Ultrasonic treatment method

Abstract

The invention relates to a method for ultrasonic treatment of plastic material, comprising the steps of placing the plastic material between a sonotrode and a support tool, bringing the sonotrode excited by means of ultrasonic oscillations into contact with the plastic material during a welding phase, moving the sonotrode and the support tool towards each other during a holding phase, wherein neither the sonotrode nor the support tool is excited by ultrasonic oscillations, so that a force F is exerted on the plastic material. In order to reduce the processing time, wherein at the same time the quality of the welding process is not affected and is even to be improved, it is proposed according to the invention to vary the force F during the holding phase.

Description

[0001] The present invention relates to a method for ultrasonic treatment of plastic materials, comprising the following steps:

[0002] A) Place the plastic material between the ultrasonic generator and the counter tool,

[0003] B) during the welding phase, bringing the ultrasonically oscillating sonotrode into contact with the plastic material, and

[0004] C) During the holding phase, the sonotrode and the support table tool are moved towards each other, wherein neither the sonotrode nor the support table tool is excited by the ultrasonic oscillations, thereby applying a force F to the plastic material.

[0005] This type of ultrasonic treatment method is generally known. The method produces a material connection between plastic material parts. The plastic material parts are located between a sonotrode and a support table tool. The sonotrode is excited with ultrasonic oscillations, so that longitudinal oscillations with a frequency of, for example, 20 kHz and an amplitude of, for example, 5 μm to 50 μm are introduced into the plastic material parts by the sonotrode. Depending on the application scenario, heat is generated due to friction between the sonotrode and the plastic material or between the contacting plastic material parts and within the molecular chains, which leads to melting of the material.

[0006] Ultrasonic welding is used, inter alia, to join injection-molded parts, to rivet different materials together and to produce form-locking connections by flanging.

[0007] During the welding phase, the sonotrode and possibly also the support table tool are excited with ultrasonic oscillations, the sonotrode and the support table tool are usually moved towards each other, while the plastic material to be processed is placed between the support table tool and the sonotrode.

[0008] Thus, during the welding phase, ultrasonic oscillations are applied to the material by means of the sonotrode and possibly also the support table tool, while these two elements (sonotrode and support table tool) are moved towards each other.

[0009] During the welding phase, the force with which the sonotrode presses on the plastic material and / or the path the sonotrode travels in the direction of the support table tool is controlled. By adjusting and varying process parameters such as the force applied to the material, the advancement speed of the sonotrode or the material temperature, the process and quality of the welded joint can be influenced.

[0010] At the end of the welding phase, the ultrasonic excitation is switched off. The material melted by the ultrasonic treatment then returns to a solid state.

[0011] It is known that the welding phase is followed by a holding phase in which a sonotrode presses the material in the direction of the support table tool with a constant force F. This prevents the material parts from separating from each other again before the material has resolidified.

[0012] For example, US 2018 / 304543 A1 describes a method for mechanically fixing a first component comprising a thermoplastic material to a second component, wherein the thermoplastic material is melted by introducing vibrations and the associated generation of frictional heat, thereby creating a bond with the second component when the thermoplastic material resolidifies during a cooling phase after melting. Similar methods are also known from EP 0 301 831 A2, EP 2 990 182 A2, EP 0 421 019 A1, US 3 022 814 A and US 4 904 319 A.

[0013] However, providing a holding phase prolongs the processing time that the sonotrode and the support table tool are in contact with the material. Therefore, during the holding phase, the sonotrode and the support table tool cannot be used to weld more material parts.

[0014] In ultrasonic processing, the goal is always to reduce the processing time while not affecting or even improving the quality of the welding process.

[0015] According to the invention, this object is achieved by varying the force F during the holding phase in step C) and by controlling or regulating the movement of the sonotrode and the support tool towards each other in step C). It has been shown that the holding phase is not only necessary for the purpose of waiting for the material to solidify, but also that during the holding phase, when ultrasonic oscillations are no longer introduced into the material, it is possible to influence the solidification process and therefore the weld joint. Varying the force F allows acting on the liquid component in a targeted manner and controlling the solidification process.

[0016] In most cases, it is advantageous to increase the force F during the holding phase. The force applied to the material supports the transition from liquid to solid phase due to the higher pressure in the material. The relaxation of the polymer chains also supports entanglement, which produces good connections.

[0017] However, as the material solidifies, the material provides a higher counter pressure. Increasing the applied force F during the solidification process ensures that even with increased solidification, sufficient force is provided on the stationary liquid component of the material to accelerate and improve solidification of these areas.

[0018] The present invention is the first to focus on the holding phase. Surprisingly, changing, in particular increasing, the forces in the holding phase significantly influences the solidification process and also the quality of the welded joint.

[0019] In a preferred embodiment of the invention, step C) comprises adjusting the sonotrode and the support table tool to move towards each other.

[0020] For example, the regulation can be performed by measuring the actual pressure at which the sonotrode and the support table tool move toward each other, comparing the actual pressure with a target pressure, and varying the movement toward each other according to the difference between the actual pressure and the target pressure. The actual pressure can be a hydraulic or pneumatic pressure of a propulsion device that moves the sonotrode and the support table tool toward each other.

[0021] Alternatively, regulation may be performed by measuring the actual force F, comparing it to a target force, and varying the movement towards each other depending on the difference between the actual force and the target force.

[0022] For example, the target force may be a time-dependent function that increases with time.

[0023] The force may also be temporarily selected to decrease again. However, the force F at time t2 after time t1 must be greater than the force F at time t1. In a preferred embodiment, a monotonically increasing function is selected as target force; ie during the holding phase, the force F does not decrease.

[0024] However, it is important to note that after the holding phase, the sonotrode must be moved away from the support table tool again in order to remove the welded material part and place the next material part to be processed between the sonotrode and the support table tool. This opening phase begins with a sudden reduction in the force F, since the direction of movement of the sonotrode is reversed. The opening phase is not part of the waiting phase, but follows it.

[0025] In another preferred embodiment, it is proposed to adjust the movement of the ultrasonic generator and the support table tool toward each other by measuring the actual distance d of the ultrasonic generator or the support table tool, comparing the actual distance with the target distance, and changing the movement toward each other according to the difference between the actual distance and the target distance.

[0026] The actual distance can be measured directly or indirectly, for example by measuring the position of the sonotrode, from which the distance to the fixed support table tool can be derived. Calculating this distance is not necessary. Since the actual distance d differs from the position of the sonotrode only by a minus factor, it is sufficient to measure the position of the sonotrode and compare it with the target position of the sonotrode.

[0027] For example, the target distance may be a function that decreases monotonically over time.

[0028] In a further particularly preferred embodiment, a function is selected as the target distance, the derivative of which is constant over time.

[0029] This ensures that the distance decreases at a constant rate.

[0030] By varying the force with which the sonotrode and the support table tool press the material to be processed, the movement towards each other can be varied according to the difference between the actual distance and the target distance.

[0031] Further advantages, features and possible applications of the present invention will become apparent from the following description of preferred embodiments and the associated drawings. The drawings show:

[0032] Figure 1 is a flow chart of a method according to the present invention.

[0033] The ultrasonic treatment method according to the invention comprises two phases, a welding phase 1 and a holding phase 2. During the welding phase, the material to be treated comes into contact with an ultrasonic generator oscillating at a high frequency.

[0034] In particular, when the material consists of different material components, high-frequency friction effects occur at the contact surfaces between the material components, which lead to local material heating and thus melting of the polymer material. Once sufficient melting has occurred, the welding phase ends; that is, the ultrasonic excitation of the sonotrode and possibly also the support table tool ends.

[0035] In principle, the workpiece can now be removed from the ultrasonic welding system. However, the polymer material still exhibits a high degree of fluidity in the molten part, so that the material parts could potentially separate again. It is therefore necessary to wait until the polymer melt solidifies again. In this so-called holding phase 2, the sonotrode applies force to the material to be processed.

[0036] During the holding phase, the liquid component of the polymer material solidifies. However, this does not happen uniformly on all liquid components, but gradually in each part. In the holding phase 2, the solidification process can be accelerated by steadily increasing the force applied to the material by the ultrasonic generator.

[0037] As the liquid portion solidifies, the resistance offered by the material to the sonotrode increases. Therefore, in order to further accelerate the solidification process and bring about better entanglement within the polymer chains, according to the invention, the force is not kept constant during the holding phase, but is varied and particularly preferably increased.

[0038] In a particularly preferred embodiment, this is achieved by measuring the relative position between the sonotrode and the support table tool and reducing the distance between the sonotrode and the support table tool at a constant rate. At the end of the holding phase, substantially all of the polymer areas liquefied by the ultrasonic treatment have resolidified, so that the sonotrode and anvil can be moved away from each other again to remove material and place the next piece of material to be treated between the sonotrode on one side and the support table tool on the other side.

[0039] Reference numerals

[0040] 1 Welding stage

[0041] 2. Maintaining phase

Claims

1. A method for ultrasonic treatment of plastic materials, comprising the following steps: A) placing the plastic material between an ultrasonic generator and a support table tool, B) during the welding phase, bringing the sonotrode excited by ultrasonic oscillations into contact with the plastic material, C) during a holding phase, moving the sonotrode and the support table tool towards each other, wherein neither the sonotrode nor the support table tool is excited by ultrasonic oscillations, thereby applying a force F to the plastic material, wherein in step C), the force F varies during the holding phase, Characterized in that in step C), the movement of the sonotrode and the support table tool towards each other is controlled or regulated.

2. The method according to claim 1, characterized in that In step C) the force F is increased during the holding phase.

3. The method according to claim 1, characterized in that In step C), the movement of the ultrasonic generator and the support table tool toward each other is regulated by measuring the actual pressure at which the ultrasonic generator and the support table tool move toward each other, comparing the actual pressure with a target pressure, and changing the movement toward each other according to the difference between the actual pressure and the target pressure.

4. The method according to claim 1, characterized in that In step C), the movement of the sonotrode and the support table tool towards each other is regulated by measuring an actual force, comparing the actual force with a target force, and varying the movement towards each other according to the difference between the actual force and the target force.

5. The method according to claim 1, characterized in that In step C), the movement of the sonotrode and the support table tool towards each other is adjusted by measuring the actual distance of the sonotrode or the support table tool, comparing the actual distance with a target distance, and changing the movement towards each other according to the difference between the actual distance and the target distance.

6. The method according to claim 5, characterized in that A function that decreases monotonically over time is selected as the target distance.

7. The method according to claim 6, characterized in that A function whose derivative is constant over time is selected as the target distance.

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