Powder coating.....how much do you loose?!

[Tootyred]

I've aways been a bit concerned when it comes to powdercoating bike frames. Whilst i spent a lot of my working life around powdered parts, these were always much thicker steel than a bike frame.

Today, i found some old tange prestige tubes taken from a frame......for those you who have never considered the thickness ( or should i say thinness) of these tubes here's a picture.....its a seat tube and the walls measure out at about 0.9 / 0.6/ 0.9. mm (I will get some more accurate measurements next time im in the workshop with decent measuring gear).

Each time a bit of metal is blasted you loose a small amount....it can't be helped. The blast media makes a big difference, the guys we use change media for the job at hand, but not everybody does. Also dirty recycled media can be a problem as it may contain particles far more abrasive than the base.

So, as an experiment im going to clean these tubes up ( these were wet painted orginally) and then ask my friendly powdercoaters to blast them with say walnut shell and something more standard and more abrasive to see how much metal actually comes off.

I honestly dont know the answer, but im keen to find out! You might be i interested, especially if your about to re-powder that tatty powdercoated frame....as that's 2 cycles of blasting!

PS this frame was door-nail dead (rotted through in 2 major places ).....so no early 90s frames were really harmed in the process!

 

[Tsundere]

Interested in the results.

I decided against getting a frame blasted for this reason, I think you should be absolutely sure the tubing is in the best possible condition before employing any mechanical techniques for stripping away surface material. I read somewhere that a lad who was using a sander with fine grit on a frame ended up sanding through a section of what was clearly an already oxygen compromised down tube.

Presumably this is something you wouldn't need to worry about at all with newer steel tubes, while older tubing of the variety we here are concerned with carries with it an obvious increased likelihood of being in possession of some kind of pre existing fragility.

Will you use weight as a measure as well as thickness?

 

[novocaine]

Technically you shouldn't loose any material thickness as the blast material is softer (wrong word I know, layman's terms) than the base metal.
there was a rule of thumb however (no idea where this one came from, but it was espoused to me at some point, or it could all be in my head) of 20-30 microns if you are close to the moh of the base metal (so steel frame with sharp steel media) and the ability of the person doing it to not overpass to many times.

so that's 3 blasts worst case for 0.1mm reduction, taking your 0.6mm wall to 0.5mm.

but rules of thumb are just guides at the end of the day, so take it with a grain of salt.

 

[EvanAL]

I work at a frame manufacturer and was just in the blast room yesterday! We blast 0.9 wall tubing for powdercoat all day. We also blasted some Zona the other day. I'll see if i can get some data for you all.

 

[mattr]

I designed a load of experiments for media blasting of various grades of steel, aluminum's, nickel based alloys and so on. Exactly to check surface damage, material loss etc etc for process optimisation.

Unfortunately, it was 20+ years ago and covered under the OSA, so I can't remember and even if I could someone might come knocking at the door...

 

[Tootyred]

The problem is your not blasting new steel its old steel. Even if corrosion isn't rampant, ive seen the frame number get fainter on frames over blasted, this is arguably partly definition loss, but if the steel has even some corrosion this does come way with blasting....with all media.....in fact there would not be much point if it didn't......

In all good theory and using the correct media, on new steel loss should be very little.....but then you might not know what media " jon down the road" is using or how much surface rot is under that paint.

We have all seen the tell- tale rust spiders creeping around under the finish!

We will see!

 

[novocaine]

if the steel has even some corrosion this does come way with blasting....with all media.....in fact there would not be much point if it didn't......

but that isn't the blast media or the blasting process at fault, that's the corrosion. the thickness you loose to that mechanism directly proportional to how fucked the metal is.

Steel corrodes instantly, the second you expose it to air the surface will start to build up a layer of iron oxide. leave it and it turns to rust (the brown stuff) which doesn't fully bond with the steel so air can still get to the base metal and the corrosion continues. so I guess what you are trying to answer isn't how much do you lose to blasting, more how much do you lose to corrosion.

as to jon down the road, he only uses aluminium oxide, I asked him last night.

 

[Tootyred]

Your completly right, but this is kinda the point....no 2 jobs are ever the same and not everybody's an expert on bike frames who does this......im not even sure many powdercoaters (or retrobikes) actually have any idea exactly exactly how thin some tubes are in sections.

Problem is " jon" may not give a rats butt, hes going to blast your frame with whatever is in the pot ( probably what he just ripped some cast up with).

The bits im using are old, have a bit of corrosion under the paint and probably are in the kinda condition your going to be thinking....."hummm, paints buggered, better get it done". If it was ok, it would be out with the t-cut.

The other reason for doing it is to see what happens if you do go at it a bit hard aka " jon", but also when you do a " proper job" like Janet.

 

[madjh]

And that is a normal Tange Prestige, the Tange ultimate Superlight is 0.6-0.3-0.6 for road racing and in MTB is 0.8-0.5-0.8 top tube
Waiting for results.
Ant yes, they are very fragile, my Fuji shout light with a "small" touch, has a dent

 

[novocaine]

It will be interesting to see anyways, so crack on.

I'm of to find Janet, she owes me a fiver.