DIP/ FLUIDISED BED/ ELECTROSTATIC COATING METHOD
FABRICATIONSteel wires of the required diameter & length are cut and bent to the desired shape and then spot welded to fabricate the required shape of the article. Jigs should be carefully designed to achieve good accuracy, repeatability and desired production rates.
The prefabricated article is de-greased, de-rusted, de-scaled, passivated and cleaned. Thin 'wire loops' should be attached at suitable locations, for ease of handling the article, so as not to mar the product appearance. These are later snipped off after the entire operation is over.
PREHEATINGThe bare article is then heated in an oven at 250°-300°C (480-570°F) for 5-10 minutes. The heating time and temperature depends on the mass of the article and the loading of the oven. This has to be practically ascertained to provide the desired production rates and coating thickness. For electrostatic coating lower temperatures may be used.
COATINGThe heated article is dipped in a tray containing the powder and the powder splashed on it, or dipped in a fluidised bed of the powder for 2-5 seconds. The powder will stick to the hot article. Once the article is coated, care should be taken to ensure that it does not rub or come into contact with any surface, to prevent damage to the coating. At this stage, a rough powdery, but uniform coating on the article is obtained. The excess un-melted powder is shaken off with a slight jerky motion or light tapping. The coating thickness increases with the duration of dipping in the powder medium. In electrostatic coating, preheating will give higher build-up.
FUSION SINTERINGTo smoothen out and provide a glossy surface, the article is re-heated in the oven for 2-3 minutes. This causes the powder to melt, sinter and flow out. Excessive heating may cause the coating to dis-colour or sag. The article should be carefully taken out of the oven, without the coating coming into contact with any other surface and allowed to cool in a clean dust free atmosphere. Sometimes water quenching is also resorted to, for wire products.
INSPECTION & POST FINISHINGThe thin 'wire loops', attached earlier for handling the article, are carefully snipped off at the base so as not to mar the appearance. The article is inspected for pin holes or blemishes. These can be filled or repaired with a little powder applied with a small soldering iron or a hot air gun.
FUNDAMENTAL NOTESThe coating thickness increases with (i) increase in temperature of the article (ii) the thickness of the wire (iii) the duration of dipping in the powder. These parameters have to be practically optimized by "trial and error" to achieve the desired maximum and minimum coating on the thicker and thinner wires of the coated article.
A coating thickness of 0.3 to 0.5 mm is normally considered sufficient. It is recommended that the oven should have a circulating fan inside it, to provide uniform temperature, and better heat transfer.