Now, dense phase powder transport, or high density low volume air (HDLV) delivery, is a new approach that is having a dramatic effect on powder coating results. Consider the following list of advantages being seen in the field:
- Increased transfer efficiency.
- Better coverage for difficult to coat parts.
- Less powder consumption.
- Easier application requiring less operator effort.
- Increased production throughput.
- Faster color change.
- Reduced maintenance costs.
This article provides a brief overview of the evolution of powder coating transport systems and how HDLV equipment works to provide these added benefits.
Traditional Powder DeliveryThose new to powder coating are always amused to watch solid powder coatings flowing like water through a hose. "Fluidized" powder in a hopper looks as though it is boiling and feels almost wet to the touch.
For the sake of simplicity, a powder delivery system can be broken down into a few major components: hopper, pump, hose and spray gun. Powder stored in a container, or hopper, is fluidized. The hopper is usually connected directly to a pump, which sends fluidized powder through hoses to the spray gun. The most common apparatus for fluidizing powder coatings is the powder coating hopper in which compressed air is introduced below the powder through a porous membrane. This air mixes with the powder to fluidize the powder. A second stream of air is introduced through a powder pump that creates a vacuum in the pump's chamber by way of the venturi effect. (This is the same effect that sucks cigarette smoke out of a moving car when the window is opened.)
In traditional systems, powder feed hoses ranging in diameter from 5⁄16 to 3⁄4" (8 to 18 mm, depending on the desired flow rate, are used to transport the powder. Improper hose sizing or inadequate airflow can cause settling out of the powder coating from the air stream.
At the opposite end of the hopper, the powder hose is connected to a powder spray gun engineered to exhaust the volume of powder/air mixture with sufficient velocity to direct a stream of powder towards the part in what is often referred to as a "cloud".
As you might imagine, just like a garden hose fitted with a nozzle, if the gun opening is too large, the powder will trickle out. Too small, and the velocity will be too turbulent. The same volume of powder is released but the spray pattern is severely disturbed.
And because powder coating is an electrostatic process, the propelled powder also passes through an electrostatic zone where fluidized powder in the proximity of a (usually negatively charged) electrode becomes charged and travels, along with free ions, in a powder cloud toward the grounded part.
Proper coverage, including wrap and the powder coating's ability to penetrate into various geometries is related to combination of electrostatic charge and the aerodynamic forces affecting the powder cloud.
In designing systems, engineers balance the desire for high powder discharge rates with the effective charging and wrap that can be achieved as the powder cloud dwells in the electrostatic zone.
Road to Dense PhaseWhile engineers realized the potential advantages of dense phase transport early on, there have been significant engineering obstacles to overcome in commercializing the technology.
Pumping powder poses problems. It is difficult to say, and it is even more difficult to fix. Gases and liquids are relatively easy to pump, and a range of piston, diaphragm, peristaltic and other designs are available from which to choose. But powder particles become tiny abrasives when they come into contact with most pump components; and these pumps used with powder could require a great deal of maintenance.
That is why most traditional powder pumps have relied on a venturi design with no moving parts, rather than piston and diaphragm pumps. But venturi pumps require a high volume of air by nature and dense phase transport is not possible with them.
One of the major goals when developing the HDLV pump was cleanability and serviceability, so the interior components of the HDLV pump are easily removed, cleaned and replaced.
To overcome the wear problem, a (Nordson) HDLV powder pump draws on material advances in high tech polymers. A few years ago, Nordson engineers collaborated with a polymer formulator in Akron, Ohio; an area rich in elastomer technology because of the tire business headquartered there. Nordson developed a polymer material for hoses that was more resistant to impact fusion and easier to clean than the industry standard materials.
While these hoses are extruded from this polymer, with some re-engineering Nordson found it could also injection-mold this material to design wear-resistant materials for the dense phase powder pump. The HDLV uses a pinch valve design to provide precise control and consistent displacement with each pump cycle.
Like a human heart, the HDLV pump operates by suctioning powder into the pinch valve, which may be opened and closed every 250 milliseconds (see "Heart of a New Machine" sidebar). The HDLV pump can be operated at a range of cycle times to deliver powder anywhere from 6 lb of powder/hr (50 g/min) to 60 lb/hr (450 g/min).
In traditional powder pumps, the powder volume can be varied by adjusting both the atomizing air and the flow rate that interact with each other. In designing HDLV, the engineers created a design on experiments to evaluate the optimum combination of air controls to deliver consistent sample volumes for any flow rate required from 0 percent to 100 percent flow. The digital control technology allows these pre-determined parameters to be used in controlling the HDLV pump with the simple adjustment of a single flow rate control knob; taking the guesswork out and building in repeatability and uniformity.
Systems Approach to Dense PhaseOne of the mistaken ideas of dense phase powder coatings has been to approach it on a component basis. An HDLV pump alone is nowhere as effective a tool as an HDLV pump with the proper control and spray equipment.
Considering that a conventional spray gun is designed to exhaust a much higher volume of powder/air it's not useable with the low air volume produced by an HDLV pump without modification. The powder entering the large cavities of a conventional gun would slow don dramatically and barely trickle out of the nozzle making it impossible to paint.
Some suppliers have attempted to fix this problem by reintroducing air at the spray gun with a diffuser. Bu this practice negates some of the biggest advantages of the dense phase approach. Many of the dense phase benefits come as a result of the efficient charging and soft powder cloud produced by the added dwell time and higher powder density at the gun tip. Added air only increases the powder velocity and dilutes the powder charge, eliminating gains in efficiency and better coverage.
Providing the benefits offered by dense phase transport required the design of a new spray gun that can handle low air volumes. Nordson's Prodigy gun is specifically designed to work with the HDLV pump and provides electrostatic charging and transfer efficiency.
BenefitsUsing a high volume of air with powder has been the status quo for years because it makes life simple. Lots of air with less powder is easy to pump and move through hoses and spray guns.
Dense phase transport uses significantly less air to do the same job. While conventional venturi style systems typically use between 3 to 4 cfm of air for delivery, HDLV systems use between 0.5 and 1.0 cfm to deliver an equivalent amount of powder. Using a smaller volume of air has some significant benefits. Hose sizes can be reduced. The hoses used for HDLV transport are typically less than 1⁄4" (6 mm) compared to 1⁄2" (12 mm) dia. Smaller diameter hoses mean a smaller cross sectional area to be cleaned during color change. This makes for much more rapid color changes with less chance of contamination.
Another quick color change feature provided by dense phase technology is that the HDLV pump - unlike traditional venturi pumps - may be operated in reverse, pumping powder from the gun back to the powder supply. This allows for the lines to be flushed rapidly without powder being exhausted into the spray booth.
But most importantly, a denser and lower velocity powder cloud results in better charging efficiency. Powder densities in conventional systems average around 0.2 pounds per cubic foot, while the density of powder in the dense phase system is 0.45 pounds per cubic foot - well over double the powder density. The associated charge density means that aerodynamic forces are secondary to the electrostatic forces and as a result a "softer" and more thoroughly charged powder cloud is achieved.
This phenomenon results in both better transfer efficiency and in better electrostatic coverage, including the ability to penetrate into more difficult recesses normally prone to Faraday cage problems.
The high efficiency of the HDLV system means that more product can be coated with the same volume of powder. It is also easier for manual operators to coat difficult parts requiring less time and muscle than conventional spray guns. Powder consumption is reduced, productivity increases, and profits are higher.
Advances in powder coatings keep coming, feeding off the controls and material science advances in other industries.
High density low volume air delivery of powder coatings is no made possible by devices like the HDLV pump and iFlow electronic controls. A properly designed Dense Phase system, including pump, hoses and a properly engineered spray gun, can provide benefits such as faster color changes, reduced contamination, better transfer efficiency, improved coverage and Faraday cage application. These advantages stemming from the reduced hose sizes, softer powder cloud and longer electrostatic dwell time bring with them other commercial benefits such as reduced labor, lower powder consumption, more uniform paint coverage and increased profitability.
Sidebar: Heart of a New MachinePerhaps the measure of how far a technology has evolved is how closely it begins to resemble its creator. Modern airplanes can virtually land and take off by themselves. Predictive cruise control in cars allows them to "look" ahead and slow down or speed up with the pace of traffic. Our computers remember our preferences and correct our spelling mistakes. The next generation of powder coating systems now contains hardware that looks very much like our own vital organs; a heart, a brain and a sophisticated circulatory system. Sound far fetched? Take a look at the latest dense phase technology.
At the center of it is the HDLV (high density low volume) pump. It resembles a human heart in that it consists of two side-by-side pumps, each with an upper and lower chamber working together to provide precise, carefully synchronized powder transport.
Like the human heart, the HDLV pump is controlled by a brain, which is able to measure changes in the system and take corrective measures to keep the pump working properly. The HDLV pump is controlled by sophisticated electronic circuitry that includes closed-loop flow control. If the surrounding conditions such as plant compressed air deliver changes, the controls provide the necessary compensation to keep the HDLV pump working consistently.