Thread: Stability of Plywood

What ply are you using? What purpose are you using it for?
I've used some *unnings CD rubbish in the past for various things, and have not been impressed, I've had no issues with some better stuff from Mr Ply and Wood though - haven't found the need for marine yet. Used for internal linings and shelves etc....

They have been purchased from both of the large hardware chains here in Canberra.
I will have to look further afield for a better quality product.

Is there any way to assess the quality when purchasing?

If it looks like rubbish, it probably is - usual building stuff is CD, so one face has cracks galore, one has filled cracks and is a bit smoother, typically there are some voids inside.

this link may help:
Plywood Grades & Applications - Australian Wood Panels - Sydney, Melbourne, Brisbane, Adelaide

It is a problem the world over - listen to Marc Spagnuolo, Shannon Rogers and Matt Vanderlist talking about "Big Box" store ply in America in episode 82 of Woodtalk Online Radio

Originally Posted by mat

I have used a fair bit of ply recently and noticed that it is less stable than the "ply of old". Almost all pieces seem to move significantly.

Are people moving up in the quality grades to get stability? How high up do you ned to go? Do you need to consider marine ply to get stable stock?

A few questions:

What sort of movement?

Linear, cupping, twisting?

Why I ask, is that with the huge humidity swings we have been seeing, I have had everything moving.

In particular, if air circulation to one side is restricted, (like a board lying on a bench), a change in RH will cause movement, due to differential expansion/contraction on opposite sides. Ply is not immune to this. Given time to equilibrate, (and some settled weather, ) it should straighten out.

I do not believe that plywood in this day and time is the same as it was even a decade ago. They make the stuff here. Down the road from my house.

Just bought a 1/4 sheet (2' x 4') G!S (good-one-side) 3/4" = 18mm? to use as a painted backdrop for a carving. It is such crud, bowed so badly, that cut in pieces, I couldn't use it for bathroom cupboard doors.

I hate to admit it but softwood plywood does not seem to lend itself to being made into things. Sheeting? Yes. The hardwood stuff, like birch, that seems to be the direction you may have to go.

In relation to the types of movement I have had everything from cupping to twisting.

I have researched some better quality options (2400 x 1200 x 12 mm). Some SE Asian rainsforest marine ply is $78, some BB grade hoop pine ply (approx $110). The hoop pine marine ply seems to cost twice as much (approx $220)

Any comments on these in terms of value for money?

I only buy ply from Mr Ply now, they have an item they call Fijian Cedar ply - nice stuff to work with, I have lined a wine cellar with it, used it for shelves and made a childs rocking horse from it - no issues at all with warping. I went through a bunch of odd shaped off-cuts yesterday, nothing warped/twisted or otherwise. Not as cheap as 'Cunnings junk, but not expensive I think - I picked up two 2400x900mm 19mm sheets a fortnight ago for $99 each.

Just curious, but why do you think the cupping and twisting is related to the quality of the ply ? What particular shortcut in the process do you think would lead to a tendancy to cup and twist ?

I can understand poor quality ply tending to delaminate (poor adhesive), have surface faults and internal voids (poor quality stock used), but wouldnt the cupping/twisting be to do with differential moisture content and thus due to poor storage and handling ?

Anyway, I dont think the meranti marine ply that you ask about is any less inclined to twist then any other. The advantage the marine ply would have is that it should be free of internal voids and should have A grade surfaces on both sides. Marine ply has the same glue as exterior ply, its the stock and the care taken in the making process which is different.

cheers
Arron

I hear the last masonite manufacturer in Oz closed down recently, is that true?

Perhaps it is the moisture content (at time of manufacture) of the lower priced ply that causes the movement.

I consider ply to be a product that is made to be stable, hence the alternating grain in layers.

It would also appear from observation of hundreds of sheets that the more costly grades tend to stay flat.

Maybe the cheaper grades are not stored/handled as well as their dearer cousins??