Cracking Edges on MDF

Q. We seem to have problems with thicker medium density fiberboard (MDF) cracking on the edges as it comes out of our ovens, and the cracking occurs down the middle of the edge. I have tried a few things, but nothing has worked too well. Do you have any advice? P.W.

A. When you heat the board it causes a release of trapped gases. Since the inner core of the board is less dense than the outer layer, there is more trapped gas in the deeper core (porosity). That can cause cracking along the edges of the board as it is heated and the gas is released. Higher resin content will help reduce the gassing effect. Check with your supplier and compare with other suppliers to make sure you have good resin content (the most expensive ingredient in the board). Higher moisture content will make it worse, but that is not likely a problem this time of year as the board would most likely have been made last fall. Board thickness up to around 1 inch is usually not too challenging, but above 1 inch it can be more of a problem. High energy for short times can make it worse.

So, what to do? If you can, slow down the line and turn down the IR output to reduce the energy input. More time with less energy will work better than higher energy levels for shorter times. Another possibility is to cut a relief groove in the back of the board near the edge with a router. It should be close to the edge and deep enough to relieve the gas and avoid the release at the edge. Another possible solution (not very practical) is to run the board once to de-gas (no coating) and then a second time to preheat and coat. A de-gas cycle will relieve the gas, and the coating will generally fill in the cracks when you apply the coating during the second pass.