MDF Problems

[Harbo]

I do not often work with MDF but had to make a CD/DVD unit for my Granddaughter who wanted it to paint to match her bedroom.
I made it out of 18mm and 12mm MDF bought from B&Q and jointed it with dowelled joints using my Joint Genie and dowel pins from Screwfix.
Everything was cut and drilled accurately but I had major problems with the glue up - the MDF split at the dowel areas!
Are there any special techniques when working with this stuff and what's the best method of repairing the splits - will superglue work?

Rod

 

[Jonzjob]

A couple of photos to show what you are trying to do would help?

 

[Paul Chapman]

Hi Rod,

The only reason I can think of is that the excess glue wasn't able to escape from the dowel holes, and the compression when you cramped it up caused the MDF to split. That sort of compression can split solid wood so the problem is not confined to MDF. I presume that the dowels were grooved? It's sometimes better, even if they are grooved, to make a couple of saw cuts along the length of the dowels so as to be sure the excess glue has somewhere to go.

I'm surprised that you used dowels - biscuits, Dominos or loose tongues are far better in my experience.

If you put a little glue (PVA would do) in the splits and cramp them up, you should be able to sort the problem.

Cheers :wink:

Paul

 

[TheTiddles]

No matter how carefull you are with the dowels, the quality of B&Q MDF is really low, though there is almost no material as cheap, it splits so easily and cuts like felt. Once you've used MR-MDF you won't go back even though it's more cash.

I normally use biscuits but sometimes a dowel or two to garantee allignment if I've done a dry fit before taking it elsewhere, in this case I don't glue them and drill them through once it's all assembled and clamped up square. I have an underlying theory as to why many people don't use domino joints and it's not because they aren't good.

As for the cracks, superglue and a clamp should do it, if it's going to be painted then no matter if it leaves marks

Aidan

 

[Chems]

Thats just MDF for you, I was upset to have some nice quality veneered MDF split when I put a 6mm panel in a 6mm groove. It was a nice snug fit but shouldn't have been snug enough to split it.

 

[Harbo]

Here's some photos:

1.6m x 0.5m to fit a dedicated space to GD's requirements.

Biscuits were not used as my machine is not that clever (cheap Axminster one).
B&Q used as was in a hurry and they will cut to a size that will fit my car. Agreed it is pretty crappy stuff! Will avoid next time.
Dowels were grooved (6 & 8's to suit thickness), glue was Titebond III.
With all those joints, the glue-up was quite fraught as my "assistant" was not available!
I kept the dowels at least 30mm from the edges?

Rod (it is a rectangle - honest)

 

[jimi43]

MDF....GRRRRRRR!!!!

 

[petermillard]

Nothing wrong with MDF (though there's MDF, and there's shed MDF…) - it's an appropriate product for a lot of furniture; flat, stable, easy to machine, available in a variety of finishes and thicknesses and takes paint well. But it's not solid wood, and techniques you use on solid wood don't necessarily transfer well to MDF; carcass screws are used for a reason!

For the splits, there's no harm in trying the 'superglue and clamp' technique, but if it doesn't work I'd be inclined to just cut the split top panel out and replace it, screwed and glued in place - it's going to be painted, presumably, so the screwheads can be countersunk and filled.

Personally, whether you replace the top panel or not, I'd lip all the face edges - it looks nicer and is easier to paint.

HTH, Pete

 

[OPJ]

I've also had the same trouble with MDF, in the past.

How far in from the outer edges of each shelf did you set your dowels? I find they usually need to be at least 40-50mm, regardless or whether you're using dowels or screws. Otherwise, it'll just split, as yours has.

What's wrong with your biscuit jointer?

If the fence isn't great, you could instead reference everything off the base of the tool using straight-edges or an L-shaped 90° jig. That is, assuming the cutter runs parallel to the base... (I remember my cheap Ferm tool didn't).

 

[Racers]

Hi,

Why not use screws if its going to be painted? a blob of car body filler on each one and you wont see it.

It horrible stuff MDF.


Pete

 

[mailee]

two things spring to mind after looking at the picture. Both have been mentioned, Cheap MDF and the dowels too close to the edge. I think you will find that the extra cost of the MR MDF is well worth it. I also always use biscuits when joining MDF panels. HTH. :wink:

 

[Harbo]

The fence arrangement on my cheapo Axminster is not that clever - has a tendency to move out of alignment. I have do quite a few test cuts to get it right and really tighten the levers. Plus how do you line up the cuts on the uprights (flat areas)?

I thought screwing into end grain MDF was a big No-No that's why I used dowels. Would have been much easier!

I am toying with the idea of lipping and agree it will look better (end grain MDF is like a sponge) but there's an awful lot of shelves?

Rod

 

[eggflan]

Glue and 50mm brads do the job in something thats only holding cds/dvds , biscuits if its for anything heavier :wink:

 

[Paul Chapman]

Looking at your picture, Rod, I reckon that was definitely compression building up because the glue had nowhere to go. A combination of too much glue in the hole and grooves in the dowels not deep enough, so the MDF split. With biscuits you tend not to get that problem because there is far more room around the biscuit for the excess glue to escape.

It's interesting that if you use a very open-grain wood like oak with dowels, you can sometimes see the glue oozing out from the end grain as you cramp up the work. The excess glue travels along inside the wood. With something like MDF that can't happen and it simply splits as the compression builds up. That's one of the several reasons I avoid dowels but if I have to use them, I always make some extra saw cuts along the length of the dowels.

I wouldn't use screws. Unless you are very careful about drilling pilot holes, there's just as much possibility of the MDF splitting.

Cheers :wink:

Paul

 

[moz]

Have you tried a dry fitted dowel joint, i.e. are you sure the drill and dowel are a good match. B&Q MDF is variable I find. Some is decent, some has the density of wheetabix. Regarding screws, you can readily screw into the end of MDF. You need a modern, parallel shanked screw with deep threads, what are described as 'carcase screws' are good. You need to drill pilot holes to at least the depth the screw will penetrate with a drill accurately sized to the unthreaded portion of the screw. Experiment with some scrap to get the torsion right and away you go. It can be a bit of a challenge assembling a case like that without something helping alignment though. Regarding the poor fence on your biscuit jointer, I wouldn't use the fence on a job like that. Just clamp some scrap spacers to the sides and use the base of the jointer to reference from. For the shelves, place bottom side down and again reference from the base. Easy peasy and you are sure that all shelves will line up.

John

 

[Steve Maskery]

I would not have used the construction method you have chosen, I'm afraid.

I would have
a) Used biscuits or Dominoes as discussed above, or
b) Routed housings for a "traditional" method, or
c) used dowels but not in the way you have:

It's quite possible to screw into the edge of MDF to make a very strong joint indeed. The way to avoid it splitting is to insert a dowel, say 10mm, into the face of the MDF close to the edge. You can then drill a pilot hole in the edge and you are really going into your dowel. You can then screw to your heart's content. I use this technique all the time on jigs and they can take some hammering.

With a good fill & paint job, they will be invisible.

Cheers
Steve

 

[studders]

Call me a cheap **** but, I use Drywall Screws for MDF etc thats going to be painted. Used with one of theseI very rarely get any splitting.

 

[prologus]

Sorry to be dumb, but what is MR MDF?

 

[Steve Maskery]

Moisture-Resistant MDF. It's often green in colour (but not always)
S

 

[jimi43]

Just a tip....

If you must use MDF...then B&Q use very large panels of it to transport their hardboard packs...they then throw it away!

I caught them doing it and they gave it to me for a token £2!

1" thick stuff...really solid and blinkin heavy....

I used it to protect my bench top!

I find if I am making something like that shelf unit...some composite pine is better...not the B&Q stuff but from a proper yard...

MDF indeed has uses...very many...I just have a natural hate for the stuff...

I haven't tried MR yet...need to get out and get some just for the experience and favourable comments often found here...

Jim