CA1184990A

CA1184990A - Method and apparatus for testing the quality of an ultrasonic weld in thermoplastic material

Info

Publication number: CA1184990A
Application number: CA000417283A
Authority: CA
Inventors: Gilbert B. Ii Chapman
Original assignee: Ford Motor Company of Canada Ltd
Current assignee: Ford Motor Company of Canada Ltd
Priority date: 1982-01-26
Filing date: 1982-12-08
Publication date: 1985-04-02
Also published as: US4410381A

Abstract

ABSTRACT
Ultrasonically welded thermoplastic materials are nondestructively inspected by sensing the infrared radiation emitted from the weld region of the material during or immediately subsequent to the welding operation.
The sensed radiation levels are compared with a predetermined value and if they exceed the predetermined value are indicative of an acceptable weld. If the sensed radiation peak does not exceed the predetermined level, the weld is deemed to be unacceptable.

Description

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METHODS AND 2iPPA`RP~TUS `FC)R. TESTING THE QUALITY
OF AN UI.TRASONIC WELD IN ~TIC MATERIAL
. _ _ The present invention is directed to the field of nondestructive testing and more particularly to the area oE
inspecting ultrasonic welds in thermoplastic material.

During the formation of parts Erom thermoplastic materials, it is common to join separate pieces by utilizing ultrasonic welding processes. However, the common forms of evaluating the quality of the weld employ destructive testing techniq~es. Pressure or pull tests are employed which determine the fracture point of the weld.
Due to the nature of any destructive test, statistical sampling techniques are necessitated, since each tested part is destroyed.
While the use of ultrasonic welds and statistical samplings have, in general, provided manu~acturers with a predictable yield rate, it is more desirable to provide an automatic nondestructive technique whereby 100% inspection can be made of the ultrasonic weld in thermoplastic 20 material to thereby provide a quality sort of the welded parts.
The inventor's analysis of ultrasonic weld failures in thermoplastic parts identified several instances where inEerior welds may result.
One oE those instances occurs when par~s are not p~ecisely positioned, due to the parts positioning equipment or due to the design of the parts themsel~es.
When ultrasonic vibrations are applied ~o misaligned parts, they will fuse only at the ab~ting surfaces.
30 Consequently, some points become welded and some do not.
Another oE those instances occurs when the parts are formed from di~ferent composition batches, wherein sm~ll differences in compositon may resul~ in different melting points. In such instances, one part will melt 35 while the other remains solid during the application of ultrasonic vibrations, thereby producing a poor weld.
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Yet another instance of inferior weld may occur when weld parame-ters, such as weld power, hold time and weld hold down pressure drift from their preset values.
The present invention provides for nondestructive inspec-tion to determine ultrasonic weld quality in thermoplasticmaterials and is easily adapted to provide 100~ inspection of welded parts during theix manufacture. The present invention involves the monitoring of inExared radiation emitted from the surface adjacent the weld region, as a result of an ultrasonic weld being performed thereon.
properly welded region will reach a predetermined fusing temperature for the particular thermoplastic material.
By utilizing destructive test methocls, it has been deter-mined that the strength of the welds are directly related to the maximum temperature reached during the welding opera-tionO Therefore~ standards are derived for the minimal temperature that must be reached before an acceptable weld is made.
In accordance with one aspect of the present invention, there is-provided an apparatus for testing the cluality of an ultrasonic-weld in thermoplastic material comprising means for sensing electromagnetic radiation within a predetermined range of wavelengths emitted from the welded material for a period of time immediately follow-ing the weld; means connected to the sensing means forresponsively producing an output signal having values directly xelated to the sensed radiation; and means receiving the output signal for comparin~ the highest value thereof occurring during said period of time with a predetermined value corresponding to that of an acceptable weld and indicating whether the highest value is above or below the predetermined value.
In accordance with another aspect of the present invention, there is providecl a method of testing quality of an ultrasonic weld in a thermoplastic material, com-prising the steps of perfoxming an ultrasonic weld on the material for a predeterminecl time period; sensing the inten-sity of electromagnetic radiation within a predetermined band of wavelengths, emitted from the welded material 4~
2a immediately following the weld step for a second period of time; producing an output signal having values directly related to the sensed radiation intensity; comparing the highest value of the output signal occurring during the second period of time with a predetermined value correspon-ding to that of an acceptable weld; and ., ~,...,~
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indicating whether the highest value is above or below ~he predetermined value.
In the present invention, therefore,the radiation emitted, usually infrared radiation, from an ultrasonically 5 welded thermoplastic material is monitored. When the monitored radiation reaches an intensity above that for which an acceptable weld has been determined to have been made, the weld is deemed to be satisfactory. If, however, the intensity of the emitted radiation does not r~a-ch the pre-10 determined level, the weld is deemed to be unsatisfactoryand the part is rejected or anal~zed to determine the reasons for the failure. Consequently, the present invention is ideally suited for instantaneous process failure analysis and 100% testing of the mechanically induced welds.
The invention is described further, by way of illus-tration, with reference to the accompanying drawings, in which:
Figure 1 is an elevational cross-section of a typical part being subjected to ultrasonic welding;
Figure 2 is a perspective view of welding and in-20 spection stations incorporating the present invention, Figure 3 is a top view of the inspection stationof Figure 2, showing a cross-section of the welded part and associated monitoring devices; and Figure 4 is a schematic block diagram of an evalua-25 tion circuit as may be utilized in the present invention.
Referring to the drawings, a thermoplastic part10 is shown in Figure 1 in position to be ultrasonically welded. The support base 12, in this case, is a cylindrically formed valve body of thermoplastic material. The base 12 30 defines a central cavity 30 and contains an upper opening defined by an annular surface 16. A cap piece 14 is formed of the same thermoplastic material to fit inside the base 12 and contains a cylindrical outer surface 24 that abuts an internal cylindrical wall 22 of the base 12.
When mechanical stress waves are applied to the upper surface 15 of the cap piece 14 via a pressure contact through surface 42 of an ultrasonic transducer ~0, the abu~tin~
surfaces 22 and 24 are caused to fuse and form a weld region.
;`~ The mechanical vibrations cause solid surfaces 22 and 24 to 4~ produce sufficient frictional heat that the thermoplastic surfaces melt, If both surfaces melt at the same temperature~ as they should, some of the melt escapes into adjacent flashing zones 20 and 26 and the region becomes fused. If, on the other hand, the surfaces are not abutting or one material melts prior to the other, the temperature of the region will not reach the known fusing temperature.
The present invention evaluates the weld hy moni-toring the amount of heat radiated from the weld region immediately subsequent to the welding operation. Weld and inspection zones A and B are respectively shown in Figure 2 as a production embodiment for the present inven-tion. In that embodiment, the part 10 is loosely assembled prior to being conveyed to weld zone A. When conveyed to the weld zone A, the ultrasonic transducer 40 make a pressure contact with the part and applies ultrasonic vibra-tions to the part for a predetermined period of time. Sub-sequently, the part lO is conveyed to an inspection zone B where a plurality of sensors 102 and 104 are positioned to receive infrared-radiation emiitted from the weld region.
The sensors 102 and 104 ~re, in this embodiment, Capintec/
Heimann S-1548 infrared thermometers with a spectral range of 8114 microns. It is forseeable that a greater number of sensors could be installed along the entire length of the weld region to increase the resolution of the inspec-tion. In this embodiment, however, the use of two dia-metrically opposed sensors was found to be adequate to detect the incomplete welds that cause part failures for the particular part shown.
A top cross--sectional view of the Figure 2 embodi-ment is shown in Figure 3 and the sensors are shown connec-ted to meter and alarm devices. In the cross-sectional view of the part 10, the weld joint formed by the abutting suxfaces 22 and 24 is shown as cylindrically encompassing the central cavity 30 of the " .,,: ,~A~

3~ -part 10. The heat produced at the weld region is conducted through the base 12 and emitted at the outer surEace of the part 10. The sensors 102 and 104 are located so as to receive a narrow band of frequencies in the infrared range and produce output signals on respective lines 103 and 105 indicative o~ the intensity of the received radiation. The sensor 102 is connected to a meter llO and a preset alarm 112 through signal line 103. Similarly, the sensor 104 is connected to a meter 114 and a preset alarm 116 through signal line 105. Of course, the meters 110 and 114 are merely for monitoring purposes while the alarms 112 and 116 symboliæe a more elaborate circuit by which the part may be automatically discarded or otherwise marked to indicate a problem resulting in substandard assembly and welding operations for that partO
A more detailed block diagram for the alarm system is shown in Figure 4, wherein the outputs from the individual infrared sensors are respectively compared with a known reference value which corresponds to the predetermined fusing temperature that must be reached in order to effect an acceptable weld. If any one of the sensors fails to receive radiation from the welded part of a sufficient intensity to indicate an acceptable weld, a "FAIL" indicator is activated along with a mechanism for separating inferiorly ~elded parts away from the acceptable parts.
~ he Figure 4 circuit incorporates separate monitoring features for each signal channel from respective sensors wherein the signal value from each sensor is separately compared with a predetermined reference until such time as the signal value starts to decrease. Such a decrease in signal value indicates that the intensity of the infrared radiation has reached its peak and is on the decline. The results of each comparison are stored and appropriately gated through a logic network to determine the acceptability or unacceptability of the welded part.
Specifically, signal line 103 is connected to a buffer amplifier 202. The output of the buffer amplifier 202 is conn~cted to one of the inputs of a comparator circuit 20~.
The other input is connected to a reference voltage source which is preselected in value to correspond to a minimum signal level attributed to intensity o~ radiation that the sensors should receive from an acceptable weld. The output of the comparator circuit 204 is normally at a low level until such time as the output from the buffer amplifier 202 exceeds the reference voltage value. At that time, the output of the comparator circuit 204 switches to a high level signal that is stored in a latch 206. The high level signal stored in the latch 206 is communicated to an input terminal of an AND gate 210.
A peak detector circuit 208 has its input connected to the output of buffer amplifier 202 parallel to the input to the comparator 204. The peak detector circuit 208 provides a high level output signal as long as the signal value output from the amplifier 202 is increasing.
~hen the signal level output value from the amplifier 202 reaches its peak and starts to decrease in value, the output from the peak detector circuit 208 switches to a low level signal. The output from the peak detector circuit 208 is fed to an inverting input terminal of the AND gate 210 so that a high level signal will be output therefrom only when the signal value from the amplifier 202 has exceeded the reference voltage ~alue and the signal from - the amplifier 202 has been determined to have reached its peak.
Similar analysis is made to the signal input on line 105 through buffer amplifier 212; a comparator circuit 214, a latch 216 and a peak detector circuit 218. The outputs of the peak detector circuit 218 and the latch 21S

are fed to an ~ND gate 220 that has its output connected to an input of ~ND gate 232.
In order to de~ermine that the radiation received has reached its highest level in each o~ the monitoring circuits, an AND gate 230 has inverting input terminals connected to receive the outputs of the peak detector circuits 208 and 218. The output oE AND gate 230 then enab.les the A~D gate 232 to provide a high level output only if the radiation rece.ived by each sensor has reached the acceptable level. In the event that any of highest signal values measured in any of the channels is less than the predetermined re~erence value, the output of AND gate 232 will be at a low level.
The output of the AND gate 232 is connected to an "O.X." latch 234 that is in turn connected to a "PART O.K."
indicator 236 to provide an evaluation of the welded part.
The output of the AND gate 232 is also connected to an inverting input terminal on an AND gate 238. Another input terminal of the AND gate 238 is connected to receive the output of ~`~D gate 230. When all the signal values have been compared, the output of the AND gate 230 provides an enabling signal to both AND gate 238 and AND gate 232. If the output of AND gate 232 i5 at a low level, and the output of AND gate 238 is at a high level, the high level signal is stored in a "FAIL" latch 240. The output of the "FAIL" latch 240 is fed to a "PART FAIL" indicator and mechanism 242, which may include a buzzer, a light or some other warning device and a conventional conveyor diversion mechanism to separate the failed part from the accepted parts.
An OR gate 250 has its input terminals respectively connected to the outputs of the latch 234 and latch 240 and responsively provides a reset signal to each of the latches 206, 216, 234 and 240 thereby conditioning the circuit to evaluate the next part positioned for inspection.
While it is apparent that many modifications and variations may be made without departiny ~rom the true spirit and scope of the invention, it is intended by the appended claims to include all such variations and modifications of the pre:ferred embodimen~.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An apparatus for testing the quality of an ultra-sonic weld in thermoplastic material comprising:
means for sensing electromagnetic radiation within a predetermined range of wavelengths emitted from said welded material for a period of time immediately following said weld;
means connected to said sensing means for respon-sively producing an output signal having values directly related to said sensed radiation; and means receiving said output signal for comparing the highest value thereof occurring during said period of time with a predetermined value corresponding to that of an acceptable weld and indicating whether said highest value is above or below said predetermined value.

2. An apparatus as in claim 1, in which said sensing means senses infrared radiation emitted within a predeter-mined band of wavelengths.

3. An apparatus as in claim 1, wherein said sensing means includes a plurality of infrared sensors spaced along said weld;
said responsive means responds to each sensor by providing output signals corresponding to respective sensors; and said comparing means compares each output signal value with said predetermined value and said indicating means indicates if any of the highest values of said signals does not exceed said predetermined value.

4. A method of testing quality of an ultrasonic weld in a thermoplastic material, comprising the steps of:
performing an ultrasonic weld on said material for a predetermined time period;
sensing the intensity of electromagnetic radiation within a predetermined band of wavelengths, emitted from said welded material immediately following said weld step for a second period of time;

producing an output signal having values directly related to said sensed radiation intensity;
comparing the highest value of said output signal ocurring during said second period of time with a predetermined value corresponding to that of an acceptable weld; and indicating whether said highest value is above or below said predetermined value.

5. A method as in claim 4, wherein said step of sensing is performed to sense the intensity of infrared radiation emitted within a predetermined band of wave-lengths.

6. A method as in claim 4, wherein said sensing step is performed at a plurality of points along said weld, said step of producing an output signal is performed for each point of sensing, said step of comparing is performed against each output signal produced and said indicating step is performed for each comparison.

7. A method as in claim 6, wherein said step of sensing is performed at said plurality of points by sensing the intensity of infrared radiation emitted within a pre-determined band of wavelengths.