NO311174B1 - Procedure for checking joints between plastic parts - Google Patents

Description

The invention relates to a method for controlling a

joint between plastic parts/details that are joined by pressure welding without the use of additive material.

In the production of objects that are formed by joining together plastic details with the desired design, one or another form of welding is often used. Since many of the plastic objects in question are containers or tanks that will later hold liquid, there is naturally a requirement that the joint between the plastic parts is tight after welding. Therefore, it is common for the containers or tanks to be checked after joining by means of vacuum or overpressure testing, so that it can be demonstrated whether the joint between the plastic parts is complete or not. Practice shows, however, that this type of inspection of welding joints with vacuum or overpressure testing is both time- and cost-consuming. Likewise, there are too many cases where the inspection is not able to detect any leaks in the welding joint between the plastic parts. Thus, the manufacturer runs the risk that undersized containers or tanks are brought onto the market with the undesirable consequences

it entails.

The technique's position is represented, among other things, by a section in the "Composites Engineering Handbook", by P.K. Mallick, who basically deals with thermal waves, as well as U.S. Patent Nos. 4,532,404, 4,594,497, 4,877,940 and 5,674,415.

In the typical case, this known technique consists of welding two metal plates together by butt welding according to traditional fusion welding methods using electrodes. The electrodes are melted into the joint between the metal plates intended for welding together, so that the metal in the weld pool formed after welding is included in the weld between the two metal plates welded together, one on each side of the joint. The control procedure consists, by known technique, in recording the temperature distribution across the welding string. On the basis of this registration, the position and advance speed of the welding electrode as well as its feed speed are then controlled.

Specifically, the U.S. patent document no. 4,532,404 a control/control system which works in real time and serves to control/control predetermined operating conditions by a welding mechanism which is movable along a joint between two metal parts to be welded together. A sensing device for radiant heat is designed to respond to radiant heat energy that flows (radiates) from the heated metal workpieces.

Both this U.S. patent document and the three other, above-mentioned U.S. patent documents as well as the above-mentioned section from the "CompositesEngineering Handbook" deal with welding methods of a completely different kind than the present invention relates to, namely pressure welding without the use of additive material, and which involves checking joints between plastic parts/details that are joined by such pressure welding.

The purpose of the present invention has thus been to provide a method for use when checking joints between plastic parts/details that are joined together by pressure welding without the use of additive material, where the above-mentioned use of vacuum and overpressure has been replaced with the use of thermography, so that heat radiation from an area around the joint of the plastic parts/details can be used as an aid to determine whether the welding joint is tight or not. This makes the inspection far more cost-effective and, moreover, far more secure against leaky joints being mistakenly approved as sealed. A further purpose is that the control should be able to be carried out at the time of production that is most appropriate for the object in question, place of production, welding equipment, etc. Either as a pre-control based on heat radiation after heating the plastic parts in a area around the joint and preferably before the plastic parts are joined together, as a follow-up check based on heat radiation of residual heat during cooling of the plastic parts in the area around the joint, or as a follow-up check after the cooling of the area around the joint has been completed based on heat radiation from heat added after cooling has been completed.

This purpose is realized by starting from a method in accordance with the introductory part of the independent patent claim 1, and also proceeding in accordance with the characterizing part in patent claim 1.

The method is thereby realized in such a way that heat radiation from the aforementioned plastic details in an area around the joint is recorded using at least one camera, e.g. digital infrared camera, and that the recorded heat radiation is compared with at least one reference value using computer equipment, so that it can be determined whether the fusion of the aforementioned plastic details in the area around the joint is complete.

Other advantageous features of the invention appear from the independent patent claims.

Preferred embodiments of the invention will be explained in the subsequent part of the description, and with reference to the set of figures, where: Fig. la-d shows a schematic outline of a first embodiment of the invention where heat radiation in an area around the joint of the aforementioned plastic parts is recorded using a digital infrared camera, and where the plastic parts are heated to the right temperature for fusion using a heat mirror, the heat radiation being recorded after the heating and before the joining of the plastic parts takes place; Fig. 2 shows a schematic outline of a second embodiment of the invention where the heat radiation in the area around the joint is recorded during cooling of the plastic parts after the fusion; and Fig. 3 shows a schematic outline of a third embodiment of the invention where the heat radiation is first registered after the aforementioned plastic details have cooled after the fusion and new heat for the registration has been supplied by means of at least one flash lamp.

The present invention relates to a method adapted for use under control of a joint between plastic details 1, 2 which are joined by means of welding. The inspection of the welding joint is done in such a way that heat radiation from the plastic details 1, 2 in the area around the joint is registered with the help of at least one camera 3, e.g. digital infrared camera. The recording of the heat radiation from the camera 3 is then transferred to a computer equipment not shown which is programmed in such a way that the recording is compared with at least one reference value. Thus, the computer equipment can determine whether the joint between the plastic parts 1, 2 meets the quality requirements set for a full fusion of these, so that produced objects with substandard joints can be distinguished. Said reference value can either be made up of an image of heat radiation taken with a corresponding camera 3, and with conditions during the welding that provide a full-fledged joint between the plastic parts 1, 2, or made up of a predetermined temperature distribution in, possibly temperatures at selected places of the plastic parts 1, 2 at specific times after the warm-up. In the latter case, the computer equipment is programmed in such a way that the real temperature distribution in, possibly the temperatures at the selected locations of the plastic details 1, 2 is calculated on the basis of the recorded heat radiation. Furthermore, the aforementioned predetermined temperature distribution in, possibly the temperatures at the selected locations of plastic details 1, 2 is obtained by means of calculations which will theoretically give a full-fledged joint after the fusion. Said calculations will also be able to form the basis for experimental tests.

The inspection of the welding joint can, as mentioned in the introduction, take place at various times, and typical examples of this are, with reference to the figures, discussed in more detail below. In fig. la-d, a first embodiment of the invention is shown where the control is in the form of a pre-control where heat radiation from the plastic details 1, 2 is recorded after the heating of these and before the joining takes place, so that the welding conditions in an area around the joint can be compared with at least one ideal reference value that provides full fusion of the plastic parts 1, 2 during welding. Said ideal reference value can either be in the form of a previously recorded image of heat radiation from corresponding plastic details 1, 2 where the heating was in order, or in the form of a predetermined (theoretical) temperature distribution in, possibly predetermined temperatures at selected locations of the plastic details 1, 2 The heat radiation from the plastic parts 1, 2 is recorded using at least one digital infrared camera 3. The camera 3 records the heat radiation as an image with different colors in accordance with the respective temperature in the plastic parts 1, 2. The recorded heat radiation is transferred appropriately from the camera 3 to the computer equipment which is programmed in such a way that the recorded heat radiation is compared with the selected reference value, be it the previously recorded image, or the relevant predetermined temperature distribution in, respectively the temperatures at the selected locations of the plastic details 1, 2. When it is compared with the relevant predetermined temperature, is computer u the device is arranged so that the real temperature distribution or temperatures at the selected locations are calculated on the basis of the recorded heat radiation. Normally, a single recording of the heat radiation during the pre-control of the plastic parts 1, 2 will be sufficient, but this does not prevent more recordings being made if necessary. The plastic parts 1, 2 are discarded, or the heating parameters are adjusted and new heating takes place, if the computer equipment during the comparison reveals that there is too great a difference between the registration in question and the respective reference value. In such cases, the computer equipment will also be able to be programmed to notify of process errors, so that the production plant can be stopped for necessary adjustments to the process parameters. The degree of process control and measures taken after recording any errors during welding will depend on the product, the production line, the customer's needs, the quality assurance system, etc.

Otherwise, fig. la-d above pre-check used in connection with welding on a filling nozzle 2 on a plastic petrol tank 1, but there is of course nothing to prevent the same being carried out for all types of welds and most products. The nozzle 2 is brought to a position above the tank 1 with the help of e.g. an industrial robot 5, after which a so-called heat mirror 4 is introduced between the tank 1 and the connection 2. The next thing that happens is that the tank 1, the connection 2 and the heating mirror 4 are brought together, cf. fig. lb, and heat is supplied to the tank 1 and the connection 2 until these have reached the required melting temperature and the desired melting bath with regard to depth, penetration and area. A differentiated supply of heat can be used depending on the specific heat capacity of the plastic and the mass to be heated, so that the correct temperature is achieved simultaneously in the tank 1 and the connection 2. The heating mirror is then removed before the connection 2 and/or the tank 1 is turned to a position where the heat radiation can be recorded using a digital infrared camera 3, cf. fig. lc. The thermal image of the connection 2 and the tank 1 can be recorded separately, or at the same time depending on how the parts look. A non-visible mirror can optionally be used for one and/or both parts, so that only a limited or no turning of the part or parts is necessary. If the subsequent comparison of the recorded heat radiation with the relevant reference value in the computer equipment shows that the tank 1 and the connection

2 has reached the intended temperature after heating, is entered

the spigot 2 against the tank 1 with the help of the robot 5, so that the welding can take place, cf. fig. Id.

Fig. 2 shows another embodiment of the invention where the inspection of the welding joint takes place during the cooling of the plastic parts 1, 2 after the welding. The temperature distribution during cooling develops according to a completely specific pattern when the heating has been in order, and the plastic parts have received the correct position and contact during welding.

i Mentioned special development of the temperature distribution during cooling will thus allow the post-check of the welding joint to be based on the registration of heat radiation from residual heat from the plastic parts 1, 2 on the desired

time during cooling. Furthermore, the registered person can

; meradiation be compared with at least one image reference which at the respective time is depicted by the heat radiation from corresponding plastic parts 1, 2 which had ideal conditions during the heating and welding otherwise, or be

compared to at least one reference value of predetermined

in temperature distributions in or predetermined temperatures in selected locations of the plastic details 1, 2. In the latter case, the computer equipment is arranged such that the temperature distribution or temperatures in the selected locations are calculated on the basis of the heat radiation recorded at the respective time point during cooling.

Otherwise, fig. 2 the same petrol tank 1 and filling nozzle 2 that are discussed in fig. 1, but here these are positioned and

welded together. Furthermore, the camera 3 is placed over the stub, like this

that heat radiation from the residual heat in tank 1 and connection 2 can

d is registered at the relevant time. Otherwise, the present embodiment corresponds to what is mentioned in connection with

the mention of the former embodiment.

Fig. 3 shows a third embodiment of the invention where the inspection of the welding joint only takes place after the cooling of the plastic parts 1, 2. Here the inspection is done using

> of heat that is supplied to the cooled plastic parts 1, 2 in the area around the weld joint. The heat can be supplied as a short pulse of heat or light, e.g. from at least one flash lamp 6. The thermal energy on the surface will then diffuse into the

the plastic details 1, 2. Possible faults in the welding joint will, however, prevent the aforementioned diffusion. The result of this is that the plastic parts 1, 2 on the surface at the fault in the weld joint will

cools more slowly and is kept warm longer. Said diffusion of the added heat after the cooling will thus allow that

the subsequent inspection of the weld joint can be based on registration

5 of heat radiation from the heat that has been added to the plastic parts 1, 2. Furthermore, the recorded heat radiation will be compared with at least one image reference that is depicted by the heat radiation from the plastic parts 1, 2 that have similarly added heat after cooling, and which were joined during o ideal conditions during the heating and welding in general, or be compared with at least one reference value of predetermined temperature distributions in, possibly predetermined temperatures in selected places of the plastic parts 1, 2. 1 in the latter case, the computer equipment is arranged so that s the temperature distribution, possibly the temperatures of the selected places, is calculated on the basis of the heat radiation recorded at the respective time or times after said heating. This type of control is preferably carried out after the weld joint has been exposed to a suitable mechanical load.

o Otherwise, fig. 3 the same petrol tank 1 and filling nozzle 2 that are mentioned in fig. 1, but here these are cooled after con-

the menstrual cycle. Heat for heating after cooling is supplied by means of two flash lamps 6. Furthermore, the camera 3 is placed at the connection 2, so that heat radiation from the heat which

is supplied to tank 1 and connection 2 can be registered at the relevant time. Otherwise, the present embodiment corresponds to what is mentioned in connection with the description of the first-mentioned embodiment.

Thus, with the help of the present invention, a method for checking welding joints has been provided which is quick during use, and which can be carried out automatically in a production line without any particular delays in production. The present method is also flexible in that one or more of the embodiments can be used at the same time, depending on how the plastic details and/or production equipment, etc. are designed. This means major cost savings as a result of the time consumption is short, that any errors can be weeded out early in production and that welded objects that have been put into use do not have to be replaced due to leakage in the welding joint. Furthermore, the control becomes very precise, whether the control takes place using only one or more of the embodiments at the same time.

Claims (8)

1. Procedure for checking the joint between plastic parts (1, 2) which are joined together by pressure welding without the use of additive material, characterized by the fact that the plastic parts (1, 2) are heated to melting temperature in the contact surfaces, where the aim is to have a uniform and sufficient temperature over the entire contact surface areas as follows that the plastic parts (1, 2) can be fused/welded tightly together over an uncut, permanent common cross-sectional area; that the temperature distribution on the plastic parts (1, 2) after this heating, but before their welding, is recorded in the form of heat radiation from them in an area around the joint during manufacture, e.g. in a production line, with the help of at least one camera (3 ), e.g. digital infrared camera, and that the recorded heat radiation is compared using computer equipment with at least one reference value, so that it can be determined whether the welding of the plastic parts (1, 2) is/is complete, i.e. as intended. 2. Method according to claim 1, characterized in that the recorded heat radiation is compared with at least one image reference previously depicted under heat radiation from corresponding plastic parts (1, 2) with ideal conditions during their heating. 3. Method according to claim 1, characterized in that the recorded heat radiation is compared with at least one reference value of a predetermined temperature distribution in, possibly predetermined temperatures at selected locations of the plastic parts (1, 2), the computer equipment being arranged so that the temperature distribution in, possibly the temperatures of the selected places on the plastic parts (1, 2) can be calculated on the basis of the recorded heat radiation. 4. Method according to claim 1, characterized in that the heat radiation is recorded at selected times while the plastic parts (1, 2) cool after welding. 5. Method according to claim 4, characterized in that the recorded heat radiation is compared with at least one image reference previously depicted at the respective time during the heat radiation from corresponding plastic parts (1, 2) with ideal conditions during the heating and welding otherwise. 6. Method according to claim 4, characterized in that the recorded heat radiation is compared with at least one reference value of the predetermined temperature distribution in, possibly the predetermined temperatures at selected locations in the plastic parts (1, 2), the computer equipment being arranged so that the temperature distribution in, possibly the temperatures at the selected locations of the plastic parts (1, 2) can be calculated on the basis of the recorded heat radiation at the respective time during cooling. 7. Method according to claim 1, characterized in that the heat radiation values are compared with at least one reference value of the predetermined temperature distribution in, possibly the predetermined temperatures at selected locations of the plastic parts (1, 2), the computer control being arranged so that the temperature distribution in, possibly the temperatures at the selected locations of the aforementioned plastic parts (1, 2) can be calculated on the basis of the heat radiation recorded after the heating of the plastic parts. 8. Method according to claim 7, characterized in that the plastic parts (1, 2) are subjected to mechanical stress before the aforementioned plastic parts (1, 2) are supplied with heat after welding.