The present invention relates to uniform and/or controlled heating process of a workpieces, tools or surfaces.
Today there are two alternative techniques in order to achieve a rapid heating of a tool surface, inductive heating and resistive heating. Both these methods are based on only to heat the outermost layer of the tool which then consists of a suitable metal or composite. The rest of the tool, if supporting structures are necessary, may consist of another material. The rapidity of the heating is achieved by substantially reducing the heated volume of the tool. In order for the technique to be industrially usable it is however required that the heating is conducted with an even or controlled temperature of the entire surface, which today, with known technique, is not possible.
Induction heating is characterized by that energy is transmitted without any contact to the workpiece by means of a high frequency electrical current driven by a coil which in turn gives rise to a magnetic field which induces currents in the workpiece. The coil is often surrounded by some type of soft magnetic core, e.g. iron powder composite (SMC) or ferrites, in order to increase the efficiency and focus the effect where the heating is desired. The result is a system where rapid and efficient heating may be accomplished, but which could involve great difficulties to heat evenly over larger areas. There are a number of solutions to even out the heating profile all with its own disadvantages and limitations, e.g. to heat thick workpieces in such a way that the heat conduction in the material becomes significant, to continuously move the workpiece with respect to the heating coil, or to use coils with a very low efficiency and have strict requirements on the distance between the coil and the workpiece. Typical heating patterns which may be achieved by traditional induction heating are shown in
Resistive Heating in General
Resistive heating works in a way that a large current is driven through the workpiece which becomes hot, in the same way as a filament in a filament lamp. Depending on the choices of material and geometry it takes very large currents which can lead to problems with electrical contacts where local overheating easily can occur. In order to achieve a uniform heating it requires both good electrical contact along two edges or lines and constant area for the current along the entire distance between the two connection lines, see
There are known and established techniques to evenly heat pipes or cylindrical objects with surrounding coils, see for example U.S. Pat. No. 5,059,762. This document discloses a number of parallel and cylindrical coils which are arranged and driven in such a way that even and controllable heating may be achieved. Its disadvantages are however that the heated object must have a closed shape, i.e. a rod, pipe or profile. To heat a non-closed shape such as a flat or curved surface, or precision heating only one part of a workpiece is not possible with this technique since all currents have to start and end up in the same point. Its surrounding coils also have to be arranged in direct connection with the workpiece in order to work satisfying.
Conventional technique is based on heating large tool quantities (with hot oil, immersion heaters, etc.) where a heat conduction has to occur before an even temperature can be achieved which requires a lot of time and energy and is therefor expensive.
The development of the present invention corresponds to a general need of rapid, efficient, and controlled heating of e.g. surfaces for industrial applications. An example of a surface may be a tool surface for pressing polymer materials where there is a need for an even heating over the entire surface and also the possibility to rapid heating and cooling. An object of the present invention is to provide improvements over prior art. This object is achieved by a technique defined in the appended independent claims; certain embodiments being set forth in the related dependent claims.
In a first aspect of the present invention there is provided an apparatus for controllable heating, comprising at least one coil system with at least one coil unit connected to a power source, where the coil unit is arranged to create a magnetic field. The apparatus further comprises at least one electric current conductor which is arranged at least partly around said coil unit, and at least one element which is configured to be heated and which is connected to the electric current conductor in such a way that the electric current conductor and the element form a closed conduit. The magnetic field of the coil unit is arranged to induce a voltage in the electric current conductor and the element, where the induced voltage creates an electric current in the closed conduit, and where the element is configured to be heated by the electric current. The above described apparatus creates a controllable and uniform heating process for plane surfaces, curved and double curved surfaces, bodies of any kind or any other object with a simple or complex shape and size. This configuration allows for a fast and precise geometrically controllable heating of the element over the entire area of interest.
In an embodiment of the invention the element is a detachable element configured to be removed from the apparatus after a heating process. The element can then represent the workpiece in a process and the arrangement thus allows for very fast and controllable heating of components in production. The setup also features high versatility and a single tool can be used for heating components with different geometries.
In another embodiment the element is a tool element, configured to heat an adjacent workpiece during a heating process. The invention can save large amounts of energy and speed up the productivity significantly compared to alternative solutions. A controllable heating pattern also ensured a high quality of the produced items.
Objects and advantages of the present invention will be clear to a person skilled in the art when reading the detailed description and viewing the drawings. The concept of the invention is defined in the independent claims while certain embodiments of the invention are defined in the dependent claims.
Embodiments of the invention will be described in the following, reference being made to the appended drawings which illustrate non-limiting examples of how the inventive concept can be reduced into practice.
With reference to
The electric current conductor 120 is partly arranged around the coil unit 111 and made of a material with good electrical conducting properties, e.g. copper, aluminum or any other suitable conductor material, as a driving system in order to induce the current through the return conductor 130, consisting of a material with a significantly higher resistivity than the return conductor, e.g. stainless steel, titanium, steel, carbon fiber or any other suitable material. The heating is therefor conducted entirely or mainly by resistive losses in the return conductor 130. The electric current conductor 120 is connected to the return conductor, (also called the heating part) with a purpose to guide the current without causing losses. A purpose of the coil unit 111 is to induce current in the electric current conductor 120 which then is guided through the return conductor 130.
Close to the return conductor 130 a workpiece W is arranged which is heated by the heat from the return conductor 130 in a controlled way. In
The invention comprises hence an induction heater or inductor construction including a coil arrangement, possibly magnetic core material and a workpiece and electric return conductor or driver. In this case the element or return conductor 130 is a tool element configured to heat the adjacent workpiece W during the heating process. The apparatus 100 may heat the desired surface/body without it being arranged in the active work area.
The apparatus 100 comprises a so called heating portion 130, which also may be referred to as a workpiece, depending on the configuration, which means the part of the conduit which shall receive a certain temperature and to which energy is about to be controlled. This surface may be a part of a tool in a process, e.g. for plastic molding, but may also be a part of an object to be heated and manufactured and after that be separated from the arrangement, see the following embodiment and
The flow conductor material is arranged within the coil unit 111, where the construction type is often named “longitudinal field” in order to focus the magnetic field and thus increase the efficiency. The flow conductor material is, with marginal benefit, able to surround the electric current conductor 120, see
If the electric current conduct 120 acts as the driving system a substantially larger distance between the coil unit 111 and the return conductor 130 may be allowed with a maintained efficiency than traditional induction heating. By making space just adjacent to the return conductor 130 (or the tool surface) it is possible to integrate an active cooler (not shown) in order to quickly be able cool the tool, e.g. when changing tools or thermal cycling. The space may also be used to thermally insulate the coil unit from the heated surface, which is an important feature at high tool temperatures, especially together with temperature sensitive material combinations such as Litz wire coils. Another example of an element that may be integrated in the space is micro mechanical actuators, piezo crystals for geometry control or vibration assisted functionality. In order to combine the inductive and the resistive contribution in a suitable way without advanced control then a proper thick sheet of a suitable conductor material, e.g. copper or aluminum be arranged in the space, which slightly dampers the magnetic field that affects the heating portion. In Table 1, to the right a 0.3 mm thick aluminum sheet has been used for this purpose with negligible losses.
With reference to
With reference to
Yet another configuration of the invention is shown in
The electric current conductor may consist of parallel wires, stripes or similar, preferably interlaced with coiling in order to obtain maximum connection. The different conductors are then connected at the heating plate. The current conductor 720 may also be provided with cooling channels or surface enlargements in the shape of e.g. flanges depending on the application.
The heat pattern may be controlled by varying the amplitude and phase shift between the currents in the different coils. To minimize the affect that the coils have on each other suitable coils may be connected in series or anti-series, alternatively parallel or anti-parallel. To maximize the controllability the coils are controlled entirely separate of each other, coils from different (equivalent) inductors however be connected to each other to reduce the transformer capacity between each other. The currents are preferably independent between the coils but interference between the magnetic fields may provide an increased controllability even if sequential drive of the coils also gives a good result. If there are several coils they may be arranged above or underneath each other, interweaved or arbitrary intersected. What is unique for the invention is that by controlling the skin depth the magnetic vector field, by combining currents with different amplitudes, frequency and phase angle, it is possible to not only heat the entire workpiece but also to supply effect along lines or points (pixels) which makes and unbeatable controllability possible.
The figure series in
A split return conductor, see
The connection between the electric current conductor and the return conductor may be accomplished by welding, soldering, screw joint reinforcement, mechanical joint or any other suitable method. Electrical insulation between the coil units, the coil unit and the core and the coil unit and the return conductor or the electric current conductor is important to avoid short circuit or electrical breakdown. Examples of an insulation material are varnish, epoxy, nomex, glass fibre, textile, fabric or any other insulating material. A construction without insulating material, i.e. with only air is also possible. The electric current conductor could be in one piece or split in one or several places for an easier manufacture/disassembly etc. It can be one or several electric current conductors in order to maximize the efficiency, the heating result or the complexity.
All described apparatus and embodiments thereof may be supplemented with active cooling between the coil unit/units and the return conductor to be able to cool the tool, e.g. at thermal cycling or a tool change in a machine. Several alternative cooling principles may be used, but where the most suitable probably is conduction through flowing gas or liquid, phase transfer from a solid or liquid state to fluid or gas by means of the Seebeck effect/thermoelectric effect through a Peltier element. By implementing a heating/cooling concept significant improvements in the evenness of temperature/controllability could quickly be achieved.
All solutions have assumed that there is a closed circuit according to the description, but some of them may also be able to work without an electric current conductor by arranging soft magnetic cores with a high permeability which encloses the coils in a similar way and which also may be a good alternative within a closed circuit. The core material compresses the electro magnetic flow and focuses the heating around its regions while the return currents only contribute with a very small heating effect. The core material is also important when magnetizing with a DC-current as it creates regions with reduced heating and may, depending on the number of coils and its geometry and placement, adopt more or less complex forms. The inactive parts of the coil may advantageously be covered in a good conductor material such as copper, aluminum or similar, to reduce the current inductance and therewith the magnetizing current, and, but not necessary, the surrounding magnetic field. The cover material may be provided with coolant channels or surface enlarging elements such as flanges.
Three Coil Inductor Implementation
The implementation or the experiments refers to verify the hypothesis that by placing a LF-inductor, with several separate controlled coils within a closed, welded construction made of a copper casing and a workpiece made of steel, the temperature over the surface may be controlled in a dimension, unlike if the inductor is placed outside the closed circuit.
The inductor consists of 5+14+9 windings of solid, insulated copper wire, 2×4.5 mm, wrapped in one layer close to each other around an insulated flow conductor core of SM2C according. The coil in the center is called coil and the two on the sides are connected in series and are called coils, each one connected to an independent electronic frequency converter and checked to have approximately the same resonance frequency. The distance between the coil and the copper casing is 0.5-1 mm and the air gap between the coil and the workpiece within the conduit is approximately 9 mm. The distance between the copper casing and the workpiece is approximately 40 mm and the magnetic field from the current in one part is assumed not to significantly affect the current in the other, which otherwise would lead to an increased heating in the center (axially)
The results clearly show that the hypothesis is correct! When the inductor is placed within the conduit it is possible to axially control the heat pattern, i.e. perpendicular to the direction of the current. Is the inductor instead placed outside the conduit then the possibilities to control disappear entirely.
The results indicate that the combination of 5+9 windings of the same current loop is not optimal in order to achieve uniform heating but the configuration is one way that with one and the same inductor is able to test different effects. An inappropriate placement of the joint between copper and steel, in combination with an extensively thick copper casing allows the workpiece to cool significantly and unnecessary much along the edges. A more well dimensioned construction with optimized coils, for best result a two-dimensional solution in order to be able to control the effect in further directions, the problem would be solved. Even a well made and return connected control is required to achieve the desired result, as well as a control of the cross-connection between the coils, but the experiments still shows the possibilities without finding any limitations, but a substantial double curved workpiece, e.g. a semi sphere, means however new challenges. The heat spread is apparent due to a relatively slow process in order to have the time to manually control the two coils and at the same time take a photograph without rising to put something on fire.
Finally, although the inventive concept has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims.
Number | Date | Country | Kind |
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1300396-7 | May 2013 | SE | national |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2014/061283 | 5/30/2014 | WO | 00 |