Patent Application
20130025793
The invention relates to a device for laser welding thermoplastic membranes.
In the context of the present invention, “absorbent thermoplastic material” refers to a thermoplastic material that absorbs electromagnetic radiation with a wavelength comprised between 800 and 1200 nm. The non-filled thermoplastics are transparent to the electromagnetic radiation at a wavelength comprised between 800 and 1200 nm. However, adding an absorbent filler, such as carbon black, even in a small quantity (0.5% by mass, for example, in the case of carbon black), makes them absorbent. Thus, in the context of the present invention, “absorbent filler” refers to a material which, by being added to a thermoplastic material, makes the latter absorbent to electromagnetic radiation with a wavelength comprised between 800 and 1200 nm.
Furthermore, uniform illumination of a surface larger than 10 mm2 is defined as followed. Let an elementary surface be 1 square millimeter. The illumination is said to be uniform over the surface larger than 10 square millimeters if the deviation between the average optical power received by that surface larger than 10 square millimeters and the power received by any of its elementary surfaces does not exceed 10%.
Simultaneous illumination of a surface is defined by the fact that at a given moment, all points of that surface are illuminated.
Direct illumination using laser diodes refers to illumination for which the optical power has not been guided by an optical fiber.
To ensure the sealing of various works, such as water retention basins, fish farming basins, waste burial pits, tunnel walls, building roofs, or concrete slabs before the placement of ground covers, thermoplastic membranes are used, in particular made from polymers such as high-density polyethylene (PEHD), flexible polyvinyl chloride (PVC-P), or polypropylene (PP).
These thermoplastic membranes generally assume the form of rolled sheets, the width of which is insufficient to cover the entire surface to be sealed. That is why thermoplastic membranes are cut from those rolls and arranged on the surface to be sealed in parallel so that they overlap over a width of 80 to 500 mm depending on the dimensions of the work, and sealing of the covering is achieved by hot welding.
To date, the following methods are known for performing the welding at the overlap area of thermoplastic membranes:
The first method consists of inserting an electrically heated boot between the two thermoplastic membranes to be assembled, so as to soften their surface enough for the pressure that is exerted by a metal roller placed behind the boot and bearing on the so-called upper thermoplastic membrane to cause their adhesion. Nevertheless, this method is not suitable for thermoplastic membrane less than 500 microns thick. Furthermore, the maximum welding speed cannot exceed 5 meters per minute.
The second method consists of inserting, between the sealing membranes, in place of the heating boot, a flat nozzle through which air is blown at a temperature comprised between 250 and 400° C. It then becomes possible to weld thinner thermoplastic membranes, but the highest performing devices used for this second method do not exceed the speed of 7.5 meters per minute, or only 450 meters per hour. This is very detrimental, since until the sealing of the ground or walls has been completely finished, the subsequent construction steps cannot start.
The welding devices used to perform the methods described above also have the following drawbacks:
It is also well known to weld thermoplastic materials using welding devices that are equipped with laser diodes and optical fibers that channel the light emitted by said laser diodes so as to transport it to the thermoplastic membranes to be welded. Nevertheless, introducing light into an optical fiber is a costly operation that requires precise optics and alignments between the light source and the optical fiber, which inevitably creates power losses.
The present invention resolves all or part of the drawbacks outlined above presented by the devices currently known for welding thermoplastic membranes.
In fact, the present invention proposes to provide a device for welding thermoplastic membranes making it possible to:
Furthermore, with a welding device according to the invention, it is no longer crucial during welding to:
Lastly, the welding device according to the invention does not require frequent cleaning as mentioned above, and above all, it has a simple design, i.e. it does not require particular and complex optics.
The welding device according to the invention for welding a first thermoplastic membrane to a second thermoplastic membrane, in which at least the first thermoplastic membrane or the second thermoplastic membrane is absorbent, is characterized in that it comprises at least one die including a plurality of laser diodes for emitting a laser beam F forming direct, simultaneous and uniform illumination on the illumination surface of the first thermoplastic membrane.
Thus, owing to the plurality of laser diodes for emitting a laser beam F forming direct, simultaneous and uniform illumination comprised by the welding device according to the invention, it is also possible to avoid using one or more movable mirrors, which are expensive and fragile, and the position of which must also be subjugated to scan in one or two dimensions, so as to generate a uniform power density when averaged over time.
The design of the welding device according to the invention is simplified as a result and gains robustness, as there are no moving parts, and the transmission of the energy to the thermoplastic membranes to be welded is optimal.
Furthermore, owing to this uniform, direct and simultaneous illumination, it is not necessary for an optical fiber to channel the light emitted by the laser diodes and transported to said thermoplastic membranes to be welded, which also contributes to simplifying the design of the welding device according to the invention.
Advantageously, the welding device also comprises at least one waveguide.
In one embodiment of the invention, the welding device also comprises at least one means for putting the first thermoplastic membrane in contact with the second thermoplastic membrane. This contact means is designed so as to exert sufficient pressure at the illumination surface by the laser beam F for the first and second thermoplastic membranes to be able to exchange calories by conduction.
Preferably, the welding device also comprises at least one calorie draining means for avoiding heat degradation of the illumination surface of the first thermoplastic membrane. Said calorie draining means has a much higher thermometric conductivity than that of the thermoplastic polymer from which the first thermoplastic membrane is made, which allows it to drain the calories generated by the laser radiation outside the upper surface of the first thermoplastic membrane.
Preferably, said laser diodes each emit a laser ray with a wavelength comprised between 800 and 1200 nm.
Advantageously, the laser diodes are laser diodes that each emit a laser ray perpendicular to the substrate from which they are made.
Very preferably, the laser diodes are laser diodes of the VCSEL (“Vertical Cavity Surface Emitting Laser”) type.
Advantageously, the welding device comprises a plurality of waveguides and a plurality of dies arranged so that at least one of said waveguides guides at least one of said laser beams F emitted by the plurality of laser diodes of said plurality of dies.
In one embodiment of the invention, the welding device comprises a plurality of waveguides and a plurality of dies arranged so that each waveguide guides a laser beam F emitted by the plurality of laser diodes of a corresponding die.
Advantageously, the calorie draining means is inserted between the laser beam F and the first thermoplastic membrane and has a coefficient of heat transfer λ much higher than that of the first thermoplastic membrane. Preferably, this coefficient of heat transfer λ is greater than 30 W/m·K at 20° C.
Advantageously, the welding device comprises at least one synthetic sapphire or synthetic diamond bar. The waveguide, the means for putting the first thermoplastic membrane in contact with the second thermoplastic membrane, and the calorie draining means for avoiding heat degradation of the illumination surface of the first thermoplastic membrane can be accumulated in such a synthetic sapphire or synthetic diamond bar. This has the advantage of simplifying the welding device according to the invention.
In one embodiment, the welding device can comprise at least one plate of transparent material that performs the functions of the calorie draining means and the means for putting the first thermoplastic membrane in contact with the second thermoplastic membrane. Preferably, the plate of transparent material is a plate of synthetic sapphire or synthetic diamond.
In one embodiment where the welding device comprises such a synthetic sapphire or synthetic diamond plate, the welding device can also comprise a glass bar that performs the waveguide function.
These embodiments thus described have the advantages of simplifying the design of the welding device according to the invention.
Preferably, the waveguide comprises at least one diopter in the form of a notch.
Preferably, the device according to the invention comprises at least one means for putting the first thermoplastic membrane in contact with the second thermoplastic membrane that includes elastic deformation elements.
Advantageously, the welding device according to the invention includes a power source that is an energy source of the battery type.
The welding device according to the invention may comprise movement means allowing it to be moved during the welding.
The welding device according to the invention may also comprise a means allowing the heat produced in the first thermoplastic membrane to propagate into the second thermoplastic membrane. Advantageously, this means is a Teflon® pad.
Preferably, the power emitted by the laser beam F depends on the welding speed of the welding device.
Advantageously, the laser beam F is made over several lines.
In one embodiment, the welding device also comprises at least one optical system designed so that the direct, simultaneous and uniform illumination formed by the laser beam F emitted by the plurality of laser diodes is in the object focal plane so as to obtain a magnification to spread the power of the laser beam F in the image plane.
Advantageously, the optical system also comprises at least two mirrors arranged on either side of the laser beam F so as to contain its optical power in a direction transverse to the movement of the welding device and spread it in the direction of movement of said welding device. Said at least two mirrors can have an adjustable spacing.
In one embodiment of the invention, said at least two mirrors perform the function of the means for putting the first thermoplastic membrane in contact with the second thermoplastic membrane.
Preferably, said optical system comprises at least one lens.
In one embodiment of the invention, the welding device also comprises at least one means for crushing the first thermoplastic membrane on the second thermoplastic membrane. This crushing means allows crushing of the first and second thermoplastic membranes at the areas softened by the heat so as to cause an interpenetration of the material between those thermoplastic membranes.
The invention will be better understood using the detailed description provided below in light of the appended drawing, showing, as a non-limiting example, one embodiment of the present invention.
a is a diagrammatic perspective view of a synthetic sapphire bar comprised by the welding device according to the invention.
b is a diagrammatic perspective view of a glass bar, as well as a synthetic sapphire plate comprised by the welding device according to the invention.
a is a front diagrammatic view of a die of VCSEL laser diodes mounted on a synthetic sapphire bar used as means for putting the first thermoplastic membrane in contact with the second thermoplastic membrane, and which is completed by other contact means in the form of springs comprised by the welding device according to the invention.
b is a diagrammatic side view of the die and the strip shown in
c is a view similar to
a is a diagrammatic view of one embodiment of the invention in which the welding device is equipped with an optical system comprising a lens.
b is a view similar to
a is a diagrammatic view of the embodiment of the invention shown in
b is a diagrammatic side view of the embodiment shown in
The welding device 21 according to the invention may comprise laser diodes called “conventional” (“edge emitters”), i.e. which emit by the edge, or laser diodes that emit a laser ray perpendicular to the substrate from which they are made. It is, however, more advantageous for the welding device 21 to comprise the latter parts, and still more preferably laser diodes of the VCSEL type.
In fact, the laser diodes of the VCSEL type have the advantage of being able to be manufactured in the form of a die or a plurality of dies with a very high density as mentioned above. In fact, each VCSEL laser diode emits over a reduced surface and parallel to the other VCSEL diodes. Using a die or a plurality of dies of VCSEL laser diodes, illuminations of several kW/cm2 can be obtained that are completely comparable to the illuminations obtained with so-called “edge emitter” laser diodes and with the following technical advantages:
Furthermore, the sum of all of the beams coming from these VCSEL laser diodes 2 procures completely uniform illumination at several millimeters from the emitting surface. Satisfying this uniformity criterion is crucial in the context of the present invention. In fact, the thermoplastic materials making up the first and second thermoplastic membranes 12, 13 to be welded are poor heat conductors. It is therefore crucial for the increase in the temperature up to the softening of those materials to be the same at each moment at all points of the surface to be welded of said thermoplastic membranes.
With a plurality of dies 1 of VCSEL laser diodes 2 aligned in a row, it is possible to generate a laser beam F (or in other words a “line”) several centimeters long and several millimeters wide at a distance from the emitting surface smaller than 100 mm. This makes it possible to dimension the welding device 21 in a very compact manner.
In one embodiment of the invention, the VCSEL laser diodes 2 emit from the bottom, which makes it possible to cool them passively, and therefore to do away with cooling using water circulation. Their energy output, i.e. the ratio of the optical power emitted to the electrical power supplied, today is in the vicinity of 50%, and developments in progress will soon make it possible to exceed 60%.
All of the dies 1 of VCSEL laser diodes 2 can be powered by a 6 V or 12 V battery capable of ensuring full-rating operation of all of the laser diodes 2 for at least 4 hours. In this way, this procures the advantage for the welding device 21 of not having to have a wired connection, which greatly facilitates its use, and in particular in locations that are often not very accessible where thermoplastic membranes are placed.
a is a diagrammatic perspective view of a synthetic sapphire bar 3a with a parallelepiped shape with a beveled lower end 4a comprised by the welding device 21.
Due to their angle of incidence and the difference between the refraction indices of the air and of the synthetic sapphire bar 3a, the beams emitted by the laser diodes 2 reflect on its walls and overlap to form a uniform illumination at the outlet of said synthetic sapphire bar 3a, i.e. in direct contact with the first thermoplastic membrane 12.
The synthetic sapphire bar 3a shown in
The synthetic sapphire bar 3a has undergone an anti-reflective treatment on the inlet surface 20 and on the surface in contact with the first thermoplastic membrane 12, which makes it possible to obtain a transmission rate close to 100% between 800 and 1200 nm.
Furthermore, the coefficient of heat transfer λ of the synthetic sapphire is 40 W/m·K at 20° C. (value 100 times higher than that of the thermoplastic membranes). That is why the synthetic sapphire bar 3a is also a calorie draining means for preventing heat degradation of the illumination surface 23 of the first thermoplastic membrane 12 of the welding device 21.
Also, as shown in
The welding device 21 according to the invention may comprise a glass bar 3b and a synthetic sapphire plate 24 as shown in
In another embodiment of the invention not shown, the welding device 21 can comprise a synthetic diamond bar of the CVD (Chemical Vapor Deposition) type, which offers transparency equivalent to that of the synthetic sapphire, but with a coefficient of heat transfer λ greater than 1000 W/m·K at 20° C. The maximum thickness currently available on the market for such a synthetic diamond bar is 1 mm. This synthetic diamond can also, like the synthetic sapphire plate 24, perform the functions of the calorie draining means for avoiding heat degradation of the illumination surface 23 of the first thermoplastic membrane 12 on the second thermoplastic membrane 13 and the means for putting the first thermoplastic membrane 12 in contact with the second thermoplastic membrane 13 of the welding device 21.
In
The helical springs 6 have a stiffness such that they can exert sufficient pressure on the first thermoplastic membrane 12 to ensure its contact with the second thermoplastic membrane 13. Furthermore, the helical springs 6 ensure permanent contact of the synthetic sapphire bar 3a on the first thermoplastic membrane 12 to offset any flatness defects of the substrate to be sealed, which represents an important advantage of the welding device 21.
As shown in
In
As shown in
Furthermore, in one embodiment of the invention not shown, the waveguide, for example in the form of a synthetic sapphire bar 3a, can have a length such that above it, several dies 1 of laser diodes 2 are aligned. In fact, as a function of the power density and the desired length of the laser line, the beams of several dies 1 can be injected into a same waveguide and overlap to form a uniform line at the outlet of said means. However, this embodiment is only appropriate when the substrate to be sealed is completely flat.
As shown in
Preferably, the first and second thermoplastic membranes 12, 13 respectively comprise two layers of thermoplastic material, i.e.:
Such bi-layer materials are commonly produced:
Their price is identical to that of single-layer materials containing carbon black.
The welding speed of the welding device 21 on such bi-layer thermoplastic membranes can reach 30 meters per minute.
However, the welding device 21 is also completely adapted to weld:
Furthermore, in the context of the present invention, the thermoplastic materials from which the first and second thermoplastic membranes 12, 13 are made are advantageously chosen among PVC, PEHD, and PP.
As shown in
The heat flow also propagates in the transparent layer 12a, but the synthetic sapphire bar 3a, which is a good heat conductor due to its high enough coefficient of heat transfer λ, absorbs the calories near the surface of the layer 12a and thus prevents the heat degradation of the illumination surface. As a result, the welding of the first and second thermoplastic membranes 12, 13 obtained is of high quality, since the crushing pressure is exerted on that layer 12a, in particular using the pressing wheel 14 as shown in
More specifically, the pressing wheel 14, due to the high pressure it exerts, forces the interpenetration of the macromolecules respectively making up the first and second thermoplastic membranes 12, 13, said macromolecules having become mobile under the effect of the heat generated by the direct, simultaneous and uniform illumination of the illumination surface 23 of said first thermoplastic membrane 12.
After cooling, a weld is obtained that is as strong as the material from which the first and second thermoplastic membranes 12, 13 are made.
Furthermore, because the heat degradation of the illumination surface 23 is avoided, as a result there is no risk of dirtying of the surface of the synthetic sapphire bar 3a or the pressing wheel 14. In this way, the present invention proposes a welding device 21 for welding a first thermoplastic membrane 12 to a second thermoplastic membrane 13 that does not encounter dirtying problems during welding.
As explained for
c illustrates another embodiment of the invention, when it involves welding two thermoplastic membranes 12, 13 in the form of absorbent single-layer thermoplastic membranes. To that end, in the embodiment of the invention illustrated in
In fact, in the particular case of thermoplastic membranes made from PEHD or PP permanently exposed to sunlight, a bi-layer material as described above is not possible, since the low ultraviolet resistance of PP and, to a lesser extent, of non-filled PEHD, may cause cracks and fissures in the transparent layer. That is why it is necessary to use single-layer absorbent thermoplastic membranes that contain an absorbent filler (for example, carbon black) when said thermoplastic membranes are exposed to ultraviolet rays.
The Teflon® pad 8 then acts as a layer transparent to the laser beam F, which transmits nearly all of the electromagnetic energy to the first absorbent single-layer thermoplastic membrane 12. The synthetic sapphire bar 3a cools the Teflon® pad 8. Teflon® being as poor a heat conductor as the first thermoplastic membrane 12, it only absorbs part of the heat produced in that first thermoplastic membrane 12 and thereby allows it to propagate downward to the second thermoplastic membrane 13 on which it bears.
Up to a certain limit, the Teflon®, through its contact, prevents heat degradation of the surface of the first thermoplastic membrane 12 by the oxygen from the air.
In this embodiment of the invention, the thickness of the first and the second thermoplastic membranes 12, 13 is limited to 300 microns, given that the heat finds it difficult to propagate in the thermoplastic membranes 12, 13, and the welding speed is significantly lower than with a transparent/absorbent bi-layer material, but it may be greater than 10 meters per minute.
It should be noted that the Teflon® pad 8 represents one possible means that may be comprised by the welding device 21 to weld absorbent single-layer thermoplastic membranes, i.e. which comprises absorbent filler such as carbon black. Other means equivalent to the Teflon® pad 8 are within the reach of one skilled in the art.
a shows one embodiment of the invention, in which an adaptive focal lens 10 is arranged across from the die 1 including a plurality of VCSEL laser diodes 2, such that a uniform illumination is found in the object focal plane and so as to obtain a sufficient magnification to spread the power of the laser beam F in the image plane.
The lens 10 used is adapted in relation to the magnification one wishes to produce between the source and the illuminated area. In the event a lens with symmetry of revolution is used, a same form factor will be obtained for the irradiated area and the source.
The lens 10 produces the image of the uniform illumination obtained by the plurality of laser diodes 2 of the die 1 on the illumination surface 23. The laser beam F is allowed to widen in the direction of the movement of the welding device 21.
However, and as illustrated in
In one embodiment of the invention not shown, the welding device 21 may be similar to that shown in
In fact, the welding device 21 may be equipped with a device for adjusting the radiated power so as to optimize the welding speed as a function of the characteristics of the thermoplastic membranes to be welded.
Preferably, the welding device 21 operates in digital mode so as to allow several adjustments to be stored.
A pressing wheel 14 is mounted at the rear of the wagon 21, aligned with its longitudinal axis so as to constitute another bearing point of said wagon 21. Advantageously, this pressing wheel 14 is made from rubber. Furthermore, this pressing wheel 14 is mounted just behind and aligned with the assembly 22 (as shown in
In this way, the rubber pressing wheel 14, by rolling without sliding, exerts strong pressure on the areas radiated by the laser beam F there in a fluid or paste state in and in the immediate vicinity of the absorbent layer 12b. This then allows the tangling (or in other words, the interpenetration) of the macromolecules between the absorbent layer 12b and the transparent layer 13a, and as has already been mentioned above.
Preferably, the width of the pad of the pressing wheel 14 corresponds to twice the width of the laser beam F to avoid leaving a hollow mark. This width remains smaller than or equal to 10 mm. This small width and the compressibility of the rubber from which the pressing wheel 14 is advantageously made make it possible to secure the sealing of the weld on irregular ground, which cannot be done with a non-deformable metal roller 50 mm wide used in the welding devices of the state of the art described above.
Preferably, the pressing wheel 14 is also mounted on a spring suspension.
If needed, a light, for example red, can be injected into the waveguides to indicate a burn risk.
The total emitted power is limited to 300 W to allow a welding speed of 30 meters per minute, or 1.8 km/h, which means that the maximum consumption of the welding device 21 is 600 W.
Furthermore, in one embodiment of the invention not shown, the welding device 21 comprises a movement sensor, which may be optical. This makes it possible to automatically and instantaneously adjust the intensity of the current powering the laser diodes 2, which guarantees a constant and optimized illumination on the first thermoplastic membrane 12 expressed in Watts per square millimeters irrespective of the variations in the movement speed of the welding device 21.
Furthermore, the welding device 21 can be provided with at least two laser lines. It is then possible to continuously or sequentially produce two parallel weld lines, which guarantees complete sealing of the work.
In another embodiment of the invention not shown, it is possible to mount the dies 1 including a plurality of laser diodes 2 with the waveguide and the contact and calorie draining means in a plastic case, and to connect them to a battery 15 using a cable. This thus makes it possible to make truck covers, advertising canvas, tent canvas, pool covers more quickly and more flexibly than with the current tools.
In one embodiment not shown, the welding device 21 can also be a manual device that comprises at least one assembly 22 as shown in
Furthermore, in one embodiment of the invention not shown, the welding device 21 can be a device not moving during operation, but under which the first and second thermoplastic membranes 12, 13 pass (for example using a conveyor belt), said welding device 21 being designed so as to weld said first and second thermoplastic membranes 12, 13 at their overlap areas.
The thermoplastic membranes to be welded can also be small surfaces, such as the thin inner membrane of a mobile telephone, for example, the plastic shell of the device acting as the second membrane.
The present invention thus procures the following advantages:
Lastly, it should be noted that the welding device 21 according to the invention can be used in multiple industrial applications, in particular including those of worksites where geo-membranes or plastic sealing layers are placed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10/52111 | Mar 2010 | FR | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/FR11/50479 | 3/9/2011 | WO | 00 | 10/17/2012 |
