AU3323699A - Method of coating articles of plastics material - Google Patents

Description

WO 99/54108 PCT/AU99/00299 METHOD OF COATING ARTICLES OF PLASTICS MATERIAL Technical Field The present invention relates to polyester powder coating of articles and more particularly to the powder coating of articles of plastics material. 5 Background of the Invention Polyester powder coating provides objects with a very durable and high quality finish. The process is relatively simple and is a "dry" process. The method of applying powder coating is an electrostatic process. The article to be coated is charged and the gun which dispenses the powder, charges the powder so 10 that it is attracted to the article to be coated. The powder coated article is then baked in an oven at a temperature that cures the powder. This is generally around 2300C. Because powder coating is an electrostatic process, it is only used in respect of articles formed of electrically conductive material, such as metal. Powder coating is not an accepted method of applying finishes to articles is formed of plastics material. This is because the articles do not readily accept a charge and secondly, the cure temperatures are too high and can result in destruction and/or distortion of the article. Accordingly, only wet paint and similar types of finishes are applied to articles formed of plastics material. These methods usually require the article to be etched, this 20 enables the paint to adhere to the article. These techniques have yet to provide a satisfactory high quality finish to items formed of plastics material. Object of the Invention It is the object of the present invention to overcome or substantially ameliorate the above disadvantages. 25 Summary of the Invention There is disclosed herein a method of forming a polyester coating on at least a portion of an article of plastics material reinforced with fibre, said method including the steps of: providing said article; 30 pre-heating said article to a predetermined temperature, which predetermined temperature is at least equal to the melting temperature of polyester powder which is to form the coating; providing a stream of said powder; directing said stream at said portion to apply the powder thereto whereupon 35 said powder melts to form said coating; and curing the coating.

WO 99/54108 PCT/AU99/00299 2 Curing of the coating may be achieved by maintaining the coating at a temperature of about 2800 for at least 20 minutes, more preferably the coating is maintained at about 2800 for approximately 30 minutes. Preferably, the curing is at a temperature of 1900C to 215oC, most preferable 5 about 206 0 C. Brief Description of the Drawing Preferred forms of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a schematic depiction of a production line to power coat articles 10 formed of plastics material; and Figure 2 is a schematic depiction of a further production line to power coat articles formed of plastics material. Detailed description of the Preferred Embodiments In the accompanying drawing, there is schematically depicted a production line 10 1is to powder coat articles formed of plastics material. The production line 10 includes a conveyor 16 having a portion 17 from which coated articles are removed, and a portion 18 at which articles are attached to the conveyor 16, for delivery to the cleaning booth 11. From the cleaning booth 11, the articles are conveyed to the oven 12 and more 20 particularly the pre-heating portion 14 of the oven 12. After pre-heating, the articles are conveyed to the spray booth 13 wherefrom they are returned to the oven 12 and more particularly, the baking portion 15 thereof. The oven 12 is provided with oven doors that are intermittently opened to enable particulars to enter the oven 12. Accordingly, the conveyor 16 is intermittently 25 operated and coordinated with the oven doors. In the following preferred embodiment, it should be appreciated that the articles of reinforced plastics material are being pre-heated to a temperature which is the same temperature at which the articles are baked after being powder coated. As can also been observed from the attached drawing, the conveyor 16 has a greater length in the 30 pre-heating portion 14 relative to the portion 15. Accordingly, the articles spend a greater length of time in the pre-heating portion 14 relative to the baking potion 15. In the following preferred embodiment the articles spend approximately three times the period that the articles spend in the baking portion 15. An article of plastics material is provided and the passed through the cleaning 35 booth 11. The article is heavily reinforced by fibres such as glass, ceramic or carbon fibres. The fibres ensure that the article does not deform beyond predetermined limits when preheated.

WO 99/54108 PCT/AU99/00299 3 The article is preheated in the oven 12 to a predetermined temperature which is determined by the properties of the polyester powder coating material to be applied to the article. The article is preheated to a temperature approximating the temperature at which the powder melts. The article is then conveyed to the spray booth 13. 5 Within the booth 13 an air stream containing the powder is directed at the article to cover at least a portion of the article with a coating. As the powder particles engage the article, the particles melt. This ensures that the polyester powder material stays attached to the article. To provide a proper coating, the stream needs to be directed to whatever area is to be coated. o10 Preheating also aids adhesion of the polyester layer to the article as "pores" on the surface of the article tend to open up as a result of heating. This allows the layer to penetrate the "pores" to ensure proper fixing of the polyester layer to the article. Melting of the polyester powder material also eliminates problems in respect of powder "drop off" evident in electrostatic powder coating processes. 15 Curing is achieved by baking the article with its layer applied thereto in the oven 12. Preferably the article with its layer would be cured at a temperature of about 2800 for approximately 30 minutes. By pre-heating the article to the baking temperature, the powder commences "curing" prior to actually entering the baking portion 15 of the oven 12. In that regard, 20 it should be appreciated that the spray booth 13 is separated from the oven 12 by means of timed doors which open on demand and are coordinated with movement of the conveyor 16. Operation of the doors helps in maintaining the optimum curing temperature. Figure 2 of the accompanying drawings is a modification of the production line 25 10 of Figure 1. In Figure 2 the same reference numerals have been employed relative to Figure 1. In the embodiment of Figure 2 the conveyor 16 may be intermittently or continuously run. If intermittent, the line would have a predetermined index rate. Clearly, the index rate or speed of a continuously running conveyor will determine the 30 amount of time each article spends at the various stations. The cleaning booth 11 of Figure 2 includes a line stem-cleaning unit. The pre-heating portion 14 heats the articles for a period of time that must satisfy two requirements. Firstly, the articles must be heated to the extend that their moisture content is zero. Secondly, as mentioned previously, the articles must have a 35 surface temperature which is at least approximately the melt temperature of the powder to be applied. Typically, an article would remain in the pre-heat portion 14 for a period of 15 to 30 minutes Typically, for the conditions in respect of pre-heating to be met, the oven 12 would need to have ambient temperature in the range of 205 to 235 0 C. The articles WO 99/54108 PCT/AU99/00299 4 would need to have a temperature of from 110 to 150 0 C, preferably 125 to 140 0 C but more preferably 130 to 135 0 C. In the baking portion 15 the articles are cured so as to have a temperature within the temperature range of 190 to 215 0 C. An optimum temperature has been to be 5 206 0 C for a baking period of 18 minutes. The curing temperature and the characteristics of the power coating material determines the curing time. Typical curing times range between 15 and 30 minutes. In the above preferred embodiments the articles being coated are typically heavily reinforced by fibres such as glass, ceramic or carbon. The fibres insure that the 10 articles do not deform beyond predetermined limits when passing through the oven 12. In the embodiment of Figure 2, the spray booth 13 is associated with a return enclosure 18 through which the conveyor 16 passes to convey the articles from the booth 13 to the oven portion 15.

Claims (12)

1. A method of forming a polyester coating on at least a portion of an article of plastics material reinforced with fibre, said method including the steps of: providing said article; 5 pre-heating said article to a predetermined temperature, which predetermined temperature is at least equal to the melting temperature of polyester powder which is to form the coating; providing a stream of said powder; directing said stream at said portion to apply the powder thereto whereupon 10 said powder melts to form said coating; and curing the coating.

2. The method of claim 1 wherein curing the coating is..carried out at a temperature of about 2800 for at least 20 minutes.

3. The method of claim 1 or 2 wherein curing is carried out at a 15 temperature of about 2800 for approximately 30 minutes.

4. The method of claim 1 wherein the items are cured at a temperature of 1900 to 250 0 C.

5. The method of claim 4 wherein the articles are cured at a temperature of 1900 to 215 0 C. 20

6. The method of claim 5 wherein the articles are cured for 15 to 30 minutes.

7. The method of claim 6 wherein the articles are cured at a temperature of about 2060 for approximately 18 minutes.

8. The method of any one of claims 1 to 7 wherein the articles are pre 25 heated to a temperature of 110 to 150 0 C.

9. The method of claim 8 wherein the articles are pre-heated to a temperature of 1250 to 140 0 C.

10. The method of claim 9 wherein the articles are pre-heated to a temperature of 1300 to 135 0 C. 30

11. The method of any one of claims 1 to 10, wherein the articles to be coated are reinforced by fibres.

12. A method of coating articles substantially as hereinbefore described with reference to Figure 1 or Figure 2.