Brown MDF and the sun.... a head's up (NOT GREEN MRMDF)

[rafezetter]

I've just learned today that a 1m square sheet of MDF, if leaned against a fence in direct sunlight (at only a moderate angle), will develop a curve, in both axis of about a 25mm offset at the centre.

In about 20 minutes.

Dunno if this is common knowledge, but I certainly didn't know.

(the good news from this is it's bowed in a way that isn't a problem for my bench top)

Just though I'd mention it in case it prevented somone else making the same mistake.

(title edited to reduce confusion)

 

[Doug71]

I learned a similar lesson a while ago.

Left a square of MDF on the floor outside in the sun, came to use it an hour later and all the corners had curled up, I turned it over and it actually flattened back out (near enough for what I needed anyway) just as quick.

It certainly isn't as stable as you imagine.

 

[MikeG.]

I almost never use the stuff, but I discovered this week that mrMDF shrinks. I made a back panel for a unit in my utility room, and it fitted perfectly. I stood it in place for a couple of days, and there is now a 3+mm gap.

 

[transatlantic]

but but ... they* always tell us its ok to glue MDF in place as it doesn't move! :O








*youtube folk

 

[Racers]

I made a door for a cupboard 2x1 and 9mm MDF brought it inside and it bent likes a banana.

Pete

 

[doctor Bob]

I almost never use the stuff, but I discovered this week that mrMDF shrinks. I made a back panel for a unit in my utility room, and it fitted perfectly. I stood it in place for a couple of days, and there is now a 3+mm gap.

I use a lot of it, yes it shrinks, but I have never seen shrinkage like that.
I ran MR MDF skirting for my new house, 3m lengths, that's shronk by about 1mm over that length. MR MDF doors shrinkage is no noticable.

 

[rafezetter]

Just as an update - I brought the MDF sheet indoors after seeing it warp and this morning it had all flattened out as it was even though it was leaned up against a wall... weird.

 

[Doug71]

I cut a large piece of MDF for the top of a big break front unit in my kitchen. When I came to fix it on next morning it was 3mm too short #-o

I think the thing is MDF seems to acclimatise very quickly, losing or gaining moisture much quicker than normal wood.

 

[AES]

Thanks for all the info. I don't use MDF all that much, but always thought it is dead stable under "all" conditions (except wet of course). You live & learn, so thanks.

 

[custard]

This thread could leave the casual reader with the wrong impression.

Moisture resistant MDF, from a big name manufacturer, that's been properly stored during the distribution pipeline, is as stable as anyone could want.

I've seen hundreds if not thousands of square metres of veneering done on MR MDF, and all of it remained sufficiently flat and stable to be used in top quality work. Yes, for some applications (like premium yacht fit outs) there are alternative substrates that are even more stable under extremes of moisture and humidity, but for virtually all internal domestic applications then MR MDF from the likes of Medite or Krononspan will absolutely get the job done.

 

[AES]

Thanks for that extra info Custard, very useful.

BTW, is there any way to tell MR MDF from the normal (apart from trade names, which don't seem to apply here)? From what I've seen of posts & videos by the likes of Peter Millard, et al, the MR version seems to have a faint slightly greener of bluer centre core, but I've never found that stuff here (but as said, not that I use a lot of it). But the stuff I buy always seems to be a uniform "sandy brown" throughout the core.

 

[MikeG.]

That brown stuff is the ordinary MDF, and the green is MR.

 

[OscarG]

Thanks for that extra info Custard, very useful.

BTW, is there any way to tell MR MDF from the normal (apart from trade names, which don't seem to apply here)? From what I've seen of posts & videos by the likes of Peter Millard, et al, the MR version seems to have a faint slightly greener of bluer centre core, but I've never found that stuff here (but as said, not that I use a lot of it). But the stuff I buy always seems to be a uniform "sandy brown" throughout the core.

You can get the MR MDF from Wickes.

I just used some for a couple of bathroom cabinets, seemed to work ok.

 

[AES]

Yeah, thanks Oscar, but not too many Wickes (or Screwfix, etc, etc) here (Switzerland)

 

[petermillard]

BTW, is there any way to tell MR MDF from the normal (apart from trade names, which don't seem to apply here)? From what I've seen of posts & videos by the likes of Peter Millard, et al, the MR version seems to have a faint slightly greener of bluer centre core

Lots of info direct from the suppliers e.g. Medite - here https://mdfosb.com/en/medite/products

As a general guide MR MDF typically has a green-tinted core, or is solid green (depends on manufacturer, and the board thickness) and flame-retardant is usually pink.

As Bob & Custard have said, MR MDF is a world away from the regular stuff, which is why I buy MR from a known supplier, with data sheets available covering everything from board stability to formaldehyde emissions, fully tested to the appropriate standards eg CARB2, EN 622-5, EN 321 etc... They’ll also clearly mark the edges of their boards e.g. “Medite MR 6412904 38 18 1220 2440 E1”

Incidentally, I’ve seen a few companies cheekily naming their MDF as ‘superior’ or ‘premium’ (and perhaps it is) but it’s not MR MDF.

HTH P