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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to thermoplastic fitting electric heat
welding methods and apparatus, and more particularly, but not by way of
limitation to methods and apparatus for electrically heat welding
thermoplastic fittings having electric resistance heating elements
disposed therein to other plastic members such as plastic pipe joints.
2. Description of the Prior Art
Electric heat weldable fittings formed of thermoplastic material have been
developed and used heretofore. Such fittings generally include an electric
resistance heating coil or element positioned adjacent the inside surfaces
of the fitting which are to be welded to one or more other thermoplastic
members such as plastic pipe sections. The electric resistance heating
element is usually a coil of resistance wire disposed in the thermoplastic
material of the fitting and is connected to electric contacts which are
attached to an outside surface of the fitting. Examples of such electric
heat weldable thermoplastic fittings are described in U.S. Pats. Nos.
4,147,926, issued Apr. 3, 1979, and 4,349,219, issued Sept. 14, 1982.
In welding the fitting to other plastic members positioned adjacent
thereto, a source of electric power is connected to the contacts, such as
by electric cable, and electric power is supplied to the resistance
heating element of the fitting. The heating element heats the fitting and
the adjacent thermoplastic members to temperatures which cause the
thermoplastic materials from which the fitting and adjacent members are
made to melt whereby they become fused or welded together.
The quality of the weld which results is primarily dependent upon the
correct quantity of electric power being supplied to the heating element
of the fitting. If too little electric power is supplied, too little
heating takes place and an inadequate low strength weld results. If too
much electric power is supplied, the fitting and plastic members to which
the fitting is welded can be deformed and overheated whereby a good weld
does not result. Other factors that affect the quality of the weld
produced include heating elements abnormalities, e.g., short-circuits,
poor fitting alignment, poor contact between surfaces to be welded, etc.
Various types of control and electric power generating apparatus have been
utilized for supplying the electric power to electric heat weldable
thermoplastic fittings. Initially, such apparatus was manually controlled
by an operator and the quantity of electric power supplied to the heating
elements of the fittings was determined by visual observation of the
fittings as they were welded. More recently, control apparatus has been
developed whereby the operator manually programs the control apparatus to
supply a predetermined quantity of electric power to the heating element
of the fitting in accordance with the particular size of the fitting.
Also, electric heat weldable fittings having heating coils and separate
resistors disposed therein have recently been developed. The values of the
resistors are chosen in accordance with the size of the fitting and the
electric power to be supplied thereto. Electric power control apparatus is
utilized with such fittings adapted to sense the values of the resistors
and automatically supply a preset quantity of electric power to the
heating coil in accordance therewith. Examples of such fittings and
control apparatus are described in U.S. Pat. No. 4,486,650, issued Dec. 4,
1984.
While the fittings including separate resistors and the control apparatus
which automatically supply the heating coils of the fittings with
predetermined quantities of electric power have generally achieved good
results, because each of the fittings must include one or more resistors
in addition to the resistance heating element disposed therein, they are
more expensive to produce than fittings with heating elements alone. In
addition, the electric power control apparatus have not included
satisfactory provision for preventing the burn-up of fittings and the fire
hazard attendant thereto when fittings having shorted-out heating coils
are encountered. Also, the predetermined quantities of electric power
supplied the fittings have not always been accurate because the
temperatures of the fittings have not been taken into account. None of the
prior electric control apparatus have been capable of detecting and
accounting for defects such as bad electrical connections, misaligned pipe
or other similar factors affecting the welding current or temperature.
By the present invention, a method and apparatus for electrically heat
welding thermoplastic fittings are provided wherein the fittings do not
require separate resistors or other devices for indicating the electric
power required. In accordance with the present invention, the entire
welding process of each fitting is comparatively monitored to insure the
fitting and other aspects of the process are not defective and that the
proper quantity of electric power is supplied to fitting.
SUMMARY OF THE INVENTION
A method and apparatus for electrically heat welding a thermoplastic
fitting having an electric resistance heating element disposed therein are
provided. The method comprises the steps of connecting the heating element
of the fitting to an electric power source; supplying electric power to
the heating element at a minimum voltage level for measuring the
resistance of the heating element without significantly heating the
element; measuring the resistance of the heating element and thereby
determining the initial temperature of the element; supplying electric
power to the heating element at a controlled voltage whereby the element
is heated; sensing the initial magnitude of the current flowing through
the heating element and comparing such magnitude and the initial
temperature of the element with predetermined current levels for heating
elements of various sizes of fittings at various temperatures to thereby
determine the size of the fitting being welded and the total time the
controlled electric power should be supplied to the heating element
thereof to insure the making of a high quality weld; continuing to sense
the magnitude of the current flowing through the heating element over the
time the controlled electric power is supplied thereto and comparing such
magnitude at predetermined time intervals with predetermined current
levels for the size of fitting being welded to thereby determine if the
welding process is proceeding abnormally at such time intervals; and
terminating the supply of electric power to the heating element of the
fitting when it is determined that the welding process is proceeding
abnormally or otherwise at the end of the total time required for making a
high quality weld.
It is, therefore, a general object of the present invention to provide
thermoplastic fitting electric heat welding methods and apparatus.
A further object of the present invention is the provision of a method and
apparatus for electrically heat welding thermoplastic fittings having
heating elements disposed therein whereby the quantity of electric power
supplied to the heating element is automatically determined and accurately
controlled.
A further object of the present invention is the provision of a method and
apparatus for electrically heat welding thermoplastic fittings wherein the
quality of the fitting is determined early in the welding process and the
application of electric power to the fitting terminated if such fitting is
defective.
Yet a further object of the present invention is the provision of a method
and apparatus for electrically heat welding thermoplastic fittings wherein
the initial temperature of the fitting is taken into account and the
temperature of the heating element is comparatively monitored along with
the magnitude of the current flowing therethrough during the welding
process to insure a high quality weld.
Other and further objects, features and advantages of the present invention
will be readily apparent to those skilled in the art upon a reading of the
description of preferred embodiments which follows when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of an electric heat weldable thermoplastic sleeve
having the ends of a pair of pipe sections inserted therein with the
electric power and control apparatus of the present invention illustrated
schematically in relation thereto;
FIG. 2 is an enlarged cross-sectional view of portions of the fitting,
plastic pipe sections and electric power and control apparatus of FIG. 1;
FIG. 3 is a partial top view taken along line 3--3 of FIG. 2;
FIG. 4 is an enlarged view of one of the connectors of the electric power
and control apparatus of FIG. 1;
FIG. 5 is a bottom view taken along line 5--5 of FIG. 4;
FIG. 6 is a graph showing the current-time relationships of different
fittings; and
FIG. 7 is a schematic illustration of the electric power and control
apparatus of the present invention connected to a thermoplastic fitting
having a resistance heating element disposed therein.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now to the drawings and particularly to FIGS. 1 and 2, an
electric heat weldable thermoplastic sleeve 10 is illustrated with the
ends of a pair of thermoplastic pipe sections 12 and 14 inserted therein.
The sleeve 10 includes a pair of electric contact connectors 16 and 18
attached thereto for receiving complementary electric contactors 20 and 22
attached to the ends of electric cables 24 and 26, respectively. The
cables 24 and 26 are connected to an electric power and control apparatus,
generally designated by the numeral 30, which will be described in detail
hereinbelow.
As best shown in FIG. 2, the thermoplastic fitting 10 includes an electric
resistance heating element 28 disposed therein adjacent portions of the
interior surface 32 thereof. The resistance heating element can take
various forms, but preferably is a coil formed of electric resistance
heating wire disposed in a spiral winding within the thermoplastic
material forming the sleeve 10 adjacent the portions of the interior
surface 32 which are to be welded to the exterior surfaces of the pipe
sections 12 and 14. The opposite ends of the heating wire 28 are connected
to upstanding electric contact pins 34 and 36 disposed within the
connectors 16 and 18. As will be understood, the complementary connectors
20 and 22 of the electric power and control apparatus 30 are adapted for
removable connection to the connectors 16 and 18 of the sleeve 10.
The connectors 20 and 22 include electric contact sockets 38 and 40,
respectively, for engagement with the electric contact pins 34 and 36 of
the connectors 16 and 18. The socket contact 38 is connected to a wire 39
extending through the cable 24 and the socket contact 40 is connected to a
wire 41 extending through the cable 26. The connector 22 also includes a
temperature sensing device 42 such as a thermister, RTD, or thermocouple
positioned in heat conducting relationship with the socket contact 40
which is connected to a multiple lead wire 43 also extending through the
cable 26. The device 42 senses the temperature of the contact pin 36 when
the socket contact 40 is engaged with the pin 36. That is, when the
connector 22 is connected to the connector 18 of the sleeve 10, the
temperature sensing device 42 senses an initial temperature which is
representative of the outside surface temperature of the entire sleeve 10.
As shown in FIGS. 2 and 3, the connector 16 of the fitting 10 includes one
or more (preferably one to nine) longitudinal code ridges 31 molded around
the inside periphery thereof. As illustrated in FIG. 3, one or more of the
ridges 31 can be enlarged so that it also functions as a position guide
for the complementary contactor 20. Each size of fitting 10 will include a
connector 16 having a particular number and/or arrangement of code ridges
31 molded therein indicating that particular size. For example, the
arrangement illustrated in FIG. 3 (5 ridges) could indicate a 5-inch size
fitting. As illustrated in FIGS. 2, 4 and 5, the complementary connector
20 includes nine longitudinal recesses 33 which are complementary to the
one to nine ridges 31 which may be included in the connector 16. Thus,
when the connectors 16 and 20 are connected together the one or more code
ridges 31 of the connector 16 are disposed within complementary recesses
33 of the connector 20. The connector 20 also includes a code ridge
sensing device, such as a micro-switch 37, disposed in each of the
recesses 33 to detect the presence of a code ridge therewithin. The
micro-switches 37 are electrically connected to leads 35 of a multiple
lead wire 27 which extends through the cable 24.
As will be described further hereinbelow, when the fitting 10 is to be
fused or welded to the pipe sections 12 and 14, the connectors 20 and 22
attached to the cables 24 and 26 are removably connected to the connectors
16 and 18 of the sleeve 10 whereby an electric circuit is completed
between the heating element 28 of the fitting 10 and the electric power
and control apparatus 30. The electric power and control apparatus 30
operates in the manner described hereinbelow to provide electric power to
the heating element 28 which causes the heating element to heat the
thermoplastic material forming the sleeve 10 and the thermoplastic
material forming the ends of the pipe sections 12 and 14 inserted within
the interior of the sleeve 10. The heating causes the thermoplastic
material of the fitting 10 and pipe sections 12 and 14 to melt and fuse
together to thereby form welds between the fitting 10 and the pipe
sections 12 and 14.
Referring now to FIG. 7, the electric power and control apparatus 30 of the
present invention is schematically illustrated connected to the heating
element 28 of the sleeve 10 by way of the cables 24 and 26 and the
connectors 16, 18, 20 and 22. The apparatus 30 includes a controlled
voltage electric power source 45 which is connected by means of the wires
39 and 41 extending through the cables 24 and 26 to the contact sockets 38
and 40 of the connectors 20 and 22. A low voltage electric power source 47
is also included in the apparatus 30 which is connected to the wires 39
and 41 by wires 53 and 55, respectively. The contact sockets 38 and 40 of
the connectors 20 and 22 are connected to the contact pins 34 and 36 of
the connectors 16 and 18 of the sleeve 10 which are in turn connected to
the heating element 28 of the fitting 10.
A current sensor 46 for sensing the current flowing from the power source
45 to the heating element 28 is electrically connected to the power source
45 by a lead 59 with the output signal therefrom connected by a lead 48 to
an electronic computer 50. A resistance sensor 57 for sensing the
resistance of the heating element 28 when minimum voltage electric power
is applied thereto is electrically connected to the power source 47 by a
lead 61 with the output signal therefrom connected to the computer 50 by a
lead 63. The temperature sensing device 42, previously described, is
connected by the wire 43 attached thereto and extending through the cable
26 to the computer 50, and the microswitches 37 of the connector 20 are
connected to the computer 50 by the multiple lead wire 27.
A switch device 52 such as a TRICAC or SCR is provided in the circuit
between the heating element 28 and the power source 45 which is operably
connected to the computer 50 by a lead 54. A similar switch device 65 is
provided in the circuit connecting the power source 47 to the wires 39 and
41 which is connected to the computer 50 by a lead 67. The computer 50
includes a readout module 51 operably connected thereto for visually
indicating various modes of operation of the apparatus 30 such as a
shutdown due to a defective fitting, etc. Also, the computer 50 is
connected to a communication interface 69 by a lead 70 which in turn can
be connected to a modem 72, a second computer 74 and a printer 16.
In operation of the apparatus 30 for electrically heat welding the
thermoplastic fitting 10 by means of the heating element 28 disposed
therein, the connectors 20 and 22 are first connected to the connectors 16
and 18 of the fitting 10. When the apparatus 30 is turned on, the computer
50 first closes the switch device 65 thereby completing a circuit between
the low voltage electric power source 47 and the heating element 28 of the
fitting 10 by way of the wires 39, 41, 53 and 55 connected therebetween.
The low voltage electric power applied to the heating element 28 is at a
minimum level sufficient for the resistance sensor 57 to measure the
resistance of the element 28, but too low to significantly heat the
element. The resistance value so measured is communicated to the computer
50 by the lead 63 and the computer 50 then opens the switch device 65.
The computer 50 determines the initial temperature of the element 28 which
corresponds to the measured resistance value, and the initial outside
surface temperature of the fitting 10 is sensed by the computer 50 by
means of the temperature sensing device 42 and the wire 43 connected
thereto. Also, the computer 50 senses the size of the fitting 10 by means
of the code ridges in the connector 16, the micro-switches 37 of the
connector 20 activated thereby and the wire 27 connected thereto.
The computer 50 next closes the switch device 52 thereby completing a
circuit between the controlled voltage electric power source 45 and the
heating element 28 by way of the wires 39 and 41 connected therebetween.
The computer 50 senses the initial magnitude of the current flowing
through the heating element 28 by means of the current sensor 46 and lead
48. The initial temperature of the heating coil 28 and the initial
magnitude of the current flowing therethrough are compared by the computer
50 with predetermined current levels for various sizes of fittings at
various temperatures in the memory of the computer to determine the size
of the fitting being welded. Such size is compared to the size of fitting
indicated by the micro-switches 37, and if the same, the supply of
controlled voltage electric power to the element 28 is continued, and the
total time such power should be supplied to the element 28 to insure the
making of a high quality weld is determined.
Referring to FIG. 6, the current-time relationship during the making of a
high quality weld for two different sizes of fittings using controlled
electric power is illustrated graphically. The top curve, designated by
the numeral 60, represents the welding process for a two-inch sleeve and
the bottom curve, designated by the numeral 62, represents the welding
process for a one-inch sleeve. As shown, the current levels are different
for the different sizes of sleeve, and each size and type of electrically
heat weldable thermoplastic fitting has a current-time relationship which
is characteristic of that fitting when a high quality weld is formed using
a proper quantity of controlled electric power.
The computer 50 includes such current-time relationship information for a
variety of electric heat weldable thermoplastic fittings in the memory
thereof whereby the computer 50 can make the comparisons described and
identify the size of fitting being welded from the initial magnitude of
current flowing through the heating element. Upon identification of the
fitting, the computer determines the total time the controlled electric
power should be supplied to the heating element for the making of a high
quality weld from the information in memory and the initial temperature of
the fitting. For example, referring to FIG. 6, if the initial magnitude of
the current flowing through the heating element of a fitting is that
designated by the numeral 64, the computer will determine that the fitting
is a two-inch sleeve represented by the curve 60. The computer will also
then determine from the curve 60 that the total time the controlled
electric power should be supplied to the heating element for the making of
a high quality weld is the time designated by the numeral 66.
The computer 50 continues to sense the magnitude of the current flowing
through the heating element of the fitting being welded over the time the
controlled electric power is supplied thereto and compares such magnitude
at predetermined time intervals with predetermined current levels for the
size of fitting being welded, i.e., for the two-inch sleeve of FIG. 6, the
computer would compare the actual current level with the current levels of
the curve 60 at frequent predetermined time intervals. As long as the
sensed current levels are substantially the same as the current levels in
memory for the size of fitting being welded, the computer continues the
welding process to the total time determined to be required for the making
of a high quality weld. If the sensed current levels deviate from the
current levels in memory, as for example the deviation shown by the dashed
line 68 of FIG. 6, the computer 50 determines the welding process is
proceeding abnormally and terminates the welding process by turning off
the electric power. The operator of the apparatus 30 is informed of the
shutdown and the reason therefor by way of the readout 51.
The initial temperature of the outside surface of the fitting 10 and the
initial temperature of the heating element 28 of the fitting 10 sensed by
the computer 50 as described above can be utilized by the computer 50 to
determine if increases or decreases in the determined total time the
constant voltage electric power should be supplied to the heating element
are required to cause the melting of the required quantity of
thermoplastic material. That is, based on such temperatures and the size
of fitting, the computer 50 can calculate the total time required for the
optimum quantity of thermoplastic material making up the fitting being
welded and the pipe sections in contact therewith to melt and adjust the
previously determined total time if necessary.
As long as the welding process proceeds normally, it is allowed to continue
to the end of the time determined to be required for the making of a high
quality weld whereupon the computer 50 terminates the supply of electric
power from the power source 47 to the heating element 28 of the fitting 10
being welded by operation of the switch device 52. Upon terminating the
supply of electric power from the source 47, the computer 50 again
measures the resistance of the element 28 and determines the final
temperature thereof in the same manner as described above for measuring
the initial temperature of the element 28.
In order to facilitate the making of a high quality weld, alternating
current is supplied to the heating element of the fitting being welded by
the controlled voltage electric power source 45 of the apparatus 30. The
frequency of the alternating current is adjusted to that frequency which
best causes the fitting being welded to vibrate as a result of the
magnetic fields produced by the alternating current flowing through the
heating element of the fitting. Such vibration facilitates and promotes
the fusing of the softened thermoplastic materials of the fitting and
other plastic members being welded thereto.
In order to concentrate the magnetic fields created by the alternating
current and amplify the vibration produced thereby, conductive metallic
material can be attached to or included in the weldable thermoplastic
fittings. For example, as illustrated in FIG. 2, iron filings 29 can be
suspended in the thermoplastic material forming the fitting 10.
During and upon completion of the welding process described above, the
computer 50 records in its memory the various temperatures, current
magnitudes and other variables sensed and determined during the welding
process. For example, the computer 50 can record the initial temperature
of the outside surface of the fitting, the initial temperature of the
heating element of the fitting, the size of the fitting, the determined
time the constant voltage electric power should be supplied to the
fitting, the magnitudes of current flowing over the time constant voltage
electric power is supplied to the heating element of the fitting, the
final temperature of the heating element, and the total time the constant
voltage electric power is supplied to the heating element. Such recorded
information can be communicated to a second computer 74 at a remote
location by way of the communication interface 69 and a modem 72 connected
thereto. The information can be printed by a printer 76 connected to the
computer 74 or utilized in any other desired way. If the supply of
electric power is terminated as a result of the welding process proceeding
abnormally, the nature of the abnormality will be apparent from the
recorded information.
Thus, the present invention is well adapted to carry out the objects and
attain the ends and advantages mentioned as well as those inherent
therein. While presently preferred embodiments of the invention have been
described for purposes of this disclosure, numerous changes in the
arrangement of steps and parts can be made by those skilled in the art,
which changes are encompassed within the spirit of this invention as
defined by the appended claims.
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