Method and apparatus for surface coating and product manufactured by means of the method

TECHNICAL FIELD: The invention relates to a method and a device for coating of objects, especially objects with an electrically non- conductive surface. The invention also comprises products, which are constituted of objects with an electrically non- conductive surface coated by the method according to the invention.

BACKGROUND OF THE INVENTION:

It is previously known to coat the surface of objects with a layer of plastic which is applied as a dry powder with the intended layer thickness and which is thereafter cured or melted to a homogenous layer through heating. The heating is normally performed in a furnace and a common temperature in this context is about 170°C The application of the powder is done by spraying of the same against the object. It is important that the powder sticks with the intended layer thickness on the surface of the object and is retained there during the subsequent heat treatment process and until the homogenous layer is formed and is thereby anchored to the surface of the object.

In order to provide for this retention of the powder electrostatic forces are utilised. The powder is charged with an electrostatic charge and the object is given a neutral state by earthing it. The charged powder is thereby attracted by the neutral surface and is stuck with sufficient force and for such a duration of time that the final anchoring by heat treatment can be performed. In order to charge the powder during the spraying it is known to either use electrical installations, which give the

powder the charge, or to frictionally charge the powder in the gun by letting it flow against a surface of such a material in relation to the material of the powder that a charge arises.

Since earthing of the object is a prerequisite for making it possible for the process with electrostatical retention of the powder to occur, the powder coating method can only be used for objects with a conductive surface. Therefore, the method has gained its major use for metal objects. In order to also be able to use powder coating for objects of non-conductive materials, the surface has been made conductive through application of a conductive varnish. However, this is not a completely satisfactory additional method. The final retention of the layer on the object is determined by the affinity of the conductive varnish to the surface and therefore one becomes dependent on finding varnishes which adhere securely to the material which is to be powder-coated.

The method is therefore not applicable for glass since until now no success has been attained in producing any paint which has satisfactory adhesion to a glass surface. On the other hand, a good adhesion of a layer of plastics applied by the powder coating method can be obtained if it is successfully accomplished. Since the surface of the glass in uncoated condition is non-conductive, said electrostatical method cannot be applied for the initial retention of the powder. In spite of this, however, coating of glass has been performed by the powder coating method, and a known method is thereby to deposit a layer of condensed water on the glass surface, which binds the powder to the same until the heat treatment has been performed. However, this method has not proved to be especially reliable and requires in any case very carefully

controlled environmental conditions during the process in terms of hygroscopic conditions and temperature differences. The method has therefore not gained any wide application worth mentioning.

However, there are other methods for changing the appearance of glass objects which have long been applied. Thus, the colour of the glass can be adapted through use of coloured glass mass. This is, however, a costly method partly because the dyestuffs in many cases demand a high price and maybe even more because at each production in one colour a batch of the coloured glass mass must be prepared and melted, due to which change between different colours takes a long time and becomes costly and must be done with high energy consumption. Another method of colouring or decorating glass is to apply a layer of glass powder or ceramic powder mixed with a paste adherent to the glass. After this, the glass is heated, causing the powder to melt on to the surface of the glass and become unified with the glass material. In connection with this the attachment paste is eliminated. However, this method requires heating to a high temperature, close to 1000°, which leads to great energy sacrifices and also to risks of deformation of the glass object. After having been burned on, the glass must, as with all heating, be subjected to a specific cooling process, which further makes the method time-consuming.

Further methods may be mentioned such as etching and glass blasting, which are used in order to change the appearance of glass objects. These methods are often used for glass which is not to be transparent, so-called opal glass. These methods are also complicated and have an undesirable environmental effect. When etching, environmentally dangerous acids must be used and when sandblasting the

working environment is very unfavourable because of the blast particles that whirl around.

As can be seen from the above, the unsatisfactory state of affairs exists, namely that objects with a non-conductive surface only in exceptional cases and with difficulty may be coated through powder coating. With glass material and also ceramic material, therefore, other and generally more costly and energy-consuming methods must be used when colour-adapting and decorating in order to obtain a satisfactory result.

Other non-conductive materials also involve difficulties and additional costs to coat. Unlike glass, and to some extent also ceramics, painting with drying wet paint may indeed give a satisfactory result, but if powder coating could be used a considerable quality improvement would be achieved in many cases. Powder coating may in fact be performed with a number of different plastic materials which can be adapted to the prevailing conditions of use regarding mechanical strength, elasticity and chemical resistance against environmental influence. The layer thickness can also be adapted in a better way than with wet paint, which in the case of thicker layers must be applied in several rounds. These favourable properties of the powder coating as a coating method have been shown in a convincing way for use on conductive material, where the said electrostatic spraying method may be used. All sorts of tools and other utility articles, as well as steel constructions, are nowadays coated by powder coating and are provided with considerably better resistance in handling and against environmental influence. Steel objects obtain a corrosion protection which is far better than all coating methods so far. The same good effects could therefore be obtained for objects of non-conductive

materials if the problem with the retention of the powder at the surface during the heat treatment process could be solved. It would be especially valuable if the powder coating method could thereby be applied for the very much used material glass where wet varnishing, as mentioned, is practically out of the question and one is therefore reduced in industrial operation to irrational, traditional methods.

The object of the invention is to solve said problems and to provide a reliable method for powder coating of objects with a non-conductive surface and also to provide a device for effecting the method.

SUMMARY OF THE INVENTION:

In the invention, as in the known method, application of the powder through spraying and subsequent heating to the curing and melting temperature, respectively, of the powder is used. Likewise, electrostatic forces are used in order to attach the powder to the object before heat treatment has been performed. Thus, during spraying the powder is emitted in a charged condition, preferably through the use of a spray gun for frictional charging. Additionally, the object is charged electrostatically with a polarity opposite to the one which the powder will obtain in connection with the application. The charging of the object occurs by surrounding air from an electrode supplied from a high voltage cascade. According to the invention, the object is separated from charge influence before the powder is applied.

When the powder is applied, it will seek and deposit itself on the surfaces of the object and may thereafter be permanently attached by heat treatment.

BRIEF DESCRIPTION OF THE DRAWINGS:

In the attached drawings the device according to the invention is illustrated in two embodiments. Based on the drawings the method and the device according to the invention will be described in the following. In the drawings,

Fig.l is a side view of a construction corresponding to the device of the invention in the first embodiment; Fig.2 in a partly sectional side view, is a construction forming the device in the second embodiment; and Fig.3 is the construction according to Fig.2 in a plan view.

PREFERRED EMBODIMENTS:

The construction according to Fig.l is shown set up for powder coating of glass bottles. The glass bottles are denoted by reference numeral 1 and two bottles are shown during passage through the construction, each placed in one of three work stations of the construction. These are a charging station 2, a powder coating station 3 and a heating station (not shown) . A conveyor belt 5 passes through the stations, which belt at an entrance end before the station 2 has a station (not shown) for placing of untreated bottles on the conveyor belt and after the heating station a station (also not shown) is provided for removal of the now coated bottles. The conveyor belt 5 is provided at regular intervals with a holding arrangement 6 for the bottles. This consists of a holder 7 for the opening of the bottles and a holder 8 for their bottom. When passing through the construction the bottles are, as shown, lying, suspended between the holders 7 and 8. The bottle is held at its opening by a cup 9, which partly

encloses the opening region, and at the bottom by a support 10 pressed into the recess of the bottom. The cup 9 and the support 10 are suitably made of a material which cannot be electrostatically charged together with the glass, whereby deposition of powder on these parts can be avoided. The holders 7 and 8 can pivot somewhat from each other in order to enable insertion and removal of the bottles.

The charging station denoted with reference numeral 2 is provided with a high voltage electrode 14. This consists of a rail, made of conductive material, inserted in an insulating holder 15, from which rail a number of tips 16 made of conductive material project. These tips are surrounded by reflectors in the rail . During charging of the objects the rail, and thereby the tips, are connected to an electrical installation 17, which emits a high voltage electrical current with a polarity that is opposite to the polarity obtained by the charged powder. The electrode is driven by direct current and has thus unchanged polarity during operation. The operation voltage may suitably be in the order of 20-40 kV.

The powder coating station 3 is placed at such a distance from the charging station 2 that the electrical field formed by the electrode 15 is considerably weakened. A spraying unit 18 for the powder is placed in the station. Here, it is assumed that the powder is charged by friction.

The unit is thereby designed with a chamber in which the powder is introduced through feeding by pressurised air. The powder and the pressurised air are thereby supplied via a hose 19. In the chamber the powder is whirled around and rubbed against the walls of the chamber. These consist of a material which, when rubbed against the material of the

powder, provides the powder with an electrostatic charge. With powders of commonly used types of plastics, such as epoxy plastic, polyethylene and amide plastics, the walls of the chamber are constituted by tetrafluoro-ethylene. The chamber suitably has a complicated shape; it can for example be spiral-formed, in order to achieve the desired rubbing effect. After having passed through the chamber, the powder is carried by the pressurised air passing out through the opening 20 of the unit. This is directed towards the object which is to be coated, i.e. the bottle 1 in the station 3. Since the powder has been charged in the gun, in the shown embodiment with a positive charge which is opposite to the charge which the material of the bottle has received by the electrode 14, the powder seeks the surface of the bottle and is stuck there. This effect is so strong that surfaces which are directed away from the opening of the spraying unit also receive a coating.

For objects with a complicated shape, for example with recesses and notches on several sides, it may, however, be necessary to use several spraying units or to rotate the object.

During the continuing movement of the conveyor belt, the powder-coated bottle enters the heating station. This has the shape of a furnace with heating elements. The heating method, that is if convection through heated air or radiation heating is used, does not have any major importance. However, the air velocity in the furnace must not be so high that the particles are displaced on the object.

In the embodiment according to Figs. 2 and 3, it is presumed that glass urns 30 are to be powder-coated on the inside but not on the outside. The powder coating, if the

glass is transparent, becomes visible from the outside and thereby obtains particular lustre, while the excellent properties of the glass when it comes to resistance against scratching and chemical influence are used for the outer surface, which often is the one most exposed to such influence. The construction here includes a round feeding table 31, which is attached to a vertical axis 32 and is driven by a motor 33. The table includes on its underside a number of positions 34 for the glass object 30. The positions consist of a central portion 35 with a circle of holes 36 therearound. A vacuum-activatable suction cup 37 is provided in the central portion 35 for holding of respective glass urns 30 with the opening downwards.

The round feeding table 31 can move the glass objects between three working stations: a formation and removal station 38, a treatment station 39 and a heating station 40. In the station 38 finished objects are removed and those that are to be treated are positioned. This may be done manually or, for example, with a robot. In the treatment station a treatment cycle is performed including charging of the object and thereafter powder application. The charging is performed in a similar way, as earlier described, i.e. by a direct current fed electrode 42. This is shown in Fig.3, bent in order to fit the curvature of the round feeding table. For the application of the powder, a spraying unit 43 of the type earlier described is located under the object.

An ionization device 44 is also provided which is located above the feeding table. It consists of an alternating current fed ionizer. Such an ionizer gives a mixture of negatively and positively ionizised particles, in this case air molecules. Air is namely drawn through the ionizer by a blower 46 which flushes the ionized air down through the

holes 36, so it sweeps around the outer surface of the glass urn 30. Finally, station 40 is a furnace 48 for heating of the glass objects to the curing or melting temperature of the plastic powder. The necessary temperature is in most cases 150-200°C.

The round feeding table moves intermittently. Its movement cycle is thereby determined by the fact that the following steps are to be accomplished in the treatment station 39:

1. Charging of the object, i.e. the glass urn 30 by the electrode 32, so the object obtains a charge opposite to the one which the powder will obtain in the spraying unit 43. The installation which supplies the electrode with high voltage direct current, may operate in front of the glass object until the object obtains the required charge in order to attract the powder.

2. The cascade installation is turned off so that the electrode becomes dead.

3. Spraying units, which have been turned off during the charging, are started and direct a jet of powder dyestuff particles against the interior of the glass urn. At the same time the cascade installation 44, which has been turned off during step (1), is in operation and sends an ionized airstream along the exterior of the glass urn. This neutralises the charge which the object has obtained on its outside by means of the electrode.

The result is that the plastic powder is deposited on the interior surface of the glass urn, which still carries the opposite charge, but it does not stick to a neutralising exterior surface. The powder that does not stick falls down and may be reused. The glass urn has now received an inner coating of a layer of plastic powder.

The object provided with the inner layer of plastic powder is now brought by means of rotation of the round-feeding table into the furnace 48, whereby the powder is altered to a homogenous layer with a strong anchoring to the interior glass surface.

In addition to the round-feeding table 31, and as is evident from the preceding description, the cascade installation, the spraying unit 43 and the neutralising ionizer 4 with its blower 46 must also work intermittently in a certain cycle. In order to achieve this a time controlling unit 50 is arranged and connected to said units.

Neutralisation on certain surfaces, which in the last described case is achieved by the device 44, is of course not mandatory. Whether such a neutralisation is to occur depends on whether all surfaces of the object are to be powder-coated or the main portion of these surfaces or if, as in the described case, only the inside is to be coated. Alternatively, coating of the outside of hollow objects may be performed in an analogous way. One can also demarcate portions on objects of another nature than hollow through using a neutralising air stream which can be directed in different ways.

The purpose of the description of the two embodiments is primarily to provide an understanding of how an important

principle of the method according to the invention may be applied. This principle is that the object that is to be coated on its non-conductive surface is given a strong electrostatic charge opposite to that which the powder will receive. This is achieved by subjecting the object to an electrostatic field with the polarity in question and with a strength and influence time adapted to the charge strength which is required in order to retain the powder particles at the surface of the object.

The charging process is performed with the surrounding air as a transport medium, which to some extent transmits the charge from the high-voltage electrode to the object, whereby the molecules of the air are also charged. However, the charging process must be separated from the operation in which the charged powder is sprayed on. The reason for this is that if the air surrounding the object is charged with opposite polarity to the polarity of the powder, the powder will at least to a considerable extent be neutralised. Only if the object is separated from the ionized air can a satisfactory result be obtained with powder application. The separation may be achieved either by moving the object from the field of charge and coating it with powder at a distance from this, possibly through the use of a dividing wall or by using a time cycle in which the ionization is interrupted a certain time before the application of the powder. This time must be long enough to ensure that the air surrounding the object loses most of its charge. This time may be shortened by making the air flow from the object and replacing it with neutral air. The first method, separation in the room between charging and powder application, is applied in the first embodiment and the second method, dividing in time between charging and powder application, is applied in the second embodiment.

Earlier attempts have been made to use a charging unit in order to achieve, when charging, a charge opposite to the one the powder has. It has then been thought that a charging unit on the opposite side of the object in relation to the spraying unit should be used. The objective is thus that the powder seeks in the direction of the electrode and thereby collides with the object. This may function well with wet dyestuffs which, in their movement towards the ionization source pass the object and thereby stick to its surface. With powder, on the other hand, there is not the same type of attraction between the object and the particles as when these are in liquid state. Even if the object is hit with the powder particles by giving them a certain direction by means of an ionization source, they will not be retained on the surface of the object by any electrostatic forces.

The method described here thereby offers unique possibilities to also apply the advantages of the coating method to objects with a non-conductive surface. As mentioned, the method has an important application for glass objects. In connection with this it may be mentioned that by powder coating of glass one does not only achieve the possibility to give it a different colour from the glass mass. The glass can also be made matt or opalescent by choosing appropriate powder qualities. Some effects may be obtained with hollow objects where the inner surface is coated in one colour and the outer surface is given a matt or shiny opalescent layer.

Materials other than glass are also interesting of course for the application of the method. It may be for example, that one wishes to powder-coat plastic objects or wooden objects. Until now one has thereby been obliged to apply a pre-varnish with a conductive varnish, thus a two-step

operation. Moreover, the adhesion to the surface can never be better than that of the conductive varnish.

Ceramics is also a material which may be coated by the method. As with glass, a side effect in addition to the decorative effect is thereby obtained through the powder coating. The strong powder layer gives a cohesive action, which gives better durability and, if the object should break, to some extent keeps the fragments together, which reduces the risk of physical damage and the spreading of fragments.

In order to obtain a good result, it is important that such a charging of the object occurs that a so-called structural charge is formed. This means that the atoms of the material in the whole mass of the object or at least to a considerable depth are electrostatically charged and not only superficially located atoms. Such a structural charge is obtained partly by using high voltages and partly by adapting the charging time to be sufficiently long in relation to the mass of the object.

Examples are given in the following of the choice of data for a treatment process for certain objects.

Example

Object: A glass urn with a diameter of 150 mm and a height of 250 mm and a glass thickness of 4 mm. To be powder-coated on its inside.

Powder material: A powder with grain size 0.3 consisting of epoxy resin with 2% pigment of metal oxides. The cascade electrode: Supplied with 40 kV direct current. Charging time: 5 seconds.

Neutralisation ionization: Ionizer is driven with 8 kV alternating current. Blower with capacity to drive an airflow with 0.5 m/s over the surface of the object. Spraying unit: Spraygun with frictional charging of the powder.

The process is driven in one cycle with, firstly, structural charging by the high-voltage electrode, thereafter an interruption of the charging, spraying of the object during operation of the neutralisation ionizer and, finally, heat treatment at 170°C.