Industrial Painting Systems 1
Painting Is A Coating Method That Adheres To A Surface, Protecting It From Mechanical And Chemical Effects While Providing An Aesthetic Appearance. It Consists Of A Solvent (Water Or Solvent), Resin, Pigment, Additive, And Filler Components. The Solvent Is Removed From The Surface During The Drying Process. Generally, Good Paint Is One With High Adhesive And Color Pigment Content And Low Additive Content.
Painting Is A Coating Method That Adheres To A Surface, Protecting It From Mechanical And Chemical Effects While Providing An Aesthetic Appearance. It Consists Of A Solvent (Water Or Solvent), Resin, Pigment, Additive, And Filler Components. The Solvent Is Removed From The Surface During The Drying Process. Generally, Good Paint Is One With High Adhesive And Color Pigment Content And Low Additive Content.
Paint Typically Derives Its Name From The Adhesive (Resin) Components It Contains. Resins Are Viscous, Sharp-Smelling, And Usually Transparent Substances That Give Paint Its Crack Resistance, Adhesion, And Strength Properties. Pigments Are Micron-Sized And Particulate In Nature, Classified As Organic, Inorganic, Or Special Pigments. They Impart Brightness And Opacity To The Paint.
Painting Systems Can Be Divided Into Two Main Groups: Powder Coating And Liquid (Wet) Painting Applications. Liquid Painting Applications Come In Single Or Dual-Component, Water Or Solvent-Based Forms.
Painting Systems Consist Of Three Fundamental Application Stages: Surface Preparation, Painting, And Drying. Surface Preparation Involves Removing Dirt And Unwanted Debris From The Surface To Cleanse It Of Oil, Grease, And Moisture. Processes Vary Depending On The Type Of Material. For Example, Sandblasting Is Used For Steel, Brushing For Aluminum, And Flame Treatment For Plastics As Pre-Treatment For Surface Preparation.
Painting Can Be Done In Four Different Ways:
- Brush Method,
- Dip Method,
- Curtain Flow Method; The Paint Flowing In Curtain Form Paints The Parts Passing Underneath.
- Spray Method.
Spray Method:
1 – Air Method:
It Is The Most Widely Used Method. Suitable For Robotic Applications. The Paint, Conveyed To The Gun Nozzle Through A Pump, Is Sprayed Onto The Part Surface Using Atomization And Fan Air To Achieve The Desired Surface Characteristics.
In Manual Applications, The Paint Applied To The Product Is Visually Checked For Quantity And Appearance, And The Desired Surface Quality Is Achieved By Adjusting The Paint Pressure/Atomization/Fan Pressures. In Robotic Applications, Visual Control Is Quite Difficult. Therefore, Adjustments Of Parameters Are Necessary Initially To Prevent Defects In The Finished Product. Monitoring These Parameters During System Operation Will Be Sufficient. Now Let’s Examine These Parameters:
Paint Pressure:
Pressure Is Directly Related To The Working Characteristics Of The Selected Pump. The Pump Should Be Selected To Provide The Required Pressure And Flow Rate For The System. When Determining The Required Pressure And Flow Rate For The System, The Material Consumption Values Of The Guns And System Losses Are Taken Into Account.
Paint Characteristics:
Viscosity Determines The Amount Of Paint Adherence To The Surface. Changes In Viscosity Directly Affect The Adhesion Of The Paint To The Surface. Similarly, Weakening Of The Chemical Bonds In The Paint Also Makes It Difficult For The Paint To Adhere To The Part Surface.
Density; Paint Density Should Be Kept Constant At The Set Values. The Density Of Each Newly Prepared Paint Should Be Measured, And The Density Of The Paint Left In The Tank For A Certain Period Should Be Checked Before Use.
Keeping The Viscosity And Density Of The Paint Within The Specified Range Is The Most Important Point To Consider In Setting Up A Paint Line. It Should Not Be Forgotten That Only Changes In Paint Viscosity Can Disrupt All The Necessary Adjustments Made For The System To Work Under Automatic Conditions. Therefore, The Paint Should Reach Approximately The Same Viscosity And Density Levels At The Gun Tip.
Atomization Air Pressure:
The Paint Reaching The Gun Nozzle Is Atomized As Soon As It Exits Into The Environment By Atomization Air. Spraying From A Very Close Distance With High Atomization Pressure Can Cause Unwanted Defects On The Part Surface.
Fan Air Pressure: Determines The Spray Width. Increasing The Fan Pressure Allows For A Wider Surface Sweep. Both Atomization Air And Fan Air Must Be Completely Free From Moisture And Oil For Painted Surface Quality.
Cabin Conditions:
Cabin Interior Temperature And Humidity Should Be Kept Constant. It Should Be Noted That Paint Quality Will Deteriorate In An Environment Above 80% Relative Humidity.
Cabin Interior Air Circulation Should Be Done Under Appropriate Conditions, And Overspray Should Be Removed From The Environment. Thus, The Formation Of Paint Defects, Especially By Adhering To Moving Surfaces And Gradually Drying And Pouring Onto The Part Surface, Will Be Prevented.
Robot Speed:
The Gun, Which Will Pass Through The Specified Points At The Programmed Speeds And With The Specified Atomization, Fan, And Paint Pressure Values, Performs The Painting Process By Opening/Closing At The Specified Points On The Part Surface. After The Process, Surface Paint Thicknesses Can Be Measured, And It Can Be Determined Where Material Quantity Should Be Increased Or Decreased.
The Guns Used In Painting Operations Are Crucial For The Performance Of The Process. Therefore, Different Types Of Guns Have Been Developed To Achieve High Paint Transfer Efficiency. These Are HVLP (High Volume Low Pressure), Airless, Air-Assisted, And Electrostatic Guns. In The HVLP Method, The Atomization Air Pressure Is About 80-90% Lower (Less Than 0.7 Bar) Compared To Normal Air Methods. The Air Consumption Flow Rate Is About 40-50% Higher. Thus, Overspray Is Reduced, And About 30% Material Saving Is Achieved. There Are Two Types Of HVLP Methods. In The First, Compressed Air Is Used As Atomization Air. In The Second Method, Low Pressure Is Provided By Electric Turbine Nozzles. Although The Turbine HVLP Is More Expensive, Its Efficiency Is Higher.
2 – Airless Method:
The Paint, Compressed At High Pressures (35-180 Bar), Is Sprayed Onto The Part Surface Through Small-Diameter Nozzles. This Method Is Known As The Airless Method. It Is Difficult To Achieve Advanced Surface Qualities Due To High Material Transfer. Usually, Secondary Processing May Be Required. Since The Paint Is Not Transported By Air, The Overspray Rate Is Lower. Therefore, A Very Powerful Cabin Ventilation System Is Not Required. However, Since The Nozzle Diameter Is Very Small, The Paint Used Must Be Very Clean.
3 – Electrostatic Method:
After The Atomization Process, The Paint Is Charged With A Negative Charge At The Gun Nozzle, And The Part Surface To Be Painted Is Charged With A Positive Charge. Thus, The Paint Is Attracted Directly To The Part Surface As Soon As It Exits The Gun. The Advantages Of The Method Are Reduced Overspray, Easy Access To Difficult Areas, And Obtaining More Uniform Paint Thicknesses. Special Guns Have Been Designed For Electrostatic Applications.
In Electrostatic Applications, Paint Consumption Is Low, And Transfer Efficiency Is High. However, It Is Difficult To Achieve High Film Thicknesses. Therefore, It Is Mainly Used In The First Coat Applications.
In The Drying Process, Baking Is Carried Out At The Desired Temperatures And For The Desired Periods Depending On The Type Of Material. It Constitutes The Easiest Stage Of Painting Systems. A Well-Designed Industrial Oven Will Provide The Expected Results.
Preferred Coating Types And Their Ratios In The Industry Are Presented In The “Coating Type” Table. This Table Shows That The Industry’s Tendency Towards Solvent-Based Paints Is Higher. From The Perspective Of The Equipment Used, The Situation Is Presented In The “Equipment Type” Table.
4 – Painted Product Defects:
Paint Manufacturers, Users, And System Builders List More Than Twenty Paint Defects. Defects May Arise From Various Reasons Such As Surface Quality Of The Part To Be Painted, Whether The Surface Preparation Process Is Correct And Sufficient, Whether The Application Parameters Are Correctly Selected, And Pre-Painting Transportation Processes. Here, The Main Paint Defects Arising From The Painting Process Are Examined.
Running Or Excessive Accumulation Of Paint On The Surface: Low Viscosity (More Fluid Paint), The Gun Is Too Close To The Surface (The Air Atomization Function Cannot Be Used Effectively), Or The Film Thickness Is High (Excessive Material). If The Runoff Continues Even When The Material Amount Is Reduced, The Atomization Pressure Should Be Increased, Or The Spray Distance Should Be Increased.
Orange Peel Or Low Fluidity: Structurally, It Appears As Bumps And Depressions Shaped Like An Orange Peel. The Cause Is High Viscosity, The Gun Is Too Close To The Surface, The Atomization Pressure Is Too Low, Or The Material Amount Is Too Low.
Overspray: Paint Reaching The Surface Is Not Enough Wet To Adhere To The Surface And Dries Very Quickly. The Main Reasons Are That The Gun Is Far From The Surface Or The Cabin’s Ventilation System Cannot Provide Enough Air Flow.
Cratering: The Main Reason Is That The Surface Tension Is High, So The Paint Cannot Wet The Surface Or There Are Active Substances (Use Of Silicone In The Cabin, Oil Mixed Into The Atomization Air, Etc.) On The Surface To Be Painted.
Every Material Surface Has A “Free Surface Energy Level” Possessed By The Molecules On The Surface. Paint On The Material Surface Shows An Escape From Low Surface Energy Regions To High Surface Energy Regions. Paint With Low Surface Tension Covers The Surface Better (Low Contact Angle). For A Paint With High Surface Tension To Have The Same Covering Property, The Painted Surface Area Should Be Reduced. If The Surface Preparation Process Is Not Correctly Applied, A Uniform Coating Cannot Be Achieved Due To This Spread. Moreover, It Becomes Inevitable That Shadowy Areas Will Form In Some Areas.
A. Part Surface Tension < Paint Surface Tension (Not Covering The Surface)
B. Part Surface Tension > Paint Surface Tension (High Covering Power)
5 – Boiling:
Factors Such As High Paint Viscosity, Excessive Film Thickness, And High Drying Rate Are The Main Causes. In Addition To Chemical Improvements, Reducing Viscosity, Reducing Film Thickness, Or Extending Drying Time May Be A Solution.
Some Of The Defects Can Be Eliminated By Improving Cabin Conditions And Maintaining Optimum Values For Application Parameters, While A Greater Part Can Be Eliminated By Changing The Additive Ratios In The Paint Content. It Is Important To Note That Making Changes In The Content Of The Paint Is Done For The Purpose Of Improvement. In Some Cases, It May Not Be Possible To Apply Some Corrective Measures Due To Concerns About Negatively Affecting The Expected Performance Of The Paint. However, Alternative Solutions Are Always Available, And There Are Appropriate Solutions.
Conclusion:
Paint Characteristics (Chemical Structure, Viscosity, Density), Application Parameters (Speed, Paint Pressure, Atomization/Fan Air Pressures), And Environmental Effects (Temperature, Humidity, Cleanliness) Are Parameters Directly Affecting All Types Of Painting Systems. When These Variables Are Considered, It Can Be Seen That In Manual Painting Systems, Intervention To Parameters Is Faster Due To Visual Control. However, Leaving The Quality Of The Product Solely To Visual Control Is A Significant Handicap For Businesses.
The Effect Of Each Parameter On Surface Quality Can Be Described Clearly. In Robotic Applications, Continuous Control Of All These Parameters Separately Is Carried Out. Thus, By Continuously Applying A Uniform Thickness Of Coating Using Only The Same Parameters (For Each Surface Segment), Material And Air Consumption Are Reduced, Processing Time Is Shortened, Labor, And Concentration Losses Encountered In Manual Applications Are Minimized, And Health Problems That May Arise In Operators Due To The Inhalation Of Solvent-Based Paints Or Enamel Powders Are Eliminated.
In Operators Due To The Inhalation Of Solvent-Based Paints Or Enamel Powders Are Eliminated.