EP1550353B1 - Composite body and method for production thereof - Google Patents

Abstract

The compound body has base body that is made of a precipitation-hardened steel and an applied heating coating. The steel is a high-alloyed steel and the base body has a round or curved surface for accommodating a heating coating. The base body can be tubular and can be a distribution or material pipe in a heating channel system. The heating coating can have several layers and/or elements. AN Independent claim is also included for the following: (a) a method of manufacturing an inventive compound body.

Description

The invention relates to a composite body having a base body of steel and a heating coating applied thereto in accordance with the preamble of claim 1 and a method for the production thereof according to the preamble of claim 17.

For various applications, thick film heaters have been developed which are fixedly mounted as a coating on the surface of a metal substrate or a steel body. The most of an array of electrical resistance tracks existing heating elements are electrically insulated from the metal substrate or the steel body by an insulating layer of dielectric material or a glass ceramic. All layers are solidified after application by baking into a composite layer, which forms a composite body together with the steel body. Examples of this are described in DE-A1-35 36 268 or DE-A1-35 45 445.

Problems always arise when the steel body has a round or curved surface and must be hardened, as is often the case, for example, in hot runner systems in injection molds. The latter usually have a branched network of distribution channels and hot runner nozzles with steel tubes made of steel, which can be exposed to extremely high internal pressures depending on the application. So that the hot mass in the distribution system does not cool prematurely, the material pipes are circumferentially provided with a heater.

WO-A1-00 23 245 proposes for this purpose to apply the heating device in the so-called fine-film printing process, wherein the individual layers are applied by means of a dispenser. Such a method is relatively expensive, far as the cannula of the dispenser for the application of the insulation and cover layers must drive the entire surface of the ceramic sleeve or the material tube exactly to produce self-contained layers. The latter therefore do not always have a uniform thickness or density, so that cracks are difficult to avoid.

Another disadvantage arises in the operation of the hot runner system, namely, when the material pipe is exposed at the operating temperature of the technologically caused by the injection molding pulsating internal pressure load. This load and the required to reach the operating temperatures heating the flow channel wall to temperatures between 300 and 450 ° C lead to elastic stretching processes that are transmitted directly to the heater. Their layers can quickly reach the range of tensile stresses, which can lead to cracks in the insulating layer, to short circuits or even to the flaking of the entire heater.

To counter this, the heating coating has been applied to an uncured steel (auxiliary) body, which is then placed on the material pipe. However, such a separate heater has no direct solid contact with the material pipe, which leads to a high heat transfer resistance and thus to a less efficient heat transfer from the heating element to the tubular flow channel. This in turn affects the overall temperature setting and the associated control effort.

From DE-A1-199 41 038 it is known to apply the Heizschichtsystem directly on the material tube and form such that it is after the burning (forming) relative to the material tube wall under a defined compression bias. This is generated by a specific mismatch of the coefficient of linear expansion of the glass-ceramic insulating layer is given to the corresponding value of the metallic hot runner tube in dependence on the strain-relevant characteristics of the hot runner tube. Such a voltage-tolerant connection withstands the elastic stretching processes in the material pipe within limits. At high loads, however, cracks or other damage may continue to occur in the insulation layer.

The aim of the invention is to overcome these and other disadvantages of the prior art and to provide a steel body with a heating coating that withstands even extreme loads permanently. The aim is in particular an equally inexpensive and easy to implement method for crack-free application of the individual, temperature changes exposed layers on a tubular or curved steel body. In particular, a heating coating should be permanently functional on a material pipe of a hot runner nozzle.

Main features of the invention are specified in the characterizing part of claims 1 and 13. Embodiments are the subject of claims 2 to 12 and 14 to 24.

As a solution, the invention provides according to claim 1, that in a composite body with a base made of steel and a heating coating applied thereto, the base body is made of a precipitation-hardening steel.

Precipitation-hardening steels have the property that intermetallic precipitates form on cooling, which, in addition to the purely temperature-related volume reduction, lead to a further reduction of the steel body volume. A precipitation-hardening steel therefore shrinks during the aging process, so that the compressive prestress of a heating coating previously applied to the surface of a base body is enhanced after hardening. The coating is always permanently bonded to the steel body surface, even when the composite is subjected to extremely high temperature or pressure loads.

By the use of high-alloy steels according to claim 2, the size and distribution of the pressure bias within the insulating layer can be adjusted very precisely, which is especially important if the steel body has a round or curved surface for receiving the insulating layer, or if the steel body a has tubular shape and the heating coating is applied to the outer wall.

The main body is a distributor or material pipe of a hot runner system. Especially in the field of hot runner technology, it is important that the injection molding compound to be supplied to a mold cavity is precisely and uniformly tempered into the nozzle or gate area. Cracks in the heating coating would immediately lead to failure of the nozzle and interruptions in the manufacturing process, which is effectively avoided by the inventive design of the composite body.

According to claim 3, the heating coating is preferably a layer composite composed of a plurality of layers and / or layer elements, which has an insulation layer applied to the base body according to claim 4. The latter is in accordance with claim 5, a ceramic or glass-ceramic insulating layer, depending on the application method and the desired layer thickness of one or - as claim 6 provides - may consist of two or more individual layers. On the insulation layer according to claim 7, an array of resistive elements is applied. The latter form a heater, which is at least partially covered by an insulating cover layer for the protection of the resistance paths (claim 8).

Manufacturing technology, it is advantageous if the insulating layer, the resistive elements and / or the cover layer according to claim 9 branded dispersions, such as thick-film pastes. These can be uniformly and precisely applied, which is important for the subsequent adhesion and functionality of the heater. Alternatively, the individual layers or partial layers of the heating coating according to claim 10 may also be formed as baked-on films.

In order to determine both the temperature distribution and their development within the heater or within the body, the formation of claim 11 provides that in the plane of the heating coating at least one temperature sensor is arranged. This is therefore housed in the layer composite, which leads to no significant volume increase. At the same time, temperature changes can be detected extremely promptly and precisely.

According to claim 12, connecting contacts for the resistance elements and / or the temperature sensors are integrated in the heating coating. The entire heater can thus be integrated directly into a control circuit.

Further important advantages arise when using a composite body according to the invention as an externally heated material pipe in a hot runner manifold and / or a hot runner nozzle. The cohesive application of the heater in layers ensures a permanently fixed connection to the wall of the body and thus for a firm grip on the hot runner manifold or the hot runner nozzle. In addition, the invention avoids extremely effective flaking or loosening of the heating, namely by the pressure bias in the Heating coating is selectively increased by precipitation hardening of the body.

Due to the small thickness dimensions achieved by the direct coating, the heating coating as a whole takes up only a small amount of space, so that extremely compact designs can be realized with virtually the same performance features as compared to conventional heating devices. In addition, the power density can be increased significantly, because the heat is generated and removed directly on the surface of the heated Heißkanaldements. Overheating of the most sensitive heating elements is reliably avoided.

In a method for producing a composite body with a base body made of steel and a heating coating applied thereto, for which independent protection is claimed, the invention according to claim 13 provides that a compressive prestress previously generated in the heating coating is reinforced by precipitation hardening of the base body.

This method, which is simple and inexpensive to implement, leads to a permanently fixed connection between the base body and the heating coating, because the latter is contracted again within definable limits by the contraction movement of the base body during cooling in the hardening process, whereby a particularly effective stress-tolerant connection is formed. All layers or partial layers of the heater have an extremely good adhesion. In particular, the insulation layer can withstand even extreme mechanical and thermal stresses permanently, so that always optimal production results are guaranteed.

According to claim 14, each layer or each layer element of the heating coating is applied to the base body, dried and baked or formed, wherein the composite body is cooled to room temperature after each baking process. In this way, all process parameters can be individually adapted to the respective heating layer, which - depending on the power requirement - can always be applied optimally.

The invention further provides in claim 15 that the steel alloy of the base body is homogenized or solution-annealed during the baking process, which has a particularly favorable effect on the process economy. This is also a claim 16, when the baking temperature is equal to the temperature for the homogenization or solution heat treatment of the base body. While the individual layers or layer elements of the heating coating are formed, the solution annealing produces stable homogeneous mixed crystals (α crystals). Separately controlled production steps are no longer necessary.

Of particular advantage is the embodiment of claim 17, according to which the individual layers can be applied by screen printing, dispensing, dipping or spraying. Thus, you can choose the optimal process for each layer. All layer parameters such as layer thickness, density, shape and the like. can be adjusted evenly and precisely, so that an always functioning heating coating is created.

In the embodiment of claim 18, each layer or each layer element is baked or formed under an air atmosphere, wherein the stoving temperature according to claim 19 is between 750 ° C and 900 ° C.

Claim 20 provides that the surface of the base body is roughened before applying the heating coating, for example by means of sandblasting. As a result, the mechanical adhesion of the insulating layer is improved. The chemical adhesion can be optimized by cleaning the base body according to claim 21 before applying the coating and oxidized.

After application of the heating coating, the steel alloy of the base body is aged or annealed in accordance with claim 22 by re-annealing. As a result, form fine inter-metallic precipitates, which lead to a targeted reduction of the body volume. Thus, within the heating coating applied to the base body, a compressive stress is created which is capable of permanently compensating mechanical loads on the base body, for example the internal pressure loads of a material pipe of a hot runner nozzle.

It is important that the Auslagerungstemperstur according to claim 23 is smaller than the baking temperature for the individual layers of the heating coating. As a result, neither the formation of the individual layers or layer elements of the heating coating nor their cohesion is disturbed. Furthermore, the pressure bias in the heating coating is optimally increased without their performance parameters or functionality is impaired. The entire process can be precisely controlled with simple means, which keeps the process costs low.

Suitably, the aging process is carried out according to claim 24 under an air or nitrogen atmosphere.

Further features, details and advantages of the invention will become apparent from the wording of the claims and from the following description of exemplary embodiments with reference to the drawings.

In a preferred embodiment of the invention is used as starting material for the production of the base body with Ni, Co, Mo, Ti and / or Al highly alloyed precipitation hardening steel, for example, X 3 Cr Ni Al Mo 12 9 2 1. The main body forms, for example Material tube with a round surface for an externally heated hot runner nozzle, which is used in an injection mold.

On the body a heating coating is applied. This consists of a directly on the body lying glass-ceramic insulating layer, an applied thereon of resistance paths as a heating element and an overlying cover layer to protect the heater against external influences. Heating coating and body are inextricably linked together and thus form a composite body.

The precipitation hardening of the material pipe usually takes place in 2 steps, namely the solution annealing of the alloy and the subsequent aging or aging,

Beforehand, however, the individual layers or layer elements of the heating coating are applied in the form of thick-film pastes and baked or formed, wherein simultaneously with the stoving of the thick-film pastes, the solution annealing of the metal alloy is carried out.

At the beginning of the process, the still uncured steel body is first sandblasted after completion of the mechanical processing in order to improve the mechanical adhesion properties for the heating coating, wherein a certain surface roughness is observed. Subsequently, the material tube is cleaned with ethanol and hot nitric acid (HNO 3) and oxidized at about 850 ° C. hereby creates a thin oxide film on the surface of the body, which improves the adhesion of the insulating layer.

After completion of the pretreatment, the heating coating is produced.

The starting material for the insulating layer is preferably a dispersion, in particular an electrically insulating thick-film paste, which is printed on the base surface with a uniform thickness in the screen printing process. Preferably, four individual layers are applied in succession, each layer being dried separately. Once the desired layer thickness has been reached, the material tube with the insulating layer is formed in a suitable kiln under an air atmosphere at about 850 ° C., so that a glass kerosene structure which is homogeneous in itself is produced.

The baking temperature corresponds to the temperature required for the homogenization or solution heat treatment of the base body. Both processes - burn-in and solution heat treatment - take place at the same time.

Furthermore, by a specific mismatching of the linear thermal expansion coefficient of the insulating layer to the linear thermal expansion coefficient of the material tube during the baking of the insulating layer within a mechanical compression bias is generated. The resulting stress-tolerant compound in the composite body sets the insulating layer as a carrier layer of the heater already in a position to withstand the technologically caused by the injection molding pulsating internal pressure loads in the material pipe within certain limits without cracks or damage to the heater occur.

If the base body with the baked insulation layer has cooled to room temperature, first the terminal contacts for the current-conducting resistance elements and optionally for a temperature sensor are applied and dried. Starting from the terminal contacts the mostly meandering or spiral resistance tracks for the heating and for the temperature sensor are applied, this - as well as for the terminal contacts - electrically conductive pastes used, which are applied either by screen printing or with a dispenser on the insulating layer. The drying takes place in each case after the application of the individual layers. All conductive layer elements are then fired together and cooled to room temperature. Also here the base body is solution-annealed again, but this has no final effect on its structure.

The cover layer is also an electrically insulating glass ceramic, which is printed by screen printing on the resistor elements, the terminal contacts and in some areas still exposed insulation layer, dried and then formed at about 750 to 900 ° C.

After the last baking process, the main body together with the already applied heating coating under nitrogen atmosphere is heated again to about 525 ° C and held for a defined time at this temperature. After the holding time, the composite body is cooled, preferably at a cooling rate of 10 K / min.

The precipitation-hardening steel shrinks by about 0.07% on all sides during cure at 525 ° C and again by about 11 ppm / K upon cooling, further pressurizing the previously applied and formed layers of the heater. The precipitation hardening thus leads to an additional compression bias, so that the entire heating coating can withstand even extreme temperature and internal pressure loads in the material pipe permanently. The hot runner nozzle is always optimally tempered by the materially applied heating in each stage of the process.

The hardness of the base body reached after the hardening process is about HRC 52.

The temperature sensor is preferably in the same plane as the resistor tracks of the heater. He is therefore integrated as well as the terminal contacts in the heating coating. The latter forms a layer composite composed of a plurality of layers or layer elements, which forms a heatable composite body in permanent detachable connection with the base body.

Due to the high TKR, the heating resistor itself can also serve as a temperature sensor. For this purpose, voltage taps are led out of desired regions of the meandering or spiral resistance paths to the outside. If the current is known, the temperature in the relevant area can be determined via the determined partial voltage.

The invention is not limited to one of the above-described embodiments, but can be modified in many ways. Thus, individual or all layers or layer elements of the heating coating can also be applied by spraying or dipping. Alternatively, however, it is also possible to use films which are baked in the same way as the thick-film pastes.

The steel alloy of the base body may also be a nickel-cobalt hot-work tool steel. It is important that the steel is suitable for a peak temperature of up to 850 to 900 ° C with respect to the baking or sintering of the heating coating. He must also withstand operating conditions temperatures of up to 450 ° C and internal pressure loads of up to 2000 bar.

It can be seen that precipitation-hardening steels are used as starting material for the steel body. In contrast to conventional hardening via carbon martensite, intermetallic precipitations take place, which can be precisely controlled by alloy choice. The contraction that occurs during curing increases the compressive stress in the insulating layer or in the entire heating coating, which substantially improves the durability and functional reliability of the heating.

Normal hardening steels can not do all this, unless one cools the steel body with critical cooling rate. However, the required high temperature and the high cooling rate destroy the heating coating, which avoids the invention in a simple and cost-effective manner.

Claims (24)

1. Composite body comprising a base body made from steel and a heating coating applied thereto, characterized in that the base body is a distribution tube or a material tube of a hot runner system, said base body being made from a precipitation hardening steel.
2. Composite body according to claim 1, characterized in that the steel is a high-alloy steel.
3. Composite body according to claim 1 or 2, characterized in that the heating coating is a layer composite constituted by several layers and/or layer elements.
4. Composite body according to claim 3, characterized in that the heating coating comprises an insulating layer applied onto the base body.
5. Composite body according to claim 4, characterized in that the insulating layer is a ceramic or a glass-ceramic.
6. Composite body according to claim 4 or 5, characterized in that the insulating layer consists of at least two individual layers.
7. Composite body according to any one of claims 4 to 6, characterized in that an array of resistance elements is applied onto the insulation layer.
8. Composite body according to claim 7, characterized in that the resistance elements are covered by an insulating top coat, at least on certain segments.
9. Composite body according to any one of claims 4 to 8, characterized in that the insulating layer, the resistance elements and/or the top coat are baked dispersions, for instance thick film pastes.
10. Composite body according to any one of claims 4 to 8, characterized in that the insulating layer, the resistance elements and/or the top coat are baked films.
11. Composite body according to any one of claims 3 to 10, characterized in that at least one temperature sensor is integrated into the plane of the heating coating.
12. Composite body according to any one of claims 3 to 11, characterized in that connecting contacts for the resistance elements and/or the temperature sensors are integrated into the heating coating.
13. Method for the production of a composite body comprising a base body made from steel and a heating coating applied thereto according to any one of claims 1 to 12, characterized in that a previously generated compressive pre-stress in the heating coating is increased by precipitation hardening of the base body.
14. Method according to claim 13, characterized in that each layer and each layer element, respectively, of the heating coating is applied onto the base body, dried and baked or formed, and that following each baking process, the composite body is cooled to room temperature.
15. Method according to claim 13 or 14, characterized in that the steel alloy of the base body is homogenized and solution-annealed, respectively, during the baking process.
16. Method according to any one of claims 13 to 15, characterized in that the baking temperature is the same as the homogenizing temperature and the solution annealing temperature, respectively, of the base body.
17. Method according to any one of claims 13 to 16, characterized in that the layers and layer elements, respectively, of the heating coating are applied by means of screen printing or dispensing, or by immersion or spraying.
18. Method according to any one of claims 13 to 17, characterized in that each layer and each layer element, respectively, is baked or formed under air atmosphere.
19. Method according to claim 18, characterized in that the baking temperature is between 750 °C and 900 °C.
20. Method according to any one of claims 13 to 19, characterized in that the surface of the base body is roughened, for example by sand blasting, before the heating coating is applied.
21. Method according to any one of claims 13 to 20, characterized in that the base body is cleaned and/or oxidized before the heating coating is applied.
22. Method according to any one of claims 13 to 21, characterized in that the steel alloy of the base body is aged or age hardened by annealing after the heating coating has been applied.
23. Method according to claim 22, characterized in that the age hardening temperature is less than the baking temperature for each of the layers of the heating coating.
24. Method according to any one of claims 13 to 23, characterized in that age hardening is carried out under air atmosphere or nitrogen atmosphere.