GB2346688A

GB2346688A - Checking of plastic details

Info

Publication number: GB2346688A
Application number: GB0000135A
Authority: GB
Inventors: Lars Magne Haldorsen
Original assignee: TRITEC AS
Current assignee: TRITEC AS
Priority date: 1999-02-02
Filing date: 2000-01-06
Publication date: 2000-08-16
Also published as: DE10004049A1; NO311174B1; NO990480D0; FR2789179A1; NO990480L; GB0000135D0

Abstract

A method for checking a joint between plastic details (1, 2) which are joined together by means of welding. A thermography technique is used wherein thermal radiation from the plastic details is observed in an area around the joint during production, by means of at least one camera (3) such as a digital infrared camera. The observed thermal image is compared with at least one reference value by computing means, whereby it may be established whether the fusion of the plastic details is adequate and of a sufficient quality. The thermal radiation is observed after heating and before joining of the plastic details, during cooling of the welded parts, or after cooling of the joint, when further heating is supplied to the formed joint by other irradiation means such as a lamp. The joint can also be thermographically imaged whilst under a mechanical load.

Description

1 2346688 METHOD FOR USE IN THE CHECKING OF PLASTIC DETAILS The invention

relates to a method for use in the checking of a joint between plastic details which are joined together by means of welding.

In the production of objects formed by joining plastic details of desired shapes, welding in some form or other is commonly used. Since many of the plastic details in question are containers or tanks which are later to contain liquid, it is required quite naturally, that the joint between the plastic details shall be tight after welding. Therefore, it is common that the containers or tanks are checked after joining, by means of testing with vacuum or positive pressure, so that it can be established whether the joint between the plastic details is adequate or not. However, practice shows that such checking of welded joints by vacuum or positive pressure testing is both time consuming and expensive. Also, there are too many cases in which the check is not able to reveal possible leaks in the welded joint between the plastic details. Thereby, the producer runs the 2 risk of inadequate containers or tanks being brought on the market with the undesirable consequences involved.

The main object of the present invention is to provide a method for use in the checking of joints between plastic details which are joined together by means of welding, wherein the above use of vacuum and positive pressure is replaced by use of thermography, so that thermal radiation from an area around the joint of the plastic details can be used as a means to establish whether the welded joint is 10 tight or not. Thereby the check becomes far more costeffective and in addition far more reliable against untight joints being approved as being tight. One further object is that it should be possible to perform the check at the moment in the production which is the most convenient one for the is object, place of production, welding equipment etc. in question. That might be either as a pre-check based on thermal radiation after heating the plastic details in an area around the joint, and preferably before the joining of the plastic details, as an after-check based on thermal radiation of remaining heat during cooling of the plastic details in the area around the joint, or as an after check after ended cooling of the area around the joint, based on thermal radiation from heat which is supplied after ended cooling. As appears from the characterizing part of the present in dependent claim, this is realized in the way that thermal radiation from said plastic details, in an area around the joint, is observed by means of at least one camera, e.g. a digital infrared camera, and that the observed thermal radiation is compared with at least one reference value by means of computer equipment, so that it may be established whether the fusion of the two plastic details in the area around the joint is adequate. Other advantageous I 3 features of the invention appear from the independent claims and otherwise from the specification.

In the following part of the specification, and referring to the set of figures, preferred embodiments of the invention will be explained.

Figs. la-d show schematic views of a first embodiment of the invention, in which thermal radiation in an area around the joint of said plastic details is observed by means of a digital infrared camera, and in which the plastic details are heated to the correct temperature for fusion by means of a heat mirror, said thermal radiation being observed after the heating and before the joining of the plastic details take place.

Fig. 2 shows a schematic view of a second embodiment of the invention, in which the thermal radiation in the area around the joint is observed during the cooling of the plastic details after fusion; and Fig. 3 shows a schematic view of a third embodiment of the invention, in which the thermal radiation is observed only after said plastic details have been cooled after fusion, and new heat for the observation has been supplied by means of at least one photoflash lamp.

The present invention relates to a method suited for use in the checking of a joint between plastic details 1, 2 which are joined by means of welding. The checking of the welded joint is done in the way that thermal radiation from the plastic details 1, 2 in the area around the joint is observed by means of at least one camera 3, e.g. a digital infrared 4 camera. Then the observation of the thermal radiation is transferred from the camera 3 to not shown computer equipment which is programmed so that the observation is compared with at least one reference value. Thereby the computer equipment may determine whether the joint between the plastic details 1, 2 meet the quality requirements set for an adequate fusion of the two, so that manufactured details with inadequate joints may be picked out. Said reference value may consist either of a picture of thermal radiation taken with a corresponding camera 3, and with conditions during welding providing an adequate joint between the plastic details 1, 2, or consist of a predetermined temperature distribution in, possibly temperatures at selected points of, the plastic details 1, 2 at specific points of time after the heating. In is the latter case, the computer equipment has been programmed so that the real temperature distribution in, possibly the temperatures at the selected points of, the plastic details 1, 2 is/are calculated on the basis of the observed thermal radiation. Further, said predetermined temperature distribution in, possibly the temperatures at, the selected points of the plastic details 1, 2, is/are provided by means of calculations which will, theoretically, provide an adequate joint after the fusion. Said calculations could also form the basis for experimental tests.

The checking of the welded joint may, as mentioned initially, take place at different points of time, and typical examples of this will be explained in detail in the following, with reference to the figures. In Figs. la-d is shown a first embodiment of the invention, in which the checking is in the form of a pre-check, in which the thermal radiation from the plastic details 1, 2, is observed after the heating of the two, and before the joining takes place, so that the welding I conditions in an area around the joint can be compared with at least one ideal reference value which provides an adequate fusion of the plastic details 1, 2 during welding. Said ideal reference value may either be in the form of a previously recorded picture of thermal radiation from corresponding plastic details 1, 2, in which the heating was right, or in the form of a predetermined (theoretical) temperature distribution in, possibly predetermined temperatures at selected points of, the plastic details 1, 2. The thermal radiation from the plastic details 1, 2 is observed by means of at least one digital infrared camera 3. The camera 3 records the thermal radiation as a picture in different colours in accordance with the respective temperature in the plastic details 1, 2. The observed thermal radiation is transferred in a suitable manner from the camera 3 to the computer equipment which has been programmed so that the observed thermal radiation is compared with the selected reference value, that be either the previously recorded picture, or the relevant predetermined temperature distribution in, or the temperatures at the selected points of, the plastic details 1, 2. When it is compared with the relevant predetermined temperature, the computer equipment is arranged so, that the real temperature distribution or the temperatures at the selected points, is/are calculated.on the basis of the observed thermal radiation. Normally it will be sufficient with one single observation of the thermal radiation during the pre-check of the plastic details 1, 2, but this does not prevent that, whenever necessary, several observations may be made. The plastic details 1, 2 are discarded, or the thermal parameters are adjusted and reheating takes place, if the computer equipment reveals, during the comparing, that there is too big a difference between the observation in question and the respective 6 reference value. Besides, in such a case the computer equipment could be programmed to give a warning by process failure, so that the production plant can be stopped for necessary adjustments of the process parameters. The extent of process control and actions after detection of possible defects during welding, will depend on the product, the production line, the requirements of the customer, the quality control system etc.

Moreover, Figs. la-d shows the above pre-check used in connection with the welding of a filling sleeve 2 to a petrol tank 1 of plastic, but there is, of course, nothing to prevent that something corresponding to this may be ca rried out for all types of welds and for most products. The sleeve 2 is brought into a position above the tank 1 by means of e.g. an industrial robot 5, after which a so-called thermal mirror 4 is inserted between the tank 1 and the sleeve 2. The next thing to happen, is that the tank 1, the sleeve 2 and the thermal mirror 4 are brought together, cf. Fig. 1b, and heat is supplied to the tank 1 and the sleeve 2 until they have reached the required melting temperature and the desired melt with respect to depth, penetration and area. A differentiated supply of heat may be used, depending on the specific thermal capacity of the plastic and the body to be heated, so that the correct temperature is reached simultaneously in the tank 1 and the sleeve 2. The thermal. mirror is then removed before the sleeve 2 and/or the tank 1 is turned into a position in which the thermal radiation can be observed by means of a digital infrared camera 3, cf. Fig. 1c. The thermal images of the sleeve 2 and the tank 1 may be observed separately or simultaneously, dependent on the appearance of the details. Possibly a not shown mirror can be used for one and/or both parts, so that only a limited I 7 turning or no turning of the part or parts will be necessary. If the subsequent comparing of the observed thermal radiation with the respective reference value of the computer system shows that the tank 1 and the sleeve 2 have reached the intended temperature after the heating, the sleeve 2 is brought onto the tank 1 by means of the robot 5, so that the welding can take place, cf. Fig. 1d.

Fig. 2 shows another embodiment of the invention, in which the check of the welded joint takes place during the cooling of the plastic details 1, 2, after welding. The temperature distribution in the cooling develops in accordance with a very specific pattern when the heating has been correct, and the plastic details have obtained the correct position and contact in the welding. Said particular development of the is temperature distribution in the cooling will thereby allow the after- checking of the welded joint to be based on the observation of thermal radiation from remaining heat in the plastic details 1, 2 at the desired point or points of time during cooling. Further, the observed thermal radiation may be compared with at least one image reference which is depicted, at the respective point of time, by the thermal radiation of corresponding plastic details 1, 2 which had ideal conditions in the rest of the heating and welding, or be compared with at least one reference value of predetermined temperature distributions in, or predetermined temperatures at selected points of, the plastic details 1, 2. In the latter case, the computer equipment is arranged so that the temperature distribution or the temperatures at the different points is/are calculated on the basis of the thermal radiation observed at the respective point of time during cooling.

8 Moreover, Fig. 2 shows the same petrol tank 1 and filling sleeve 2 which is described in Fig. 1, but here the details have been brought into position and welded. Further the camera 3 is placed above the sleeve, so that the thermal radiation from the remaining heat in the tank 1 and the sleeve 2 may be observed at the relevant point of time. Furthermore, the present embodiment corresponds to that mentioned in connection with the description of the former embodiment.

Fig. 3 shows a third embodiment of the invention, in which the checking of the welded joint takes place only after the cooling of the plastic details 1, 2. Here the check is carried out by means of heat supplied to the cooled plastic details 1, 2 in the area around the welded joint. The heat is may be supplied as a short pulse of heat or light, e.g. from at least one photoflash lamp 6. The thermal energy on the surface will then diffuse into the plastic details 1, 2. However, any defects in the welded joint will prevent said diffusion. The result of this is that on their surface, where the defect is, the plastic details 1, 2 will cool more slowly and stay warm longer. Said diffusion of the supplied heat after the cooling, will thus allow that the after-check of the welded joint may be based on the observation of thermal radiation from the heat supplied to the plastic details 1, 2.

Further, the obser-ved thermal radiation will be compared with at least one image reference depicted by the thermal radiation from the plastic details 1, 2, to which heat has been supplied in a corresponding manner after the cooling, and which were joined under ideal conditions in the rest of the heating and welding; or be compared with at least one reference value of predetermined temperature distributions in, possibly predetermined temperatures at selected points I I 9 of, the plastic details 1, 2. In the latter case the computer equipment is arranged so that the temperature distribution, possibly the temperatures at the selected points, is/are calculated on the basis of the thermal radiation which is observed at the respective point or points of time after said heating. This type of check is preferably carried out after the welded joint have been subjected to a suitable mechanical load.

Fig. 3 shows, furthermore, the same petrol tank 1 and the filling sleeve 2 which are described in Fig. 1, but here they are cooled after the welding. Heat for the heating after the cooling is supplied by means of two photoflash lamps 6. Further the camera 3 is placed by the sleeve 2, so that the thermal radiation from the heat supplied to the tank 1 and is the sleeve 2, can be observed at the relevant point of time. Otherwise, the present embodiment corresponds to that mentioned in connection with the description of the embodiment first mentioned.

Thereby, by means of the present invention, there is provided a method of checking welded joints, which is quick in use, and which may be carried out automatically in a production line, without any particular delays in the production. Moreover, the present method is flexible in the way that one or more of the embodiments may be used concurrently, dependent on how the plastic details and/or production equipment etc. are/is formed. This means great savings in expenses as a consequence of the time consumption being short any defects may be weeded out early in the production, and the welded objects put to use, do not have to be exchanged because of leakage in the welded joint. Further, the checking will be very accurate, whether the checking takes place by means of just one or by several embodiments at the same time.

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Claims

C L A I M S

1 A method of checking a joint between plastic details (1, 2) which are joined together by means of welding, c h a r a c t e r i z e d i n that thermal radiation from said plastic details (1, 2) is observed in an area around the joint during production, e.g. -in a production line, by means of at least one camera (3), e.g. a digital infrared camera, and that the observed thermal radiation is compared by means of computer equipment with at least one reference value, so that it may be established whether the welding of the plastic details (1, 2) will be/is adequate.

2. A method according to claim 1, c h a r a c t e r i z e d i n that the thermal radiation is observed after the heating and preferably before the joining of said plastic details (1, 2).

3. A method according to claim 2, c h a r a c t e r i z e d i n that the observed thermal radiation is compared with at least one image reference depicted earlier by thermal radiation from corresponding plastic details (1, 2) by ideal conditions during the heating thereof.

4. A method according to claim 2, c h a r a c t e r i z e d i n that the observed thermal radiation is compared with at least one reference value of a predetermined temperature distribution in, possibly predetermined temperatures at selected points of, the plastic details (1, 2), the computer equipment being arranged so, that the temperature distribution in, possibly the temperatures at the selected 12 points of, the plastic details (1, 2) can be calculated on the basis of the observed thermal radiation.

5. A method according to claim 1, c h a r a c t e r i z e d i n that the thermal radiation is observed at chosen points of time while said plastic details (1, 2) are cooled after welding.

6. A method according to claim 5, c h a r a c t e r i z e d i n that the observed thermal radiation is compared with at le ast one image reference depicted earlier at the respective point of time in the thermal radiation from corresponding plastic details (1, 2), by ideal conditions in the rest of the heating and welding.

7. A method according to claim 5, c h a r a c t e r i z e d i n that the observed thermal radiation is compared with at is least one reference value of the predetermined temperature distribution in, possibly the predetermined temperatures at selected points of, the plastic details (1, 2), the computer equipment being arranged so, that the temperature distribution in, possibly the temperatures at the selected points of, the plastic details (1, 2) can be calculated on the basis of the thermal radiation observed at the respe ctive point of time in the cooling.

8. A method according to claim 1, c h a r a c t e r i z e d i n that the thermal radiation is observed only when said plastic details (1, 2) have been cooled after the welding, heat being supplied by means of a pulse of heat or light, e.g. from at least one photoflash lamp (6), before the thermal radiation is observed.

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9. A method according to claim 8, c h a r a c t e r i z e d i n that the thermal radiation observed, is compared with at least one image reference depicted earlier by thermal radiation from the heat supplied to the plastic details (1, 2) after cooling, and with ideal conditions in the rest of the heating and welding.

10. A method according to claim 8, c h a r a c t e r i z e d i n that the thermal radiation values are compared with at least one reference value of predetermined temperature distribution in, possibly the predetermined temperatures at selected points of, the plastic details (1, 2), the computer equipment being arranged so, that the temperature distribution in, possibly the temperatures at the selected points of, said plastic details (1, 2) may be calculated on is the basis of the thermal radiation observed after the heating of said plastic details (1, 2).

11. A method according to claims 8-10, c h a r a c t e r i z e d i n that said plastic details (1, 2) are subjected to a mechanical load before heat is supplied to said plastic details (1, 2) after the welding.