adds perhaps ten percent to the price of the autoclave in mid-size range installations. Fluid life is estimated to range from five to fifteen years, depending upon length of exposure to the highest temperatures as well as the maintenance of internal cleanliness. As this chemical is a constituent of food products, such as ice cream, there need be no toxicity concerns.
It has approximately the same specific gravity as water, so pumping is straightforward. Since there is no phase change, coils don't build up scale deposits.
Fluid life is very good if air is kept out of the loop. Propylene glycol must be used without water dilution, and stainless steel plumbing is not necessarily mandatory. The cost of propylene glycol is not trivial, so the amount of coolant in the loop has to be balanced between the interests of economy and heat dissipation.
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It once unpleasantly surprised one large autoclave customer to learn that closed-loop water cooling systems were strictly regulated in his locale. The price tag on this nasty surprise was in the five-figure range.
In some localities, dumping pristine and clean cooling water down the drain may be illegal. In general, not using water for cooling can have a number of real advantages. Circulation[ edit ] External motor drive on small autoclave, shaft seal, during construction. Unless the autoclave uses steam injection, the circulation fan carries the burden of assuring temperature uniformity throughout the working chamber. Since the heat flows from the source, whether electric resistance, steam coil, or firing tube, into the circulating air stream and then into the workload, the greater the airflow turbulencethe better the heat transfer, particularly with workloads that are heavy and dense.
The fan drive must be sized for the conditions creating the greatest load on the fani. Ideally, this means backward-incline fans; these are more efficient than radial impeller and forward-curve types. The purpose of circulating the air or inert gas through the autoclave is to assure effective heat transfer and temperature uniformity. Vigorous circulation and careful attention to where the airflow actually goes are the best ways of accomplishing this.
As a rough rule of thumb, do not consider less than feet per minute average air speed through the empty workspace of the autoclave. More than this will make heat transfer more effective. The aircraft industry has specifications relating directly to temperature uniformity. Even if the application is non-aerospace, one of these specifications may be worth adopting to assure process quality and reliability. Fan drives can be internal or external. Internal drives have the motor inside the autoclave in an unheated chamber.
A thick wall of insulation keeps the heat out, and the motor is under full autoclave pressure. External drives require a shaft seal to carry the drive shaft through the pressure vessel wall. Internal drives are simpler, result in slightly less floor space taken, and impose a small but essential cooling load; external drives require more elaborate drive shaft arrangements and use high pressure seals. The service life of the high-pressure shaft seal can be difficult to predict, and it may safely be assumed that the seal will cost a good deal more than the motor itself.
Access to the motor room on an autoclave with an internal fan drive is through the back door or a manway. The autoclave working space is not reduced, as the pressure vessel is made slightly longer to accommodate the fan drive.
The accessibility of the hardware in the back is of the essence. Eventually, the maintenance personnel will need to get at it, and access then suddenly becomes very much an issue. Although this does add to the initial price, well-designed autoclaves feature removable back ends which provide easy and unrestricted access to the hardware in the unheated area.
It is difficult to realize how valuable this is until it is suddenly necessary to remove a sixty-horsepower motor that weighs well over half a ton through an opening just barely large enough for it to pass through. Some autoclaves have the circulation fan, complete with motor, mounted in an end-bell of reduced diameter.
While this allows the assembly to be removed easily, it also means that the fan is undersized in diameter and thus less efficient. If the fan impeller is mounted on the motor shaft or an extension of it direct drivethen the fan speed is constrained to that of the motor, typically rpm, and that will most likely result in suboptimal fan operation, given the sensitivity of fans to rotational speed.
Fans are like airplane propellers; the larger they are and the more slowly they turn, the better they are. Some applications allow doing without the circulation fan and air heating altogether.
If the parts being processed are fairly simply geometrically, it may be feasible to use molds which are themselves integrally heated. For example, it is feasible to manufacture flat and single-curvature light aircraft landing gear springs on inexpensive aluminum molds with electric heating pads bonded directly to the bottom of the mold. This eliminates the cost of the motor and fan as well as the air heaters and uses much less electricity than a comparable electric autoclave would require.
In this way, the autoclave provides consolidation pressure alone.KC Controls: How can Autoclave Tubing benefit you?
There are limitations to this approach, such as mold complexity. Sometimes, the parts are heated from one side only; sometimes, the mold has top and bottom parts, each fitted with heaters. Although not usually feasible in a job shop, this type of autoclave can afford significant savings when only a small range of comparatively simple parts is being made. Since failure of the circulating fan will have immediate and unhappy consequences for the heat exchanger or heating elements, detection of circulation fan failure is vital.
This can be done in several ways. First, monitor the surface temperature of the heater, whether coil or element. If the airflow fails, this will rise suddenly, and the control system can execute an immediate shut-off. Second, install at least one and preferably two airflow sensors. Since the airflow may be at very high temperatures, this can be done with remotely mounted pressure switches that connect to the high and low pressure sides of the fan by means of stainless steel tubing long enough to put the switches well inside the cool area of the inside of the vessel.
These switches should be wired in series on one side for the control circuit, such that either one opening will disrupt the heater power, and in parallel on the other side so that the computer can detect which one has changed state.
The substantial mass of the pressure vessel provides assurance of pressure containment, but it represents an equally massive heat sink which must be heated and cooled cyclically as the autoclave runs. Steam autoclaves are necessarily insulated on the exterior, making this heat loss unavoidable. Autoclaves using air or another gas employ thermal insulation on the interior, and this incurs a one-time penalty in the cost of the pressure vessel and a slight operating cost resulting from the somewhat greater internal volume to be pressurized.
The insulation, which is protected behind a metal shell, is sized to keep the heat loss within an acceptable range and to keep the temperature of the outside surface of the vessel below that which would affect worker safety. Depending upon company policy on energy conservation, this temperature may be set even lower.
Both mineral wool and fibreglass are used in autoclaves. The thickness varies with internal temperature, ranging from a bare minimum of two to three inches up to three to four times that, the rough rule of thumb being one inch per hundred degrees F. Economically, the biggest effect is to raise the pressure vessel cost by increasing its diameter. This effectively discourages over-specifying the insulation thickness. One minor factor is ensuring that the insulation can "breathe", since air flows into and out of it as the pressure in the autoclave changes.
Additionally, the sheet metal retaining the insulation requires some provision for thermal expansion. Even an autoclave as short as twenty feet experiences considerable movement across a temperature difference of several hundred degrees.
Pressurization[ edit ] The choice of pressurizing agent is driven by the process. Air may be acceptable for autoclaves operating at comparatively low temperatures,  but it may be wholly unacceptable beyond that. The flammability of the materials often used in composite parts increases under pressure, as the partial pressure of oxygen rises.
Thus, nitrogen or carbon dioxide may be used for pressurization. Hydroclaves use water as the pressurizing medium. Since the boiling point of water rises with pressure, the hydroclave can attain high temperatures without generating steam. While simple in principle, this brings complications. Substantial pumping capacity is needed, since even the slight compressibility of water means that the pressurization stores non-trivial energy. Seals that work reliably against air or another gas fail to work well with extremely hot water.
Leaks behave differently in hydroclaves, as the leaking water flashes into steam, and this continues for as long as water remains in the vessel. For these and other reasons, very few manufacturers will consider making hydroclaves, and the prices of such machines reflect this. Vacuum[ edit ] Parts processed in an autoclave are often vacuum bagged to enable the pressure to operate isostatically on the workpieces.
In simplest form, the workload is fully contained inside a loosely fitting bag made of resilient plastic capable of withstanding the temperatures involved. When vacuum is drawn, the bag is compressed by atmospheric pressure and compacts the components inside.
Between the parts and the bag, an absorbent material provides a channel for the evacuation of the air and wicks up the excess resin squeezed out during curing. In autoclave processing of composite parts, the vacuum bag functionality may be where the greatest variety may be found. Some shops will leave the bag under full vacuum from lay-up to post-cure tear-down. Others will hold the vacuum only until the autoclave reaches full pressure.
Yet others will back-fill the vacuum bag with inert gastypically nitrogen, at zero pressure. The role played by the vacuum bag's internal pressure can be critical in the consistent production of high-quality multi-ply composite parts. A SAMPE paper  describes the benefits of controlling the vacuum and pressure under the vacuum bag in a lay-up. By following the vacuum in the bag with pressurization,  the formation of voids in the resin matrix is suppressed, reducing the microscopic flaws which seed cracks and other matrix failures.
Installing this capability on an autoclave involves additional control valving and software, and, in light of the benefits in performance of the composite materials, does not have an unreasonable effect on machine price. To eliminate oxygen from the interior and replace it with a pressurizing agent that does not support combustion, the simplest way is to remove nearly all of the air and then introduce the nitrogen or carbon dioxide. It should also be noted that autoclaves have to be designed for such vacuum service, since the vessel itself may require stiffeners to withstand the external pressure, and ordinary access doors and man-way covers often are rated for internal pressure only and will not be able to withstand the external pressure that results from interior vacuum.
The vacuum is delivered to the work-pieces by manifolds and individual tubes fitted with quick-disconnects on the interior walls. The simplest vacuum system consists of a pump and gauge on the outside and a single quick-disconnect port on the inside. In more elaborate arrangements, there may be a dozen or more individual vacuum supply lines going into the machine, each to a separate QD port, with as many gauge lines coming back out to vacuum sensors wired to the control system, and an inert gas backfill under controlled pressure for when the pump is shut off and the vacuum lines vented during the cure cycle.
The flow capacity of the vacuum pump is less important than its peak vacuum level. Any significant flow means that there is a leak in the vacuum bagging, and a powerful pump will try to overcome this, resulting in a smaller measured vacuum loss than would be the case with a smaller pump. This would serve to hide the bag failure, because the vacuum drop would be harder to detect. Since at operating equilibrium there should be no air flow at all, it is better to draw a higher vacuum than to try to maintain a high flow rate.
There should be a properly sized vacuum receiver tank which can maintain system vacuum if the pump fails during a production run. This will be feasible only if the vacuum plumbing is virtually leak-free.
This is why vacuum leakage is one of the major concerns in the pre-acceptance testing of the machine. Note that this receiver must be ASME stamped for the full operating pressure of the autoclave, since it is conceivable that a vacuum system failure could result in this much pressure being vented into the receiver. When multiple parts are processed, it may be beneficial to have a separate vacuum line for each, reducing the potential loss if one vacuum bag leaks during cure.
This is easily accommodated with multiple supply tubes from a single manifold on the outside of the autoclave. Additionally, it is necessary to decide whether each line is to be monitored individually and how this is to be done. If there is a single vacuum sensor, whether an electronic transducer connected to the control computer or only a sensitive gauge checked visually, determining which vacuum bag is leaking means shutting each one off in turn and watching for a slight change in vacuum manifold pressure.
Given the airflow restrictions in the average lay-up and vacuum lines, even a complete vacuum bag failure may show up as little more than a small change in vacuum level. Putting a sensor in each vacuum line takes care of this, but costs an additional several hundred dollars per line. Some applications involve two vacuum lines per bag. One is connected to a bag penetration at one end of the part being processed and supplies vacuum from the pump and manifold.
The other line returns from a separate bag penetration at the other end of the vacuum bag and through a line back through the vessel wall to a sensor which measures the vacuum level in the bag rather than the level at the manifold.
This is preferred, since it provides an accurate indication of the real vacuum seen by the part as a whole. Individual monitoring of each vacuum line does not necessarily require gauge lines, nor does inert gas backfill. However, the extra cost of providing for gauge lines is not much. If vacuum bag pressure control is used, then the appropriate valves and regulators must be built in.
If the process generates considerable resin flow out of the workpiece, the process specification may demand resin traps. Some materials lose enormous amounts of highly mobile resin during heat-up, and this flow can sometimes work its way back through the vacuum plumbing perhaps far enough to plug critical components. It is far easier to prevent this sort of damage than it is to repair it. Some resins, such as polyestersgive off significant amounts of volatiles during cure.
These will carry out through the vacuum ports and sometimes cause damage to the pump. The better vacuum pumps use oil reservoirs and oil recirculation, and these volatiles can quickly turn the oil into a revolting mush.
They also attack the seals of vacuum valves and cause deposits to build up over time. To prevent this, it may be necessary to install condensers on the vacuum port lines.
A small chiller will add several thousand dollars to the cost of the autoclave, plus another several hundred dollars per port for the condenser and separator. The stainless steel resin traps should be designed and made to be easily disassembled and cleaned. Naturally, they must also be fully accessible.
Controls and instrumentation[ edit ] While much of the operation of a simple autoclave can remain manual, temperature control is virtually always automated, as this is easily done at low cost. The hardware and software available for industrial process automation makes fully automatic operation of an autoclave affordable and reliable. It is realistic to design and implement such automation without the services of an outside vendor in many cases.
Temperature[ edit ] As with the other parameters, the required precision of temperature control depends upon the process specification.
The autoclave should exceed this capability by a margin sufficient to preclude all chances of inadequate or excessive temperatures in the workload.
Too hot and the parts can be damaged or undergo thermal excursion;  too cold and the full structural properties may not be realized. Equally vital is the avoidance of variation in temperature throughout the working volume of the autoclave.
Aerospace specifications include maximum allowable variation as well as how to test for uniformity. This kind of precision can be achieved with indirect gas-fired heating, but not as easily. The electric power drawn by the heating elements can be controlled to bit  precision by SCR devices driven by analogue signal from the temperature controller. The low mass of the heating element makes it responsive, and sudden and dramatic changes in output - although not generally necessary - can be achieved.
The need for accuracy and precision  in the measurement of the air temperature inside the autoclave places importance upon the selection and implementation of the sensor. The cheapest and easiest is a single thermocouple placed somewhere in the airstream. For better results at a trifling price, two or three averaged RTDs work better, with higher precision and less drift.
While RTDs will respond to sudden temperature changes less quickly than thermocouples will, this does not matter, since sudden temperature changes in autoclaves simply do not happen. A third can be placed close to the middle. Sensing air temperature close to a wall surface will usually incur a boundary layer error, or, worse, a stagnation error.
Pressure[ edit ] Control of pressure presents the fewest challenges. Given a source of air or gas of sufficient pressure and flow capacity, the autoclave control system opens the pressurization valve and shuts it once the internal pressure has reached the setpoint. Depressurization occurs when the dump valve is opened. On large autoclaves, a silencer or muffler may be needed.
As the temperature rises, the gas expands, driving the pressure upward. A trim valve releases the excess, maintaining the setpoint. In some applications, the precision of pressure control directly bears upon the success of the process.
For instance, core materials have limited compressive strength at elevated temperatures; even a small over-pressure can collapse the core  and ruin the workload. In a poorly designed autoclave, oscillation of the pressure can result in chattering of the inlet and outlet valves. One means of preventing this is utilizing large valves for filling and dumping and small valves for trimming at and near the setpoint.
Alternatively, modulating valves will avoid this phenomenon. Vacuum[ edit ] Often the least controlled factor in an autoclave, the vacuum may or may not require modulation. In some instances, it is not automated at all and involves little more than a connection to the plant vacuum system, a few manual valves, and a gauge.
At the other extreme, the vacuum control system may be considerably more complex than that of the air temperature. Safety assurance[ edit ] Safety is always a concern with autoclaves. The ASME code is extremely conservative; as a result, pressure vessels are among the safest, least risky types of machine in use today.
However, this doesn't mean that safety can be taken for granted. While this triggering of the valve will relieve any possible overpressure in the vessel, it must also be able to keep the pressure source, whatever it is, from pushing enough air, inert gas, or steam in to bring the pressure back up to an unsafe level even with the safety valve wide open.
The valves are mounted on a manifold that allows multiple pressure vessel outlets to feed multiple safety valves, each one of which can handle the entire air dump by itself, even if one pressure vessel outlet is accidentally blocked by debris from an internal failure. The added cost of the redundant safety valves is approximately one tenth of one percent of the machine price. Air or nitrogen from the source of pressurization is not the only potential cause of sudden over-pressure. An autoclave fire is guaranteed to raise the internal pressure, and this may exceed the safety valve's ability to vent fast enough.
The solution is oversized safety valves and rupture disksand more than a single one of each. Doing this right depends upon providing the correct data to the autoclave manufacturer. This matching of the safety relief valving to the plant compressed air capacity is an example of how the autoclave is regarded not as an isolated entity but as an integral part of the plant in which it is sited and operated.
Composite parts and the materials used in their curing are often flammable, even if not readily so at room temperature and atmospheric pressure. The elevated temperatures and pressures involved in curing increase the risks of potential combustibility. The risk may also be too high in extremely large and expensive cure loads.
In such applications, nitrogen can be used, since it is both inert in that it normally won't support combustion and readily available. In bulk liquid form, it costs less than soft drinks. The simplest and most cost-effective safety device is the rupture disk.
Incorporated into the pressure vessel in fabrication, this is simply a port in the vessel closed off by a finely machined plate that will burst at a predetermined pressure. This plate may be made of either aluminum or carbon. The disk is fairly inexpensive and can be replaced easily. The rupture disk should be used to back up the safety valve and sized to drop internal pressure as quickly as possible.
Autoclave fires can release considerable energy into the air inside, resulting in a sudden pressure spike. The rupture disk is designed to release at a pressure slightly above that of the ASME safety valve and well below that of the hydrostatic test; it is never called into play unless there is a sudden pressure increase beyond the capacity of the safety valve.
The very modest cost of even a pair of fairly large rupture disks makes this an extremely attractive option. The rupture disk should be not less than twice the diameter of the inlet to or the outlet from the vessel, whichever is larger. The type of door will determine whether it needs its own safety device. Every autoclave will have this much; it's the absolute least that is legal. However, a prudent autoclave operator ought not to be willing to settle for the very least that is required.
Even the best made components are not perfect, so the conservatively designed autoclave uses backup interlocks in both hardware and control software to reduce risk to the lowest level reasonably attainable. For example, if the control system senses any pressure in the vessel, it blocks the door opening cylinders with fail-safe valves, thus precluding any attempt to bypass the safety interlock manually. This is in addition to the code-mandated interlock. If desired, an additional interlock can be installed on a T-bolt door, too.
Another safety consideration is how sensors are wired. If a device is capable of failing in a particular state, then the failure should be such that a false indication of pressure is given. This is far better than a false indication of no pressure. Nevertheless, checking the pressure as a condition to opening an autoclave is simply not safe enough. These steps usually follow a strict procedure because they have been found to affect the final properties of the composite [ 15 ].
Similar to other composite manufacturing techniques, the prepreg materials are stacked or laid-up to yield a laminate of the required dimensions. The entire lay-up including prepreg plies, release film, and breather cloth is then covered with a vacuum bag as shown in Figure 1. Since the pressures in the bag and autoclave are different, a compacting force that corresponds to this pressure difference is exerted on the laminate.
Through this feature and the flexible characteristics of the vacuum bag, the pressure is always acting normally to the laminate surface, which leads to a uniform pressure distribution. This uniform pressure distribution, which is applied continuously throughout cure, reduces the risk of void formation, especially when dealing with thermosetting prepreg materials [ 16 — 21 ]. Autoclave entire lay-up [ 14 ].
After completion of the vacuum bag, the laminate can be autoclaved using a certain temperature and pressure profile. In composite fabrication, the autoclave is usually pressurized with nitrogen so that fire hazards imposed by the exothermic prepreg materials are reduced [ 22 ].
Heat is applied using heat exchangers in conjunction with fans that assist heat transfer [ 31423 ]. Generally for autoclaves, the usual processing procedure for thermosetting prepreg curing is to firstly increase pressure and right afterwards heating the autoclave at a chosen heating rate to the desired temperature. According to most fluids behaviour, the viscosity of the matrix will decrease with this temperature rise. As a result, at elevated temperature, the resin will freely flow, facilitating the consolidation process until eventually the chemical cross-linking starts occurring and forming gelation.
At this point, the resin will soon change from a liquid into a solid, and it will start preventing viscous flow. It is, therefore, important that the chemorheology of the prepreg resins is known in order to complete consolidation and volatile removal prior to resin gelation. A typical autoclave cycle containing two isothermal dwells is shown in Figure 2. The first dwell is performed to prolong the time range for consolidation and eventually to prereact the matrix to reduce the risks of large exotherms.
The second dwell is the actual cure step. The application of pressure assists the consolidation and helps suppress voids in the laminates. Typical diagram of temperature and pressure profile for curing in a clave [ 14 ].
Nevertheless, high pressures might eventually drive too much resin out of the fiber bed leading again to void formation, resulting from resin starvation this time. Even though much research has focused on the autoclave consolidation and processing optimization, the developed models are of limited use. So, many variables such as prepreg type, part dimensions, vacuum bag materials, lay-up, and processing conditions influence the consolidation that the composites industry has determined almost all operating conditions by trial and error.
Consequently, the aerospace industry has tried to employ standardized procedures. In addition, one distinguishes between high- intermediate- and low-flow prepreg systems.
Considering this information, the lay-up can be adjusted such that the optimum part qualities can be accomplished [ 31415181923 ]. Pressurized Vessel The repair clave Figure 3 is a semiportable pressure vessel, designed to provide controlled temperature, vacuum, and pressure for repair of composite and metal-bonded parts that require higher pressures than can be achieved by vacuum only.
Unlike a conventional autoclave, there are no internal heating elements in the vessel; so, the heat for curing is derived from heat blankets or other heating elements.
This study focuses mainly on the cures that are done with the help of heat blankets. Although the main difference between a repair clave and an autoclave comes from the fact that one is mainly dedicated to repairs of composite parts while the other for manufacturing, it is believed that a repair clave could become a useful tool in manufacturing simple parts at a much more affordable cost.
Currently, autoclaves could be used for repairs also, if extensive rebuild of a part is needed. The repair clave can be referred to as an affordable autoclave, as it reaches almost the same pressures, but it permits the use of localized heat which results in much more economical cures.
However, one needs to investigate if the quality of a repaired part can be maintained at the standards of a part manufactured through an autoclave. There are of course many differences that put the two claves apart, but by understanding these differences, one can see at which common point the repair clave might be used as a possible future autoclave for manufacturing simple composite parts Table 1.
Repair clave and autoclave advantages and disadvantages [ 142829 ]. Experimental In order to investigate the role of pressure during manufacturing, when it is applied separately from the temperature, and how it affects the final material, four different plates were manufactured under four different process pressures.
The four different pressures were 0, 30, 50, and 70 psi. Each panel had eight plies of woven carbon fiber prepreg type: Dimensions of the panels were inches with an approximate thickness of 1 mm. After being manufactured, the panels were cut to appropriate samples for dynamic mechanical analysis and differential scanning calorimetry analyses.
They were cut to the desired dimensions and orientation. After that, the lay-up took place. The plies were placed in the desired configuration, and on top of them, all the layers described in Table 2 were applied. Lay-up before curing in repair clave [ 14 ]. When the desired vacuum is accomplished, the laid-up panel enters the vessel. The vessel is sealed, and the pressure can be adjusted to the desired level.
Since the pressure is at the level required by the experiment, the temperature profile by controlling the heat blanket initiates curing. All the measurement parameters can be controlled by a data acquisition system attached to the vessel. After two hours in the isothermal phase, the temperature starts reducing with a rate of 2. The pressure is released, and the panel is ready after removing all the lay-up extra layers. Three different temperature elevating rates were used, 0. Hence, eventually, experiments of the 4 different panels manufactured at 0, 30, 50, and 70 psi were tested in 3 different rates, that is, 0.
Scanning Electron Microscopy SEM Characterization The manufactured samples are also characterized by scanning electron microscopy, through visual examination of surfaces resulting after manufacturing. Samples were cut in dimensions of mm3 so that the cross-section mm2 can be observed in the microscope. Although carbon fibers are conductive, the matrix reduces the conductivity for SEM observation, and for this reason, higher conductivity was achieved through low-vacuum sputter coating with gold.
Samples are exposed to an electron beam, which comes through a tungsten heated filament electron gun. The samples were tested in 3-point bending experiments.
The thickness was approximately 1. Flexural Mechanical Testing The specimens were tested in a screw Instron machine model More than five specimens were tested for each of the four different CFRP panels of 0, 30, 50, and 70 psi manufactured.
Specimens of mm2 were cut from the CFRP panels the length mentioned is the gauge length. A 5 kN load cell was utilized to perform these experiments. Compression Testing Furthermore, specimens from the panels were also tested under compression testing in the same Instron machine that flexural testing took place. The thickness is again proportional to the psi that the CFRP panel was manufactured.
The crosshead of the machine in the compression test had a speed of 1. Results and Discussion 5. Differential Scanning Calorimetry At first, uncured samples of the prepreg with which the panels were manufactured were tested in DSC. Figure 4 shows the DSC scans for all uncured samples at 0. It can be easily noticed that with an increasing heating rate, the area under the peaks, which by integral represents the heat of the exothermic reaction needed for the cure, is increasing as well.
Thus, the higher the heating rate, the higher the heat of cure [ 24 — 26 ]. This happens due to the fact that the macromolecules of the polymer need more time to respond to the temperature elevation when the later increases rapidly, due to the curing kinetics of the thermoset.