Convection Curing

INTRODUCTION

The coatings and paint industries strive to provide high technology coatings while reducing volatile organic compounds and energy consumption to produce a finished coating. Conventionally Convection ovens are used to cure the coatings. But this process which uses electric heaters is not an optimal process and is associated with various disadvantages. Improved technologies are available today, which can either replace or improve the convection curing process. Infrared Curing is such a technology which uses Infrared rays emitted by an Infrared emitter to provide the required cure. 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 cross linking begins quickly once the surface is exposed to the emitter. Infrared emitters are often custom manufactured to suit the production demand. The various aspects of Infrared curing and convection curing and the possibility of combining these two technologies into a singe system will be discussed in this seminar.

CURING

Curing is a process of baking surface coatings so as to dry them up quickly. Curing is a broad term which means all the techniques employed for the finishing operations incurred during part production. Curing essentially involves either the melting of the coating or evaporation of volatile fluids present in the coating by the application of heat energy.

Curing is given to a wide range of materials both organic and inorganic .Usually curing is given to materials like ,

" Paints
" Enamel
" Laquer
" Powder coatings
" Varnishes
" Epoxy coatings
" Acrylic coatings
" Primers Etc

Curing is also given to Rubber and Latex .The principle used for curing can also be used for drying rice and grains.

CONVECTION CURING

Convection ovens are usually used for curing purposes. Traditional convection ovens use heated forced air to provide the necessary cure. Convection ovens consist of a chamber lined on the inside with Electric heaters. The shape of the chamber will be in accordance to the shape or geometry of the part being cured. A series of blowers circulate the heated air around providing the required cure. This process depends on convection to transfer heat from hot air to body surface and conduction to transfer heat to the interior of the surface. The air being delivered is held at temperature using closed-loop control, which provides predictable, repeatable results. Typically a temperature of around 250-500 degree farenheit is required for paint or powder. Though convection ovens are widely used today they have certain disadvantages, which chokes the overall productivity of a company

Disadvantages of convection ovens :

" Fairly long heating times:-
Convection is a slow process. It takes a considerable amount of time for the heaters to heat up and raise the temperature of air to the required level. This causes a lag in the process and hence the curing time increases. Longer curing time spells reduced assembly line movement. This in turn reduces productivity.

" High energy consumption:-
A convection column dryer uses around 2000 BTU(British Thermal Unit) of energy to remove 1 pound of moisture. They use around 7.7 KW of electrical energy to dry a ton of rice. These are significantly larger figures for any company trying to bring energy consumption under control. The additional use of blowers and compressors further increases energy consumption.

" Large floor area required:-
Convection ovens are bulky in nature. Due to the presence of compressors and blowers, additional space is needed, which in turn increases the floor area requirement.

" Air circulation is required:-
Convection heating requires a medium for transmission of heat. Hence blowers are employed for good circulation of heated air. This increases the overall cost of the equipment.