Does respraying a frame weaken it?

Something I’ve wondered for a while. Any experts?

If it’s an aluminium frame and you use a pro company – say Orange – how many times is it safe to respray?

If you don’t strip off the old paint then surely it will strenghen it with each additional layer…….probably 😉

Really? Why would it? The metal is what does the structural shit, the paint is for prettification.

Depends how you strip the paint and apply the new.
If for example you get the old paint shot blasted off and then get it powercoated it “could” weaken / damage the frame.
Depends how its stripped / re-coated really.

I’ve heard (and seen images) of powdercoating weakening the frame as it heats it up, which affects the welds…

DrP

What he said. Done by someone competent you could repeat indefiately, done by a monkey with a shotblaster and it’ll be scrap in 20 seconds.

Thanks. You’d imagine that Orange would be fine then wouldn’t you?

Stove enamelling also heats the frame

Fairly sure orange (as in the bike co) know what they’re doing with re-sprays. They do it all in-house don’t they?

[EDIT] there is potential that the heat used in both powder-coating and stove enammeling could affect frame strength by (in simple terms) altering the entire materials heat treatment state, not just the welds.

I suspect there is more to this question than meets the eye 😉

Powder coating is done at 160-210 celsius, doubt that temperature makes any difference to the properties of the metal.

If this was a question about powder-coated bike frames in general, not specifically Orange.
Then, not all powder-coat is the same type of plastic, and removing plastic powdercoat is quite a harsh process.
If you took a bike frame to a powder coater to get it re-powdercoated, they can only guess what type of powdercoat/ plastic it is. The process to remove the old powdercoat (or paint) can be a chemical dip to soften the plastic / paint then blast clean, or a pyrolytic burnoff oven then blast clean.
The blasting of the frame is also to get a surface that the powdercoat can hold on to , key the surface.
http://www.youtube.com/watch?v=5kHCh1E6REo

For an aluminium frame this is quite a brutal process.

6061 aluminium is generally age precipitation hardened (heat-treated)at 160 to 190 degrees C.

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MA6061t6

Powder coating is done at 160-210 celsius, doubt that temperature makes any difference to the properties of the metal.

yes but often companies (like armourtex) use a pyrolysis oven (for steel frames only – alu frames are chemically dipped) to burn the old paint etc off…

not sure what temp they run at… would of thought more than the powder bake oven.

No more detrimental to properties than welding the frame in the first place.

It is a bit of a loaded question ell_tell but not in the way I think you think.

I cracked a frame that had been resprayed once. It’s a prototype Orange and I’d have binned it if I could get another one the same size (I can’t). Anyway, I’ve had it rewelded by BETD and I’m thinking of getting it resprayed and building it up again. When I was riding yesterday one of my mates brought his Nomad along that has been resprayed and now has a couple of holes in it… it wasn’t done by Orange but the company are pretty reputable.

http://www.eastoncycling.com/bike/wp-content/uploads/2010/04/fab_instructions-7005_6061.pdf

I wasn’t going to do this as last time I got jumped on but here is what I know and what I think about powder coating (somewhat simplified and a few things left out):

Background. I’m a materials science/metallurgy graduate who has worked on and off in the aluminium and steel industry for the the last 17 years. I have done an awful lot of work with fatigue and fracture of aluminium and heat treatment. I am by no means a world expert on this but I know a fair bit about this topic and people pay me for my expertise.

The risk: Only with treatments that involve heat, and mostly only in aluminium alloys.

Explanation:

1) Aluminium alloys internal structure begins to be affected by heat at about 120 celcius, it would take a long time at 120 celcius for anything to change (like months or years) but up that to 200 or 250 Celsius and the changes can happen in hours. Many aluminium alloys are heat treated at 200 -250 degrees where the temp is controlled very very carefully and the time at that temp is also controlled very very carefully. This is done in a high tech heat treatment plant.

2) Typical powder coating temps are between 140-250 Celsius depending on the powder and treatment. But important to note that the powders tend to be time and temp tolerant within a certain range and that powder coating plants have nothing like the strict level of control found in a metal heat treatment plant.

If you look at the graph enclosed for 6061, especially the 205 celcius line you can see that if the alloy is left in the heat at 205c for more than about an hour the peak strength starts to diminish.

I have heard of powder coating plant where the operator says – “I know you want it tough mate, so I left in the oven all weekend, the powder will be well baked on..”

Or they up the temp a bit, or the just don’t have an accurate enough control mechanism to keep the temp within range. For example I’ve tested a powder coat oven for a client who had exactly this problem. The oven had its thermostat near the door, and near the dial, which they keep below 140c. But with a thermocouple on a probe I mapped the heat in the oven. In the middle it was getting to 325c, and ruining the heat treatment of the aluminium within minutes.

Either way it is a risk.

If you find a clever powder coat plant who are aware of these issues (and have the knowledge to control it, for example do they have a verified thermal map of their oven?) then you will be fine, but personally I would stick to paint and avoid powder coat, unless you are convinced that they are going to stick to lower temps and can control their oven..

{runs away and hides}

Wow, thanks toys19. I think that’s the first time anyone has managed to explain something so technical in a way that I understand 🙂

*Stands and Applauds*

materials science/metallurgy graduate professional who has worked on and off in the aluminium and steel industry for the the last 17 years

FTFY. Jolly good show that chap.

toys19 I can understand why you’re hiding. Using facts, figures, actual evidence and graphs to explain something is not the done way here. 😉
Have you not got some random story of a mates brother once got his alloy rims powder coated and they turned to dust the first time he put them on his escort? 😆

Yes good graph, with significantly yeild strength.

bigyin
since you mention alloys for cars. Not all alloys of aluminium are heat-treatable: not all can be precipitation hardened. Thats why car wheels could survive the sort of treatment that would be too much for bike frames.

Also, pure aluminium has a higher fracture toughness than high-strength aluminium alloys.

Thats why car wheels could survive the sort of treatment that would be too much for bike frames.

Well it depends on the cast alloy being used (as it does with wrought alloy used in bike frames) as many of the cheap wheels the boy racers put on their cars are made from for example A356 (also known as LM-25)(clicky). (Or LM-9 another heat treatable casting alloy)

Now A356 is an Al-Mg heat treatable alloy, and can easily become more brittle with the application of heat. See this paper on pag 15 table 4 shows that for alloys at the upper end of the Mg content (its limits are between 0.25 and 0.45 wt%) will show a marked reduction in impact strength at longer aging times at 180C.

My real point is that all Aluminium alloys are thermally influenced in some way at similar temps to powder coating/stove enamelling and unless you know what the alloy is, what it’s previous thermo-mechanical history is, exactly what the heat treatment is going to be during coating and how well controlled it is, then you are playing the lottery with your bike frame/wheels/golf clubs,scaffold tower…

I remember reading that heating the frame during paint baking can affect it.

I don’t have any reference or evidence 🙁

Use light sand to shot blast and then spray by pressure?

I have an old race frame which was redone by Argos and it’s still strong
as mutts nuts.

Isn’t search handy.

I was just about to ask about powder coating a Reynolds 653 frame. I’ve had two steel frames fail shortly after powder coating and I don’t know if this is just coincidence.
The Inbred cracked on a chain stay and my old 531 crosser just folded.

Should I stump up for a pro enamel job?

They stove cure enamel so it’ll still be subject to heat

Raw is the way to go

Oldgit – steel is pretty slow to to change at powder coat temps, the above discussion was aimed at aluminium alloys.

Rationale:

You can think of it like this: most metal generally wont have changed appreciably after cooling if it only gets to about 1/3 of their melting point (in kelvins), but by 1/2 of the melting point then lots of rearrangement is going on.

Steel Melts (in kelvins) at approx 1773K
1/3 of this is about 590K
Powder coating is carried out around 473K(but I have seen an oven go to 600k) so you wouldn’t expect to see a huge effect on steel.

Aluminium alloys melt at around 933

1/3 is 311 and 1/2 is 466. So you can see 473K (200c) is over half the melting point of Aluminium and there are some interesting changes likely to occur. (regardless of whether its a heat treatable alloy or not)

Therefore I would imagine your steel frame failures are due to something else, although without investigation of the metallurgy of your frames before and after heating, I would not rule it out.

The higher the strength of the steel the faster it will react to heat, so a normal structural steel (yield strength 275 MPa) will be slow to change even approaching 1/2 melting point (it will change eventually though). But as the strength increases there is more potential energy in the system to be released to drive the changes.. Do you want me to go on?

So a 4130 or 531 frame I would not be worried about. A 953 frame will experience small changes in metallurgy, is it enough to effect fatigue life or yield strength? Dunno, unlikely even, (if it does it’s as likely to be positive or negative) but if it was mine I would get it room temp paint sprayed just to be sure. Your 653, hmm have a chat with powder coaters about time and temp control and if they can minimise it then it will be fine. With steel I would really only be worried if the line was run by grunts and they were wild with the temps..

Thank you for the info on the Al alloys….I for one will not be having any of my alloy frames recoated.
Your comments about thermocouple location and the powder coaters just living in ignorance is compelling.

PaulD

oldgit, i wonder if the problem wasn’t internal corrosion? most steel frames i have come across that have failed have done so inside out. shot blast frame to get old paint off and you might exaggerate any weakness?