Glueing 2 boards back to back

[motownmartin]

I have come across a slight problem, whilst making a top for my router table I glued 2 boards back to back, one board is chipboard (kitchen worktop) and the other MDF but to my astonishment they have dished, the chipboard being concaved and the MDF convexed, is this because the chipboard has shrunk slightly or is it because the MDF has expanded slightly :?:

If I were to use 2 MDF boards would they stay flat :?:

I would like to understand this before I go and buy more materials, any advice is greatly appreciated.

Many thanks

Martin

 

[ByronBlack]

different materials expand and contract at different rates, so this is what I guess is causing the dishing along with the different densities of the two boards. I've glued MDF together for various jobs and they always seem stable to me.

I'm sure someone more knowledgeable than I can give you the real science behind it.

 

[jasonB]

The brown backing on worktops is a balancing veneer not just "brown paper" so all the moisture from the glue will have gone into the MDF and caused it to swell.

JAson

 

[motownmartin]

Thanks for the replies so far.

Jason, there was no brown paper on the underside of this worktop, but I am thinking that because the chipboard is more porous, the board has shrunk as the glue has dried.

Martin

 

[jasonB]

Thats unusual to find one with bare chipboard, most have some forn of covering, but it does answer the question of cupping. The top of the worktop is covered with laminate which will not allow moisture in or out, but the lower surface is open to changes in humidity and any added moisture in the form of glue.
Jason

 

[bjm]

You have inadvertently created an unbalanced board in gluing two different materials together. When the humidity changes the MDF face will shrink/expand whilst the worktop side will remain relatively unchanged due to the laminate. Hence, the combined board has cupped - if you placed the glue-up in a dry environment for a few days you should see the opposite effect as the MDF face dries out and causes that side of the board to cup.

It's best to glue similar boards to make up a thickness so any net changes in movement are balanced and surface both sides with similar laminates.

Brian

 

[motownmartin]

Thanks Byron, Jason & Brian, it looks like another visit to B&Q to get some more MDF Today

Martin