Flux Characteristics and Transfer Systems in Electrostatic Application – Part 3

Nov 30
2017

flux-characteristics-and-transfer-systems-in-electrostatic-application

 

Flux Powder Fluidization:

In an effort to develop a flux with more desirable properties for electrostatic application, the first step is to qualify criteria. In summary of the above, it is apparent that fluidization is one of them. There is standard equipment available on the market to quantify fluidization characteristics. However, when we tested these fluidity indicators, we found the fluidization ability of flux powder to be so poor that the results were meaningless unless a vibration unit was attached to the equipment. A photo of the modified installation can be found in the attachments. We combined a Binks-Sames powder fluidity indicator (AS 100 – 451 195) with a Fritsch vibration unit (L-24). The equipment consists of a fluidizing cylinder with a porous membrane on the bottom. The cylinder is mounted to a vibrator with a fixation plate. After the sample material (250 g) is placed in the cylinder, the vibration is turned on (via the vibrator control unit) and a consistent flow of dry nitrogen (via the fluidity meter control unit) is forced through the porous membrane. Depending on its potential to fluidize, the powder will start to expand until an equilibrium is reached (one minute). Measurements of the original and the fluidized height are taken at different points (see attachment).

3-1-fluidmeter-vibrator-control-unit

Powder Fluidity Indicator

3-2-indication-of-the-locations-for-the-measure-of-the-height-of-the-powder

Indication of the locations for
the measure of the height of the
powder in both fluidized and
non fluidized condition.

3-3-collecting-powder-as-it-comes-out-of-the-calibrated-hole

Collecting powder as it comes out of the
calibrated hole.

The second parameter determined with this device is the weight of powder flowing through a small hole on the side of the cylinder (as can be seen on the picture). Similar to the above procedure, the sample is fluidized in the cylinder. The side hole is then opened for 30 seconds, and the powder flowing out is caught in a beaker and weigh.

The spray factor is a combination of the expansion factor and the powder flow. Especially in dry flux application, where the material transport depends on fluidizing properties, the spray factor presents an important relative figure for powder evaluation.

To be continued …

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Flux Characteristics and Transfer Systems in Electrostatic Application – Part 2

Oct 27
2017

flux-characteristics-and-transfer-systems-in-electrostatic-application

 

The background of electrostatic flux application:

When controlled atmosphere brazing with non-corrosive fluxes was introduced, the only realistic method for using the flux was wet application. This strategy was supported by the physical and chemical properties of flux powder.

Non-corrosive fluxes for aluminum brazing consist of potassium fluoroaluminates (inorganic fluorides) with low water solubility. The majority of the flux products on the market are manufactured by precipitation in aqueous solution. These show a rather fine particle size distribution, i.e. from one to fifteen micrometers (1 – 15 µm) for most of the grains (50% and more) and reaching from 0.5 – 50 µm with an average particle size between four and ten micrometers (4 – 10 µm). This type of powder is ideal for slurry application, as the fine particles prevent the flux from settling too fast. Also, when sprayed on a clean surface under wettable conditions, they present a uniform, very thin and fully adhesive coating after drying. As mentioned earlier, the flux slurry needs to be agitated continuously and the concentration must be monitored in order to guarantee consistent flux loading (i.e., flux weight per surface area).

The most significant problem in wet application is waste water. With stricter requirements and limitations for trace impurities in waste water, the pressure to reduce water consumption increases. At the same time, production capacity is expanding worldwide. Waste water treatment is expensive, and some brazing operations have limited experience in this field. In addition, more and more facilities are constructed in areas where water appropriately treated for flux slurry preparation is scarce and costly.

The challenges of electrostatic flux application:

Electrostatic powder coating has been standard technology for many years, and it was only a question of time before it was also realized in flux application. The following will focus on essential flux properties and basic equipment arrangements.

Some material characteristics of non-corrosive brazing fluxes make it difficult simply to transfer the normal powder coating equipment to the fluxing area and use it there. Most powders utilized for electrostatic application are either designed with special properties or already contain them. Important elements are:

  • Particle shape and particle size distribution
  • Ability to accept and to hold electrical charge

Particle size distribution has a significant influence on the ability of a powder to fluidize and to flow. Better fluidization characteristics lead to better equipment performance. Consistent flux transfer and the ability to flow through pipes and plastic hoses is directly affected by fluidization. Additionally, it has been observed that good fluidizing material shows less tendency to build up in the equipment. Buildup can quickly result in interruptions of the flux flow. When this buildup is expelled the nozzle may release an excessive amount of flux. This excess will in turn be deposited on the surface of the part, resulting in non-uniform flux distribution. It is possible to induce charge on flux when it travels through an electrical field. However, the powder, by its chemical and physical nature, displays instantaneous charge decay when it hits the grounded heat exchanger. Therefore, the forces that adhere the flux to the part are not electrostatic forces, but are more likely Van der Waals forces. In dry flux application, the following complications have been described by users when operating conventional flux qualities:

  • Fluidizing the powder and material transport is difficult. Vibration or stirring is necessary to improve on these characteristics
  • Problems with consistency of flux flow and uniformity of applied flux
  • Adhesion of deposited flux is inferior when compared with wet application
  • High humidity causes physical adsorption of water molecules to the fine powder dust in the booth. This may result in agglomerations
  • Recovering, recycling and reusing flux requires special attention

To be continued …

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Forward to part 3

Flux Characteristics and Transfer Systems in Electrostatic Application – Part 1

Sep 29
2017

flux-characteristics-and-transfer-systems-in-electrostatic-application

 

Introduction:

This article summarizes some experimental results of a project on electrostatic application of non-corrosive fluxes for aluminum brazing. The objective is to qualify and quantify flux powder properties and equipment parameters with positive effect for dry flux technology.

For more than 30 years, controlled atmosphere brazing (CAB) [NOCOLOK ® Flux brazing] has been the leading technology for the manufacture of aluminum heat exchangers for the automotive industry.

The most common flux application method is by spraying an aqueous suspension. Constantly agitated flux slurries with concentrations of approximately 10 – 35% solids are pumped from tanks to fluxing booths. All aluminum surfaces involved in the brazing process are coated with the slurry, resulting in a uniform flux layer. Excess flux slurry is removed with a high-volume air blow; the excess is then collected, recycled and reused in the fluxing booth. Before going into the furnace, the heat exchangers are pre-dried in a separate drying oven to remove residual moisture.

In wet flux application, the following are critical factors and need specific observation by the user:

  • Flux slurry concentration
  • Consistency and uniformity of applied flux
  • Flux loading on heat exchangers
  • Drying step

Depending on the particular brazing operation, flux slurries may become contaminated with dust, metal particles, rust and organic compounds. The used slurry also contains the soluble portion of the flux (i.e., small levels of potassium, fluoride and aluminum), and must therefore be treated and then disposed of in accordance with environmental regulations.

Over the past five years, some users of NOCOLOK brazing technology have implemented dry flux application methods. Based on the principles of powder paint technology, an alternative application technique was introduced in the brazing industry.

The benefits of electrostatic application are directly related to the problems of wet application:

  • No need to mix slurries
  • No need to monitor slurry concentration
  • No need for a surface wettability concept (i.e., surface treatment or wetting agent)
  • No separate drying step required to remove moisture
  • No waste water effluent

Particularly when dry fluxing is used in connection with evaporative oils and lubricants, the objective is to eliminate or significantly reduce water consumption during the process.

To be continued …

Forward to part 2

Forward to part 3

Electrostatic Fluxing – Additional Information

Jul 21
2014

Additional information to the Article Flux Application: Electrostatic Fluxing

In dry flux application, the flux powder is electrostatically charged (typical voltage is ~ 100 kV) and applied to a grounded work piece. An electrical field results in flux deposition of the work piece. In practice, anisotropic distribution of the electric field can influence the homogeneity of powder coverage. At edges powder may accumulate, while penetration of powder into deep/thick fin packages (e.g., in case of double row tubes) can be limited by the Faraday cage effect.

Flux powder is electrically charged in the gun. However, it loses charge relatively fast when it hits the grounded heat exchanger. Therefore adhesion of the flux on the work piece is established rather by relatively weak Van der Waals forces than by electrostatic forces. Fine flux particles adhere better on the surface – but they are more difficult to operate with in the dry powder feeding system.

The relatively fine flux particles are more difficult to handle in dry powder feed systems compared to coarser paint powders – therefore the equipment used for electrostatic flux application is adapted to meet the specific requirements. Venturi pump, hose diameter, air flow and spray nozzle suitable for flux application are designed to minimize the possibility for powder buildup and clogging in the system. Powder transport within the hose system and the spray nozzle is further enhanced by introduction of additional air streams. The direction of the powder flow should always be from top to bottom. Sharp changes in flow direction must be avoided. In critical areas additional vibration units are installed to avoid powder buildup.

There are two types of powder feed systems established on the market (see the illustrations in the article):

The first type starts with the flux powder being fluidized in a fluidization vessel by compressed air that is fed through a porous membrane at the bottom of the fluidization vessel. The air going through the flux makes it behave like a fluid, since the powder is essentially diluted with air. A pick up tube attached to a Venturi pump is extended into the fluidized flux. Powder dosage is controlled by the volume of air flow through the pump. To optimize fluidization the vessel may additionally be equipped with a stirrer.

This type of feed system works perfectly well for classic electrostatic paints powders that are easy to fluidize, however, it may be difficult to establish a stable fluidization with ‘standard’ flux powder (i.e. the flux powder quality offered for wet/slurry-based flux application).

Fluctuations in density of the fluidized bed can result in inhomogeneous spray pattern (splashing) and might be a source for flux buildup within the system.

NOCOLOK® Flux Drystatic is optimized to minimize the challenges of powder feeding, while providing sufficient fine particle fraction for good adhesion properties.

The second type of powder feed system works on the principle of feeding the powder by a rotating helix screw (see the illustrations in the article above). Because of the mechanical displacement of the flux powder from the hopper, such devices minimize fluctuations of flux powder flow.

Most mechanical type of dry flux feed systems work with standard quality NOCOLOK® Flux as well as with special ‘Drystatic’ grade NOCOLOK® powder.

To achieve flux distribution patterns for specific process needs (e.g., higher flux loads in tube to header areas, coating from both sides of thick cores), multiple spray nozzles are arranged for deposition of the necessary flux load at different locations of the heat exchanger.

Dry fluxing booths must be equipped with a filter system to collect the overspray. The overspray material is recycled within the booth. To avoid accumulation of impurities within the recovered flux, it is necessary to take care of the booth environment (i.e. avoid dust, fumes, and high humidity level) as well as for the quality of the compressed air used. Contamination introduced by the heat exchangers or the transport belt must be prevented as well.

Due to the relatively weak flux adhesion (compared with wet- or paint- application methods), handling of dry fluxed parts should be done with special care to avoid flux fall off, especially at higher flux loads. To reduce flux fall off, some users perform electrostatic fluxing on heat exchangers with evaporative oils still present on the surfaces. Thermal degreasing in this case takes place after fluxing – just before the parts enter the brazing furnace.

Flux Application: Electrostatic Fluxing

Aug 31
2012

This technique, also known as dry fluxing is gaining popularity as an alternative fluxing practice and therefore will be described here in some detail. Dry fluxing is a technology whereby the flux is electrostatically charged and applied to a grounded work piece, in our case a heat exchanger or individual heat exchanger components. The electrostatic attraction causes a layer of flux to be deposited on the work piece. A typical flux application system consists of a powder feed system, the electrostatic spray gun, the gun control unit, the grounded work piece and finally the flux recovery system.

The advantages of such a system over conventional wet fluxing are evident. Since the flux is applied dry, there is no need to prepare flux slurries, hence no need to monitor flux slurry concentrations. There is also no wastewater generated therefore more environmentally friendly. The dehydration or dry-off section of the furnace may be eliminated since the heat exchangers enter the furnace already dry. However, one must keep in mind that this is a relatively new fluxing technique and there are some minor drawbacks. Flux adhesion is not as good compared to that of wet fluxing. The flux also tends to accumulate on the leading edges of the heat exchanger and because of the Faraday cage effect, may have some difficulties in coating into corners or more specifically, in tube to header joints.

Powder Feed Systems
Presently, there are two types of powder feed systems on the market. The first type begins with the flux being fluidized in a hopper. Dry compressed air is fed through a porous membrane in the bottom of the hopper. The air rising through the volume of flux makes it behave like a fluid since the powder is essentially diluted with air. A pick up tube attached to an air pump is extended in the fluidized flux. Powder flow is then regulated by controlling the air-flow to the pump which is then delivered through the feed system to the spray gun. This type of feed system works perfectly well for powder paints. However, the flux has very different physical characteristics than powder paints (particle size, morphology) and so is difficult to fluidize. This must be taken into consideration when the manufacturer designs a powder feed system that relies on fluidization.

Dry Fluxing – Mechanical Flux Transport

The second type of powder feed system works on the principle of mechanical delivery or positive displacement. This means that the powder feed rate to the air pump is controlled by a screw or auger. The flux is contained in a main feed hopper and delivered mechanically at a controlled feed rate to the air pump. Powder flow is thus regulated by controlling the auger feed rate. This powder feed system does not rely on the flux being fluidized. Nonetheless, modifications over conventional mechanical powder feed systems are still necessary to overcome the differences between the flux characteristics and conventional powder paints.

Dry Fluxing – Powder Fluidization

Japanese heat exchanger manufacturers have used the technique of dry fluxing for many years now. Within the last few years, North American and European manufacturers have also installed electrostatic fluxing stations. Experience with this technique is being accumulated at a rapid rate, given that the equipment manufacturers and flux suppliers are taking an active role in improving the technology.

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