Warping Plywood - Woodworking Talk - Woodworkers Forum
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post #1 of 22 Old 07-07-2014, 10:06 PM Thread Starter
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Warping Plywood

I have a large cabinet I am building, sides are 31" w x 8' tall and rear is 30" wide. I have a top rabbit and a bottom dado at 4.5 inches and 3 more dividers decently spaced between the 2. However when I mock up this assembly I see that the sides are warped, the plywood is bending and I dont know if the shelves in the middle of it will hold it straight or not.

I have dado'd everything so it is pretty strong, but even when clamped it seems as if its just going to stay bent. Now I did leave the plywood sit in my garage for a couple months, it was laying flat on the ground, the top piece was pretty warped but they got better as it went to the ground and I used the top piece for the dividers so that they could be held straight.

Can I do anything or just get new wood and not have it sit there. I live in AZ, its very hot here and been humid the last 3 days too.

Thanks for your help in advance.
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post #2 of 22 Old 07-08-2014, 11:30 AM
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Where did you buy your plywood? Over the past few years I have on occasion bought a few sheets of plywood from the big box stores and watched it warp after cutting it up. On close examination the cross section revealed that the ply were overlapped in many spots resulting in uneven stresses. I was lucky if I could get any straight pieces cutting it down to 2' x 3' sections. This is imported junk and most of it got thrown away.

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post #3 of 22 Old 07-09-2014, 02:25 AM Thread Starter
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I purchased it from Arizona Hardwoods and honestly I expected better from them since they are a local hardwood store.

Looking further at it there are inconsistancies and actually it was made in China the sales rep told me so and I didnt really think twice about it. There area actually gaps in the ply as well.
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post #4 of 22 Old 07-09-2014, 07:17 AM
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Plywood is a weird product, as it's made from multiple laminations. I don't think a blanket statement applies just to imported goods, but rather how those goods were packed, handled and stored.

The box stores get quite a bit more inventory than a local hardwoods supplier. How old the product is, and how dry the plies were when laminated can have a great effect on how stable it is.

Once you get your order, whether one sheet or 10 sheets, I wouldn't lay the first piece on the floor directly, especially if the floor is bare concrete. Laying the goods flat would be the best way if you have the room. I would sticker the first and all subsequent sheets especially if you won't be getting to them for a while. Keep weight on the top sheet to keep them all flat. Being stickered allows airflow around the entire sheet.

If you don't have the room you can store them on an 8' edge, as vertical as you can, and sticker in between them and clamp the top edge of all the sheets together tightly. The sheets will stand on their own, and you can do this near a wall, allowing as much floor space as possible.

When cutting parts, visually determine any bowing or warping and try to configure the usage to your advantage. Example..if a piece is to be used for a shelf, or top, a bow up will likely be a more physical attribute, to resist deflection.

Most all my sheet stock comes from hardwood and plywood distributors, not the box stores, and there is still that problem that we try to prevent if at all possible. If you keep the sizes with reasonable dimensions that would help.






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post #5 of 22 Old 07-09-2014, 07:33 AM
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When I was using Chinese made plywood I had problems with warpage. I finally gave it up when it started delaminating too.
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post #6 of 22 Old 07-09-2014, 08:24 AM
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laying flat without stickering allowed the moisture to come and go on one face only, causing the warpage in my opinion. try laying it concave face down on the ground (outside) with the hot sun above.
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post #7 of 22 Old 07-09-2014, 09:12 PM Thread Starter
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Thanks guys some really good thoughts, Im assuming by stickering you mean some kind of gap between the boards.

I definitely will try putting the sides in the sun for a bit, Im assuming so the curve is going into the ground so the top could dry and shrink a bit?

I did not put weight on the boards, I can do that too, especially next time.

Another question, if you cut the board right after getting it, assuming its flat, has anyone had them do strange things after assembly? Obviously long board will be more prone to warpage, but just on balance.
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post #8 of 22 Old 07-10-2014, 08:00 AM
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It's a good idea to put the crown side of the plywood in the sun but keep a close eye on it this time of year. It shouldn't take very long before it starts flattening. Also the pine will darken some from being in the sun so if you are going light you might have to thoroughly sand the plywood after you flatten it.

Usually when you build a cabinet or something with warped plywood once it is fitted with other parts in a perpendicular direction it flattens it and you don't have any more problems with warpage
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post #9 of 22 Old 07-10-2014, 10:11 AM
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It may have something to do with the basic plywood assembly process. When North Central Plywood was up and running, they could squeeze 145 sheets of 3/4" at full press capacity.

First, the logs are cooked in a steam chest. Then the logs go onto the lathe and the veneer sheet is cut. When that sheet ribbon comes onto the outfeed table, the direction of the original core of the log is downwards. In the glue up, all the veneers are still in that orientation.

Beyond sanding, trimming and grading, each sheet of plywood has an "inner" log face and an "outer" log face, whether it looked like it or not. I have a sneaking suspicion that this style of assembly leads to warping. If alternate sheets of veneer were flipped over in assembly, the mechanical properties would even out.
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post #10 of 22 Old 07-10-2014, 10:28 AM
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Quote:
Originally Posted by Robson Valley View Post
It may have something to do with the basic plywood assembly process. When North Central Plywood was up and running, they could squeeze 145 sheets of 3/4" at full press capacity.

First, the logs are cooked in a steam chest. Then the logs go onto the lathe and the veneer sheet is cut. When that sheet ribbon comes onto the outfeed table, the direction of the original core of the log is downwards. In the glue up, all the veneers are still in that orientation.

Beyond sanding, trimming and grading, each sheet of plywood has an "inner" log face and an "outer" log face, whether it looked like it or not. I have a sneaking suspicion that this style of assembly leads to warping. If alternate sheets of veneer were flipped over in assembly, the mechanical properties would even out.
The core veneer (if it's a veneer core plywood) is most likely not the same species as the veneer used for the faces.






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post #11 of 22 Old 07-10-2014, 11:21 AM
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I've watched the process many times at NCP. That's how I can describe it.
That is not to say that all mills do the layups in the same way.

When McBride Forest Industries (spawn of Zeidler Forest Products) was up and running, all they did was cut veneer sheet. Truck loads of that went to plywood mills. However, it wasn't hard to see the uniform cupping in every sheet in every bundle.

Seems to me that lumber core plywood should have good dimensional stability for cabinet work.
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post #12 of 22 Old 07-10-2014, 12:33 PM
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Quote:
Originally Posted by Robson Valley View Post
Seems to me that lumber core plywood should have good dimensional stability for cabinet work.
It's not bad.






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post #13 of 22 Old 07-10-2014, 03:51 PM
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I took a few pictures of a section of 3/4 baltic birch purchased from the big box stores (made in China) plywood. Take a look at the overlapping of the plys and you get an idea of why the sheet warps. This layup is throughout the entire sheet. I was told by the store section manager that they no longer buy from this importer but, there made by the same Chinese Mills. Sorry about the blurry image and I understand that the void is common but, in the blurred image the overlapping was everywhere in the sheet.




Last edited by jacko9; 07-10-2014 at 03:56 PM. Reason: edit images
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post #14 of 22 Old 07-10-2014, 04:24 PM
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Quote:
Originally Posted by jacko9 View Post
I took a few pictures of a section of 3/4 baltic birch purchased from the big box stores (made in China) plywood. Take a look at the overlapping of the plys and you get an idea of why the sheet warps. This layup is throughout the entire sheet. I was told by the store section manager that they no longer buy from this importer but, there made by the same Chinese Mills. Sorry about the blurry image and I understand that the void is common but, in the blurred image the overlapping was everywhere in the sheet.



That doesn't look like any Baltic Birch I've ever seen. If BB is sold in the box stores...or made in China...news to me.






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post #15 of 22 Old 07-10-2014, 04:56 PM
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Straight from Home Depot in Northern California. Your right about it not looking like BB and their Oak plywood is just as bad as far as layup goes - "junk".

Jack
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post #16 of 22 Old 07-10-2014, 05:52 PM
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"Baltic" should give you a clue as to it's origin.

That looks like regular Chinese birch. When I cut up a sheet, it came apart, not delaminated because there wasn't any glue to delaminate! I took it back to the lumber yard for credit. I buy enough of their other items to eat up the credit in short order.

Pictures...
https://www.google.com/search?q=balt...2F%3B800%3B737
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post #17 of 22 Old 07-10-2014, 06:13 PM
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Originally Posted by MT Stringer View Post
"Baltic" should give you a clue as to it's origin.

That looks like regular Chinese birch. When I cut up a sheet, it came apart, not delaminated because there wasn't any glue to delaminate! I took it back to the lumber yard for credit. I buy enough of their other items to eat up the credit in short order.

Pictures...
https://www.google.com/search?q=balt...2F%3B800%3B737

I just used the term "Baltic" as a generic description of the thin multi layer design. The pictures are Chinese plywood - not Baltic Birch.

Jack
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post #18 of 22 Old 07-10-2014, 09:07 PM
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Quote:
Originally Posted by jacko9 View Post
I just used the term "Baltic" as a generic description of the thin multi layer design. The pictures are Chinese plywood - not Baltic Birch.

Jack
Pretty sure all plywood is a multilayer design mate. Baltic birch is a specific type of plywood made in the baltics, a region surrounding the baltic sea. Its a birch veneer throughout, with the plys being uniform in thickness throughout and thicker than most other plywood. Most plywoods are fewer, thicker plys of cheaper woods, usually spruce, for or particleboard in some cases, with a very thin layer of hardwood veneer. What's usually in big box stores is the second type, true baltic birch cab be a beyotch to find
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post #19 of 22 Old 07-10-2014, 09:26 PM
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Quote:
Originally Posted by epicfail48 View Post
Pretty sure all plywood is a multilayer design mate. Baltic birch is a specific type of plywood made in the baltics, a region surrounding the baltic sea. Its a birch veneer throughout, with the plys being uniform in thickness throughout and thicker than most other plywood. Most plywoods are fewer, thicker plys of cheaper woods, usually spruce, for or particleboard in some cases, with a very thin layer of hardwood veneer. What's usually in big box stores is the second type, true baltic birch cab be a beyotch to find
I didn't want to get into a debate as to what constitutes "Baltic Birch" plywood, I was using as an example some sheets sold in the big box store that were made in China with a very shoddy construction technique which, I assert contributes to or actually causes the warping observed by the OP.

Jack
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post #20 of 22 Old 07-11-2014, 01:09 AM
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If possible, just stay away from anything offshore from China.
Their ideas concerning plywood are off the wall.

Last edited by knotscott; 07-11-2014 at 08:17 AM. Reason: edited by moderator to remove disallowed content
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