I had a requirement to extend a length of 18mm MDF by butt jointing and glueing with biscuits. However before this I thought I would see how butt jointed MDF would perform with out biscuits.
I cut 4 250mm by 250mm samples cut each in half on the table saw and then glued them back up. Before gluing I noticed on the cut edge when checking with an engineers square that the centre core of the MDF was slightly raised making the edge slightly convex, one pass with the plane gave a perfect edge. I first tried 3 PVA based glues: WUDCARE 5 minute super fast PVA, EVOSTICK W and TITEBOND 3. I brushed on a a fairly thick layer on each mating edge and clamped, waited 24 hrs and carried out a crude strength test, I placed one half of each sample in a vice with the join just level with the top of the vice. I suspended a 2 pound (old money) hammer on a cord attached to the roof above, and set it so the face of the hammer was in the centre of the top half of the panel.
I pulled the hammer back in an arc to 600mm from the test piece and let go, and not surprisingly all 3 samples failed at the clue joint. I then reapplied the same glue back on each of the joints and repeated, I could not fail the joint even when I smashed with the same hammer using both hands, the MDF finally failed randomly but not on the glue joint. Perhaps this is not a surprise as the edge of MDF is very adsorbent and I guess the first application just primed the surface.
However there was a further positive surprise, I repeated the test but this time with SOUDAL pro + Polyurethane rapid adhesive, I could not fail the joint with just one application of the adhesive.
So my very crude test results when subjected to a heavy instantaneous bending load, the 3 PVA glues failed with one application, but a further application provided a stronger joint than the MDF, however the Poly gave the same joint strength with just one application. It should be noted that a sustained bending force or continuos tension test may yield different results.
I cut 4 250mm by 250mm samples cut each in half on the table saw and then glued them back up. Before gluing I noticed on the cut edge when checking with an engineers square that the centre core of the MDF was slightly raised making the edge slightly convex, one pass with the plane gave a perfect edge. I first tried 3 PVA based glues: WUDCARE 5 minute super fast PVA, EVOSTICK W and TITEBOND 3. I brushed on a a fairly thick layer on each mating edge and clamped, waited 24 hrs and carried out a crude strength test, I placed one half of each sample in a vice with the join just level with the top of the vice. I suspended a 2 pound (old money) hammer on a cord attached to the roof above, and set it so the face of the hammer was in the centre of the top half of the panel.
I pulled the hammer back in an arc to 600mm from the test piece and let go, and not surprisingly all 3 samples failed at the clue joint. I then reapplied the same glue back on each of the joints and repeated, I could not fail the joint even when I smashed with the same hammer using both hands, the MDF finally failed randomly but not on the glue joint. Perhaps this is not a surprise as the edge of MDF is very adsorbent and I guess the first application just primed the surface.
However there was a further positive surprise, I repeated the test but this time with SOUDAL pro + Polyurethane rapid adhesive, I could not fail the joint with just one application of the adhesive.
So my very crude test results when subjected to a heavy instantaneous bending load, the 3 PVA glues failed with one application, but a further application provided a stronger joint than the MDF, however the Poly gave the same joint strength with just one application. It should be noted that a sustained bending force or continuos tension test may yield different results.
