What's a good way to join MDF squares to make a frame-shaped wall decoration?

Question

I have a bunch of 20x20x1.7 cm MDFs, and I'm trying to make a wall decoration that looks like ONE of these:

_{(image from here)}

...except my wood pieces are small and thus I need to use many squares to construct one edge of a bigger rectangle. So I put together this shape that I want to build with my squares:

• The total weight of the squares that will be used is less than 5 kg
• I want it to tolerate things like weight of plants

The question is: How do I appropriately join these squares as such they're very durable? Glue doesn't seem to be strong enough for this and I think wood dowels wouldn't help with the vertical joints.

A related question is also: how do I eventually hang this thing on a wall?

Answer 1

Sorry in advance, there's no TL;DR for this.

I wouldn't choose to make up frames like this from squares, but if I had to I would not be overly concerned about the strength of the finished item; I don't think the warnings of certain failure are entirely justified. I would actually be more concerned about the aesthetics of the joins.

Some of the dire warnings are (justifiably) based on the assumption of meh quality MDF, as you'd typically find sold in a big-box store. But it bears repeating that MDF is a product type only, a class of material, and not a specification. So the product can and does vary, and better grades are streets ahead of bottom-tier MDF.

Partly based on this, glue-only joints in MDF (except when joined face to face) are regularly considered to be weak. But it's not as simple as that.....

....post #1 in this thread and reply #2 in this thread are worth a read (and then some). I imagine more than a few woodworkers are surprised right about now, I sure was.

Now I would personally still choose to reinforce just to be on the safe side, and my reinforcement of choice would be dowels.

Dowels, done well, add tons of strength¹. They are easy to relatively easy to install, and cheap, requiring no expensive jig, no specialist drill bit or driver bit and no proprietary/specialist screws (I'm looking at you Kreg). Instead you use the drill you already own, possibly a drill bit you already own, and some dowelling. The only extra things you really need are a pencil or small stick (bamboo skewers or coffee stirrers are good) for glue application and a saw that you may also already own.

Normal dowel joinery, using hidden dowels, does require careful drilling. You have to position each pair of holes accurately, ensure every hole is perpendicular to the surface, not wallowed out and drilled to sufficient depth. This can be achieved by careful freehand drilling (as it was regularly done before the days of power drills) but it is much easier and more repeatable using a jig of some sort. And given the number of holes you have to drill here — bare minimum 12, two per edge — I would highly recommend using a jig. You can buy dowelling jigs of many kinds, the simpler ones being inexpensive, or you can make one.

You'd use a jig for all the holes in the middle of the shelves and uprights you wish to make up, but not for the corner joints as I'll outline below.

You expressed concerns about the strength of the dowel joints in the vertical sides and obviously these two butt joints have to resist most of the weight here (both the weight of the plant pot you want to support and that of the MDF below this joint, i.e. half the total weight of the item). But in order for these joints to fail the edge joints in the MDF must fail along with every single dowel joint. IME nothing short of destruction will withdraw a well-glued dowel from its hole; even before the glue has set it can be extraordinarily difficult to pull one out!

Some basic pointers:

• 'Size' the cut ends of the MDF. This means applying some glue prior to application of the main glue and final assembly.
• Use hardwood dowelling if you can get it. Softwood could be sufficient, but hardwood should be stronger².
• For 17mm material 8mm dowel should be about perfect. 5mm or 6mm dowelling could be used if it's all you can get but make them longer (100mm?) and consider using more dowels per joint.
• If you are buying in person select your dowel(s) carefully, you want the ones with the straightest grain. Also check the ends and reject any that are obviously ovoid and not circular.
• Add some means to allow glue to easily escape the holes to prevent hydraulic lock. A single saw cut along the side of a dowel can be sufficient, or see previous Answer.
• Ensure your drill bit's diameter and that of the dowel match closely. Don't assume, check.
• Freshly sand the outside of your dowel just prior to assembly — only fresh wood surfaces glue strongly and reliably.
• Don't skimp on glue.
• Clamp the joints firmly together and leave in the clamps for at least a couple of hours, preferably overnight.
• Chamfer both ends of every dowel. You can do this by sanding or paring/whittling with a sharp chisel or knife, you might even be able to use a pencil sharpener. This makes inserting them a lot easier.
• Add a small countersink to every drilled hole. This gives a small pocket for excess glue to collect, reducing squeeze-out.
• Drill each hole slightly deeper than half the length of the dowel. This ensures none will bottom out and hold the joint apart, and gives another space for excess glue.

Corner joints
One might think the attachment of the sides to the shelves presents the same basic difficulty as the joint midway in the sides, but actually it's quite different. Here I would recommend you use through dowels, which allow for an extra level of reinforcement — by angling some of them you get a dovetail-like mechanical advantage, which directly resists pulling forces.

So at minimum you'd use three, installed like this: / | \ or this / / \ If you use four you'd do them like this / \ / \ or like this / / \ \ And if you want to go OTT here you'd install five of them like this / \ | / \

TBH I think with only three that bottom shelf ain't going anywhere given the planned-for load. But since through dowels are so quick and easy to install you lose little adding more for extra peace of mind.

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¹ Done right a dowel joint in solid wood can exceed the strength of a mortise and tenon, which is often held up as the gold standard of joint strength.

² If you must use softwood select the dowels with the tightest spacing of grain lines.

Answer 2

Ok, I think I found a good solution - but it will only work if you have a lot of those squares:

Just glue them in two layers - like a brick wall!

And then screw vertical and horizontal "doubleboards" together.

This will be strong, it should not be difficult and it will probably look much better (like in your photo) than single-board box.

(Edit - glue surfaces highlighted by green lines)

Answer 3

So, the fundamental problem here is that you want to edge join mdf and have it be faintly strong. You could dowel or biscuit join, but these have no hope of standing up to any load bearing work.

If I had to build this using the constraints given, I'd make the big rectangle first. Butt join the corners with screws (predrilled and countersunk). To join the long edges, I'd prefer biscuits (mostly alignment; some strength) over dowels (because dowels are more finicky for alignment in my experience). Glue all these joints.

Since the edge joined mdf is extremely weak, use full length pieces of timber to frame the back (so 40/60/40/60cm long pieces) and the same amount of wood to frame the front. You could be fancy and groove (rabbet/dado) the wood to hold the mdf more effectively, or you could simply glue and screw the wood to the edge of the mdf. Predrill and countersink, of course. Be sure to put screws at the beginning and end of each discrete chunk of mdf. Size-wise, I'd use 1x3 (~18mm x 63mm) and hang the excess to the inside of the box. That'll be useful for mounting on the wall. For the front, I'd lean toward a 1x2 (~18mm x 37mm). You could push the excess in whichever direction you'd like -- hang to the inside and you have a lip to prevent things from falling out; hang to the outside and you have a clear line to the interior 'floor'. Or split the difference.

If you chose to make 20x20 squares, you wouldn't have any of these problems. Maybe that could be a good entry to these sorts of projects, and the next one could use longer pieces that don't require edge joining.

Answer 4

How do I appropriately join these squares as such they're very durable? Glue doesn't seem to be strong enough for this and I think wood dowels wouldn't help with the vertical joints.

Your intention seems to be to slice some of your 20x20cm squares into strips and to join them at the ends to make longer strips that you can then use to form a larger rectangle. The problem is that butting the ends of MDF strips together will never give you a strong joint. Dowels would help a lot, and they'll work just fine for the vertical parts as well (if you use a dowelling jig to assure accurate hole placement and nice clean holes). However, even with dowels, you're not going to get a joint that anybody would describe as "durable."

I think the only way to get close to what you're asking would be to mount the strips to some sort of substrate that provides some real structure. If you want your project to be 40x60cm, get a single piece of plywood or even MDF that size, and then glue your strips to that. You might not even really need screws; you'll have glue all along the long edges of the strips, which should provide quite a lot of area for a strong glue joint. But if you like, you can add screws through the back into the strips. This way, the substrate will provide all the strength that's needed to hold whatever objects you're planning to display. If you paint the interior the same color as the wall, it'll be hard to notice the substrate at all.

I know you're looking to use up the MDF pieces that you have, not to buy more, but by themselves the pieces you have simply aren't suitable for what you have in mind.