The Impact of Infrared Curing
Effect of Infrared Radiation on Powder.
We have been considering a conversion from our current 2k polyurethane liquid paint to a polyester powder coating to reduce VOC and waste disposal. We have some very large steel parts (4’ x 5’ x 25’) and some that are very small (3” x 4”). The gauge of the metal is a light as 16 gauge but we also have some very heavy gauges include one part that is 4” x 4” x 3’ long and weighs over 1,000 pounds. Since powder needs to be cured in a high temperature oven we are concerned getting a good cure with consistent appearance could be challenging. Can we use infrared curing to overcome the different metal masses and cure the very heavy parts? Do you know of anyone successfully doing the variety of parts we have to coat in powder? What other cure options are there for our situation? RT
Thermoset powder coatings require substantial thermal energy to crosslink and cure. Most powders need a metal temperature of 175º to 200° C to reach full cure and the peak metal temperature must be held at this level for 10 to 15 minutes. When working with different metal shapes, sizes and weights it can be challenging to get uniform heat up and cure.
Infrared (IR) curing can be very helpful for part variation, especially when a single product has different metal masses on the same assembly. The energy from an IR oven can be absorbed by the coating and it can also provide rapid temperature increase at the metal surface. This can radically reduce the time needed for full cure of the powder when compared to convection heating alone.
There are some cautions that need to be evaluated for the use of IR. Any technology that can create rapid reaction can also cause an over-reaction if not used correctly. Convection ovens cure by heating the surrounding air to a set temperature and then gradually heating the metal substrate. IR ovens cure by emission of electromagnetic energy and a much more direct transfer of thermal energy to the coating and part surface. The amount of absorption is closely related to the amount of part surface exposed and the geometry of the part. The more exposed surfaces will heat up much faster than areas that are hidden from the direct line of energy transfer.
There are different wavelengths of infrared (short, medium, long) and different types of emitters (gas, electric, catalytic, quartz lamp tubes, etc.). The right IR emitter is important and so is the design of the oven. It is important to position the emitter correctly and get the amount of time and exposure correct. Testing is recommended to determine the best possible use of IR for powder cure. With careful testing it can be determined how much IR to use and what type of emitter to use. Generally speaking, I works best when the there is a lot of surface in line with the emitter. In addition, consistent part size and shape is more conducive to the use of IR. The things that reduce the amount of IR that can be used effectively are just the opposite. Very little exposed surface and widely different part shapes. A simple geometry with a lot of surface makes it possible to use more intensity for a shorter period of time. A complex geometry with limited exposed surface requires a lower level of energy for a longer period of time and creates more risk of non-uniform cure.
So, IR will definitely help with part mass and variety, the question is how much can used and what type is best. There are many coaters that use IR in connection with convection ovens. The IR gives them the rapid rate of cure on some heavy areas and the convection takes over when the amount of IR that can be used has been optimized for time and efficiency. There are many lines that use both technologies. For example, a large powder line used for agricultural equipment may feature two minutes of infrared oven followed by convection. The convection oven can be cycled for 30 minutes or 60 minutes using a power and free conveyor system. The cycle is based on the mass and shape of the metal.
IR can provide more rapid cure and help overcome some metal thickness issues. In order to work right it must be tested and designed for optimum used based on a specific group of parts.