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Powder coating machined cast iron: gassing problem
May 25, 2011
Q. How do we prevent the carbon present in cast iron from adversely affecting the powder coating process.The components are fully machined but require painting
MARTIN WALKERENGINEER - UNITED KINGDOM
^- Privately contact this inquirer -^
May 27, 2011A. I've not noticed carbon being a problem in powdercoating cast iron.
What problem symptoms are you experiencing?
Geoff Crowley
galvanizing &
powder coating shop
Glasgow, Scotland
June 1, 2011
I suspect that the problem you seek to avoid is more to do with gas entrapped in the casting than Carbon content.
My experience with CI parts is to treat them pretty much the same way as with HDG parts.
De-gas the parts by exposing them to a temperature ~20-30° C metal temperature above powder cure temp for ~20-30 mins.
Then make sure you are using a de-gas powder (often referred to as casting grade).
The object is to drive out as much gas as possible before pre-treatment and then to extend the Gel time of the powder during the cure cycle to allow any residual gas to escape.
Hope this helps.
Regards,
Bill
Trainer - Newcastle NSW Australia
June 3, 2011The issue of "gassing" is an interesting one. Seems to occur almost exclusively on casting (both iron and aluminium) and HDG.
We are embarking on a research project soon to try to get to the bottom of this phenomenon.
There are many opinions as to what causes it, what gas it really is, how to avoid it, or to cause it to evolve prior to curing.
"Ant-gassing" powders gel slower, allowing time for evolved gas to escape and the coating to reform smoothly after a bubble pops though. They have (I understand) more wax in their formulation.
The common "cures" for gassing are not foolproof.
Anti-gassing powder sometimes don't cope.
"de-gassing" doesn't always work.
These factors seem to have some influence:
steel chemistry; In HDG, the thickness of zinc; The hanging orientation; the colour; the metal thickness; The pretreatment route.
I'm keen to get as many opinions as possible on this phenomenon.
Geoff Crowley
galvanizing &
powder coating shop
Glasgow, Scotland
June 8, 2011Dear Geoff,
Yes we would be very pleased to get involved with your company and local university in an endeavour to discover how and why gassing occurs during the baking of a powder coating film when applied to a galvanized substrate. By including waterborne primer into the University programme we may obtain some answers to why it seems no longer necessary to pre-heat or T-wash a galvanized substrate prior to powder coating.
I believe getting involved with this exercise will help us to understand why/how a waterborne primer works in this application. Could it be that the wetting agents in the formula (included to wet a metal substrate and help with pigment dispersion) allows the waterborne primer to apply evenly over a metal substrate but also allows the primer to sink into the galvanized coating and displace any air/gas that may be en-trapped within?
I understand that you will be contacting us for samples when the University is ready to begin its investigatory exercise.
Terry
Terry Hickling
Martex Paints Ltd
Birmingham, United Kingdom
June 9, 2011The problem with castings is relatively easily explained ie porosity.
William Doherty
The problem with HDG a little more interesting.
I observe that BS 6497 calls for air quench.
I always thought this was more to do with Sodium Dichromate used in the water quench to delay White rust onset and which interferes with Powder adhesion.
Maybe air quench also allows entrapped gases to escape?
I wonder what comparative results would be achieved if the metal was IR heated rather than convection heated at the de-gassing phase?
This subject should be attracting a wider audience to provide as much independent input as possible.
The recognised contributing factors might be more fully elaborated on to assist us all as well.
For Example: how does hanging orientation affect the phenomenon?
Let's get together on this one for everyone's benefit.
Regards,
Bill
Trainer - Australia
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January 31, 2012
I would be interested in any insight into the outgassing of gray cast iron during powder coating. The occurrence is sporadic, but apparently far more frequent in gray iron than in nodular iron. Our process is to shot blast the surface, then pre-heat to 220-280° F prior to painting, then curing that 310 ° F. - Chattanooga, Tennessee, USA February 3, 2012 I've recently started working with Geoff Crowley at his Galvanising and Powdercoating shop in tandem with a local University and I'm looking into the Pinholing/ Gassing issues associated with powdercoating metals.  Rikki Speakman - Cumbernauld, Strathclyde, Scotland |
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February 12, 2012
Hi Rikki,
You must be feeling a little let down by the lack of replies?
Trust me, it is not because of a lack of interest.
This is one of the two major bugbears confronting the Powdercoat Industry.
Geoff suggested in an earlier post that he had identified a number of influencing factors.
Perhaps if you were to enumerate those it will inspire more robust contributions.
Many of us have theories but have not had the luxury of a purely scientific and analytical programme to test these.
Regards,
Bill Doherty
Trainer - Newcastle NSW Australia
February 17, 2012
Hi Bill,
I think the issues Geoff talked about on June the 3rd last year on this thread sum up many of the potential causal effects, maybe the original steel processing (will tie up with the original cast issue of the thread) and stand down times between parts of the process are a couple of possibilities worth adding to the list.
We don't have a huge amount of quantification just now on the impact of these individually, hopefully we can develop that over time. Certainly though the vast majority of pinholing in my first few weeks has been seen on thick metal parts and structural steels, so hopefully get my teeth stuck into those swiftly,
Regards,
Rikki
- Cumbernauld, North Lanarkshire, Scotland