WO2006048500A1

WO2006048500A1 - Laser welding method

Info

Publication number: WO2006048500A1
Application number: PCT/FI2005/000469
Authority: WO
Inventors: Anssi Jansson
Original assignee: EMMECON Oy; VTT Technical Research Centre of Finland Ltd
Current assignee: EMMECON Oy; VTT Technical Research Centre of Finland Ltd
Priority date: 2004-11-03
Filing date: 2005-11-02
Publication date: 2006-05-11
Anticipated expiration: 2007-05-03
Also published as: FI20041414A0

Abstract

A laser welding method for welding together two plastic components, a first plastic component (1) i.e. one that is permeable to laser radiation and a second plastic component (2) i.e. one that is non-permeable to laser radiation, in which method a laser beam (3) is directed through the first component into the second component and the laser beam is moved along a predetermined path (4); the components are held substantially in engagement with one another at least in the area of welding and during the entire welding, resulting in that as the laser beam is absorbed in the second component, the latter starts to melt in the area of radiation, thereby melting the first component so that the molten areas of the components adhere to one another forming a seam, and the components are welded together; criteria for an acceptable seam are determined; and a product is accepted when the criteria for an acceptable seam are met . According to the invention, the laser beam is moved several times along a predetermined path; the melting of the components and the formation of the seam are monitored substantially continuously using image processing technology as the welding proceeds through the first plastic component; criteria for the image of an acceptable seam are determined, and the welding is stopped as the criteria are met.

Description

LASER WELDING METHOD

FIELD OF THE INVENTION

The invention relates to a laser welding method as defined in the preamble of claim 1 for at- taching together two plastic components that are dif¬ ferent as their permeability to laser radiation is concerned, i.e. one permeable to radiation and one non-permeable to radiation.

BACKGROUND OF THE INVENTION

In welding together two plastic components, often a more massive component and a plastic film, by means of a laser beam, the welding is performed using rivet welding so that the laser beam permeates the topmost plastic transparent to the wavelength of the laser, but not the lower plastic non-transparent to the wavelength of the laser. In that case, the lower plastic absorbs the laser radiation directed to it so that it will heat and melt, thereby starting to melt the topmost transparent plastic that is in a suitable pressure contact therewith. When the plastics are mol¬ ten in the area of the weld being produced and suffi¬ ciently mixed together, heating is stopped. After the weld has cooled, the plastic parts have been fixedly welded together.

The requirement of a successful weld is that the plastics that are to be welded together are in a close contact with one another in the entire area of the weld to be made, that is both along the length and the width during the entire welding process. Due to this, welding flaws are easily produced in case, irre¬ spective of the compression together, the plastics are not sufficiently well attached to one another, for ex- -amp-le-_r- because -of—an—ai-r bubble-_?- -t-ra-sh-,- a—f-l-aw- i-n t-he- material or some other fault. This kind of welding uses mainly semi- simultaneous welding, in which the laser beam is de¬ flected along a predetermined path by means of scanner mirrors, for example, under the control of a numerical path guide tens and even hundreds of times consecu¬ tively. When the cycle length of the path is suffi¬ ciently short, the welding path will heat more and more with each cycle until the surfaces of the parts to be attached together will melt; the molten parts will be mixed with one another and welded together along the entire path almost simultaneously.

The problem with the prior-art technology is the quality inspection. Usually it is not until the welds are completed that they are inspected and defec- tive ones eliminated because the prior-art technology does not offer any possibilities to monitor how the weld is produced and to adjust the welding process when making the weld.

The problematic prior-art technology referred to above has been described, for example, in publica¬ tion Al Habaeih et al, " A novel approach for quality control system using sensor fusion of infrared and visual image processing for laser sealing of food con¬ tainers", Measurement Science and Technology, 15 (2004) , pp. 1995-2000, publish. 20/8/2004, ISSN 0957- 0233. In this publication, it is not until a few sec¬ onds from the completion of the seam that it is moni¬ tored based on the heat radiation emitted from it, and the products with a defective seam are discarded.

OBJECTIVE OF THE INVENTION

The objective of the invention is to elimi¬ nate the aforementioned disadvantages of the prior- art. One specific objective of the present invention is^" to^""dTsclose ^~a^~hew kind^" of method that enables moni¬ toring the formation of a seam in real time and ad¬ justing the welding so that flaws in the seam can be prevented, and when making the seam, flaws in the seam can be fixed as they appear prior to the completion of the seam.

SUMMARY OF THE INVENTION

The laser welding method of the invention is characterised by what has been described in claim 1.

The laser welding method of the invention is used for welding together two plastic components, a first plastic component i.e. one that is permeable to laser radiation and a second plastic component i.e. one that is non-permeable to laser radiation. In the method, a laser beam is directed through the first component into the second component; the laser beam is moved along a predetermined path and the components are held substantially in engagement with one another at least in the area of welding, that is to say in the area of the seam being produced, and during the entire welding. In that case, as the laser beam penetrates the first component and is absorbed on the surface of the second component, the latter starts to melt in the area of radiation, while at the same time melting the surface of the first component at a corresponding point so that the molten areas of the components are mixed and adhere to one another, and when the radia¬ tion is stopped and the components cool, the compo¬ nents have been welded together. When acceptability criteria have been imposed on the seam to be produced, the seam can be accepted as the criteria are met. Ac- cording to the invention, the laser beam is moved along a predetermined path several times in a row, and the melting of the components and the formation of the seam are monitored substantially continuously through the first plastic component using image processing technology. When acceptability criteria have been im¬ posed on the image of the seam to be obtained using image processing technology, the laser welding and formation of the seam proceed until the criteria are met, whereby the welding is stopped.

In the method of the invention, the formation of the seam is monitored substantially continuously, 5 i.e. either in a continuous and uninterrupted manner of monitoring or suitably periodically and at short, given intervals as the welding proceeds, and the image taken of the seam is compared to given criteria that determine the finished seam. Thus, one proceeds with

10 the welding as long as the criteria are not met and stop the welding as the criteria are met. On the other hand, in case the criteria are exceeded, i.e. one has welded too much so that, for example, too long a seam has been produced, the product is discarded.

15 Depending on the dimensions and shape of the seam to be produced, as well as the entire component to be processed, the laser beam can be moved either in parallel along a given path or back and forth along a given path.

20 In a smooth and flawless jointing process, the weld is usually simultaneously formed to be of uniform quality and width along its entire length. Be¬ cause, however, there is a possibility to existing im¬ purities or other deviations at least in the area of 25 the seam, in one embodiment of the invention, the in¬ tensity of the laser beam can be adjusted at the dif¬ ferent points of the^' seam to be produced, in its lon¬ gitudinal or lateral direction, to form a seam that meets the criteria.

30 Alternatively or simultaneously with the ad¬ justment of the intensity of the laser beam also the effect time of the laser beam can be adjusted at the different points of the seam being produced to form a seam that meets the criteria. Further in addition to -3-5 the-—a-foreme-nt-i-oned— ones— -or—aite-rnati-vei-y—together- with them, because one uses a laser beam that sweeps over the seam several times, the effect times of the laser beam at the different points of the seam to be produced can be adjusted to form a seam that meets the criteria.

In a preferred embodiment of the invention, 5 the seam is welded using semi-simultaneous welding in which the laser beam is moved very fast along a prede¬ termined path several times in a row to achieve suffi¬ cient melting. One keeps circling the path or moving back and forth along it so fast that the warming of 0 the plastic to be heated is substantially uniform and continuous and the seam being produced will not get cool while the laser beam is circling the path.

The method of the invention for monitoring a seam is a continuous online process in which, as far 5 as the success of the welding process is concerned, the system is tuned to the monitoring of the important issues, such as the development of the width of the weld that can be seen in the image area in the form of a stripe, along the entire stretch of the weld; the 0 uniformity of the stripe or some other typical quan¬ tity. If during the processing, it seems like the weld is not going to be uniform throughout, the process pa¬ rameters can be changed adaptively so that uniformity is reached, or at least a uniform outcome is achieved. 5 If changing the process parameters will not help and the final quality of the weld remains below the ac¬ ceptability limit, then the system will provide this information for discarding the article in question.

Depending on the shape and dimensions of the 0 seam, the application, the use requirements or the like, the criteria for an acceptable image of a seam can include the following facts: the width of the seam, the uniformity of the width along the entire length of the seam, the uniformity of the contours of -5- the—seam,—-t-he -str-ai-ghfcness -of—the -seam -contours, —the- uniformity and continuity of the seams and/or the uni¬ formity of the colour of the surface of the seam area. The image processing technology to be used in the method of the invention is not limited in any man¬ ner; instead various techniques known per se and obvi¬ ous to a person skilled in the art can come into gues- tion. In that case, monitoring the criteria of the seam and the determination of the criteria can vary freely depending on the known image processing tech¬ nology used. As one example we mention an embodiment in which the seam being produced has been divided into several small sections in respect to its surface area, and each of these sections is inspected separately. There are certain numbers of minimum and maximum pix¬ els determined for each section, and in an acceptable seam, the number of pixels in the completed seam shall fall within these values. Other kinds of image proc¬ essing techniques that monitor the process of produc¬ ing an image of the seam and compare it to given val¬ ues or reference images are also possible.

The method of the invention has several ad- vantages compared to the prior-art technology. In welding together plastic components, the invention en¬ ables continuous monitoring and process control during the production. Thanks to this fact, the production process is accelerated, the number of faulty articles is almost eliminated and the need for inspecting the products after the process is eliminated. The products to be rejected can be identified and discarded from the process in an individual manner, resulting in that there is no need to wholly eliminate or demolish big- ger production lots either, as it often is the case at present. Just the defective articles are eliminated. Hence, the invention has an important economic role in the laser welding industry of plastic components, par¬ ticularly in huge mass production. As an example of appl-yi-ng—the -invent-i-on- —in— pr-aeti-ee—we— mention —the welding of a transparent display screen to the casing of a mobile phone. In the following section, the invention will be described with reference to the drawings, in which

Fig. 1 is a schematic view illustrating a system in which the method of the invention can be used; and

Fig. 2 represents a series of images illus¬ trating the formation of a seam using the method of the invention.

As shown in Fig. 1, the system comprises a control unit 5 having a laser beam source 6 attached thereto, a guide system such as mirror equipment 7, for guiding the laser beam along a desired path, a camera 8 for taking an image of the seam being pro¬ duced, as well as a light source 9 for illuminating the object being imaged. In the embodiment of the fig¬ ure, as the plastic component 2 impermeable to a laser beam serves the casing of a mobile phone and as the plastic component 1 permeable to a laser beam a trans¬ parent display screen to be welded to the mobile phone casing.

The method functions as follows. Programmed in the control unit 5 is the path along which the la¬ ser beam is guided, using the mirror equipment 7, along the surface of the components 1 and 2 to be at- tached to one another. The control unit 5 is similarly informed of an acceptable seam, that is of those cri¬ teria which need to be met and which cannot be ex¬ ceeded in order that the seam being produced can be accepted. After one has placed on top of the casing 2 the display screen 1 to be attached thereto, the lat¬ ter is held suitably in place, for example, by press¬ ing it by the very edges against the casing 2. There¬ after, the laser beam 3 is directed by means of the mirror equipment 7 to the casing 2 through the display screen—1---wh±-le- t-he-beam—keeps- constant-ly -moving -and circling a predetermined path 4. In that case, the en¬ ergy of the beam is absorbed in the casing, resulting in that the latter starts to heat and melt on the sur¬ face. This melting also melts the display screen 1 at the corresponding point, so that the molten areas of the components 1 and 2 are united, thereby forming a seam that welds the components together. The formation of the seam is continuously monitored using the camera 8 and by comparing, using a suitable image processing technique, the image of the seam being produced to the image of an acceptable seam included in the control unit 5. If necessary, the seam can be further illumi¬ nated with a lamp 9 using visible light of suitable colour or other radiation so that the image taken of the seam is better and sharper. When the image taken of the seam being produced matches, with sufficient accuracy, with an image of an acceptable seam, the welding is stopped.

In a normal situation, the laser beam keeps circling at a regular speed and with a regular power for a sufficient period of time, e.g. 0.5 to 2 sec- onds, after which one has obtained a smooth seam of uniform width along the entire length about the dis¬ play screen. If, however, the components are not uni¬ formly attached to one another; there is air or trash between them, etc., there can be points of non- uniformity in the seam. In that case, the laser beam is guided so that at a point of non-uniformity, the intensity of the laser beam is increased; it is moved more slowly over this place; or the laser beam is even guided back and forth just in this place for a while so that the seam will be of desired width and will meet the criteria defined for it also in the place in question. In case the seam cannot be made to meet the acceptability criteria in suitable time, or if as a result of the processing, the seam exceeds the crite- ria- imposed—on i-t i-n some -r-espeets-y the—component -will be discarded, unless it cannot be accepted irrespec¬ tive of the exceeding. Fig. 2 represents a series of images illus¬ trating the formation of a seam. The weld or seam be¬ ing produced as shown in images a-h develops and gets wider at the same time with each sweep or scan of the laser beam. Finally, a sufficiently wide weld or seam that meets the given criteria is formed.

The area of a weld being produced as shown in image i can be divided into pixels, whereby the forma¬ tion of the weld is monitored for each pixel specifi- cally. Hence, when the weld occupies the area of suf¬ ficiently many pixels, the scanning can be stopped. To achieve the desired weld quality, the density of the pixels shall be selected to be sufficient .

Image j , in turn, shows how too high a laser intensity in relation to the scanning speed has done damage to the weld and produced bubbles in the middle of the weld. Here it is apparent that by keeping the individual pixels sufficiently small so that the size of the pixel corresponds to the size of the bubble, also these kind of defects in the weld can be noticed by examination of the pixels.

Image k shows a fault situation in which there is in the weld an air gap preventing a suffi¬ ciently wide and uniform welding. This kind of defect will be noticed even with a rather big pixel size.

The invention is not limited merely to the examples referred to above; instead many variations are possible within the scope of the inventive idea defined by the claims.

Claims

1. A laser welding method for welding to¬ gether two plastic components, a first plastic compo¬ nent (1) i.e. one that is permeable to laser radiation and a second plastic component (2) i.e. one that is non-permeable to laser radiation, in which method, a laser beam (3) is directed through the first component into the second component; and the laser beam is moved along a prede- termined path (4) , the components are held substantially in engagement with one another at least in the area of welding, and during the entire welding, whereby as the laser beam is ab- sorbed in the second component, the latter starts to melt in the area of radiation, thus melting the first component so that the molten areas of the components adhere to one another, thereby forming a seam, and the components are welded together, criteria for an acceptable seam are deter¬ mined; and the product is accepted, as the criteria for an acceptable seam are met, c h a r a c t e r i s e d in that the laser beam is moved several times along a predetermined path; the melting of the components and the for¬ mation of the seam are monitored substan- tially continuously using image processing technology as the welding proceeds through the first plastic component; criteria for an acceptable image of a seam are determined, and - the welding is stopped as the criteria are met .

2. The method as defined in claim 1, c h a r a c t e r i s e d in that the laser beam is moved in parallel about a predetermined path.

3. The method as defined in claim 1, c h a r a c t e r i s e d in that the laser beam is moved back and forth along a predetermined path.

4. The method as defined in any one of claims 1-3, c h a r a c t e r i s e d in that the formation of the seam is monitored continuously and without stop- ping during the entire time the laser beam is focused.

5. The method as defined in any one of claims 1-3, c h a r a c t e r i s e d in that the formation of the seam is monitored periodically and at given inter¬ vals.

6. The method as defined in claim 1, c h a r a c t e r i s e d in that the intensity of the laser beam is adjusted at the different points of the seam to be produced to form a seam that meets the cri¬ teria.

7. The method as defined in claim 1, c h a r a c t e r i s e d in that the effect time of the laser beam is adjusted at the different points of the seam to be produced to form a seam that meets the criteria.

8. The method as defined in claim 1, c h a r a c t e r i s e d in that the effect times of the laser beam at the different points of the seam are adjusted to form a seam that meets the criteria.

9. The method as defined in any one of claims 1-8, c h a r a c t e r i s e d in that the seam is welded using semi-simultaneous welding in which the laser beam is guided along a predetermined path sev¬ eral times in a row to achieve a sufficient melting.

10. The method as defined in any one of claims ^~l-^~9^", ^~c h^~ a^~r ^"aCt^~e^" r^~'i^~~s ^~e^~d^~~~±n "that: ^~the^~ seam- being produced is illuminated to improve the image taken of it.

11. The method as defined in any one of claims 1-10, c h a r a c t e r i s e d in that the cri¬ teria for the image of an acceptable seam include: the width of the seam, the uniformity of the seam, the uniformity of the contours of the seam, the straight- ness of the contours of the seam, the uniformity and continuity of the seam and/or the uniformity of the seam area.