The painting process, as everybody understands, serves the dual purpose of protection of surface as well as enhancement of aesthetic value. There are paints which are cured at elevated temperatures only. However, forced drying of air drying paints gives many benefits such as time between the coats gets reduced from 6 to 8 hours to 20-30 minutes. In practical terms, it means that the second coat can be completed on the same day and material can be packed.

The practice of high temperature drying is most commonly accepted for all industrial products. However, for heavy components like valves the weight of component poses problems in designing a conventional hot air oven, which have now been overcome by infrared heating technology introduced in India by Litel. These ovens are already in operation in many leading multinational valve manufacturing companies in India. (Reference available on request).

Why Infrared?

Infrared heating is radiant heating i.e. the infrared emitters heat the object without heating the air in-between. These emitters have very low thermal inertia; hence they are instant On/Off type (i.e., heat-up/cool down time about 1 second). Such a technology has significant benefits for the users, such as:

1. No need to heat the entire mass; no need to heat the oven. In a typical convection oven it becomes absolutely essential to heat all the mass of the object, leading to a very long cycle which is further worsened by the need to pre-heat the oven. Infrared ovens can be switched on just as the object enters the oven and temperature on the surface is attained in a short time even before rest of the mass begins to get heated. Such switching on/off is controlled automatically through PLCs. "Energy Savings"Coupled together, both these features mean that the actual energy consumed is very small and cost of drying is less even as compared to gas fired ovens.

2. Reduction in work-in-process inventory and space required for inventory. Because of just-in-time nature of operation, there is minimum WIP inventory and so is the space required for inventory. Also, problems of dust settlement on the components during such period are eliminated.

3. Clean heat. There is no combustion by-products, so the product is not contaminated. Low (or zero) air velocities reduce the possibility of surface contamination by airborne dirt.

4. Floor space savings. For continuous conveyorised operations, a shorter heating time means a shorter oven length. This helps in reducing initial costs as well as valuable floor space which itself is expensive for a bulky equipment like oven.

5. State-of-the-art controls and intelligent operation. Because of fast response property, electric radiant heaters can be effectively synchronised with a PLC operation, making it an "intelligent" operation.

How Does Infrared Work for Paint Films?

Being purely radiant, IR cures the paint on the surface itself thus the entire mass of the component doesn't get heated. The infrared rays heat the object directly without heating the air in between. The IR rays penetrate through the film, hit the metal surface and are partly absorbed in paint film, surface and partly reflected. During reflection they reheat the film. Thus temperature throughout the film has more uniformity and heating is faster. Also, the paint gets thoroughly heated from the bottom to the top. As against this, in a convection oven the heat transfer is through the paint film which in itself is a bad conductor of heat. This slows down the entire curing process.

Typically Infrared Heating Takes Only One-third the Time of Convection Heating

By virtue of the property of infrared to heat the film thoroughly, better quality of coated film, higher gloss and higher adhesion are achieved.(See figure)

Practical Results

Results vary from paint to paint. But a very common practice is to use zinc rich primers/epoxy primers followed by polyurethane topcoats. For this combination, 20-30 min hard- to-dry finish for the primer coat followed by another 15-20 min of cooling followed by same time for the top coat has been achieved in most cases.

Summary

In a competitive economy, it is imperative to use equipments which add value to the components, at the same time reducing overall costs and increasing profitability. Infrared paint drying/ curing offers key benefits such as :?

??Just-in-time operation
??Increase in productivity
??Reduced inventory cost
??Clean operation
??Improved finish
??Space savings
??Better controls

(Contact: Mr P B Kulkarni, General Manager, Litel Infrared Systems Pvt Ltd, Mobile: 094220-30166, Email: sales@litelir.com)