Gluing Timber, detailed instructions for best results.

 

In order to use the glue, to obtain the very best results wood pieces should:

• Have a similar moisture content.

• Have a similar structure, density and mechanical behavior

• Be prepared at the same time

• Joints where these requirements cannot be met should still work very well.

To prepare surfaces for gluing, make sure they are:

• Straight and even to obtain close contact between the work pieces

• Not “compressed” by machining (looks shiny)

• Able to absorb glue

• Free of oil, dust and other impurities

• Freshly prepared to limit oxidization of the natural resin/oil content of the wood

• Cross grain sanding is better than planning

 • Don't use stearated sandpapers (white, grey or violet).  They will contaminate the surface.

Moisture of Content of wood varies:

• Most kiln-dried wood is 8 to 12%

• On the east coast of Australia, 12% is the usual moisture content of air dried timber, during very dry weather, inland and in the west of Australia it can be a lot lower.

• The glue reaction in the glue line will accept moisture content in the range of 10 to 20 %

• When gluing open grain wood, wood with up to 25% moisture content may be acceptable

For wood with low moisture content (less than 10%), you must:

• Spray or wipe water onto the surface to be glued without soaking the wood

• Leave time for the moisture to distribute before applying the glue

• Apply glue soon after dampening an open grained wood

• Sand the moistened surface lightly before applying glue on closed grain wood or high-density wood types in order to remove any oil/ resin emulsion and to create a larger gluable surface

• Leave some time after dampening a closed grain wood or high-density wood types to allow surplus water to evaporate

• Be careful, because by adding water, the open time may be reduced quite a bit

• Remember: moisture does not speed up the final curing process but allows for a chemical reaction to take place

Clamping pressure:

• Varies according to wood structure and density, and may vary from job to job and from wood type to wood type.  Clamp hard.

• Is usually 1 kg/cm2 to 7 kg/cm2 for softwoods. ( 100 kPa to 700 kPa)  ( 14 psi to 100 psi ).

• Is usually 5 kg/cm2 to 15 kg/cm2 for hardwoods ( 500 kPa to 1.5 MPa ) ( 70 psi to 200 psi )

• Clamping pressure should always be adjusted to secure close contact, and always be sufficient to secure minimum or no foam buildup in the glue line

• Clamping pressure must be distributed evenly over the glue line area

• Generally, clamping time has expired when the foam pressed out from the glue line feels dry and somewhat crisp

• Foam in a glue line has no structural strength whatsoever, and must not be considered as a gap filler

• Vacuum bagging ( 15psi 100kPa ) works well with most softwoods, test first with hardwoods.

Joint Design.

• Joint design is always important, joints in compression and shear are stronger than a joint in tension. A joint in peel is the most challenging for a glue. A joint in peel with a narrow start point could fail due to overload of the glue at the peel start point.  End grain gluing does work quite well, but the strength of the timber can be an issue.  In high performance situations, please ask us for guidance.