Abstract:
In the manufacture of double-walled corrugated extruded pipe it is desirable to form an integral connecting cuff that is of a single wall thickness and typically of a large diameter. The mold blocks and process parameters for forming of the cuff as part of an otherwise double-walled pipe requires a transition as the cuff moves past the die outlets. The present invention allows for accurate sensing and control of air pressure and temperature as the cuff moves past the die outlets. Improvements in both the die tooling and the method of manufacture are disclosed.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to systems and methods for forming corrugated pipe and in particular for forming double walled corrugated pipe having connecting cuffs. 
       BACKGROUND OF THE INVENTION 
       [0002]    Corrugated pipe is commonly manufactured as a continuous process with the corrugated pipe having elongate corrugated sections that are separated at pre-determined intervals by an integral cuff that allows pipe sections to be connected in an end to end manner. These cuff sections downstream of the corrugator are typically cut to separate the pipe sections from each other. The cuff sections are of a greater cross section and are designed to sleeve over the corrugations of the pipe. Different sealing arrangements can be provided. 
         [0003]    A pipe corrugator cooperates with die tooling which extrudes one or more envelopes of plastic that form the walls of the pipe. It is common to have an air pressure outlet associated with the die outlets of the tooling to assist in displacing the extruded plastic envelope outwardly and to have it engage and be drawn into the corrugations of the mold block. The amount of air pressure provided is a function of the diameter of the pipe, the thickness of the walls, the extrusion temperature and other factors. An operator will adjust the air pressure to provide satisfactory results. 
         [0004]    The physical configuration of the mold blocks for forming the cuff significantly changes the air pressure required to force the plastic outwardly. The cuff cavity in the mold blocks defines a fairly large volume and if the air pressure used to form corrugations was maintained, the extruding plastic envelope would effectively balloon in an upstream direction and not provide a smooth displacement of the plastic envelope into the cavity of the mold block defining the cuff. It is known to sense the position of the secondary mold blocks that define the cuff and to reduce the air pressure for forming the cuff. Typically there is an air pressure regulator provided outside of the die tooling that provides processed air at the desired pressure to bias the plastic envelope into the mold blocks in the desired manner. Additionally the mold blocks include vacuum channels which further act to draw the plastic into conformity with the mold block cavities once there has been some contact with the plastic envelope. 
         [0005]    The problem of accurately forming the cuff becomes more acute when the die tooling is designed for forming double walled pipe. In this case there is an outer wall of corrugations formed from a first plastic envelope extruding from a first die outlet and an inner wall of the pipe is formed by a second plastic envelope extruding from a second die outlet downstream from the first die outlet. Processed air is used to blow the first extruded plastic into the corrugations and typically the die tooling includes a cooling plug that biases the plastic of the second die outlet against the corrugations while forming a smooth inner wall of the pipe. 
         [0006]    This process works quite well and is consistent when forming the pipe sections of the corrugations and the inner smooth wall. As the mold blocks for forming the pipe cuff start to move first past the first die outlet, and then past the second die outlet, the air pressure urging the first plastic outwardly against the corrugations must be reduced to avoid blowing or ballooning of the plastic envelope effectively upstream as opposed to biasing it into the cuff cavity of the mold blocks. Therefore pressure balance points are necessary where the air pressure is sufficient to bias the extruded plastic envelope outwardly and into the corrugations (a first balance point) or into the pipe cuff (a second balance point) and yet of a sufficiently low pressure to avoid ballooning of the plastic envelope in an upstream direction. 
         [0007]    It has been found that improvements can be made with respect to the forming of the pipe cuffs for double walled pipe by accurate measurement of the pressure adjacent the first die outlet and providing feedback to a pressure regulator outside of the die tooling. In addition, a second pressure can be appropriately controlled for assisting the plastic envelope being extruded from the second die outlet to merge into the cuff cavity of the mold blocks without ballooning in an upstream direction. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a pipe corrugator and associated die tooling for forming pipe having elongate pipe sections separated by integral connecting cuffs provided at predetermined locations in the length of the formed pipe. The pipe corrugator includes two opposed series of circulating mold blocks that abut to form an inlet to a mold tunnel and remain in abutment until an exit to the mold tunnel where the mold blocks separate and are returned to the inlet. Each series of mold blocks includes first mold blocks for forming the elongate pipe sections in the mold tunnel and second mold blocks for forming the connecting cuffs in the mold tunnel. The die tooling includes two die outlets located in the mold tunnel adjacent the inlet with the die outlets separated by a process air cavity formed in a recessed portion of an exterior wall of the die tooling open to the mold tunnel and connected to a first process air supply providing air under pressure to a first process air outlet located in the process air cavity. A first air pressure transducer is located in the process air cavity detecting air pressure in the process air cavity, and a second supply of process air supplying process air to a second process air outlet is located immediately downstream of the second die outlet. A controller receives a signal of the air pressure sensed by the first transducer and based thereon provides process air at a first air pressure to the first process air outlet appropriate for forming corrugations when the first mold blocks pass over the first and second die outlets and the controller controls the first process air supply and selectively operates the second process air supply to provide process air at a second air pressure reduced relative to the first air pressure for forming a cuff when the cuff cavity of the second mold blocks pass over the die outlets. 
         [0009]    According to an aspect of the invention, the controller receives positional information of the second mold blocks relative to the die outlets and based on the positional information of the second mold blocks determines when a leading wall of the pipe cuff cavity is about to move past the first die outlet and reduces the pressure of the air supply cavity to a the second air pressure. The controller based on the positional information determines when the leading wall of the pipe cuff cavity is about to move past the second die outlet and provides air pressure at a cuff forming pressure generally equal to the second air pressure via the second air supply. The controller maintains the air pressure at the lower cuff forming pressure and the second air pressure until a trailing wall of the pipe cuff cavity passes the second die outlet and then returns the corrugation forming pressure in the process air cavity and removes air supply pressure through the second inlet. 
         [0010]    In a further aspect of the invention, a second process air outlet is located immediately downstream of the second die outlet and connects with a second process air supply conduit extending in a length of the die tooling and supplies regulated process air under pressure to the second process air outlet immediately downstream of the second annular die outlet. The controller is connected to a pressure transducer adjacent the second annular die outlet detects an air pressure to the exterior of the die tooling at the second process air outlet. 
         [0011]    Die tooling for use in forming double wall corrugated pipe according to the present invention comprises a die tool body having a first annular die outlet and a second annular die outlet located downstream of and separated from the first die outlet by an air processing cavity located in a recess of the die tooling and opening outwardly. The first and second annular die outlets are connected through the die body to extruded plastic inlets. The air processing cavity includes a first process air outlet located in the cavity. The first process air outlet connects with a first process air supply conduit extending in a length of the die tooling and supplies process air under pressure to the first process air outlet. 
         [0012]    The air processing cavity includes an air pressure transducer located in the cavity and produces an air pressure signal in accordance with the air pressure of a variable chamber defined between the die tooling including the air processing cavity and plastic extruding through the die outlets and connecting with each other. The air pressure signal is provided to a controller at a position upstream of the first and second annular die outlets. 
         [0013]    A regulated air supply controlled by the controller and connected to the first process air supply conduit is used to regulate the air pressure of the variable chamber between at least a corrugation forming pressure and a lower cuff forming pressure. 
         [0014]    In an aspect of the invention the die tooling includes a second process air outlet located immediately downstream of the second die outlet and connecting with a second process air supply conduit extending in a length of the die tooling and supplies regulated process air under pressure to the second process air outlet immediately downstream of the second annular die outlet. The controller uses a pressure transducer adjacent the second annular die outlet to detect an air pressure to the exterior of the die tooling at the second process air outlet for regulating the pressure. 
         [0015]    In a further aspect of the invention, the controller includes an operator adjustment for varying the corrugation forming pressure and varying the lower cuff forming operating pressure used to form a single wall cuff of the pipe from plastic extruding through both of the die outlets. 
         [0016]    In yet a further aspect of the invention, the controller for the second process air outlet includes a minimal operating pressure used during the forming of corrugations connected to an inner smooth wall of a pipe and a higher cuff forming operating pressure for forming a single wall cuff of the pipe from plastic extruding through the die outlets. 
         [0017]    In an aspect of the invention, the controller for the second process air outlet includes a minimal operating pressure used during the forming of corrugations connected to an inner smooth wall of a pipe and a higher second operating pressure for forming a single wall cuff of the pipe from plastic extruding through the die outlets and wherein the higher second pressure is generally the same as the lower cuff forming pressure. 
         [0018]    A pipe corrugator and associated die tooling for forming pipe having alternating long pipe sections separated by alternating integral connecting cuffs provided at predetermined locations in the length of the formed pipe according to the present invention includes two opposed series of circulating mold blocks that abut to form an inlet to a mold tunnel and remain in abutment until an exit to the mold tunnel where the mold blocks separate and are returned to the inlet. Each series of mold blocks includes first mold blocks for forming the elongate pipe sections in the mold tunnel and second mold blocks having a cuff cavity for forming in the mold tunnel the connecting cuffs. The die tooling includes a die tool body having a first annular die outlet and a second annular die outlet located downstream of and separated from the first die outlet by an air processing cavity located in a recess of the die tooling and opening outwardly. The first and second annular die outlets are connected through the die body to extruded plastic inlets. The air processing cavity includes a first process air outlet located in the cavity, with the first process air outlet connecting with a first process air supply conduit extending in a length of the die tooling and supplying process air under pressure to the first process air outlet. The air processing cavity includes an air pressure transducer located in the cavity and produces an air pressure signal in accordance with the air pressure of a variable chamber defined between the die tooling in an area including the air processing cavity and plastic extruding through the die outlets and connecting with each other. The air pressure signal is provided to a controller at a position upstream of the first and second annular die outlets, and a regulated air supply controlled by the controller and connected to the first process air supply conduit regulates the air pressure of the variable chamber. 
         [0019]    In an aspect of the invention, a second process air outlet is located immediately downstream of the second die outlet and connects with a second process air supply conduit extending in a length of the die tooling and supplies regulated process air under pressure to the second process air outlet immediately downstream of the second annular die outlet controlled by the controller using a pressure transducer adjacent the second annular die outlet detecting an air pressure to the exterior of the die tooling at the second process air outlet. 
         [0020]    In yet a further aspect of the invention, the controller receives positional information of the second mold blocks relative to the diet outlets. The controller, based on the positional information of the second mold blocks, determines when a leading wall of the pipe cuff cavity is about to move past the first die outlet and reduces the pressure of the air supply cavity to the second air pressure. 
         [0021]    The controller, based on the positional information determining when the leading wall of the pipe cuff cavity is about to move past the second die outlet, provides air pressure at a cuff forming pressure generally equal to the second air pressure via the second air supply. 
         [0022]    The air pressure at the lower cuff forming pressure and the second air pressure is maintained until a trailing wall of the pipe cuff cavity passes the second die outlet and then returns to the corrugation forming pressure in the process air cavity and removes air supply pressure through the second inlet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    Preferred embodiments of the invention are shown in the drawings, wherein: 
           [0024]      FIG. 1  is a schematic view of a pipe corrugator and die tooling; 
           [0025]      FIG. 2  is a partial sectional view showing details of a corrugator and die tooling for forming double walled corrugated pipe; 
           [0026]      FIG. 3  is a view similar to  FIG. 2  with the second mold blocks for forming of a cuff portion partially overlapping with die outlets; 
           [0027]      FIG. 4  is a similar view to  FIG. 2  with the mold blocks for forming the cuff portion generally centered over the first die outlet; 
           [0028]      FIG. 5  is a similar view with the mold blocks for forming the cuff portion about to move past the first die outlet; 
           [0029]      FIG. 6  is a similar view with the trailing portion of the cuff portion approaching the second die outlet while the first die outlet is extruding plastic into corrugations of the mold block; 
           [0030]      FIG. 7  is a sectional view showing the pipe cuff portion of the mold blocks positioned downstream of the second die outlet; 
           [0031]      FIG. 8  is a modified sectional view of a preferred embodiment that includes additional temperature sensors allowing improved control of the extrusion process; 
           [0032]      FIG. 9  is a further modified arrangement that includes downstream sampling of temperature and pressure in formed corrugations; 
           [0033]      FIG. 10  is a partial sectional view through a portion of double wall corrugated pipe having a mechanical sensor for measuring repeating distortions (if present) in an inner wall caused by incorrect pressure at extrusion of the corrugations; and 
           [0034]      FIGS. 11 and 12  are similar to  FIG. 10  where the mechanical sensor moves and detects low pressure distortions of the inner wall. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0035]    The pipe corrugator  2  shown in  FIG. 1  includes die tooling  4  positioned adjacent the moving mold tunnel generally shown as  40 . The mold tunnel includes an inlet  42  where the two series of mold blocks  44  and  46  come into abutment with each other and the moving mold tunnel has an exit  48  where the first and second mold blocks separate and are returned to the inlet. The die tooling has associated therewith a first air pressure supply source  50  and a second air pressure supply source  52 . Each of these include their own regulator  54  and  56  respectively. The pipe corrugator also includes a controller  60 . Double walled corrugated pipe  62  is generally shown at the exit to the corrugator. 
         [0036]    The controller  5   a  is connected to mold block positioning sensing unit  61  and first and second air pressure signal unit  63  for receiving air pressure signals detected adjacent plastic extruding outlets of the die tooling  4 . 
         [0037]    In the partial sectional view of  FIG. 2 , the mold blocks of the moving mold tunnel are moving across a first die outlet  14  that is extruding a first plastic envelope  16  and are moving past a second die outlet  18  extruding a second plastic envelope  20 . The first plastic envelope  16  will form the corrugations of the outer wall of the corrugated double walled pipe and the second plastic envelope  20  forms the inner smooth wall of the pipe. A cooling plug  21  is shown downstream of the second die outlet  18  and the cooling plug biases the extruded second plastic envelope  20  into contact with the inner walls of the corrugation thus attaching the inner and outer walls. 
         [0038]    It should be understood that  FIG. 2  shows first mold blocks  6  which are of the type to form the corrugations of the double walled pipe as well as the inner wall of the corrugated pipe. There will be many of these mold blocks forming long sections of corrugated double walled pipe of this configuration.  FIG. 2  also illustrates second mold blocks  8  which cooperate to form a cavity  29  for forming a cuff of the corrugated pipe. It can be seen that the cavity  29  is quite large and is of a cross section similar to the outer walls of the corrugations and perhaps slightly larger. This cuff can be inserted over the corrugations of the pipe to connect one pipe section to the other. Different corrugators allow for either the insertion of mold blocks  8  during the cycling of the mold blocks to form a pipe cuff at a desired location or the corrugator may have a fairly large number of mold blocks and a cuff is formed at predetermined intervals. 
         [0039]    In  FIG. 2  it can be seen that the die tooling  4  includes a cavity  26  that is open to the interior of the corrugations of the mold block. This open cavity in the die tooling is immediately downstream of the first die outlet  14 . During the normal manufacture of a double walled pipe the first plastic envelope is extruded through the first die outlet  14  and is biased (by air pressure) into the corrugations of the mold block to form the corrugated pipe. To encourage the movement of the envelope  16  into the corrugations, air under pressure is introduced through the inlet  28  and provides a bias force displacing the envelope outwardly as it continues to move with the mold blocks. A first pressure transducer  30  monitors the pressure in the open cavity  26  and as will be subsequently described, is used to detect two different pressures. In  FIG. 2  a pressure Y 1  is shown, which is a higher corrugation forming operating pressure that provides the bias force forcing the first envelope into the corrugations. The second envelope  20  passes out the second die outlet  18  and is brought into contact with the inner walls of the corrugations to form a connection therewith, and the cooling plug  28  biases this inner wall against the corrugations and forms the smooth inner surface of the double walled pipe. 
         [0040]    In the forming of double walled corrugated pipe with outer corrugations and a connected inner smooth wall, it is known to adjust the pressure Y 1  to achieve the desired results. If this pressure is too great, the first plastic envelope will balloon in an upstream direction past the die inlet and will cause significant problems. If the pressure is too low the first plastic envelope will not be brought into full contact with the corrugation forming cavities of the mold blocks and deficiencies in the formed pipe. The first type mold block  6  as well as the second type mold blocks  8  include vacuum channels which assist in drawing the plastic into contact with the cavities of the mold blocks once the envelope has been generally brought into close proximity with the cavities. 
         [0041]    The present invention additionally addresses a further problem that occurs when the second type mold blocks  8  that include the cavity for forming the pipe cuff, move past the die outlets. In particular, there is a requirement to change the pressure as the initial leading wall of the pipe cuff cavity starts to move past the first die outlet  14 . This is shown in  FIG. 3  where the lead wall  31  of the pipe cuff cavity  29  has moved past the first die outlet  18 . The pipe cuff cavity  29  is quite large and there is a large gap now formed between the pipe cavity  29  and the first die outlet  18  through which air can flow in an upstream direction. This large open cavity does not provide any substantial resistance to air flow which would cause the hot extruded plastic envelope  20  ballooning outwardly and upstream. To overcome this tendency the pressure within the open cavity is reduced to a level Y 2 . This reduced pressure still encourages the first plastic envelope  20  to be displaced outwardly and form the walls of what will be the pipe cuff by being pressed into the shape of the cavity  29  of the mold blocks  8 . The second die outlet  22  in  FIG. 3  is continuing to form the inner wall of the double walled corrugated pipe and this inner wall is being pressed against previously formed corrugations. The cooling plug  21  continues to force the inner wall against the corrugations. 
         [0042]    In  FIG. 4  the mold blocks  8  continue to advance past the die outlets. As shown, the die outlet  16  is extruding plastic into the pipe cuff cavity  29 . The pressure has been reduced to ensure that the extruded plastic envelope  18  does not balloon in the upstream direction. The pressure is still sufficient to force the extruded plastic envelope  16  out into contact with the mold cavity  29  as the mold blocks move downstream. The second die outlet  20  has just finished forming the inner wall against the last corrugation. Any subsequent movement of the mold blocks of the moving mold tunnel will require that the second plastic envelope  22  will now be displaced outwardly to form part of the pipe cuff. 
         [0043]    This aspect can be appreciated from a review of  FIG. 5  where the second plastic envelope  22  leaves the die outlet  20  and is displaced outwardly against the cavity  29  forming the pipe cuff. It is generally at this point that additional processed air is provided through the processed air outlet  32  and is generally at a pressure similar or equal to Y 2 , namely the reduced pressure in the open cavity  26 . In this way there is a bias force moving the second plastic envelope outwardly into the deeper cavity for forming the pipe cuff. The generally equal pressures provide the outward bias without undesirable ballooning or collapse of the plastic envelopes. 
         [0044]    As can be seen in  FIG. 5 , the trailing wall  33  of the second mold blocks  8  is about to move past the first die outlet  14 . This trailing wall has moved past the first die outlet  14  in the view of  FIG. 6  and the first plastic extrudate is now forming the corrugations of the pipe. The pressure within the open cavity  26  is still at a reduced level but at sufficient level to encourage the first plastic extrudate to follow the shape of the corrugations. The cavity  29  of the pipe cuff is now generally over the second die outlet  20  and this cavity is at the reduced pressure Y 2  as processed air is being provided thereto. Again there is a balance between the pressure encouraging the first plastic extrudate to form the corrugations and the second plastic extrudate which is presently forming an inner portion of the pipe cuff. 
         [0045]      FIG. 7  shows how the cavity  29  forming the pipe cuff has now moved past the second die outlet  18 . At this point the second plastic envelope is returning to form the inner wall of the corrugated pipe sections. The first plastic envelope is forming the corrugations of the pipe. The cavity  29  that forms the pipe cuff has now moved past the inlets and the pressure within the open cavity  26  can now be returned to the higher Y 1  pressure. No air pressure is being provided downstream of the second die outlet  20 . 
         [0046]    It is common to cut the formed pipe at the end of a cuff at two locations to remove a short transition portion from cuff to corrugations. 
         [0047]    It has been found that sensing of the pressure via the first pressure transducer  30  located in the open cavity  26  of the die tooling  4  provides improved information and regulation of the pressure between Y 1  and Y 2  that is important for accurately forming of the pipe cuff. Similarly the second pressure transducer  34  senses air pressure at the second inlet of the cavity forming the pipe cuff as it is moving past the second die outlet  18 . It is preferable that each of these pressure transducers has an inlet that is not directly exposed to the flow of processed air or is at least downstream thereof to more accurately sense the pressure in the cavity as opposed to pressure caused by the air flow directly contacting the transducer. 
         [0048]    As described in  FIGS. 2 through 7 , the position of the second mold blocks which form the pipe cuff as they move past the die outlets of the die tooling is important. The position of these mold blocks is tracked by the controller shown in  FIG. 1  and the controller  59  can receive the signals from the air pressure transducers to accurately determine the pressure provided to the pipe cuff as it moves past these die outlets. Regulators for each of the air pressure supplies  50  and  52  are provided and are used to provide the desired air pressure at a particular point in time. It can be appreciated from the description above, the air pressure and processed air used in association with the second die outlet is typically only operated or operated at a significant level to move the second plastic envelope outwardly and into contact with the pipe cuff cavity of the mold blocks. During the forming of corrugated pipe with the smooth inner wall, this air is typically cut off or not provided to any substantial extent. 
         [0049]      FIG. 8  shows a modified corrugator  102  that in addition to sensing of the pressure adjacent the extrusion outlets also advantageously senses temperature at the extrusion outlets. With this arrangement more information with respect to the extrusion process at the extrusion outlets is known and the operator can use this information for better control of the extrusion process. 
         [0050]    For example, the smooth inner wall bridges across adjacent corrugations of the outer wall and forms sealed cavities filled with air between corrugations. The air is at the pressure measured by pressure transducer  130 . When the pipe cools the smooth inner wall may deform into the cavity between corrugations due to a reduced pressure caused by cooling. These cavities at the time of forming relative to the cooled cavity after manufacture have the general relationship 
         [0000]    
       
         
           
             
               
                 
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                 T 
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         [0051]    The difference between T 1  and T 2  is typically in excess of 100° C. It is desired for V 1  to approximately equal V 2  to maintain the straight smooth wall, however if P 2  is too low, V 2  may decrease by inward buckling into the cavity. By appropriate control of P 1  inward buckling can be reduced or avoided. It is preferable to automatically adjust pressure based on the sensed conditions. It is also possible to have the operator adjust P 1  based on the pipe being produced. Operator adjustment is also an effective approach as plastic material, extruder operating conditions and other factors can affect this relationship. Monitoring the temperature allows the operator further information and for example may increase cooling if the sensed temperature is too high. 
         [0052]      FIG. 9  shows a further embodiment where one wall of a double wall corrugated pipe  250  is shown having a smooth inner wall  252  and outer corrugated wall  254 . The inner wall  252  is preferably of a generally constant internal diameter to provide a smooth flow through the pipe and reinforce the corrugations. One of the sealed corrugations  254   a  is about to be sampled by a device  200  with respect to temperature and pressure substantially downstream of the die tooling. The cut line  260  indicates a substantial gap. In this way some cooling has already occurred and the temperature and pressure will have changed. This measured information can be provided to the automated program and depending on the information adjustment of the die tooling (pressure, temperature, etc.) may be required. Resealing of the sampled corrugation can also be completed. It is preferable that both temperature and pressure be sampled and this information provided to an automated program controlling the extrusion process. 
         [0053]    Downstream sampling of corrugation temperature and pressure is programmed at certain intervals to push the hollow needle  202  into a corrugation. Pressure and temperature are measured as previously indicated. 
         [0054]    One suitable location to measure the air pressure with respect to polyethylene or polypropylene double wall corrugated pipe is downstream of the pipe after cooler. 
         [0055]    This control and adjustment of pressure based on sensed temperature and/or other factors is used to provide a smooth inner wall to the double walled pipe of a generally consistent internal diameter. As an alternative to temperature and pressure sampling the internal diameter may be measured either continuously or intermittently and this information provided to the automatic control. If the internal diameter deforms inwardly into the pipe too much pressure is present whereas deformation outwardly indicates too little pressure at the time the corrugation is sealed. 
         [0056]    A device  300  for sensing the linearity of the inner wall of a double wall corrugated pipe is shown in  FIGS. 10 through 12 . The sensing device  300  is located after the cooling plug and typically the structural member  302  will be attached to or associated with the trailing edge of the cooling plug or other upstream structure. At the end of the structural member  302  is a lever finger  304  having a roller  306  pivotally secured to the finger and lightly biased against the inner wall of the corrugated pipe. This roller will follow any changes in the internal shape of the inner wall and in particular senses any deformation of the inner wall that spans each corrugation. The position of the roller  306  relative to the distance sensor  308  is tracked and processed. 
         [0057]    In  FIG. 10  it can be seen that an ideal pressure has been achieved at the die outlets (time of sealing of the corrugations) as the inner wall  320  that spans the various corrugations  322  is linear. With this arrangement the roller  306  essentially stays at the same distance from the distance sensor  308  (a zero calibration point). 
         [0058]    In  FIGS. 11 and 12  the inner wall of the double wall extruded pipe has been pulled into the individual corrugations  322  due to low pressure. In  FIG. 11  the roller  306  is in a zero or neutral position as the roller is contacting the joined inner and outer wall between corrugations. The thickness of these two walls and the position thereof does not appreciably vary as it is primarily determined by the die tooling and corrugator. 
         [0059]    In  FIG. 12  the double wall extruded pipe has moved and the roller  306  is now centered in the distorted inner wall  320 . The distance between the roller  306  and the distance sensor  308  has increased and thus the amount of distortion of the inner wall is accurately tracked. 
         [0060]    The type of distortion shown in  FIGS. 11 and 12  is caused by low pressure at the time of extrusion and measuring this distortion by a sensing arrangement provides a feedback signal preferably used to automatically adjust the air pressure in the cavity of the die tooling. As can be appreciated, each corrugation is assessed by this sensing device and variations in the sensed movement (i.e. the position of the roller  306  relative to the distance sensor  308 ) will repeat and an average maximum distortion signal can be used particularly for air pressure adjustment. 
         [0061]    The sensing arrangement of  FIGS. 10 through 12  provides an alternate arrangement to downstream pressure and temperature sensing. The alternate embodiment measures the linearity of the inner wall and the extent of any deformation. Deformation caused by low pressure as shown in  FIGS. 11 and 12  and deformation caused by excessive air pressure in the corrugations are determined and appropriate automatic adjustments can be made. 
         [0062]    The mechanical sensing arrangement  300  provides an alternative to downstream sampling or the operator merely making a visual assessment of the condition of the inner wall after the pipe is cut into sections. The mechanical sensing of the condition of the inner wall and/or the pressure and temperature sensing by sampling downstream provide a feedback signal used to automatically adjust the air pressure to maintain a consistent inner wall as generally shown in  FIG. 10 . Other arrangements for sensing the linearity of inner wall can also be used. 
         [0063]    For PVC pipes the air pressure preferably is sampled approximately at a midpoint between the corrugator and the pipe cut off device that cuts the formed pipe into discrete lengths. 
         [0064]    As in the earlier figures, the corrugator  102  includes die tooling  104  and has schematically illustrated a moving mold tunnel  106 . The moving mold tunnel includes a corrugated portion  108 , a coupling portion  110  and a following corrugated portion  112 . The extruded plastic forming the pipe has been omitted for greater clarity. During the molding of a pipe coupling when the pipe coupling portion  110  is moving past the first extrusion outlet  120 , accurate control of the pressure is desirable to urge the plastic extruding out of outlet  120  to move against the outer wall of the moving mold tunnel. As can be seen, a pressure port  124  is provided downstream of the extrusion outlet  120  and this port is connected to a regulated pressure source that can be adjusted depending upon what portion of the moving mold tunnel is passing the extrusion outlet  120  and can also be adjusted to modify the extrusion process in accordance with sensed conditions or results. 
         [0065]    The modified corrugator  102  also preferably includes the additional temperature sensor  136  that provides a measurement of the temperature of the plastic being extruded through the extrusion outlet  122 . Again, for the reasons discussed in association with the other figures, a pressure sensor  138  is also valuable for sensing the pressure immediately downstream of the extrusion outlet  122 . 
         [0066]    Knowledge of the sensed air temperature by temperature sensor  132  in what has previously been referred to as cavity A provides information with respect to the temperature and the pressure of the air that will be effectively sealed between the inner wall of the pipe and the outer wall of the pipe with respect to each corrugation. Each corrugation is effectively sealed by the plastic being extruded through the extrusion outlet  122  as it contacts the inner portion of the corrugations. This trapped air is locked in each corrugation once the inner wall has been secured to the outer wall. As the double wall corrugated pipe starts to cool down the volume of air within each corrugation remains the same (unless distortion occurs) however the temperature and pressure within the sealed corrugation thereof continue to decrease. The reduction in pressure can cause the inner wall to deform inwardly and thereby reduce the volume to partially compensate for the reduced pressure that otherwise would occur within the corrugation. This inward distortion of the inner wall causes the inner wall of the pipe to have a wavy or distorted surface and it is difficult to maintain a straight inner liner face (generally consistent internal diameter) which is the intended result. 
         [0067]    With the knowledge of the internal volume of the corrugation, the temperature of the air and pressure of the air when the corrugation volume is effectively sealed, it is possible to reduce any unwanted inward distortion of the inner wall. 
         [0068]    With respect to the sensing of pressure and temperature adjacent the inner wall extrusion outlet  122 , a different area of control is addressed. The temperature of the extruded plastic can significantly affect the fusion of the inner wall to the opposed portions of the corrugated wall. Thus the fusion of the inner and the outer wall can be assessed by monitoring the air temperature as plastic is extruded through the extrusion outlet  122 . This preferred extrusion temperature can be from about 210° C. down to about 150° C. depending upon the type of plastic being extruded and the particular conditions of the extruder. 
         [0069]    It is also desirable to include an ability to heat the die tooling adjacent the outlet  122  for example by circulating hot air through the die tooling or by providing a separate heat arrangement which can be electrically controlled to maintain a desired heat temperature. The sensing of both pressure and temperature generally associated with the extrusion outlet  120  and the extrusion outlet  122  allows the operator to understand the actual operating conditions of the extruder and make appropriate adjustments to achieve the desired quality and consistency of the extruded plastic pipe. 
         [0070]    Sensing of these temperatures and pressures allows an automated program or an operator to respond to actual extruding conditions and appropriately modify the extrusion process in an efficient manner. 
         [0071]    Although various preferred embodiments of the present invention have been described herein in detail, it will be appreciated by those skilled in the art, that variations may be made thereto without departing from the appended claims.