Method and apparatus for infrared welding of polymer composites

Abstract

A system and method for welding carbon fiber reinforced thermoplastic workpieces (10, 12) comprising upper and lower tools (1, 2), each tool having a holder (4.1, 2.1) for holding the respective workpiece (10, 12) in a cantilevered manner during heating (6), and a pusher (4.2, 2.2) movable relative to the holder (4.1, 2.1) for pushing the workpieces (10, 12) once the workpieces are brought together by the tools (1, 2). The heated regions (10.1, 12.1) of the workpiece are pressed against each other. This arrangement allows the heated ends (10.1, 12.1) to be surrounded by air, providing a smaller, more uniform thermal gradient over the thickness of the respective workpiece (10, 12). This also helps to reduce the heat dissipation from the workpiece (10, 12) to the tool (1, 2), in particular in materials with high thermal conductivity. Ideally, heating is provided by a movable infrared heater (6) cooperating with the shielding screen (8).

Description

Technical Field

[0001] The present invention relates to a manufacturing process for welding of polymers. More specifically, the present invention provides a solution for welding polymers by infrared welding. Background Art

[0002] Infrared welding is a technique that is well used to bond plastic materials together. It involves placing two workpieces in a holder (such as in a forging die) and, after illuminating appropriate portions of the workpieces with infrared light, applying a force between the two workpieces to bond them together.

[0003] Figure 1 This technique is shown. System 1 comprises a lower tool 2 and an upper tool 4 movable vertically (direction z). A series of infrared lamps 6 are movable laterally (along x), together with shielding 8 preventing radiation from the lamps 6 from reaching the entire workpieces 10 and 12 to be joined together.

[0004] Figure 1 The four main steps of infrared welding are shown: in step (a), the lamp is switched on and starts radiating; in step (b), the parts 10.1, 12.1 of the workpieces 10, 12 start to melt or decompose (and thus increase in width); in step (c), the heating lamp and shield are retracted, and in step (d), the tools 2, 4 are moved towards each other to apply welding pressure.

[0005] However, this known technology is not suitable for welding operations of materials with high thermal conductivity. For example, carbon fiber reinforced thermoplastics (CFRTP) have a thermal conductivity 5 to 10 times higher than that of some other polymers (0.71.5 W.m -1 .L -1 ). This means that temperature variations across the workpiece can significantly affect the structure (porosity, crystallization rate) of the final product.

[0006] This problem becomes a particular concern for workpieces with a thickness greater than a few millimeters. The lack of a uniform thermal gradient during welding can induce amorphism at the interface. Structural inhomogeneities can be observed along the weld bead. Figure 1 In the example of , heat is conducted from the heated workpieces to the tools 2, 4 and it can happen that the workpieces do not reach the desired welding temperature due to the heat being dispersed into the tools. In addition, the workpieces are heated unevenly and they cool rapidly in an uneven manner (there is therefore the risk of obtaining an amorphous and therefore unbondable structure).

[0007] When one wishes to ensure that the portion of the workpiece to be heated is properly heated at the proper welding temperature or does not cool too quickly, one may be tempted to overheat the workpiece. However, CFRTP materials do not allow overheating because they degrade and may become thermoset, thereby preventing them from being weldable thereafter.

[0008] Therefore, there is a need to provide a welding system and method for welding CFRTP by infrared radiation. Summary of the invention

[0009] The present invention aims to provide a system and a method which overcome the above-mentioned drawbacks and finally enable to properly bond two CFRTP workpieces by infrared radiation, with a high quality bond.

[0010] The invention relates to a system for infrared welding a first workpiece made of a carbon fiber reinforced thermoplastic polymer CFRTP to a second workpiece made of CFRTP, the first workpiece and the second workpiece being substantially flat at least in their respective regions intended to form a weld joint between them, the system comprising: an upper tool for holding the first workpiece; a lower tool for holding the second workpiece, wherein the upper tool and / or the lower tool are movable in a vertical direction to change the distance between them; and a movable infrared heating device, the movable infrared heating device being configured to move in a transverse direction between an operating position and a rest position, wherein the movable infrared heating device is movable in a transverse direction between an operating position and a rest position. In the operating position, the heating device is capable of simultaneously heating the corresponding parts of the first workpiece and the second workpiece; wherein the upper tool and the lower tool each include: a holder, which holds the first workpiece or the second workpiece in a cantilever manner, and their corresponding parts to be heated form the free end of the cantilever attachment; and a pusher, which can be vertically moved relative to the holder between a rest position and an operating position, wherein in the rest position, the pusher is away from the first workpiece or the second workpiece so that the corresponding parts to be heated of the workpieces can be surrounded by air during heating, and wherein in the operating position, the pusher contacts the heated part of the first workpiece or the second workpiece and applies welding pressure in the vertical direction.

[0011] In essence, by enabling the isolation of heated portions of the workpiece with an air gap, the system of the present invention enables smaller gradients to be achieved along the thickness of the workpiece, thereby enabling infrared welding of CFRTP workpieces.

[0012] According to a preferred embodiment, the system further comprises an upper shield and a lower shield, which are configured to move in a lateral direction independently of the heating device, and the upper shield and the lower shield are configured to move between an operating position and a rest position, wherein in the operating position the shield partially shields (shields) the first workpiece or the second workpiece from radiation emitted by the heating device.

[0013] Compared to prior art systems where the shield is kinematically fixed to the infrared lamp, this embodiment of the invention is able to preheat a wider portion of the workpiece than the only portion to be welded. This allows for smaller in-plane thermal gradients and better control of interface crystallization. The crystallization state at the weld interface is more uniform.

[0014] According to an advantageous embodiment, the upper and lower shields are configured to move in opposite directions. This provides greater flexibility in the path the lamp must take to reach its operating position, thereby reducing the time gap between heating the workpieces and joining them together. As mentioned above, the high thermal conductivity of the material also means that every second that passes between heating and pressing affects the quality of the final product.

[0015] According to an advantageous embodiment, the system also comprises springs interposed between the pushers and the holder of each tool to keep the pushers in their rest position, and actuators to compensate for the force of the springs in order to move the pushers into their operating position.

[0016] According to an advantageous embodiment, the system further comprises heating means for heating the holder and / or the pusher.This also reduces the heat dissipation from the workpiece by conduction and improves the temperature uniformity during and after heating of the workpiece.

[0017] The present invention also relates to a method for infrared welding a first workpiece made of a carbon fiber reinforced thermoplastic polymer (CFRTP) to a second workpiece made of CFRTP, the first workpiece and the second workpiece being substantially flat at least in their respective regions intended to form a weld joint therebetween, the method being performed using a system according to any one of the above-described embodiments, the method comprising: when the first workpiece and the second workpiece are held in a holder in a cantilevered manner, and when a pusher is held away from the first workpiece and the second workpiece, heating corresponding portions of the first workpiece and the second workpiece using an infrared heating device so that the heated portions are surrounded by air; moving the heating device away from the heated portions of the first workpiece and the second workpiece; moving the holder closer to each other; and applying pressure on the first workpiece and the second workpiece using the pusher so that the corresponding heated portions are combined together and form the weld joint.

[0018] According to an advantageous embodiment, the system further comprises an upper shield and a lower shield, which are configured to move in a lateral direction independently of the heating device, and which are configured to move between an operating position and a rest position, wherein in the operating position the shield partially shields the first workpiece or the second workpiece from the radiation emitted by the heating device, and the method further comprises a preheating step before the heating step, during which preheating step the heating device is in its operating position and the shield is in its rest position.

[0019] According to an advantageous embodiment, during the preheating, the first workpiece and the second workpiece are brought to a uniform temperature of approximately 100° C.

[0020] According to an advantageous embodiment, during preheating, the heating device delivers energy sequentially, for example by flashing infrared light.

[0021] According to an advantageous embodiment, after preheating and before heating, the heating device is switched off, but remains in its operating position for a period of time such that the temperature is homogenized throughout the first and second workpieces.

[0022] According to an advantageous embodiment, before the heating step, the shield is brought into its operating position.

[0023] Benefits of the Invention

[0024] Several aspects of the invention ensure, to varying degrees, that high quality (aerospace quality standards) carbon fiber reinforced products can be obtained by infrared welding. The system and method of the invention also allow strong products of various thicknesses (even more than 12 mm thick) to be obtained.

[0025] The system and method of the present invention has been shown to have no weld burrs, so no cleaning step is required. Controlled crystallization means that no crystallization solidification is required after welding. Therefore, the system and method of the present invention results in a faster welding process.

[0026] Using the system and method of the present invention, crystallization occurs from the molten state without the need for cold crystallization via post-solidification.

[0027] Crystallinity of greater than 18% was observed throughout the thickness, whereas the prior art had no crystallization or a maximum of about 2%.

[0028] The system and method of the present invention is compatible with PAEK polymers and is applicable to woven or UD tapes.

[0029] The system and method of the present invention are applicable to existing welding machines.

[0030] Further benefits may become apparent with respect to the discussion of the following figures. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 A method according to the prior art is shown;

[0032] Figure 2 The same sequential steps performed using the system of the present invention are shown;

[0033] Figure 3 A variation with an additional step of preheating is shown;

[0034] Figure 4 Comparison results of the temperature gradient through the thickness of the workpiece are shown;

[0035] Figure 5 A diagram highlighting the effect of pre-heating is shown. DETAILED DESCRIPTION

[0036] The following examples and drawings are given for illustrative purposes only. The present invention is not limited by these examples, but only by the appended claims. The various parts of the system can have various attributes or be embodied in various ways. Unless otherwise explicitly mentioned, each variation of each part of the system can be combined with each variation of any other part of the system.

[0037] The drawings are schematic and not drawn to scale. Some elements of the system are not shown, such as, for example: elements for assembling the various components together (flanges, screws, etc.), elements for properly ensuring the sealing of the various compartments (seals, etc.), or elements for controlling the system (wires, sensors, actuators, valves, safety devices, etc.).

[0038] The above background technology discusses Figure 1 .

[0039] Figure 2 The various steps (a) to (d) of the system and method for infrared welding of the present invention are shown.

[0040] In step (a) of heating, the infrared lamp 6 is positioned in an operating position 6 . 1 between the two tools 2 , 4 holding the workpieces 10 , 12 .

[0041] The workpieces 10, 12 are held in the respective holders 2.1, 4.1 of the tools 2, 4 so that they are held in a cantilevered manner. This results in the workpieces 10, 12 presenting free ends 10.1, 12.1, which correspond to the portions of the workpieces intended to be heated to join the workpieces 10, 12 together. For example, the size of the portions 10.1, 12.1 may be one tenth or one thousandth of the size of the workpieces 10, 12. The workpieces 10, 12 are substantially flat at least in the vicinity of the portions 10.1, 12.1 (a few centimeters). The thickness of the workpieces 10, 12 may be 1 to 20 mm.

[0042] In order to concentrate the heat on the portions 10 . 1 , 12 . 1 , a shield 8 is provided to prevent radiation from the heating device 6 from reaching other areas of the workpieces 10 , 12 .

[0043] In step (b), the heating process continues and the free end portions 10.1, 12.1 of the workpieces 10, 12 begin to melt and decompose.

[0044] During heating, the end portions 10.1, 12.1 are surrounded by air 14. This is made possible by the design of the non-vertical contact end portions 10.1, 12.1 of the tools 2, 4: the tools 2, 4 are made of two kinematically different parts, which are kept away from each other by springs 2.3, 4.3. Therefore, the pushers 2.2, 4.2 are kept away from the workpieces 10, 12 during heating.

[0045] In step (c), the heating devices 6 are moved to their rest position 6.2 to allow one or both of the tools 2, 4 to move vertically to ensure contact between the two workpieces 10, 12, more specifically between their melted portions 10.1, 12.1.

[0046] Sufficient pressure (in excess of the force required to compensate for the force of the springs 2.3, 4.3) may be applied for an appropriate duration to ensure that the weld joint 16 is formed.

[0047] Figure 3 Shown as Figure 2 In addition to the step of preheating (a1), an additional step of preheating (a1) can be foreseen, i.e. Figure 2 before step (a).

[0048] In the preheating step (a1 ), the shield 8 is positioned in its rest position compared to the heating step (a). This enables the heating device 6 to radiate over a wider portion of the workpieces 10 , 12 .

[0049] During preheating, the heating device generates a lower amount of power (or generates the same amount of power for a shorter amount of time, such as sequentially). This ensures that the workpieces 10, 12 slowly reach a uniform temperature below the melting temperature.

[0050] Optionally, after preheating, a homogenization step may be performed, wherein the heating means is switched off for a given duration to allow the heat to be homogenized throughout the workpiece (by internal conduction).

[0051] After preheating (a1) and before heating (a), the shielding element 8 is brought into its operating position (see Figure 2 (a) in Figure 1). They advantageously move in opposite directions.

[0052] In a variant not shown, the holder 2.1, 4.1 and / or the pusher 2.2, 4.2 are heated by suitable heating means (eg resistors) in order to further reduce the temperature gradients within the workpiece.

[0053] Figure 4 is a graph showing the temperature gradient through the thickness of the workpiece 10 as a function of time.

[0054] The top curve shows that within 20 seconds, Figure 1 The temperature gradient of the system shown reaches 50° C. Another comparative example is shown, in which a mica insulating layer is interspersed between the workpiece 10 and the tool 4. The temperature gradient reaches about 20° C. within 10 seconds, but then shows a certain degree of homogenization.

[0055] The bottom curve shows the gradient measured with the system of the invention. The maximum measured gradient is about 10°C, thus two to five times better than with the prior art system.

[0056] This low temperature gradient obtained with the system and method of the present invention makes it possible to speed up the entire process without damaging the material: greater than 20°C.s -1 The heating slope is 7°C.s instead of the prior art system. -1 This results in a reduction in the duration of the heating step from 50 seconds to 20 seconds. The maximum temperature at the outer surface is well controlled at 400°C, instead of 450°C as in the prior art, which requires heating for a longer duration.

[0057] Figure 5 The effect of preheating on the temperature gradient through the thickness of the workpiece is highlighted. The curve is the gradient difference between the gradient obtained without preheating and the gradient obtained with preheating. The curve shows that the temperature gradient is reduced by about 30 to 45°C over most of the thickness of the workpiece. The point of the curve with coordinates (0;0) is understandable because the temperature at the interface between the two workpieces is generated by heating and is a boundary condition common to both methods (with or without preheating).

[0058] Exemplary thin composite materials and Figure 2 The process was used together with the TECHNOLOGY process to demonstrate its ability to provide better results than known technologies: Toray Cetex 1225, 6 layers, 1.86 mm thick, presenting a quasi-isotropic layup: [0° / 45° / 90°]. The carbon fabric was a 5-harness satin made of T300JB carbon filaments. For a heating step with a duration of about 22 seconds and a welding pressure of about 3 bar, the porosity content measured over the entire weld area was less than 0.5%. The tool was heated at 200°C.

[0059] In another example, thick composite materials Figure 2 process and Figure 3Used together with the supplementary preheating step: Toray Cetex 1225, 36 layers, 11.16 mm thick, presenting a quasi-isotropic layup: [0° / 45° / 90°]. The carbon fabric is a 5-harness satin made of T300JB carbon filaments. Preheating is carried out at 50% power during 25 seconds, followed by 10 seconds at 15% power for homogenization. The heating step is carried out at 80% power for 14 seconds. Welding pressure of 3 bar. The porosity content is less than 0.5% measured over the entire weld area. Tools 2, 4 are kept at room temperature.

[0060] Known techniques result in porosity greater than 5%, which means that the final product is weaker and the dimensions of the final product are more chaotic in the area of ​​the weld joint.

[0061] Single lap shear testing showed that the standard deviation of the weld joint was less than 0.7 MPa (compared to more than 4 MPa with known techniques) when performed with the system or method of the present invention.

[0062] The present application is particularly directed to welded joints of two flat surfaces, such as ribs for aircraft wings, but the invention is not limited to this particular example and may be applied to weld beads of various types or geometries.

[0063] Furthermore, those skilled in the art will appreciate from the teachings of this application that the present invention can be practiced with alternative heating devices (other than infrared) and with materials other than carbon reinforced thermoplastics.

Claims

1. A system (1) for infrared welding a first workpiece (10) made of carbon fiber reinforced thermoplastic polymer (CFRTP) to a second workpiece (12) made of CFRTP, the first workpiece (10) and the second workpiece (12) being substantially flat at least in their respective regions intended for forming a weld joint (16) therebetween, the system comprising: - an upper tool (4) for holding the first workpiece (10); - a lower tool (2) for holding the second workpiece (12), wherein the upper tool (4) and / or the lower tool (2) are movable in a vertical direction (z) to change the distance between them; and - a movable infrared heating device (6) configured to move in a transverse direction (x) between an operating position (6.1) and a rest position (6.2), in which the heating device (6) is capable of simultaneously heating respective portions (10.1, 12.1) of the first and second workpieces (10, 12); Wherein, the upper tool (2) and the lower tool (4) each include: - a holder (2.1, 4.1) which holds the first workpiece or the second workpiece (10, 12) in a cantilevered manner, their respective portion to be heated (10.1, 12.1) forming the free end of the cantilevered attachment; and - a pusher (2.2, 4.2) which is movable vertically (z) relative to the holder (2.1, 4.1) between a rest position and an operating position, wherein in the rest position the pusher (2.2, 4.2) is away from the first workpiece or the second workpiece (10, 12) so that the corresponding portion to be heated (10.1, 12.1) of the workpiece (10, 12) can be surrounded by air (14) during heating, and wherein in the operating position the pusher (2.2, 4.2) contacts the heated portion (10.1, 12.1) of the first workpiece or the second workpiece (10, 12) and applies welding pressure in the vertical direction (z).

2. The system (1) according to claim 1, wherein: The system further comprises an upper shield (8) and a lower shield (8), wherein the upper shield (8) and the lower shield (8) are configured to move in a transverse direction (x) independently of the heating device (6), and the upper shield (8) and the lower shield (8) are configured to move between an operating position and a rest position, wherein in the operating position, the shield (8) partially shields the first workpiece (10) or the second workpiece (12) from radiation emitted by the heating device (6).

3. The system (1) according to claim 2, characterized in that The upper shield (8) and the lower shield (8) are configured to move in opposite directions.

4. A system (1) according to any one of claims 1-3, further comprising a spring (2.3, 4.3) inserted between the pusher (2.2, 4.2) and the holder (2.1, 4.1) of each tool (2, 4) to keep the pusher (2.2, 4.2) in its rest position, and the system comprises an actuator that compensates for the force of the spring (2.3, 4.3) to move the pusher (2.2, 4.2) to its operating position.

5. The system (1) according to any one of claims 1 to 4, further comprising a heating device for heating the holder (2.1, 4.1) and / or the pusher (2.2, 4.2).

6. A method for infrared welding a first workpiece (10) made of carbon fiber reinforced thermoplastic polymer (CFRTP) to a second workpiece (12) made of CFRTP, the first workpiece (10) and the second workpiece (12) being substantially flat at least in their respective regions intended for forming a weld joint (16) therebetween, the method being performed using a system (1) according to any one of claims 1 to 5, the method comprising: - when the first workpiece (10) and the second workpiece (12) are held in the holder (2.1, 4.1) in a cantilevered manner and when the pusher (2.2, 4.2) is kept away from the first workpiece (10) and the second workpiece (12), heating (a, b) corresponding parts (10.1, 12.1) of the first workpiece (10) and the second workpiece (12) by means of the infrared heating device (6) so that the heated parts (10.1, 12.1) are surrounded by air (14); - moving (c) the heating device (6) away from the heated parts (10.1, 12.1) of the first workpiece (10) and the second workpiece (12); - moving (c) said holders (2.1, 4.1) closer to each other; and - applying pressure (d) to the first workpiece (10) and the second workpiece (12) by means of the pusher (2.2, 4.2) so that the corresponding heated portions (10.1, 12.1) are joined together and form a welded joint (16).

7. The method according to claim 6, wherein: The system (1) further comprises an upper shield (8) and a lower shield (8), wherein the upper shield (8) and the lower shield (8) are configured to move independently of the heating device (6) in a transverse direction (x), and the upper shield (8) and the lower shield (8) are configured to move between an operating position and a rest position, wherein in the operating position, the shield (8) partially shields the first workpiece (10) or the second workpiece (12) from radiation emitted by the heating device (6), and the method further comprises a preheating step (a1) prior to the heating step, during which the heating device (6) is in its operating position (6.1) and the shield (8) is in its rest position.

8. The method according to claim 7, characterized in that During the preheating (a1), the first workpiece (10) and the second workpiece (12) are brought to a uniform temperature of about 100°C.

9. The method according to claim 7 or 8, characterized in that: During preheating (a1), the heating device (6) delivers energy sequentially, for example by flashing infrared light.

10. The method according to any one of claims 7 to 9, characterized in that After preheating (a1) and before heating (a), the heating device (6) is switched off but maintained in its operating position for a duration that allows the temperature of the entire first workpiece (10) and the second workpiece (12) to be homogenized.

11. The method according to any one of claims 6 to 9, characterized in that Prior to said heating step (a), said shielding member (8) is brought into its operating position.

12. The method according to any one of claims 6 to 9, characterized in that: The retainer (2.1, 4.1) and / or the pusher (2.2, 4.2) are heated during at least one of the following steps: preheating (a1), heating (a), moving (c) the heating device (6) away, moving (c) the retainers (2.1, 4.1) closer to each other and / or applying pressure (d).

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