Abstract:
A dwell time control system and method for automatically adjusting the selection and timing of a sequence of pressures used to drive the plunger in a parison mold during the parison forming process. The timing of characteristics of the observed press curve from one or more previous parison forming cycles are ascertained and used to control the timing of the changes in pressure during a subsequent parison forming cycle. The timings of these changes of pressure are determined as predetermined percentages of the timings of the characteristics in order to prevent the blow mold from being forced open and in order to prevent the occurrence of an overpressed finish.

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0001]    The present invention relates generally to the pressure used to operate the plunger in a parison mold, and more particularly to a dwell time control system and method for automatically adjusting the selection and timing of a sequence of pressures used to drive the plunger during the parison forming process. 
         [0002]    Glass containers are made in a manufacturing process that has three parts, namely the batch house, the hot end, and the cold end. The batch house is where the raw materials for glass (typically including sand, soda ash, limestone, cullet (crushed, recycled glass), and other raw materials) are prepared and mixed into batches. The hot end begins with a furnace, in which the batched materials are melted into molten glass, and from which a stream of molten glass flows. 
         [0003]    The molten glass is cut into cylinders of glass called gobs, which fall by gravity into blank molds, sometimes referred to as parison molds. In the blank molds, a pre-container referred to as a parison is formed, typically by using a metal plunger to push the glass into the blank mold, or alternately by blowing the glass from below into the blank mold. The parison is inverted and transferred to a mold, where it is blown out into the shape of the container. An annealing process performed in an annealing oven or Lehr heats the containers and then slowly and evenly cools them over an extended time period to prevent them from having weakened glass caused by stresses caused by uneven cooling. The equipment at the cold end of the glass container manufacturing process inspects the containers to ensure that they are of acceptable quality. 
         [0004]    The present invention is concerned with the parison formation process using a plunger to push the glass into the blank mold. Parisons are molded in a blank mold in an inverted position. The blank mold has two halves, and completing the finish portion are two neck ring molds located below the blank mold halves, with an upwardly oriented plunger extending through the neck ring halves and into the bottom of the blank mold halves. The blank mold halves are open at the tops thereof, and a gob of molten glass drops through this opening into the blank mold halves. A baffle is placed on top of the blank mold halves to close the opening at the top thereof, and the plunger is raised to force the gob to fill the entire cavity defined by the blank mold halves, the neck ring halves, and the baffle, thereby forming the parison. Upon completion of the cycle, the baffle is removed and the mold halves open, with the neck ring halves then transporting the parison to the blow molds. 
         [0005]    Plunger contact time or dwell time is a particularly important parameter when producing in a narrow neck press and blow glass container manufacturing process or in a press and blow production in general. The full contact of the plunger with the glass in the gob that occurs during plunger contact or dwell time influences the characteristics of parisons produced for use in further steps in the glass container forming process. While dwell time depends on a number of parameters including friction in the movement of the plunger and glass temperature, it can also be strongly influenced by the pressure driving the plunger in its upward motion. 
         [0006]    The plunger was formerly driven by a hydraulic system, as shown for example in U.S. Pat. No. 4,662,923, to Vajda et al. and U.S. Pat. No. 4,867,778, to Pinkerton et al., both of which are assigned to the assignee of the present patent application, and both of which are hereby incorporated herein by reference in their entirety. Both of these patents used feedback to monitor the position of the plunger and to use plunger position information to control the parison formation process to improve parison uniformity and quality. 
         [0007]    In order to reduce the risk of fire associated with the use of hydraulic fluid in the operation of the plunger and other system components, pneumatic systems using compressed air were adopted, as illustrated in European Patent No. 0691940, to Plater et al., and in U.S. Pat. No. 5,800,590, to Pilskar, both of which are assigned to the assignee of the present patent application, and both of which are hereby incorporated herein by reference in their entirety. The &#39;940 patent used a proportional control valve operated by a microcontroller dependent upon position and pressure feedback signals from the plunger drive piston and cylinder. The &#39;590 patent used an initial higher pressure for a short time followed by a succeeding lower pressure that was approximately 70% of the initial higher pressure to operate the plunger. 
         [0008]    The operation of the plunger was further refined by controlling the movement of the plunger, as illustrated in U.S. Pat. No. 6,050,172, to Schwegler et al., and in U.S. Pat. No. 7,290,406, to Anheyer, both of which are assigned to the assignee of the present patent application, and both of which are hereby incorporated herein by reference in their entirety. The &#39;172 patent controls the timing of valves providing compressed air to both sides of a piston driving the plunger, and the &#39;406 patent provides a feedback control system for driving the plunger at desired speeds. 
         [0009]    After comparing the determined value with the desired dwell time, past closed loop controller increased or decreased the pressure for driving up the plunger until the resulting dwell time corresponds to the desired dwell time value has been achieved. However, simply increasing the pressure for plunger movement resulted in bottle defects, especially during dwell time. An alternative solution was moving the plunger up with different pressures (high, medium, low). However, this alternative presented problems in selecting when to switch from a higher to a lower pressure. 
         [0010]    An illustration of such a problem is found in European Patent No. 1466871, to Krumme, which is hereby incorporated herein by reference in its entirety, describes a method of operating the plunger that somewhat varies the teachings of the &#39;590 patent to have second and third different lower pressures following an initial higher pressure to operate the plunger. The second pressure is controlled to bring the plunger to completely fill the cavity defined by the mold halves, the neck ring halves, and the baffle at a fixed time at which point a fixed pressing time at the third pressure begins, which third pressure may be less than (in the primary embodiment) or greater than (in an alternate embodiment) the second pressure. Thus, the duration of the applications of the first and third pressures is predetermined (meaning that the duration of the second pressure is also predetermined since the overall machine is operating at a predetermined speed), with the only variable being selecting the second pressure to be sufficient to completely fill the cavity by the end of application of the second pressure. 
         [0011]    A key deficiency of the &#39;871 patent is that the detection of the point at which the plunger has completely filled the cavity is made by detecting that the plunger has reached a predefined position rather than actually detecting when the plunger has completely filled the cavity (see paragraph 0012 and claim 2 of the &#39;871 patent). Measuring the position of the plunger may be performed, for example, using the device disclosed in U.S. Pat. No. 6,185,829, to Geisel, which is hereby incorporated herein by reference in its entirety. Further, since the first pressure is only maintained for a short period of time, the operation of the plunger with the second pressure must be sufficiently high to reach the predefined position in the required time period, but not so high that it will drive open the mold halves (see the last sentence in paragraph 0010 of the &#39;871 patent). This is a compromise that necessarily cannot result in optimizing system performance. Due to the difficulties associated with multi-pressure pressing, most glass container manufacturing plants still press with only a single pressure level that is sufficiently low to prevent the related defects, but also certainly less than an optimal solution. 
         [0012]    It is accordingly desirable that the present invention provide an improved dwell time control method and system that results in the ability to control the dwell time (the time that the plunger is in full contact with the parison). It is also desirable that the improved dwell time control method and system automate the pressure switching process without requiring operator input once the process has been initiated. It is further desirable that the dwell time control method and system prevent the inadvertent opening of molds due to the occurrence of overpressure situations. 
         [0013]    The dwell time control method and system of the present invention must also be of construction which is both durable and long lasting, and it should also require little or no maintenance to be provided by the user throughout its operating lifetime. In order to enhance the market appeal of the dwell time control method and system of the present invention, it should also be of inexpensive construction to thereby afford it the broadest possible market. Finally, it is also an objective that all of the aforesaid advantages of the dwell time control method and system of the present invention be achievable without incurring any substantial relative disadvantage. 
       SUMMARY OF THE INVENTION 
       [0014]    The disadvantages and limitations of the background art discussed above are overcome by the present invention. With this invention, the operation of the plunger is controlled to optimize the dwell time of the plunger in contact with the parison. It results in the ability to fully automate the pressure switching process without requiring operator input once the process has been initiated. It also prevents the blank molds from being inadvertently forced open due to the occurrence of overpressure situations in the operation of the plunger. 
         [0015]    The dwell time control method and system of the present invention uses three consecutive pressures to operate the plunger to form the parison from the glass gob in the blank mold. The total time for operating the plunger to form the parison is predefined and unchangeable time period since it is established by the operational cycle timing of the I.S. machine, so the timings that are variable are the time that the pressure is changed from the first pressure to the second pressure, and the time that the second pressure is changed to the third pressure. The dwell time control method and system of the present invention bases these times on the observed press curve from one or more previous parison forming cycles. 
         [0016]    The timing of two characteristics of the observed press curve from one or more previous parison forming cycles are determined: the time at which the upper part of the mold becomes filled with glass from the glass gob that causes an increase in the resistance encountered by the parison is detected by the occurrence of a nonlinearity in the press curve; and the time at which the mold becomes completely filled with glass from the glass gob that results in a slowing in the movement of the plunger below a particular level. By ascertaining these times (each of which is measured from the initiation of the parison forming cycle), the times at which pressure changes can be determined. 
         [0017]    The time that the pressure is changed from the first pressure to the second pressure is a first predetermined percentage of the ascertained time at which the upper part of the mold becomes filled with glass, and the time that the pressure is changed from the second pressure to the third pressure is a second predetermined percentage of the time at which the mold becomes completely filled with glass. The two characteristics from one previous parison forming cycle may be used, or more than one previous parison forming cycles may be used by averaging the ascertained times from the previous parison forming cycles. The first predetermined percentage is less than one hundred percent in order to prevent the blow mold from being forced open, and the second predetermined percentage is less than one hundred percent in order to prevent the occurrence of an overpressed finish. 
         [0018]    In a method of implementing the dwell time control method and system of the present invention: the position of the plunger in the blank mold is monitored with respect to time during at least one parison forming cycle beginning at a time t 1  and ending at a time t 4 ; a time t 2  is determined in each monitored parison forming cycle at which a first characteristic of the movement of the plunger during the parison forming cycle is detected; a time t 3  is determined in each monitored parison forming cycle at which a second characteristic of the movement of the plunger during the parison forming cycle is detected; during each parison forming cycle, after a gob is loaded into the blank mold, applying a first pressure from time t 2  to time t p2 , a second pressure from time t p2  to time t p3 , and a third pressure from time t p3  to time t 4 ; wherein the time interval between time t 1  and time t p2  is a first predetermined percentage of a time interval based upon the time interval between time t 1  and time t 2  for one or more previous parison forming cycles; and wherein the time interval between time t 1  and time t p3  is a second predetermined percentage of a time interval based upon the time interval between time t 1  and time t 3  for one or more previous parison forming cycles. 
         [0019]    Pursuant to this method: the first characteristic of the movement of the plunger may be a nonlinearity exhibited by the movement of the parison with respect to time which is indicative of an upper part of the mold having been filled with glass from the glass gob, and the time t 2  in each monitored parison forming cycle is the time at which an upper part of the mold has been filled with glass from the glass gob; and the second characteristic of the movement of the plunger may be a movement-related characteristic of the plunger falls below a preselected level which is indicative of the glass from the glass gob has been distributed throughout the entire blank mold to completely fill it, and the time t 3  in each monitored parison forming cycle is the time at which the glass from the glass gob has been distributed throughout the entire blank mold to completely fill it. 
         [0020]    In a system for implementing the dwell time control method and system of the present invention: a position sensor monitors the position of the plunger in the blank mold versus time during at least one parison forming cycle beginning at a time t 1  and ending at a time t 4 ; a control system determines a time t 2  in each monitored parison forming cycle at which a first characteristic of the movement of the plunger during the parison forming cycle is detected, determines a time t 3  in each monitored parison forming cycle at which a second characteristic of the movement of the plunger during the parison forming cycle is detected, and operates the source of a pressurized medium during each parison forming cycle, after a gob is loaded into the blank mold, to apply a first pressure from time t 1  to time t p2 , a second pressure from time t p2  to time t p3 , and a third pressure from time t p3  to time t 4 ; wherein the time interval between time t 1  and time t p2  is calculated by the control system to be a first predetermined percentage of a time interval based upon the time interval between time t 1  and time t 2  for one or more previous parison forming cycles; and wherein the time interval between time t 1  and time t p3  is calculated by the control system to be a second predetermined percentage of a time interval based upon the time interval between time t 1  and time t 3  for one or more previous parison forming cycles. 
         [0021]    It may therefore be seen that the present invention teaches an improved dwell time control method and system that results in the ability to control the dwell time (the time that the plunger is in full contact with the parison). The improved dwell time control method and system automates the pressure switching process without requiring operator input once the process has been initiated. The dwell time control method and system also prevents the inadvertent opening of molds due to the occurrence of overpressure situations. 
         [0022]    The dwell time control method and system of the present invention is of a construction which is both durable and long lasting, and which will require little or no maintenance to be provided by the user throughout its operating lifetime. The dwell time control method and system of the present invention is also of inexpensive construction to enhance its market appeal and to thereby afford it the broadest possible market. Finally, all of the aforesaid advantages and objectives of the dwell time control method and system of the present invention are achieved without incurring any substantial relative disadvantage. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0023]    These and other advantages of the present invention are best understood with reference to the drawings, in which: 
           [0024]      FIG. 1  is a schematic cross-sectional view of a blank mold and an associated plunger mechanism illustrating a glass gob in the blank mold with the plunger in the loading position in the blank mold; and 
           [0025]      FIG. 2  depicts two time-aligned plots associated with the dwell time control method and system of the present invention, with the top plot showing the pressure supplied to the plunger mechanism illustrated in  FIG. 1  to press it into the glass gob to form a parison, and the bottom plot showing the actual position of the plunger in the blank mold. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    Referring first to  FIG. 1 , a blank mold and an associated plunger mechanism are illustrated. The mold includes two mold halves  30  and  32 , which are shown as being closed atop two neck ring halves  34  and  36 . A plunger  38  is shown extending upwardly into the bottom of the mold halves  30  and  32 , with the plunger  28  being in the loading position in the mold halves  30  and  32 . A glass gob  40  is shown loaded into the mold halves  30  and  32 , with a baffle  42  shown atop the mold halves  30  and  32  and closing the top ends thereof. 
         [0027]    The plunger operating mechanism is housed by a lower cylinder  44 , and upper cylinder  46  on top of the lower cylinder  44 , and a cylinder cap  48  that is mounted on top of the upper cylinder  46 . A hollow sleeve  50  has a cylindrical upper portion  52  that extends upwardly through the cylinder cap  48  and into the area between the bottom portions of the neck ring halves  34  and  36 . The sleeve  50  has a central portion that includes an outwardly extending circular flange  54 , and a bottom portion  56 . A guiding ring  58  is located at the top of the upper portion  52  of the sleeve  50 , and the plunger  38  extends through the upper portion  52  of the sleeve  50  and the guiding ring  58  and into the bottom of the mold halves  30  and  32 . 
         [0028]    The bottom of the plunger  38  is mounted onto the top of a plunger base  60 , which is slidably mounted in the interior of the sleeve  50 . The bottom of the upper cylinder  46  has a reduced diameter aperture located therein in which a piston rod  62  is slidably mounted. The piston rod  62  is connected at its top end thereof to the bottom of the plunger base  60 , and at its bottom end to the top of a piston  64  that is slidably mounted in the bottom cylinder  44 . It will be appreciated that movement of the piston  64  in the lower cylinder  44  will drive the plunger  38 . 
         [0029]    A cooling tube  66  extends from the closed bottom of the lower cylinder  44  upwardly through the piston  64  and into the hollow interior of the piston rod  64  to provide cooling fluid thereinto. A spring  68  is mounted in the upper cylinder  46 , and extends between the top side of the bottom of the upper cylinder  46  and the bottom side of the circular flange  54 . The spring  68  functions to bias the plunger  38  to its loading position as shown in  FIG. 1  by driving the circular flange  54  of the sleeve  50  into contact with the underside of the cylinder cap  48  in the absence of any downward pressure on the piston  64  in the lower cylinder  44 . 
         [0030]    Pressurized fluid (typically compressed air) may be supplied to drive the piston  64  and the plunger upwardly through a first or lower inlet  70 , and pressurized fluid may be supplied to drive the piston  64  and the plunger  38  downwardly through a second or upper inlet  72 . It should be noted that in order to drive the plunger  38  downwardly from the loading position it is illustrated in  FIG. 1  it is necessary to overcome the force of the spring  68 . This will also cause the sleeve  50  and the guiding ring  58  to be lowered from their respective positions illustrated in  FIG. 1  to somewhat retract them from the neck ring halves  34  and  36 . 
         [0031]    Pressurized fluid is supplied from a first pressure source  74  with both the flow of pressurized fluid from the first pressure source  74  and the pressure at which the pressurized fluid is supplied to the lower inlet  70  being controlled by a first proportional valve  76 . Similarly, pressurized fluid is supplied from a second pressure source  78  with both the flow of pressurized fluid from the second pressure source  78  and the pressure at which the pressurized fluid is supplied to the upper inlet  72  being controlled by a second proportional valve  80  (although a simple on/off valve will also suffice since the function is simply to drive the piston  64  to retract the plunger downwardly). 
         [0032]    The operation of the first proportional valve  76  and the second proportional valve  80  are controlled by a control system  82 , which stores programmed information and data in a memory  84 . The operation of the control system  82  may be monitored on a display  86 , and controlled using an input control  88 . Information regarding the position of the plunger  38  is provided by a position sensor  90  that monitors the position of the piston rod  62 , the movement of which corresponds with the position of the distal end of the plunger  38  in the mold halves  30  and  32 . The position sensor  90  uses the relative positions of the piston  64  and the piston rod  62  with respect to the cooling tube  66  to provide an input regarding the position of the plunger  38  to the control system  82 . 
         [0033]    Referring next to  FIG. 2 , an exemplary use of a three-pressure operation to drive the plunger  38  (shown in  FIG. 1 ) from the loading position (in which it is illustrated in  FIG. 1 ) to form the parison from the glass gob in the blank mold is illustrated. According to the teachings of the present invention, the three consecutive pressures, referred to herein as p 1 , p 2 , and p 3 , are cumulatively applied during a time period beginning at time t 1  and ending at time t 4 . It will be appreciated by those skilled in the art that a single cycle of the blow molding process lasts for a predefined and unchangeable time period that is determined by the operational speed of the I.S. machine (typically one full cycle lasts for approximately four to five seconds). Similarly, the time period beginning at time t 1  and ending at time t 4  is a predefined and similarly unchangeable time period that is established by the timing of the cycle of the operations of the I.S. machine (typically this time period is approximately one second). 
         [0034]    The dwell time control method and system of the present invention detects two events that occur during the time period that begins at time t 2  and ends at time t 4 , with the respective times at which these two events occur being time t 2  and time t 3 . The first of these events, which occurs at time t 2 , is when the plunger  38  (shown in  FIG. 1 ) has forced the glass gob  40  (also shown in  FIG. 1 ) to hit the baffle  42  (also shown in  FIG. 1 ), at which point a non-linear increase in resistance to further movement of the plunger  38  due to the upper part of the mold having been filled with glass from the glass gob  40 . 
         [0035]    This may be seen in  FIG. 2  in the bottom plot which shows the position of the plunger  38  in the blank mold at the point identified by the intersection of the plot with the time t 2 . At the point where the upper part of the mold is completely filled with glass from the glass gob  40 , there is a readily observable nonlinear characteristic or “knee” in the plot of the position of the plunger  38  in the blank mold. This time t 2  may be detected by the dwell time control method and system of the present invention by monitoring the first and second derivatives (velocity and acceleration) of the position of the plunger  38  in the blank mold. 
         [0036]    The second of these events, which occurs at time t 3 , is when the first and second derivatives (velocity and acceleration) of the plunger  38  have fallen below preset levels, which generally occurs when the glass from the glass gob  40  has been distributed throughout the entire blank mold, completely filling it. This may be seen in  FIG. 2  in the bottom plot showing the position of the plunger  38  in the blank mold at the point identified by the intersection of the plot with the time t 3 . The time period from time t 1  to time t 3  is parison forming time and is also referred to as the “pressing time.” During the time period beginning at time t 3  and ending at time t 4 , the final pressing of the glass in the mold into a parison occurs. This time period, which is commonly referred to as the “dwell time,” is generally at least a certain time period, for example approximately between 400 and 600 milliseconds. 
         [0037]    Thus, what can be varied by the dwell time control method and system of the present invention are the time at which the first pressure p 1  is changed to the second pressure p 2 , which time will be referred to herein as time t p2 , and the time at which the second pressure p 2  is changed to the third pressure p 3 , which time will be referred to herein as time t p3 . The present invention uses the measured times t 2  and t 3  of two detected events from the plot of the position of the plunger  38  in the blank mold during previous cycles as the triggering events to calculate the time t p2  at which the pressure applied to the plunger  38  will change from p 1  to p 2 , and the time t p3  at which the pressure applied to the plunger  38  will change from p 2  to p 3 . 
         [0038]    The first pressure p 1  is highest since higher pressure is needed to overcome initial friction and to accelerate the movement of the plunger  38 . However, this higher first pressure p 1  must be removed before the glass in the glass gob  40  hits the baffle  42  in order to prevent the blow mold from being forced open. In order to ensure that this does not happen, the time interval between time t 1  and time t p2  after which the pressure applied to the plunger  38  will change from p 1  to p 2  is selected to be a percentage of the measured time interval between time t 1  and time t 2  for one or more previous I.S. machine cycles (if this time interval is measured for more than one machine cycle, the measured times may be averaged). 
         [0039]    In a preferred embodiment, the time interval between time t 1  and time t p2  can vary from approximately sixty percent to approximately ninety-five percent of the time interval between time t 1  and time t 2 . In a more preferred embodiment, the time interval between time t 1  and time t p2  can vary from approximately seventy percent to approximately ninety percent of the time interval between time t 1  and time t 2 . In a most preferred embodiment, the time interval between time t 1  and time t p2  is approximately eighty percent of the time interval between time t 1  and time t 2 . 
         [0040]    The number of prior cycles over which the time interval between time t 1  and time t 2  can be measured and averaged may be varied from one cycle (in which case no averaging is needed) to one hundred cycles or even more in preferred embodiments, with consideration being given to a balancing of only recent cycles being used and a greater number of cycles being used. In a more preferred embodiment, this balancing uses a number of cycles that is between approximately three cycles and approximately twenty cycles to calculate the average, and in a most preferred embodiment, this balancing uses approximately eight cycles to calculate the average. In each case, the measurements of the time interval between time t 1  and time t 2  are used for the given number of immediately preceding cycles, so that a new average value is calculated for each succeeding cycle. 
         [0041]    The third pressure p 3  may be lower than the second pressure p 2  in order to have a higher pressure p 2  to complete the pressing time of the glass gob  40  in the blank mold quickly and to have a lower pressure p 3  in order to prevent the occurrence of an overpressed finish. In this case, this higher second pressure p 2  should be removed before the glass in the glass gob  40  fills the blank mold in order to prevent the finish from being overpressed. In order to ensure that this does not happen, the time interval between time t 1  and time t p , after which the pressure applied to the plunger  38  will change from p 2  to a lower p 3  is selected to be a percentage of the measured time interval between time t 1  and time t 3  (alternately, it could instead be a percentage of the measured time interval between time t p2  and time t 3 , or even a percentage of the measured time interval between time t 2  and time t 3 , although these alternatives are not the most preferred implementation of the dwell time control method and system of the present invention). 
         [0042]    In a preferred embodiment, the time interval between time t 1  and time t p3  can vary from approximately fifty percent to approximately ninety percent of the time interval between time t 1  and time t 3 . In a more preferred embodiment, the time interval between time t 1  and time t p3  can vary from approximately sixty percent to approximately eighty percent of the time interval between time t 1  and time t 3 . In a most preferred embodiment, the time interval between time t 1  and time t p3  is approximately seventy percent of the time interval between t 1  and t 3 . 
         [0043]    The number of prior cycles over which the time interval between time t 1  and time t 3  can be measured and averaged may be varied from one cycle (in which case no averaging is needed) to one hundred cycles or even more in preferred embodiments, with consideration being given to a balancing of only recent cycles being used and a greater number of cycles being used. In a more preferred embodiment, this balancing uses a number of cycles that is between approximately three cycles and approximately twenty cycles to calculate the average, and in a most preferred embodiment, this balancing uses approximately eight cycles to calculate the average. In each case, the measurements of the time interval between time t 1  and time t 3  are used for the given number of immediately preceding cycles, so that a new average value is calculated for each succeeding cycle. 
         [0044]    If the first alternate embodiment mentioned above is used instead, the time interval between time t p2  and time t p3  can vary from approximately forty-five percent to approximately eighty-five percent of the time interval between time t p2  and time t 3 . In a more preferred embodiment, the time interval between time t p2  and time t p3  can vary from approximately fifty-five percent to approximately seventy-five percent of the time interval between time t p2  and time t 3 . In a most preferred embodiment, the time interval between time t p2  and time t p3  is approximately sixty-five percent of the time interval between time t p2  and time t 3 . 
         [0045]    In some instances (such as, for example, producing wide mouth glass containers) it may be desirable to have p 3  be greater than p 2  (and also to have p 2  be greater than p 1 ). This may be done because during the dwell time the plunger  38  is in contact with the parison in the glass gob  40  in the blank mold, and as such is either not moving or moving at such an exceedingly low rate that it has essentially no momentum. As such, it may be possible for the dwell time pressure to be higher than the second pressure p 2  used during the pressing time, although this alternatives is generally not the most preferred implementation of the dwell time control method and system of the present invention (except perhaps in the production of wide mouth glass containers). 
         [0046]    Since the time period that begins at time t 1  and ends at time t 4  is fixed, and since it is desirable to have a dwell time beginning at time t 3  and ending at time t 4  that is at least a minimum time period long, such as, for example, between approximately 400 and 600 milliseconds long, it is possible in an alternate embodiment to have the objective of defining a desired value for the time t 3 . By varying the values of either the second pressure p 2  only, or by varying the values of both the first pressure p 1  and the second pressure p 2  with them in a fixed relationship (e.g., the first pressure p 1  is equal to 1.12 times the second pressure p 2 ), this objective for a dwell time beginning at a desired value for the time t 3  can be realized in relatively few parison forming cycles. 
         [0047]    Depending upon the specific mold design, various loading possibilities, and the variations possible in other parameters, virtually every possible combination of p 1 , p 2 , p 3  levels could, in some instances, make sense. All possible combinations are thus viewed as being encompassed by the improved dwell time control method and system. 
         [0048]    It may therefore be appreciated from the above detailed description of the preferred embodiment of the present invention that it teaches an improved dwell time control method and system that results in the ability to control the dwell time (the time that the plunger is in full contact with the parison in the gob). The improved dwell time control method and system automates the pressure switching process without requiring operator input once the process has been initiated. The dwell time control method and system also prevents the inadvertent opening of molds due to the occurrence of overpressure situations. 
         [0049]    The dwell time control method and system of the present invention is of a construction which is both durable and long lasting, and which will require little or no maintenance to be provided by the user throughout its operating lifetime. The dwell time control method and system of the present invention is also of inexpensive construction to enhance its market appeal and to thereby afford it the broadest possible market. Finally, all of the aforesaid advantages and objectives of the dwell time control method and system of the present invention are achieved without incurring any substantial relative disadvantage. 
         [0050]    Although the foregoing description of the dwell time control method and system of the present invention has been shown and described with reference to particular embodiments and applications thereof, it has been presented for purposes of illustration and description and is not intended to be exhaustive or to limit the invention to the particular embodiments and applications disclosed. It will be apparent to those having ordinary skill in the art that a number of changes, modifications, variations, or alterations to the invention as described herein may be made, none of which depart from the spirit or scope of the present invention. The particular embodiments and applications were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such changes, modifications, variations, and alterations should therefore be seen as being within the scope of the present invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.