GB2116386A

GB2116386A - Protecting a powder coating plant against the danger of explosion

Info

Publication number: GB2116386A
Application number: GB08303365A
Authority: GB
Inventors: Walter Holl
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1982-03-02
Filing date: 1983-02-08
Publication date: 1983-09-21
Also published as: GB8303365D0; JPS58159859A; DE3207402C2; DE3207402A1; CH658804A5; FR2522539B1; FR2522539A1; GB2116386B

Abstract

To protect a high voltage powder coating plant 10 against the danger of explosion of flash-over due to electrical charging of the articles 18 to be coated, the existence of an arc with an inadmissibly high energy between the articles (18) and their supporting devices (19) is inhibited by applying a test high voltage 24 between the article (18) to be coated and earth (25), before the spray area is reached. Insulation which may be produced by contamination in the earth part 19, 20, 21, 22 is broken down thereby due to an electrical breakdown of lower energy. When the applied voltage is not sufficient to burn off the insulation, the test current falls below a predetermined minimal value (28) which leads to shut down (32) of the plant. <IMAGE>

Description

SPECIFICATION Method of monitoring a powder coating plant against the danger of explosion STATE OF THE ART The invention originates from a method for monitoring a high voltage powder coating plant against the danger of explosion by flash-over due to electrical charging of the articles to be coated in accordance with the preamble to the main claim.

Such methods known by the term "earthing control" normally operate so that the electrical resistance between an article to be coated and the supporting device carrying it is checked with a low DC voltage wherein the transfer resistance must not exceed a value of 1 Mohm. With a high transfer resistance, the danger exists that the capacitance formed by the insulated article to be coated in association with the high voltage present during the powder coating stores sufficient energy to ignite the powder-air mixture to be sprayed by a flash-over. It is frequently impossible, particularly with small work pieces, to meet the requirement of keeping within the prescribed resistance.Due to the relatively low weight of the articles and the unavoidably thin insulating film on the supporting device during the powder coating there is no certainty of retaining the transfer resistance reliably below 1 Mohm which leads to frequency stoppages of the plant.

On the other hand, it is known that arc discharges having an arc energy below 5 mWs are not dangerous as regards the danger of explosion in that kind of powder coating plant. Such arcs are not such as to ignite the powder cloud and are therefore not dangerous so that even with a transfer resistance below 1 Mohm measured with a low voltage the plant need not necessarily be stopped.

ADVANTAGES OF THE INVENTION As opposed to the measuring of the transfer resistance between the article to be coated and its supporting device, the method in accordance with the invention with the characterising features of the main claim has the advantage that dangerous insulating layers on the supporting device can be distinguished from insulating layers which are not dangerous without having to stop the plant. With the proposed measures, it is ensured in a very simple and economical manner that insignificant insulation deposits between the article to be coated and the supporting device are burnt away by an arc with a pre-determined limited energy.If the article to be coated is then subjected to the high voltage applied during the powder coating, no arc with a higher energy content can occur either so that, on the one hand, the danger of explosion is excluded and on the other hand a continuous working operation is ensured although pure measurement technology would allow an inadmissibly high transfer resistance to be established. The plant is only stopped when a predetermined minimum measuring current is not reached since falling below this minimum current indicates the presence of inadmissibly thick insulating layers which cannot be burnt off the applied test high voltage.

Advantageous further developments and improvements of the method set forth in the main claim are made possible by the measures set forth in the sub claims. The use of a DC voltage has a test high voltage has proved particularly desirable and which is preferably poled in the same sense as the high voltage DC voltage applied to the spraying pistol which is used. By the use of DC voltages in both cases and especially even by the use of a similar polarity, there is the guarantee that the arcs occurring in the powder coating plant cannot release energy higher than an arc possibly occuring in the test phase.For this purpose, a negative DC voltage is preferably used both for the powder coating and for the testing because, on the basis of physically known procedures, it is easier to control having regard to insulation than a positive DC voltage with which larger insulating distances must be maintained. With capacities between the insulated suspended article to be coated and the supporting device occurring in the normal manner test DC voltages between 1 and 5 kV have proved desirable although higher test voltages are permissible without reaching the critical ignition energy for the powder-air mixture.

DRAWING An embodiment of the invention is illustrated diagrammatically in the drawing and is explained in detail in the following description.

DESCRIPTION OF THE EMBODIMENT In the Figure, a coating chamber is referenced 10 wherein a powder coating is carried out with its spray pistol 1 The powder inlet to the spray pistol 11 is referenced 12 and its air inlet is referenced 1 3. A high voltage electrode 14 is arranged inside the spray piston 11 to which a negative DC voltage of about 80 to 100 kV is supplied from a DC voltage source 1 5. A cloud of coating powder and air leaves at the powder outlet 1 6 of the spray pistol 11 wherein the powder particles are negatively charged due to the applied high negative DC voltage. The powder particles are referenced 1 7.

The articles to be coated in the chamber 10 are provided with the reference numeral 1 8 and are illustrated diagrammatically just like the remaining parts of the plant. In this case, it can be a question of small articles such as headlamp reflectors or of large articles such as for example the housings of refrigerators or similar domestic apparatus. The articles 1 8 to be coated are suspended from supporting devices 1 9 which are fixed to a transport chain 20 which is advanced in the direction of the arrow. The articles 1 8 to be coated and the supporting devices 1 9 and the transport chain 20 consist of electrically conductive material, particularly of metal. The current circuit is connected to an earth terminal 22 through a sliding contact 21.

A likewise negative test high voltage is supplied through a further sliding contact 23 to the parts 1 8 to be coated before entering the coating chamber 10, the test voltage being at a level of up to 20 kV, but preferably at a level below 5 kV between the sliding contact 23 and an earth terminal 25. The test high voltage 24 is generated in a test apparatus 26 which includes a predetermined internql resistor 27 for current limiting, a current measuring element 28 and a DC voltage source 29. The output from the test apparatus 26 is connected through conductors 30 and 31 to a switching apparatus 32 which serves to stop the coating plant.

The described arrangement operates in the following manner: Before the articles 18 to be coated enter the coating chamber 10, they are provided with a negative test high voltage of for example 1.5 kV through the sliding contact 23.

Using a maximum permissible current flow of 10 mA as a basis for safety reasons, a transfer resistance of 1 50 kOhms is calculated which lies well below the maximum permissible transfer resistance of 1 MOhm; in the embodiment, 5 mA is established for the lower limit value of the test current which can be detectable by the current measuring element 28 and which corresponds to a total resistance of 300 kOhms. Below this test current, an inadmissible insulation between the article 1 8 to be coated and the supporting device 19 or the transport chain 20 is signalled whereupon the switching apparatus 32 stops the coating plant.In the case of a test current which is too low and thus a transfer resistance between the article 18 to be coated and its metallic support which is too high energy which is too high would be stored before the ignition of the arc during a subsequent application of the coating DC voltage of for example 100 kV due to the capacitance of the article to be coated and which could lie above the ignition energy of the sprayed coating powder.

The capacitances between the articles 1 8 to be coated and the supporting device 1 9 measured during the occurrence of an insulating layer lie within the range of about 20 to 80 pF. From this capacitance and the applied voltage the released energy in the arc can be calculated as 1/2 CV2.

The given capacitance applies to the various articles which are normally lacquered in accordance with the high voltage powder coating method. This method operates in known manner so that the charged powder particles adhere to the metallic article 1 8 and are then melted and hardened to form a lacquer layer in a subsequent process at temperatures of 1 200C to 1 800C.

On the basis of a maximum permissible arc energy of 5 mWs with a capacitance of the work pieces of 20 to 80 pF, a maximum permissible test voltage 24 of 10 to 20 kV is calculated according to the size of the work piece. However, in order to obtain additional safety, the test voltage is limited to a value of 1 to 5 kV, preferably to a value of 1.5 to 3 kV. At this test voltage, thin insulating coatings which arise between the articles 1 8 and the supporting device 1 9 and if necessary also between the supporting device 1 9 and the transport chain 20 are burnt away so that, despite the insulating coating originally provided, the plant need not be stopped.By applying a high voltage likewise negative in the coating chamber 10 proper, no high arc energy can occur so that the ignition of the powder-air mixture is excluded and with it the danger of explosion. The method in accordance with the invention is unaffected by dirt and guarantees that the charge on the articles 1 8 to be coated in the coating chamber 10 remains below the predetermined value although with a pure DC voltage resistance measurement an inadmissibly high insulation would result which, with the known test method, would lead to stoppage of the plant.

In the embodiment described, a negative DC voltage has been used both for the coating operation and also for the test operation. The use of DC voltages in the same direction ensures that during a subsequent occurrence of a DC voltage at the same level of the contact with earth 22 is guaranteed so that no arc can form with an energy higher than the predetermined energy. However, whilst considering the different kinds of insulation conditions, a positive test voltage could also be used instead of the negative test voltage 24 which is used. Furthermore, it is also possible to use an alternating voltage as a test voltage 24 wherein however, for safety reasons, the current flowing through the internal resistor 27 must be limited to a value below 4 mA. Again with a test voltage of 1.5 kV this would correspond to an internal resistor of 375 kOhms and a transfer resistance of 375 k0hms at the most when the plant is stopped at currents 42 mA.

Claims

1. A method of monitoring a high voltage powder coating plant against the danger of explosion by flash-over due to electrical charging of the articles to be coated, wherein said articles can be connected to an earth terminal with a limited transfer resistance, preferably through a circulating supporting device, characterised in that, the existence of an arc with an inadmissibly high energy between the article (18) to be coated and/or its supporting device (19) and an earth terminal (22) is inhibited by the application of a test high voltage (24) between the article (18) to be coated and earth (25) wherein, insulation which may be produced, is broken down by an electrical breakdown at a lower energy or in any case the powder coating plant (10) is shut down when the test current falls below a predetermined minimum test current (28) due to a transfer resistance which is too high.

2. A method according to claim 1, characterised in that, in accordance with the capacitance occurring in the case of an insulation between the article (18) to be coated and parts (19, 20) lying adjacent to earth (22), a test high voltage (24) is applied which limits the arc energy to a value below 5 mWs in the case of a breakdown of an insulation which is provided or a flash-over to a part connected to earth.

3. A method according to claim 2, characterised in that, the test high voltage (24) is a DC voltage in the range below 20 kV, preferably in range of 1 to 5 kV.

4. A method according to one of the preceding claims, characterised in that, the test voltage (24) is only applied temporarily to each article (18) to be coated.

5. A method according to one of the preceding claims, characterised in that, the test voltage (24) is applied to the article (18) to be coated by means of a resilient sliding contact (22).

6. A method according to one of claims 3 to 5, characterised in that, the test voltage (24) is applied with the same polarity as the coating high voltage (15).

7. A method according to claim 6, characterised in that, negative DC voltages (15, 24) are applied for testing and for coating.

8. A method according to one of the preceding claims, characterised in that, the test voltage (24) is applied before the article (18) to be coated enters the coating chamber 10.

9. A method of monitoring a powder coating plant against the danger of explosion, substantially as herein described.

10. Apparatus for carrying out the method according to any preceding claim.

11. Apparatus according to claim 10, substantially as herein described with reference to the accompanying drawings.