What is the difference between Particle Board and MDF?

[Lorrie]

I'm just wondering what the differences are between particle board and MDF?  MDF is more expensive and I was wondering why.  What applications would you use them differently? I'm a beginner woodworker and thought that you could help me understand the difference so that I can purchase the right material for the projects.  I'm always looking to save money. . .  any help you can give will be much appreciated.  Thank You.

[Ken Darga]

The boards sag as result of ''overloading".

Most particle boards, included with the metal storage framed shelving, is 3/8" thick.

A 24x36 rack will support an 800lb load---evenly distributed amongst all the 4-5 shelves.

3/8" thick is adequate to support a 200lb evenly distributed load, per shelf 24x36".

Option: 

Install thicker boards, just lay it atop the thinner boards.

NOTE: If using sheet material, it'll need to be cut to size, to fit between the upright frame members.

BTW

The back end of an automobile will sag, and perhaps collapse the springs, when the trunk is loaded/filled with 80lb bags of concrete.

RichardB said:

I have some shelves from Home Depot. The frame is heavy metal, but the shelves are either MDF or particle board. All the shelves that had any weight on them are all sagging. I've replaced most of them with pine boards. I wouldn't waste any more of my money on either of those products. They have no strength - weight/humidity/moisture is their enemy, and they cave far to easy. 

[RichardB]

That stuff seems to sag from its own weight. I've stored items that are very light in weight, only to see the shelves sag. 

Rita B. said:

Lorrie, about your particle board shelving, you will want to add some additional bracing because they will sag over time.  Attaching a straight board across the front of each shelf helps too.   I purchased metal strips designed exactly for that purpose on some of my heavy use shelves.   I learned how quickly they can sag the hard way :)

[Ken Darga]

I have particle board shelving, 3/4 x 12 x 4-8 ft sections, on shelf brackets spaced at 16"---

Some shelving has been up for over 25 years---no sagging problems.

Some shelves are loaded with books.

Some shelves support tool boxes, and the like.

To support "heavy loads", 2 such shelves, (double thickness of 3/4" thick material), can be used.

Any shelving material, whether wood or metal, to prevent sagging, needs to be adequately constructed, to support the weight of the material being stored on the shelves.

Supporting heavy loads, such as books, a 1x3 cleat can be installed on the wall or back edge, on the underside of the shelf, to resist sagging.

A support, such as a 1x2 along the front edge of the shelf, orientated 90 degrees to the shelf edge, and located on the underside of the shelf will provide additional support of the shelving, so as to resist sagging.

There are various suitable joinery methods that can be used to add support to shelving, to increase their load carrying capabilities.

[Rick]

No.  What kind of adhesive?

Ken Darga said:

Rick,

Have you ever used thinned down waterproof adhesive, to seal the exposed surfaces of particle board material?

[Rick]

Like Ken said, PB needs support every 16 in.  The fact that long boards are used also helps. 

I use plywood for nearly all utility shelving.  It helps to put a couple coats of poly on it to prevent stains.

3/4 in thick boards are slightly stronger than 3/4 in thick plywood.  Note that plywood is much stronger with the grain than against it.  The guy who built my sun porch obviously didn't know that since he ran the sheathing parallel to the rafters.  Ugh!

Even though each shelf material has its own modulus, note that its resistance to sag is a cube function.  In other words, under similar load conditions, doubling the thickness of the shelf reduces its sag by 8 times.  This, of course, only works if the shelf is one member.  Stacking 2 similar boards only reduces the sag in half.

RichardB said:

That stuff seems to sag from its own weight. I've stored items that are very light in weight, only to see the shelves sag. 

[Ken Kortch]

I once found this on-line "Sagulator" ... if you find it useful:

http://www.woodbin.com/calcs/sagulator.htm