Hello everyone,
For several years we have been making laminated wood panels using a glue line ripsaw. We would like now to use our moulding machine to prepare the wood to be glued. Before starting full production we conducted some joint solidity tests which so far have not been conclusive.
We made some test blocks using 8/4 sugar maple (acer saccharum), the moisture content was 6 to 8%. For the test we compared the glued joint created by the glue line ripsaw to the joint made with two types of cutter heads, either, straight knives Tersa style , or a multi knives spiral style. Both cutters had new knives.
In order to test the solidity of the joint, we hammered a wood chisel into the glued joint. Before gluing, we did a dry assembly to evaluate the quality of the joint, which were
impeccable.
The results are very disappointing. As expected, the glued joint in the test block made with the ripsaw never failed. However, the glued joint in the test block made from the moulding machine failed most of the time.
We suspect that the glue is either evacuated from joint when it is pressed or the wood is "glazed" and does not absorb the glue.
We decided to redo the test using slightly dulled knives. The glue line was not nice but the joint did not fail.
Would you have any suggestions to solve this challenge?

Since the ripsaw never failed and is likely much quicker than the molder why would you change?

In order to keep this thread short, I did not elaborated all the details. We are not abandoning the ripsaw production. The molder production will be complementary.

I never glue machine planed wood without sanding first. The surface after machine planing is scalloped, not truly flat, and the fibers are compressed and won't absorb glue well enough to bond properly.

John

The gold standard for using pva and wood has always been sharp knives on a jointer.
pva likes smooth surfaces and thin glue lines.

I suspect the problem is clamping pressure. You are squeezing the glue out of the milled joint. The sawn surface provides enough space for the glue to not be squeezed out.

PVA glues do not work by “soaking” into the surface. There is a problem with mitered/end grain joints “sucking” the glue out of the joint leaving a “starved” glue line that is prone to fail.

Most of the failures people see with epoxy is not abrading the substrate(wood, aluminum, composite) with 80grit. Also way too much clamping force. Epoxy likes rough surfaces and wide glue lines.

The idea that you need to abrade wood as prep before bonding with pva glues is false.

A couple of years ago this question or a similar question came up and I suggested glue starving of the joint by over-clamping. I decided to do a test of how much is too much clamping. It can't be considered definitive in any way because I only used two short samples of cherry for each of rub joint no clamps,moderate clamps typical of a regular glue up and extreme clamping with heavyduty F clamps at 4" intervals with maximum force.
The conclusion was that there was glue line failure on the rub joint sample but the moderately clamped and extremely clamped samples had mostly wood failures
I think glue starving for PVA is not a factor - remember the ripsawn glue up is using the same clamping.
It's been decades since I read Bruce Hoadley's Understanding Wood. I recall there being a chapter on gluing that was extensive.
The suggestion that the scallops left by the moulder would interfere with the glue bond is intriguing but I suspect unless the moulder is cranked up to maximum feed the scallops would be in the range of .001" or less.
I would like to see if the OP tried passing a sanding block over some moulded staves then gluing and seeing if there is a benefit.
One last thing. Are you gluing the parts immediately after machining in both methods? A delay would allow distortion of the surface however small due exposure of the fresh face to drier conditions.

It could be a couple different issues. My first guess is your moulder cannot hold the tolerances for a glue line edge. This could be how well the knives were cut or tolerances on the head and spindle. It could also be your feed rate/speed. Too slow and the wood can glaze over, too fast the knife marks per inch will be too spread not allowing for good contact and not producing a glue line edge. And as someone already mentioned, sharp knives will also help your case. Clamp pressure is also important. Too much pressure will force too much glue into the wood, not leaving enough to fill the bond line. But you also want to make sure you have enough pressure. Another possibility is you are using too much or too little glue, but I doubt this is the case because you are already having success with the rip sawn edges. I put a blog together not too long ago on this exact topic. It mentions glue line edge tolerances and other basics. It's a really quick read, so its worth the read.

Link