Electric heating – Heating devices – Combined with container – enclosure – or support for material...
Reexamination Certificate
2002-02-27
2004-09-28
Pelham, Joseph (Department: 3742)
Electric heating
Heating devices
Combined with container, enclosure, or support for material...
C219S521000, C219S534000, C219S543000, C219S544000, C392S478000
Reexamination Certificate
active
06797925
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an electrical heating device for hot runner systems, in particular for hot runner manifolds and/or hot runner nozzles, and a method of manufacturing such a heating device.
BACKGROUND OF THE INVENTION
Electrical heating means for hot runner systems are usually separate component parts with tube-shaped heating elements which are integrated in detachable jackets for peripheral mounting onto flow ducts that commonly are tube-shaped. As disclosed e.g. in DE-U1-295 07 848 or in U.S. Pat. No. 4,558,210, the jackets may be rigid structures whose radii of curvature match the flow duct, additional holding or clamping means being provided for fixing them on the tube periphery in an axial direction. Alternatively, they form flexible heating strips or heating blankets between electrically insulating layers which may have different heat conduction properties and which are fixed onto the tube periphery of the flow duct. EP-B1-0 028 153 provides heat conducting adhesive strips for the purpose, whereas WO 97/03540 employs flexible heating tapes having velcro or other snap fasteners.
Heating devices which in principle are mechanically detachable have the important drawback that heat transition from the heating element to the tube-shaped flow duct is frequently rather inefficient. For compensation it is necessary to enlarge the overall dimensions of the heating device, causing larger heat capacities. The resulting big thermal masses lead to prolonged heat-up and cool-down periods of time, whereby the growth of productivity rates is limited. Moreover, there are problems regarding linear temperature distribution within the walls of the flow duct which rarely feature a constant temperature throughout the length of the flow duct. In the region of the nozzle tip, in particular, sufficient heat transition and thus a sufficient level of temperature can be attained with large expenditures only. This, in turn, affects the entire temperature setting as well as the effort required for controlling means.
SUMMARY OF THE INVENTION
It is an object of the invention to overcome these and other disadvantages of the prior art and to create an electrical heating device for hot runner systems providing, between the main hot runner portion and the nozzle, a heat transition and temperature distribution pattern that is generally improved and permits individual precise adjustment. The device is to be designed for easy operation without much effort for control means.
The invention further aims at providing, for hot runner systems, positively and non-positively integrated electrical heating means of compact design which are adapted to be non-detachably mounted onto a flow duct wall such as a mold mass flow tube, a rod, a manifold branch, etc. and which will permanently withstand even extreme mechanical and/or thermal loads.
Another important object of the invention is the development of a method of manufacturing heating devices for hot runner systems, especially for hot runner manifolds and/or hot runner nozzles, requiring a minimum of effort but permitting simple and economical performance.
Principal features of the invention are defined in claims
1
and
19
relating to an electrical heating device and its manufacture, respectively, for use in hot runner systems including manifolds and/or hot runner nozzles with at least one mold mass flow tube associated to a flow duct. The invention provides that at least one insulating dielectric layer is applied by direct coating in an adherent manner onto a wall of the flow tube and is coated by at least one heating layer having heating conductors.
Adherently depositing layers of the heating device results in a permanently fixed connection with the wall of the flow duct and thus in a secure fixing on the hot runner manifold or on the hot runner nozzle. The heating device requires little room owing to the small thickness dimensions achieved through direct coating, whereby in comparison to conventional heating devices, and with almost equal features of performance, extremely compact embodiments can be realized. Moreover, the power density can be distinctly increased since heat is produced and carried off directly at the usually curved surface of the hot runner element to be heated. Together with the direct fixing of the heating device on the flow tube wall in a mechanically non-detachable manner, all of this warrants an always optimal heat transition from the heating layer via the insulating layer onto the wall that is heated most uniformly and precisely. There Is no need for expensive control means which would have to cope with reaction delays caused by thermal masses. The device allows quick and accurate heating and cooling-off again, too, with favorable effects on the entire producing sequence of injection molding.
Another advantage is that the heating device is reliably protected against moisture absorption. Conventional heating devices employing tubular heaters or helix tube cartridges pose, in addition to mounting problems, also insulation problems due to absorption of moisture in a hygroscopic insulating material, as penetrating moisture may cause shortcuts. In order to avoid this, additional control means are required for dewatering by initially operating the heating device under reduced heating power. The heating device of the invention does without that. Rather, it is joined to the flow duct in an absolutely tight and self-captivated manner so that the conventionally necessary effort for mounting and control is completely dispensed with. This has positive effects on the purchase and mounting costs.
Specifically, the at least one insulating layer may be a dielectric layer comprising glass, vitreous ceramics or ceramics. Preferably during the firing process, a pressure pretension is produced within this insulating dielectric layer relative to the flow tube wall, effected by a mismatch in that the linear thermal expansion coefficient (TEC
DE
) of the baked dielectric layer is smaller than the linear thermal expansion coefficient (TEC
M
) of the flow tube wall, the difference between the linear thermal expansion coefficients (TEC
DE
−TEC
M
) amounting to at least 5.0 10
−6
K
−1
. This further important feature of the invention results in a tension-relief connection between the insulating dielectric layer and the hot runner tube which under operating temperature is exposed to a pulsating interior pressure load technologically caused by the injection molding process. Such load, and the need to heat the flow duct wall up to temperatures between 300° C. and 450° C. in order to reach and maintain operating temperatures, entail elastic expansions and contractions which are directly transferred to the heating device. The actual degree of deformation will depend on material-bound factors (e.g. elastic modulus) and on technical boundary conditions (operating temperature, tube wall thickness, level of interior pressure). Layers conventionaly applied onto a steel tube will, under the co-influence of the said factors, be freely exposed to varying tensile stresses. The invention, by contrast, avoids or reduces this reliably as the pressure pretension within the dielectric layer will compensate delamination forces occurring under the interior pressure load the magnitude of which varies depending on the respective radii. The heating device as a whole will thus have an extraordinarily good bonding strength on the usually tube-shaped flow tube wall and will permanently withstand even extreme mechanical and thermal loads. Thus optimum production results are always warranted.
The insulating dielectric layer preferably comprises a system of materials including preformed glass, vitreous ceramics or ceramics suitable for wetting, at a predetermined baking temperature, the surface of the flow tube wall which commonly is of metal, said insulating dielectric layer assuming at least partially a crystalline state. The system of materials may include at least one further glass which will not become crystalline under predetermined baking
Günther Herbert
Kretschmab Christel
Otschik Peter
Clark & Brody
Gunther Heisskanaltechnik GmbH
Pelham Joseph
LandOfFree
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