GB2294898A

GB2294898A - Control of a fusion process by determination of specific heat variation

Info

Publication number: GB2294898A
Application number: GB9419334A
Authority: GB
Inventors: Masaru Kumagai
Original assignee: Toa Kokyu Tugitevarubu Seizo Co Ltd; Tokushu Kougyo Co
Current assignee: Toa Kokyu Tugitevarubu Seizo Co Ltd; Tokushu Kougyo Co
Priority date: 1994-09-26
Filing date: 1994-09-26
Publication date: 1996-05-15
Anticipated expiration: 2014-09-26
Also published as: GB9419334D0; FR2725658A1; FR2725658B1; GB2294898B; DE4435393A1; DE4435393B4

Abstract

In a method of automatically controlling the fusion process of thermoplastic resin articles a current is fed to a resistor for fusing together the resin surfaces facing one another. Electrical characteristics of the resistor is calculated upon increase of the power to a given level, also calculating specific heat of the resin through ON-OFF control of the resistor. The power is then raised through stages based on the characteristics and the specific heat, to determine variation coefficients of the characteristics and specific heat, at each stage and through further ON-OFF control. The power is stopped as the variation coefficient becomes smaller than a given value, with the ON-OFF control continued until the specific heat becomes equal to or lower than a preset value to cease the power supply. The method may be applied to pipe joints (Figures 2 - 4) or fusible tapes (Figures 5 and 6). <IMAGE>

Description

2294898 A METHOD OF AUTOMATICALLY CONTROLLING THE FUSION PROCESS BETWEEN

THERMOPLASTIC ARTICLES

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to a method of au- tomatically controlling the fusion process between ther moplastic articles.

The so-called electro-fusion joints ( hereinafter referred to as 'EF joints') are and has been used to fixedly adjoin articles one to another. Examples of those articles made of thermoplastic resins include water pipes, drain pipes and gas pipes. Each of the EF joints comprises a body and an electric heating wire embedded therein, with the body being also a piece made of an appropriate thermoplastic. An electric current will be supplied to the electric heating wire to heat and fuse in part the thermoplastic joint and the pipes so that they become integral with each other.

PRIOR ART

Electro-fusion controllers ( hereinafter referred to as 'EF controllers') of the prior art types have some control patterns adapted to energize the electric heating wire. Those patterns corresponding to the types of EF joints are stored in memory devices in the EF controllers. The most proper pattern will be selected for the EF joint in use, in order to automatically control the electric power for the fusion process.

However, electrical characteristics of the EF joints from various manufactures differ from one another in the resistance of their heating wires ( viz. resistors) and temperature coefficients thereof. One EF controller matches only one type of the EF joints, so that piping workers must have a large assortment of the EF controllers, suffering from an economical loss.

SUMMARY OF THE INVENTION

An object of the present invention is therefore to provide a method of automatically controlling the fusion process for all the types and for a variety in each type of the thermoplastic pipes and joints.

The method provided herein comprises the steps of: (1) supplying a resistor with an electric current in response to a start signal indicating the start of fusion process, the resistor serving as a heater to fuse together thermoplastic resin surface layers facing one another and in close contact with each other, calculating the electrical characteristics of the resistor upon increase of the electric power to a predetermined level, and also calculating specific heat of the resin by conducting ONOFF control of the resistor; (2) subsequently raising the electric power stepwise at successive stages and based on the values of electrical characteristics and specific heat which are already measured at the first step (1), so is as to detect a variation coefficient of electrical characteristics at each stage, and also detecting a variation coefficient of specific heat at each stage by further conducting ON-OFF control of the resistor; and (3) finally discontinuing to increase the electric power when the variation coefficient of specific heat measured at the second step (2) will have reached or become smaller than a prescribed value, with the ON- OFF control being continued until the specific heat itself becomes equal to or lower than a prescribed level so that the supply of electric power is ceased.

The third step (3) may be modified such that the supply of electric power is ceased when a length of time will have passed after the start of ONOFF control at said step (3), wherein the length of time is calculated using the value of specific heat measured at the second step (2).

According to the present invention, an optimal level will be determined for the electric power heating and partially melting the resin articles, based on the resis tor's electrical characteristics and the resin's physi cal properties which are being measured at the same time during the fusion process. Thus, articles made of any resin included in any type can be fused together successfully and accurately.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings showing some embodiments of the - 3 invention:

Fig. 1 is a control diagram for automatically controlling the fusion process in which thermoplastic articles are fused together in an embodiment; Fig. 2 is a perspective front elevation of an EF joint and an EF controller used in the embodiment; Fig. 3 is a cross section of a socket portion of the EF joint; Fig. 4 is a cross section of another EF joint; Fig. 5 is a front elevation of a fusible tape; and Fig. 6 is a front elevation of another fusible tape.

THE PREFERRED EMBODIMENTS An EF joint and an EF controller shown in Fig. 2 are used as examples in the method of automatically control ling the fusion process of thermoplastic articles, and the process is controlled according to a control diagram shown in Fig. 1. The T-shaped EF joint I has socket portions 3 in which ends of thermoplastic resin pipes 2 are to be fixedly adjoined. A pair of terminal pins 4 protrude outwardly from end surfaces of each socket portion. The EF controller generally denoted by the reference numeral 5 has an installed CPU ( viz. central processing unit) used for the automatic control of the fusion process. A control code 7 extending from the controller has, at its extremity, connectors 6 to mate the terminal pins 4. A cable 8 from the controller has at its f ree end a plug 8 to be inserted in a power supply socket. The controller 5 further comprises a start switch 10, a stop switch 11 and an indicator 12.

The pipes 2 can be fixed in the respective socket portions 3. Each of the socket portions 3 of the EF joint 1 has an inner periphery in which an independent electric heating wire 13 is embedded. Each heating wire 13 as the aforementioned resistor is composed of a core covered with a coating of the same thermoplastic material as the material of the EF joint 1, as shown in Fig. 3. The electric heating wire 13 folded down to form a U-shaped end and a pair of parallel legs extending therefrom. The bent-down wire 13 is coiled around and embedded in the inner periphery of the socket portion 3 such that the legs' free ends are secured to the terminal pins 4.

The two connectors 6 of the EF controller 5 mate with the respective terminal pins 4.

In operation, the plug 8 of the EF controller 5 will be plugged in the power supply socket, and the ends of the pipes 2 will be inserted in the socket portions 3 of the EF joint 1. After the connectors 6 are connected to the terminal pins 4 jutting from the socket portions 3, the start switch 10 will be turned on to initiate the automatic control of the fusion process for the EF joint 1.

As the start button 10 is turned on, a start signal will be given to the CPU to initiate the control process as shown in Fig. 1. In detail, a power of a certain - 5 initial intensity is thus supplied to the electric heating wire 13 to raise its temperature from an ambient temperature T1. The CPU continues to detect the voltage and current ( viz. ampere) and count the time.

A first step of the automatic control method is as follows. At a point when the electric power has reached a predetermined level, the variation coefficient of resistance will be calculated as the electrical characteristics. Then, ON-OFF control of the heating wire will be continued for a given time, at regular intervals, so that the specific heat of the relevant resin is determined by for example the so-called PID ( viz. proportional-integral-differential) calculation. The resistance coefficient depending on temperature of the heating wire as well as the specific heat of the resin will be estimated roughly at this stage. The ON-OFF of the heating wire is done three times in the illustrated example, whereby the temperature of said wire is repeatedly raised and lowered between T2 and T3 ( more exactly, between a temperature a little higher than T2 and another temperature a little lower than T3, due to a certain delay in the system). It will be seen in Fig. 1 that such an ON-OFF operation is started when the wire 13 has risen to the temperature T2 which is low enough to protect from the resin from melting. ( The reference symbols P1, AI1, and A T1 - &T5 denote a peak of electric power, a difference between the peak and a bottom, and period of time between each peak and the following bot- tom.) A second step of the automatic control method is as follows. Here, the electric power supplied to the heating electric wire will subsequently be raise6 step wise at successive stages, based on the specific heat and the variation coefficient of resistance which have been measured at the first step. During this step, the variation coefficient of resistance is further calculated at each stage A variation coefficient of specific heat will also be determined at each stage by further conducting ON-OFF operations of the heating wire. The roughly estimated values of the heating wire's resistance coefficient depending on temperature and the resin's spe cific heat obtained at the first step are thus judged as to whether they are accurate or not. If judged 'No', then those values can be corrected at this step. The most appropriate heating condition will be established for the resin, at this step. In the illustrated exam ple, the electric power is raised four times up to higher and higher levels, with one ON-OFF operation being done at each of such stages. In detail, the temperature of the heating wire 13 rises from T2 to T4, before subse quently lowering to T5. Then, the temperature is con trolled to rise again beyond T4 to T6, before temporarily descending to T7. The temperature further ascends therefrom to T8 before descending to T9, and subsequently ascends to T10 and then descends to T11, before finally converging to the target temperature T10. The last stage, i.e., fourth stage in this example, is important to confirm the most proper heating condition. There may be cases wherein only one and the first stage is enough to establish such a heating condition most appro- priate in the fusion process, or five or more stages are required for this purpose. It is highly preferable to limit the maximum number of repeated stages for example to 10 (ten). The power supply has to be stopped at the end of the final stage, i,e, the tenth stage to emit an alarm. Any way, the described intermittent increasing of the power supply to the heating wire 13 will be effective to protect the resin from being heated to such a temperature that will cause heat deterioration of the resin. The symbols P2 - P5, A12 -,&14, and AT6 - A T8 denote differences each between adjacent peaks of electric power, differences each between one peak and the next bottom, and periods of time each between the adjacent bottoms.) A third step of this method is as follows. The increasing of the supplied power will be stopped when the variation coefficient of specific heat ( more exactly a rec 1procal thereof throughout this specification) measured at the second step just has reached or become smaller than a prescribed value. The ON-OFF control will be continued until the specific heat itself becomes equal to or lower than a prescribed level so that the supply of electric power is ceased. In the illustrated example, the specific heat variation coefficient will become smaller than the prescribed value, when the increasing power supply has heated the wire 13 to the optimal tem perature T10. Thereafter, the increasing of power supply will be ceased and the ON-OFF thereof will be continued for a while to keep the temperature of the heating wire 13 at the optimal one T10 - During such a leveling-off operation, a differential electric power observed in one cycle of ON-OFF operation will gradually decrease, as temperature.

resin heated the resin temperature approaches the wire This means that the specific heat of the by the wire 13 gradually decreases towards almost 0 (zero), indicating the melting of resin. The necessary surface portion of the resin article will be molten in this manner, and the power supply will be shut off to finish the fusion process.

The third step may be modified such that the supply of electric power is ceased when a length of time has passed after the start of ON-OFF control, wherein the length of time is calculated using the value of specific heat measured at the second step.

Fig. 4 is a cross section of the EF joint la of another type to which the method is employable. This joint la. is socket-shaped in its entirety, having opposite ends receiving the ends of pipes 2a to be adjoined thereto. A heating electric wire 13a in this case is such that both the pipe ends are simultaneously molten to become integral with this joint. The electric wire 13a is a bare wire helically embedded in the joint la, courser in an intermediate region and denser in end regions thereof. Attached to ends of the wire 13a are terminal pins 4a. protruding sideways and outwards the joint la. The connectors 6 of the EF controller 5 likewise mate the terminal pins 4a, and electric power will be supplied in the same manner as in the case described above.

Fig. 5 shows a fusible tape 14 made of a thermoplastic resin. This tape is 'composed a f ront and back base layers 15a and 15b, and two heating electric wires 16 sandwiched therebetween. The wires 16 meander transversely of the tape and extend over the full length thereof. In use, an appropriate length will be severed from the endless tape, and two ends of the wires 16 protruding out of one extremity of the severed length will be electrically connected one to another. This severed length of tape will then be adhered or otherwise secured to a portion of a thermoplastic article or to portions thereof to be fused together. The other two ends of the wires 16 at the other extremity of the severed length then mate the connectors 6 of EF controller 5 which will subsequently be energized to carry out the fusion process. This tape 14 is useful in fusing any thermoplastic articles that have no heating wires embedded therein, and is applicable to any surface shape of said articles. In a particular use to provide a large-sized consolidated and impermeable sheet to cover an underground garbage pit, thermoplastic resin - 10 unit sheets of a given width may be fused together along their parallel sides by means of several lengths this tape 14. Such a large sheet will be effective to prevent various hazardous materials from migrating into the earth. Even if the fusible tape 14 remarkably varies in its length due to the variety in shape and dimension of said unit sheets, the automatic control method provided herein will operate successfully and accurately.

Fig. 6 shows another fusible tape 14a. This tape comprises a knit fabric of carbon fiber as the base layer 17. A pair of electric wires 18 serving as a cathode and an anode are adhered or otherwise fixed to the longitudinal sides of the base layer. on ends of the wires 18 will be connected to the connector 6 of the EF controller 5 to heat the base layer 17. This tape 14a may also be used in the same manner as the first described tape 14, with the base layer being molten.

In summary, the present invention provides an au tomatic control method for fusion process wherein an optimal operating condition will be established for the electric wire to heat and partially melt the resin arti cles, based on the wire's electrical characteristics and the resin's physical properties which are being measured at the same time during the fusion process. Thus, articles made of any resin included in any type can be fused together successfully and accurately.

Any additional resistors of varied values of resistance, which have been embedded in the EF joints so as to 11 - be previously detected to know the type thereof, are needed no longer in the method proposed herein. Thus, the EF joints can now be produced more inexpensively, and the EF controllers can be of a simplified structure as to their connectors.

Further, a drawback that a variable gap between the prior art EF joint and the pipe has been a disturbing factor causing a poorer fusion is now eliminated, because the specific heat of the resin is measured during the fusion process so that any gap or clearance will never adversely affect the process in an unpredicted manner.

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Claims

What is claimed is:

1. A method of automatically controlling the fusion process of thermoplastic articles, the method comprising the steps of:

(1) supplying a resistor with an electric current in response to a start signal indicating the start of fusion process, the resistor serving as a heater to fuse together thermoplastic resin surface layers facing one another and in close contact with each other, calculating the electrical characteristics of the resistor upon increase of the electric power to a predetermined level, and also calculating specific heat of the resin by conducting ONOFF control of the resistor; (2) subsequently raising the electric power stepwise at successive stages and based on the values of electri cal characteristics and specific heat which are already measured at the first step (1), so as to detect a variation coefficient of electrical characteristics at each stage, and also detecting a variation coefficient of specific heat at each stage by further conducting ON-OFF control of the resistor; and (3) finally discontinuing to increase the electric power when the variation coefficient of specific heat measured at the second step (2) will have reached or become smaller than a prescribed value, with the ON- OFF control being continued until the specific heat itself 13 - becomes equal to or lower than a prescribed level so that the supply of electric power is ceased.

2. A method of automatically controlling the fusion process of thermoplastic articles, the method comprising the steps of:

(1) supplying a resistor with an electric current in response to a start signal indicating the start of fusion process, the resistor serving as a heater to fuse together thermoplastic resin surface layers facing one another and in close contact with each other, calculating the electrical characteristics of the resistor upon increase of the electric power to a predetermined level, and also calculating specific heat of the resin by conducting ONOFF control of the resistor; (2) subsequently raising the electric power stepwise at successive stages and based on the values of electrical characteristics and specific heat which are already measured at the first step (1), so as to detect a variation coefficient of electrical characteristics at each stage, and also detecting a variation coefficient of specific heat at each stage by further conducting ON-OFF control of the resistor; and (3) finally discontinuing to increase the electric power when the variation coefficient of specific heat measured at the second step (2) will have reached or become smaller than a prescribed value, with the ON- OFF control being continued until a length of time will have passed after the start of ON-OFF control at this step (3) so that the supply of electric power is ceased, wherein the length of time is calculated using the value of specific heat measured at the second step (2).

3. A method of automatically controlling the fusion process of thermoplastic articles substantially as herein described with reference to the accompanying drawings.

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