An improper spray pattern can be caused by several issues. The first thing to consider is the spray angle designation on the tip itself. Spray tips are available in a wide variety of configurations. Your choice of spray angle is dependent on two factors: the tip spacing on your spray bar and viscosity of the materia. Tips are calibrated with water so an 80/70 designated tip means an 80-degree spray angle with 7 gpm flow rate at a set pressure and the viscosity of water. All sealcaoting contractors have experienced firsthand the wide range of material viscosity in the field so each operator needs to test and know his own material. If you were to test an 80/70 spray tip with a gradual increase in viscosity you would see the actual spray pattern getting a little narrower with each increase. The opposite would be true with each reduction.
The second consideration affecting spray pattern is the pressure the material is under. When material is pushed through a fixed orifice (spray tip) at a reduced pressure the spray angle will be reduced proportionately. If the pressure is too high then the spray angle will be distorted and atomization of the material will occur, possibly causing overspray problems. The spray system needs to have a controlled, constant material pressure to maintain the spray angle. This also will address the inconsistent coverage issue too. (Every time the material pressure changes so does the flow out of the tip.)
Viscosity and Its Importance
Viscosity is the measurement of material thickness in its liquid form. The first benefit to measuring viscosity is to protect you and your supplier against a bad batch of product. It is not uncommon for anomalies to occur in the manufacturing process. Even the best suppliers can have occasional problems. If you cannot measure your product then how do you know it meets your high standards? As an applicator your reputation is on the line if the material fails to last. It’s a smart practice to periodically check your equipment to ensure it is in good condition, why not do the same with your material? Measuring viscosity can be accomplished easily and inexpensively in the field with the right tools.
The second benefit to knowing your viscosity is it helps you decide how to apply the material. Knowing the viscosity will help you determine the tip size, speed of the application, and the desired thickness. Once you know the viscosity setting up your machine for spraying is easy.
The spray tip is an orifice. Pushing a constant viscosity material through a fixed orifice requires a specific pressure and will produce a measured output. What the heck does that mean?
The pumping system should provide a consistent supply of material at a given pressure with the least amount of pressure ripple. If you want to apply material at a specific application rate you can very easily change your spray tip size.
Here is an example: Viscosity is approximately 44,440 centipoises, pump pressure is 22 psi, and tip size is 80/70. The flow out of each spray tip will be approximately 4.9 GPM. The nozzle spacing directly effects the application rate so factor that into your spread rate formula. Using desired rate, volume per nozzle, and overlap you can calculate the application speed of your equipment. There is equipment on the market that can do the math for you.
At the end of the day you want to have supplied a quality job to your customer, have a project you can be proud to show your family, and have a well-earned profit in your pocket.
Randy Tattershall is sales manager/senior engineer for Rayner Equipment Systems, Sacramento, CA; 916-381-8033.