Expansion of plastic problem?

[fizzy]

I just bought an 8x4 sheet of 9mm Stokbord - a recycled plastic similar to polypropylene. £60 inc delivery so not bad.

I was going to use it as an underfloor for my trailer instead of wood. It will be exposed to all the road spray so don't want something that will rot.

It is only really a spacer to sit on the cross pieces - there is a sheet of 6mm ali going over the top. The sheet will sit down between the 2 outer rails. You can see the sheet of epoxy coated ply I was intending to use.

The whole floor will be riveted to the cross pieces 6mm angle - then the plastic or ply sheet - then the ali.

I thought the plastic would be a great solution - until I saw a warning about it's thermal expansion. The whole sheet will expand or contract by 2.5mm over 20C! They say not to screw it down tightly and to make screw holes oversize.

I don't think 165 x 6.4mm rivets will allow for much movement...........

Any thoughts? I should imagine the stresses could get quite large if the thing isn't allowed to expand?

I don't want to go back to my sheet of ply but might have to.

 

[zx9]

I read that as 2.5mm expansion on an 8 ft length or approximately 1mm per M or around 0.1% is that anything to worry about especially given that you don't know the expansion coefficient of the steel frame or aluminium plate?

 

[fizzy]

I read that as 2.5mm expansion on an 8 ft length or approximately 1mm per M or around 0.1% is that anything to worry about especially given that you don't know the expansion coefficient of the steel frame or aluminium plate?

I just don't know that is the problem. Wood has a very low coefficient of expansion. Steel and ali a bit more but plastic is a lot higher.
I don't want to find welds cracking or whatever in hot or cold weather. Perhaps I am just overthinking things as usual...........

 

[zx9]

Probably won't help but here is something for you to play with http://www.utausa.com/metalconverter.html have fun.

 

[Gareth J]

Stockboard gets pretty floppy when warm. It won't have the compressive strength to do any damage to your trailer (unless the trailer is held together with 4 zip ties or something) it'll sag/warp.

 

[rikrobson]

Id insulate the ali and steel frame and not use the plastic. Id also paint or underseal the bottom. If you dont seal the plasic and ali sandwich then waster will get in

 

[gordon stephenson]

If you are going to use the plastic Can you just trap it under a rim around the edge and let it float?, Maybe a decent fixing in the centre to locate it.

 

[captain-destructo]

cut the plastic 2.5mm short and drill the holes 1mm oversized in the plastic as well. That way it has room to move but bolt rivet everything else as you were going to originaly

 

[Pete.]

1mm oversize would not be enough. You'd need to make the hole radius bigger than the bolt by the expansion amount. It will move away from the most securely anchored part so if that's at one end the other end will shift a lot.

 

[Hitch]

Why not just forget the plastic and fit the ally straight on? Couple of spacer strips to make up the difference between sides and cross members..

 

[Gareth J]

Honestly, it'll be fine! I've tek screwed (ie zero hole clearance) sheets to steel framed doors before, they don't start ripping themselves apart in the sun.

It's plastic, yes it will try to expand/contract but it's also very soft. It will deform before your trailer falls apart like a clown car. What you can expect is, after some hot days, some slight sag between your supports as the plastic expands, softens and droops. If you're concerned about it buckling up the Ali sheet, use more fixings.

 

[normspanners]

6mm ally with all those x members aint going anywhere as it is, without the plastic my car trailer was two wheel troughs and open centred, for the centre 3.5 ft between the troughs I put a few extra x members in at no where near as close centres as yours are, and I used 2mm steel sheet and that stands all kinds on it, have had 3.5 ton of scrap on it many times and it hardly moves, if using an under belly, I would have used 3mm ally !!