Fluid Tip Selection
The Air Atomized Tip
The function of the fluid tip is to meter and direct the material into the air stream. As previously stated, the fluid tip is also the seat for the fluid needle. This fluid seat and needle shuts off or restricts the flow of material.
The fluid tip and needle are a matched set and must be purchased as such. The user selects an appropriate sized (orifice opening) fluid tip based on the material viscosity.
High viscosity materials flow slowly and require a large orifice and high fluid pressure to obtain the necessary flow rates. On the other hand, lower viscosity fluids will require smaller openings and less pressure for optimum performance.
Pressure feed and siphon feed guns use fluid tips of slightly different designs.
Table of Comparison Micron Values for Common Micronic Objects
Pressure feed guns require a tip which is tapered to guide the fluid needle into place and allow it to fit snugly for positive shut-off.
Siphon feed guns require a fluid tip in which the inside walls are constructed at a wider, more open angle. This extra opening allows the easy passage of material necessary in a siphon gun. Since the fluid needle does not create a positive shut-off in siphon guns, the needle need only seat itself tightly enough to prevent dripping from the tip.
The selection of the diameter of the fluid tip and needle will vary with the material being sprayed. The following table designates tip sizes for materials of commonly used viscosities.
Fluid Tip Size Chart
|Fluid tip orifice size (in inches)
|.020 - .030
|.040. - .050
|.055 - .070
|.085 - .110
|.125 - .300
The two materials most commonly used in the manufacture of fluid tips are hardened steel for solvent based fluids and stainless steel for water based paints and chemicals.
Delivery in Ounces Per Minute Chart
|At 8 P.S.I
|At 12 P.S.I.
|At 18 P.S.I.
A rule of thumb that can be followed with standard materials when setting pressures is that with the air supply cut off, the paint will flow in a solid stream.
This fluid stream should begin to bend down about three feet from the spray gun. Anything in excess of three feet means too high a fluid velocity. (See figure below).
Tips are selected by their orifice size (.007 to .072 inches) and fan angle (10 to 80 degrees). The proper selection is determined by the fan width required for the specific job and the orifice size that will supply the desired amount of fluid and accomplish proper atomization of the material.
For low viscosity fluids, smaller orifice tips generally are desired. For higher viscosity fluids, larger orifice tips are preferred. The quantity of fluid sprayed is determined by the size of the orifice; the thickness is determined by the orifice and the fan angle. Note that two nozzle tips having the same orifice but different spray angles, deposit the same amount of paint over a different area.
Note that orifices are not circular but are elliptical in shape. The diameters referred to are equivalent to a circular diameter. A good rule is to determine the largest fan angle and the smallest orifice that is practical for your specific fluid and application method.
How It Works
- Three basic methods used in finishing application to atomize materials: air spray, airless spray (hydrostatic) and centrifugal.
- Air assisted airless concept uses the air and airless methods.
- A pump is used to force material through a small orifice or tip at low hydrostatic pressure.
- Airless pressures are typically 2000-3500 psi. Air assisted airless operates at pressures generally under 2000 psi.
- Usually, low airless spray pressures form "tails."
- To complete the atomization and eliminate tails, low pressure (10-30psi) compressed air is added to the airless spray by an air cap.
With the air assisted airless concept, materials can be atomized with full spray patterns at low pressures. This provides a low turbulence of the material on the part surface and overspray conditions are minimal, increasing the transfer efficiency of the material.
Fluid Tip Identification
- The orifice size of all Graco airless tips is determined by the last two digits of the six digit part number.
- Example: 182-211 = 0.011 Orifice 182-215 = 0.015 Orifice
- The minimum fan pattern width and degree of the angle of the fan is designated by the fourth digit
- Double the fourth digit for minimum fan width
- Example: 182-211 = 4" minimum
- Add two inches for the maximum fan width: 182-211 = 4"+2" =6" maximum All dimensions are based on 12" from work surface.