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Plywood is assembled, at least from I can see in this video, so the sheets are arranged perpendicular to each other so that the directional durability of the grain is complimented to give an overall unidirectional strength.

When paper is milled it has a grain direction. The paper is stronger along one direction than the other. You can tell this by both looking at the paper and bending it along its lines of symmetry.

Does the manufacturing process of MDF give the resulting boards a grain direction?

If I were to glue two MDF boards together would the orientation matter for stability?

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    Since I don't KNOW I'm putting this as a comment. I don't think so, it is just sawdust and glue. Paper is actual fibers laid down. – bowlturner♦ May 12 '15 at 0:41
  • @bowlturner - That's my assumption as well, but don't know for sure either. – Pᴀᴜʟsᴛᴇʀ2 May 12 '15 at 0:42
  • @bowlturner I believe you are right as well... thought it was a curious question nonetheless. – Matt May 12 '15 at 0:57
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Does the manufacturing process of MDF give the resulting boards a grain direction?

Yes and no.

The wood fibres in MDF are said to be randomly oriented, and it is plainly visible if you study the surface that they are random. However, during manufacture MDF is laid down in a continuous horizontal sheet and then pressed down to thickness which does leave a sort of grain-like structure or stratification between the faces. This leads to the well-known tendency of MDF to split when screws are driven in at the edge, while if driven through the face there is far less tendency towards splitting.

If I was to have two boards of MDF glued together would the orientation matter for stability?

No, stability is entirely uniform as far as this goes.

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  • during manufacture MDF is laid down in a continuous horizontal sheet is the reason I was asking the question. Thanks for the confirmation. – Matt May 12 '15 at 12:59
  • Can't disagree with this. In my answer I considered length vs. width, and the strength of MDF is quite uniform in that plane. But you're right that it's compressed in only one direction -- across its thickness -- and that leads to a difference in that direction. – Caleb May 12 '15 at 16:40
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Does the manufacturing process of MDF give the resulting boards a grain direction?

No. MDF is made of short fibers that are randomly oriented. If you cut a circle out of MDF with a router, for example, you'll notice that the cut edge has the same appearance along the entire circumference.

There are manufactured products that are stronger in one direction than in the other, though. OSB, or oriented strand board, is one example.

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  • I am humbled to learn that OSB isn't just another word for plywood. Thanks, all this time I thought they were interchangeable terms. – Peter Grace May 12 '15 at 13:07
  • That's interesting @PeterGrace. They're definitely different products, but OSB is often used for the same applications as some kinds of plywood, like CDX. I think there's even more confusion between MDF, which is nothing like plywood, and MDO, which is a type of plywood. – Caleb May 12 '15 at 16:48

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