A.Charles Stout, director of Laboratory and Certification Services at the Composite Panel Association, responds:

When a fastener enters wood, the fibers in the fastener's path are displaced into air spaces that surround each fiber. But MDF is a compressed cellulose mixture; when a fastener enters it, the material in the fastener's path has nowhere to go but up and around the fastener and out — the so-called "mushroom." The degree of mushrooming...

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