Spray Equipment Selection & Setup
May 18th, 2013 | author: Phil Stevenson
There are hundreds of spray gun options to choose from when evaluating new spray equipment selection & setup. When asked what my favorite spray gun is, I answer that the best gun is the gun that is designed to perform the best with the coating I am spraying. The spray gun that performs the best in the application of the coating is the preferred spray gun for the product.
With the vast array of options, purchasing the right spray gun and setting it up to optimum performance can be a daunting task. The choice of equipment should be broken down into two categories: equipment for hand spraying and equipment for automatic applications.
There are two types of material categories used in most wood finishing shops. Spray to color stains, shaders,and toners, and heavier bodied materials such as sealers, primers, and top coats.
Spray to color coatings are low in viscosity and require little atomization energy to effectively atomize the materials and heavier materials require higher energy.
HVLP (high volume low pressure) gun technology is normally recommended for the application of these types of materials that are spray to color only. HVLP gun technology uses a large volume of air at low pressure to atomize the coating material. The forward velocity of the stain particles traveling to the part is relatively slow compared to other higher pressure atomization technologies. Therefore, HVLP technology applies the staining materials more effectively into corners and recesses, which will minimize areas where the stain is not completely covering the substrate.
HVLP guns require a considerable volume of compressed air to operate properly. Always check with your equipment supplier to ensure the equipment is set up with adequate compressed air requirements. When the spray guns are starved for air, excessive air pressures will be required to sufficiently atomize the coatings. This will result in excessive overspray and uneven stain application. (See side bar for HVLP set up recommendations.)
For manual application of higher viscosity primers and topcoats, air assisted airless (AA) application technology should be considered. This preferred application technology uses hydraulic pressure at relatively low pressures of 400-900 hydraulic psi. A small amount of compressed air for final atomization is used to provide a very soft spray pattern. This technology provides exceptional atomization energy to break up higher solids/viscosity coatings. Good flow rates, minimal overspray, high finish quality, and excellent transfer efficiencies are the features of AA spray technologies. (See side bar for set up of manual AA spray technologies.)
Side Bar: Manual Spray Gun Set UP
HVLP Set-up for spray to color stains, shaders and toners
Tip size 1.2 to 1.4 mm
Atomization air pressure: 15-22 psi
Typical flow rate range: 48 oz per minute
Average fan pattern width: 8-10 inches
AA set-up for sealers, primers, and top coats:
Tip size .013 for sealers for and clear coats (Kremlin 09.-)
Tip size .015 for primers and paints (Kremlin size: 12.-)
Hydraulic fluid pressure range: 400-600 psi
Atomization air: 12-20 psi
Average fan pattern width range: 8-10 inches
Typical gun to part distance: 7-8 inches
Please note that all the above settings are general depending on part size and coatings sprayed.
Most automated spray finishing in wood is finished on a flat line reciprocating spray machine. Therefore, our discussion will be focused on automated flat line finishing.
When choosing automated spray gun technology, the same guidelines are generally followed for selection of gun technologies as described for hand spraying. Air assisted airless for paints and clear coats and HVLP for spray to color materials.
The first consideration when setting up automated spray guns is how fast the line speed will be operating. Slower line speeds equal higher finish quality, and higher line speeds equal lower finish quality. Typical flat line speeds range from 3-8 meters per minute. Paints and clear coats are most commonly run at 6 meters per minute. Stains are generally run at 3-4 meters per minute. Where premium finish quality is not a priority, line speeds may log in excess of 8 meters per minute. It is normally recommended to run at the slowest production speed possible that provides the desired production through-put and an acceptable finish quality.
Line speeds dictate the number of spray guns required to apply the coating to the specified mil thickness. Industry guidelines specify one gun for every meter the line speed is running for high quality spray application results. Spray guns must be paired in sets of four guns. Therefore a four gun set up will be adequate at line speeds of up to 4 meters per minute. At over four meters per minute, an eight gun set up will provide the best application process.
The alternate set-up for very high speed production may require the use of airless gun technology. Airless gun technology uses high fluid pressure only to atomize the coatings. Airless gun technologies may be considered a viable technology to use with line speeds greater than six meters per minute.
A four gun airless set up can be considered to consistently apply many coatings using high line speeds. Not all coatings work well with airless – be sure to consult with your coatings supplier to verify airless application technology viability.
Choosing Tip Size: Using four guns versus eight guns will determine the range of tip sizes that should be used. Typically, the four gun set up will require larger tip sizes than an eight gun set up if line speed and mil thickness requirements are equal. Also, faster line speeds require more material to be applied in a given time than slower speeds. Therefore, tip sizes should be selected according to coating characteristics, number of guns used, and the line speed to hit the targeted mil thickness.
Fan Pattern: The width of the gun fan pattern should be selected as wide as possible for most automated spray applications. This is especially important when a four gun set up is used. Wide fan patterns will make it easier to achieve a consistent 50% overlap in the spray pattern to provide uniform film build. The reciprocating speed of the spray machine must be set at the precise number of cycles to provide a 50% overlap. The reciprocating speed should be set at lowest value while maintaining uniform coverage of the coatings. The faster the reciprocator runs, the more the amount of overspray increases, transfer efficiency is reduced, finish quality is lowered, and the cleanliness of the spray cabinet is affected. Therefore, the entire set up process should be maximized to eliminate these negative impacts on the application process.
Spray Direction: Spraying in the pushing or pulling mode or in both directions should be evaluated closely. With most automated spray applications, the proper set up of the equipment will provide the best results in the pulling mode. Spraying in the pushing mode can cause the fan pattern to exhibit excessive turbulence or a snow plowing effect that can cause the spray pattern to bounce up into the spray cabin rather than effectively coating the edges and profiles of the product. Loss of coating material and finish quality can be attributed to spraying in the pushing mode. Most application processes should be set up in the pulling mode only to provide the best finish quality and transfer efficiency. Maintaining good air flow in the spray cabin with a slightly negative air pressure will help the spray pattern in the pulling mode to wrap around the edges and profiles of the parts.
Gun Tip to Belt Distance: When using air assisted airless guns, the gun tip to belt distance should be set at approximately six inches for parts up to one and half inches high. For thicker parts, the guns should be raised to six or seven inches in height from the tip of the gun to the surface of the belt. For spraying stains with HVLP guns, the height of the gun should be set at approximately seven and a half inches from the belt to the tip of the gun.
Downward Angle: The downward angle of the gun for the air assisted airless guns should be set at approximately 50-55 degree angle. HVLP guns are normally set at around a 60 degree angle for most stain applications.
Swivel orientation: The swivel orientation of the gun in relation to the travel of the belt can be set in two different positions depending on coating material and product type being sprayed. The two options are at a 40 degree orientation to the direction that the belt travels or parallel to the direction that the belt travels. It is recommended that you try both settings to determine the best orientation to achieve the best coating uniformity and edge coverage.
Air Pressure: The atomization air pressure for both HVLP and AA guns should be set at the lowest value to effectively break up the coating material into satisfactory particles sizes.
Fluid Pressure: For both spray application technologies, the fluid pressure should be set at a value that delivers a flow rate to meet the requirements of the finish application process. Excessive high pump pressures with the AA guns may over-atomize the coating material. Changing to a large tip and lowering fluid pressure will be necessary. Too low a fluid pressure may cause particle size to be too large to provide adequate finish quality. Therefore, a smaller tip should be used and pump pressure will need to be increased.
Whether you are manually spraying or using automation, always remember that the best application process will result in a very calm spray pattern and minimal over spray. Using excessive atomization pressure, fluid pressure, or reciprocating speed is the enemy of fine finishing.
There are no recommendations for setting your equipment that fit all application requirements. However, if best practice standards are followed, an outstanding finish quality and high efficiency can be achieved.
Side Bar: Automated Spray Gun Set Up
Automated HVLP Set Up
Line speed: 4-6 meters per minute
Number of guns: eight
Tip Size: .6 – 1.0 mm
Flow Rates: Varies by stain type and determined by adjusting pressure settings to hit target color
Fluid Pressure and Pressure Regulator: 3-8 psi
Delivery pump pressure: 12-18 psi
Fan Pattern Width: Fully wide open
Atomization air: 15-22 psi
Gun Downwards Angle Setting: 60 degrees
Gun Tip to Belt Distance: 7 ½ inches
Gun Swivel Orientation: 45 degrees to belt feed direction
AA Spray Gun Set Up
Line speed: 6 meters per minute
Tip Size: .012 to .018
Fan pattern Width: 12-14 inches
Gun Tip to Belt Distance: 6 – 6 ½ inches
Atomization Air: 15-20 psi
Pump Pressure: 450 – 800 psi
Gun Downward Angle: 55 degrees
Gun Swivel Orientation: 45 degrees to parallel with belt feed direction
If you would like more information about this topic or to schedule an interview, please contact Kenny Stevenson at (856) 696-8467 or firstname.lastname@example.org. For online information please visit AWFI.org