EP1767668A1

EP1767668A1 - Method for enamelling cast parts of standard hypoeutectic gray iron

Info

Publication number: EP1767668A1
Application number: EP05108791A
Authority: EP
Inventors: Candido Dal Mas
Original assignee: Cosma SpA
Current assignee: Cosma SpA
Priority date: 2005-09-23
Filing date: 2005-09-23
Publication date: 2007-03-28

Abstract

The present invention refers to a method for enamelling cast parts made of standard hypoeutectic gray iron, comprising following steps:
- preparing a cast part of standard hypoeutectic gray iron;
- pre-treating said cast part in order to promote mechanical bond and adherence of the enamel substances to the surface of the part to be enamelled;
- applying porcelain-enamel powders onto the thus pre-treated part by dry electrostatic spray technique;
- vitrifying the enamel powders by exposing the part to a heating and cooling sequence occurring according to such a heating and cooling pattern versus time as to cause the part to dwell at a temperature between 700°C and 780°C for 480 seconds ± 5%.

Description

The present invention refers to a method for enamelling cast parts of standard hypoeutectic gray iron.
Enamelling is one of the most effective systems available today for coating metal surfaces in view of protecting them against corrosion induced by chemical and weather agents, making them impermeable and, of course, more attractive and appealing aesthetically.
Normally used enamels, in particular the so-called porcelain enamels, are generally constituted by alkaline-earth, alkaline and aluminium silicates mixed with borates, fluorides and fluoborates, as well as substances having an opacifying function (antimony and stannous oxides, cryolite, feldspar, calcium phosphate) and a dyeing or colouring function (iron, chromium, titanium, copper, cobalt, manganese and similar oxides).
Currently, for enamelling cast-iron grates used as pan supports on cooking surfaces, i.e. cooktops, use is made of a method generally referred to as a "wet process".
This process consists in mixing enamel powders together with a liquid medium, usually water, which acts as a carrier for the same enamel powders, and applying the thus obtained suspension, generally termed "slurry" in the technical parlance, to a pan-support grate, which will have been previously pickled mechanically, by spraying it under pressure thereonto.
Thereafter, the pan-support grate is sent through a drying oven, in which the water is caused to evaporate from the thus applied liquid enamel film. Subsequently, the grate is fired in a so-called vitrification oven at a temperature of up to 780°C. In this oven, the grate is caused to undergo heating and cooling cycles occurring according to such a heating and cooling pattern versus time as to practically cause the grate to constantly dwell at a temperature above 700°C for approximately 10 minutes.
This "wet" enamelling process has however a number of inherent drawbacks.
One of these drawbacks arises from the need for the enamel coating sprayed onto the pan-support grate to undergo a drying process aimed at causing the water to evaporate from the enamel film before the coating itself is fired for vitrification. This of course contributes to further increase the energy usage and cost of the entire enamelling and vitrification process.
Another drawback derives from the environmental pollution effects connected with the enamel "slurry", as determined by the use of the liquid carrier medium (water) for the enamel powders, as well as the spray application of the resulting suspension, wherein it should be further emphasized that the enamel "slurry" contains also the colouring substances used to give the enamel a desired colour.
The quite high costs for the disposal of the "slurry" in special dumps, upon the required preliminary chemical analyses having been duly completed, contribute to making the enamelling process particularly expensive and rather complicated to manage.
A further drawback derives from the considerable space requirements of the equipment used to mix the enamel powders with the liquid carrier medium (water), to spray the resulting suspension under pressure onto the articles to be coated, and to collect and dispose the resulting wastes.
A still further drawback lies in the fact that producing each single enamelled pan-support grate turns out to be a particularly labour-intensive process owing to the number and complexity of the operations being required.
It therefore is the object of the present invention to provide a method for enamelling cast-iron parts, which does away with the afore-indicated drawbacks and disadvantages of prior-art approaches.
Within this general object, it is a purpose of the present invention to provide a method that is simple to carry out and ensures a vitreous-enamel coating on cast-iron parts having characteristics in terms of surface-finish, colour, adhesion, adherence, resistance to mechanical and thermal shocks that are at least comparable with or even superior to the ones ensured by prior-art enamelling processes.
Another purpose of the present invention is to provide a method that ensures a maximum extent of reliability and safety.
According to the present invention, these aims, along with further ones that will be apparent from the following description, are reached in a method for enamelling cast parts of standard hypoeutectic gray iron, as well as cast parts finished by this method, incorporating the features and characteristics as recited in the appended claims 1 et seq.
Features and advantages of the present invention will anyway be more readily understood from the description of a preferred, although not sole embodiment that is given below by way of non-limiting example.
The method for enamelling cast-iron parts according to the present invention comprises pre-treating a cast part of standard hypoeutectic gray iron in order to facilitate the enamel substances mechanically binding to the surface of the part to be enamelled, without giving rise to blisters; applying porcelain-enamel powders onto the thus pre-treated part electrostatically; vitrifying the enamel powders by exposing the part to heating and cooling cycles occurring according to such a heating and cooling pattern versus time as to practically cause the part to constantly dwell at a temperature between 700°C and 780°C for 480 seconds ± 5%.
A number of experiments have been carried out in view of developing and eventually providing an enamelling process, which would ensure, as a final result, a vitreous-enamel coating effectively anchored in a continuous and even manner to the surface to be coated of a cast-iron support, such as for instance the pan-support grates of cooking surfaces and ranges.
In the course of the tests performed during the above-cited experiments, the most suitable materials for the process to be able to be carried out satisfactorily have been selected on the basis of a thorough assessment of the physical and chemical characteristics of the vitreous-enamel coatings that were each time obtained.
It has in this way been found that cast parts of standard hypoeutectic gray iron with a carbon content of between 2% and 4.2%, to be used as supports for the enamel coating layer, turn out to be particularly satisfactory, as far as chemical compatibility is concerned, when they are coated with porcelain-enamel powders based on alkaline borosilicates, e.g. of the type of those currently produced by Escol Products Limited.
The method according to the present invention includes a pre-treatment of the surface of the support to be enamelled in view of ensuring good adherence between the cast-iron part to be coated and the porcelain-enamel powders sprayed thereonto.
A pickling operation is provided for in view of removing the residues of casting and moulding sands, rust particles, corrosion products and other contaminants from the surface of the metal parts.
The pre-treatment process comprises a mechanical operation, which is adapted to impart a certain roughness to the surface of the cast-iron support to be coated, so as to favour mechanical adherence of the porcelain-enamel powders to the surface of the support. Various methods are known in the art to be suitable to achieve such result. These include sandblasting, i.e. a process that is used in this case, which consists of blowing appropriate enamel-grade metal grit, i.e. particles of metal capable of being enamelled, against the item to be enamelled.
Porcelain-enamel powders are then applied electrostatically onto the surface of the item to be coated in a "dry" process, i.e. a process that does not make use of any liquid carrier medium. In this phase, use is made of the electrostatic forces that arise between the positively charged powders and the item, i.e. the cast-iron part to be coated, which, duly grounded, is charged negatively. Due to electrostatic attraction, the enamel powders are caused to settle onto the entire surface of the cast-iron support.
According to an embodiment of the inventive method, the enamel powders are applied by means of a plurality of spray guns, which charge the powders at approx. 70-90 kVolt. Other systems known as such in the art may of course be used, too.
The enamel powders are caused to settle onto the item to be coated in such a manner as to form a coating layer having a thickness situated between 70 µm and 200 µm, depending on the specific chemical composition of the porcelain-enamel powders suitable for use in connection with the particular application onto cast parts of standard hypoeutectic gray iron.
The thus coated part is placed, by means of appropriate equipment, in a so-called firing or vitrification oven, the temperature inside which is adjustable by means of an automatic-control system. Thereinside, the coated part is moved through various zones of the oven in which different temperatures prevail, so that it is heated and cooled successively according to a pre-determined heating and cooling pattern versus time.
A number of experiments have been carried out with the aim of adjusting and setting the most appropriate vitrification temperatures in the oven. In particular, these experiments have included varying the heating and cooling rates of the coated part being fired, by changing the forward moving speed of the coated part through the oven, as well as varying the maximum vitrification temperature reached in the oven, while eventually assessing the thereby achieved results on the basis of the characteristics of the enamel having been so vitrified on the support.
The assessment of the enamel quality has been made by comparing the test results with the existing standards concerning surface quality, colour, adhesion, adherence, resistance to mechanical and thermal shocks of enamels applied onto supports according to prior-art techniques.
Through a sequence of successive empiric fine-tuning adjustments, it has been determined that, for vitreous enamel coatings to be obtained with properties that are comparable with or even superior to those of existing, prior-art enamel coatings, the most suitable process parameters call for a highest allowable temperature of 780°C ±8°C to be used for heating the coated part being fired, as well as such a heating and cooling pattern versus time to be followed as to enable the coated part itself to dwell for a period of 480 seconds ±5% at a temperature ranging from 700°C to 780°C.
It will of course be readily appreciated that the above-indicated thermal characteristics of the vitrification process can be also obtained with the use of different types of ovens that are known as such in the art, such as for instance those ovens which operate by having the coated part to be fired standing still inside the oven, while allowing for an extremely quick-acting variation of the heating and cooling temperature, which the same part is exposed to.
Fully apparent from the above description is therefore the ability of the the present invention to effectively reach the afore cited aims and advantages by providing a method for enamelling cast parts of standard hypoeutectic gray iron, which is particularly simple to carry out.
The method according to the present invention is much more advantageous and profitable than prior-art processes from an energy usage standpoint, thanks to its having done away with the operation needed to first dry the enamel coating applied on the part, i.e. the cooktop grate in the case considered, since enamel powders are applied under dry conditions in this case, without using any liquid carrier medium, such as water, to this purpose.
An important advantage of the method according to the present invention derives from the fact that it drastically cuts environmental pollution effects, as well as the costs in connection to waste disposal tasks.
A further advantage of the present invention derives from the fact that it has extremely simplified equipment requirements, not only thanks to the afore-cited elimination of a drying phase of the enamel coating. But also - and above all - thanks to the elimination of the systems needed to collect and recover the polluting effluents that are produced when applying the coating by spraying the enamel under pressure in the traditional enamelling processes.
It shall be appreciated that the method according to the present invention is subject to a number of modifications and may be embodied in a number of different manners without departing from the scope of the present invention as defined in the appended claims. It should further be noticed that the materials used, as well as the sizing thereof, may each time be selected so as to more appropriately meet the particular requirements or suit the particular application.

Claims

Method for enamelling cast parts made of standard hypoeutectic gray iron, characterized in that it comprises following steps:
- preparing a cast part of standard hypoeutectic gray iron;

- pre-treating said cast part in order to promote mechanical bond and adherence of the enamel substances to the surface of the part to be enamelled;

- applying porcelain-enamel powders onto the thus pre-treated part by dry electrostatic spray technique;

- vitrifying the enamel powders by exposing the part to a heating and cooling sequence occurring according to such a heating and cooling pattern versus time as to cause the part to dwell at a temperature between 700°C and 780°C for 480 seconds ± 5%.
Method according to claim 1, characterized in that said standard hypoeutectic gray iron comprises a carbon content of between 2% and 4.2%.
Method according to claim 1 or 2, characterized in that said enamel powders are deposited onto the part to be coated such that a coating layer having a thickness ranging from 70 µm and 200 µm is formed.
Finished cast part obtainable with the method according to any of the preceding claims 1 to 3.
Finished cast part according to claim 4, comprising a cooktop pan-support grate.