MDF and water

Re: MDF and water

Brent English

That's because when wood fibers get wet they swell across their thickness, but not in length. For instance, as boards dry, they get smaller in width and thickness, but not length.

Structurally, think of MDF as very thick paper. The wood fibers in are all pretty much laying down in a single plane. So when it gets wet it increases a lot more in thickness.

I saw this same swelling pattern......

JamieDonaldson

....in the top panel of the stand I have in my show booth. It became wet at a show last year, and the corner where the water puddled swelled to almost twice the original thickness, and the top surface was a wood veneer. This reminds me that I need to be looking for a replacement before the next show!

Re: MDF and water

Keith Newton

I'm not sure if it's due to any grain orientation, as much as being due to the compression of the loose fiber is all between the faces, and not from the edge.

I suspect that if the same loose fiber were compressed into a cube from all sides, that when it got wet, it would swell in all 3 axis. But I could be wrong.

Re: MDF and water

Brent English

Keith, I have a background in these materials. You are right in how the fibers are compressed, however, when they are laid out in the mat that gets compressed they are predominantly horizontal. They are "air laid". The fibers are made much the same way paper fibers are made, in something called a refiner or defibrator that "disassembles" the wood into its constituent fibers. A binder (a version of CA glue called MDI or sometimes phenol formaldehyde) is then spray applied at something around 2-3% by weight to the fibers as they move through an air stream. The fibers then air laid into a fluffy mat that gets compressed in huge hot presses to make the board. When the fibers are laid into the mat that are almost all horizontally oriented. Preferentially, fine fibers are laid into the faces for a smooth finish, and coarser fibers are used in the core. If the fibers were just tumbled up and compressed, you would be right about the expansion, but they are not, and the process gives a very definite bias to the fibers being all horizontal. Just look at the edge of a MDF board and you'll see what I mean.

Thanks for sharing Brent, I'm glad to learn,

Keith Newton

About how long does a sheet stay in the press, and how much heat and pressure?

Re: Thanks for sharing Brent, I'm glad to learn,

Brent English

Time in press is longer or shorter based on thickness - the heat has to transfer from the press plates (platens) to the center of the board. As wood is a natural insulator it takes some minutes (say 5-15) even with plates around 500F. Modern presses are also continuous with large heated moving bands, top and bottom, doing the pressing as the MDF travels through the equipment. Saws trim the edges and a flying saw moves across the sheet to cut it to length. The panels then set in big cooling racks before getting stacked for shipment. Waste gets reground and fed back into the process. There are some YouTube videos you can watch to see these in action. Particle board and OSB are made in a similar fashion.

Aside.....

Barry Irby

Back in the day I was builder. There was a nearby plant that made wood fiber siding. When things were rocking and rolling and demand was high we would occasionally get a stack of siding directly from the factory and the pieces in the middle would be so hot you couldn't hold your hand on them.

Sadly, water had a similar effect on the siding.