Description Technical Field

The present invention relates generally to process control and, more particularly, to methods and apparatus for recognizing a change in state of a material, such as the curing of phenolic resin in a plastic laminate curing press.

Background Art

In a typical process for manufacturing plastic laminate, multiple stacks of phenolic resin-impregnated sheets of paper-like material are heated in a curing press at least until the phenolic resin cures, and are then cooled. Each stack thus ultimately forms a single sheet of finished plastic laminate material. A typical sheet size is 4 feet by 8 feet (1.2 meters x 2.4 meters). The press applies a pressure of approximately 4000 psi (276 bar) . A typical curing temperature, which varies depending upon the particular formulation, is within the approximate range of 270° F to 300° F (132° C to 149° C) . Conventionally, superheated hot water at approximately 400° F (204° C) is employed to heat the press, although any suitable source of heat may be employed.

For efficient press utilization, it is common to assemble a pile including multiple stacks of resin-impregnated sheets, for example fourteen stacks, each including five or six sheets of phenolic resin-impregnated paper-like material. The stacks actually are placed back-to-back in double stacks separated by release paper. More particularly, the pile includes alternating double stacks of resin-impregnated sheets and metal separator sheets, such as stainless steel sheets. The entire pile, including the multiple stacks, is heated between the press plates to produce a finished sheet for each stack.

For efficient press utilization, it is also desirable to minimize the length of time required for each curing cycle, thus maximizing the production rate, over time, of each press. However, if a press heating cycle is terminated prematurely, then the phenolic resin is not properly cured, resulting in defective product. Accordingly, it is conventional to make a press heating cycle longer than the minimum required in order to ensure the phenolic resin is completely cured. Thus it is considered better to sacrifice some production efficiency in order to minimize the risk of producing a bad batch of plastic laminate product due to an incomplete cure. Nevertheless, if the heating cycle is made unnecessarily long, production efficiency adversely suffers. In order to determine when curing is completed, for typical prior art press operation disposable thermocouples are employed, for example "J"-type thermocouples, inserted into one or more of the stacks among the resin-impregnated sheets. The thermocouple is connected through appropriate signal conditioning and interface circuitry to a controller, which is thus able to sense the temperature of the resin-impregnated sheets during the curing cycle.

In typical prior art operation of a press, superheated hot water at approximately 400° F (204° C) circulating through heat exchangers is employed to heat the press plates until temperature of the material, as sensed by the thermocouple, reaches 260° F (127° C) . Heating then continues for a predetermined interval based on time, for example, eight minutes, during which interval temperature continues to rise at a rate of approximately 3° F (1.6667° C) per minute. At the end of the eight-minute additional interval, the material is assumed to be cured, there being an adequate margin to ensure the phenolic resin is completely cured. Total heating time is typically approximately twenty eight minutes. The heat is then turned off, and the press and stacks are then cooled down,

which takes approximately thirty five to forty minutes. Cool-down is forced by pumping cooler water through the press and the heat exchangers.

There are two disadvantages in particular of this prior art technique for operating a plastic laminate curing press. One disadvantage is that, when the phenolic resin cures, the thermocouple is permanently embedded in the resultant plastic laminate product, and accordingly cannot be reused. Thus, the process consumes thermocouples. Another disadvantage, perhaps more significant, is that press production efficiency is not as high as possible due to the intentional prolonging of the heating cycle beyond the minimum required in order to ensure the phenolic resin is completely cured. In part, this results from the indirect manner in which curing of the phenolic resin is sensed; it is simply assumed that, once a particular temperature is reached, curing is assuredly complete.

While the present invention is concerned most directly with the curing of phenolic resin in a plastic laminate curing press, the invention disclosed herein has broader applicability and, in particular, is applicable to a variety of processes where a material changes state. Thus, by way of example and not limitation, in addition to the change in state of phenolic resin changing from liquid to solid form, the invention relates to the change in state of other resins from liquid to solid form, such as the change in state of an epoxy resin from liquid to solid form.

Disclosure of Invention

Briefly, and in overview, the invention is based both on the discovery that many materials, while changing state, produce a measurable voltage which decreases to approximately zero when the change of state is completed, as well as on a recognition that this measured voltage can be applied in a practical manner to process control

applications, such as the control of a plastic laminate curing press.

In accordance with a more particular aspect of the invention, there is provided a method for controlling a plastic laminate curing press having a pair of press plates between which at least one stack of phenolic resin-impregnated sheets is heated. The method includes the steps of employing a pair of metallic elements on either side of the stack as electrodes. Preferably, these metallic elements employed as electrodes are metal separator sheets employed between stacks of resin-impregnated sheets in a pile including alternating stacks of resin-impregnated sheets and metal separator sheets. In accordance with the invention, voltage between electrodes is monitored while the stack is being heated, and heating of the stack is terminated when the monitored voltage decreases below a predetermined threshold voltage, that is, when the monitored threshold voltage decreases to near zero. Preferably, in order to avoid premature termination of the press heating cycle as sensed voltage fluctuates around zero at the beginning of the press heating cycle, a predetermined delay interval must elapse before heating of the stack can be terminated.

In accordance with another more particular aspect of the invention, there is provided a controller for controlling a plastic laminate curing press having a pair of press plates between which at least one stack of phenolic resin-impregnated sheets is heated. The controller includes a heat control element for controlling heating of the stack, a sensing circuit connected to electrodes on either side of the stack for monitoring voltage between electrodes, and a control device connected to the heat control element and to the sensing circuit and operable to initiate heating of the stack and operable to subsequently terminate heating of the stack when the voltage between the press plates decreases below a predetermined threshold voltage. Preferably, the control

device is operable to subsequently terminate heating of the stack when the voltage between the press plates decreases below the predetermined threshold voltage after a predetermined delay interval has elapsed. More generally, the invention provides a method for recognizing a change in state of a material, such as the curing of a resin from liquid to solid form, for example, a phenolic resin or an epoxy resin. Thus, the terminology "change in state" is employed herein in a broad sense to include, but without limitation, polymerization reactions. The method includes the steps of placing a pair of electrodes in contact with the material, monitoring voltage between the electrodes while the change in state is expected to occur, and recognizing the change in state when the monitored voltage decreases below a predetermined threshold voltage.

In some cases, what is herein termed a "change in state" is accompanied by an increase in voltage. In other cases, a peak or a dip in the voltage signal monitored over time indicates that a particular change in state has occurred, for example the onset or completion of a constituent boiling out of a complex material, such as alcohol boiling from phenolic resin. Thus, in accordance with the invention, a voltage signal is developed, and the rate of change of the voltage signal is determined. The change in state is recognized when the rate of change reaches zero.

The invention thus provides methods and apparatus for control of a plastic laminate curing press for increased production efficiency. The invention eliminates the need for thermocouples inserted within the stacks of resin-impregnated sheets during the manufacture of plastic laminate. The invention further provides methods and apparatus for more directly sensing when a change in state of a material has occurred, such as the change in state of a resin, such as a phenolic resin or an epoxy resin, from liquid to solid form.

Brief Description of Drawings

While the novel features of the invention are set forth with particularity in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings, in which:

FIG. 1 depicts generalized apparatus for recognizing a change in state of a material;

FIG. 2 is a graph plotting temperature and voltage as a function of time during a typical phenolic resin curing cycle as recorded during operation of the apparatus of FIG. 1; FIG. 3 is a highly schematic representation of a plastic laminate curing press and a controller in accordance with the invention; and

FIG. 4 is a simplified flowchart of a control program implemented in the controller of FIG. 3.

Best Modes for Carrying out the Invention

Referring first to FIGS, l and 2, in FIG. l a beaker 10 contains a quantity of test material 12, such as phenolic resin, and is heated over a hot plate 14. In contact with the material 12 are a pair of electrodes 16 and 18, such as stainless steel electrodes, connected via respective leads 20 and 22 to a strip chart recorder 24. In addition, a temperature sensing device 26, such as a thermocouple, is connected via a test lead 28 to the strip chart recorder 24. The particular strip chart recorder 24 employed has at least two channels, and is arranged to plot voltage and temperature as a function of time.

FIG. 2 is a plot or graph depicting a typical strip chart recording produced as the material 12 is heated. In FIG. 2, temperature is plotted as a dash line 30, while voltage between the electrodes 16 and 18 is plotted as a solid line 32.

As a phenomenon not fully understood, a measurable voltage, as represented by the plot line 32, is produced between the electrodes 16 and 18 as the phenolic resin 12 is heated and cures. This voltage eventually decreases to essentially zero when the resin 12 is completely cured, as indicated at a point 34 in the plot of FIG. 2.

While it is a DC voltage which is produced, the polarity is unpredictable. Thus, during successive test runs, either the electrode 16 or the electrode 18 is positive with reference to the other of the electrodes 16 and 18. Moreover, at the beginning of a run, as indicated generally at 36, the voltage may undergo fluctuations, including a polarity reversal, before settling out with one relative polarity or the other.

It may be noted that the two electrodes 16 and 18 are of the same material, such as stainless steel. The magnitude of the voltage depends to some extent on the area of the electrodes, and can range from a maximum of five or ten millivolts in the case of small electrodes placed in a test beaker 10, to more than ten volts in the case of electrodes of many square feet (several square meters) in a plastic laminate press embodiment. Thus, the phenomenon is believed to be something other than a simple battery effect.

Example Ordinary grocery bags were cut into eight ten-inch square (25.4 cm by 25.4 cm) pieces of paper, which were soaked in phenolic resin. A squeegee was employed to remove excess phenolic resin. The eight pieces were stacked and then sandwiched between two eleven-inch by twelve-inch (27.9 cm by 30.5 cm) stainless steel sheets, with a J-type thermocouple placed midway in the stack. One input of a strip chart recorder was connected to the thermocouple to record temperature, and another input of the strip chart recorder was connected to the two stainless

steel sheets to measure voltage across the stainless steel sheets. The paper sandwich, initially at a temperature of 50° F (10° C) , was placed horizontally in an oven heated to 300° F (149° C) , and a twenty-pound (nine kilogram) weight placed on top of the sandwich.

Heating of the sandwich commenced, with an initial recorded temperature of 50° F (10° C) and an initial recorded voltage of 1 millivolt. There was an initial voltage peak of 8 millivolts at 10 minutes, with a recorded temperature of 90° F (32° C) . Voltage decreased to approximately 2.5 millivolts at 25 minutes, with a recorded temperature of 130° F (54° C) . Voltage remained at approximately 2.5 millivolts for a further 10 minutes, beginning to rise at 35 minutes, with a recorded temperature of 150° F (66° C) , believed to coincide with the onset of alcohol boiling from the phenolic resin. There was a second voltage peak of 11 millivolts at 57 minutes, with a recorded temperature of 190° F (88° C) , believed to coincide with the substantial completion of alcohol boiling from the phenolic resin. Voltage decreased, reaching essentially 0 millivolts at 160 minutes, with a recorded temperature of 275° F (135° C) , at which time the phenolic resin was completely cured.

While experimental efforts have primarily been focused on the curing of phenolic resin as employed in the manufacture of plastic laminate, a variety of other materials have been tested, and many are found to generate to produce a measurable and useful voltage when a pair of electrodes are placed in contact with the material during a change in state. Such materials include resins in general, such as epoxy resins which cure when two components are mixed, latex paint, and others. The voltages are believed to be produced by the chemical reactions which effect the change in state. As noted hereinabove, the terminology "change in state" is employed herein in a broad sense to

include, but without limitation, polymerization reactions such as the curing of various resins.

Referring now to FIG. 3, in a practical embodiment of the invention a conventional plastic laminate curing press, generally designated 50, includes an upper press plate 52 and a lower press plate 54 which in turn include respective heat exchangers 56 and 58 and which are controllably heated by superheated hot water circulating through a fluid flow loop 60 heated by a controllable heating device 62. The heating device 62 is conventional, and includes a pump for circulating hot water through the loop 60. In addition, although not indicated in the drawing figure, the heating device 62 is also capable of circulating cooling water for cooling down the press 50 near the end of a curing cycle.

In accordance with conventional practice, a pile 64 of alternating double stacks 66, 68 and 70 of resin-impregnated sheets and metal separator sheets 72, 74, 76 and 78 is placed within the press 50, compressed between the press plates 52 and 54 with a pressure in the order of 4000 psi, and heated. Each of the double stacks 66, 68 and 70 conventionally comprises two individual stacks (not separately shown) of five or six resin-impregnated sheets each placed back-to-back and separated by release paper. On the front of each individual stack is a pattern sheet (not shown) placed directly against one of the metal separator sheets 72, 74, 76 and 78 to produce a glossy finish. The heating cycle commences and proceeds as is described generally above under the heading "Background of the Invention". It will be appreciated that, for purposes of illustration, the elements of the pile 64 are spaced and that, during actual press operation, the elements are all in contact.

Each of the illustrated double stacks 66, 68 and 70 results in the production of two sheets of plastic laminate, which are typically four feet by eight feet (1.2 meters x 2.4 meters) in size. Although a total of three

double stacks 66, 68 and 70 are illustrated, this particular number is for purposes of illustration only, as each pile 64 placed into the press 50 typically comprises seven double stacks, resulting in fourteen sheets of plastic laminate product being produced. An actual plastic laminate production press has from eighteen to thirty vertically-arranged openings (not shown) , and each opening receives one pile of double stacks. Thus, a typical plastic laminate production press with twenty-two openings produces three-hundred eight finished plastic laminate sheets per batch.

As discussed hereinabove, in the prior art approach control is based on temperature as sensed by a thermocouple (not shown) placed among the individual sheets of one or more of the stacks 66, 68 and 70. In accordance with the invention, rather than employing a thermocouple, two or more of the separator sheets 72, 74, 76 and 78 are employed as electrodes, such as the separator sheets 74 and 76. In FIG. 3, sensing leads 80 and 82 are connected to the separator sheets 74 and 76 at respective connection points 84 and 86, across which a voltage is developed, as represented by the voltmeter 88 connected to these points 84 and 86. The voltmeter 88 is included in FIG. 3 for purposes of illustration only, and would normally not be included in actual apparatus implementing the invention.

For controlling press operation, a controller 90 is provided, which may comprise an Allen-Bradley P.L.C., such as a Model 2/30, with an analog input card depicted in FIG. 3 as an interface circuit 92. The interface circuit 92 serves the function of applying suitable signal conditioning to voltage signals conducted along the leads 80 and 82, and converting the voltage to digital form for further processing in the controller 90. The interface circuit 92 is set up for high impedance voltage sensing, although a 10 K Ohm load resistor (not shown) may be connected across the input leads 80 and 82 for noise

reduction purposes. A two-second integration time is typically employed for filtering purposes.

The controller 90 is microprocessor-based, and is programmed in a conventional manner. It will be appreciated that the controller 90 is appropriately and conventionally interfaced to generally control operation of the press 50, such as opening and closing of the press 50, as well as control of the heating device 62, via a representative control line 94. Operation of the FIG. 3 controller 90 is represented in the program flowchart of FIG. 4. Briefly, the FIG. 4 control program is entered in box 100. In box 102, press operation is commenced by, among other things, turning on the press heat by activating the heating device 62. In order to allow time for adequate voltage to be developed as the chemical reactions associated with curing proceed, and additionally to avoid ambiguity caused by initial polarity reversals as indicated in FIG. 2, decision box 104 implements a predetermined delay interval, such as twenty minutes, before the controller 90 even begins to consider whether to terminate the heating operation. Thus, even though an increase in production efficiency is desired, it is assumed that under no circumstances could curing be completed in less than twenty minutes. Thus, in decision box 104, the question is continually asked whether the delay interval has elapsed. So long as the answer is "no", then execution flow path 106 is taken, implementing a wait loop.

When the answer in decision box 104 is eventually "yes", indicating the exemplary twenty-minute delay interval has elapsed, then, in box 108, the voltage across the electrodes comprising the separator plates 74 and 76 is measured via the interface circuit 92. In decision box 110, this voltage is compared to a threshold voltage selected to determine when the measured voltage is near zero. So long as appreciable voltage between the electrodes comprising the separator plates 74 and 76 is

sensed, the answer in decision box 110 is "no", and program execution loops back along execution flow path 112 back to box 108 where the voltage is again measured.

When curing of the phenolic resin is completed, the answer is decision box 110 is then "yes", and box 114 is entered to initiate press cool down by turning off press heat and, typically, instituting active cool down in a conventional manner as indicated in box 116. When cool down is completed, which takes approximately thirty five to forty minutes, the press is opened, and execution terminates at 118.

While specific embodiments of the invention have been illustrated and described herein, it is realized that numerous modifications and changes will occur to those skilled in the art. It is therefore to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of the invention.

Industrial Applicability The way in which the invention is capable of being exploited and the way in which it can be made and used will be apparent from the foregoing. The invention thus provides methods and apparatus for control of a plastic laminate curing press for increased production efficiency and without requiring thermocouples inserted within the stacks of resin-impregnated sheets.