EP1607685B1 - Coated telescopic pull out rail - Google Patents

Description

The invention relates to a method for producing a telescopic extension system with telescopic rails, preferably a telescopic extension system for Gargutträger in a cooking oven, wherein the base material of the telescopic extension system structural steel and / or stainless steel and which is at least partially provided with a PTFE (polytetrafluoroethylene) based coating.

State of the art

Garages, by which all types of ovens and microwave ovens as well as combined oven systems are to be understood, are increasingly being equipped with telescopic extension systems, in which the food support is moved in and out of the cooking space via telescopic rails. Telescopic rails usually consist of several elongated, mutually displaceable rail elements. The rail elements can be mounted against each other via ball bearings or roller bearings. In simpler embodiments of telescopic rails, so-called slide rails, the rail elements are arranged to slide against each other directly slidable. As so-called partial extracts trained telescopic rails usually comprise two or three rail elements and can be extended to a total length which is not greater than their total length in the retracted state. Telescopic rails designed as so-called full extensions comprise at least three rail elements and can be extended to a total length which is at least twice as large as their overall length in the retracted state.

For the installation of telescopic rails on a carcass wall, a fastening device is required. The fastening device may be a simple hooking mechanism. In Garöfen telescopic rails for attachment to the inner side wall of the cooking chamber are often attached to grid structures by welding or screwing. These mounting grid are then attached to the side wall of the cooking chamber by hanging or screwing. As a rule, several pairs of telescoping rails located at the same height are mounted in several levels of the cooking chamber, so that several food supports can be introduced into the oven at the same time or a change of the level of the food support in the oven with the advantages of the telescopic extension in every level is possible. Fastening devices in the sense of this invention can also be attachment angles which are known in the field as so-called "verticals".

Known telescopic extension systems for cooking ovens are made of steel and / or stainless steel with optionally chrome-plated surfaces. During operation, the telescopic drawer systems are contaminated by splashes of grease, vapors and other residues of the treated food, which are burnt in at the temperatures prevailing in the oven and difficult or impossible to remove. To improve the cleanability of rail systems in Garöfen proposes the DE 102 11 470 A1 respectively. EP 1 344 984 the provision of a PTFE-based coating on the rails. Such PTFE-based coatings are already known as non-stick coatings in frying pans and pans and allow very easy cleaning of the coated articles from soils.

The DE 17 71 323 describes a method of coating metal surfaces with a fluorine-containing polymer by selectively roughening, chrome plating and then electrolytic etching metal surfaces, and then applying a fluorine-containing polymer.

The DE 42 15 594 describes a method for coating metal cookware whereby a first metal coating is applied by arc spraying onto which successive two PTFE coatings are applied. The dishes have a high hardness.

While the PTFE-coated areas of frying pans and other Gargutträgem are relatively large, continuous and smooth surfaces, have telescopic extension systems, consisting of several rail elements and possibly a grid-like fastening device, many curves, corners and edges. In addition, many surfaces of such telescopic extension systems are heavily stressed, e.g. Surfaces on which the food support is placed, and the running or sliding surfaces of the telescopic rail elements. A PTFE coating for such telescopic extension systems must therefore meet high demands on the load capacity, in particular a good adhesion to the coated substrate and a high scratch resistance. In addition, the coating must withstand the high temperatures in a cooking oven and the frequent and severe temperature changes. Finally, the coating must also be food safe.

The known PTFE coatings, as used for example in frying pans, generally do not meet the requirements for a PTFE coating for telescopic extension systems in terms of adhesion and scratch resistance. It is therefore usually recommended by the manufacturer to work in frying pans with PTFE coating not with metal cutlery, but with cutlery made of wood or plastic, so as not to damage the coating.

The DE 102 11 470 A1 Although the basic idea is to apply a cleaning-friendly non-stick coating made of PTFE to the rails of a telescopic extension system, it does not describe whether this PTFE coating has a practical adhesion and scratch resistance and how the PTFE coating was applied.

Object of the invention

The object of the present invention was therefore to produce a telescopic extension system with a PTFE-based coating, which overcomes the disadvantages of the prior art and in particular has a PTFE coating of high adhesion and scratch resistance.

Description of the invention

1. a) wenigstens auf Teile, deren Grundmaterial Baustahl ist, eine ein- oder mehrlagige Grundierungsbeschichtung aufbringt, deren äußerste Lage eine Chromschicht ist,
2. b) alle Teile, die mit einer PTFE-Beschichtung zu versehen sind, einer Temperaturbehandlung bei einer Temperatur über 300°C für wenigstens 10 min unterzieht,
3. c) die gemäß Stufe b) erhaltenen Teile einer Oberflächenbehandlung unterzieht, bei der die Oberfläche eine Rauheit (Mittenrauhwert) R_a von mindestens 2 µm erhält,
4. d) auf die gemäß Stufe c) erhaltenen Teile eine oder mehrere Schichten einer PTFE enthaltenden Lösung oder Suspension aufbringt,
5. e) die gemäß Stufe d) erhaltenen Teile bei einer Temperatur zwischen 300 °C und 450 °C brennt.

This object is achieved by a method for producing a telescopic extension system with telescopic rails, preferably a telescopic extension system for food support in a cooking oven, wherein the base material of the telescopic extension system structural steel and / or stainless steel and which is at least partially provided with a PTFE (polytetrafluoroethylene) based coating in which one

1. a) at least on parts whose base material is structural steel, applying a single- or multi-layer primer coating whose outermost layer is a chromium layer,
2. b) subjecting all parts to be provided with a PTFE coating to a temperature treatment at a temperature above 300 ° C for at least 10 minutes,
3. c) subjecting the parts obtained according to step b) to a surface treatment in which the surface has a roughness (average roughness) R _a of at least 2 μm,
4. d) applying one or more layers of a PTFE-containing solution or suspension to the parts obtained according to step c),
5. e) the parts obtained according to step d) at a temperature between 300 ° C and 450 ° C burns.

In the method according to the invention for applying a PTFE coating, a distinction is made between the base materials of structural steel and stainless steel. In a preferred embodiment, the telescopic extension system comprises a fastening device, preferably a mounting grid to which the telescopic rails are fixed or removable and which is designed for mounting on a cabinet wall, preferably for mounting on a side inner wall of a Garofens. The telescopic rails expediently consist of an inner rail and an outer rail for a so-called partial extension or of an inner rail, an outer rail and one or more middle rails for a so-called full extension. In the Production of such systems, the individual rail elements are coated separately and connected only in a final step to an extendable telescopic rail. In ball-bearing telescopic rails, for example, ball cages are used in this last step with the balls therein, the rail elements pushed together and secured against sliding apart, for example by bending of Auszugsbegrenzer serving as tabs at the ends of the rail elements. When permanently connected to the fastening device or the mounting grid telescopic rails usually the inner rail elements, for example by welding, connected to the mounting grid. The further treatment and coating then follows. In a preferred embodiment, the base material of the fastening device or of the fastening grid and the base material of the inner rail fastened thereto are structural steel. The other rail elements, ie the outer rail and optionally one or more middle rails are suitably made of stainless steel as a base material. Of course, all these components of the telescopic extension system can be made entirely of mild steel or stainless steel as the base material. However, the production of the mounting grid and attached inner rail of structural steel has manufacturing advantages and is less expensive than stainless steel.

Structural steel of quality ST 37 or ST 37-2 (according to DIN 17100) is preferably used as the basic material for the production of parts of the telescopic extension system made of structural steel. Austenitic stainless steel or ferritic stainless steel, particularly preferably austenitic stainless steel of quality 1.4301 or ferritic stainless steel of quality 1.4016 (according to DIN EN 10088-1) is preferably used for parts of the telescopic extension system made of stainless steel as base material. These aforementioned steel grades have proven to be particularly suitable in the PTFE coating process according to the invention. Austenitic grade 1.4301 stainless steel is particularly preferred for its high food grade suitability and corrosion resistance. The ferritic quality 1.4016 stainless steel is also very suitable and slightly cheaper than the austenitic stainless steel.

In the method according to the invention, in a first stage a), a single-coat or multi-layer primer coating, the outermost layer of which is a chromium layer, is applied at least to the parts whose basic material is structural steel. For parts whose base material is stainless steel, this first primer coating is not essential, but it can also be applied to stainless steel parts. Preferably, however, the primer coating is only applied to structural steel parts. For cost reasons, parts made of stainless steel are preferably not provided with the primer coating. The primer coating with an outermost chromium layer has the advantage that it provides corrosion protection and its surface can be roughened well for the subsequent PTFE coating. For parts made of stainless steel is the primer coating Not absolutely necessary, because Edle steel is already very corrosion resistant and its surface can roughen well without primer coating.

In a preferred embodiment of the method according to the invention, the primer coating from step a) is applied by electroplating. In an alternative embodiment, the primer coating from step a) is applied chemically. It is particularly advantageous if the primer coating from stage a) has at least one or more, preferably two or three further metal layers between the base material and the chromium layer. A primer coating has proven to be particularly suitable which, starting from the base material, has a first nickel layer, above which a copper layer and above this a second nickel layer, followed by the chromium layer. As a result, a particularly suitable adhesion of the subsequently applied PTFE coating is achieved on the base material. A primer coating has also proven to be particularly suitable which, starting from the base material, has a first nickel layer and immediately above it a second nickel layer, followed by the chromium layer. Advantageously, a so-called matt nickel layer is applied as the first nickel layer and a so-called bright nickel layer is applied as the second nickel layer. In a further embodiment, only a first nickel layer and above the chromium layer is applied directly.

The primer coating from stage a), if it comprises several metal layers, is expediently applied in a total thickness of 10-30 μm. Preferred is a thickness of 16-24 microns, more preferably a thickness of about 20 microns. The outermost chromium layer of the primer coating is preferably very thin over the entire primer coating and has a thickness of 0.2-1.0 microns, preferably a thickness of 0.3-0.7 microns, more preferably a thickness of about 0.5 microns on. Preferably, a first nickel layer has a thickness of about 1-3 microns, more preferably about 2 microns. If a copper layer is provided over the first nickel layer, this preferably has a thickness of approximately 6-8 μm. A second nickel layer preferably has a thickness of about 8-12 μm.

In a second stage b) of the process according to the invention, all parts which are to be provided with a PTFE coating are subjected to a temperature treatment at a temperature of more than 300 ° C. for at least 10 minutes. This temperature treatment is applied both to parts whose basic material is structural steel and which are provided with a primer coating according to step a), as well as to parts whose base material is stainless steel and which have not been provided with a primer coating according to step a). The temperature treatment according to the invention serves to clean the parts to be coated with PTFE, in particular the removal of fat residues, which are from the manufacturing process of the rail elements and the fastening device on the parts, as well as fat residues, as for example be left on the material by fingerprints. The conditions of the temperature treatment are to be selected so that the surfaces of the parts after treatment are substantially completely free of grease. This treatment step has proven to be essential, as even small residues of grease or other contaminants on the surfaces of the parts to be coated have proved to be devastating to the adhesion of the later applied PTFE coating. The removal of fat residues and other impurities by the temperature treatment according to the invention also provided a significantly better adhesion of the later applied PTFE coating than a comparison carried out chemical degreasing of the parts. In addition, a thermal treatment compared to a chemical degreasing is cheaper and requires no polluting and disposal-intensive solvents. Advantageously, the temperature treatment according to the invention in step b) at a temperature in the range of 300 ° C to 600 ° C, preferably in the range of 350 ° C to 450 ° C, more preferably in the range of about 400 ° C. The duration of the temperature treatment will depend on the treatment temperature set in each case. The person skilled in the art will be able to adjust the temperature and duration of treatment as a function of the result achieved, ie the degree of purification achieved or the degree of degreasing achieved. Particularly suitable has been a temperature treatment for a period of 10 minutes to 3 hours, preferably for a period of 30 minutes to 2 hours, more preferably for a period of about 1 hour in the specified temperature range proved.

A further process step c) essential for the process according to the invention, which follows the heat treatment of step b), is the roughening of the surface in such a way that the surface has a roughness (average roughness) R _a of at least 2 μm. If the surface roughness R _{a is} less than 2 μm, the adhesion of the subsequently applied PTFE coating is insufficient. A roughness of the surface R _a is preferably from 2 μm to 5 μm, particularly preferably a roughness R _a from 2.5 μm to 3.5 μm. Particularly advantageous for the adhesion of the PTFE coating, a surface roughness R _a of about 3 microns has been found.

Most preferably, the aforementioned roughness is provided by subjecting the parts in step c) to surface treatment by sand blasting. As a result, a particularly good adhesion of the later applied PTFE coating is achieved. It is believed that the particularly good adhesion of the PTFE coating when roughening the surface by sandblasting is achieved in that the roughening of the surface during sandblasting is punctiform and without orientation in a certain direction in the plane of the surface. In the comparative experiment, a corresponding PTFE coating was applied to a ground material with comparable surface roughness. In the wear test, however, the adhesion of the PTFE coating to the ground material was inferior. The PTFE coating lifted from the surface in some places and burst. It is believed that this is due to it is that when grinding, especially when using commercially available pre-ground material, grinding marks in an orientation on the surface are the reason for the poorer adhesion. In an alternative embodiment for surface treatment by sandblasting with sand, the irradiation can also be carried out with glass beads or similar particles. When roughening the surface of the primer coating from step a), ie the surface of the outermost chromium layer, by sandblasting, the conditions of the treatment should be such that the chromium layer is not damaged in such a way that the underlying layer, such as the nickel layer, is exposed becomes. A partial removal of the chromium layer would reduce its corrosion protection effect. In addition, it has been found that the subsequent PTFE coating with exposed areas of the nickel or copper layers underlying the chromium layer can undergo reactions which result in undesirable staining in the PTFE coating. In addition, partial removal of the chromium layer affects the adhesion of the PTFE in these areas.

In a next process step d), one or more layers of a PTFE-containing solution or suspension are applied to the parts obtained according to step c). Preferably, one or two layers of a PTFE-containing solution or suspension are successively wet-sprayed. It is expedient to spray on the PTFE-containing solution or suspension by means of an electrostatic spray gun. Suitable commercially available PTFE-containing solutions or suspensions are from Industrielack AG (ILAG), Lachen am See, Switzerland, under the name ILAFLON Resist R-GI (Base Coat) and ILAFLON R (Top Coat) as a two-coat system and under the name DURIT Resist 3Gi available as a three-layer system. The PTFE-containing solutions or suspensions are suitably applied in a layer thickness of about 75 μm, based on the moist solution or suspension.

Subsequently, the parts obtained according to step d) are fired in a further stage e) at a temperature between 300 ° C and 450 ° C. Appropriately, the firing takes place in a continuous furnace, wherein the method is not limited thereto. Particularly advantageous is a firing temperature between 340 ° C and 400 ° C, preferably between 350 ° C and 380 ° C, proved. Surprisingly, particularly good results were achieved at a stoving temperature of about 360 ° C. Surprisingly, because a baking temperature of about 380 ° C to 400 ° C is recommended by the processors of PTFE coating systems in order to achieve particularly good anti-adhesion properties for the PTFE coating. However, these baking temperatures can lead to undesired optical effects in the inventive method, such as color irregularities and staining. At the most preferred baking temperature according to the present invention of about 360 ° C, these optical disturbances did not occur, surprisingly similar or equally good properties of the PTFE coating in terms of their non-stick properties and even better adhesion on the ground than at the higher temperatures.

It is particularly preferred if the PTFE-based coating after baking according to stage e) has a layer thickness of 10-40 μm, preferably a layer thickness of 15-35 μm, particularly preferably a layer thickness of about 25 μm. When the layer thickness is too low, the wear resistance in continuous operation is low, and there is a risk that abrasion of the PTFE coating leads to the exposure of the underlying substrate. Excessive layer thickness of the PTFE coating causes problems with the adhesion of the coating to the substrate.

Definitions and methods Roughness R _a

The surface roughness given in connection with this invention refers to the average roughness R _a [μm] according to DIN 4768. The average roughness value R _a is the arithmetic mean of the absolute amounts of the distances y of the roughness profile from the middle line within a measuring path. The roughness measurement is carried out with electric stylus devices according to DIN 4772. For the measurement of the average roughness R _a , the measurement conditions are defined according to DIN 4768 T1.

corrosion resistance

The corrosion resistance of the telescopic extension systems according to the invention was determined in the salt spray test according to DIN 50 021-SS (1975). This standard specifies the requirements for corrosion testing devices and corrosive agents that must be met in the salt spray test. In the salt spray test, a sodium chloride solution adjusted to a pH value of 6.5-7.2 with a concentration of 50 ± 5 g sodium chloride per liter in a closed chamber at 35 ± 2 ° C is continuously forced through a nozzle to the one under test Test piece sprayed. After a certain period of treatment, the tested parts are cleaned and evaluated by corrosion products.

The evaluation of the corrosion resistance was carried out in the present investigations by optical evaluation of the treated surfaces. If no corrosion of the surface was observed after 48 hours of salt spray test, the corrosion resistance was evaluated as good.

wear

To investigate the wear or abrasion of the PTFE coating, PTFE-coated telescopic pull-out systems, consisting of a furnace side grille with partial pull-out rails, were installed in an oven and an endurance test for 15,000 load changes was carried out. The endurance test was carried out with the following parameters: Load change [number] Temperature [° C] Test load [kg] Speed [sec./loadchange] miscellaneous 2000 210 10 5 --- 2000 210 10 5 + 0.5 h grill at 280 ° C without movement and load 1000 250 10 5 + 0.5 h grill at 280 ° C without movement and load 5000 200 3 5 + 0.5 h grill at 280 ° without movement and load

In the test, after 0, 2,000, 5,000 and 10,000 load changes, the motive force over the entire extension was determined by means of an Erichsen force gauge and the lowering of the extended baking sheet at a test load of 10 kg (in the middle). Furthermore, the abrasion of the PTFE coating, the running behavior of the telescopic rails and their side play were determined.

adhesiveness

The adhesive strength of the PTFE coating was examined in the cross-cut test according to DIN EN ISO 2409 (1994). In this test, a standard blade cutter is passed over the PTFE coating under specified conditions. For the present tests of adhesion, an "Elcometer 107 Cross Hatch Cutter" with 6 blades was used. The cutting guide is repeated at an angle of 90 ° to the previous cutting test, so that the cuts made in the surface by the blades form a grid. Subsequently, a standardized transparent self-adhesive tape with a bond strength of 10 ± 1 N per 25 mm wide adhered to the surface and peeled off. The cut edges are then examined for spalling of the coating. The rating of the test results is in cross hatch values of 0 to 5, the cross hatch value of 0 means that no chipping was detected.

Telescopic drawer systems coated with PTFE with a cross-cut characteristic value of "2" would still be well suited for practical applications. In the case of the PTFE coating applied according to the invention, however, the cross-cut characteristic value of "1" was generally not exceeded.

layer thickness

The layer thickness of the PTFE coating on parts with stainless steel as the base material was determined by the eddy current method with a conventional film thickness gauge (e.g., Fischer MP40). For parts with structural steel as base material, the layer thickness was measured inductively. In the inductive measurement, the result is the layer thickness up to the first magnetic layer, which in the case of a layer sequence Ni / Cu / Ni / Cr / PTFE is the first nickel layer seen from the PTFE coating. The result thus comprises the thicknesses of the PTFE coating and the thin chromium layer, the latter being deducted for determining the PTFE layer thickness.

Examples

In comparison tests attachment grid with welded inner rails (mild steel) and outer rail elements (stainless steel) were provided by the process according to the invention with a PTFE coating. For comparison purposes, corresponding tests were carried out with deviating process steps. The process conditions and test results are given in Table 1 below.

Figur 1

zeigt in verschiedenen Ansichten ein Befestigungsgitter mit daran angeschweißten Innenschienenelementen für den Einbau in einen Backofen.

Figur 2

zeigt in verschiedenen Ansichten ein Außenschienenelement einer Teleskopschiene.

The following abbreviations are used in the table: BG + IS = Mounting grid + inner rail (welded) AS = Outer rail element BS St 37-2 = Structural steel of quality St 37-2 ES 1.4016 = Quality 1.4016 stainless steel ES 1.4301 = Quality 1.4301 stainless steel iO = okay (= practical) nok = not in order (= not suitable for practical use)

FIG. 1

shows in various views a mounting grid with it welded inner rail elements for installation in an oven.

FIG. 2

shows in different views an outer rail element of a telescopic rail.

FIG. 1 shows a fastening grid 1 with inner rail elements 2 welded thereto. The fastening grid 1 is suspended by means of hooks 3 on the inner side wall of a cooking oven. The basic material of the attachment grid is structural steel, which has been coated with PTFE by the process according to the invention. After coating with PTFE, the inner rail elements are combined with ball bearings and an outer rail and optionally one or more middle rails to form a telescopic rail. In operation, a food support is placed on top of the outer rail (not shown). The mounting grid 1 further comprises horizontally extending pairs of bars 4, in which additional food support, such as baking grates or baking trays, can be inserted. FIG. 1 shows in the left-hand illustration the fastening grid with inner rail elements from the side and in the right-hand representation in FIG. 1 with a view from the rear.

FIG. 2 shows an outer rail element 5 from the side (upper illustration) and from the rear (lower illustration). On the upper side of the outer rail element 5, a protruding pin 6 is provided, which is intended to secure an applied food support against displacement and slipping on the upper side of the outer rail element 5. For this purpose, a corresponding opening for the insertion of the pin 6 is provided on the food support. Furthermore, the outer rail element 5 at the end, which faces the Garofenrückseite in the installed state, a straightened tab 7. The straightened tab 7 prevents an applied on the outer rail element Gargutträger strikes against the rear wall of the oven when inserted into the oven. The outer rail element 5 is made of stainless steel as a base material and coated with PTFE according to the method of the invention.

Claims (16)

1. A process for the production of a telescopic extension system comprising telescopic rails, preferably a telescopic extension system for cooking material carriers in a cooking oven, wherein the base material of the telescopic extension system is structural steel and/or high-grade steel and which is at least partially provided with a coating based on PTFE (polytetrafluoroethylene), wherein

   a) a single-layer or mlti-layer priming coating whose outermost layer is a chromium layer is applied at least to parts whose base material is structural steel,

   b) all parts which are to be provided with a PTFE coating are subjected to a temperature treatment at a temperature above 300°C for at least 10 minutes.

   c) the parts obtained in accordance with step b) are subjected to a surface treatment in which the surface acquires a roughness (mean roughness value) R_a of at least 2 µm,

   d) one or more layers of a PTFE-containing solution or suspension is applied to the parts obtained in accordance with step c), and

   e) the parts obtained in accordance with step d) are calcined at a temperature of between 300°C and 450°C.
2. A process according to claim 1 characterised in that each telescopic rail has an inner rail and an outer rail (partial extension) or an inner rail, an outer rail and one or more central rails (full extension).
3. A process according to one of the preceding claims characterised in that the telescopic extension system has a fixing arrangement, preferably a fixing grid, to which the telescopic rails are fixedly or dismantleability secured and which is designed for mounting to a body wall, preferably for mounting to a lateral inside wall of a cooking oven.
4. A process according to one of claims 2 and 3 characterised in that the base material of the inner rails of the telescopic rails and/or the fixing arrangement is steel, preferably structural steel, particularly preferably structural steel of the quality ST 37 or ST 37-2.
5. A process according to one of claims 2 to 4 characterised in that the base material of the outer rails and/or the central rails of the telescopic rails is high-grade steel, preferably ferritic high-grade steel or austenitic high-grade steel, particularly preferably ferritic high-grade steel of the quality 1.4016 or austenitic high-grade steel of the quality 1.4301.
6. A process according to one of the preceding claims characterised in that the priming coating of step a) between the base material and the chromium layer has at least one or more and preferably three further metal layers.
7. A process according to one of the preceding claims characterised in that the priming coating of step a) between the base material and the chromium layer, starting from the base material, has a first nickel layer, a copper layer and a second nickel layer.
8. A process according to one of the preceding claims characterised in that the priming coating of step a) is applied galvanically.
9. A process according to one of the preceding claims characterised in that the priming coating of step a) is of a thickness of 10 to 30 µm, preferably a thickness of 16 to 24 µm, particularly preferably a thickness of about 20 µm, wherein the chromium layer is of a thickness of 0.2 to 1.0 µm, preferably a thickness of 0.3 to 0.7 µm, particularly preferably a thickness of about 0.5 µm.
10. A process according to one of the preceding claims characterised in that the temperature treatment in step b) is carried out at a temperature in the range of 300°C to 600°C, preferably in the range of 350°C to 450°C, particularly preferably in the region of about 400°C.
11. A process according to one of the preceding claims characterised in that the temperature treatment in step b) is carried out for a duration of 10 minutes to 3 hours, preferably for a duration of 30 minutes to 2 hours, particularly preferably for a duration of about 1 hour.
12. A process according to one of the preceding claims characterised in that the parts in step c) are subjected to a surface treatment in which the surface acquires a roughness Ra of 2 µm to 5 µm, preferably a roughness Ra of 2.5 µm to 3.5 µm, particularly preferably a roughness Ra of about 3 µm.
13. A process according to one of the preceding claims characterised in that the parts in step c) are subjected to a surface treatment by sand blasting.
14. A process according to one of the preceding claims characterised in that the parts obtained in accordance with step d) are calcined at a temperature of between 340°C and 400°C, preferably between 350°C and 380°C, particularly preferably at about 360°C.
15. A process according to one of the preceding claims characterised in that the PTFE-based coating after the calcining operation in accordance with step e) is of a layer thickness of 10 to 40 µm, preferably a layer thickness of 15 to 35 µm, particularly preferably a layer thickness of about 25 µm.
16. A telescopic extension system comprising telescopic rails, preferably a telescopic extension system for cooking material carriers in a cooking oven, produced according to one of claims 1 to 15.

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