MDF Density

[Patty Hann]

I have two pieces of 3/4" MDF bought at different times from the same timber supply place.
(A reputable lumber yard -indoor- in the Phoenix metro area - and neither a Big Blue building nor a Big Orange building.)
I cut each one to measure (for my needs) 32 x41 inches.
While doing this I noticed that one piece was significantly heavier than the other.
I held one [cut] piece while stepping on the bathroom scale, then did the same with the other piece, also cut to size.
There was a 12 lb difference.
This is 9 sq ft of 3/4" MDF weighing in the first case about 16 lb and in the second piece about 28 lb.
Is there that much variation in MDF?
I am a woman who weighs about 135 and am not super strong.
The extra 12 lbs in weight will make a difference in my being able to handle the mdf during construction without help.
Why would there be such a difference?
I now have to locate the type of MDF that weighs less but I am not certain what to ask for .
Thanks for any help with this.

[Matt Day]

There are indeed different densities of MDF sold (same as melamine, etc), and one is marketed as lightweight. Nothing is very consistent these days either!

[Jim Becker]

MDF is a compressed "wood" and resin product and there are indeed both variations in manufacturing at the same place and between makers as well as specially delineated versions for particular purposes like Mett mentions. The lightweight product, for example, is a staple for folks who run CNC machines and use MDF for their spoilboards as it "flows" vacuum better than the denser versions. There are also exterior rated MDF products that are moisture resistant and those are even heavier. In your case, the lumber yard could be selling product from multiple manufacturers or each sheet from the same manufacturer could have come from a different batch/lot in production. Or each was made at a different phase of the moon... :D

[Bill Dufour]

I suppose it also depends on what species of wood the sawdust came from.
Bill D

[John TenEyck]

According to this company the density of MDF can vary up to 50%, though the variation from mean density is listed as +- 10%. I take that to mean there are several grades within what's generically sold as MDF, and you must have some near the extremes.

https://pioneerpanelproducts.com/plain-mdf-specification/

Weyerhauser lists the nominal density of their MDF products, but not the tolerance, so who knows. https://www.weyerhaeuser.com/application/files/1715/2631/5929/WEY-MDF-Standard-Product-Specifications.pdf

John

[Edwin Santos]

I think I know the lumber yard you purchased from. They stock three types of MDF as far as I know, ultralight, regular (medium density) and high density under the Plum Creek brand name. Each has its pros and cons. I suspect you got one sheet of ultralight and one sheet of regular or maybe Plum Creek. The latter is quite dense and heavy, but the advantage is it machines very well in terms of routed edges. You should give the customer service people there a call and they should be able to look up your order and tell you what you received in each order which will explain. Either way, they can tell you more about what they stock.

FYI, recently supply disruptions have caused issues where they have not had all types available and might be filling orders with whatever they happen to have in stock. This has happened to me.

[johnny means]

I think a lot of places are mislabeling products speced to be sold as "Light" or "Ultralite.

[Edwin Santos]

I think a lot of places are mislabeling products speced to be sold as "Light" or "Ultralite.

Just being sloppy, or bait and switch?

[Bill Dufour]

I saw a armored truck at the wrecking yard. the inside was lined with what looked like MDF bolted to the sheet metal walls. There was a broken off piece. It weighed a ton. similar density to iron at a guess.
Bill D

[Patty Hann]

I think both Jim Becker and Edwin Santos nailed it (HAHA.. little carpentry pun there... very little:rolleyes:).
The timber place also supplies the shop where I take my WW class. Said shop has a CNC machine that has full size 4x8 spoiler board on it.
That is where I first encountered that particular weight of MDF and thought it was "normal/regular" weight MDF.
So as Edwin suggested I will return to the timber place and see about getting the lighter version (if they currently have it), now that I know what to ask for.
The nice thing is that one of the youngsters in my class works there part time.
He has always been very helpful (especially with loading ply or MDF ) and will be more than willing to help me get the right MDF.
Thanks to everyone who answered my questions and offered advice. :)

[johnny means]

Just being sloppy, or bait and switch?

I think it's just how it works when you're selling to a certain market. Retailers get what they can at the best price they can. Borg shoppers just aren't as concerned with thickness, density, and hardness as they are with price. Light and medium are fairly loose terms and probably not a selling point for many. My local supplier knows that their commercial buyers know the difference and that it matters.

[Tom Bender]

The lighter variety should be cheaper to manufacture since it uses less material, especially adhesive.

I would think density would have more importance than weight; machinability, surface finish etc.

[johnny means]

Raw material is probably a tiny fraction of the cost of a sheet of MDF. Likely, getting that sheet to the end user is a greater portion of the final price than raw material cost.
It might not matter, but keep in mind that Ultralight material is often thinner by a couple of mill as the industry uses it more often as a substrate for veneered and laminated materials.

[Frank Pratt]

I don't have adequate words to describe my dislike of light weight MDF. I don't care for MDF in general, but for some things it really is the best choice. If I gotta use it, I'll spend a bit more to get a water resistant grade like Medex. It is so much better than standard MDF and even more so than light weight.

[Jim Becker]

The reason for light-weight MDF is for specific applications where it presents an advantage. "Technically", one should use the variety that's best for what they are doing. The OP appears to have run into a situation where the product got mixed at the supplier for whatever reason and honestly, many folks are not aware there are multiple versions of the stuff!

Frank, I do agree that I like working with the exterior format MDF better based on experience with it for a client project a couple years ago.