Spray Training

Spray Training

Spray TrainingTheoretical & Technical AspectsAgendaAbout Airless Spraying

Coating Characteristics

Airless Spray Pump Components

Setup for Application

Quality Control

Safety

Spraying TheoryJohn William Strutt Rayleigh (1842 1919)

First put forward the theory that materials break up or ATOMISES (this is what we see as a fan pattern) when forced to travel at great speed.

Therefore it is the SURFACE TENSION and not the VISCOSITY Which governs how easily a fluid can be sprayed.

Achieving the desired level of atomization requires maintaining a balance of the fluid viscosity and fluid flow rate on one side with atomization energy on the other side. Once the system (or operator) achieves the desired level of atomization, a change in any parameter will affect the atomization. Balancing the equilibrium with an opposing change can return the atomization to the desired level.

Spraying TheoryViscosity

This is the resistance of a fluid to flow. Highly viscous or thick fluids flow less easily than low viscosity or thin fluids. http://www.planetseed.com/flash/science/lab/liquids/visco_exp/en/viscosity.swf

Surface Tension

This is the tendency of liquids to reduce their exposed surface to the smallest possible area. The phenomenon is mainly due to cohesion, the attractive forces acting between the molecules of the liquid.In other words, we need to overcome the materials built in resistance to break into fine particles in order to spray it.

Thixotropy

A fluid is said to be Thixotropic if its viscosity reduces when it is agitated or stirred i.e. it is subjected to shear force.

Specific Gravity/Density

The specific gravity of a substance is a comparison of its density with that of water. A substance with an S.G greater than 1 (water) is heavier than water and a substance with an S.G. less than 1 is lighter than water.

Two units of measurement for viscosity are Poise and Seconds.

Surface Tension

Surface Tension is an effect within the surface layer of a liquid that results in a behavior similar to an elastic sheet. When a liquid interfaces with a solid to produce this effect, it is referred to as surface tension. When this effect is found between two liquid phases, it's referred to as interfacial tension. Surface tension results from an imbalance of molecular forces in a liquid. At the surface of the liquid, the liquid molecules are attracted to each other and exert a net force pulling themselves together. High values of the surface tension means the molecules tend to interact strongly. Lower values mean the molecules do not interact as strongly. This property causes sheets or thin ligaments of liquid to be unstable; that is, they break up into droplets, or atomize.Water has a very high value of surface tension because it has a high degree of hydrogen bonding. Organic molecules with polar groups such as iodide and hydroxyl have a slightly lower surface energy than water. Pure hydrocarbons are even lower, while fluorinated compounds are very low because the fluorine atom won't share electrons very well so only dispersion interactions occur.In surface science, surface tension is measured in newton's per meter (mN/m) Surface tension is caused by the attraction between the liquid's molecules by various intermolecular forces. In the bulk of the liquid, each molecule is pulled equally in all directions by neighboring liquid molecules, resulting in a net force of zero. At the surface of the liquid, the molecules are pulled inwards by other molecules deeper inside the liquid and are not attracted as intensely by the molecules in the neighboring medium (be it vacuum, air or another liquid). Therefore, all of the molecules at the surface are subject to an inward force of molecular attraction which is balanced only by the liquid's resistance to compression, meaning there is no net inward force. However, there is a driving force to diminish the surface area, and in this respect a liquid surface resembles a stretched elastic membrane. Thus the liquid squeezes itself together until it has the locally lowest surface area possible.

Thixotropy

Thixotropic is that fluid whose viscosity decreases over time at a constant shear rate, like honey for instance

Specific Gravity

Specific gravity is defined as the ratio of the density of a given solid or liquid substance to the density of water at a specific temperature and pressure, typically at 4C (39F) and 1 atm (14.7 psi) , making it a dimensionless quantity below. Substances with a specific gravity greater than one are denser than water, will sink in it, and those with a specific gravity of less than one are less dense than water, and so will float in it.

Spraying TheorySPEED is the key word !

By forcing a fluid through the spray tip, we cause it to rapidly increase its speed, otherwise known as its velocity.

At a critical speed, governed by the surface tension of the fluid, the friction caused by the collision of the fast flowing fluid with the surrounding air molecules is sufficient to overcome its viscosity and the fluid breaks up or atomises.

The source for air spraying to occur is pressure. The higher the pressure, the higher the speed of the coating through the tip, and the higher droplet sizes formed. The atmosphere provides resistance and tends to break up the stream of fluid. This resistance tends to overcome, in part, the fluids properties of surface tension, viscosity, and density. Lets summarize the effect of these factors on atomization.The relative velocity between the fluid and the air also affects droplet sizes. The fluids velocity iscreated by pressure in the nozzle. As the fluid pressure increases, velocity increases and the average droplet size decreases. And conversely, as fluid pressure decreases, velocity is lower and the average droplet size is larger.

Spraying TheoryAirless spray utilizes equipment which creates high pressure (energy source) within the fluid which then converts that pressure to velocity to atomize the fluid. The material then gains momentum as it leaves the spray tip, thus delivering an even coating over a designated surface, by forcing it through a small orifice.

The source for air spraying to occur is pressure. The higher the pressure, the higher the speed of the coating through the tip, and the higher droplet sizes formed. The atmosphere provides resistance and tends to break up the stream of fluid. This resistance tends to overcome, in part, the fluids properties of surface tension, viscosity, and density. Lets summarize the effect of these factors on atomization.The relative velocity between the fluid and the air also affects droplet sizes. The fluids velocity is created by pressure in the nozzle. As the fluid pressure increases, velocity increases and the average droplet size decreases. And conversely, as fluid pressure decreases, velocity is lower and the average droplet size is larger.

Atomisation

Advantages of Airless SprayHigh production rate

Pump can operate direct from the supplied container

Reduced overspray and blowback

Good coating uniformity

Better coating penetration and surface wetting

Airless spraying has several distinct advantages over air spray methods. This method is more efficient than the air spray because the airless spray is softer and less turbulent, thus less material is lost in bounce back. The droplets formed are generally larger than conventional spray guns and produce a heavier pattern in a single pass. Airless spray applies most types of coatings faster than any other manually operated method, so production rate is increased with airless spraying.As the speed developed is very high, coating may be driven into crevices, cracks, corners and hard-to-reach areas easier than with conventional methods but note it is not guaranteed! Standard rules still apply.The equipment for airless spray can be powered by AIR, ELECTRICITY, and HYDRAULICS. The guns are very versatile. Some of them are lighter than others, or are designed for better grip, etc.

Disadvantages of Airless SprayThe major disadvantage of the airless spray is that the tip tends to clog and can be dangerous to use or clean because of the high pressures involved.

Because of the speed/outgoing pressure of the fluid, an accidental skin injection is extremely dangerous, as well see later on.

Again, because of the high pressure levels involved, operators must be fully trained to operate the equipment and service it.

It must be borne in mind that airless spray is a pneumatic system. A fluid cannot be compressed unlike air. The airless spray system must be fully purged with a fluid before reaching the required spray pressure.Airless Spray EquipmentPump (s)

Fluid Lines

Gun

Spray Tips

Reservoir / hopper System

Airless Spray Pumps

It might sound contradictory, but compressed air in airless spray is used to power the unit, but air, as we have already discussed plays no role in atomizing the product. The unit will convert incoming air pressure into outgoing fluid pressure!!!! Therefore, controlling the incoming air pressure provides a means of controlling the outgoing pressure of the product! This is whats referred to as PRESSURE RATIO!! And its a ratio of the effective area of the air motor piston to the effective area of the pump piston!Lets say we have two pumps, one is a 60:1 King pump, and the other one is 20:1. If the incoming air pressure is 50 psi, what will be the outgoing fluid pressure? 3,000 psi for 60:1, 1,000 psi for 20:1Based on this, which pump do you think has the piston with the largest diameter? 20:1 as the piston area will be the largest!

Airless Spray Pump

Airless Spray Pump

Double Acting Reciprocating Pump

Displacement rod Wet cupPacking's Displacement rodChrome cylinder Piston ballInlet ballInlet seatPistonCartridgeBuilt-in-filterFilter capFilter capCylinder pinOutlet plugQuick Access TM Intake

Lets have a closer look at the cross sectional area of a pump and built-in filter.

Displacement rod. Its connected to the motor rod and can be easily disconnected by removing a quick disconnect coupler, as we can see here! The coupling cover is removed first. This reduces maintenance time!Wet cup: Formally called packing nut Its manually adjustable and tightens the packing'sCartridge: Holds packing's. Its easily removed!Packing's: Prevent any leaking, well see how to arrange them appropriately later on this presentation. Chrome cylinder: Houses all the different pump components and it is made out of hard chrome over stainless steel.Cylinder pin: This pin locks the cylinder into place.Piston and inlet ball: They are made out of steel, well talk about them shortlyInlet seat: Made out of tungsten carbide, this is where the inlet ball sits.PistonBuilt-in-filter / Manifold: Well discuss about this later. Quick Access Intake: Allows for quick on-the-job maintenance only using a hammer as we can see in this picture.

Double Acting Reciprocating PumpPiston reciprocates when the trigger is pulled. One cycle consists of upstroke and down stroke.

Upstroke

Down stroke

Material is sucked in and pumped out. Material is pumped out.

AIRLESS SPRAY COMPONENTS

Spray Pump Strip Down

Plural Component Spray

Plural ComponentAirless Spray Hose

Rated for high pressure - up to 517 MPa (7,500 psi).

Electrically continuous and grounded.

Check for damage and discarded if defective.

Tightly connected.

Length should not exceed 45m (150 ft.).

Fluid Hose Safety

Hoses should be stored in neat coils.

Before each use, check for any damage (kinks, scuffs).

Check chemical compatibility between cleaning material and hose.

Hose connections should be tight.

Always check for electrical continuity.

Airless Spray Equipment

Fluid hoses provide the means of getting the fluid from the pump to the spray gun.

ENSURE that only hoses with a pressure rating compatible with the maximum output of the pump are used.

DISCARD any hose with kinks or outer sheath damage.

NEVER attach a hose without the use of the correct wrench.

NEVER disconnect a hose unless you are absolutely sure the internal pressure has been relieved.

Airless Spray Pressures

Range from 55 - 450 Bar (800 to 6,500 psi)Most commonly use 100 - 240 Bar (1,500 to 3,500 psi).

Ensure you are using the correct hose for the capacity of the airless pump that you are using!

Airless Spray Equipment

Spray Tip Safety Guard

Trigger Guard

Trigger with Safety Lock

Gun Filter

Gun Swivel Fitting

Safety Catch

Reversible Airless Spray Tip

Trigger Safety

The most important aspect of the spray gun is the safety mechanism.The most common type shown below, is a simple design and prevents accidental release of high pressure fluid.

If You Are Not Spraying, Then The Safety Catch Must Be Engaged, No Matter How Brief The Stoppage.

Trigger Safety

NEVER use a spray gun with a faulty safety catch.

Always make sure the gun is correctly rated for thepressure that can be developed by the pump.

Do not use a low pressure rated gun on a high pressure pump. Ensure the gun is compatible with the pump being used.

Airless Spray Gun Safety

All airless spray guns must be fitted with a safety guard to prevent accidental injection.

Never use a gun without a guard.

Using The Spray Gun

Arcing reduces penetration into the blast profile and reduces the adhesion, as well as poor thickness control.

Using The Spray Gun

50% overlap provides uniform coverage.

Overlapping.

Overlap each stroke 50% over the preceding one. Move the gun at a constant speed while the trigger is pulled, since the material flows at a constant rate. A 50% overlap will provide uniform coverage

Spray Gun Safety

KEEP THE SAFETY CATCH ON AT ALL TIMES!

Only release it when you are wanting to squeeze the handle and put product on the job!

Its a loaded gun!

Airless Spray Tip Components

Flow Rate Spray Tip Fan Width Spray Tip Flat Reversible

Know that the tip must be chosen accordingly as it controls the flow rate thro the orifice and the fan width, creates atomization. By doing this, it actually dictates the wet film thickness of the material being applied! If your fan width is too narrow, youll be spraying the same amount of product onto a small area! The speed at which the material leaves the tip can be as high as 280 miles/hr, so never put your hand in front of the tip, as the product can penetrate the skin and cause serious injuries. Spray tips can be flat and reversible. The guard avoids any direct contact as it widens the flow of product

Spray Tips

Tip Seal Safety Guard Tip HolderSpray Tips

The tip aperture is elliptical in shape and the aperture size designation does not refer to either the long or short axis length but to the diameter of a round pattern orifice that would have the same cross sectional area and flow rate.Spray Tip Part NumbersXHDType of spray tipRefers to fan angle and widthadd 0 to give 50o angle ordouble 5 to give 10 fan width at 12 from the tip.

Last two digits refer to the tip aperture sizein thousandths of an inch.501210523Spray Tip Wear

Correct Pattern / Partially Worn Tip / Badly Worn TipAs the tip wears out, the aperture increases and the spray shape gets distorted. The more worn out, the more the material wastage as theres more material passing thru the tip.

Airless Spray Tip size

CoatingTip Size (inches)Tip Size (millimetres)Fan Width (inches) Fan Width (millimetres)Low Viscosity0.0110.286, 8, 10150, 200, 250Industrial Primer0.015 to 0.0210.38 to 0.536 to 12150 to 300High Build 0.019 to 0.0250.48 to 0.638 to 14200 to 350Flake Glass Systems0.025 to 0.0400.63 to 1.010 to 14250 to 350Spray Tip Selection

Film Thickness Requirement

Table of Independent Variables


XHD-313, desired FT= 150 m. New requirement: 60 fan angle.


Desired FT = 190 m Spray trial with XHD-515 FT=145 m

Troubleshooting

If the coating is failing to atomize you should check the following:

Compressed air source

Material temperature

Flow rate at the gun without a tip

Spray tip size (may need reducing or increasing)

Troubleshooting

Stall Out TestRaise the pressure at the pump, trigger the gun and check both strokes for even flow.

Upstroke fails

Down stroke fails

Note: Displacement rod should be continuously lubricated with a compatible TSL oil.

Check displacement rod packings and top ball / top ball seatCheck pump inlet valve / bottom ball / ball seat Troubleshooting

Mixing

Settlement is likely to occur in storage.

Mechanical mixing should be used to agitate the base prior to adding the catalyst / hardener.

Plan your mixing program.

If using a pot, change regularly to avoid part cured material from being drawn into the pump.

Spray Application

Apply coatings with correct spray technique.

Purge system before spraying.

Adjust gun to give correct spray pattern.

Overlap of 50% on passes.

Measure wet film thickness (brushed out).

Focus on difficult areas.

Do not arc spray gun.

Complete within pot life of mixed coating.

Airless Spray Technique

Gun moves parallel to surface

50 % Overlap spray passes.

Trigger gun at each end of the pass.

Select tip size and fan angle to suit configuration of work piece.Airless Spray Technique

With any spray applied coating it is essential that correct technique is used, but it is particularly important with Polyglass as the large nozzle requirement and dense flake structure may cause sag when Polyglass is incorrectly applied.

Polyglass should be applied by the wet on wet multi-pass technique as follows:

Before spraying job surface, set pump pressure to give good spray pattern and atomisation at the LOWEST ACCEPTABLE PUMP PRESSURE. Remember the higher the pressure, the shorter the pot life and the longer the thixotropic recovery time on the surface.

When a suitable pattern and atomisation have been achieved, start work by spraying vertical passes (spraying from top to bottom/bottom to top). Do not try to achieve full wetting out of the surface at a single pass, but mist coat the area with several vertical passes until at least 90% coverage is achieved. Move to the adjacent area and carry out the same procedure, then return to the previously vertically sprayed area and with horizontal passes, build up the coating thickness. After building this area, return to the adjacent area with horizontal passes. Check wet film with suitable gauge and, where necessary, build film thickness by alternate vertical and horizontal passes until desired wet film is achieved. On completion, move to the next adjacent area to be coated, again starting with vertical passes.

REMEMBER GOOD INITIAL WETTING WITH VERTICAL PASSES IS ESSENTIAL FOR BUILDING THICK FILMS WITH THIS PRODUCT.Airless Spray Fingering

Increase the pressure until the Fingers just disappear.The Unit is then set at the ideal pressure for atomisation.

Coating Techniques

Stripe Coat Extra coat applied to cover sharp edges, welds, and vulnerable areas.

Full Wet Coat Maximum wet film application.Pausing During Spray Application

Engage Safety Catch.

Remove & Clean Spray Tip.

Flush System.

De-pressurise.

Turn Off The System.Finishing a Spray Application

Engage safety catch.Remove spray tip.

Flush system.

De-pressurise Reduce pressure to zero.

Open the system pressure drain valve.

Clean spray tip.

Strip & rebuild wet end.

Application Quality Control

Wet-Film Thickness (WFT) checks.

Dry-Film Thickness (DFT) checks.

Environment / Climatic Measurement

Air TemperatureThermometer

Steel TemperatureMagnetic gauge or electronic surface gauge (laser is good but can be easily confused by shiny surfaces!)

Humidity and Dew point (Calculated)

Steel Temperature Must be >3C above Dew point.

Application - Inspection

During application;

Product batch number recording

Start / finish of coating

Overcoat windows

Environment conditions

Surface and air temperature

Relative humidity and dew point

Coating thickness

ProblemsInspection of Spray Application

Monitor equipment and materials

Observe preparation and mixing

Observe application technique

Measure results

Record all inspection outcomes, including visual observations.

Workmanship

Pinholes (in this case outgassing)

Workmanship

Improper Thickness

Sagging

Workmanship

Fish Eyes

Blistering

Workmanship

Particulate Contamination

Solvents

Highly flammable and may cause dizziness with over exposure.

Personal Protective Equipment

Some General Precautions

Eliminate all fire and explosion hazards.

Ground all equipment and accessories.

Always engage the safety catch if the gun is left idle.

Always have a Safety Data information available.

Airless Spray Safety

The high pressure fluid that exits the gun is sufficient enough to penetrate human skin.

Correct solvent PPE must be worn before using cleaning solvents.

You should avoid spraying cleaning solvents


Do not leave pressurized unit unattended.

Use only high-pressure-rated hose and identical fittings.

Always lock the gun when not actively spraying.

Observe safety guidelines.


Because of the risk of fire / explosion, always ensure that all equipment and product drums are all earthed at all times, and that any couplings have continuity straps fitted.

NEVER use plastic containers that contain product or solvents that feed the pump (Metal buckets are essential).

Injection Injury

Injection Injury

A relatively small injury could be very serious.


Should an injection injury occur;

Injured person should be taken to hospital immediately.

The person(s) should take with them the literature (MSDS) on the material they have been injected with, and recommendations for treatment. Injection Injury: looking at Statistics

Injection injuries happen most commonly to men aged 21 to 59.

Over 75% of injections are to the index finger not holding the gun.

The second most common injury site is the fleshy area below the thumb of the hand not holding the gun.

Oil based products cause more injury than water-based, but both can cause rapid tissue death.

Treating Injection Injuries

Injury site may appear to be a small puncture, but it can lead to amputation or death.

Other complications include infection, chronic pain and permanent contraction of the injured limb.

The injured site shall be treated as a serious injury, leading to surgery in most cases.

Physical and occupational therapy is critical after the treatment to help restore the injured area as much as possible.

STAY SAFE !

Engineering

Spray Training