A production method for producing molded articles from fiber composites, wherein a strip-shaped starting material comprising reinforcing fibers is continuously pushed or pulled forward, is known by the designation pultrusion and/or extrusion. In the case of this prior art, the starting material is typically prepared from dry reinforcing fibers, which are passed through a resin bath in order to impregnate them with resin. The starting material prepared in this way is then fed to a mold, in which it is molded and cured with respect to its resin component as it passes continuously through. The finished molded article leaves the mold in the form of an endless profile, which is then divided up into portions of the desired length. The forward-pulling force on the starting material is in this case applied in the region of the endless profile by a drawing tool. A variant of pultrusion is based on semifinished products known as prepregs as the starting material, which comprise reinforcing fibers and resin which, as it passes through, is at least preheated by a heating device before the prepregs enter the mold. Further heating of the resin for curing the same may take place in the mold. In the case of pultruders that are currently in use, the starting material is heated upstream of the mold by means of infrared radiation, a naked flame, hot air or hot inert gas. The heating in the mold takes place by way of the mold, which is for its art heated by heating cartridges, water or induction coils.
JP 63035334 A discloses heating the starting material in a forward region of a mold of a pultruder, in that a shaping inner core of the mold is heated by induction by means of an outer magnetic coil. In addition, heating of the rear part of the mold takes place by an electric heater. Therefore, the starting material is ultimately heated from the inside and the outside by contact with the mold.
The known methods of producing molded articles from fiber composites, wherein a strip-shaped starting material comprising reinforcing fibers and a thermoset or thermoplastic resin is continuously pushed or pulled forward, also include tape laying and tape winding. Basically the same techniques as those described above for heating upstream of the mold in the case of pultrusion have so far been used for heating the starting material. In addition, the starting material may be “ironed” onto the part of the molded article onto which it is laid or wound in this production method, being pressed against it with a hot tool.
All the methods of heating the starting material in the production of molded articles from fiber composites on the basis of strip-shaped starting materials described above entail the disadvantage that they do not allow quick, and nevertheless controlled, heating of the starting material to elevated temperatures, as are required for example for high-quality thermoplastic resins, such as PEEK, to form molded articles from these resins at a cost-effectively high rate. All these techniques require very high contact temperatures for heating at a high heating rate, and such temperatures directly entail the risk of damaging the starting material.
In a production method known from U.S. Pat. No. 3,960,629 an alternating magnetic field in the range from 60 Hz to 5 MHz, in particular from 1 kHz to 2 MHz, is coupled into the starting material, in which electrically conducting reinforcing fibers are present. The alternating magnetic field induces eddy currents in the electrically conducting reinforcing fibers, the electrical resistance of the reinforcing fibers having the effect that these currents lead to induction heating of the reinforcing fibers, and consequently of the starting material comprising the reinforcing fibers. However, it is found that the known production method requires very high frequencies of the alternating magnetic field in order to introduce the latter effectively into electrically conducting reinforcing fibers of a small diameter, unless eddy currents can be induced over a number of reinforcing fibers that are in electrical contact with one another. However, such eddy currents over a number of reinforcing fibers are scarcely realistic in the case of unidirectionally arranged reinforcing fibers such as those that are frequently used in the case of a pultrusion process. High frequencies of the alternating magnetic field, however, result in low depths of penetration of the alternating field into the starting material, so that it becomes difficult to heat the latter in volume. The production method known from DE 26 03 540 stipulates as a basic requirement that selection of the reinforcing fibers should take their electrical conduction properties into consideration, which greatly restricts the degrees of freedom in the selection of the reinforcing fibers in terms of achieving the otherwise desired properties of the molded articles produced.
It is known from U.S. Pat. No. 4,871,412 to bond thermoplastic layers that are reinforced by conducting fibers, such as for example carbon fibers, by introducing an alternating magnetic field into the fibers in order to induce electric currents in them. The electric currents are converted by the resistance of the conducting fibers into heat, which transfers itself to a resin component of the thermoplastic layers. The induction of the electric currents takes place only at a relatively high frequency of the alternating magnetic field above 1 MHz, and it requires that the unidirectionally running conducting fibers of a number of layers run in different directions, in order that eddy currents can flow over the fibers. However, in the case of the starting materials of the methods of the type described at the beginning, the reinforcing fibers are generally only present with unidirectional orientation in the direction of advancement of the starting material in a single layer. Moreover, heating of the resin over the reinforcing fibers likewise entails the disadvantage that the entire heat that is to be supplied to the resin must be transferred to the resin from outside, i.e. here from the conducting fibers. This limits the possible heating-up rate that is possible under controlled, non-damaging conditions.
A semifinished product known from U.S. Pat. No. 5,248,864 comprises electrically conducting reinforcing fibers, which however are specifically not intended to become coupled to an alternating magnetic field that is introduced. Rather, magnetic particles intended to serve as a preferential heating material when the alternating magnetic field is introduced are added to the resin of the semifinished product for this purpose. The magnetic particles may consist of a magnetic oxide. The magnetic particles are formed in such a way that the Curie point of the magnetic particles is not reached by the time the melting temperature or the curing temperature of the resin is reached, because it is assumed that no further significant heating occurs above the Curie point as a result of the alternating magnetic field introduced. The resin of the known semifinished product may be polyether ketone ketone (PEKK) or polyether ether ketone (PEEK), that is to say known high-temperature thermoplastics. Production methods in which a strip-shaped starting material comprising reinforcing fibers and resin is continuously pushed or pulled forward, or a corresponding strip-shaped starting material, are not mentioned here, or considered in any other way.
It is known in principle from U.S. Pat. No. 7,147,742 to add nanoscalar particles with ferromagnetic, ferrimagnetic, superparamagnetic or piezoelectric properties to an adhesive composition, so that, when an alternating magnetic field is introduced, the adhesive composition is heated in such a way that, in the case of thermoplastic adhesives, the softening point of the thermoplastic binder is reached or exceeded and, in the case of reactive adhesives, a temperature that brings about crosslinking of the binder matrix by way of the reactive groups of the binder is reached. The superparamagnetic particles are so-called “single-domain particles”. In comparison with paramagnetic particles, these are distinguished by the fact that they have no hysteresis. This has the result that energy dissipation of the alternating magnetic field introduced is not induced by magnetic hysteresis losses, but instead the heat generation is attributable to an oscillation or rotation of the particles in the surrounding matrix that is induced during the time in which the alternating field is acting, and is consequently ultimately attributable to mechanical friction losses. This leads to a particularly effective heating rate of the particles and of the matrix surrounding them. U.S. Pat. No. 7,147,742 is not concerned with the production of molded articles from fiber composites, in particular not concerned with production methods in which a strip-shaped starting material comprising reinforcing fibers and resin is continuously pushed or pulled forward.
There still is a need of a production method for producing molded articles from fiber composites and of a semifinished product, that allow for a high production rate even when resins which must be heated to a high temperature are used, without involving the risk of thermal damage to the reinforcing fibers of the starting material.