June 2014 Archives

Although similar to the fluidized-bed coating method, this method does not immerse the paint subjects in the fluidized layer of powder paint. Rather, the electrically charged powder paint particle layer that forms on top of the fluidized layer is electrically deposited onto the subjects. The system, as shown in [Fig.1], has a perforated plate with electrodes, and high voltage DC of 30~100kv is applied between the electrodes and the earth. As the result, a strong electrical field is generated between the grounded paint subject and the electrodes. The air in the tank is ionized by the corona discharge from the electrodes. The powder paint particles are electrically charged, repelling each other, floating on the top section of the fluidized bed, getting absorbed onto the paint subject as the opposing pole. The powder paint amount to be used would suffice at 50~100mm high from the perforated plate, much less compared with the fluidized-bed coating method. The powder paint of 20〜100μm granularity is used to obtain 50〜150μm thick coating layer.

The characteristics of this method are as follows.

	1.	Since preheating is not required, loss of heat energy is little.
	2.	Since preheating is not required, paint subject's heat capacity and heat conductivity are not of concern.
	3.	Requires 1/10 of the paint amount compared with the fluidized bed coating method.
	4.	Since the film is electrically formed, thick film such as fluidized bed method can not be obtained.

The method is applicable on steel pipe fittings, metal mesh, resistors, clasps, sprigs, and wire, etc.

Cost comparisons are shown on [Table 1]. As can be seen, the powder painting is highly economical compared with the conventional solvent based painting. Furthermore, when considering solvent pollutions on the work and general environments by the conventional painting, switching to powder painting can be considered as urgent.

[Table 1] Cost comparisons of painting methods

Fluidized Bed Coating Method is also known as fluid bed method. As shown in [Fig.1], powder paint is put in a fluidized tank with a perforated bottom plate. and compressed air or inert gas is pressure fed. The powder paint whirls up through the perforated plate and the tank is fluidized. When a paint subject preheated to temperature above the melting temperature of the powder paint is immersed, the melted (or semi-melted) paint forms a film on the subject's surface. The paint subject then is heated afterward to be finished as needed. The perforated plate is made of ceramic, expanded form resin, sintered metal, filter cloth, and etc. with uniform sized perforations of 10〜60μm based on required particle size for the paint to be used. The factors for the fluidized bed coating method are said to be [1] preheating temperature of the paint subject, [2] immersion duration, [3] shape and heat capacity of the paint subject, [4] particle size (80〜150μm) and melting point. The factor that determines the coating film thickness is the immersion time duration, and is typically 5~20 seconds.

Examples of process conditions are shown in [Table 1].

[Table 1] Fluidized Bed Coating process condition examples

The characteristics of this method are as follows.

	1.	It is possible to obtain 250〜1500μm thick film in 1 coat.
	2.	Less paint losses.
	3.	Spray booth and recovery device are not needed.
	4.	Simple and economical system device.
	5.	Preheating is required.
	6.	Limitations on paint subject sizes and shapes.
	7.	A certain amount of paint is needed even for a small job.