Actually, function of curing oven in powder coating is to heat material’s surface or its coating in order to activate physical and chemical processes involving several stages: gradual shift of powder particles into viscous state, melting, wetting of a primer, spreading, gaseous inclusions removal, the cure (for thermoset combinations).
Cameras or curing ovens are the basic parts of any coating line. An urgent task of modern economical painting equipment is searching of new methods and means of heat treatment which must be aimed on improving of curing process. It is a physical fact that any heating occurs due to mechanism of heat transfer. But the channel of energy transfer can be different: heat exchange, connective or radiant. Each of these methods is widely implemented in equipment. More and more manufactures began to look for more effective use of infrared radiation’s features. It is connected with serious rise of energy cost. However there are some obstacles for wide industrial application of IR technology. On the one hand it is growing pains, from the other hand it is lack of specialized scientific and engineering divisions, which work in this direction. The creation of new modern IR systems requires both analytical and experimental thorough study of energy transfer processes in absorbing environment.
The knowledge of these rules as well as reducing time of transition from experimental models to industrial installations will allow to solve problem with IR technology’s introduction scientifically. It is clear that this issue is relevant in paint production, where modern technologies require improved methods of heat treatment concerning both liquid and powder coatings.
For the first time infrared rays have been used in 1934 at the Ford plant. Since that time basics of drying process by selected spectral range of infrared radiation have been laid.
«Drying» process in liquid paints means removal of moisture and heating the surface in order to perform curing process. Unlike this the purpose of heating in powder painting is to thermo activate physical and chemical processes listed at the beginning of this article.
Therefore, it is necessary to determine scope and sequence of technological problems while using of IR heating.
Infrared radiation contributes to acceleration of technological process as it causes increasing of natural vibrations of atoms groups in molecule, which results in radiation energy transformation into heat energy. This difficult question is still being studied. Since infrared rays irradiation is not only method of heat treatment, but a huge impact on material’s nature.
The efficiency of IR heating is determined by the ratio of reflected, absorbed and transmitted energies. These characteristics depend on range, type and physical properties of polymer and primer. It is known that for the most of coating materials 85% of radiant energy in the spectrum of radiation penetrated into material to a depth of 30-50 micron. Thus air environment has almost no effect on radiant flux transmission. This is a fundamental difference between mechanisms of IR and convective heating. There are two different directions of temperature gradient. At the convective heating temperature gradient is directed from the coating surface to primer, and at the IR heating coating is partially heated by primer. This mechanism affects the quality of formed powder coating significantly, primarily, increases its adhesion strength.
While developing of our equipment the first task we faced was to consider all features of IR heating and to choose the optimal spectral range and emitter’s type. One more important criterion was an economical and operational characteristic of developed equipment.
Rigorous mathematical modeling of heating process in IR ovens is very difficult task. Analytical description of heating kinetics is finding links between power density, maximum temperature and maximum permissible heating rate. In this situation basis of the model can be the differential equation of energy balance. With its help you can set compliance rules which link «emitter-product». It can be done only for particular cases with significant assumption and approximations. Moreover, great disadvantage of approximate calculations is complete absence of analytical connect between temperature functions, effective flows with optical properties and parameters of mutual arrangement of system’s structural elements.
That is why developers mainly rely on the results of mathematic modeling, which are confirmed by experimental testing.
Theoretical analysis and physical experiments with different combinations of emitters and powder samples confirmed that the best results are achieved by using IR radiation with wavelength in the range of 0.78-2.00 microns. Lamp halogen infrared emitters work in this range. Their filament temperature is lower that it is in lightning. Accordingly flux fraction emitted in the IR area is more than in the visible one. In addition these sources allows creation of high energy light condition, have long lifetime and stability, very low thermal inertia, simplicity of bases devices, high efficiency, mechanical strength and water resistance. As an example, the figure shows graphs of spectral energy distribution of the lamp radiation. We have chosen quartz halogen emitters with working temperature in 240 °C. The most rational technically available and economically feasible method of heat treatment is the oscillating mode of energy supply with heat flux relaxation. For its implementation it is necessary to identify causal and functional interrelation between parameters of IR radiation and coating layer parameters.
For practical implementation of different modes of heat treatment flexible system of automatic control of infrared energy supply was designed.
It allows maintaining of maximum and minimum materials temperature. In such way possibility to use computer technologies which ensure the software automatic regulation of thermal modes in working camera was appeared.
It allows to verify a hypothesis about general rules of impulse IR thermal energy input control in the process of powder coating. The correctness of our choice concerning ideas and technical solutions embodied in our IR heaters and emitters control system was confirmed by time, consumer’s interest in our developments and expanding of their application scope.
As an example the figure shows dynamic characteristics of curing oven.
The size of oven (7,3-2,2-2.2 m).
Maximum power – 66 Wt.
Download – 56 shaped tubes with weight near 1500 kg.
You can meet main drawback of IR systems in literature. It is their directed radiation, which makes them unsuitable for creating powder coatings on difficult subjects. Our research refuses this common opinion.
First: infrared heating is inextricably connected with convective one, due to heat transfer;
Second, we use reflectors in curing oven. They have specially treated surface which creates resonator. There equable flux of diffuse radiation is created.
Thirdly, shortwave emitters due to high density of radiant energy are able to warm up inacceptable parts of product.
Fourth, wavelength of radiation increases in thermal stabilization mode, which leads to an additional connective component formation.
And the last, an even transfer of energy to complex volume is ensured by using of IR radiation.
We consider it necessary to raise one question that we often face: the harmful effect of IR equipment on worker’s health. It is absolutely harmless. Moreover medical studies in Japan, China, and the USA proved the curable factors of IR heating. For example, it is wide distribution of IR saunas.
Properly designed and constructed IR device is a complex engineering structure. Its development must be carried out only on basic of scientifically grounded choice of emitters, their quantity, modes and location. Therefore, you shouldn’t trust advertising and low prices when you choose equipment. It is important to pay attention to the popularity of manufactures. Make sure that developer knows methodological approaches and fundamentals of engineering calculations of IR system. Self-respected manufacture will give you an offer with a list of references where previously supplied equipment will be mentioned.