Patent Application
20100068331
The present invention relates to an electric heating unit for hot runner systems, in particular hot runner nozzles and/or hot runner manifolds.
Hot runner systems are used in injection molds in order to feed a flowable material such as a plastic melt at a given temperature and under high pressure to a separable mold insert. Such systems typically comprise a material feed pipe fitted with a flow duct, said pipe ending in a nozzle mouth. The nozzle mouth subtends at its end a nozzle discharge aperture terminating through a gate aperture into the mold insert (mold nest). To prevent the flowable material from prematurely cooling within the material feed pipe, one or more electrical heating elements are used to ensure a temperature as uniform as feasible as far as into the nozzle mouth.
The electrical heating unit illustratively may be a separate component with a helicoidal heating element integrated into the tubular casing and circumferentially deposited on said material pipe. As disclosed for instance in the German patent document U 295 07 848 or in the US patent document U.S. Pat. No. 4,558,210, said casing may be a rigid structure affixed in the axial direction by additional retaining or tightening elements on the material pipe. Alternatively the heating unit may be designed in the form of flexible heating strips or as a flexible heating mat between electrically insulating layers, where called for with different thermal conductivities, that are affixed to the outside of the material feed pipe. The European patent document EP B1 0 028 153 uses thermally conducting adhesive strips whereas the document WO 97/03540 uses flexible retention tapes with velcro or snap-in locking means.
One drawback of these known heating units is their substantial bulk, as a result of which much room is required for hot runner nozzle installation; this aspect is undesirable in most cases, especially where small nest spacings are required. Such comparatively large dimensions also entail more heating power with attendant increased energy consumption. Again, the large thermal inertias prolong the heating and cooling phases, thereby limiting high production rates.
The German patent document DE A 199 41 038 proposes to eliminate such problems by depositing on at least one wall of a material feed pipe associated with a flow duct at least one insulating layer and at least one heating layer comprising thermally conducting tracks by direct and integral deposition, in other words, to make the heating unit and the material feed pipe integral. Direct deposition illustratively can be carried out by the techniques of film making, thick layer or screen printing techniques, where, following deposition, the layers each shall be baked, hereafter “fired”, separately or simultaneously. Such an integrated stratified heating unit deposition assures permanent, firm affixation to the flow duct wall and hence to the hot runner manifold or the hot runner nozzle. Because of the small sizes made possible by direct deposition, the heating unit in turn requires only little space, and in this manner, extremely compact design—compared to the previously cited heating units—shall be attained at nearly the same outputs. Such outputs also may be increased because the heat is directly generated on/and dissipated from the surface of hot runner element to be heated. In this manner overheating the generally delicate heating elements is avoided. Moreover, the nozzle may be heated quickly and accurately and be cooled rapidly, allowing improved production runs.
However said heating units incur the drawback that the selection of the heating layer material is restricted to those of such low firing temperatures which will not affect the microstructure of the material feed pipe that typically is made of tool steel and that also is exposed to the firing temperatures while the firing procedure is unchanged. Accordingly the firing temperatures of the heat conducting layers may not exceed the processing temperature of the feed pipe's raw material.
The objective of the present invention is to circumvent the above and other drawbacks of the state of the art and to offer an improved heating unit which can be made in simple and economic manner. In particular the present invention relates to a corresponding manufacturing method, to an improved hot runner system and to an improved hot runner nozzle.
The main features of the present invention are defined by the claims 1, 25, 27, 28, 31 and 32. Embodiment modes are the objects of claims 2 through 24, 26, 29 through 30 and 33 through 40.
An electric heating unit for hot runner systems, in particular for hot runner nozzles and/or hot runner manifolds and defined by the present invention comprises at least one tubular or muff-like support bearing at least one heating conductor constituted by a(n electric) resistance wire.
Due to the tubular or muff-like design of the said support, all of the heating unit can be slipped onto a material feed pipe of a hot runner nozzle, resulting in a design in two parts, namely the heating unit and the material feed pipe. The heating unit is commensurately exchangeable. Alternatively the heating unit also may be constituted directly on the material feed pipe or on a manifold to attain greater compactness, assuring a durable firm connection and hence firm affixation on the hot runner manifold or on the hot runner nozzle. In either case the heating unit as a whole requires little space whereby, compared to other heating units and at nearly the same outputs, the hot runner nozzles may be kept exceedingly compact. Furthermore, the heating unit of the invention may be manufactured simply and economically and thus manufacturing costs may be lowered.
At least one electrically insulating cover layer is provided above the resistance wire constituting the heating conductor, covering and electrically insulating from the outside the heating conductor and the support. Advantageously this covering layer shall be the heating unit's outermost layer.
Preferably, at least one temperature sensor of which the electric resistance depends on temperature is used to detect the temperature in the flow duct. On account of the changing resistance, the temperature is continuously detectable and the measured values may be used to control the heating unit of the present invention. The design of the temperature sensor may be conventional or it may be in the form of an electrically conducting layer of which the electrical resistance is temperature dependent. The instantaneous temperature can be continuously monitored on account of the varying electrical resistance, said measurements allowing regulating the heating unit of the present invention. The layer serving as the temperature sensor preferably shall be a PTC (positive temperature coefficient) or an NTC (negative temperature coefficient) material. Alternatively, a thermocouple may be used as a temperature sensor with the same structure as an ohmmeter and the measurement site being near the nozzle tip.
The temperature sensor and the heating conductor advantageously are radially situated in a common plane whereby the space occupied by the heating unit of the present invention may be reduced additionally.
Preferably, the support is made of a sintered material such as a ceramic or a sintered metal. However, a metal, a metal alloy, a steel or a steel alloy also may be used. Using a ceramic is advantageous in that the heating conductor respectively the resistance wire can be directly mounted on the support. If a metal is used, for instance, a tool steel, a hard metal or the like, or also a steel, then an insulating layer will be inserted between the support and the heating conductor in order to electrically insulate from each other said resistance wire and said support. Such an insulating layer however can also be present on a ceramic support, illustratively to improve adhesion.
Both the cover layer and the insulating layer acting as the intermediate layer preferably are vitreous and/or a ceramic dielectric layer which, following at least one firing process, shall be pressure-prestressed relative to the support, so that, when said support is internally pressure loaded, radius-depending delamination forces occurring at different amplitudes will be compensated within the insulating layer. Preferably pressure-prestressing is generated so that, as a function of the support's elongation parameters and depending on the particular case, a specific mismatch between the linear expansion coefficient of the ceramic insulating layer TECDE respectively the linear thermal coefficient of expansion TECDEA of the cover layer and the corresponding value of the support TECM shall be predetermined, the differential expansion TECDE−TECM respectively TECDEA−TECM not exceeding a value of 5·10−6K−1.
Furthermore, the insulating layer and/or the cover layer preferably shall exhibit the property of wetting the support surface during the particular firing. Under certain conditions, it will be advantageous that the material shall at least partly pass into the crystalline state.
The insulating layer and/or the cover layer may exhibit a vitreous or vitreous-crystalline structure containing at least one preformed glass that at a predeterminable temperature of firing shall wet the support's surface. The material structure moreover may contain at last one preformed glass which at a predeterminable firing temperature shall at least partly pass into the crystalline state.
Additionally respectively alternatively the material structure may contain at least one further glass that does not crystallize under firing conditions and/or at least one a priori crystalline compound, where, by optimizing the quantitative portions of the preformed vitreous and crystalline components of the material structure and taking into account their particular TEC increments under the conditions of the particular firing, a ceramic dielectric layer will be prepared having a TEC value in the range between 0 and 13·10−6 K−1.
A compensation layer also may be configured between the support and the insulating layer and illustratively is made of chemical nickel. Such a compensating layer also serves to compensate the different expansion coefficients of the support and the insulating layers. This compensating layer may be deposited the same way as the insulating and cover layers.
Preferably, the insulating layer and/or the cover layer and/or the compensating layer shall be a fired foil or a fired thick film paste. The layers however also may be deposited by detonation coating and/or by thermal coating. Alternatively, dip-coating with ensuing firing may be used.
Appropriately, the resistance wire constitutes a heating conductor helix, where the design and/or the configuration is matched to the particular heat output needed. Consequently, the coils or turns of resistance wire to be mounted in a region requiring much heating will be illustratively very tightly configured with each other in order to apply more heat to that region. If moreover the resistance wire is helical or meandering, its pitch may be selected smaller or larger in segments to vary the heat energy output. Again, the resistance wire's cross-section may be varied as called for.
A contacting layer may be configured in each case between the insulating layer, the heating conductor and/or the temperature sensor. Again, as regards said contacting layer and/or the temperature sensor and/or the intermediate (insulating) layer configured between the support and the heating conductor, they are preferably selected fired foils or fired thick-film pastes. These may be deposited in simple and economic manner, in particular regarding structuring and handling, as a result of which an exceedingly reliable and advantageous heating unit may be manufactured.
Altogether, the insulating layer and/or the cover layer and/or the compensating layer and/or the contacting layer and/or the layer acting as the temperature sensor constitute the compound layer imbedding the resistance wire, as a result of which a compact design of the heating unit of the present invention shall be attained. This applies in particular when the resistance wire forming the heating conductor is imbedded into the insulating layer and/or in the contacting layer.
The present invention furthermore relates to a hot runner system, in particular a hot runner nozzle or a hot runner manifold fitted with an electric heating unit of the above described kind. The tubular or muff-like support in this design is deposited/slipped onto a material feed pipe, a bar, a manifold arm, a nozzle or the like.
Alternatively, the tubular or muff-like support per se is or constitutes a material feed pipe, a bar, a manifold arm, a nozzle or the like. This feature improves the heat transfer from the heating unit to the object being heated.
The present invention also relates to a hot runner nozzle comprising an electric heating unit of the present invention, the tubular or muff-like support being deposited at a fit subtending a predetermined play on a material feed pipe. In this instance too the support itself may constitute the material feed pipe, thereby advantageously affecting the heat transfer.
Lastly, the present invention relates to a method for manufacturing an electric heating unit of the present invention used for hot runner systems, in particular for hot runner nozzles and/or hot runner manifolds, the resistance wire constituting the heating conductor being deposited on the support and subsequently the cover layer being deposited by foil printing of screen printing. Depositing the resistance wire as well as the cover layer is easily controlled and in combination is surprisingly advantageous. The heating unit made in this manner operates accurately and always reliably; this feature advantageously affects altogether the temperature distribution and the service life of the heating unit elements.
Depending on support design, and before the heating conductor forming the resistance wire is wound in place, the insulating layer is deposited by foil or screen printing onto said support, the resistance wire being appropriately imbedded into the insulating layer.
The layers deposited by screen printing advantageous shall be in the form of pastes deposited by wraparound printing, assuring overall procedural economy. Each layer may be deposited separately and then be fired; the firing temperature may vary with each layer. However the firing temperature may be varied individually for each layer and lower it after each firing step. In another implementation of the method of the present invention, all layers are deposited separately and are fired simultaneously (co-fired). The firing temperature range is between 800 and 1,400° C.
Further features, details and advantages of the present invention are defined in the claims and discussed in the following description of illustrative embodiments in relation to the appended drawings.
Below the same references denote similar components.
The hot runner nozzle 12 sketched in
A heating unit 10 is mounted on the external surface of the wall 16 of the illustratively steel material feed pipe 13. Said heating unit comprises a casing-like ceramic support 20 serving simultaneous as an electric insulator, further a resistance wire 23 constituting a heating conductor helix 22 wound on said support, where, as schematically indicated in
In order to monitor and control/regulate both the rise and the time function of the temperature within the material feed pipe 13 respectively within the wall 16, a PCT temperature sensor 28 is configured between the support 20 and the cover layer 24, its resistance increasing with rising temperature (
Like the heating conductor helix 22, the temperature sensor 28 also may be made of a resistance wire 29 (
Preferably the cover layer 24 and/or the contacting layer 26 are integrally deposited by direct coating on the support 20 and then are fired under the conditions required by the particular material, thereby producing an integral compound constituting the heating unit 10. Because the resistance wire 23 of the heating conductor helix 22 and the individual functional layers 24, 26 (possibly also 28) are unusually adhesive with respect to each other, the heating unit 10 per se withstands durably even extreme mechanical and/or thermal stresses.
The heating unit 10 is raised from below onto the material feed pipe 13 at a predetermined play selected in a way that said heating unit in its hot operational state shall not be damaged by the thermally expanded material feed pipe while nevertheless optimal heat transfer between the support 20 and the material feed pipe 13 is assured.
Improved heat transfer may be attained by additionally roughening the inside of the support respectively the outside of the material feed pipe.
Again, to improve the heat transfer, the inner side of the support element respectively the outer side of the material feed pipe may be fitted with a dark or black layer. This layer may be made of a black paint as used in radiation heating units. Alternatively or additionally, a dark substance may be used for the support, such as black aluminum oxide. If a metallic support is used, the firing process may induce dark tarnishing of the metal on the pipe inner side.
The foil printing and the thick-film screen printing techniques are suitable for the coating method of depositing the individual functional layers, further for the deposition of the insulating layer or the cover layer, where appropriate also for detonation coating or thermal coating procedures. Preferably, however the thick film screen printing technique shall be used while applying wraparound. Layer firing may be carried out individually or jointly.
The electric terminals 23′ and 29′ for the resistance wires 23, 29 of the heating conductor helix 22 and the temperature sensor 28 also may be made by the thick film technique or conventionally, the pertinent contacts being designed to allow feeding power respectively data transmission using plug-in cable connections.
A first thick-film dielectric paste 34 is deposited on the surface of the support 32—where said surface was roughened beforehand in known manner to improve adhesion—to generate the dielectric layer by the wrap-around procedure. The solid part of said thick-film dielectric paste illustratively may be a glass crystallizing in-situ in a temperature range above 900° C. and having as its main components BaO, Al2O3 and SiO2 of the approximate molar composition BaO Al2 4SiO2. Subsequently to firing, the dielectric layer exhibits a TEC of 6·10−6K−1 in the temperature range from 20 to 300° C.
Because the resultant TEC mismatch between the metal wall 16 and the dielectric layer 34 is of the order of magnitude of 5·10−6 K−1, then it is to be expected when cooling the dielectric-coated wall 16 of the support 32 in the temperature range of purely elastic deformation, namely the glass transformation temperature between about 700° C. and room temperature, that there shall be a buildup of compressive stresses of about 3,500 bars (assuming a Young's modulus of 2·106 bars for the dielectric 34). The magnitude of the pressure prestressing does not yet reach the critical boundary range of the intrinsic compressive strength of the dielectric starting at 6,000 bars. Said magnitude however is sufficient to reliably preclude tensile stresses in the dielectric layer 34 and hence also in the subsequent layers when the wall 16 of the support 32 is cyclically stretched at a load of 2,000 bars.
The present invention is not restricted to one of the above embodiments. Instead, it allows modifications and changes without thereby transcending the scope of protection defined by the appended claims.
All features and advantages, inclusive construction details, spatial configurations and procedural steps, explicit and implicit in the claims, specification and drawings, may be construed inventive per se and also in arbitrary combinations.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 049 669.8 | Oct 2006 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2007/007433 | 8/24/2007 | WO | 00 | 4/17/2009 |
