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What type of glue and do you nail or screw? It seems to be to soft for nailing or screwing.
I think more details are needed here, but MDF can split easily if you are going into the edges of it. You definitely want to predrill with screws, moreso on the edges but it also helps if you predrill faces also.
Fibrecore (veneer covered MDF) is easier to work with than raw MDF. After seeing alot of the plywood that I deal with at the cabinet shop I work for though Im beginning to like fibrecore because it stays flatter. Ive had warped plywood actually bend solid lumber on a cabinet side.
I agree with Orange, I have a cabinet shop and use a good woodworking glue, brad nails and particleboard screws, you have to predrill for all your screws.
Veneer sheets with particle or mdf cores are way flatter and smoother than plywood cores. They are also quite a bit heavier, I guess everything has its pros and cons, good luck.
I'm certainly not in the league as the two prior members, with their cabinet making professions, but I've found that my amateur adventures with MDF to be challenging. Still, I've built a couple of things with it.
What I've found is that it's weight and sag are something you need to account for. Butt joints are a pain, but a good quality glue appears to work in conjunction with screws made for the task. Coarse threads worked better for me, then conventional wood screws and certainly you have to pre-drill. Otherwise, the stuff just seems to open up into ugly and unstable splits. The challenge is to have the screw thread bite into the material without acting like a wedge to separate it. Even then, I usually apply glue to the hole, in an effort to keep the stuff together. Basically MDF is little more than "dust" that is glued together under pressure.
Likewise I've found that if you rabbit the edges, you provide much better support for shelving. Keep the shelves under 30-inches wide for book cases, or else use cross support or a face frame of real wood, in an effort to minimize sag. Even then, you probably should rabbit the edge of the face frame, in order to provide "under-the-lip" support, as just "edge" fastening may still cause MDF to split under weight and time.
I've found this to be equally true on vertical members too. I built a small book case (25.5" wide x 72" tall x 10" deep). The top, bottom, and middle shelves were rabbited, screwed and glued on the edges, with the remaining shelves pin-adjustable for spacing. The sides have remained straight and true, and 24" width shelves are sagging about an 1/8th inch. That sag's not terrible, but it is noticeable. On the other hand, the same design, made with pine, is absolutely flat, and much lighter in weight.
The second MDF project was a bit more challenging, as I made it as a surround to a wall heater. It measures 46" wide x 86" tall x 12" deep. It has two 10.5" wide x 30" high storage cabinets on each side of the heater (the 20 x 25 vented wall heater is positioned just above the floor). There are five 44" width shelves above it, which are supported in the middle. The sides are edge glued and screwed at the very bottom and top, and at a position 30-1/2 from the bottom (across the top of the cabinet areas). The shelves contain techical and computer manuals, which are fairly heavy.
The two side areas were initially build with MDF doors. They were almost immediately replaced with pine, as they just can't be held on the edge with hinges without splitting... too much weight. After a couple of years, all the shelves are showing about a 1/8" sag across a 21-inch unsupported span. So at a total width of 44-inches, with a support in the middle they look a bit gull-winged.
The biggest problem is the sides, which though supported as described, are bowing out considerably across the 56" span between the top and middle points where they are laterally fastened. I think I need to replace a middle shelf with pine or poplar, so that I can fasten the sides to it for better lateral support.
The last point of failure is the very top support shelf, which is now showing a split on the right edge. Oddly, this top piece acts only as a lateral support. Sitting right next to the room's ceiling, it has no weight on it at all (other than it's own). The side pieces are screwed and glued to it at their top edges, and 2" in from the front and back edges as well as the middle. However, the forward right edge is now showing a split. I could just fill the crack and repaint the area, but that only covers the failure and doesn't remedy it.
So, it's good to learn from such projects. MDF has it's advantage because it's relatively cheap, it's sides are very smooth, and it takes paint or laminate nicely. But I hate it's weight, the dust from cutting, it's edges are like a sponge, and it has very little integrity with too much flex. Still if you go into it with these things in mind, it can be worked with.
But, I still have to look at both of these as "learning to work with MDF" lessons. Both projects were inexpensive, to-be-painted, ventures. The first was okay but not as stable as the pine bookcases of the same design. The second, heater surround, was a failure in my eyes. For maybe a $50 difference (if that), I could have built it out of pine, it would have looked the same, been more a joy to work with, and would after two years be as straight and stable as the day it was built.
The funny thing is that I see these programs like "Trading Spaces" where they use MDF for almost everything. Such projects look uniquely nice, but I wonder what they look like six months later.
MDF stands up well to being glued and screwed, but never rely on the screws to hold the shelf up, it'll just split. If you're going to use MDF for the shelves, you need to dado it into the upright pieces, then screw through to keep it tight. Also, never rely on it for long spans, it will sag.
I did the built-in bookshelves below using 3/4" MDF for the case, and poplar faceframes/doors. The shelves inside the cabinets sag a little bit, but since it's hidden, I wasn't too worried. I used a tongue-and-groove oak front for the exposed shelves for strength--those don't sag.
I used pocket hole screws for most everything. No dados were used; all of the butt-joints are horizontal (and hidden), meaning gravity is helping me. I'm not sure how standard the cleats are in the second picture; I was making this up as I went along.
If I did it over again, I'd more carefully consider using plywood for one main reason...the dust. My shop is in my basement; though I have a dust collector and Delta air cleaner, it still gets all over the house, much to my wife's chagrin.
Is that smoked glass on the sides of the tall cabinets?
No, it's dark stained 1/4" oak plywood, just like the "countertop" under the open shelves. (It looks like a bit of a reflection on the left side...maybe a camera flash artifact.) It's fastened to the 3/4" MDF underneath.
Next step is to build shelves on top of the section in pictures 2 and 3!