MDF core plywood vs lumber or veneer core

Chad

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I'm in the process of standing up a laser cutting service for the rocketry hobby. One thing I'm learning is finding 1/8" baltic birch plywood that is not laughably warped is very difficult. In all the places i've looked around town and orders i've placed online only around 10-20% of it has been usable. The situation for 1/4" is a little better but you're still hand picking pieces out of a stack which is very time consuming especially for a material inventory.

I placed an order tonight for some samples of MDF core plywoord, basically MDF with a veneer of Maple on both sides. MDF, while not as strong, is at least more stable and hopefully flatter than what i've been able to find so far. Then the Maple veneer hopefully means sanding/finishing/painting is a similar experience to the standard wood we deal with all the time. Does anyone have experience building with MDF core plywood? I wonder how much the difference in strength would affect a build.

(btw, no formaldehyde in the MDF i'm trying out so should be safe to cut with a laser)
 

tfish

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I just got done building a 6" ARLISS.org style rocket with my local high school STEM program. To save money I used MDF snap together flooring. I got lucky in that the local "Goodwill Building Supply Store" has a good selection and $1 a piece. I was surprised at how many layers. Thickness. Widths and weights they come in..
We just used it for centering rings and bulkheads.
It's cheap abd does the job.20220131_180205.jpg
20220131_180316.jpg
20220202_110912.jpg

Tony
 

heada

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MDF is really heavy and while dimensionally stable, it has a much greater flex than any normal plywood. Getting construction grade ply that is flat is difficult but you can get cabinet grade plywood that is much higher quality. You'll pay more for it.

If looking for flat ply-like sheet goods that are cheap, look for 1/8" door skin. 3 ply with a balsa core and oak veneers on either side. Light weight, flat, cheap and available at most bigbox stores like Home Depot.
 

QFactor

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Here are pictures of the plywood I use in my rockets. They are Baltic Birch; 6-ply (1/8") and 12-ply (1/4").

The 1/4" comes flat as a pancake. I buy a 4'x4' sheet and have it cut into in 24"x24" pieces for shipping.

The 1/8" sometimes has a touch of warp. But the parts I cut from it are so small the warp does
not appear in the parts.

This plywood is a little heavier than the 5 and 3-ply we see used in rocket kits because of all the gluelines.

I've not had anyone laser-cut parts from this plywood. With all those gluelines it may offer a small
challenge for most hobbyist laser systems. You can "hear and feel" the difference in its cutting and
routing when compared to the 5 and 3-ply material

IMG_5043.JPG IMG_5040.JPG IMG_5037.JPG IMG_5038.JPG

I get my plywood from Boulter Plywood.
 

dhbarr

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Aircraft Spruce isn't cheap but it also isn't trash. They've got some really thin ply stuff that I've been tempted to sandwich with a glass line in the middle but keep the wood look on the outside.
 

waltr

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I'm in the process of standing up a laser cutting service for the rocketry hobby. One thing I'm learning is finding 1/8" baltic birch plywood that is not laughably warped is very difficult. In all the places i've looked around town and orders i've placed online only around 10-20% of it has been usable. The situation for 1/4" is a little better but you're still hand picking pieces out of a stack which is very time consuming especially for a material inventory.

I placed an order tonight for some samples of MDF core plywoord, basically MDF with a veneer of Maple on both sides. MDF, while not as strong, is at least more stable and hopefully flatter than what i've been able to find so far. Then the Maple veneer hopefully means sanding/finishing/painting is a similar experience to the standard wood we deal with all the time. Does anyone have experience building with MDF core plywood? I wonder how much the difference in strength would affect a build.

(btw, no formaldehyde in the MDF i'm trying out so should be safe to cut with a laser)

What is the use in the rocket of these parts??
What size rocket?

This matters greatly on the plywood selected.

I have used 1/8" MDF for centering rings and bulk heads on low power rockets (up to BT55 & a few oz). For larger rockets good birch is needs for cord attachment. CR's can be less critical when fins are through wall to MMT.
Same with fins...but warp is an issue to have the rocket fly straight with no spin. Also speed of rocket.
 

cls

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We could not find any good stuff locally, so we ordered a bunch of 1/8" Baltic birch plywood from Aircraft Spruce. It was all warped, barely a 3" usably flat section on a 12x24 piece. Terrible.

So I called and asked, the knowledgeable man said it's all warped, too bad.

There are tricks to unwarp plywood. Water, or steam, on inside curve, and a strong press.

That extra processing work makes up for wasting time with crummy 3 layer that just falls apart eventually.
 

Chad

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What is the use in the rocket of these parts??
What size rocket?
This is a service similar to sendcutsend.com but a lot smaller, I'm doing this with my middle schooler since he seems interested in stuff like finance, marketing, and running a business and not writing code like his old man haha.

I've been thinking about how to incorporate fiberglass in a scalable way. If you can use fiberglass or some other composite to strengthen and straighten warped wood in a way that still works with my 50w laser then that could be a real contender. Also, it would be really cool to come up with a new material that works well for fins and rings that lands somewhere between plywood and fiberglass. Similar to how bluetube is a great material for body tubes when you want more than a cardboard tube but less than a full fiberglass one.
 

Sandy H.

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This is a service similar to sendcutsend.com but a lot smaller, I'm doing this with my middle schooler since he seems interested in stuff like finance, marketing, and running a business and not writing code like his old man haha.

I've been thinking about how to incorporate fiberglass in a scalable way. If you can use fiberglass or some other composite to strengthen and straighten warped wood in a way that still works with my 50w laser then that could be a real contender. Also, it would be really cool to come up with a new material that works well for fins and rings that lands somewhere between plywood and fiberglass. Similar to how bluetube is a great material for body tubes when you want more than a cardboard tube but less than a full fiberglass one.

Very cool to give your young son an opportunity to learn, succeed and fail (not negative, it is the process!!!) at such a young age. You are setting him up to gain knowledge that might let him retire from 'work' and simply pursue opportunities for gainful transactions 10-15 years before his peers. Great parenting, IMO!

As far as materials go (I also run a laser, but not much wood anymore), I personally believe you're going to cull 20% of the wood you buy without extra effort. This is based on bulk orders for various ply back in 2018 and it is possible that things have gotten worse due to everything that has happened since then.

I have access to a decent size press (50 ton) and have tried a few methods for flattening. Thin stuff (1/8" or less) seems to flatten pretty easily and could probably be done with a home built screw press (guessing, not sure) but thicker stuff seems to be a bit harder. I've tried plain water, veneer softening solutions and an effort at steam, but it wasn't well executed, so I can't say it is perfect or terrible.

In my experience, the veneer softening solutions sprayed on lightly and pressed gave a huge improvement, but it was never dead flat. It is probable that doing something smarter than what I've tried would yield better results, but at this stage I find it likely that the proper solution is to buy whatever batch you buy, pull the perfect stuff, kinda good stuff and the 'yikes!!!' stuff and segregate. Store the perfect stuff flat, keep the kinda good stuff for smaller parts where warp is a minimal issue (centering rings, for instance) and the 'yikes!!!' stuff gets flattened into whatever it will flatten to, usually between the other 2 classes, but extra work for sure and I can't say for sure how it will react 5 years later. So far, it looks good, but I am climate controlled and storing flat, not a cut rocket fin sitting in a 100 deg trailer for half the year. I keep that stuff for jigs in-house and if you're doing laser work, you know in-house jigs is a big part of the process.

All of the above is my opinion only. Your material suppliers may have different materials and you might simply be smarter than me. If anything I said above seems to be positive, that is how it was intended. If it seems negative, I apologize, as I think what you are doing is cool and I wish you and your son all the success in the world.

Sandy.
 

Sooner Boomer

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I've been thinking about how to incorporate fiberglass in a scalable way. If you can use fiberglass or some other composite to strengthen and straighten warped wood in a way that still works with my 50w laser then that could be a real contender. Also, it would be really cool to come up with a new material that works well for fins and rings that lands somewhere between plywood and fiberglass. Similar to how bluetube is a great material for body tubes when you want more than a cardboard tube but less than a full fiberglass one.

Honeycomb core materiel?


Should be easy to cut with laser. It's designed to be the center core in a laminate
 

Chad

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Honeycomb core materiel?


Should be easy to cut with laser. It's designed to be the center core in a laminate
I've seen this, if i'm ever doing a very large fin I think you'll have to use something like this otherwise the you're going to be dealing with a lot of mass. You have to combine it with leading/trailing edge material though. For smaller fins i think it's just too expensive.
 
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