Abstract:
A method and apparatus for filling preforms with resin or slurry includes a tank for holding the slurry and an autoclave for holding the preforms. The tank and autoclave have a common vacuum system connected thereto, and the tank is disposed above the autoclave. This arrangement provides a compact casting system which utilizes a vacuum and gravity to transfer slurry from the tank to the preforms within the autoclave.

Description:
BACKGROUND 
     The present invention is directed to an integrated apparatus and method for filling a porous composite preform with a slurry or resin. 
     The filling of a porous composite preform with particulate slurries or resins has until now been done in separate steps with separated pieces of equipment. As a result, such conventional systems and methodologies have been time consuming and expensive both to operate and construct. 
     SUMMARY OF THE INVENTION 
     The present invention involves the provision of and use of three separate chambers interconnected by one common vacuum system. A tank holding slurry is preferably disposed above an autoclave with the tank and autoclave being connected to the common vacuum system. With this arrangement, slurry can be easily and efficiently transferred between the tank and autoclave by the use of the vacuum and gravity. 
     In addition, since the uninfiltrated slurry/resin remains viable, it can be drained, again by the vacuum and gravity into a holding tank. The slurry/resin in the holding tank can then be pumped up to the feed tank, but since the vacuum level of all tanks are the same, the slurry/resin has not degraded due to atmospheric effects, nor has it gained entrapped air. 
     A similar process is used in the polymer composites industry, whereby polymers are infiltrated into organic fiber or glass fiber preforms. However, a differential vacuum is used to transport polymer from one tank to the other. 
     This invention integrates the operations of 1) slurry/resin preparation, 2) preform preparation, 3) slurry/resin transfer to preform, 4) slurry/resin removal from preform, 5) recycling of slurry/resin, 6) initial drying of slurry/resin. It also allows pressure application during preform filling if required. 
     Aside from the conventional ball mill that will be used to batch the slurry/resin, all operations are performed in different chambers or containers that are connected to the same vacuum system. This allows the transfer of slurry/resin between containers to be done with the use of the vacuum and gravity. When pressure is needed in the filling step, the main chamber will be isolated from the vacuum system and then pressurized. 
     The inventive apparatus and method are especially suited for filling turbine shrouds, turbine engine liners, and for other SiC products. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a slurry/resin casting system in accordance with the preferred embodiment of the present invention; 
     FIG. 2 shows in block schematic form the main components of the slurry/resin casting system; 
     FIG. 3 is a block diagram of the control architecture for the slurry/resin casting system; 
     FIG. 4 is a flow chart describing the slurry processing cycle; 
     FIG. 5 is a flow chart showing the resin processing cycle; and 
     FIG. 6 shows clean-up cycles for the slurry/resin casting system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows slurry/resin casting system  8  to include autoclave  11 , and tank  10 , surrounded by platform  13  and access ladder  12 . Also shown in FIG. 1 is valve palate  14  which provides a centralized location for the valve connections between tank  10 , autoclave  11  and other equipment discussed below. As shown in FIG. 1, tank  10  and autoclave  11  are integrated into a relatively compact and accessible arrangement. 
     The work platform  13  and ladder  12  provide access to tank  10 . The structure should conform to all applicable OSHA and local building code specifications. 
     The slurry/resin casting system  8  is shown in greater detail in FIG.  2 . In FIG. 2, tank  10  is shown to be interconnected to autoclave  11  and through diaphragm transfer pump  21  to ball mill  20 . Ball mill  20  is shown to be remotely located, on the other side of a wall, from tank  10  and autoclave  11 . 
     A vacuum system (not shown) is connected through valves  22  and  23  to tank  10  and through additional valve  24  to autoclave  11 . The connection between tank  10  and autoclave  11  is made through pinch valve  25 . Autoclave  11  is also connected to the vacuum system through a set of valves  26  and to a nitrogen source (not shown) through a set of valves  27 . Autoclave  11  is vented through a set of valves  28  which are in parallel to oxygen sensor  29  connected in series to a set of valves  30 . Finally, room air and compressed air are supplied to autoclave  11  through a pump  31  with the compressed air also being supplied through a set of valves  32 . 
     Interlocks are provided to prevent the improper opening of process valves when the autoclave vessel pressure is above ambient pressure. For example, an over pressure condition within autoclave  11  will cause a system fault and close the nitrogen supply lines. 
     The presence of oxygen within the autoclave  11  above a programmed threshold limit will cause a system fault and prevent the delivery of power to the platen heating elements. Oxygen sensor  29  has a range from 0 to 25%, and preferably the sensor should tolerate oxygen excursions above 25% without sensor damage. 
     An over temperature condition with respect to the heated platen will cause a system fault and result in power removal from the platen heating elements. 
     An interlock in the autoclave door opening mechanism prevents human incursion into the autoclave interior until the interior pressure is within a small tolerance band about ambient pressure and the oxygen content within the autoclave has been restored to levels exceeding 19.5% (normal is 20.9%). Autoclave  11  can be vented to the atmosphere through valve  45  and tank  10  can be vented to the atmosphere through valve  46 . 
     All of the above-described interlocks are hard wired and do not employ a software controlled system to enforce the interlock. 
     The ball mill  20  is provided for mixing the SiC slurry. No particle size reduction is required in this process. 
     A Naigene® part container  35 ,  36  is placed, on a rolling cart  38 . Cart  38  is moved using a hand truck (not shown). Preforms will be placed in part container  35 ,  36  and the entire package rolled off the hand truck into autoclave  11 . Transfer pipe  39  connects the slurry supply, for example, to part container  35 . 
     A stainless steel diaphragm pump  21 , with a neoprene diaphragm, provides the pressure to pump the slurry from ball mill  20  to tank  10 . Ball mill  20  is connected to pump  21  through valve  34 , and a set of valves  33  supplies atmospheric air to pump  21 . Flexible piping  44  connects discharge valve  43  at one end and pinch valve  42  at the tank end. 
     Two degassing methods are possible. First, tank  10  can be evacuated before transfer of the slurry from the ball mill  20 . As the slurry enters tank  10 , it will be directed to the wall of the tank to promote degassing. Alternatively, the entire charge of slurry (approximately 20 gallons) can be transferred under normal conditions and degassed in bulk. 
     A four blade paddle wheel mixer  40  along with four internal tank baffles (not shown) provide for the stirring of the slurry in tank  10 . 
     Under vacuum conditions (28″ Hg), slurry will be metered into autoclave  11  through automated pinch valve  25 . An operator can judge the slurry level in autoclave  11  through sight glass  37  and manually close pinch valve  25  when sufficient slurry has been transferred. 
     A nitrogen source (not shown) is used to apply pressure to autoclave  11 . The pressure level is programmed and controlled through a system programmable logic controller (PLC)  50 , shown in FIG.  3 . In the exemplary embodiment the maximum pressure is 125 psi. 
     The pressure is ramped to ambient. A blower (not shown) provides adequate flow through the autoclave to ensure safe oxygen levels before allowing the door (not shown) to open. A whistle valve with interlock is provided, as noted previously, to prevent premature opening of the door when autoclave  11  is still under pressure. 
     A drain port  47  allows removal of excess slurry from the part container  35  after autoclave  11  is opened. The excess slurry can be, for example, drained into portable recycle tank  41 . 
     As shown in FIG. 6, clean up of the system can be easily accomplished in three independent processes  96 ,  97  and  98 . The ball mill  20  and diaphragm pump  21  are cleaned in process  95  by using flush water disposed through a drain valve into a container. All other hose and transfer lines are quick disconnect for remote clean-up as shown by processes  96  and  97 . 
     Ball mill  20  has a total mill capacity of 90 gallons, and a slurry capacity of 60 gallons. Tank  10  is preferably made of stainless steel and has a capacity of 75 gallons. Autoclave  11  is preferably made of carbon steel and has a capacity of 48″×48″×30″. Recycle tank  41  preferably has a 25 gallon capacity and is made of stainless steel. 
     Cam and groove fittings with replaceable flex hose are used for slurry transfer from the diaphragm pump  21  to slurry tank  10 . All valves in contact with slurry are pinch valves to reduce the possibility of clogging or jamming. All other valves for services are ball, globe, diaphragm, or solenoid valves. 
     Resin mixing is done in a remote resin handling laboratory. Resin is dispensed from a portable tank (not shown) into the part container  36 . Autoclave  11  processing preferably involves the following settings or parameters: Vacuum—28″ Hg vacuum capability; Pressure—125 psi N 2  pressure capability; Heat—2 hours to 150C, 30 minutes hold at 150C, turn off and cool under pressure; and Removal—by venting to atmosphere, and opening autoclave  11  and removing part container  36 . 
     Slurry transfer from ball mill  20  to tank  10  involves the following: slurry raw materials are manually weighed on a scale; weighed raw materials are transferred to ball mill  20 ; slurry mixture is agitated in ball mill  20 ; and the material is transferred from ball mill  20  to tank  10  via air-powered diaphragm transfer pump  21 . 
     Controls to start and stop pump  21  are accessible at a control station (not shown) near pump  21  and at a control station (not shown) at tank  10 . 
     Tank  10  employs an interlock to prevent overfilling and is vented through valve  46 . The system has the ability to vacuum degas during the transfer operation and subsequent to the transfer operation. Slurry holding tank mixer  39  may be started and stopped independent of the state of the transfer operation or the degas cycle. 
     The Slurry Processing Cycle, as shown in FIG. 4, includes steps  60 - 73 . Step  60  involves mixing the slurry in ball mill  20 . Step  61  involves transferring the slurry to holding tank  10  and degassing the slurry. Step  62  involves bringing autoclave  11  pressure from ambient conditions to the programmed vacuum level. Step  63  involves equilibrating the vacuum in autoclave  11  and slurry holding tank  10 . Step  64  involves, under operator command, opening valve  25  between slurry holding tank and autoclave  11 . Step  65  involves isolating slurry holding tank  10  from autoclave  11 . Step  66  involves releasing the autoclave vacuum (bleed to atmospheric pressure). Step  67  involves applying blanketing nitrogen pressure. Step  68  involves the initial bleeding of the autoclave. Step  69  involves final depressurization of autoclave  11 . Step  70  involves purging the nitrogen atmosphere and confirming restoration of normal oxygen levels. Step  71  involves opening the autoclave door and removing parts from the autoclave. Step  72  involves, external to the autoclave, draining excess slurry into portable recycle tank  41 , Finally, step  73  involves, external to the autoclave, allowing the preform to air dry. 
     The Resin Processing Cycle, as shown in FIG. 5, includes steps  80 - 91 . Step  80  involves preparing resin in a mixing room, and transferring the resin to one or more mixing tanks. Step  81  involves transporting the tank to autoclave  11 . Step  82  involves, external to the autoclave, pressurizing the mixing tank to transfer the resin from the tank to part container  36 . Step  83  involves opening the autoclave door, rolling the resin filled part container  36  into the autoclave, and closing the autoclave door. Step  84  involves, if a vacuum cycle is desired, applying a vacuum to the autoclave and running the programmed vacuum cycle (ramp to vacuum and dwell). If a vacuum is not desired, the process skips to step  86 . Step  85  involves releasing the autoclave vacuum (bleed to atmospheric pressure). Step  86  involves applying blanketing nitrogen pressure. In step  87 , if the autoclave oxygen content is below the combustion threshold the programmable temperature cycle is initiated. Step  88  involves applying the pressure cycle per programming within the pressure controller, and performing the initial bleed of the autoclave pressure. Step  89  involves performing final depressurization to near atmospheric pressure. Step  90  involves removing the electrical power from the heating elements, purging nitrogen atmosphere, and confirming restoration of normal oxygen levels. Finally, step  91  involves opening autoclave  11  door and removing parts from autoclave  11 . 
     The control system, shown in FIG. 3, comprises three control zones: autoclave zone  100 , slurry holding zone  200 , and slurry preparation zone  300 . PLC  50  is used to manage the interlocks within and between the control zones, execute the sequential processing steps, monitor the status of hard-wired safety interlocks, enable manual control of system elements, and collect process data. Preferably, the system uses a GE 30/90 PLC to execute the above-described functions. 
     The system employs a graphical operator interface terminal (OIT)  55  to control and view the status of control elements and the setpoint and process variable for control loops within each process zone. OIT  55  is used to initiate and interrupt process operations for all three process zones. In particular, OIT  55  is used to initiate the vacuum pressure cycle for the slurry holding zone and the vacuum, pressure, and heating cycles in the autoclave zone. OIT  55  provides a graphical display of all hard-wired safety interlocks to examine the sensor(s) state associated with each interlock. 
     OIT  55  also provides alarm information for error controls or out of control process loops. Individual, single loop process controllers  51 ,  52 ,  53 ,  54  are used to regulate process conditions between and in the autoclave and slurry holding zones. 
     A video graphical recorder  56  is provided to record the actual process variables during the process cycle. The video graphical recorder  56  is provided with an Ethernet interface and PCMCIA memory card to facilitate transfer of process data from the recorder to a PC (not shown) or corporate network (not shown) for archiving or further analysis. Setpoint parameters for process conditions will be entered via OIT  55 , transmitted to PLC  50 , and relayed to individual process controllers  51 - 54 . 
     A communications interface is provided between PLC  50  and individual process controllers  51 - 54  capable of supporting transmission of process cycle parameters to process controllers  51 - 54  and process status and error conditions from process controllers  51 - 54  to PLC  50 . The communications interface between PLC  50  and process controllers  51 - 54  is preferably Modbus. 
     The Autoclave Zone  100  incorporates software programmed interlocks to manage the process transition between vacuum, positive pressure, heating, venting, and restoration of an OSHA safe oxygen level. The Autoclave Zone  100  also incorporates hard-wired safety interlocks to mitigate the impact of the following fault conditions: over pressure; application of electrical heating in the presence of an oxygen level capable of supporting combustion; over temperature condition for heated autoclave platen; attempt to open door in presence of autoclave pressure that is not near zero gauge pressure; attempt, to open autoclave door in the absence of a safe oxygen level (&gt;19.5% O 2 ); attempt to open valves between the slurry holding tank and the autoclave when the autoclave pressure is greater than atmospheric pressure; and attempt to open autoclave&#39;s high volume vent and blower valves when autoclave pressure is greater than atmospheric pressure. 
     The autoclave  11  incorporates disperse control station  58 , which is accessible by the operator when viewing autoclave  11  through sight glass  37 . The operator has the ability from the control station to turn the autoclave illumination on/off, to equilibrate the vacuum pressure in autoclave  11  and slurry holding tank  10 , confirm that both zones are at equal pressure, and control the release of slurry into autoclave  11  at high and low rates. The control station will provide a fault lamp to prompt the operator if a system fault condition exists. The autoclave zone further includes process controllers to regulate the ramp rate and dwell time for the vacuum cycle, pressure cycle, and heating cycle. 
     The process cycle includes a software programmed interlock to prevent the restoration of normally oxygenated atmosphere until autoclave  11  is below a target temperature. This interlock helps prevent a potential combustion hazard due to the simultaneous presence of combustible fuel (uncatalyzed resin), oxygen, and ignition source (high temperature) being simultaneously present. The autoclave  11  incorporates automated control valves to control the flow into and out of the autoclave. These materials include slurry under vacuum, vacuum balance between autoclave and slurry holding tank, pressurization via nitrogen, vacuum, venting under various pressure condition, and normal atmosphere interchange. 
     Control valves are provided so that the rate of change vacuum and the vacuum pressure can be digitally controlled via a process controller. Likewise, control valves and devices are provided so that the rate of change of pressure and the pressure can be digitally controlled via a process controller. Separate process controllers control vacuum and pressure. 
     The system includes the ability to perform ramp and dwell cycles in the vacuum regime and pressurization cycles in the positive gauge pressure regime. Pressure switches are used to detect the transitions between the vacuum, ambient pressure, and above ambient pressure regimes for purposes of process sequencing and enforcement of safety interlocks. 
     The autoclave supports up to four Type-K thermocouple temperature sensors. Three temperature sensors are dedicated for use in regulating the part process temperature. Two sensors are used in a cascade control scheme that measures both the tooling surface temperature and the temperature of the component. Another temperature: sensor is used for an independent over temperature interlock. The remaining temperature sensor is available for general process monitoring. 
     Pressure within the autoclave is monitored with a compound range pressure transducer capable of sensing from −14.7 psig to 150 psig. The pressure transducer supplies a BRAIN-enhanced, 4-20 mA signal. The pressure transducer incorporates a local pressure display. The autoclave pressure is visible via OIT  55 . 
     Oxygen sensor and associated alarm outputs are used to display the current oxygen content within the autoclave and oxygen status if the autoclave atmosphere is safe for human exposure or insufficient for combustion. The heating controls are sized sufficiently large to support a maximum heater wattage of 16 kW. 
     The Slurry Holding Zone  200  incorporates a process controller to determine the vacuum ramp rate and regulation at vacuum pressure. No specific interlocks based on the slurry holding tank are provided to prevent or limit the transfer of slurry from the slurry preparation area to the slurry holding tank if the tank pressure is less than or equal to atmospheric pressure. A hard-wired interlock is provided to prevent the valve between the slurry holding tank and the slurry preparation area from opening if the slurry holding tank pressure is greater than atmospheric pressure. 
     The slurry holding tank is outfitted with four limit switches to detect the fill level within the tank. These switches are the basis for a low—low, low, high and high-high level alarms. The normal maximum fill level is the high alarm. Status of the level switches is displayed on OIT  55 . 
     The slurry mixer  40  is activated from OIT  55 . The mixer  40  is outfitted with a sensor to confirm that the, mixer shaft is turning. The mixer is driven by a 3-phase AC induction motor with gearbox. 
     Pressure within tank  10  is monitored with a compound range pressure transducer capable of sensing −14.7 psig to 150 psig. The pressure transducer supplies a BRAIN-enhanced, 4-20 mA signal. The pressure transducer incorporates a local pressure display. The slurry holding tank pressure is visible via OIT  55 . 
     Slurry Preparation Zone  300  incorporates transfer control station  59  to facilitate the transfer of resin from ball mill  20  to tank  10 . The operator station incorporates a light indicating “Ready to Transfer.” This light implies that tank  10  is not full, that a slurry dispense is not in process (balance valve closed and slurry pinch valve closed), and that tank  10  is not at a pressure above ambient. Pushbuttons start and stop the transfer process. 
     The pumping rate is controlled via the supplied air pressure. A manual regulator is provided to change this air pressure. A “transferring” light Vindicates that air is being supplied to diaphragm transfer pump  21  and slurry tank pinch valve  42  is open. A solenoid valve  33  is used to start and stop air flow to diaphragm pump  21 . A pressure switch confirms operation of solenoid valve  33 . 
     When the level in slurry holding tank  10  reaches the high or high-high limits, pump  21  is automatically stopped. Failure of the actuators in the slurry process results in a “Transfer Fault.” An indicator lamp is provided at the slurry preparation control station. The OIT  55  displays information for all the feedback devices. 
     An identical transfer control station  59 ′ is provided at tank  10  to start/stop transfer when the operator is near quick disconnect valve  43  for flex hose  44  connecting the slurry preparation area to tank  10 . 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.