Lexan or plexiglass for fins?

[rocketsonly]

Hello. How do Lexan and plexiglass differ? Or are they the same thing?

Are they suitable for fins on mid-power to high-power rockets?

Thanks.

 

[Justin Horne]

From what i've read on here before, lexan is preferred.. I don't remember why, but I remember someone stating that it was better...

EDIT: Here's where I read that:
link

 

[Stymye]

plexi can crack or even shatter when you try to cut or drill it

lexan is a much better choice

 

[DynaSoar]

Originally posted by rocketsonly
Hello. How do Lexan and plexiglass differ? Or are they the same thing?

Are they suitable for fins on mid-power to high-power rockets?

Thanks.

Lexan is pretty much shatter proof. It's far tougher than plexi. I don't know about its machinability, but when it's molded, it's tough stuff. I used to sell cassette tapes made of lexan. You could run over them with a car and whack on them with a hammer. Fins molded out of lexan would far outperform AT's "high impact" styrene fins. Lexan is also heat resistant; one of the selling points of the tapes was that you could leave them in a closed car in the summer and they wouldn't warp.

Plexi is too heavy. You'd need too much to make it stiff enough to not flutter. It's prone to crazing and can shatter. I might make a plexi rocket just to do it as a show off piece, but not for anything serious.

 

[Micromeister]

Guys:
Lexan is the trademark name for G.E. produced Polycarbonate plastic. Plexiglass is the Trademark name for Acrylic plastics produced by Autofina, formally Auto-Hass which was for years Rohm and Hass.
Acrylics are very brittle not a good choice for fins at all. any of the Polycarbonates are very tough, cold formable, can be drilled, punched, threaded, and riveted. CAN be broken in cold weather, but will not shatter like acrylics.
Most Sign supply houses and Plastic distributors will have at least one of the many brands of polycarbonate plastics in sheets in thicknesses from .010" in clandered (frosted) material .030" clear, .064" clear, .080" clear, .125", .1875", .250, .375", .500" and up to 3" thick material used as bullet proofiing, I've seen a 44mag stopped cold in a 1.25" sheet of polycarbonate.. very impressive stuff.
Just about every model i've ever converted with Clear fins use Lexan, While I have popped a few off, I have yet to actually break one. The best adhesive for attaching Polycarbonate to polycarbonate is a material called Weldon-16. a clear adheasive that will from a bond so strong i've lap jointed two sections of .063" clear, let it sit overnight and cold formed an over 90degree bend at the joint without breaking or weakening the joint. Best method for attaching polycarbonate fins to cardboard body tubes is 15 - 30minute expoxy using the epoxy rivet method.
Dee-Piggy's fins are .063" clear Polycarbonate butt mounted with 15minute epoxy rivets to the polypropylene bank body and epoxy fillets. After about 30 D12-3 flights and they are still as solid as the day they were applied.
Hope this helps.

 

[astrowolf67]

I just built a crayon rocket a couple of weeks ago. Unable to find Lexan locally, I used acrylic. I had one heck of a time cutting this stuff. I'd make some deep score lines, as directed, then try to "snap" along the line, and this stuff was TUFF!!! Then, with the rocket complete, in the back of the van Thursday night (we went to visit relatives, and stopped at a launch on the way home Saturday), I leaned in to grab something, and SNAP!!! Broke a fin like it was nothing. But, medium CA held it quite well. Well enough to fly on a G80. I'd take advice from others, use Lexan, if you can find it.

 

[rbeckey]

Home Depot carries Lexan sheets.

 

[gregzo]

MicroMister nailed it. John - you must be a fellow plastics geek to rattle off all that crap..... (You gotta be one to spot one!)

As far as weight - there is not enough of a difference to worry about. It's close enough to one another.

Plexiglas is acrylic and is darn tough stuff. It does bond well with CA. If you can cut with abrasive cutting methods, you are probably better off. Scoring and breaking will most likely just lead to blood loss.

Polycarbonate is great stuff. I.e. better stuff!! As I said, plexi is tough, Polycarb is incredibly tough stuff. It is very machinable. It can be drilled and tapped. (Do NOT use any cutting fluids - it will craze). Epoxy sticks well. If you can get your hands on MEEK - it is a solvent that is the absolute best for Polycarb to Polycarb bonding. It is not a "glue". It bonds at the molecular level. It is very NASTY stuff however!!!
The down side of PC is crack propagation. Once a "sharp" forms - it will propagate the sharp as a crack. If you drill holes, counterbore both sides of the hole with a 45-degree chamfer. It you get a knick somewhere (watch those leading edges), round file or sand out the nick so it is nice and "smooth". If you get a crack that starts to form, you can head off the crack by drilling holes at each end of the crack and putting a chamfer on the hole(s). (yes - drill right through the fin at the end of the crack.. You can fill the hole with epoxy if you want - but the drilled hole will stop the crack from continuing.)

Other sources to get PC sheet that I see mentioned here all the time is McMaster Carr. www.mcmaster.com They have good thickness selection and sheet size. They also sell black, but only in 1/2" or thicker. Black is much more UV resistant. UV shouldn't be a problem for most rockets, but PC will yellow over time with sun exposure and that will lead to a lower impact resistance. Painting the fins also helps the UV thing. (But then you do not have beautiful clear fins....) Glass fiber filled PC is darn near indestructable (but not clear). It would make awesome fin stock of molded fins. Aerotech should shoot their fin molds with glass PC. It's a few bucks per pound versus the sub $1.00/lb impact styrene they use, but the fins would really survive.

Bulletproof "glass" is usually laminated PC sheets with PolyUrethane binder layers. Like a bullet proof vest, the layers/fibers are meant to fail, but each layer distributes more energy to cause it to fail. As the bullet destroys layers, there is less energy for destruction of the next layer. So hopefully it stops the round before it gets through the matrix.

Mid & HPR use? Absolutely! You should use thicker unreinforced (clear) PC sheet material than you would normally use for G10 fins. Use .093" PC when .063" G10 would work. Use .125" PC in place of .093" G10 and so on.

Good Luck

 

[Micromeister]

Originally posted by Gregzo
MicroMister nailed it. John - you must be a fellow plastics geek to rattle off all that crap..... (You gotta be one to spot one!)

Yea! as a former PA for a large illiminated Sign manufacturer, I have to know a little bit about alot of materials Sure helps sometimes with the hobby thou

 

[LampertRocketry]

Originally posted by Micromister

Originally posted by Gregzo
MicroMister nailed it. John - you must be a fellow plastics geek to rattle off all that crap..... (You gotta be one to spot one!)

Yea! as a former PA for a large illiminated Sign manufacturer, I have to know a little bit about alot of materials Sure helps sometimes with the hobby thou

What is the best/easiest way to bend the stuff. I am going to guess heat and bend... what would one wish to use to heat it up?

Thanks.
Murray

 

[Stymye]

the ideal way is with an element type heating rod
you want to catch it before bubbles start to develop for a pretty job, there is equipment made specificaly for it.
you can also cut a v groove( to the inside) before bending for a sharper bend

It can also be cold bent, but thats a bit harder to do

I have used a small propane torch but its hard to do that without bubbles.

you can click on my WWW below to see alot of (lexan) work I've done to safety guard new machines
(look under "pictures")

I use alot of extrusion for framework and corners so I don't have to do a ton of bending.

 

[gregzo]

Yes - heat and bend.

Now controlling the heat is critical. Too much heat and it will thin out and be weak. Too little heat and you will stress it and it becomes weak. I would play with scraps to get the feel of it.

Masking the area to be bent with an insulator helps control the heat and the bend. (Wood, plywood, wall board, etc.) Make sure the area of the bend that sees the heat is uniform. Clamp on the insulators so the exposed area is even.

A paint stripping heat gun would work well.
A propane torch has a very concentrated heat. It would be my last choice. PC (polycarbonate) will burn!!

The larger the radius of the bend the less induced stress you put into the bend.
It's also good to bend around a formable surface. A Home Depot wood dowel would work. Heat the uniform exposed area. Then once it starts to flow/move, form it around the rod. You will probably imprint the surface of the form/rod/cone into the plastic so a polished metal "mold" would be best, but anything would work. It is a "feel" thing. Practice!!

Standard PC has a HDT (Heat Deflection Temperature) of around 280 degree F. It melts around the mid 500's. You'll need to heat the material to somewhere between these two numbers. The closer you can stay to the HDT number the better. You may need more than two hands for this. If one person works the heat while the other SLOWLY bends the material, you should get really good results.
Good Luck

 

[Stymye]

agree on the torch part,
I wouldn't recommend a torch either
I've wanted to make some guick and dirty bends with small scrap peices for one time use, and didnt have the need to make something really nice, so I just used the torch in a pinch..


I also leave the protective (plastic) sheeting on when I bend
after it cools I start peeling and spray wd-40 behind the sheet as I peel it,it seems to leave a clearer finish at the bend.
obviously this wont work with the paper coated sheets

I'm not sure if thats the proper way to do it, but it works for me

 

[Newtons 3rd]

We use .125 Lexan fins on our flying outhouse and this material requires NO heating and can be bent cold on a sheet metal break.. Incredible strong and easy to form/drill/machine. We have kits that have more than 25 flight without any problems, and this from a 1 lb kit using streamer recovery on hard packed soil...

 

[Micromeister]

Heat bending polycarbonates is a special procedure, If you simply apply heat to the material "it will develop bubbles that will not go away" the material must be Heat "dryied in an oven at 250 degrees for a minimum of 5.8 hours. Than it can be thermoformed at 415 degrees. Torch heating will ruin the material unless you don't mind the bubbles and opaque semi-white to brown crazed look.
A much better alternative for fin work is to use a slighly thinner material and cold form your bends with sheet metal tools. Ductbill visegrips are excellent for this task. .125" material and below can be cold formed by hand, 3/16" will require a bench vise. either way the material CAN be struck with a 2lb sledge hammer without fear of breaking along the fold line. If one hammers the line to sharply the flange may crack under stress, let it roll around the corner a little.

Also keep in mind this stuff is tough as nails in hot weather, but can and will crack and break in very cold below freezing weather. if you put a piece in your freezer for a half hour or so, take it out and hit it with a hammer on concrete or something solid backing the polycarbonate will shatter, that why GE no longer calls their Lexan unbreakable as they did with it's introduction...Sorry my bad but you should have seen the factory rep's face when I shatter his brand new baby
Hope this helps.

 

[LampertRocketry]

Originally posted by Newtons 3rd
We use .125 Lexan fins on our flying outhouse and this material requires NO heating and can be bent cold on a sheet metal break.. Incredible strong and easy to form/drill/machine. We have kits that have more than 25 flight without any problems, and this from a 1 lb kit using streamer recovery on hard packed soil...

Flew my 'Lil Johnny' at BUFFROC V this past weekend. Twice. First time on a G80.... the thing did loops. A real crowd pleaser! Came in under power. Only damage was the epoxy came loose in some spots. Second flight on an F something. Good straight flight. I love those PC fins.... and that, honestly, is what got me to thinking about using PC and making a bunch of OddRocs over the winter.

Thanks to all for their suggestions on bending. Is the .125 Lexan that you are using any special type to permit cold bending? Or just standard .125 Lexan? It sure seems plenty strong to me and I would have little problem putting it on an OddRoc.

Lil Johnny, BTW, is awesome. We are going to add some noseweight, reinforce the top with some foam and fly it on a totally rude-sized motor. My kids ... in fact, ALL of the kids, were jumping up and down when it flew.

Regards,

Murray Lampert
BRS

 

[Newtons 3rd]

Thanks for the nice words.... The kits have been fun to design build and fly... Not sure why a G did loops as we fly them all the time. Be sure to use no more than a 1/4" tape thrust ring, CG and CP can get a little close



Mike
N3