Question Acrylic expands??!?! (Rob Markoff?)

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LeMieuxGalleries

LeMieuxGalleries

CGF, Certified Grumble Framer
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i was just reading through a really awesome old thread in which Rob Markoff mentioned that plexi may (will) expand over time. what's up with that? :party: never heard of this before...
 
His discussion was about expansion/contraction with temperature changes (not time). No definitive comparison to frame rates of expansion/contraction has been part of the discussion.
 
okay, well expanding with temp change, travel, environment, etc. still unaware that it expanded at all, and considering it is used often for shipping would love some input as to what precautions people take to deal with this...thanks!
 
Indeed....

Actually, almost all materials expand and contract dependent upon temperature. Greg Fremestad of FrameTek has written a bit on this subject with expansion / contraction rates for a variety of materials that we use in the framing industry. Perhaps he'll comment here as well as Rob.

John
 
I think your better off making sure you have a strict 1/8 allowance. I know sometimes corners are cut (no pun intended) and sometimes we make do. Just be aware of the consequences.

You have examine and judge how many things you have returned for repair because your corners have popped or come apart, that would be the correct measure of the issue.
 
Acrylic expansion

From the FACTS standard on framing allowance:

Acrylic sheet (any thickness) will expand/contract with changes in temperature. An 8 foot long piece changing temperature 50° F will expand (or contract) 0.192 inches ( 3/16 inch = 0.187)

A piece of glass 8 feet long changing temperture 50°F will expand 0.025 inchess (1/32 inch = 0.0312)
 
Tru Vue has some recommendations on their acrylic spec sheets, but as has been previously noted, if you are framing for indoor use, the standard allowance should prevent problems.
 
Acrylic

Experience has shown that acrylic sheet in frames performs well, even when the frames have been shipped on long plane flights, where the temperature can drop to around 40 degrees F. The traditional 1/8" allowance was used. A more pressing problem can occur, when acrylic is used to glaze a sealed package and the exterior conditions go dry. There, the acrylic can warp inward, toward the art, raising issues for transport.



Hugh
 
preservator said:
.... when acrylic is used to glaze a sealed package and the exterior conditions go dry. There, the acrylic can warp inward, toward the art, raising issues for transport.
Hugh
Click to expand...

Framers note: I believe when Hugh Phibbs says "sealed package", he probably is not referring to the custom framer's typically-closed frame with filler boards and dustcover or taped edges. That standard fitting/finishing technique does not actually create a seal, as moisture and air can still transfer in/out of the frame in response to ambient changes. The tighter the closure, the slower the rate of change inside the frame, but this type of framing adapts to its environment.

Hugh probably is referring to a frame that has been sealed quite literally, to the point of being submersible, using Marvelseal and other components. That is, the transfer of moisture and air in/out of the frame has been halted, and this frame responds differently to environmental changes.

Of course Hugh's point still stands, as acrylic will respond, unlike glass, to a change of moisture/humidity that would create a differential between the inside and outside surfaces of the sheet. However, in the custom framer's typically-closed frame described above, the propensity to bow inward might not be as severe as if the frame were literally sealed, as conditions inside and outside of the frame can begin to equalize.

Hugh, please correct me if any of this is mistaken.
 
Evonik states that their acrylic sheets (all thickness) expand roughly 1/32 of an inch for every 20 degrees Fahrenheit temperature increase. So a change from 10 degrees to 90 degrees ought to increase the size by about 1/8th inch. Not a big worry in most cases.
 
05 said:
Not a big worry in most cases.
Click to expand...

Hate to disagree with you, but from experience I would say that it IS a big worry. Unfortunately I know first hand that large pieces of acrylic will expand more than 1/8" in less extreme temperature changes and with enough strength and force to pop miters.

I think you may have left out an important detail:

The thermal expansion coefficient of acrylic is 1/32 of an inch per foot for each 20 degrees in temperature change. You also need to consider changes in relative humidity as the sheet will also expand .3% @ 80% rH. Acrylic absorbs water!

Help me with the math here-

Say we have an 8' long piece. That would be 8' x 1/32 or 1/4 of an inch of expansion over a 20 degree temperature change. So if a room goes from 65 degrees to 85 degrees (very possible in a room that was air conditioned during the day/week and then the AC is turned off for the weekend (as in an office building)) the piece of acrylic would expand 1/4".

So, if we have a 4' piece of acrylic, 4' x 1/32 = 1/8" . If you only have 1/8" allowance and your expansion is 1/8" for a 48" piece of acrylic, where does an expanded piece of acrylic larger than 48" go? It either "pillows" out (or in) or pops the corners of the frame. Pillowing is less of an issue for thicker pieces of acrylic and we use 3/16" for most larger pieces - so the possibility of popping the corners is even more likely.

If the "standard" allowance is 1/8" (and the lip of most mouldings is 5/16" wide) - how can you NOT expect problems if you build a frame with an 1/8" allowance?

How can you build a frame with an allowance greater than 1/4" if the lip is only 5/16" wide?

I recently had an oversized piece that I know had "ample" allowance in the frame - that came back (well, we had to rent a truck and go get it) because all four corners opened up.

When we removed the acrylic, it was almost 1/8" wider and taller than the net frame size and I know it was at least 1/8" smaller than the frame size when it was fit.

We ended up opening up the rabbet under the lip so there was additional room for expansion and then rebuilt the frame with a 1/8" allowance.
 
Rob Markoff said:
The thermal expansion coefficient of acrylic is 1/32 of an inch per foot for each 20 degrees in temperature change.
Click to expand...

The numbers from the Acryite Physical Properties specifier are a little different : http://www.thepaperframer.com/acrylic/ffpp.pdf

Per the specifier : A 48” panel will
expand and contract approximately .002” for each degree fahrenheit change in temperature.

Rob Markoff said:
The thermal expansion coefficient of acrylic is 1/32 of an inch per foot for each 20 degrees in temperature change.
Click to expand...

I don't know the source of this, but the Acrolite Specifier differs.

For a 20 degree swing that is .0005/inch or .006"/foot. This is less than 1/128" ( 1/128" = 0.0078125"). Over 8' this works out to be .048" or about 3/64" (3/64 = 0.0468750" ).

Rob Markoff said:
Say we have an 8' long piece. That would be 8' x 1/32 or 1/4 of an inch of expansion over a 20 degree temperature change. So if a room goes from 65 degrees to 85 degrees (very possible in a room that was air conditioned during the day/week and then the AC is turned off for the weekend (as in an office building)) the piece of acrylic would expand 1/4".
Click to expand...

3/64" is a lot less than 1/4" ( 16/64");

Since a 1/8" allowance is 8/64", that means that this is less than 1/2 a 1/8" allowance.
 
A couple of other thoughts:

I go through 5-10 4x8' sheets of acrylic a month.

When I first started, I stored my acrylic in an outdoor shed. In the wilds of PA, that meant a temperature range of below zero to over 100 degrees. Never an issue with the finished product. I would bring in a sheet, cut (standard allowance) it and put in the frame before it had a chance to defrost (winter) or cool down (summer). Most of the time the acrylic would sit in the frame long enough to expand or contract. I never noticed a problem with expansion/contraction.

My shop for the last 8 or 9 years is kept at 50 degrees in the winter.

Everything is always cut with a 1/8" allowance and some of my frames are over 4' wide. I have never had a customer complaint about the acrylic being too large or small.

Unless it's for an outdoor application, I agree with Hugh and U5; it's not much of a worry. Can anyone point to an indoor application for normal (less than 4' wide or tall) framing with a standard allowance where this has been an issue?
 
Here is what Plexiglas says:

For indoor applications where temperature normally remain the same (+/- 20 degrees F), acrylic sheet does not generally require special considerations for expansion and contraction other than providing for a snug rather than tight fit since its movement is approximately 1/32 of an inch per foot length for each 20 degrees of temperature change.

Here is what Tru-Vue says:

Allow at least 1/16th” of clearance for each 12” of frame length. In conditions of extreme humidity or temperature, greater allowances may be necessary.

There are many other references on the Web who have similar coefficients.

My experience confirms this.

I asked for help with the math - but do not understand your response:


"A 48” panel will expand and contract approximately .002” for each degree fahrenheit change in temperature."

So for a 20 degree change, wouldn't it be .002 x 20 = .04 ? Where does the .0005/inch come from?
 
acrylic

Jim did a good job in expanding onmy post. We can think of the performance we expect acrylic sheet to stand up to and paper being preserved should not be exposed to high temperatures, so siginficant acrylic expansion should not be a design essential. Very low temperatures may only be expected in power outages and it may be wise to check frames that have gotten very cold, to see the position of the glazing, before moving them. When frames are made very large, it is helpful to build in larger rabbet widths and with those can come larger allowances, since all of the components of the frame (except for glass and metal parts) are hygroscopic.



Hugh
 
And the bottom line, you are not in control to where or what conditions the frame/art will be placed. I remember one 60" long piece that popped that was exposed to direct sunlight, in a glass room and roof, was very hot when I went to investigate. I'd done the piece in the middle of winter ~60deg and I'd guess the plexi temp had to be over 120deg in the sunlight.

The larger the piece the more room you need and picking a frame with a space to allow larger than 1/8 space is wise.

framer
 
Sorry, made a mistake with my math but I believe that my source is the correct one. I will show the correct mat later, but here is more on the sources.

My source is Cyro's official specifier "Physical Properties of Acrylite FF Acrylic Sheet". The is the acrylic that we currently use from Tru-Vue.

acr1.jpg


You can view the spcifier on the manuals page at http://www.thepaperframer.com/acrylic/ffpp.pdf

This specifier was issued by Cyro - which was later purchased by Evonik. The Acrylic that we buy from Tru-Vue continues to be made by Evonik - I believe. It might be that Tru-Vue manufactures this under license from Envonik. In any case the physical properties will be the same.

This is the official specifier. As far as I know, nothing has superseded this specifier but if you can show a later source that proves me wrong....

Here is what the specifier says about expansion and contraction :

acr2.jpg


and if you go to Evoniks site at http://cyro.custhelp.com/app/answers/detail/a_id/400/kw/thermal%20expansion it says the same thing - with a little more explanation.

acr3.jpg


This is the manufacturer's official position - unless someone can show otherwise.

Now to correct my math.

.002" per degree for a 48" piece of acrylic = .005" per foot (I said per inch before). .002"/4 = .005" per foot per degree.

Multiplying that times 20 for 20 degrees gives us .005: * 20 = .01" per foot or 1/100th inch per foot for a 20 degree swing.

Multiplying that times 8 gives us .08" for 8 feet (.01 * 8 - .08") which is a little more than 1/16" over a full 8 feet.

Multiplying that times 5 for a full 100 degree swing gives us .4" which is in line with what the specifiers say above. That is for a full 100 degree swing. If the acrylic is cut at a temperature somewhat in the middle of the 100 degree swing then the overall expansion/contraction from the cutting temperatures will be about +-.2" over the full 8 feet.

But for those of use working with normal temperatures - a 1/16" over 8 feet for a 20 degree swing is not significant.

I also googled "expansion of acrylic" but didn't find any significant sources that shows the above to be off-base.

Here is a calculator I found if you want to do the math in metrics using expansion coefficients : http://www.pmma.dk/Acryl_Expansion.aspx?Lang=en-GB

BTW, I'm not trying to be an a** about all this; this is an important subject and I want to be sure we all agree about this. Prove me wrong with some new official data from the manufacturers and I will admit it.
 
Larry Peterson said:
Now to correct my math.

.002" per degree for a 48" piece of acrylic = .005" per foot (I said per inch before). .002"/4 = .005" per foot per degree.
Click to expand...

Using the data supplied, you would get an "F" in math class, as even though your answer is correct your work is wrong. .002/4 = .0005, as you stated in your original post...
 
David N Waldmann said:
Using the data supplied, you would get an "F" in math class, as even though your answer is correct your work is wrong. .002/4 = .0005, as you stated in your original post...
Click to expand...

I need to fire my calculator - it shows things in scientific format below a certain threshold.

Edit - Nevermind - its my keyboard that's at fault. My final answer of .01/foot/20 degrees is correct. I just typed in .005 rather than .0005
 
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