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05-04-2019, 07:29 AM
(This post was last modified: 05-04-2019, 07:32 AM by KyleD.
Edit Reason: fixed spelling
)
I must admit my eyes glazed over when I tried to read that study.
I will further admit to using a lot of MDF in shop projects over the years. My experience is empirical rather than scientific and it agrees with the summary that MDF is weak in screw edge holding. The reason I have experienced this weakness is that MDF is made of many layers of paper across its thickness. These layers split apart easily. Screws act like a double wedge. The body of the screw wedges and as the screw tightens the threads pull up forcing the MDF further apart. Once the MDF splits-there goes the holding power. This is why the proper clearance hole is important. It helps reduce the wedging effect of the screw body.
I have had some luck slightly angling the screws on the edge as this catches many more layers of the MDF at once. I wonder if the screws that have little reliefs at the tip to act like a pre-drill help reduce the wedging effect?
Proud maker of large quantities of sawdust......oh, and the occasional project!
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The study focused on the math to come up with an equation to calculate or predict holding strength. But that aside, the helpful part is the table of screw size, length, pilot hole and resulting strength. It seems to me that 200-600 lbs of pulling force per screw is quite a bit of strength (I weigh less than 160). I suppose its time to find an equivalent chart for holding power in plywood, softwood and hardwoods. Perhaps the complaint of screws pulling out is due to not using sheet metal screws with the short taper, finer threads and not using a long enough screw?
Train to be miserable...
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A friend of mine builds a lot of jigs and things for his shop using MDF. He will drill holes in the face of the MDF and glue in dowels when he wants to ensure the holding power of screws into the edges. And of course he pre-drills for the screws. Some of his jigs require threaded holes for nylon bolts used as leveling screws and the hardwood dowel takes the threads much better than the MDF.
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The Ready To Assemble manufacturers' association in Europe commissioned a test on fasteners for all sheet goods (plywood, particle board and MDF).
Unlike this test they were mostly interested in racking strength (side to side flexing). If you are building kitchen cabinets where the wall is a structural member of the cabinet, that is less of an issue.
Screws performed better than dados and the metal dowels with quarter turn fasteners in that regard. But not nearly as well as Confirmat screws, which with their wide barrel body performed like screws and dowels.
The lowly and old fashioned dowel performed best.
Regardless of the fastener, it should be placed no closer to the end of the panel than 2 inches. The fastener needs support on both sides or it will split the sheet goods apart.
The test was basically a 3/4" thick sheet shelf mounted on one end only to another 3/4" thick sheet (or nearest metric equivalent). So this was basically only testing racking strength not pull out strenght.
You will have to decide if racking or pull out is more important.
I switched to dowels after reading the report. I use two confirmat screws to hold the panel in place plus an appropriate number of dowels. I just recently switched to through dowels which is faster and seems to be stronger.
No animals were injured or killed in the production of this post.
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I had screws pull out of the edges of MDF that were holding bifold hardware hinges. The only fix that worked was to drill out the hole the screw went in to the point that the threads just barely gripped the MDF.
I then forced in the holes slow setting epoxy and carefully tightened the screws. I left enough bite that the screws would pull into the holes.
Slow setting epoxy is preferred as it allows it to remain liquid long enough to saturate into the MDF.
After about on hour the screws were set and solid and I never had an issue with the hinges again.
No animals were injured or killed in the production of this post.
