Abstract:
A sensing system is provided for sensing the pressure of liquid in a vacuum chamber ( 7 ). The system includes a pneumatic microswitch ( 14 ) having an operating element ( 15 ) arranged for movement in response to movement of a diaphragm ( 10 ) operated by a change in pressure and a plate means ( 16 ) in contact with the diaphragm ( 10 ) and with the operating element ( 15 ).

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to sensing systems, and has for its object the provision of an improved form of pneumatic sensing system. 
         [0002]    Catalysed resin is commonly used in the composites industry for low pressure filling of an injection mould which is held together with vacuum. Such a mould has an injection point and at least one vacuum exit point. There is a specific need for providing a means of determining when the mould is filled. The normal procedure is to calculate the amount of resin that will be required, meter the amount of resin being injected and then stop the machine when this predetermined amount has been injected. 
         [0003]    This predetermined resin amount cannot be accurately set as slight variables of viscosity due to ambient temperature changes, atmospheric pressure variations and humidity changes are all factors which alter the amount of a specific mould fill capacity. 
         [0004]    It is an object of the present invention to provide a more reliable and simpler method of determining when such a mould is completely filled. 
         [0005]    Furthermore there is a need to control the flow rate of injecting such moulds in order that the mould is filled at a rate not so great that in-mould resin pressures exceed the vacuum clamping force. 
         [0006]    It is a further object of the present invention to provide a pressure sensing system that can be used for this purpose. 
         [0007]    The liquids may be volatile and inflammable and it is a more specific object of the present invention to provide a sensing system that can be used in connection with the filling of a mould with a resin that does not include any electrically operated components. 
         [0008]    It is an object of the present invention to provide a precision liquid pressure sensing system for use within a sealed vacuum container or mould cavity. The signal of the sensor may be used to provide an alarm and/or automatically stop an injection machine over-filling a mould or control the speed of mould fill subject to in-mould pressure constraints 
       SUMMARY OF THE INVENTION 
       [0009]    According to the present invention there is provided a sensing system for sensing the pressure of liquid in a vacuum chamber, the system including a pneumatic microswitch having an operating element arranged for movement in response to movement of a diaphragm operated by a change in pressure and a plate means in contact with the diaphragm and with the operating element. 
         [0010]    The diaphragm is preferably movable between first and second operative positions, with the arrangement such that, when the level of liquid in the chamber increases to a predetermined level, the resultant pressure increase causes the diaphragm to move from its first operative position into its second position. 
         [0011]    As applied to the controlling of the filling of a mould with resin, upward movement of the diaphragm preferably causes the microswitch to operate and generate a signal when the pressure increases in response to the resin partially filling the chamber so as to cause the resin injection machine to stop. At this point, the resin pressure will decay and the atmospheric pressure above the diaphragm will cause the diaphragm to be pushed downwardly to release the pressure switch such that the machine will once more be able to start to pump. 
         [0012]    Furthermore, as is common to composite injection moulding under vacuum, it is preferable to have a mould cavity vacuum catchpot arrangement to act as a resin trap at the mould final fill point for excess resin from the mould fill, thus guarding against resin entering the vacuum system pipework. 
         [0013]    The system of the present invention is accordingly preferably located adjacent to the catchpot to pre-empt the mould filling, i.e. the injection machine will stop before the resin has reached the final fill point, which is the catchpot. However, signalling the machine to stop just prior to the resin reaching the catchpot is an advantage as, if stopped after the resin reaches the catchpot, there is always the possibility of a degree of over-run and thus wastage of resin. 
         [0014]    The use of the sensing system of the present invention thus allows moulds to be filled with no prior knowledge of the mould fill capacity as when the resin reaches near to or is art the actual mould final fill point, the sensing system will signal the machine to cancel the injection command and thus stop the machine automatically . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0015]      FIG. 1  is a cross-sectional view of a sensor mounted on a closed mould cavity for sensing the pressure change of liquid within the sealed vacuum/pressure mould cavity and for generating a signal when the presence of liquid in the mould causes a pressure change of a predetermined value. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    The device shown in the drawing includes a closed mould having a cavity  3  between a lower mould face  2  and an upper mould face  1 . The upper mould face  1  includes a bonded-in mould insert  4 . A sensor main body  6  fits into the insert  4  and has a sealing engagement with the insert  4  by means of a sealing ring  5 . 
         [0017]    A sensor clamping ring  12  has threaded engagement with the upper portion of the sensor body  6  and the clamping ring  12  is sandwiched between a lower housing member  24  and an upper housing member  23 . The sensor body  12 , which is preferably made entirely from PTFE material, is formed with a cone-shaped chamber  7  positioned directly over the mould cavity  3 . At the top end of the chamber  7 , a narrow duct  8  is provided and this leads upwards to and communicates with a second or upper sealed chamber  9 . The top of this upper chamber  9  is sealed by a flexible diaphragm  10 . The sensor clamping ring  12  serves to seal the diaphragm  10  against the body  6  by clamping pressure exerted upon a sealing ring  11  located in a groove in the upper surface of the sensor body  6 . 
         [0018]    The lower and upper housing members  24  and  23  are held clamped together by fixing screws  25 . 
         [0019]    Within the assembled housing formed by housing members  23  and  24  is fixed a micro pneumatic switch  14  with its rear mounting hole  21  acting as a pivot point such that a limited amount of pivotal movement of the switch  14  about the axis of the hole  21  is permitted. The microswitch  14  has an actuator  15  which is positioned centrally relative to the diaphragm  10  with an amplifier plate  16  interposed between the two. As shown in the drawing, the amplifier plate  16  is a rigid flat circular disc. 
         [0020]    The position setting of the microswitch actuator  15  is set by adjustment of the position of a yoke  17  which is engaged with a pin  20  projecting from the body of the micro switch  14 . The position of the yoke  17  can be adjusted rotation of a screw  18  access to the head of which is obtained via a central access hole  22  of a fitting  19  which is mounted in a recess in the upper housing member  23 . When the actuator  15  is operated, this switches the microswitch  14  between its “off” and “on” conditions and the air supply is switched between its “off” and “on” conditions. When the air supply is “on”, air flows from an air supply feed pipe  26  to an output signal pipe  27 . As shown in the drawing, the feed pipe  26  and the output signal pipe  27  are connected to the body of the microswitch  14 . 
         [0021]    A compression spring  13  is mounted in the upper chamber  9  and acts on the diaphragm  10  to urge it upwardly, i.e. in a direction such as to move the actuator  15  to its “on” state under normal atmospheric pressure conditions. However, when the pressure within the mould cavity  3 , and thus within the chamber  9 , falls a predetermined amount below atmospheric pressure, the diaphragm  10  is drawn downwards against the action of the spring  13  such that the actuator  15  is free to move into its “off” condition of switch  14 . 
         [0022]    In this manner it is possible to set, to a fine degree, the point at which the microswitch  14  actuates between “on” and “off” by the rating of spring  13  and final fine adjustment of the set point level by means of the screw  18 . 
         [0023]    A mould cavity vacuum catchpot arrangement (not shown) is provided to act as a resin trap at the mould final fill point for excess resin from the mould, guarding against resin entering the vacuum system pipework. 
         [0024]    The insert  4  is accordingly located adjacent to the catchpot to pre-empt the mould filling, i.e. the injection machine will stop before the resin has reached the final fill point, which is the catchpot. However, as described above, signalling the machine to stop just prior to the resin reaching the catchpot is an advantage as, if stopped after the resin reaches the catchpot, there is always the possibility of a degree of over-run and thus wastage of resin. 
         [0025]    The system shown in the drawing is sufficiently sensitive for it to be able to recognise the presence of resin passing by in a closed mould vacuum infusion/injection process and provide a positive pneumatic signal of this event. It is thus possible to control the injection machine output speed relative to in-mould pressure user setting limits. 
         [0026]    The sensing system can be used to determine when the mould has been filled and thus signal the injection machine to stop injecting. The point at which the sensing system switches is, as described above, determined by the rating of spring  13  and by the setting of the range adjustment screw  18 . 
         [0027]    It has been found that, if the range setting is set to switch at a level just above the cavity pressure when no resin is present, (e.g. a cavity pressure of 500 mb absolute and a switch setting of 550 mb absolute) then the microswitch  14  will be switched on almost immediately once the resin flowing past has covered the conical face of the sensor body  6 . 
         [0028]    However, if the sensor range is set to a higher range, (e.g. a cavity pressure of 500 mb absolute and a sensor switch setting of 700 mb absolute) then the switch-on point will be later and thus the resin will have flowed a longer distance towards the catchpot, if not right up to the catchpot. It is thus possible to adjust the switch-on point relative to the position at which the user wishes to generate the signal to switch off the machine.