TECH TIPS - SPRAY GUNS

Q. How much does the air cost that I use in my spray gun?
A. Is air free?  Technically, the air we use is free, but the cost to compress that air is not.  Factors to consider are:
  • Local electrical costs
  • Type of compressor
  • Pressure used for spraying
  • CFM consumed
  • Amount of time the trigger is being pulled
  • Number of coats being applied
  • Efficiency of the gun/cap/tip combination

Before getting into the cost calculation, let's cover some of the not so obvious factors.

The consumer type compressors that are bought from the big box stores typically do not provide enough air volume/pressure for individual applications.  Two stage compressors provide the higher pressures and volumes required.

The air pressure supplied to the spray gun should be set at the lowest pressure that will allow the atomization required and still allow the operator to accomplish the production required.  Higher than required pressures result in higher air consumption, therefore higher electrical costs.

In general, it is more economical to apply the finish coat in one pass than using two thinner coats.  Less paint is used and less electricity is used.  Be sure to check your coatings Technical Data Sheet for proper application and film builds.

To calculate the daily electrical costs for the air in a spray gun use the following formula:

DC X HC X ED X CFM X .228

DC = DUTY CYCLE = PERCENT OF TIME TRIGGER IS BEING PULLED
HD = HOURS IN WORK DAY
EC = ELECTRICAL COST PER KILOWATT HOUR (.0935 NATIONAL COMMERICAL AVERAGE, JULY '07
CFM = CFM RATING OR AIR CONSUMPTION OF THE GUN
.228 - ELECTRICAL CONSUMPTION OF 1 CFM IN KWh

For example, what is my annual cost to supply the air for my 18 CFM air cap.  How much would I sve if I lowered my pressure to a pressure that consumes 12 CFM or use a different air cap that consumes 12 CFM?

My factors are:

  • Duty cycles = 50%
  • Work day = 15 hours
  • Cost per KWh = $.10
  • Days per year = 250
  • 18 CFM Air Cap

.5 x 15 x .1 x 18 x .228 x 250 = $769.50 annual electrical costs

Compare the above to a gun/pressure combination consuming 12 CFM

.5 x 15 x .1 x 12 x .228 x 250 = $513.60 annual electrical costs

Lowering air consumption by using lower air pressures or using an air cap with less air consumption would result in an annual savings of $256.50 per spray gun.

The above calculation are based on typical efficiencies and are approximate. The change in cost is

Q. I cannot trigger my spray gun. What causes this problem?
A.
  1. Trigger safety is in the SAFETY position. Put trigger safety in SAFETY OFF position. Refer to your Instruction Manual for the operation of your particular spray gun's safety.
  2. The gun housing may be blocked from pumping a highly viscous material with a lot of filler that has packed behind the gun needle and will not allow the gun needle to move. Disassemble and thoroughly clean the spray gun.
Q. What parts of the spray gun require lubrication?
A. The fluid needle packing A, the air valve packing B and the trigger bearing screw C require daily lubrication with a non-silicone/non-petroleum gun lube. The fluid needle spring D should be coated lightly with petroleum jelly or a non-silicone grease (i.e.. lithium). Lubricate each of these points after every cleaning in a gun washer.  
Q. What causes a top or bottom-heavy spray pattern?
A.
  • Horn holes plugged - clean holes with non-metallic point (i.e.. toothpick)

  • Obstruction on top or bottom of fluid tip - clean

  • Cap and/or tip seat dirty - clean

 
Q. What causes the spray pattern of my airless gun to feather?
A. The problem you refer to as feathering is also known as tailing. This is due to low fluid pressure at the tip. To avoid feathering or tailing, increase the fluid spray pressure by increasing the pressure at the pressure control knob. If the pressure control knob is turned all the way up you may have to thin the paint or reduce the length of airless hose being used.  
 
Q. How should the air cap be cleaned?
A. Remove the air cap from the gun and immerse it in clean solvent. If necessary, use a bristle brush to clean dried paint. Blow it dry with compressed air. If the small holes become clogged, soak the cap in clean solvent. If reaming the holes is necessary, use a toothpick, a broom straw or some other soft implement. Cleaning holes with a wire, a nail or a similar object could permanently damage the cap by enlarging the jets, resulting in a defective spray pattern.  
 
Q. How can I make my tips last longer? What causes tip wear?
A.

Fluid tip wear on airless/air assist airless guns is caused by several factors.

  1. The pressure being used.
  2. The size and shape of the tip.
  3. The abrasiveness of the material being sprayed
  4. The tip material

Fluid pressure
The higher the fluid pressure, the greater the wear.

Tip size and shape
Smaller tip sizes result in shorter tip life with other factors being equal.
Larger tip sizes result in longer tip life with other factors being equal.
Shorter pattern sizes give longer tip life than taller pattern tips.

Material abrasiveness
Highly abrasive materials will result in a shorter tip life when compared to low or non-abrasive materials.

Tip materials
The quality of the materials will help determine tip life. Higher quality materials like carbide, while more expensive to manufacture, will not wear as quickly as carbon based material.

How does tip wear affect my setup?

Flow rate increases
As the tip wears, the physical opening in the tip increases.  An increase from .015” to .017” (two one-thousands of an inch) may result in a 33% increase in flow rates. How quickly this happens depends on the factors listed above.

Pattern size decreases
The tip will wear out in the top and bottom portions of the tip opening. This will result in a smaller pattern size. It will continue to decrease in size as the tip wears.

How does tip wear affect my spraying?

Tip wear is gradual, usually over days or weeks. The operator will attempt to compensate by doing the following:

  1. Increase fluid pressure (an attempt to achieve an acceptable pattern). This will increase fluid delivery even more.
  2. Back away from the part (an attempt to achieve a larger pattern). This may result in a dryer spray pattern.
  3. Increase gun speed (an attempt to prevent runs and sags).


 

Q How long should a gun last before needing to be rebuilt? What can I do to make it last longer?
A This depends on what material you're spraying and how many gallons sprayed
per day. For example, with lacquers, guns don't need rebuilding as often because lacquers don't have solids in them. In contrast, the high solids in blockfillers are abrasive and require more frequent gun rebuilding.

One way to increase gun life before repacking is to thoroughly clean your gun at the end of every day. Be sure to trigger the gun before removing the diffuser and when installing the diffuser. If you don't, the diffuser will score around the ball on the new needle which can lead to premature wear. Your gun will develop a leak and this will cause spitting.
 
Q. Why did my airless gun just stop spraying?
A. The tip may be clogged. Turn the black plastic arrow shaped handle to the unclog position (arrow facing rear of the gun). Aim the gun into a waste paint container. Squeeze the trigger for a second. Return arrow shaped handle to the spray position (arrow facing forward) to begin spraying. Always set the spray gun trigger safety when working with the spray tip.
 
Q. How long should a gun last before needing to be rebuilt? What can I do to make it last longer?
A.

This depends on what material you're spraying and how many gallons sprayed per day. For example, with lacquers, guns don't need rebuilding as often because lacquers don't have solids in them. In contrast, the high solids in block fillers are abrasive and require more frequent gun rebuilding.

One way to increase gun life before repacking is to thoroughly clean your gun at the end of every day. Be sure to trigger the gun before removing the diffuser and when installing the diffuser. If you don't, the diffuser will score around the ball on the new needle which can lead to premature wear. The gun will develop a leak and this will cause spitting.
 

Q. My gun is spitting a small paint stream of paint after releasing the trigger and additional material is splattering on the surface. What's the cause and solution?
A. The cause of the problem is that the needle is not seating properly in the seat. You will need to either purchase a kit for the gun needle and seat or you may only need to clean the needle and seat assembly. Residue or debris may cause the needle to move off to the side before seating.

Tip extensions, extension poles and tip filters can also cause spitting or a stream of fluid after the gun is released due to decompression of the paint trapped between the gun and the tip. Fluid pressure dissipating through the tip causes this spitting or paint stream. An automatic shut-off valve will prevent this problem.
 

Q. How do I know what size needle and nozzle is best for the job?
A. Although every job may have slightly different requirements, for most materials it is best to choose a mid-size, or No. 3, needle and nozzle.  If your paint is thicker than standard oil-based enamel, you may want to consider a larger size.  Remember that there is no one tip that is perfect for all jobs.  Needles and nozzles are quick and easy to change out.  So try different sizes until you find what works best.
 
Q. Why am I getting a bad spray pattern, two thick lines at the top and bottom of the spray pattern, or meager flow from my tip?
A. 1. Tip is partially plugged. Reverse the spray tip and trigger the gun to clear clog per instructions in your manual. Remove and clean the tip, but do not use any metal objects to clean it with because the tip carbide could be chipped and damaged.
2. Pressure switch is set too low. Increase pressure by turning the knob clockwise.
3. Filters in sprayer or gun are plugged. Remove and clean the filters or replace as needed.
4. Tip is too large or worn out. Replace the tip with proper size for the material being sprayed and within the rated tip size of the sprayer.
5. Material is too thick. Let the paint warm to room temperature if the paint is cold or thin the paint according to the manufacturers instructions.
6. Paint hose is too long and/or too small diameter. Use shorter and/or larger diameter hose.
7. Extension cord is too long or not a heavy enough gage. Use as short of an extension cord as possible and it should be at least 12 gage or larger.
8. Valve seat, ball or gasket is worn out or valve may have been loose, causing leakage and damage. Replace valve.
9. Pressure switch is defective/worn out. Replace pressure switch.
10. The suction hose is blocked, kinked, or cracked and leaking air or suction filter is blocked. Examine the hose and filter and clean or replace as necessary.
 
Q. Why do I get streaks in my spray pattern?
A. Streaks in the spray pattern, especially heavy bands at the outside edge, is an indication of low pressure at the tip. Turn up the pressure control knob until these bands are eliminated. If the sprayer is already at maximum, you may have to use larger diameter hose or shorten the length of the hose to reduce the pressure drop. Also, make sure any paint filters in the system are clean, because there will be a pressure drop across a restricted or plugged screen.

Sprayers are rated for a maximum tip size. Using a tip that is larger than the maximum size or a tip that is worn larger will cause low pressure. The tip should also be the proper size for the type of material being sprayed.


 

Q. How do I choose an airless tip?
A.

The size of an airless fluid tip will depend on the following:

  1. The fluid delivery requirements.
  2. The size of the pattern required.
  3. The viscosity of the material being sprayed.
  4. The type and amount of solids in the material beings sprayed.
  5. The type of filtration in the system.
  6. The pump pressure available

Let’s go through basic tip sizes first.

Fluid tips generally have a size listed in thousandths of an inch. The size in normally the diameter of an equivalent circle. Thus we may have 4 or 5 tips that have identical opening sizes (ie .015”). While the sizes may be all the same, the shape of the opening is not. Imagine taking a 15 in flexible circle and changing the shape from a circle to an oval. How many different size ovals could we make?

Tips may also have an indication of the pattern size or angle of spray. That will vary with the manufacturer.

Looking at Figure 1, the areas of all the circles are the same size.

Tip “A” would provide:

The least amount of resistance
The least amount of "tip plugging"
The least amount of tip wear
The shorter / rounder pattern          

Tip “E” would provide:

The highest amount of resistance
The highest chance of "tip plugging"
The highest amount of tip wear
The taller / narrower pattern

                                   Figure 1

Fluid Delivery Requirements

Most manufacturers have charts available that give flow rates for various tip measured using oils of a specific viscosity at specific pressures. A good rule of thumb for fluid delivery is:

Doubling the fluid pressure will result in about a 40 percent increase in delivery.

Pattern Size Requirements

The choice of pattern size typically correlate directly to the job at hand. Most manufacturers have several pattern sizes available for a given tip opening. Match the size to your job requirements.

Material Viscosity

Higher viscosity materials will result in lower flow rates. Another factor to consider is the temperature of the coating material. Higher solids materials will be easier to spray and atomize better if heated. Check your Technical Data Sheet for the coatings maximum recommended temperature

Material Solids

Material solids directly affect the viscosity of the material. Higher solids material typically require larger tip openings. The particle size of the pigment will also determine how frequently the tip ‘plugs’. Use larger tip sizes if tip plugging happens frequently.

Filtration

Due to the narrow openings of most tips, filtration is highly recommended. Passing the material through a coarse strainer may be adequate on the shorter pattern tips, but a fine filter usually is required for the taller pattern tips.

Pump pressure

Use a pump that has enough pressure available to do the job. You should try to size a pump for no more than 70% of its rated pressure range
 

HVLP Spray Guns
Q. Can latex be sprayed with an HVLP Spray Gun?
A. Definitely. While most latex paints require some thinning for best results, the HVLP is a great tool for applications.  Consult your paint supplier for recommended dilution amounts.  Also, look for models with pressure-fed paint supplies because they work best with latex and other thick coatings.
 
Q. My HVLP gets very hot after running for awhile.  Is this normal?
A. It is perfectly normal.  The turbines inside HVLP units run at a very high rpm, which generates a large amount of heat. HVLP units are designed to run effectively at high temperature levels.  However, before using the unit, always refer to the owner's manual for safety warnings.
Q. How does HVLP differ from conventional air spray?
 
A. HVLP utilizes the volume of air available at the air cap rather than pressure to atomize. You will notice that all the orifices in an air cap are larger in an HVLP air cap in order to allow the volume to pass. HVLP air pressures are lowered internally in the spray gun air passages. Conventional air spray gun will normally have the same pressure at the air cap that was input to the spray gun.
 
Q. If HVLP means High Volume Low Pressure, why am I using High Pressure (40 to 80 psig) at the gun inlet?
A. When using compressed air, the lower the pressure, the smaller the volume of air available. For example, air compressed to 40 psi contains 5 times the original amount of volume. Air compressed to 100 pounds contains 8 times the original volume. Obviously at 10 psi to the spray gun inlet, very little volume would be available for atomization.
 
Q. Is HVLP available for all fluid feed systems (suction, gravity & pressure)?
A. Yes. Keep in mind that a suction feed gun depends on pressure at the fluid tip/air cap junction to create a negative pressure in relation to the cup atmospheric pressure. When using HVLP with 10 psi or less at the air cap, very little suction is created. Use only low viscosity materials with an HVLP suction feed gun
 
Q. Are other HVLP technologies available?
A. Yes. Industrial HVLP guns are commonly called conversion HVLP guns. They use higher pressure air and convert it inside the spray gun. Another form of HVLP is the turbine unit. Turbines are more typically used in architectural or commercial applications. (Think of a shop vacuum in reverse)
 
Q. Is HVLP required in all industries?
A. Not at this point in time. Even though some federal regulations restrict the use of conventional air spray guns, it is typically only required for "major emitters" as defined by the enforcing agency. Keep in mind that any local jurisdiction may have their own requirements (ie. Southern California)
 
Q. Is it possible to exceed 10 psi at the air cap?
A. Yes. HVLP air caps are typically rated for a maximum gun inlet pressure to achieve 10 psi at the air cap. For example if 50 psi inlet achieves 10 psi measured at the air cap, then an inlet pressure of 60 psi would exceed 10 psi at the air cap.
 
Q. How can I be sure I do not exceed 10 psi at the air cap?
A. 1. Operator training
2. Use an air cap test kit to measure the pressure at the air cap
 
Q. Do I need to use 10 psi at the air cap in order to  atomize?
A. Not necessarily. Use only enough air pressure (determines atomization quality) and fluid pressure (determines production capability) to do the job. Any pressure higher than that will increase over spray and lower gun efficiency
 
Q. Are all HVLP guns the same?
A. No. Just like conventional guns, HVLP guns are available in different feed methods, production and atomization capabilities.
 
Q. Can I interchange air caps on HVLP guns?
A. Yes and no. It depends on the manufacturer and gun model
 
Q. What are some reasons HVLP might not work?
A. Not enough air supplied. Restrictive quick disconnects and small inside diameter air hose are the common culprits. Additionally undersized compressors and low cfm regulators may cause problems. Another problem is not matching the equipment to the application. For example, trying to use a lower cfm air cap to spray high volumes of coatings.
 
Q. Is operator technique different for HVLP?
A. Spray gun to part distance typically is reduced to 6"-8" for HVLP compared to 8"-10" for conventional air spray guns
 
Q. How do you stop an airless spray gun from clogging?
A. The most common techniques for preventing clogging of the spray gun are to properly filter the paint, keep all dust and dirt from the surrounding air from depositing into the paint reservoir, constantly agitate the paint (without introducing air bubbles), and use the correct tip size for the paint being used.  When the gun clogs, take a small sample of the paint that is clogging the gun and look under low-power magnification to see if you can determine what is in the paint that is causing this problem. 

Typically, a stainless steel mesh filter is placed at the end of the paint pickup tube.  For even better filtration (finer mesh), you might consider wrapping some pantyhose around the stainless steel filter, but first ensure that the pantyhose is not sensitive to any of the paint ingredients, especially solvents.  Some airless spray guns provide a filter in the handle of the spray gun.

Ensure that you maintain the filter and keep it clean.  Also look to see there are not breaks in the mesh.  You can also purchase small filter washers that fit behind the fluid tip.  The mesh size on your airless spray gun might be to large, but you can often purchase other sizes, such as 30, 60, and 100 mesh.  If you use a wooden stick or paddle to mix or stir the paint, do not allow any old dry paint to enter the paint reservoir.  Regular and proper maintenance is the best preventative aid for clogging problems.
 

Q. What criteria should be known when selecting a Spray Gun?
A.

The selection of a spray gun is an important decision to make in the design of a spray system. Matching the gun and fluid source to the application is of utmost importance. Failure to choose the correct equipment generally results in lower efficiencies, increased coating costs, increased emissions, higher booth filter costs, etc.

To match the spray gun to the application, the following criteria must be known:

Fluid Viscosity:
Generally listed as high, medium and low - manufacturers' charts will usually indicate the capabilities of their air caps and fluid nozzles in terms of viscosity. If one knows the viscosity in centipoise or time in a viscosity cup, it is a simple matter of conversion to the criteria listed.

Fluid Flow:
Generally listed in ounces, cc, ml or gallons per minute - spray gun charts will generally indicate a range of flow rates for a given fluid tip / air cap combination.

Production Rate:
Production speed will dictate what the flow rate of the equipment should be. Obviously, a line running a 30 feet per minute with 6 square foot parts will require a higher flow rate than a line moving at 10 feet per minute with 2 square foot parts.

Available Air (psi & cfm):
In most plants, the available air is adequate to supply the necessary air required by a spray gun. Problems usually show up when:

  • Restrictive quick disconnects are used
  • Small I.D. air hoses are used (i.e.¼”)
  • Air hose longer that necessary is being used.
  • Compressors too small in size
  • Keep in mind, a typical industrial air cap consumes 20+ CFM

Fan Pattern Size:
Pattern size is one parameter listed for an air cap. It should be matched to the part being sprayed. Pattern sizes range from as small as one inch to several feet.

Atomization Required:
A “Class A” finish is not required for all applications. Applying a finish to a garden utensil would not necessarily be the same as the hood of an automobile. A stain applied to wood that is to be wiped off would dictate a very low level of atomization. If a lower level of atomization is required, an air cap with a lower CFM ($$$) would be selected.

Atomization Type:
Air Atomized:
Conventional Air Spray
Conventional air spray is the lowest in efficiency of the different atomization types, but yields good atomization with difficult to atomize coatings. Conventional air spray may not be permitted by Federal, state or local rules for some industries.

HVLP (High Volume Low Pressure)
An air atomized spray gun with a limit of 10 psi at the air cap. Since there is a legal limit (10 psi) in many cases, choosing the proper air cap for the above criteria is critical.

Hydraulically Atomized:
Airless
Uses high fluid pressure (1000-5000 psi) to atomize. Capable of very high flow rates, lower in atomization capabilities than air atomized equipment.

Air Assist Airless
Uses high fluid pressure (300-1500 psi) to atomize. At these lower pressures (compared to airless) the top and bottom of the pattern have a difficult time atomizing. An air cap is added for the sole purpose of eliminating the “tails”. Quality of finish is between air atomized and airless guns.

Fluid delivery system:
Suction Feed
Suction feed guns have been around for a century. While they are simple to operate, they are the most inefficient of the group. The range of motion is somewhat restricted since the pick-up tube must remain in the fluid to function. It is available in conventional air spray and HVLP

Gravity Feed
Gravity feed guns depend on gravity (cup is on top) to supply fluid to the gun. As a result, some control of the gun is gained over suction feed. Maximum use of the material due to gravity is also a plus.

Pressure Feed
Allows maximum control of the required pressures (air and fluid). The source of the coating is dependent on the amount used during a given time period. Available sources include pressurized attached cups, pressure pots (.5 to 60 gallons) and pumps (both low and high pressure)

Given the above criteria, a competent equipment supplier can match the equipment to the application.

 

 

Q. What are the advantages of "triggering" a spray gun?
A.

Triggering a spray gun refers to momentarily releasing the trigger when the spray gun has passed the end of the part and is moving to the position for the next spray pass. The amount of paint wasted does not appear to be significant for those who do not release the trigger between passes. The following example shows otherwise:

Example:

Operator spraying 50% of 7 hour day
Flow rate = 10 oz/min
Coating Cost = $10.00 / gallon
Trigger time  = 1/5 (.2) second

Using the above figures:

Wasted Coatings

1 cc lost at each spray stroke end
Part size requires 6 gun strokes
1 gun triggers per stroke, 6 Total
1 cc x 6 strokes = 6 cc lost per part
Production rate = 2 parts/minute
420 minutes x 2 parts / min. = 840 parts
840 Parts x 6 cc = 1.33 Gallons/Gun/Day

3,785 cc / gal
1.33 Gal./Day x 240 Days / Yr. = 319 Gal./Yr.
$10 Cost x 319 Gal. = $3,190 Finish Waste Per Year
$3,190 Lost For One Gun

VOC Reduction

30%  Solids, 70% VOC
8 Lbs. / Gal x 319 Gal. = 2,552 Lbs. Liquid Sprayed / Yr.
2552 Lbs. X 70% = .9 Tons
2,000 Lbs ./ Ton
.9 Tons Emissions Generated by One Gun (applies against your VOC permit)

 

Waste Disposal Considerations

319 Gal. X 30% Solids = 95.7 Gal. Solid Waste

          95.7             = 1.74 Drums of Waste

55 Gal. / Drum
$200 / Drum Disposal Cost = $348
$348 Waste Cost Generated by one Gun

Total Cost of 1 cc Wasted per Gun

Finish Cost
 
$3,190
 
VOC Permit
$22
@ $24 / Ton
Disposal
$348
 
Total Cost of 1 cc
$3,560
 

Obviously higher flow rates and higher coating costs would increase the figures above. In addition to the above, increased booth filter usage would be an issue.


 

   

 

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