Exploding track!

[pskipper]

Just in case anyone else decides to do the same...

I've been cutting up old broken bits of mamod track as I'm fitting together faller play train points and mamod track (the joys of a tight budget). It turns out that the quality of the alloy is quite poor and the heat from cutting the track is enough to trigger small explosions as it oxidises. The most frustrating one was a long piece of track blowing it's self in half and shattering as it hit the worktop.

I've been wearing goggles and gloves since that one happened.

[Big Jim]

Never come across this before, one to watch out for.

I wonder if it is the localised heating causing expansion in parts of the track but not others leading to fracturing.

The other possibility is the old chestnut of not using a disc on aluminium after it has been used on steal due to the possibility of a thermite reaction. (don't know if it is possible but this tale has been doing the rounds of the motor trade for years)

[GTB]

It turns out that the quality of the alloy is quite poor and the heat from cutting the track is enough to trigger small explosions as it oxidises.

Zinc dust is pyrophoric (ignites in air) and has caused at least one documented fatality. The aluminium and magnesium in the zinc alloy used for diecastings may make it even more reactive.

The other reason for not using a cutting disc on Zamac diecastings, is that the stuff easily clogs abrasives and you run the risk of the disc jamming and disintegrating.........

Have to ask why are you cutting it with a cutting disc? A junior hacksaw might take a bit more elbow grease, but it doesn't make very fine dust and won't get hot enough to initiate an explosion, or turn into shrapnel.........

The thermite reaction is a different one, but well documented. It happens when someone uses a grinding wheel on aluminium and doesn't bother cleaning up the area. When the next person comes along and grinds steel, they have an 'Oh, dear' experience when the red hot iron oxide dust coming off the wheel reacts with the aluminium powder. The reaction in a workshop is usually due to build up of dust behind the grinder.

Regards,
Graeme

[RylstonLight]

Exploding track!?

Where is Mr Isle when you need him?

[Dr. Bond of the DVLR]

On that note he has just arrived at DLR Dan's. We should be hearing from him soon...

[Keith S]

Well that's a relief. I thought he was dead!

[Big Jim]

Zinc dust is pyrophoric (ignites in air) and has caused at least one documented fatality. The aluminium and magnesium in the zinc alloy used for diecastings may make it even more reactive.

Ahh, that is rather interesting!

Regarding thermite reactions, I mentioned this to a new employee of mine today who actually claimed to have seen a 9'' grinder catch fire due to this

[pskipper]

Have to ask why are you cutting it with a cutting disc? A junior hacksaw might take a bit more elbow grease, but it doesn't make very fine dust and won't get hot enough to initiate an explosion, or turn into shrapnel.........

I wasn't using a cutting disc, the hacksaw heated the track sufficiently ! About the only thing which didn't cause this was when I was filing areas smooth.

The explosions are caused by discrete pockets in the material, I guess from the posts so far that these are pure unalloyed zinc?

[GTB]

The explosions are caused by discrete pockets in the material, I guess from the posts so far that these are pure unalloyed zinc?

My apologies, I formed the impression you were using a cutting disc.

Zinc only burns if it is finely divided, so grinding dust can react, but the bulk metal doesn't (same as steel wool burns, but a nail doesn't). I worked in an industry that used ingots of the stuff by the thousand. The only risk from handling pure zinc in bulk form was putting a wet ingot into a molten metal pot. We regularly drilled ingots of 99.99% zinc in my lab to take samples and never had problems.

This one has me beat. I've machined diecast parts in the past and they are full of voids and holes inside, but I've never come across areas that explode. Given Mamod track may have been used outside, maybe water has got into the metal and caused some sort of reaction.....

Certainly worth keeping your experience in mind when working with diecast parts, as it's not something you'd expect to happen.

Regards,
Graeme

[Chris Cairns]

There were at least one bad batch of late 1980s Mamod track that is very brittle and will tend to splinter into pieces when broken rather than a clean break. Some of this track has been reported not to survive airmail to Australasia, with the box full of unusable shattered track on arrival.

Chris Cairns

[GTB]

There were at least one bad batch of late 1980s Mamod track that is very brittle and will tend to splinter into pieces when broken rather than a clean break. Some of this track has been reported not to survive airmail to Australasia, with the box full of unusable shattered track on arrival.

That sounds like 'zinc pest', a form of intergranular corrosion that is the bane of toy collectors. Caused by contamination of the diecasting metal.

More common with pre 50s castings, but it still happens now and again. Generally, the affected zinc parts just crumble like a soggy biccy, I've not heard of it exploding.

http://en.wikipedia.org/wiki/Zinc_pest

Graeme