Powder Coating Done Right

Most coaters know what a throat pipe is, but many don’t know how important this component is to successful powder coating.

When powder coating with a venturi injector, the throat pipe is the device that, along with the internal nozzle, provides the negative (suction) pressure inside the injector that “sucks” powder out of the hopper or box and propels it out of the injector, through the powder hose to the powder gun. While it looks like an innocent piece of plastic, it is actually a relatively low-cost, precision-engineered and precision-manufactured component designed to deliver optimal powder flow with the least amount of conveying air. Built from materials such as nylon, Delrin or Teflon, these throat pipes are designed to resist, but not eliminate, erosion from powder particles flowing through them. 

It is essential to note that throat pipes are precision engineered and precision manufactured. By design, the entry angle, the primary inner diameter (orifice), and the exit angle are all set to precise values that, along with the nozzle diameter and the main air pressure, define the suction pressure that pulls powder particles into the injector and directs them through the powder hose to the gun with the lowest amount of air producing a gentle low velocity spray onto the part being coated. Note: When engineered properly, supplemental air may need to be introduced downstream of the throat pipe to maintain a velocity that keeps these particles in suspension until they exit the gun. More on this in a future column.

Here's where issues start to arise: As the powder starts to erode the throat pipe, the “precision” aspect of it degrades quickly. Degradation is in the form of erosion of the entry or exit angles or enlargement of the primary inner diameter. Erosion of the entry or exit angle results in turbulent flow through the throat pipe and a significant reduction in powder flow. Enlarging the primary inner diameter reduces the injector suction pressure, thereby reducing powder flow. Coaters typically compensate for this reduction in powder flow by increasing main air pressure, which does increase powder flow back to “normal” levels, but it does so at much higher pressure, which results in higher powder velocities coming out of the gun. Higher exit velocity has the adverse effect of causing powder “blow back” that then reduces transfer efficiency and increases coating costs.

It is recommended that the throat pipe be inspected daily when the injector is being cleaned. Any evidence of wear or erosion, no matter how insignificant it may seem, should result in immediate replacement. Delaying replacement of an eroded throat pipe will result in loss of transfer efficiency and increased cost of powder coating. Waiting weeks, or even days, to replace will cost you much more than the relatively low cost of immediate throat pipe replacement.

John Cole is president of Parker Ionics—a powder coating application and systems supplier—and sits on the Powder Coating Institute board of directors.

The venturi injector contains a throat pipe made of nylon, Delrin or Teflon. As the powder starts to erode the throat pipe, it degrades quickly, in the form of erosion of the entry or exit angles or enlargement of the primary inner diameter. See the progression of the erosion in this photo series. Photos courtesy of Parker Ionics.