WOODnetWork Discussions: Glue

      The correct amount of clamping pressure is vital to a strong bond - 1998

Editors note: There were many messages received at WOODnetWork regarding glue and gluing techniques. A number of these messages are "reprinted" below, and we think you'll find them interesting and helpful.

A question was asked about clamping pressure. As a partial answer, let me provide the following information. The role of pressure when gluing is to spread the glue on both surfaces, developing a glue line thickness between 0.006 and 0.002 inches thick any thinner and there isn't enough glue; any thicker and the joint strength goes down. Pressure also squeezes out any extra glue-in fact a little squeeze out is a good thing from a visual quality aspect. Pressure does NOT squeeze glue into the wood; pressure does not affect the cure rate or joint strength (except as mentioned above).

With this concept of the role of pressure, then, most folks will shoot for 50 to 75 psi on the joint. Softer woods are more forgiving than harder woods. In any case, the wood surfaces to be glued must be perfectly mated, which is probably more important than the pressure. Perfect mating is achieved when we machine initially; but let the pieces sit around for an hour or two and if their moisture content changes, the surfaces won't be true anymore!

Gene Wengert
Professor and Extension Specialist in Wood Processing
Department of Forestry
1630 Linden Drive
University of Wisconsin-Madison
Madison, WI 53706
Phone 608-262-3455
FAX 608-262-9922
e-mail: wengert@calshp.cals.wisc.edu



Firstly, thanks to Gene Wengert for providing such great support and careful data. Gluing and clamping are areas where trial and error, guesses and misinformation are common. It's great to get some solid data. I would like to ask for more. I have read that both the optimum thickness of the glueline and the pressure required to achieve the optimum glueline thickness vary with the type of glue. Is this true? Woodworkers don't use a tremendous range of glues, but comparisons of aliphatic (yellow) PVA, white PVA, plastic resin, Urea-Formaldehyde, cyano-acrylate and epoxy would be most interesting.

Bruce Hoadley, on page 179 of his book, Understanding Wood, states,

"Clamping pressure should be adjusted according to the density of the wood. For domestic species with a specific gravity of 0.3 to 0.7, pressures should range from 100 psi to 250 psi. Dense tropical species may require up to 300 psi. [...] In gluing woods with a specific gravity of about 0.6, such as maple or birch, 200 psi is appropriate."

I have wondered about these figures, ever since I first read them. Most of us have experienced glue-starved joints at pressures well below those recommended by Hoadley. Dense woods seem more prone to glue-starving than softer woods, a problem exacerbated by higher clamping pressure. The pressures suggested by professor Wengert seem much more in line with my experience. I hope he will comment on professor Hoadley's figures.



Dear Prof. Wengert:
I appreciate your response to the posting on the clamping topic. I am, however, a little confused by some of the data you cited. I had been informed by our adhesive supplier, Franklin Titebond, that pressures of 100-250 were in order, given film thicknesses similar to those you wrote of. The product we generally use is Franklin Titebond 50, a production adhesive.

They informed us that the pressures were a function of the density of the materials being bonded, which roughly correlate to the common softwood/hardwood designations common in our industry. Specifically 100-150 psi for low density (softwoods, IPB, MDF), 150-200 psi for medium density, and 175-250 for high density. Interestingly, high pressure laminate to IPB is recommended at 50-75 psi, to minimize telegraphing.

Precision of fit is paramount, as you pointed out, but I would add that there are gap-filling adhesives, in aliphatics specifically, to contend with problem situations. This posting is not intended to contradict or refute your comments, but to further the discussion and assist in understanding of the dynamics of adhesives and clamping by all, including myself.

Thank you for your comments!



Let me add further information with respect to clamping pressure.

My original statement included the statement that the ROLE of Pressure is not to squeeze glue into the wood. I did not mean to imply that glue doesn't go into the wood. But such penetration is not helpful in developing a good glue bond or strong joint. Therefore, my statement could have been more clearly stated as "it does no good to squeeze glue into the wood, and therefore that is not an objective of putting pressure on the joint." I have some excellent microscopic photos of wood cells and the glue distribution. So, the role of pressure is to squeeze glue into all the nooks and crannies, create a uniform film, etc. compress the high spots of the wood surfaces at the joint and thereby provide more of the wood surfaces within the 0.002 to 0.006 inch gap. (Some people go as high as 0.008 inches gap maximum.) A softer wood will compress more easily than a harder wood; hence the suggestions of higher pressures on higher density woods. But When the surfaces are not well mated (i.e., perfectly smooth), then pressure on the joint will my EXPERIENCE is that over 100 psi is excessive for North American woods; higher pressure is trying to correct for non-flat surfaces, but does so very poorly in many cases. (May I share a pet peeve? Get rid of all steel tapes that measure to 1/32-inch; get everyone micrometers or other devices that measure to 0.001 inches (or equivalent metric) and get true straight edges; etc.)

Some adhesives, most notably epoxy and hot melts, have excellent gap filling properties-that is, they have high strength even with a large gap. (But hot melts are not so strong and soften with heat.) (Note: When repairing a loose chair rung or other poor fitting joint, because the joint often has gaps over the 0.006 limit, we must use a strong, gap filling adhesive.) Except for hot melts and construction adhesives (rubber type cements), all glues used on wood can provide a joint stronger than the wood itself!

Gene Wengert
Professor and Extension Specialist in Wood Processing
Department of Forestry
1630 Linden Drive
University of Wisconsin-Madison
Madison, WI 53706
Phone 608-262-3455
FAX 608-262-9922
e-mail: wengert@calshp.cals.wisc.edu


Regarding brushing glue onto a surface. I hope that the following comment isn't too elementary or too brief: If a joint will be assembled quickly, brushing the glue is okay, but brushing, especially when it results in a thin coating (instead of a thick bead or thick spread), can lead to rapid drying of the glue and adsorption of the water in the glue by the wood. If the joint isn't assembled quickly enough, the result is a starved joint or a pre-cured joint.

Gene Wengert
Professor and Extension Specialist in Wood Processing
Department of Forestry
1630 Linden Drive
University of Wisconsin-Madison
Madison, WI 53706
Phone 608-262-3455
FAX 608-262-9922
e-mail: wengert@calshp.cals.wisc.edu



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