MDF screw holding study

[jppierson]

I stumbled across this interesting study concerning the holding power of sheet metal screws in MDF. At the end of the four page report is a chart with the edge holding power of various screw sizes, lengths and thickness of MDF. The bottom line (my weak analysis) is that the holding power in the face of MDF is about double that of the holding power in the edge. The study does not address strengthening the hole using CA or epoxy. The study does not address the impact of screwing into the edge of face laminated MDF, such as a workbench top. So I don't know if one would use the holding power results of a single laminate vs the entire laminate. It is a fairly easy read, and has recommendations for pilot hole size for the different screws (there is a 13% reduction in holding strength without a pilot hole). Unfortunately there is no convenient chart for face holding but as I stated before it looks to be about double that of edge holding.

https://www.agriculture.purdue.edu/fnr/f...98805a.pdf

[KyleD]

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?

[jppierson]

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?

[DaveR1]

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.

[Cooler]

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.

[Stwood_]

 The reason I have experienced this weakness is that MDF is made of many layers of paper across its thickness.

 MDF is made from ground wood chips (fiber) and lots glue, not layers of paper.

https://www.youtube.com/watch?v=ZRE1L7anoss

[Cooler]

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.

[KyleD]

 MDF is made from ground wood chips (fiber) and lots glue, not layers of paper.

https://www.youtube.com/watch?v=ZRE1L7anoss

I am sure you are right. I should have stuck to what I know. It has a definite grain and that grain is very prone to splitting.

[Cooler]

I am sure you are right. I should have stuck to what I know. It has a definite grain and that grain is very prone to splitting.

It has no real structure.  If you grab a corner of a sheet with a pair of big (really big) pipe pliers you can snap it off.  It will split.  It absorbs moisture so you can't use it in a bathroom or near water. If you use it in the damp basement the material will absorb moisture.   Pretty much all  of this applies to particle board too.

However, if it is wisely selected for a specific application it is a very good choice.

Melamine laminated particle board makes perfectly satisfactory kitchen cabinets.  That is because the Euro style hardware is designed to mount on this material.  And kitchen cabinets get much of their structure from the walls that they are mounted on.  Cabinets made from melamine laminated particle board are quick to build, do not require any finishing and the material is far cheaper than plywood.

MDF is similar but with the ability to take paint well.  I call it a "young man's material" because it is so heavy to carry into the house.

[MichaelMouse]

I am sure you are right. I should have stuck to what I know. It has a definite grain and that grain is very prone to splitting.

Paper faces are characteristic to MDO/HDO, which is a plywood.  Maybe the source of the confusion.