Don't rely on screws to hold MDF together.

Learnt this lesson the hard way.

My kitchen was revamped in 1995, and the doors and drawers all have vacuum formed laminate fronts and backs and timber trim on the edges.
The most frequently used under bench door is a bifold corner door. Fancy Hafele hinges hold it to the cabinet. The screws that hold the top Hafele hinge to the back the back the door pulled out , left the door supported only by the bottom Hafele hinge.

So to avoid more damage or the bottom Hafele hinge failing (not designed for lateral stresses) I unscrewed the 2 screws that hold the bottom hinge to back of the door and took the door off the hinges.

Called who I thought was a cabinet maker (who I thought would know how to fix the door properly) , I didn't want to risk ruining the door if I tried to do the repair myself ( high risk I'd drill through the door , which would create a large and expensive repair , likely requiring all the doors and drawer fronts replaced with new - I don't fancy doing that at this time).

He suggested drilling out the holes , filling with BUILDERS' BOG which he assured me would dry rock hard and be as good timber , this I agreed to , $65 later door is back on the hinges and 4 days later , the same screws have pulled out under the weight of the door that was left open while my wife was putting washed dishes and plates away …. NOT IMPRESSED , the bog had little or no thread shear resistance and the screws had simply pulled out with bog in the threads ( very crumbly ).

So back where we were , door off , and I've asked him to come back , this time I've done my homework and instructed him I want the bog drilled out , and timber flutted dowels coated in HIGH STRENGTH 2 part ARELDITE ( epoxy ) NOT WOOD GLUE ( that stuff requires clamping to achieve strong bonds ) inserted , and allowed to set for 3 days before returning to reattach the hinges to the door.
I can't find a better solution , other then replacing a block of MDF in the door core with timber or replacing the door entirely .

I know high strength ARELDITE works , as I've used in before in other projects ( ie bonding winch mounts , sandspikes to fishing rods , literally have to destroy the rod to remove them once the glue hardens ).

So my advise is if you must use MDF, don't rely on screws to fix to the MDF , THE SCREW HOLES WILL EVENTUALLY FAIL , insert dowels into the end of the MDF and glue these in place with HIGH STRENGTH ( not quick set ) ARELDITE and not with wood glue if you want a strong fitting.

Also don't screw into BULDERS' BOG if there is likely to shearing stresses applied to the screw (threads) , the BOG will fail.
I espect nailing into BUILDERS' BOG will have the same effect. IT IS NOT AS GOOD AS TIMBER.

MDF is a pain to build with. I only use it for router templates. I try to avoid it otherwise.

However you can conquer it, just need the right stuff.

Wood glue actually does a good job of holding mdf together, the problem is in the glue application. Since it's so porous you need to use quite a bit more glue than youd think, and of course good clamping pressure. Typically, with a good glue up, the material itself becomes the weak point, not the glue joint. You can glue two MDF boards together and try to break them apart, it will be the MDF itself that breaks, not the glue joint. Assuming you glued it properly.

When I glue MDF I actually put a coat of glue on my joints and let it sit for a few minutes to soak up, then I apply more glue and then I join the pieces.

In regards to screws, screws will hold MDF OK also, they just need to be the right screws. Typical cabinet screws wont work well. MDF has way less screw holding strength than solid wood or even plywood. You want a very deep thread on your screws, akin to what youd see in decking screws.

But, for all it's worth, I hate the stuff and I hate working with the stuff and it's probably the worst "sawdust" you could breath in.

Anyways, hopefully what you came up with lasts longer than the 4 days your handyman got you lol.

-Brandon

Yeah, I avoid any kind of particle core--especially melamine.
IME, it all depends on the application.

Handy man back to take door away to work on it.

Wanted to try Hafele plastic dowels but I told I'd rather something stronger (no confidence on dowels held in by friction only).

I've instructed him to
> drill out the Builders' Bog ( need access to clean MDF core material )
> enlarge the holes if necessary
> apply 2 part high strength ARELDITE to the hole let soak in
> use TIMBER FLUTED DOWELS coated in 2 part high strength ARELDITE
> press the coated dowels into the holes
> let stand and to set for 3 days.

MrSpectrum":2t5mjsxi said:

Yeah, I avoid any kind of particle core--especially melamine.
IME, it all depends on the application.

Click to expand...

Seems very common practice here in door manufacture , when they claim "SOLID TIMBER CORE" it nearly always means MDF core , and some even have untreated particle board cores ( desasters waiting to happen , don't last long if moisture gets inside a seam or under the laminate outer surfaces or part the timber frame ).

Very hard to find timber block cored or plywood cored doors.

I can find them; I just can't afford them! :roll:

So , it appears the Turbo Builders Bog which is supposed to set superhard ( and emulate solid hardwood timber ) never set properly , the handyman said it was only hard on the surface and very soft otherwise , was able to remove the bog plugs using drill bit by hand .

MrSpectrum":2v1h90ao said:

I can find them; I just can't afford them! :roll:

Click to expand...

I never use it either. I have the style and rail bits and two different styles of panel bits for my router table so I can make my own.

To repair that door ...you are absolutely correct King, fluted wood dowels would be my choice to repair it but good wood glue will hold very well.

That wood putty dries to hard and is NOT able to hold any type of screw under any condition.

Gormagon":2czi03ne said:

MrSpectrum":2czi03ne said:

I can find them; I just can't afford them! :roll:

Click to expand...

I never use it either. I have the style and rail bits and two different styles of panel bits for my router table so I can make my own.

To repair that door ...you are absolutely correct King, fluted wood dowels would be my choice to repair it but good wood glue will hold very well.

That wood putty dries to hard and is NOT able to hold any type of screw under any condition.

Click to expand...

I inherited a full set of router bits and a router from my granddad (who was a very skillful lay carpenter) when he died about 30 years ago, I've only used it a few times and some of the bits I've never used .

I opted for araldite because it doesn't need to be compressed to get a very strong bond and I've used it before . Wasn't so confident about wood glue being up to the task.