Painting & Powder coating

Painting & Powder Coating

Painting & Powder Coating Expert

Meghan M Manjrekar

Meghan M Manjrekar

A qualified Paint Technologist having an experience of around 32 years in the field of Surface Coatings with wide experience and expertise in various fields, such as Paint Manufacture, Paint Technical Service, Paint Application, Application Technology, Powder Coatings Manufacture and Applications. Special Expertise in Automotive & Commercial Vehicles finishing.

A Certified Protective Coating Inspector with SSPC USA Certification PCI level 2

Activities & Attachments

• Visiting Faculty - D E Society’s Technical Institute, Pune
• Managing Committee Member - D E Society’s Technical Institute
• Member - The Society for Protective Coatings (SSPC), USA
• Member - Colour Society - Pune Chapter,
• Associate Technical Editor - Coatings Bhaskar - A Journal for Coatings Application
• Visiting Faculty Member - Film & Television Institute of India, Pune
• Consultant & Faculty - National Institute of Coating Technology,
• Mentor - BYST of India
• Syllabus Committee Member - for Board of Technical Examinations, Maharashtra State
• Visiting Faculty - National Training Institute ‐ Oman

All Questions

78 Questions

1
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We have powder coating line in place,but during assembly we got some scratches and to repair them we use quickdry spray paint bottles for touch up.After 10 days we found shade mismatches where we using spray.Colour - Orange.Can you please let us know the solutions for this.

Thank you for contacting us. Our expert has replied:
1. You need to avoid the scratches. In a car plant they take utmost care by not wearing any sharp object like watches, rings etc. avoid this. 
2. If the scratches are too often, your powder curing is not sufficient. It is remaining undercured and so soft. Please consult your powder supplier and conduct a TTR (travelling temperature recorder)and have correct baking. 
3. The touch up to be done by PU paint for outdoor.  The touch up tins are not suitable for outdoor use.
Hope you found this answer useful,
Team Ecafez
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is salt spray testing approriate for powder coating and paints?

Yes, Salt spray test ( ASTM B 117 ) is perfectly appropriate. This test would inform you about
the corrosion resistance of the coatings. This would not give you an exact outdoor / anti-corrosion
life, but can be predicted as, better than the other sample in comparison.

 

Thank you for contacting us.

Team Ecafez

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How to inspect pretreatment process ? Which chemicals used for it? What is peel off?

Pre-treatment for different metals are different. Please refer to the webinar "Pre-treatment master
class with module on new generation Nano Coatings" on youtube link - https://www.youtube.com/watch?v=Z-YHn78vLek&t=16s

All the questions would be answered. Thank you for giving us a chance to serve you.

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what all materials being used for painting and powder coating

This is a very general question, but our expert has answered:

These are mostly the organic materials with different compositions of raw materials like: 
Pigments - are the coloring materials
Resins or Binders - are the main film-formers
Solvents dissolves the binder and are thinning agents ( In powder coating solvents are not there.)
Additives give special properties

The paints or powder materials are basically applied for Aesthetics and Protection against corrosion 
and service abuses.

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How to inspect pretreatment process ? Which chemicals used for it? What is peel off?

How to inspect pretreatment process ?


The pretreatment types are different, depending on the substrates used. Usually for CR chemical pretreatment is suggested, and it is phosphating. There are two types of phosphating process, Zinc Phosphate and Iron Phosphate. Both should give a uniform grey coloured coat.
For HR usually recommended process is Blasting.

For phosphating, there are in stage inspections, like time, temperature and concentration ( pointages). Pointages are checked by titrations as recommended by the chemical supplier. The pointages would vary for different suppliers and their products. Then you have to have visual checks like water break test after rinse followed by degreasing etc etc. Ultimately the texture of the phosphate needs to be visually checked as well as the phosphate coating weight, to be checked, by the supplier at his lab.

Which chemicals used for it?


The degreasing chemicals are alkalies and surfactants, The derusting chemicals are acids, for phosphating, phospheric acid based mixtures are used. And for passivation Zirconium/titanium saslt based chemicals are used. They are to be mixed in the proportion recommended by the supplier, in good quality of water. The water quality has to be good for the entire process, to get good results.

What is peel off?


Peel off is the delamination of the coating either from the substrate or from the earlier layer.

The reasons are enormous, but the main few reasons are as below
 The oil from the surface is not removed before applying the first coat during the pretreatment.
 The recommended Over-coating interval after primer is not observed, rather it is exceeded.
 The Stoving Paint / Powder coating is uncured.
 The mix ratio in case of 2K system not observed.


These are some of the regular occurring reasons, but as I have mentioned there can be many more….

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Sir , how can we avoide rusty problem in MS part in built of AL part pretreatment part is only suitable for AL part can you suggest me any process which will be suitable for both AL and MS parts for more clarity kindly refer attached photograph thanks

Dear sir,

Actually the traditional pretreatments (PT) for MS (Mild Steel) and Aluminum are different. MS demands for a "7-tank" treatment/process, which has De-rusting, as the third tank which takes care of the rust formed in the process, and then Phosphating. Aluminium needs passivation.

There is a new multi-metal PT called Nano PT. This can be utilised, but the standard process flow for that does not have derusting stage. I feel you have to get in touch with the chemical supplier to give you a off-line derusting activity, then neutralising before taking the jobs for the Nano process.

 

Answer given by Meghan Manjrekar, expert in Painting & Powder Coating

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1
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1)what type of chemicals used for paint stripping? 2)How do paint Aluminum Castings forAutomobile application?

Paint stripping from jobs can be done with strong alkalies, if the jobs are of Mild Steel and the coating is of Alkyd or similar coatings. They can also be removed with very strong solvents. For jobs coated with Epoxy or PU you need to use Dichloromethane (Methylene chloride) or similar other 
strong solutions which are quite hazardous and extreme care needs to be taken while using. While on Aluminium, alkalies can not be used as they react.

Answer supploed by Meghan Manjrekar, Paint & Powder Coating Consultant

1
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Which type of phosphating to be used on cast iron object and how is it works and how we can recognise that it has done or not?

It is not recommended to use phosphating for MS castings, as the water soluble salts usually get absorbed in the tiny blow holes of castings, which are difficult to rinse. These water soluble salts would initiate blistering in the long run. Better to go for blasting ( shot or grit ) and paint or 
powder coat. While powder coating the air from these blow holes generate pinholes, so better to preheat 
and coat while hot.

Answer by Meghan Manjrekar, Painting & Powder Coating consultant

1
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How do eliminate over spray with out no of empty jigs during color changing in liquid painting?


There is no thumb rule for this. This will have a direct co-relation of your conveyor pitch and speed.
Also the Booth Balancing. In case if the air direction is moving inside the flash-off zone, you will 
have paint spray particles moving inside the flash-off and will get deposited on the previous colour. 
Ideally you should ask your spray booth supplier to have proper balancing of the spray booth. You can 
check the air direction by attaching threads at the cut out between the booth and the flash off zone, 
have them installed permanently. Ideally you should be able to paint the next colour, when the last jig 
painted with previous colour exits the booth.

Answer given by Mr Meghan Manjrekar, Painting & Powder Coating Consultant

1
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is zinc hot dipping possible on Aluminium Castings?

Usually not very easy as the melting point of Aluminium is 659 ºC and that of zinc is 419 ºC. So the 
controls required are too many. Secondly it will not offer any great advantages as on MS.

Answer by Meghan Manjrekar, Painting & Powder Coating Expert

1
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Sir , we are facing earlier Blister problem in Humidity test in Alloy wheel black color

The reason may be the water soluble salts not getting removed before coating. The last tank has to be D M Water to dissolve these salts. Please maintain the D M Water conductivity. Also control the water quality in the Chromatising process. Handling after chromatising upto coating also should be good. Does this happen with other shades or only with black?

Answer by Meghan Manjrekar, Painting & Powder Coating Co0nsultant

1
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IS POWDER COATING POSSIBLE ON ACRYLIC SURFACE?

No, powder coating can be done on metal , glass or plastic which can withstand temperature Of 200 degree C for min 30 Minutes.

1
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IS POWDER COATING POSSIBLE ON GLASS SURFACE?

Yes, same powder can be applied on glass surface with same gun, after some time without surface preparation, but some time etching with sulphuric acid and cleaning with water is

1
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HOW WE CAN ASSURE OVEN TEMPERATURE?

Oven temperature can be checked by TTR ( Travel Temperature Recorder ) having 6 sensor and calibration of oven could be done.

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WHAT IS IDEAL OVEN TEMPERATURE FOR CURING?

Oven should be at recommended temperature by powder supplier usually 180 degree C, while objects are being placed in oven ( 185 to 200 degree C ). Trolly of coated piece were entered in the oven, during this  temperature would go down say upto 150 degree C, close door and let the Temperature raise upto 180degree C again. Start air circulation ( In case of Electrical and Gas oven ).Start stop watch for recommended time by powder supplier.

1
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HOW WE CAN REMOVE CONTAMINATION OF POWDER ?

Contamination of powder is basically lack of cleaning of Booth , pipes and gun. Fresh powder may be OK, but you may get contamination in recovered powder. 

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WHY DIFFERENCE IS COLOUR IS MEASURED AS dE?

Delta-E (dE) is a single number that represents the 'distance' between two colors.

The idea is that a dE of 1.0 is the smallest color difference the human eye can see. So any dE less than 1.0 is imperceptible (as in turn the lights off and head to the pub) and it stands to reason that any dE greater than 1.0 is noticeable (as in put the coffee on, we're going to be here a while). Unfortunately - and probably not surprisingly - it's not that simple. Some color differences greater than 1 are perfectly acceptable, maybe even unnoticeable. Also, the same dE color difference between two yellows and two blues may not look like the same difference to the eye and there are other places where it can fall down.Details can be read on link;

 

http://www.colorwiki.com/wiki/Delta_E:_The_Color_Difference

1
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WHAT IS COVERAGE OF POWDER ON OBJECT?

Theoretical coverage = (( 1000/( thickness in micron X Specific gravity ))x 0.97 Thickness can be evaluated on coated object in micron Specific gravity can be considered from QC REPORT supplied by supplier of powder coating. 3 % process loss is considered

1
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HOW TO JUDGE COATING THICKNESS ?

Coating thickness on coated object can be tentative judge as

In single up and down coating with all regular parameter, 20 to 25 micron can be Applied, so for 60 micron 3 to 4 up and down application are required. There is no thumb rule, it depend on person to person also. With slow hand motion thickness could be more and with fast hand motion thickness could be low, with all same

1
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HOW TO TEST IN COMING POWDER COATING?

Testing of In coming powder can be done by applying powder and curing can be done on test panel of 3inch X 6 inch and thickness 0.5 mm. After curing following testing are must   THICKNESS , GLOSS, CROSS CUT, COLOUR

1
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WHAT IS PARTICLE SIZE OF POWDER?

Powder coating is not of only one size of powder, it is particle size distribution. The Correct size of powder can be defined by sieve analysis.

            

Powder passing through  32 mesh         28 – 32 %

                       through  63 mesh         70 – 80 %

                       through  80 mesh         90 – 95 %

                       through  100 mesh       99.9 %

1
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HOW WE CAN REWORK IF WE FIND FINISH NOT OK AND REWORK IS REQUIRED?

Rework can be done after applying sand paper rubbing or buffing the cured coated surface and apply powder recoating. You can use paint stripper to remove coated surface and fresh coating can be applied after surface treatment.

1
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HOW WE CAN COAT ONE OBJECT WITH TWO DIFFERENT COLOUR ON TWO SIDE ?

You can coat one side with one colour with masking other side with masking tape, cure it ,While coating other side you should apply masking tape on coated side and apply powder on other side,after removing masking tape, cure it.

1
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HOW WE CAN DEFINE FINISH BY GLOSS LEVEL?

Finish can be defined by Gloss value as ( AT 60 degree head )

 DEAD MATT FINISH     :     0 TO 5

 MATT FINISH               :     10 TO 15

 SATIN FINISH               :     20 TO 40

 SEMI GLOSSY FINISH   :     70 TO 80

 GLOSSY FINISH            :     ABOVE 90  

1
Answer

What is the root cause of the orange peel?

In liquid paints it usually depends on the type of spray gun used and the distance of spray gun from the job. If it is too far away, the thinner will evaporate. The drying time also needs to be checked as it is different in different seasons.

1
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We do not have blasting or power tool facility. We are using 3 in 1 chemical. Is it okay to use 3 in 1 chemical before priming?

In case of high-end corrosion resistance only the cross hatch test is not applicable, we have to do one more test which is the adhesion pull-off test, where we check the hydraulic force. The 3 in 1 will not give the right pull-off test.

1
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Can air dry paint be baked in oven or not?

Not all, but there are some paints which you can put in the oven. It is not called baking, but is known as force drying. Usually the temperature is 70-80°C. Reactions will speed up at higher temperatures.

1
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What is recommended mixing ratio of powder coating with recycle powder? Thickness is 70 ums.

Usually 80:20 is the ratio recommended by all the powder suppliers, especially if it is glossy powder. If it is plain finish or matte finish, blacks or greys, the ratio can go up to 70:30.

1
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Can PU DTM paint be used for 800 hr. salt spray instead of current Epoxy ZP primer + PU Topcoat?

These are new kinds of paints which are getting promoted in Europe and also India. DTM is direct to metal and PU has good resistance. You can avoid epoxy but ultimately the decision has to be made between the customer and you.

1
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How do we decide the rod distance from surface in compressed air quality testing?

It is 18 inches as mentioned in the ASTM standard. You don’t have to decide, you have to follow. 

1
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EPT is installed, but not being used efficiently. How we can contribute to bring to notice of the government on broad level so that we can save our environment and reduce the pollution?

There are people from Pollution Control Board to look into these matters. You can definitely inform the local PCB and they’ll take necessary action.

1
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At 140micron thickness we are facing waviness problem, what could be reason for that?

If the base metal imperfections are in mm, 140 microns is probably 14% of 1mm. So it’ll not hide those waviness problems fully. The problem must be coming from the base metal. Just check the incoming material.

1
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How to dispose the liquid paint sludge?

There are regulations by CPCB regarding this.

1
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3 in 1 Cleaning is recommended method of pre-cleaning or not?

3 in 1 should be done on something which cannot be dipped into the chemical tanks or which cannot be blasted.

1
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We are using Al-Zinc 2 mm, 1.5mm for our product. In case of IP23 compartment, where is this Al-Zinc sheet used, at edges we find rust. What should be the Coating process to avoid the rusting in this case?

The cutting method is important because if you cut it with flames or lasers, sometimes the Zinc will melt and come to the edges which will avoid the rust. If you shear it these kinds of problems arise and with coating only you have to do it. In case of powder coating edge thickness is better to avoid rust.

1
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Is there any other process to resist rust?

Yes, there are many methods to resist rust such as electroplating.

1
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Can we use the solvents on forged pieces before powder coating?

If your powder coated pieces do not have sharp edges, you can use. Otherwise the lint from the cloth will get stuck in that and problems will be caused.

1
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What is the fastest method to clean surface?

Automatic blasting machines are available which can give results very fast.

1
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Is de-rusting process required for online process where there are no chances of rust in parts?

No, in fact you should avoid that. There you can go for offline de-rusting if few of your parts are having problems. These chemicals are available with chemical suppliers.

 

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WHAT IS THE BEST PROCEDURE TO REMOVE OIL WHICH IS SEEN AFTER SHOT BLASTING FOR CASTING PARTS?

You are not supposed to remove oil after blasting. You have to remove the oil before blasting. There is a standard called SSPCSP-1 which is mandatory before shot blasting for any kind of mechanical cleaning which you have to do and after that you’ll not face this problem. So don’t try to remove the oil afterwards, try and remove it before.

 

1
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For large frames 19 ft X 10 ft can we grit blast the frame before powder coating? If yes then what should be the surface finish and profile?

If these frames are made of structural steel or HR, you have to blast it. If made of CR it need not be blasted, you can go for chemical pre-treatment. So depending on that you can select the pre-treatment before coating. Surface finish ideally, if it is blasted, should be SE 2.5 which is standard. You’ll find it in ISO XY01 and the profile should be less than 40% of your first coat DFT.

 

1
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WHAT IS THE REASON IF WE USED RECYCLED POWDER OF YELLOW COLOR THERE ARE LOTS OF DEFECTS SEEN AS COMPARED TO BLACK COLOR?

Usually it should not happen. If you are facing certain problems then the cleaning efficiency of your booth is a questionable thing. Then it is contaminating your recycled powder which in turn gives you defects. That is probably the black colour that is coming and in black the yellow will get totally dampened. So you’ll not see any kind of contamination in black but in yellow you will definitely find contamination from black colour.

1
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Primary cyclone take 1.5 hour as it is multi-cyclone but my booth base powder is not get by cyclone blower

You need to check your design because there is some problem with the total design of your booth and cyclone and in your total powder coating system. Cyclone is hard over system and second important part in your oven. If they are good then you’ll get good results and good output commercially and technically. 

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I am facing problems in passing the impact test of structure powder. What are the check points for passing the impact test?

What is the impact resistance you are looking at? Usually for structure powder the impact is lower. Sometimes 50kg/cm and sometimes 80kg/cm if you are doing it at par with your normal glossy powders you will not get it. It’ll be lower than that.

1
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How can the SST of foundry parts be checked? Can actual parts be used for SST checking instead of regular popular MS panels? What is the general practice in foundry industry?

Usually the jobs are put in the salt spray chamber. In many places the chambers are bigger, which can accommodate the parts and you can do the salt spray there. Actual results on job are more important than the results on MS panel. So usually people have bigger size of salt spray chambers used for jobs.

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What is the mixing ratio of virgin powder to used powder?

Usually it is 80:20 with virgin being 80% and used/recycled powder being 20% in case of glossy, structured texture. But in certain cases like matt black or satin black we can go up to 30% for recycled/used powder because there is hardly any gloss difference seen.

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Is aging period different based on type of powder?

No usually as a thumb rule anything which you are curing with heat i.e. maybe liquid or powder coating, the time period is 48 hours and in case of air drying it is 7 days. So everything will have different period of aging but normally we consider 48 hours as maximum.

1
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Is UV coating effective?

Yes it is but UV cured powders are not readily available in India. UV is successful on flat jobs only and not on jobs like tables, chairs or automotive components or frames because of the distance between the UV source light and the job has to remain same. Then only will the curing happen. There cannot be any shadow areas where the UV light is not reaching, the curing will not happen then. Otherwise it is effective. We use UV coating for the jobs which we cannot heat i.e. on plastics or wood.

1
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What should ideal DFT for structure finish powder on TEST PANEL (75mm x 150mm x 0.8)?

It’s very difficult to check DFT on structure finish powder because in that you have ups and downs. But with normal DFT gauge you get 90-100 microns or 90 plus minus 10 microns as standard DFT for structure or texture type finish.

1
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What is the difference between phosphate and oxinol process?

Basically phosphate is a normal conversion coating where you are forming crystalized structure of zinc phosphate onto metals. Specifically ferrous metals. Oxinol is a different material. It is silicon based polymeric coating which is formed on the surface. It is not crystalline. It’s an amorphous coating. So it is a very loosely hit coating, even 50-200 nanometers thickness of coating is good enough to give you a prolonged corrosion resistance against phosphate. Phosphate is usually 2-10 microns.

1
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Is there any process to reduce cleaning time of cyclone and booth? Is it possible to flow the powder from booth base to cyclone?

The correct design is that the cyclone is going to suck the entire powder from booth bottom/base. Definitely there is some problem in the design of your booth and cyclone. Industry standard time for cleaning is 30-45 minutes from black to white. Nowadays the new generation plastic booths have hardly any deposition on the walls. So cleaning time is reduced to less than 5 minutes but they are very expensive and need to be imported. If the walls are stainless steel and highly polished, the time can be reduced to 20-30 minutes from black to white.

1
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Mild steel or stainless steel,which one is recommended for hanger?

Mild steel is okay because even on stainless steel you have to strip it down. Only thing is that if it is a continuous conveyor line where your hangers are passing even through the pre-treatment tanks, then recommended is stainless steel as it will not react with any of the chemicals inside that.

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After 6 month we have found rusting in outdoor panel,outer surface area.

Yes obviously because your queuing is a problem. MEK test is failing, so naturally the powder is not cured properly. The powder is giving away much earlier and you are facing these problems. Normally with any powder under any conditions it should not rust for the first 2 years at least. Correct your oven issues.

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Sometimes our MEK test fails.How can it be improved?

MEK test is when we take MEK on a cotton swab and rub it onto the panel so one wipe down is one cycle. Usually 15-25 cycles again depending on the resin system whether epoxy or pure polyester or hybrid even. If that colour gets transferred onto the cotton swab we say it has failed. If it doesn’t it is accepted. Now this is an indication of curing of powders. If powder is cured properly i.e. baked properly, the colour will not get transferred. If colour transfer happens MEK test fails. One of the reasons is due to under-curing of powder job. You have to improve your baking schedule. So get your oven calibrated, the TTR (Travelling Temperature Recorder) or data/temperature logger type machines are available for use. You have to calibrate the entire oven and depending on that you have to set your time and temperature. Also try and find out what is the heat role calculated or given to the oven supplier at the time of installation and whether you are sticking to the same amount of weight.

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Do we have instrument to measure humidity during powder coating? If yes, what is the cost?

I will not be able to tell you the cost but there are various instruments and electronic displays available. I think they will not cost more than Rs. 1000-2000 which will give you temperature and humidity readout. But these are not technically correct and you will find a 1-2% difference. They will also not get calibrated and they are just indicators. However, you can get instruments up to psychrometer but it can be quite expensive. Electronic psychrometers are also available which can be sourced out from technical instruments or testing instrument suppliers.

1
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Is there different bake times for different powders? What is an ideal time?

There is a difference between resin systems and these resin systems will demand different temperatures at different times for baking/polymerization. It’s a chemical reaction so you cannot expect all the chemical reactions happening in all the resins to be at the same level but what we have done is we have tried to keep the time same and vary the temperatures. Like in the case of epoxy it is 180*C for 10 minutes but in case of pure polyester it is 210-220* for 10 minutes. So if it is a conveyer system, everything will remain same but the only change will be an increase or decrease in temperature.

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Is peel off issue found due to any material defect (physical & mechanical properties)?

Yes it can be. Usually if your powder is very old or not kept properly you find this peel off issue. The main reason is either pre-treatment or under-baking or not fully curing the powder.

1
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How to find out peel off location?

I think you mean to find the location where the adhesion is not really good. It’s very difficult but if you look at it against the light, sometimes you can see some kind of imperfection or some kind of upraised material. Then you can find out that adhesion is not really good. But it is very difficult if there is a thin layer of oil or thin layer of lubricant on which powder coating happened, you’ll not be able to see it visually but if you do it with regular adhesion test you will realize that the adhesion is a failure. So it’s very difficult to find out.

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How to handle powder coated Aluminium Extrusions during assembly, without damaging coatings?

It basically involves good practices because what happens is in auto industries, car plants, you cannot wear rings, watches or anything that has sharp edges and which has the capacity to damage the coated jobs. Usually you cannot wait for 48 hours like in packing i.e. the aging period. The cars will move immediately to the assembly shop. Normally they will have an inspection on the screw driver edges, whether they are really sharp or not in order to avoid scratching and handling problems at assembly level. So you will have to adopt these kinds of practices and there are no textbooks available for this, neither are there any standard practices. You have to derive your own system and come to the conclusion.

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Would it be advisable to add flow control agents during application in order to avoid lump formations?

I don’t think it’s possible because addition of flow additives needs to be done in very precise quantities which is very difficult to add at application stage. Then they have to get merged or embedded into the resin component. If you don’t post-mix it properly with those resins then you’ll not get very good flow property so it wouldn’t really be advisable and I don’t think that will avoid the lump formation. It occurs because of certain other reasons.

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What is the ROOT CAUSE OF OPACITY ON EDGES (SPECIALLY YELLOW POWDER)?

Now there are pigments known as chromium pigments which are inorganic pigments having very good opacity. Now however, since chrome is out, we have to replace it with certain other yellow colours and in case of bright yellow colours especially the colour yellow used in earthmoving industries. These yellows are made with certain organic pigments which do not have very good opacity. Now on edges especially you get greenish or blackish lines or patches and this is a technical limitation. Because on the edges the powder will slide down when it is under heating, specially the top edge corners and there the DFT will be reduced to lower than 20-25 microns. But on the edges you cannot check the DFT. So it is very difficult to check it there also and usually you see that opacity problem there.

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What is the effect of Re-Phosphating on Powder Coated part? In case of Re-working, will this affect SST?

Certainly it will affect SST. Re-phosphating is not recommended. On powder coating the phosphating will not have any effect. You can only coat certain areas for reworking if you have done some sanding and all, that area can be coated and you can use it but re-phosphating on bare job is not recommended.

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Which pretreatment is better - dip type or spray type?

I think it depends upon the job configuration and job type. Both are good technically speaking and can be adjusted, controlled, managed equivalently. But in some cases such as that of a motorcycle petrol tank, it cannot be dipped as a lot of air and energy is required to dip it inside so it is better to go for spray phosphating inside. Sometimes we have to do rocking and rolling also if the jobs have very odd shapes or cavities, so that the air should not interact. When taken out of the tank, the water or chemical should get drained out as fast as possible because we cannot allow the chemicals in the pretreatment zones to dry. If it dries then you’ll start getting different problems such as rusting. Another problem with spray type is that if you have very odd jobs and if you don’t hang them properly, the spray can get bounced and go into another compartment. Sometimes you have very short drip gap in between two baths or spray zones, so usually the carryover is much more in case of spray phosphating. Thus the design of spray phosphating has to be done very carefully.

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When can we pack the powder coated jobs after oven curing?

Ideally after 48 hours. This is known as the ageing period and we have to give some amount of ageing so that we can pack it otherwise you will get some markings especially if you pack it in bubble wraps. Those bubble marks will appear on the job. Better to wait 48 hours before packing.

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If after finish job we do recoating for a specific area followed by reheating is it a correct process?

I won’t say it is wrong but it is very difficult to match the finish. You’ll find a different finish in that particular spot where you do the touchup. Usually recommended to do the total job coating again for a second time and bake it together so that there is the same finish throughout the job. But in case of spot touchup the original coating will be baked twice and naturally the gloss level will go low and you’ll always find a patch there.

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What is the difference between metal temperature and oven temperature in general?

There are two things – one is oven temperature and actual metal temperature which we call EMT (Effective Metal Temperature). Usually when jobs are put in the oven, the jobs are at room temperature or ambient temperature, which is usually 30*C or so under Indian conditions. From 30*C it has to go to 180*C. Inside the oven the construction of oval is very important. The hot air will pass over your jobs and they will start accepting or absorbing that heat. So the air reaching back to the heat exchanger will have a lesser amount of heat. We usually have a TC or temperature control there, which is regularly monitoring the temperature of the air. Now this particular temperature naturally balances through the inlet air i.e. 200*C in both cases, the oven heating will shut off. When it goes below the set temperature which is probably 195*C or 190*C, the burner will be put on. So what happens is that there is always a difference in oven temperature i.e. oven set temperature and job temperature and this should not be more than 5*C or 10*C because this is a hot air temperature which we are controlling for burner shutoff conditions.

1
Answer

Is there any procedure to strip the powder coating completely from the substrate if the defect found is not acceptable?

There is no recommended process but if it is a very thick job, HR jobs, more than say 3mm thickness, we can recommend blasting and removing the jobs. Of course it is not as easy as regular blasting but it can be used to remove the powder. A lot of people are doing it chemically by using very highly concentrated phenolic materials. They are dangerous to handle and store but again none of the text books say how to remove the powder or how to strip the powder coating completely. You can go ahead with whatever suits you but industry standards for first coat rejection or FTP shouldn’t be less than 2%. Ideally less than 1.5%. In that when you recoat some jobs and you come down to 0.2 or 0.3% of virtual actual rejection which needs to be stripped. So try to reduce your rejection, then probably the damage will not be so much.

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Answer

What material can be used for reducing deposition of powder over hangers? (Pins always get thick)

There are no good materials available as such. You have to keep it under cleaning on a regular basis. Hangar cleaning should be a regular activity because of the deposition that happens. People have tried using silicon tubes on the hangars but that silicon degrades and creates some other problems on the powder. So usually we do not have any good solution for this. The only thing that can be done here is regular cleaning of the hangars so that you have earthing resistance not exceeding 1 mega ohm, which can be checked on a normal multimeter. 

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Answer

What are the best method for touch up in the power coating available in market at present?

Nothing, in the sense that there is no right way to do touchup for powder coating. Because if it is a spot touchup or a brush touchup then it’s okay but if it’s a bigger touchup, you cannot match the finish of powder in the first place, secondly what people are using for touchup in most cases are aerosol cans. There are usually resins in that or the polymers in that is thermoplastic acrylic. These are pre-polymerized and have no outdoor resistances. Nor do they have any good corrosion resistance. So till the time it reaches your customer, it’ll be looking exactly the same as before. But if you want it to have long term touchup then it is not a recommended process. There are certain clients of mine who are into commercial vehicles and farm equipment. They are talking of touchups with equivalent polyurethane paints. My recommendation would also be to use polyurethane of the shame shape and gloss level. But again a small touchup and not a big one should be done, where you will be able to see a difference in the finish. 

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Answer

Is DFT directly affected on salt spray life?

It is both a yes and no. Yes because if the DFT is very low, there will be a failure or early failure in salt spray life. But if you go for the recommended DFTs you’ll get the salt spray life. Again if the DFT is higher than twice the recommended DFT there are chances that the film is not fully cured from inside and then it may peel off or delaminate or it may remain soft on the inside which happens mostly in case of liquids but it can happen in powder also. And there could be early failure of salt spray life. So you have to stick to the DFTs recommended by your powder supplier.

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Answer

What is the minimum time for which the phosphated parts are to be used for further painting/ powder coating processes? Will keeping it for long time affect the coating life?

Usually we recommend 3-4 hours. There is no minimum time, you can immediately put it for coating but maximum time should not be more than 4 hours. In case of high humidity it cannot be kept for a long time because over a period of time there will be a growth of corrosion or rust inside which cannot be seen with the naked eye. And if the coating is applied over that, then the early failure of your coating is certain.

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Answer

On Same Bath preparation we use multiple GI material, but oil patch especially observed in only one GI part only.

This needs a root cause analysis and investigation. Because it is not happening on other GI material but only on 1 part of GI. Please check what lubricant is being used while pressing. Now drawing lubricants are available which are miscible in water. In north India, I have observed that people are more comfortable using mustard oil as a lubricant for press shop but that mustard oil doesn’t react with any of these pretreatment chemicals. So avoid such oils or avoid oils which are really tough on these things. Just check if the oil you are using is miscible in water or not. There are water miscible lubricants which are available and please try to shift to those so that these oils get removed very fast without adding a load onto to your degreasing tank. The oil content doesn’t increase in the degreasing tank as well.

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Answer

In case of Steel Products Pretreatment. What If the Fe contents increases in the phosphate bath/tank?

The ferrous content usually increases after you use it but it doesn’t give you so much of a problem. In fact, if the ferrous content is low you start getting a yellowish or tarnished phosphate layer. Usually 6 months on continuous productivity, the phosphate tank ferrous content doesn’t increase so much as to be really detrimental to your coating but after that usually your chemical suppliers recommend changing the bath.

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Answer

In Pretreatment process how do we identify whether the bath is affected by Silicon contamination on using anti spatter spray for parts?

It is usually very difficult to find out, we have to check it chemically. Send your samples to your chemical suppliers and get an analysis done to determine whether silicon contamination is there or not. And please avoid using silicon anti-spatter spray. There are non-silicon or silicon free anti-spatter sprays available in the market. Please try them and they work very well. The second problem is that people are using silicon on the nozzles of welding torches and that gets onto the job which contaminates the jobs. Again these sprays are very close to the application area. They may come and settle on the jobs before coating and they may cause certain problems such as cissing.

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Answer

In GI Sheet oil patches observed after coating. Before coating how would we be able to identify that defect. (We are already using NanoBonderite technology. Oil content less than 1gm/sqm. Still we are facing the issue.)

I don’t know if you have an entire line of spray phosphate and whether the jobs are moving on to the conveyers. But if they are handled between coating i.e. pre-treatment and powder, just check the cleanliness of your workers, whether they are wearing caps and gloves or not. Because, majority of workers, I have observed, apply oil in their hairs. Then body sweat and other contaminations may give you these kind of defects afterwards. Another source of oil could be the oil coming in from compressed air and it especially happens during the monsoon season. Usually at the start of monsoon there is condensation and this condensed water usually comes in through the pipelines driving the oil and bringing it to the application equipment. 

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Can you please explain aluminium pretreatment -chromatising and how to evaluate chromated surface?

In case of aluminium the same treatment that is used for galvanizing or galvanized steel (GI) can be used for aluminum because both behave equivalently. What we do now is degreasing. Again this particular degreasing for aluminium   or GI is little less strong as alkalis are less and we have higher amount of surfactants in that. So emulsifiers are more and then you have water rinse. After that if your aluminium or GI has oxidation in the process, like what we call white rust formation, usually we have a de-oxidation stage. Again these are mild acids, mild in the sense that they are not dilute, these are usually organic acids. So you usually clean the upper layer of the white rust and then you treat it for a process called chromatising. Now this chromatising will not create a layer, it will only etch the substrate. Now evaluating the chromated surface is usually a visual thing. Also take it under an electronic microscope and see the etching levels and compare it with visual standards and then pass it. Because of the RoHS and other compliances chrome is not available anymore. So we are doing this etching treatment with non-chrome type of materials. But still the process is known as chromatising even though chrome is not used anymore.

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Answer

How much Ra value of material will be optimum?

In case of automotive where you need a very high end finish/gloss, the Ra value recommended is less than 2 microns and with the first coat that is CED we try to bring it below 1 micron Ra value. But in case of powder coating we usually do not talk about Ra value of the substrate. 

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Answer

What are the check points to pass the impact test ( 100 kg.cm ) on structure powder?    

Thank you for your question.

Please watch

https://www.youtube.com/watch?v=0UTBntfPaSE

It is explanatory

ASTM D2794-93(2010) 

Full Description

1.1 This test method covers a procedure for rapidly deforming by impact a coating film and its

substrate and for evaluating the effect of such deformation.

1.2 This test method should be restricted to testing in only one laboratory when numerical values

are used because of the poor reproducibility of the method. Interlaboratory agreement is

improved when ranking is used in place of numerical values.

1.3 The values stated in inch-pound units are to be regarded as standard. The values given in

parentheses are mathematical conversions to SI units that are provided for information only and

are not considered standard.

1.4 This standard does not purport to address the safety concerns, if any, associated with its use.

It is the responsibility of the user of this standard to establish appropriate safety and health

practices and determine the applicability of regulatory limitations prior to use.