Choosing the oven suited for a particular application depends on all sorts of variables, including the product size, shape, thickness, material composition, production throughput, operating cost and type of powder to be used must be considered.
As powders are cured or hardened, time and temperatures are critical considerations. Within certain limits a period at a lower temperature may be equivalent to a shorter exposure period at a higher temperature. However it must be emphasized that the curing temperature specified by the powder supplier refers to the article temperature itself and not to the air or panel temperature of an emitter.
Ovens should be designed to ensure efficient use of production labour and minimum maintenance is required. Maximum use should be made of fuel and the efficient recycle of hot air maintained where possible.
Temperature should be exactingly controlled, if possible within ±3oC of the specified curing temperature.. Temperature indicating heads should be situated as near as possible to the path of the workpiece in the oven.
Due allowance should be made for possible extension of the oven and/or possible changes in the type and flow of workpieces being processed.
Care should be taken that air velocities within a forced convection oven should not be so strong as to blow powder off or to move workpieces so that they come into contact with each other during stoving. Air velocities which are acceptable lie in the 1-2 metres/second range.
If the oven has relatively small openings normal exhaust seals will be adequate, but with larger openings seals that re-circulate to minimise heat loss are effective.
Ovens available to cure powders differ widely using a variety of heating methods.
This is by far the most popular method where gas or oil-fired heaters are used to heat air. This is then circulated through an enclosed space where it provides the required temperature. The powder coated pieces are then carried into this space where they absorb heat, reach the surrounding temperature and are held at this temperature for the specified time.
In a direct fired oven, the products of burner combustion are carried directly into the heat holding zone, whilst in an indirectly fired oven the burner products pass through a heat exchanger so that only clean air enters the heat holding zone. In the latter case, the hot air is free of combustion products which can in some instances interfere detrimentally with the powder during the curing cycle.
The quantity of air exhausted from a convection oven must be sufficient to keep the atmosphere within the oven below the lower explosion limit of the gases being released in the oven.
Care must be taken, especially in the case of direct fired ovens, to regularly inspect and clean the oven interior as foreign material can build up on the inner oven sections, which can from time to time flake off and adhere to the workpieces which in turn can lead to rejection.
Basically, infra-red heat energy is emitted by a hot body and transferred in straight lines until it makes contact with another body when the heat is absorbed by the latter causing it to rise in temperature. The main advantage of radiant heating is that it produces a rapid rise in object temperature. As the temperature of the heating source is increased, so the proportion of heat transferred by radiation as opposed to convection increases appreciably. The amount of heat energy radiated from any source depends upon its area, its temperature and its emissivity.
Medium temperature radiation offers the most effective source of radiant heat for curing thermosetting powder. Usually the emitter panels are gas-fired giving a panel surface temperature of 850-950oC, or alternatively sheathed electric panels of a surface temperature of 750-850oC. The workpiece is maintained at about 30cm from the emitter panel surface with a minimum distance of 15cm.
The color of the powder is an important consideration – light colors can reflect (depending on powder composition) proportionately more infra-red radiation and do not heat up as quickly as the darker colors. This means that each and every powder must be evaluated to determine the optimum curing conditions.
As radiant heat tracks in straight lines, coated articles of intricate design, or having sections shielded from the radiation source cannot be processed using this method. Ideally infra-red radiation is used for simple workpieces such as relatively flat surfaces.