Electrostatic powder coating method and apparatus

0001Title: Electrostatic Powder Coating Method and Apparatus

0002Description:

0003The present invention relates to a method and apparatus for electrostatically powder coating products.

0004Powder coating, which utilises electrostatic charging of powder particles, is a well-established process, which is now a preferred method for the painting of the majority of manufactured components. The process is based upon the charging of the coating powder by means of electrostatic corona discharge or frictional triboelectric charging, and the deposition of the charged powder on to an earthed (or grounded) product, due to the forces of electrostatic attraction.

0005The deposited powder is then melted using heat and either allowed to cool and solidify, as is the case with thermo plastic-powders or ceramic glaze, or to solidify and cure by chemical reaction using heat or radiation as is the case with thermo set or UV cured powders.

0006It is accepted practice that the ability to coat a metallic or conductive component satisfactorily and consistently requires the component to be suitably earthed or grounded as non-earthed parts will generally attract little or no charged powder. This is due in part to the fact that a non-earthed part will take on a charge of the same polarity as the charged powder and therefore repel the charged powder instead of attracting it.

0007Most usually the component to be coated is suspended f om an earthed carrier. For production purposes the components pass through a spray chamber suspended from an endless conveyor that is earthed and to which the components are directly earthed. The process may be carried out manually without the use of a conveyor by suspending the product from an earthed hanger or placing it on an earthed support.

0008It is recognised that non-conducting components such as glass or ceramic may be successfully powder coated by placing an earthed item behind the component i.e. on the opposite side of the component to the spray gun or inside the component as in the case of bottles or and other hollow vessels. .

0009The application of powder coating to partially conductive materials, such as wood based products such as medium density fibreboard (MDF), has followed the same principles as that for metal. Thus MDF board is mounted on earthed metal jigs, which are suspended from an earthed overhead conveyor. The process may be aided by attention to the conditioning of the MDF as described in GB Patent 2333725 or by other methods such as the use of conductive or ionic primers, or additives used in the manufacture of the MDF board.

0010Electrostatic powder coating has the advantage that the electrostatically charged particles are attracted to the entire surface of the product irrespective of whether the surface is in the direct line of fire of the application gun. However, there are instances in which selective coating of only selected parts of the component is required. Traditionally this has necessitated masking of parts of the component that are to remain uncoated.

0011A requirement has arisen to be able to powder coat MDF board but to leave one face uncoated. With that in mind it has been proposed to place the MDF board face down onto a conductive conveyor belt with the aim of coating the entire component except for the face that is in contact with the conveyor belt. This method is known for coating metallic components and has been successfully used for the coating of metallic components. However, when tried with MDF it has been found that the bottom edge of the MDF sides do not coat. Originally, this was thought to be caused by the belt taking the powder preferentially over the less conductive MDF, but it is now thought to be caused by charge polarity reversal and repulsion of the powder due to electrostatic capacitive effects.

0012It is an aim of the present invention to solve the above-mentioned problem. It is another aim of the invention to provide a method and apparatus for powder coating that utilises a flat bed conveyor.

0013Accordingly, a first aspect of the present invention provides an electrostatic powder coating apparatus comprising earthing means, a non-conductive substrate overlying the earthing means, and charged powder depositing means, characterised in that when in use, the product to be coated is placed in contact with the non- conductive substrate.

0014Preferably a non-conductive substrate is interposed between the product to be coated and earthing means. Preferably the product is in contact with the non- conductive substrate. Preferably the non-conductive substrate comprises an endless conveyor belt. Preferably it is a flat bed conveyor belt. More particularly the non- conductive substrate is backed up by and in contact with a support surface therefor. The support surface may comprise a metallic earthing means. More preferably the support surface is non-conductive and has earthing means associated therewith. Preferably there are a plurality of discrete earthing means disposed spaced apart over the area of the support means, or at least over the area that is traversed by the product to be coated. Preferably a plurality earthing elements are accommodated in respective apertures of the support surface. Preferably the earthing means are disposed to contact the aforesaid non-conductive substrate. Preferably the upper surface of the earthing means lie flush with the upper surface of the support means.

0015Another preferred embodiment of the invention sees the earthing elements being movable relative to the non-conductive substrate. Preferably, the earthing elements are mechanically movable. Means for mechanically moving the earthing elements may be provided by way of a platen having the earthing elements thereon being arranged to move relative to the non-conducting substrate. The platen may be adapted to oscillate, reciprocate, translate or vibrate relative to the non-conducting substrate. Movement of the platen, where provided, relative to the non-conducting substrate may be accomplished using any suitable actuator, for example, one or more motors, solenoids, electromagnets, hydraulic or pneumatic assemblies.

0016Additionally or alternatively, the earthing effect of the earthing elements may be made to move.

0017A first example of this embodiment of the invention sees the earthing elements being arranged to swivel or orientate in a plurality of directions. Thus, the earthing effect may be moved relative to the non-conducting substrate by continuously swivelling or pointing the earthing elements in different directions. It is anticipated that this would have the effect of moving the electromagnetic field in the vicinity of the earthing elements.

0018A second example of this embodiment of the invention sees earthing elements being individually earthed or electrically- connected to one another in groups. The groups could be interspersed on an imagingary grid or randomly positioned beneath the non-conducting platen. A swithch may be provided between each earthing element or each group of earthing elements. Actuation of the switch or switches in sequence or at random would have the effect of changing the electromagnetic field in the viscinity of the product being coated, and may have the same effect as physically moving the earthing points relative to the non-conducting substrate. Where provided, the switches may be manually or electronically actuated, although a preferred embodiment of the invention sees the switching being computerised.

0019Another aspect of the invention provides electrostatic powder coating apparatus comprising earthing means, a non-conductive substrate overlying the earthing means, and charged powder depositing means, and wherein, in use, the ^' product to be coated is placed in contact with the non-conductive substrate.

0020Preferably the non-conductive substrate is in the form of an endless belt and is made from a non-solid material, i.e. one which is porous/permeable. A woven plastics or apertured plastics material is preferred. The belt is preferably part of a flat bed conveyor system. The substrate is preferably supported by support means, which may be a non-conductive material and accommodating a plurality of earthing elements as aforesaid.

0021The aforesaid described apparatus is particularly suitable for coating flat board, especially the likes of MDF or other wood based board products that are required to have one surface uncoated.

0022It has been found that supporting the MDF board on an earth plate which is significantly smaller than the bottom surface of the MDF results in the top and sides being fully coated with little or no powder wrapped round the underside. Thus where the earth is smaller than the component to be coated and/or is spaced from the edge of the component satisfactory coating of the edges can be achieved. More particularly, it has also been found that placing an insulating dielectric material between the earthed plate and the MDF results in the top and sides being coated with little or no powder wrapped around the underside. Thus it has been established that the MDF does not require a direct earth contact in order to be successfully powder coated.

0023Performing the same tests on conductive metal components demonstrated that these too could be successfully coated without a direct earth connection. A capacitive coupling to an insulated earth proved quite satisfactory. Hence the present invention is not limited to the coating of partially conductive products.

0024Accordingly it has now been found that direct earthing is not required, contrary to the established teaching in the art.

0025The present invention will now be described further hereinafter by way of example only with reference to the accompanying drawings; in which:-

0026Figure 1 is a simplified perspective view from above, one end and one side of powder coating apparatus embodying the present invention,

0027Figure 2 is a perspective view of a support sheet used in the present invention

0028Figure 3 is a fragmentary cross-sectional view taken on I-I of Figure 1.

0029Figures 4 and 4a show a partial cross section and perspective view of a support for the invention having moving earthing points respectively,

0030Figure 5 shows a partial section through an embodiment of the invention having swivelling earthing points, and

0031Figure 6 shows a partial section through an embodiment of the invention having grouped, switched earthing points.

0032The present invention is described by way of example only in relation to the powder coating of MDF board. The application to the coating of other materials, be they conducting, partially conducting or non-conducting will be apparent from the foregoing. The invention is also described in relation to a flat bed conveying system which has advantages in relation to fully automated product handling, but alternative orientations of the product are not to be excluded that embody the invention.

0033Referring firstly to Figure 1, there is illustrated electrostatics powder coating apparatus comprising a flat bed conveyor which in the illustrated embodiment comprises four sections 1, 2, 3, 4. Each section comprises an endless conveyor belt entrained around respective end rollers 5, 6 at the leading and trailing end of the conveyor sections. The number of sections may be increased or decreased and may involve additional processes such as pre-treatment of metal components prior to spraying or preheating of MDF components prior to spraying. Conveniently there is a separate conveyor for each section.

0034Each section represents a work station. Thus section 1 represents a loading station, section 2 the powder application station, section 3 the curing station, and section 4 the unloading station. The sections are driven in synchronism. The powder application station further comprises a spraying chamber 21 made from plastics or any other non-conductive material which incorporates one or more powder spraying guns 23 (two in the illustrated embodiment). The spray guns and the means for feeding powder to the spray guns is conventional and well known to one skilled in the art and is not described in any further detail. Corona discharge or Frictonal triboelectric charging systems may be used. Means 25 is provided for extracting oversprayed powder from the spraying chamber. A suction means is preferred. Overspray remaining on the belt is conveniently removed by suction means 26. Section 2 of the conveyor belt comprises a non-conductive belt 27, which is made from a material having a low dielectric strength. A woven or porous plastics material is preferred. The belt 27 is supported by a plastic or other non-conductive support sheet 29. In the illustrated embodiment the support sheet occupies substantially the entire width of the belt 27 and extends between the leading and trailing ends of belt

003527. In practice it needs to be sufficiently large to support the product to be coated and may comprise one sheet or a plurality of sheets. Preferably the support sheet 29 has a plurality of through apertures 31 disposed at spaced apart and occupying part of the area thereof. Each aperture accommodates an earthing plate 32 of metal and which is preferably arranged to have the surface lie flush with the upper surface of the support sheet 29. This is illustrated in Figure 3 to which attention is directed in this respect and from which it will be apparent that the non-conductive belt 27 is interposed between the product 33 to be coated. (Examples of the product to be coated are shown at 33 and in this example comprise discrete boards that are placed one face down onto the conveyor surface to leave the edges and its other (upper face) exposed.)

0036The spraying chamber 21 is generally rectangular in the illustrated embodiment and overlies the belt 27. The opposite ends of the chamber are cut away to allow product to enter at the leading end and exit at the trailing end. In practice the conveyor may have apertures in the opposite ends that are dimensioned to allow the conveyor to pass through the spraying chamber.

0037Section 3 comprises the curing station 37. Curing may be by the action of heating or light or chemical action as dictated by the type of powder coating material being used.

0038Positioning of the products on the conveyor at the loading station 10 and removal at the unloading station 4 may be done manually or using automated handling machinery. For- example robots having suction pads are particularly suited to handling the board type products of the illustrated example. In operation, the products to be coated are loaded onto the conveyor at the loading station 1 and they are moved to the spray station 2 transferring to belt 27 in the process. Belt 27 passes over the support plate 29 and the associated earthing plates 32 which is sufficient to produce the capacitive coupling between the product and the earth plate for the operation of the spray guns to apply the desired coating to the exposed upper surface and all the edges.

0039Excess powder is removed by the suction means 25, including that lodged on the belt 27. A solid dielectric material may be used for the belt but one that is porous/peπneable is preferred. This may be achieved by a woven construction or by incorporating apertures in the belt by any convenient means, for example by piercing. The use of a non-solid belt 27 is advantageous as it lessens the amount of powder that can be deposited on the belt and it lessens the likelihood of the product retaining a charge — by providing the desired non-direct earth leakage. The apertures can be relatively small.

0040In the illustrated embodiment the product is shown as being moved on the conveyor belt past the support surface, which incorporates the plurality of earthing elements. This is not essential. The product, the earthing means and the interposed non-conductive layer may have a fixed relationship. Each product may have its own discrete earthing means and interposed non-conducting element. I.e. one for each product, but more usually the earthing means and interposed non-conducting element will be common to a plurality of products - by virtue of the fact that the plurality of products are presented to the earthing means in turn one after the other by operation of the conveyor.

0041An overhead conveyor may be used to move the products over and passed a conductive layer that is disposed other than horizontally, eg inclined or vertical and which overlies the associated earthing means. Whilst the earthing means has been described as comprising a plurality of earthing elements in the illustrated embodiment it may take alternative forms - e.g. a single earthing plate. A porous or permeable non-conductive belt has the advantage that it retains little or no charge, and attracts minimal powder compared to a belt made from solid dielectric material.

0042Figures 4 and 4a show an alternative embodiment of the invention whereby the earthing plates 32 are mounted on a rotating disc 40. The disc 40 is recessed into an aperture 41 in the support plate 27 and is rotated at a desired speed using a motor (not shown).

0043Figure 5 shows another embodiment of the invention whereby the earthing plates 32 are mounted on shafts 43 that are adapted to rotate and swivel by virtue of a ball and socket joint 45 on the shaft 43. By moving the earthing plates 32, the electromagnetic field can be made to move as desired.

0044Finally, Figure 6 shows a further embodiment of the invention whereby the earthing plates 32 are grouped 32a & 32b. The groups 32a & 32b are connected to earth via a switch 47. By constantly moving the switch from position A to position B means that the work piece is earthed alternatively via earthing plates 32a and 32b, respectively. The switch is most conveniently computer controlled (not shown).

0045The embodiments of the invention depicted schematically by Figures 4, 4a, 5 and 6 aim to minimise repulsive electrostatic forces (that cause the powder to coat thinly), which occur when an edge of the workpiece coincides with an earthing plate 32.