Method, device for heating fiber-reinforced semi-finished products with different wall thicknesses and a heating arrangement with such a device

Abstract

Method for heating fiber-reinforced semi-finished products (1) of different wall thicknesses to a required temperature above the glass transition zone or the matrix melting temperature of a plastic matrix of the semi-finished product (1) to be heated, wherein in a first step the semi-finished product (1) to be heated is heated by means of heat conduction of a device to below the glass transition zone or the matrix melting temperature, wherein the device has a flexible heat conduction layer (2), wherein a first, preferably upper, and a second, preferably lower, tool half (3, 4) are provided, between which the semi-finished product (1) to be heated can be arranged, wherein the first and the second tool half (3, 4) each have a heatable base plate (5, 6), and wherein at least one, preferably both tool halves (3, 4), has a flexible heat conduction layer (2);and wherein, in a further step, the remaining amount of heat required to reach the temperature above the glass transition zone or the matrix melting temperature is introduced by means of thermal radiation or thermal convection.

Description

The invention relates to a method, a device for heating fiber-reinforced semi-finished products with different wall thicknesses, and a heating arrangement with such a device. To achieve lightweight construction, fiber-reinforced plastics are increasingly being used. Back-injection molding or the functionalization of unidirectionally fiber-reinforced plastics (tapes) or organosheets is of great importance in this context. For example, in the tape laying process, various tapes of different lengths, widths, and orientations are laid down and fixed together. The mechanical properties of this tape layup are determined by the individual fiber orientations of the tapes. Thus, the tape laying process can be used to create a semi-finished product that subsequently meets the required specifications. This load-path-oriented design typically results in semi-finished products with varying thickness profiles. To form such a semi-finished product, it must be heated to a temperature above the glass transition temperature of the thermoplastic polymer matrix of the tapes. The use of electromagnetic radiation, particularly infrared radiation, for heating thermoplastic semi-finished products and films is state of the art. Electromagnetic radiation is emitted from at least one radiation source. This radiation is absorbed by the semi-finished product being heated. Absorption primarily occurs near the penetrating surface of the electromagnetic radiation. The penetration depth and extent of absorption depend on the chemical structure of the semi-finished product. After absorption of the thermal radiation, the heat energy is transported and distributed within the semi-finished product via thermal conduction.A uniform distribution of heat within a semi-finished product is essential to achieve a consistent temperature. Since plastics are generally poor conductors of heat and the absorbed radiant energy decreases with increasing distance from the emitting radiation source, maintaining a constant distance between the radiation source and the absorbing semi-finished product is crucial for homogeneous heating. If a semi-finished product has multiple wall thicknesses, homogeneous heating using infrared radiation is extremely difficult. On the one hand, the distance to the radiation source must remain constant, and on the other hand, areas with greater / lesser wall thicknesses require a higher / lower amount of heat. To enable homogeneous heating, each wall thickness of the semi-finished product would need its own temperature control zone and a precisely contoured radiation medium. These requirements are generally not feasible. Firstly, the radiation medium cannot be precisely fitted to the contour. Secondly, this would result in numerous temperature control zones, all of which would influence each other. Consequently, a stable, repeatable control process is impossible. To enable the heating of semi-finished products with varying wall thicknesses in practice, despite these challenges, temperature control is achieved by monitoring the temperature at the critical point(s) (those areas where overheating is imminent). This means that the entire control of heating time and power is based on the thinner areas, which reach the set temperature more quickly than the thicker areas. This increases the overall cycle time because, once the thinner area reaches the set temperature, the thicker areas only receive the heating power necessary to protect them from overheating. Therefore, the control system treats the semi-finished product as if it had no wall thickness variations, resulting in an inhomogeneous temperature distribution across areas with different wall thicknesses. DE 10 2009 024 789 A1 and US 2018 / 0 104 866 A1 each describe devices and methods of the same type. The object of the invention is to provide a method, a device and a heating device which, compared to the prior art, enable geometry- and wall-thickness-independent heating of a semi-finished product in order to achieve a significantly better temperature distribution and a reduction in the overall cycle time. This problem is solved by a method having the features of claim 1, a device having the features of claim 6, and a heating device having the features of claim 11. The invention employs various heat transfer mechanisms. The first step involves heat input via conduction (contact heating), the second step involves heat input via thermal radiation (preferably infrared radiation) or convection. To form a thermoplastic semi-finished product (a semi-finished product with a thermoplastic matrix), sufficient heat energy must be supplied to reach at least the glass transition temperature for amorphous plastics or at least the matrix melting point for semi-crystalline plastics. To enable faster and more homogeneous heating, the heating process is divided into at least two steps, preferably carried out in two separate devices, which can form heating stations of a heating system. In the first step, the semi-finished product to be heated is placed in a device according to the invention and heated by heat conduction until it is below the glass transition temperature or the matrix melting temperature. To achieve the most homogeneous temperature distribution possible within the semi-finished product, all thickness variations present in the product must also be taken into account in the device. This ensures uniform heat input and results in a homogeneous temperature distribution within the semi-finished product. This enables a flexible heat conduction layer that is able to adapt to the surface of the semi-finished product. In a further step, the material is transferred to a device where the remaining heat required to reach the temperature above the glass transition zone or the matrix melting temperature is introduced by means of thermal radiation. This transfer must, of course, occur before the semi-finished product has cooled significantly. Since a large portion of the heat required for formability has already been introduced in the preceding process step, a semi-finished product with varying wall thicknesses can be considered flat when heat is introduced by thermal radiation or convection. Because the largest amount of heat is introduced in the first step, an exceptionally homogeneous temperature is established across the cross-section or surface. This results in a more favorable temperature distribution in the subsequent step of heat introduction by thermal radiation or convection.Heat convection offers the advantage that only a small amount of heat needs to be introduced, thus maintaining temperature homogeneity. Furthermore, by dividing the process into a multi-stage procedure, a significantly shorter heating time is possible. If the semi-finished product is a matrix material made of an engineering plastic, it is preferably heated to a temperature up to 50 °C below the glass transition temperature or the matrix melting temperature, and particularly preferably to a temperature up to 30 °C below the glass transition temperature or the matrix melting temperature. If the semi-finished product is a high-temperature plastic, the temperature may also be more than 50 °C below the glass transition temperature or the matrix melting temperature. The device according to the invention consists of a first and second, preferably upper and lower, tool half, each of which has a heatable base plate and, individually or jointly, a flexible heat-conducting layer in the form of, for example, a flexible tool insert. For optimal heat transfer between the insert and the semi-finished product, pressure can be exerted on the semi-finished product via the two tool halves. The base plate and counter plate are preferably arranged to be axially movable relative to each other, so that an opening and closing movement can be performed and a force can be applied to a semi-finished product placed on the base plate. In a preferred embodiment of the invention, the base plate is designed to be immovable, while the counter plate is designed to be axially movable. In an alternative embodiment, the counter plate is designed to be immovable, while the base plate is designed to be axially movable. Alternatively, both plates can be designed to be movable relative to each other. The base plate, which is heated and transports the heat to the flexible insert via thermal conduction, has a particularly high thermal conductivity. The flexible heat conduction layer consists of a material that can compensate for wall thickness differences of a few millimeters, preferably less than 5 millimeters, and particularly preferably less than 3 millimeters. Elastomers, and especially silicones, are preferably used as materials. To increase the thermal conductivity of the flexible heat-conducting layer, it can be filled with fillers. Depending on the application, the flexible heat-conducting layer can have a compact volume or an open- or closed-pore structure. Alternative designs can be implemented using a fluid, a fine material (aluminum powder, glass dust, etc.) encapsulated by a membrane, or another flexible and thermally conductive material. To optimally utilize the installation space, in an embodiment of a heating device according to the invention, the device for introducing the heat quantity by means of heat conduction (first heating station) is preferably located above the device for introducing the heat quantity by means of heat radiation or heat convection (second heating station). A method for heating a semi-finished product with a heating device described above is carried out, for example, as follows: The semi-finished product to be heated is placed on the flexible / moldable, thermally conductive tool insert. The counter plate and base plate are then moved relative to each other, thus closing the device. The closing movement can be achieved by moving the counter plate, the base plate, or both plates. Preferably, a defined pressure is exerted on the semi-finished product to be heated during the closing movement. This pressure serves to fix the semi-finished product, adapt the flexible insert to the contour, and further improve heat transfer. Since the plates are preferably heated continuously, the heating process begins as soon as the plate touches the semi-finished product. The heating process is carried out for a specific period of time, determined by the required heating time for the thickest part of the semi-finished product. The heating time can be selected, for example, so that the temperature distribution across the thickness profile of the semi-finished product varies by less than 10 °C, preferably less than 5 °C. The controlled temperature depends on the matrix material of the plastic matrix of the semi-finished product to be heated and is below the glass transition range or the matrix melting point of the plastic. After the heating period has elapsed, the preheated, still warm semi-finished product is placed in the second heating station, where heat is introduced via thermal radiation or convection. In this station, the semi-finished product is heated to a temperature above the glass transition temperature or melting point. Once the target temperature is reached, the temperature is maintained for a certain period to ensure that the entire cross-section is heated uniformly. This period is adjustable and depends on the thickness of the semi-finished product. A heating device according to the invention is shown in Fig. 1 and Fig. 2. The heating device 7 comprises a first heating station 8 and a second heating station 9. The second heating station 9 serves to introduce heat by means of thermal radiation or thermal convection. It has two plates 10 between which the semi-finished product 1 can be placed after preheating in the first heating station and does not require further description as it corresponds to the prior art. The plates 10 can have known heating elements. The first heating station 8 is shown in an open position in Fig. 2a and in a closed position in Fig. 2b. The first heating station 8 has a first (upper) tool half 3 and a second (lower) tool half 4. These each consist of a heatable base plate 5, 6 and a flexible heat-conducting layer 2 arranged thereon. The devices required for heating the base plates 5, 6 are not shown, as they correspond to the prior art. A comparison of Figures 2a and 2b shows how the flexible heat-conducting layer 2 adapts to differences in the wall thickness of the semi-finished product 1 to be heated. Instead of providing a flexible heat-conducting layer 2 on both tool halves 3, 4 as shown here, providing only one flexible heat-conducting layer 2 on one of the two tool halves 3, 4 could also suffice. The flexible heat-conducting layer(s) 2 would not necessarily have to be formed in one piece, as depicted. Reference symbol list: 1 Semi-finished product 2 Flexible heat conduction layer 3 First tool half 4 Second tool half 5 Base plate of the first tool half 6 Base plate of the second tool half 7 Heating device 8 First heating station 9 Second heating station 10 Plates of the second heating station

Claims

Method for heating fiber-reinforced semi-finished products (1) of different wall thicknesses to a required temperature above the glass transition zone or the matrix melting temperature of a plastic matrix of the semi-finished product (1) to be heated, wherein in a first step the semi-finished product (1) to be heated is heated by means of heat conduction of a device to below the glass transition zone or the matrix melting temperature, wherein the device has a flexible heat conduction layer (2), wherein a first, preferably upper, and a second, preferably lower, tool half (3, 4) are provided, between which the semi-finished product (1) to be heated can be arranged, wherein the first and the second tool half (3, 4) each have a heatable base plate (5, 6), and wherein at least one, preferably both tool halves (3, 4), has a flexible heat conduction layer (2);and wherein, in a further step, the remaining amount of heat required to reach the temperature above the glass transition zone or the matrix melting temperature is introduced by means of thermal radiation or thermal convection. Method according to claim 1, wherein in the first step the semi-finished product (1) to be heated is heated by means of heat conduction to a temperature of up to 50 °C below the glass transition range or the matrix melting temperature, particularly preferably to a temperature which is up to 30 °C below the glass transition range or the matrix melting temperature. Method according to at least one of the preceding claims, wherein in the first step pressure is exerted on the semi-finished product (1) to be heated. Method according to at least one of the preceding claims, wherein in the first step the heating process is carried out for a time which is determined by the required heating time for the thickest area of ​​the semi-finished product (1) to be heated. Method according to at least one of the preceding claims, wherein in a further step, after reaching the required temperature, this temperature is maintained for an adjustable time to ensure uniform heating of the semi-finished product (1). Device for heating fiber-reinforced semi-finished products (1) of different wall thicknesses, wherein the device has a flexible heat conduction layer (2) which is adaptable to wall thickness differences of the semi-finished product (1) to be heated and through which heat energy can be transferred to the semi-finished product (1) to be heated by means of heat conduction, wherein a first, preferably upper, and a second, preferably lower, tool half (3, 4) are provided, between which the semi-finished product (1) to be heated can be arranged, and wherein the first and the second tool half (3, 4) each have a heatable base plate (5, 6) and at least one, preferably both tool halves (3, 4), has a flexible heat conduction layer (2). Device according to claim 6, wherein a pressing device is provided for pressing the flexible heat conduction layer (2) onto the semi-finished product (1) to be heated, so that the flexible heat conduction layer (2) adheres to the surface of the semi-finished product (1) to be heated, which has wall thickness differences. Device according to claim 6, wherein the pressing device comprises the first and a second tool half (3, 4) which are movable relative to each other for pressing. Device according to one of claims 6 to 8, wherein the flexible heat conduction layer (2) is designed as a vacuum mat and, after the flexible heat conduction layer (2) has been applied to the surface of the semi-finished product (1) having wall thickness differences, can be fixed in its shape by applying a vacuum. Device according to one of claims 6 to 9, wherein the flexible heat conduction layer (2) consists of an elastomer, particularly preferably a silicone, wherein it is preferably provided that the flexible heat conduction layer (2) has fillers to increase the thermal conductivity. Heating device (7) comprising a first heating station (8) in the form of a device according to one of claims 6 to 10 and a second heating station (9) in which the semi-finished product preheated in the first heating station can be heated by means of heat radiation or heat convection.

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