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The next stage after coating the parts with powder is applying heat to the parts so that the powder coating ‘cures’. In this sense the definition of Curing is the cross linking of the resin structures within the make-up of the powder so that the powder coated film hardens.

The most important factor to take account of is that of Metal Temperature. The curing instructions on the powder manufacturer’s data sheet will say something like ‘Cure for 12 minutes at 180 degrees centigrade. This does not mean put the parts in the oven for 12 minutes at 180 degrees C. It means that first the component has to reach curing temperature and then it has to cure for the required amount of time. The most common fault encountered with powder coating is under curing of parts.

If parts are not given sufficient heat for the correct amount of time the powder coated surface does not harden correctly and will chip or flake off. Special account must be taken regarding the thickness of the materials making up the component, it not unusual to see parts these days made of a variety of thicknesses of metal, certain parts can be 4mm, others 6mm and even thicker at 8mm or 10mm. In these instances care must be taken to cure the parts to the thickest material as this will take longer to reach curing temperature.

To be absolutely sure of the time taken to heat parts up to the curing temperature there are Meters available that have probes attached to them. The meter is put into a heat proof container and the probes attached to various parts of the component. The uncoated component can then be put into an oven with the Meter and heated up for a period of time.

When cooled the meter can be retrieved and it will have recorded how long the various areas of the component took to heat up to curing temperature. So for instance it may show that the 8mm thick parts of the component took 6 minutes to heat up to 180 degrees centigrade, so the curing of these parts would be 6 minutes plus 12 minutes, 18 minutes in the oven in order for the curing to take place.

There are various types of oven in which curing can take place; the two main types are Box or Static Ovens and Conveyorised Ovens.

Box Ovens are as the name implies a box type structure with doors and some means inside it to hang work from. It will usually have a gas process heater attached that blows heated air into the oven. There will also be thermostats and controls so that a desired curing temperature can be chosen and the time for parts to be in the oven. Powder Coated parts are hung in the oven, the temperature is set and the timer, the doors are closed and the curing cycle started, normally some sortof bell or alarm will sound when teh desired time has elapsed so the oven can be turned off, doors opened and the parts can cool.

Conveyorised Ovens are larger structures with an open aperture at each end and a motorised overhead conveyor running through it in a loop. Normally a Spray Booth will be located at some point on the conveyor. The parts to be coated are hung from the conveyor and moved through the booth at a pre-determined speed, sprayed with powder in the booth by one or sometimes two spraying personnel or by automatic reciprocators with the powder spray guns attached.

Reverting to the Curing Time Issue with conveyorised ovens there are two factors determining the speed of the conveyor, firstly the speed at which the parts passing through the booth can be sprayed and two the time that the parts need in the oven to effect a cure. In some case the parts can be sprayed faster and there is a temptation to increase the speed of the conveyor to increase production throughput. This can cause problems as although production is increased it will be at the cost of properly cured parts.

That being said Powders these days are resilient to higher temperatures so for instance 12 minutes at 180 degrees C curing is equivalent to 8 minutes at 210 degrees C. This means it is possible to increase the temperature inside the oven and lower the time that parts must remain in the oven, always remembering that the metal must heat up to the Curing Temperature so that parts can be fully cured.