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Combination Dry-off/Cure Ovens
The interior size of an oven for a conveyorized system will depend on the largest part size and the production line speed. The actual length of the conveyor will be the cure time required plus the bring-up time of the substrate, times the line speed. If the metal of the part takes 5 minutes to bring up to the design temperature and the powder takes 15 minutes of cure time, the oven cycle time will be twenty minutes. If the line is travelling at 10 feet per minute (3 meters/min.) the oven will need 200 linear feet (60 meters) of conveyor. bring-up time + cure time = cycle time cycle time x line speed = length of conveyor in oven Oven Balancing Even air flow and temperature balance in a convection oven is important. If there are areas that are not even in temperature the coating may not be uniformly cured and color and gloss may vary. To make sure that the oven is evenly heated, a temperature recording device should be run through the oven at least once a month. Using several probes to read both air and part temperature from top to bottom, the operator can determine if any problems are likely to occur and where the temperature variance in the oven is located. They can also determine how long it takes to bring the metal to temperature so that the cure cycle time is known. Adjustments to the discharge ductwork can then be used to create even distribution of the oven energy supplied by the heat source and conveyor speed can be adjusted to provide the proper time in the oven.
Combination Dry-off/Cure Ovens
Some manufacturers offer a single oven with one route through for drying and one route for curing. This arrangement saves capital equipment cost and some manufacturers may claim that it saves energy. However, moisture drying and curing operations should not be done in the same oven. Moisture evaporation can affect the cure process and uncured powder can drift onto parts in the drying section and cause dirt defects. Air flow and temperature requirements for drying and curing are not the same. As for energy savings, the amount of fuel required to dry and cure the parts will be the same regardless of whether it comes from one burner
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Combination Dry-off/Cure Ovens
or two. A dual-purpose oven with one burner box will require as much burner capacity and fan capacity as an oven with separate heat sources. In fact, a dual-purpose oven may cost more to operate because the dry-off temperature must be the same as the cure temperature. In many cases the dry-off oven temperature can be much lower than the cure temperature. It is economical to build the two ovens side by side and separate them with a wall due to reduced radiant heat loss. They should always have their own fan, burner and controls to provide the best control of the process and the most effective results. Fuel Gas, electricity and steam can all be used to fuel process ovens. Which one to use depends on the cost of the fuel and the supply. If it available, natural gas or propane is by far the most economical fuel source. Gas Gas is most commonly used in process ovens because it is readily available; it is easy to control and typically cheaper than other fuels. A high quality gas burner has a 40:1 turn down ratio. This means that the oven burner output will respond to the conditions in the oven interior and modulate the volume of gas burned to provide the correct temperature. The more product that is put through the oven, the more gas it will burn. When the oven has a light load or it is empty, the burner will modulate to low fire and idle until the thermocouple indicates a need for more Btus. The fan can be a suitably sized plug fan or a Double Width, Double Inlet (DWDI) fan. A DWDI fan is the most commonly used. The gas burner is mounted in the side of the burner box, upstream from the fan and the fan discharges into the oven transition and into the supply duct. The burner box can be mounted on top, on the side, on the end, or under the oven. The position of the discharge duct and the return air opening should consider the conveyor path and avoid disturbing uncured powder or swinging the parts on the racks. Air flow and temperatures in the cure zone must be designed with the understanding that parts will travel through the oven. These parts must not be disturbed during the cure process and the temperature must be uniPowder Coaters Manual 1/98
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Infrared Curing
Figure 5-3 Recirculating Burner Box
form and consistent. Ovens should be specified to provide a maximum temperature variance of no greater than 15 F (-9.4 C) from the set point in the cure zone. Electricity An oven can function quite well with electricity and the design is very similar to the gas burner box. The electric heating elements take the place of the gas burner in a package type heater with a built in centrifugal recirculating fan. Electric ovens will take longer to bring up to temperature and to cool off, and they are at least 3:1 more expensive to operate than gas. No by-products of combustion are generated with electricity. Steam Like gas and electric, a steam heating system is a recirculating convection type. The return air should be filtered with high efficiency air filters to prevent clogging of the steam heat exchanger. The steam heat exchanger should be used with a self-operating regulator with an indicator. The regulator should have a strain for valve protection. A bypass line should be installed for proper heat control as well as a throttling valve self-acting steam control valve. Steam is typically not practical for curing powder coating unless a facility already has a boiler with excess capacity.
Infrared Curing
Infrared curing applies light energy to the part surface by direct transmission from an emitter. Some of the energy emitted will be reflected off the surface, some is absorbed into the polymer and some is transmitted into the substrate. This direct transfer of energy creates an immediate reaction in the polymer and crosslinking begins quickly once the surface is exposed to the emitter. V/8
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