evidence in case of Lexan vs acrylic

devout · Trick Member Joined: 15 Dec 2006 Location: the atl

Since the Lexan vs acrylic debate seems to be a recurring topic, here's some technical info that might be useful:

There are dozens of physical properties that might be of interest for plastics, but as far as ddr pads are concerned, you can probably make your choice based on just two of them: flexural modulus, and impact strength.

I took a trip to my local Lowes, and they sold 3 types of plexiglass: Plaskolite Optix, Plaskolite Duraplex, (both are brands of acrylic) and GE Lexan XL10. A quick google search turned up the property values for these materials. Once you know the brand names/product nubmers of your choices you can find the properties on www.matweb.com, www.geplastics.com for Lexan (need to create an account) or just google them. (A word of warning, you need to be specific with what type of material you're using. If you search for Lexan on www.matweb.com you will see that there are over 400 types of Lexan, and the properties vary WIDELY. Dont buy plastic online if it just says "Lexan" and doesnt give an actual product number. Also note that Lexan is a brand name of polycarbonate, not a type of material itself. Real
Lexan will say Lexan on the protective coating.) Heres how the lowes products stack up: (in both cases, higher is better) (values are from manufacturers websites)

Flexural Modulus (psi)
Optix: 490,000
Duraplex: ???
Lexan: 340,000

Flexural Strength (psi)
Optix: 17,000
Duraplex: 8,300 to 13,690 (depending on grade)
Lexan: 13,500

Izod Impact Strength (ft*lb / in)
Optix: 0.4
Duraplex: 1.1 to 0.6 (inversely related to Flex strength... high flex str = low impact str)
Lexan: 12 to 16

What do these numbers mean?

Flexural modulus is a measure of how much the plastic will bend if you support it at the edges and put a weight in the middle.

It is expressed in psi (pounds per square inch). You can see how it's measured here: http://www.matweb.com/reference/flexuralstrength.asp Flexural strength is just a measure of how much force it takes to produce a 5% deflection. So it's directly related to the Flex modulus. You could calculate the flex modulus of the duraplex from its flex strength if you really wanted to.

You can calculate the deflection for a weight applied at the center of a piece of plexiglass supported at the edges from this equation:

Deflection = (Force * Length^3) / (4 * Width * Thickness^3 * Flexural Modulus)
Where Force is in lbs, Flex Modulus is in psi, and all other values are in inches.

Note that in the equation, deflection is inversely proportional to thickness cubed, so if you double the thickness, you'll reduce the deflection by 2*2*2 = 8x!

Calculated deflections for a 150 lb person standing on a 11" square piece are as follows:
.25" Optix: .59"
.25" Lexan: .85"
.375" Optix: .18"
.375" Lexan: .25"

Also note that these are "worst case" values, as your arrows wont be supported right on the edges, your supports might actually only be 9" apart or less. Also, the person wont be applying their whole weight directly to the center of the step. They might only have half their weight on the arrow, and it will be spread out over a larger area. Finally, the corner brackets will be supplying a downward force on the edges that will offset some of the flexing. However, this should give you some idea of how much each material flexes. The Optix (which btw is just a brand name of acrylic) actually flexes a lot less than Lexan does. However, keep in mind that there are many other types of Lexan out there. If you search on matweb you will find Lexans with a flexural modulus over 1,000,000 psi!.

(FYI, The "acrylic overview" on matweb lists the range of Flexural Modulus for extruded acrylics as 479,000 to 493,000 psi. So the Optix from Lowes is typical for acrylics.)

So what is so great about Lexan then? Must be the impact strength...

Impact strength is less well understood than flexing, so no equations here. These values come from the Izod test: hitting a piece of material with a golf club to snap it in half, and measuring how much it slows down the golf club. (Ok, its a little more complex than that... details here: http://www.matweb.com/reference/izod-impact.asp)

But it's easy to see from the values above that Lexan is about 40x more resistant to breaking under a sharp impact than acrylic. (Note, the 0.4 value is pretty much universal for all acrylics. Lexan might range from values of 2 to 20+ depending on the type).

So... conclusions: if you want stiff panels, use acrylic or thick lexan. If you want panels that wont crack, use Lexan. My personal recommendation would be to use acrylic since it's cheaper and stiffer, and if you ever break a panel replace it with Lexan. If your arrows are supported by something soft (like weatherstripping) and people arent totally stomping your pad, .25" acrylic probably wont crack anyway (from my own experience). If you're not doing a lighted pad, use thin lexan to resist cracking over a more durable material, like wood or metal.

Final thought: How do you calculate the strength of multiple layers of plexiglass? Two 0.25" pieces are NOT the same as one .5" piece. The reason is they can slip past each other. Think of it like this: it's easy to bend a phone book in half around it's vertical axis, because the pages slip right past each other. But you cant bend it in half around the horizontal axis because the spine keeps the pages from slipping. I dont know off the top of my head how to calculate this, but if you want really stiff arrows, I'd say layer 3/8" and 1/8" around your graphic. That'll be stiffer than 0.25" and 0.25".

ChilliumBromide · Trick Member Joined: 16 Jun 2006 Location: Beaverton, OR

Wow, good job on all this; requesting sticky.
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devout · Trick Member Joined: 15 Dec 2006 Location: the atl

probably not sticky worthy, but I'm writing a tutorial for the pad I'm building... when it's done I'll include this on that site

trob xD · Trick Member Joined: 18 Jul 2006 Location: Ohio

Ok. Well instead of reading all that, at the very bottom of your post, could you tell us which one is better. LOL

ChilliumBromide · Trick Member Joined: 16 Jun 2006 Location: Beaverton, OR

I think that it's important for people to be able to fully understand why they're picking one material over another when buying or building pads. I doubt this will be a huge discussion topic, and it's not like this forum's exploding with stickies.

Perhaps those of us who know about the actual statistical qualities of commonly used DDR pad materials should work to make a full material guide. I've been hanging out here for the last 6 months almost every day, and I still don't fully understand what makes some things better than others.
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devout · Trick Member Joined: 15 Dec 2006 Location: the atl

ChilliumBromide · Trick Member Joined: 16 Jun 2006 Location: Beaverton, OR

Oh, hey, now that you mention it, how's Lexan with blacklights? Would you know?

I'm planning on building my own pad with Blacklights in the non-button panels and high-powered LED's in the buttons, but I hadn't thought of whether or not the blacklights will actually work with the Lexan.

Also, does anyone know where I could get clouded (translucent) glass or acrylic?
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devout · Trick Member Joined: 15 Dec 2006 Location: the atl

Not really sure about Lexan and blacklights... the XL10 that lowes carries is meant to block UV, so obviously you wouldnt want to use that. However it just uses a separate UV-blocking coating on one side, so im not sure how much plain Lexan would block.

I have a blacklight cold cathode light, and I believe the casing is some kind of polycarbonate (like Lexan), and it blocks a little of the UV, but not much. But If you were going to try something like lighting through an edge like someone showed on the main homepad thread a while back, even a little blocking will add up quick. So I would experiment before you buy a large amount.

(Actually, if you were using the edge lighting technique, you might be able to use the XL10 if you put the uv blocking site facing outward, since you only need the UV to go through one side of the sheet.)

Whenever i'm looking for a specific type of material I like www.mcmaster.com because they have a really user friendly site and they let you search by material properties. A search for plastic >> sheets >> white translucent >> 1/4" * 12" * 12" gives $5 polypropylene sheets and $500 fluoropolymer sheets. I recommend the polypropylene

If you just search for "plastic" and scroll down you can see all the colors they offer. (Of course this is for all plastics, doesnt guarantee they'll offer them in sheets.)

They do offer "fluorescent tint" acrylic... that paired with a blacklight could be very cool. (might need to paint one side white to give it something to reflect off of)

ChilliumBromide · Trick Member Joined: 16 Jun 2006 Location: Beaverton, OR

I'll keep that in mind; I plan on just having a cheap blacklight bulb under each one.
Actually, I don't really care about the UV white-glow effect so much as the gnarly colour, so maybe I'll just use some 100watt bulbs and purple Lexan.
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revenGmyass · Trick Member Joined: 01 Sep 2006

Maverickdebater · Trick Member Joined: 10 Jul 2006

In this specific instance which would be better?
I'm building the DJ3 and the layering inside each panels are Wood > Sensor (Matrlx) > Wood > Acrylic
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devout · Trick Member Joined: 15 Dec 2006 Location: the atl

idConstruct · Trick Member Joined: 12 Mar 2007 Location: Green Bay, WI

ChilliumBromide · Trick Member Joined: 16 Jun 2006 Location: Beaverton, OR

Actually, the pads I plan on putting them in will have a cooling system all around them and they'll be inside 10" arrow wells, so I'm not too worried. I will look into blacklight cathodes though. I didn't realise they had them.
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idConstruct · Trick Member Joined: 12 Mar 2007 Location: Green Bay, WI

lol nice

http://svc.com/v12inch2ccflblack.html
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Mushroooshi · Trick Member Joined: 20 Aug 2007

Sorry for bump:

How would 1/8 in lexan with 1/4 in wood underneath do?

I'd guess the biggest person would be 150 pounds?
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