What is Colour coated steel & Colour coating process.

Q- Colour coating is a term used to describe the application of a decorative and/or protective organic coating to steel substrate supplied in coil form. Colour coated steel is also called as pre-painted steel.

CHAPTER I     –       WHAT ARE COIL COATING ?

CHAPTER II   –        THE RANGE OF COATINGS.

CHAPTER III  –        PRIME METAL TEMP.  (OR PEAK METAL TEMP)

CHAPTER IV   –      TESTING METHODS      (COVERS 15 TESTS)

CHAPTER V    –       CCL FLOW CHART

CHAPTER VI   –      PROCESS SECTION LAYOUT

CHAPTER VII  –       DEFECTS,CAUSES & REMEDIES

What are coil coating ?

coating layer

There are two main technologies,

1.Solution coatings – use resins dissolved in solvants. They show 40-65% solids.

2.Plastisol coatings – are virtually 100% solids content.

1. Solution coatings

    These coatings are normally based on thermosetting systems. The formulation usually includes,

  1. Resins : They provide the basic properties of the coating,flexibility,hardness, durability & corrosion resistance. Polyster,epoxy,acrylic,melamine,PVDF etc. are used as binders.
  • Pigments : Which provide colour,opacity,gloss,reflectivity & corrosion resistance.
  • Extenders : They influence gloss & opacity.
  • Solvants : Organic solvents are used to dissolve the resins and control viscocity.
  • Additives : modify the coating performance, for example :       

        -Flow promoters improve paint flow

                                         -Catalysts promote the cure reaction

                                         -Bubble bursters help to reduce foaming

                                         -Slip aids can modify the surface friction characteristics.

  • Curing : Involves evaporation of the solvent, film formation through cross-linking of the resin.

                     This gives the coating its full physical properties.

  • Plastisol coatings

In the curing process heat changes plastisol from liquid to a tough thermoplastic coatings by a physicochemical process. In this process Resins used is normally PVC

THE RANGE OF COATINGS.

  1. POLYSTER :

                           A family of coatings based on polyster. These products can be flexible to meet forming processes, but also built very hard, tough films. All levels of gloss are available from full matt to very high gloss. Polyster based products show a good balance of surface properties ( elasticity, scratch resistance ) and economy.Durability is classified as a medium level with an anticipated life expectancy of 12-15 years outdoors in temperate climates.

  • PRIMERS :

                        The principle purpose of a primer coating is to maximize adhesion between the pretreated substrate and the topcoat  and giving the full corrosion protection for the complete coating. This must also be accompanied by adequate flexibility. Sometimes primers impart a desired surface texture, or base coat colour. The choice of the resin is responsible for adhesion and flexibility. It is essential that the primer selected is compatible with both the substrate and the topcoat.                    

Polyster and polyurethane based primers are standard due to their adhesion, corrosion protection and high flexibility, which is far above the former epoxy primers.Especially Epoxy based primers show high technical properties and very good efficiency. Corrosion properties of primers depend on both, resin and pigment selected.

PRIME METAL TEMP

The PMT required to cure the coating and the time taken to develop its properties fully are important characteristics. The PMT will also influence the energy consumption of the line.                  PMT is governed by line speed, Zone temperature in the oven, Mass of metal being processed. The cure reaction of all solution coatings has a temperature/time relationship and is dependent on the total energy imparted to the coating during the cure reaction.

230°C AVERAGE PMT(AVG PMT MAINTAIN OPTIMUM COLOUR)

BELOW 230° C HOW PMT AFFECTS THE FOLLOWING PROPERTIES ABOVE 230° C
MEK TEST ¯
­ + GLOSS 60° ¯
¯ PENCIL HARDNESS ­ +
¯ ADHESION(IMPACT/CUPPING) ¯
¯ BEND TEST ¯
­ + COLOUR DIFFERENCE ­ +
BLUER COLOUR YELLOWISH

­(+) INCREASE

(-)DECREASE

TESTING METHODS

  1. VISUAL INSPECTION :                    

                                               ONLINE VISUAL EXAMINATION OF STRIP AT EXIT SECTION FOR SURFACE DEFECTS LIKE ROUGH SURFACE,CISSING,OVER BECKED,PAINT LUMB,GLOSSY LINE,BRAKE LINE,SHADE VARIATION,BLISTER,DENT/SCRATCHES FROM OVEN & ROLL, DIRT STICK,FLOW LINE etc…..

                                               ALSO GALVANISED (SKIN PASSED) SURFACE DEFECTS ARE TAKEN INTO CONSIDERATION, LIKE DROSS,OVER COATING,SPANGLE VISIBLE,NOZZLE LINE,NOZZLE CLEANING MARK,ZINC FLOW LINE,BUCKLING,FLATNESS etc…..

For Defects,Causes & Remedies see chapter VII.

  • DIMENSIONAL :

                                       CHECK THE ACTUAL SIZE OF THE STRIP ie. THK,WIDTH,COIL ID.

FOR THICKNESS MEASUREMENT CONSIDER THE ACTUAL GP THICKNESS BY REMOVING PAINT FILM WITH NaCl & MEK SOLUTION.

  • COLOUR DIFFERENCE – DE :

                                        THIS METHOD IS USED FOR DETERMINATION OF COLOUR DIFFERENCES OF COATED STRIP. ie MEASURE DIFFERENCE IN VALUES OF DL, Da, Db BETWEEN STANDARD (buyer sample) & ONLINE SAMPLE.

DE  =        (DL)2 + ( Da)2 + ( Db)2             IF,   DL  Positive  = Lighter than std.

                                                                        DL Negative = Darker than std.

                                                                        Da  Positive  = Redder than std.

                                                                        Da Negative = Greener than std.     

                                                                        Db  Positive  = Yellower than std.

                                                                        Db Negative = Bluer than std. 

D Means- Delta

DE SHOULD BE ALWAYS LESS THAN ONE FOR PERFECT COLOUR SHADING.

APPARATUS USED TO FIND OUT ABOVE VALUES DIRECTLY CALLED AS COLORIMETER.

REMARK :- TO CHECK THE COLOUR DIFFERENCE BETWEEN STANDARD & SAMPLE.

  • GLOSS 20°,60°,85° :

APPARATUS :-  20’,60’,85’ ANGLE GLOSS METER AND ONE CALIBRATION PLATE.

PRINCIPLE :- REFLECTOMETER OR GLOSSMETER OPERATING IN ACCORDANCE WITH THE

                        REFLECTION PRINCIPAL I.E. REFLECTION CAUSED BY THE SPECIMEN IS    

                        MEASURED. IN ACCORDANCE WITH INTERNATIONAL STANDARD COTAINS

                        20’,60’,85’ MEASUREMENT GEOMETRY.

METHODS :-

  • CALIBRATE THE INTRUMENT IN ACCORDANCE WITH WORKING STANDARD PLATE
  • PLACE REFLECTOMETER ON STANDARD PLATE.
  • PRESS MEASUREMENT KEY ON THE CALIBRATION POTENTIOMETER AND SET THE CALIBRATION VALUE SPECIFIED ON THE STD FOR THE RESPECTIVE MEASUREMENT.
  • TAKE COATED FLAT TEST SAMPLE OF ABOUT 6”X4” SIZE.
  • PLACE INTRUMENT ON TEST SAMPLE IN SUCH A WAY THAT THE APERTURE OF THE INTRUMENT IS TO BE COVERED BY THE SAMPLE
  • TAKE READING AT THE THREE DIFFERENT PLACES BY PRESSING THE MEASUREMENT KEYS AT 60’ ANGLE OR AS PER THE REQUIREMENT OF THE CUSTOMER.
  • NOTE DOWN THE READING IN RESPECTIVE FORMAT.

REMARK :-  GLOSS IS DEFINE AS A LIKE MIRROR IMAGE OF PAINT FILM WHICH IS MEASURE AT DIFFERENT ANGLE FOR DIFFERENT PAINT APPLICATION. ( ie. AT 20°- HIGH GLOSSY FOR AUTOMOTIVE PURPOSE,AT 60°- SEMI GLOSSY FOR COIL COATING & AT 85°- FOR MATT FINISHES.)

  1. IMPACT TEST :

APPARATUS :- THE INSTRUMENT CONSISTS OF,

  1. A GRADUATED TUBES (0-100 CMS) WHICH IS ADJUSTABLE IN HEIGHT IS MOUNTED VERTICALLY IN ITS BASE PLATE.
  2. A WEIGHT OF 1000 GMS FITS INSIDE THE GRADUATED TUBE WITH A SCREW HANDLE.
  3. A PUNCH OF TEMPERED STEEL BALL OF DIA. 15.87 MM (0.625”) WITH A SCREW HANDLE.
  4. A BASE WITH TEMPERED DIE OF 16.25 MM DIAMETER.

PRINCIPLE :- THIS APPARATUS DETERMINE THE IMPACT RESISTANCE OF THE COATING

                         PAINTS, VARNISES, GALVANISED AND ALUMINIUM SUBSTRATE TO TOUGH

                         PLATED OR LAMINATED COATING.

METHODS :-

  1. TAKE SAMPLE OF 50 X 300 mm
  2. PLACE SAMPLE IN APPARTUS WITH SURFACE EVALUATING DOWNWARD (REVERES IMPACT)
  3. DROP LOAD FROM 100 CMS FOR 1.0MM THICKNESS, 50CMS FOR THICKNESS OF 0.5 MM, 20 CMS FOR THE THICKNESS OF 0.20 MM AND SO ON.
  4. REMOVE SAMPLE AND VISUALLY INSPECT FOR CRACKING.
  5. APPLY CELLO TAPE OF 16-20 MM WIDTH FIRMLY ON THE DEFORMED AREA AND PULL OF IMMEDIATELY
  6. EXAMINE LOSS OF ADHESION. IN CASE LOSS OF ADHESION REPEATS THE SAME METHOD FOR 2-3TIMES
  7. NOTE DOWN THE READING IN RESPECTIVE FORMAT

REMARK :- TO CHECK PAINT FILM FLEXIBILITY BY APPLYING A PARTICULAR WORKDONE

AGAINST PAINT FILM.

  • MEK TEST :   ( METHANE,ETHANE,KETONE )

 

DETAILS :- MEK, SURGERY COTTON

METHODS :-  (TOP 100 RUB & BOT 50 RUB AS PER ECCA)

  1. TAKE THE TEST SAMPLE OF 10”X3” SIZE FREE FROM DEFECTS.
  2. TAKE THE COTTON AND WET WITH MEK.
  3. PLACE THE TEST SAMPLE ON FLAT SURFACE AND START RUBBING THE TEST SAMPLE IN BACK AND FORTH MOTION WITH A STROKE AT LEAST 8” LONG USING MODERATE TO HEAVY PRESSURE. APPROXIMATELY, WEIGHT OF PRESSURE SHOULD BE ABOUT 1KG. A FORWARD AND BACKWARD STROKE TO BE COUNTED AS ONE DOUBLE RUB (DR).
  4. CONTINUE UNTILL REACHING THE REQUIRED NO. OF DOUBLE RUB OR TILL IT FAILS.
  5. OBSERVE THE REMOVAL OF FILM TO EXPOSE THE BASE MATERIAL AT ANY SPOT ALONG THE CENTER.

REMARK :- TO CHECK THE ABILITY TO RESIST ATTACK OF SOLVENT ON PAINT FILM.

  • BEND TEST :

APPARATUS :-  BENCH VICE.

PRINCIPLE :-  THIS METHOD DEFINES THE  PROCEDURE FOR TESTING THE RESISTANCE TO CRACKING OF AN PAINT FILM WHEN BENDING THROUGH 180°.

METHODS :-

                1)    TAKE A SAMPLE PARALLEL TO THE ROLLING DIRECTION.

2)    TAKE A SAMPLE OF 25 X 300 mm SIZE SHOULD BE FLAT AND FREE FROM ANY

                      SURFACE DEFECTS.

  • INSERT ONE END OF THE SAMPLE IN VICE JAWS & PRESS THE OTHER END 

                       WITH THUMB PRESSURE IN 90° ANGLE.

  • REMOVE THE SAMPLE FROM JAWS & AGAIN INSERT THE V-SHAPE SAMPLE IN THE VICE JAWS. TIGHT THE JAWS WITH HANDLE TO FORM THE FIRST BEND 180° CALLED AS 0T.
  • REPEAT THE SAME METHOD WITH SAME SAMPLE TO FORM SECOND BEND CALLED AS ½ T AND SO ON. (0T, ½T,1T,1½T,2T)
  • CHECK FOR CRACKING OF THE PAINT FILM WITH THE CELLO TAPE.
  • PERFORMED THE SAME METHOD TO BOTH THE SIDE ie TOP AND BOTTOM.

REMARK :- TO CHECK THE FLEXIBILITY OR EXCESSIBILITY & ADHESION OF PAINT FILM.

  • DRY FILM THK. :

APPARATUS :-  IT CONSISTS OF 256 MODELS ELCOMETER, STANDARD THICKNESS SET,

                     TWO PROBE FOR FERROUS AND NONFERROUS SUBSTRATE.

PRINCIPAL :-  THE COATING THICKNESS ON MAGNETIC SUBSTRATE IS DETERMINED BY

MEANS OF AN ELECTRICAL  PROBE PLACE ON THE COATING AND DEVELOPING ELECROMAGNETIC FIELD IN THE BASE METAL. THE POTENTIAL VARIATION

OF THIS FIELD WHICH IS FUNCTION OF DISTANCE BETWEEN THE PROBE AND BASE METAL IS MEASURED IN MICRONS. THE COATING THICKNESS ON NONMAGNETIC SUBSTRATE IS DETERMINE BY MEANS OF AN ELECTRIC PROBE PLACE ON THE COATING AND PRODUCING EDDY CURRENT IN THE BASE METAL. THE VARIATION OF AMPLITUDE OF THESE CURRENT WHICH IS THE FUNCTION OF THE DISTANCE BETWEEN PROBE AND THE BASE METAL.

 METHOD :- 

  1. PRESS AND HOLD DOWN THE ON/BATT BUTTON. IF THE READING IS HIGHER THAN 100, PROCEED FURTHER OTHERWISE CHANGE THE BATTERY.
  2. CONNECT THE RESPECTIVE PROBE.
  3. DO THE INITIAL ADJUSTMENT OF THE INSTRUMENT ON THE ZERO AS WELL AS OPTIMUM VALUE OF THE MEASURING SCALE BY MEANS OF CALIBRATED STD OF KNOWN THICKNESS.
  4. PLACE THE PROBE ON FLAT SAMPLE OF SIZE 50 X 300 mm AND READ THE COATING THICKNESS.
  5. REMOVE THE COATING BY USING MEK & NaCl ON SAME PLACE AND PLACE THE PROBE.
  6. DIFFERENCE BETWEEN READING IS THE COATING THICKNESS.
  • NOTE DOWN THE RESULTS IN THE RESPECTIVE FORMAT.

REMARK :- TO MEASURE THE FILM THK IN MICRONS (TOP/BOT) AS SPECIFIED BY THE        BUYER. ( General practice, primer coat top/bot 5micron +/-2, finish coat top 18-20micron +3/-2, finish coat bot 5micron +/-2)

  • PENCIL HARDNESS :

       DETAILS :- A SET OF KOH-I-NOR DRAWING PENCILS OR THEIR EQUIVALENT IN THE

                     FOLLOWING RANGE 6B-5B-4B-3B-2B-B-HB-F-H-2H-3H-4H-5H-6H-7H-8H-9H.

                                                         Softer                                                     Harder       

        PRINCIPLE :- THIS METHOD DEFINES THE PROCEDURE FOR TESTING THE RELATIVE                        

                        HARDNESS OF COATING. THE HARDEST LEAD, WHICH DOES NOT PENETRATE

                        THE COATING , DETERMINES THE DEGREE OF HARDNESS.

         METHODS :-

                1)    TAKE A SAMLE OF 6”X4” SIZE SHOULD BE FLAT AND FREE FROM ANY

                      SURFACE DEFECTS.

                2)    SHARP THE PENCILS AT LEAST 6MM OF LEAD FREE FROM NICKS AND

                      PERFECTLY CYLINDRICAL INSHAPE. THEN TRIM THE LEAD ON EMERY PAPER

                      TILL OBTAIN THE LEVEL AND PERFECTLY CIRCULAR SECTION.

                3)     HOLD THE PENCIL AT 45 DEGREE ANGLE TO FILM SURFACE AND PUSH

                       FORWARD ABOUT 500 GMS PRESSER WITHOUT BREAKING THE LEAD.

                4)     SUCCESSIVE TRAIL ARE MADE UNTIL HARDEST LEAD WHICH DOES NOT

                      REPTURE THE PAINT FILM IS DETERMINED.

                5)     NOTE DOWN THE REFERENCE NO. OF HARDEST LEAD WHICH DOES NOT

                      REPTURE THE PAINT FILM.

REMARK :- TO CHECK THE FILM HARDNESS AS SPECIFIED BY THE BUYER.

                      (HB TO 6B – SOFTER & H TO 9H – HARDER )

  • CROSS HATCH :

APPARATUS :-  CROSS HATCHING DEVICE OR CUTTING TOOL.

PRINCIPLE :-  A SAMPLE IS DEFORMED BY PRESSING UNDER A GIVEN CONDITION AFTER IT HAS BEEN CROSS HATCHED WITH CUTTING TOOL OR SHARP EDGE.

METHODS :-

  1. TAKE A SAMPLE OF 6” X 4” SIZE SHOULD BE FLAT AND FREE FROM ANY SURFACE DEFECTS.
  2. FOR COATING LESS THAN 60mm : AT LEAST SIX PARALLEL CUTS ARE MADE 1mm APART,TO A DEPTH TO REACH THE SUBSTRATE . AT LEAST 6 SIMILAR CUTS ARE MADE PERPENDICULARLY TO THE FIRST.
  3. FOR COATING GREATER THAN 60mm : TWO PARALLEL CUTS ARE MADE 5mm APART, TOGETHER WITH TWO ADDITIONAL SIMILAR CUTS AT RIGHT ANGLES TO FORM A CENTRAL 5 X 5 mm SQUARE.THE CUTS SHALL JUST REACH THE METAL, AND EACH CUT SHALL MEASURE AT LEAST 50mm IN LENGTH. 
  4. APPLY CELLO TAPE ON THE SQUARE AREA AND PULL OF IMMEDIATELY.
  5. EXAMINE LOSS OF ADHESION. IN CASE LOSS OF ADHESION REPEATS THE SAME METHOD FOR 2-3TIMES
  6. NOTE DOWN THE READING IN RESPECTIVE FORMAT

REMARK :- TO CHECK THE ADHESION OF COATED FILM AFTER INDENTATION BY SLOW DEFORMATION.

  • CUPPING TEST :

APPARATUS :- THE INSTRUMENT CONSIST OF

  1. A DIAL GAUGE GRADUATED IN 1/ 10 MM.
  2. A CUP FORMING ASSEMBLY CONSISTING OF CYLINDER

PRINCIPAL :- THIS TEST COVER THE DRAWABILITY OF COATING FILM AS WELL AS SUBSTRATE IN MM               

METHOD :-   

  1. TAKE SAMPLE OF 50 X 300mm
  2. PLACE IT IN APPARATUS WITH SURFACE EVALUATING UPWARD.
  3. TURN THE DIA HOLDER TO THE CLOCKWISE DIRECTION WITH THE HELP OF

         TWO SMALL KNOBS FOR FIXING THE SAMPLE .

  • TURN THE FOUR HANDLE KNOB IN CLOCKWISE DIRECTION AND DRAW THE

        SAMPLE BY READING DIAL GAUGE AS PER CUSTOMER REQUIREMENT

  • MOVE SAMPLE AND VISUALLY INSPECT FOR CRACKING.

                          APPLY CELLO TAPE OF 16-20 MM WIDTH FIRMLY ON THE DEFORMED AREA            .          AND PULL OF IMMEDIATELY.

                 6)    EXAMINE LOSS OF ADHESION. IN CASE LOSS OF ADHESION REPEATS THE

         SAME METHOD FOR 2-3 TIMES.

  • NOTE DOWN THE READING IN RESPECTIVE FORMAT

REMARK :- TO CHECK THE DRAWABILITY OF COATED FILM.

  • SCRATCH HARDNESS :

APPARATUS :-  PRESSING DEVICE AND SHARP INDENTOR.

PRINCIPLE :-  TO OBSERVE A SCRATCH BY APPLYING A PARTICULAR LOAD ON COATED FILM.  

METHODS :-

  1. TAKE A SAMPLE OF 50 X 300mm SIZE SHOULD BE FLAT AND FREE FROM ANY SURFACE DEFECTS.
  2. THE SAMPLE IS CLAMPED ON PLATFORM TIGHTLY WITH NUT & BOLT.
  3. APPLY 1 KG LOAD ON INDENTOR.
  4. START MOVING INDENTOR ON SUBSTRATE  IN PARALLEL DIRECTION UPTO APROX. 80 – 100mm DISTANCE.
  5. REPEAT THE SAME METHOD FOR 2-3TIMES
  6. EXAMINE NUMBER OF SCRATCHES ON COATED FILM
  7. NOTE DOWN THE READING IN RESPECTIVE FORMAT

REMARK :- TO CHECK THE PAINT FILM BRITTLENESS.

  • ANTIFLUTING CONICAL :

APPARATUS :-  CONICAL MANDREL.

PRINCIPLE :-  THIS TEST COVER THE ANTIFLUTING PROPERTIES OF COATED FILM AS WELL AS SUBSTRATE.  

METHODS :-

                1)    TAKE A SAMPLE FROM TRANSVERSE  DIRECTION TO THE ROLLING.

  • TAKE A SAMPLE OF 25 X 300 mm SIZE SHOULD BE FLAT AND FREE FROM ANY

SURFACE DEFECTS.

  • INSERT ONE END OF THE SAMPLE IN CONICAL MANDREL & TURN THE SAMPLE IN CIRCULAR DIRECTION WITH HANDLE.
  • REPEAT THE SAME METHOD ON OTHER END.
  • CHECK THE ANTIFLUTING FOR TOP & BOT COATING SURFACES

REMARK :- TO CHECK THE PAINT FILM BRITTLENESS.

  1. SALT SPRAY RESISTANCE TEST :

APPARATUS :-  SALT SPRAY UNIT (5% NaCl )

PRINCIPLE :-  THE METHOD CONSISTS OF EXPOSING A SUBSTRATE  TO A SALT FOG FOR A PREDETERMINED TIME, TO EVALUATE POSSIBLE CORROSION.

METHODS :-

  1. TAKE A SAMPLE OF 200 X 150 mm SIZE SHOULD BE FLAT AND FREE FROM ANY

SURFACE DEFECTS.

  • INSERT THE SAMPLE IN SALT SPRAY TRAY WITH COVERING EDGES .
  • THE PERIOD OF EXPOSURE SHALL BE  500hrs FOR TOP COAT & 150hrs FOR BOT COAT.
  • REMOVE THE SAMPLE FROM CABINET.
  • CALCULATE THE AREA/PERCENTAGE OF WHITE RUST/CORROSION.

(THE LOSS IN MASS SHALL BE RELATED TO THE SURFACE AREA EXPOSED TO CORROSION)

REMARK :- FOR TESTING THE RESISTANCE OF COATED FILM TO SALT SPRAY FOG.

  1. HUMIDITY RESISTANCE TEST :

APPARATUS :-  HUMIDITY CHAMBER.

PRINCIPLE :-. THIS TEST CONSIST OF EXPOSING A SUBSTRATE TO A ATMOSPHERIC MOISTURE FOR A PREDETERMINED TIME.

METHODS :-

               1)    TAKE A SAMPLE OF 200 X 150 mm SIZE SHOULD BE FLAT AND FREE FROM ANY

                       SURFACE DEFECTS.

               2)     INSERT THE SAMPLE INTO THE CHAMBER WITH COVERING EDGES .

               3)     THE PERIOD OF EXPOSURE SHALL BE  1000hrs FOR TOP COAT & 500hrs FOR 

                       BOT  COAT.

  • REMOVE THE SAMPLE FROM CABINET.
  • CALCULATE THE EFFECTIVE AREA IN PERCENTAGE & NOTE DOWN IN RESPECTIVE FORMAT.

REMARK :- TO CHECK DISCOLORATION OF COATED FILM & RESISTANCE TO HUMIDITY.

DEFECTS,CAUSES & REMEDIES

DEFECT TYPE POSSIBLE CAUSES REMEDIES
INCORRECT COLOUR Incorrect film thickness (top coat &/ or primer) Incorrect cure  / PMT Incorrect colour of wet paint Color range due to roll shear   Adjust coater rolls Adjust oven temperature Contact coating supplier Contact coating supplier
ROUGH SURFACE       Paint quality Improper filter External dust Viscosity Coating thickness Applicator roll crack   Contact paint supplier Clean the filter Clean the coater room Check actual viscocity of paint Adjust wet film thickness Replace applicator roll
FLOW LINES   Speed variation betn applicator & pick up roll   Adjust uniform speed of applicator & pick up roll
BLISTER Contamination of air or water bubbles Because of  Improper drying after surface treatment.   Adjust chromate dryer temp. Increase oven temp.  
STREAKING       Paint not thoroughly stirred Change of barrel or batch – different viscosity /temperature Incorrect coater Set  up – insufficient. ‘dam’ in tray   Restirred , check for settlement     Modify set up  
MOTTLED APPEARANCE Incorrect roll speeds Applicator roll diameter too small Low film thickness Poor opacity /low pigment to binder ratio   Adjust speed Change roll Adjust coater rolls Contact coating supplier
INCORRECT GLOSS Wrong Primer Primer PMT too low Incorrect film thickness Incorrect top coat PMT Paint settlement in drum Incorrect gloss of wet paint High fume level in oven Incorrect diluent used Surface of substrate out of specification   Change primer Increase temperature Adjust coater rolls Adjust oven temperature Re-stir Contact coating supplier Adjust oven air flow Contact coating supplier Change substrate  
LOW SOLVENT RUBS Normally Under-cure / low PMT   Over- cure / High PMT, normally with yellowing & gloss reduction   Check dwell time  / PMT – adjust as necessary Check dwell time / PMT – adjust as necessary
DEFECT TYPE POSSIBLE CAUSES REMEDIES
POOR ADHESION  TO METAL Inadequate pre –treatment Conversion coating film weight too high Inhomogeneous pretreatment Incorrect primer Incorrect prime cure / PMT Surface of substrate out of specification   Check pretreatment section for blocked sprays Check solution strengths Check solution strengths Change primer Adjust as appropriate Change substrate
POOR INTERCOAT ADHESION Primer PMT too high Top coat PMT too low Wrong or incompatiable primer Contamination of primer surface   Reduce oven temperature Increase oven temperature Change primer Check all pass rolls  
EDGE DELAMINATION Poor Pretreatment at edges of strip Incorrect primer used Poor oven air flow Metal corroded edges     Check pretreatment sprays Change Primer Check temperature distribution Change oil
POOR FLOW Paint viscosity too high Rolls speeds too low Applicator roll too small Incorrect coater set up Paint configuration not optimum     Add thinners Increase roll speeds Replace with larger roll Adjust Contact coating supplier – Flow additive  may help
CISSING / CRATERING (Random ‘fish-eye’ effect) Surface contamination Oil mist from compressed air Paint contaminated   Check possible sources- Air & hydraulic lines Replace with new barrel
AERATION May be small circular hollows in the coating a line  of these hollows. (Do not confuse with cissing) Bubble in the paint Foaming in the tray Cascade from the tray into the barrel Incorrect coater set up by bubble guards (‘dams’) Insufficient depth of immersion of rolls in paint tray   Contact  coating supplier for bubble burster Return down side of the barrel Re-set, install foam barriers   Increase paint level
SOLVENT BOIL Mass of bubbles over the  cured surface       Film thickness too high Viscosity too low (wet film too thick) Oven settings incorrect-too much heat too quickly Reduce thickness Reduce thickness Try new barrel Change zone temperatures-step value  
VERY FINE BUBBLES (do not confuse with ‘bits’ or solvent boil) Roll speeds too high Roll pressure too low (pressure bubbles)     Reduce roll speeds Increase roll pressures
DEFECT TYPE POSSIBLE CAUSES REMEDIES
THICK EDGES Substrate edge burr Any of the following for primer or top coat: Film thickness too high Viscosity too low Roll speeds too high Roll pressures too small Roll diameters too small Tracking lines on applicator roll Oven settings wrong Coating formulation   Change coil     Adjust film thickness Try new barrel Reduce roll speeds Increase roll pressures Replace applicator roll Replace applicator roll Adjust temperatures contact coating supplier-try flow additives
FINE ‘BITS’ OR PARTICLES Oven ‘soot’ Airborne dust Coarse grind of primer or top coat Primer thickness too high Primer coater configuration wrong Paint not filtered Poor paint dispersion   Clean ovens &ducts Keep coater room clean Contact coating supplier Reduce film thickness Change set up Fliter/contact supplier Contact coating supplier