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The learning curve continues, 1/8" acrylic

 
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IMMark
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Joined: 18 Nov 2010
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Location: Columbus, Ohio

PostPosted: Tue Jul 12, 2011 12:23 pm    Post subject: The learning curve continues, 1/8" acrylic Reply with quote

This is a different project, so I started a new thread.
I am trying to pull .110 white translucent acrylic, sheet size 24x32.
I am using my previous mold, which is approx. 24 x 5 x 6, nice arch and plenty of draft.
My first try, I took the acrylic up to 310 degrees on the backside (took temp from below opposite of heating elements). From reading; the forming range is 290-320 degrees. At 310, i have virtually no sag at all. I was hesitant to let it go much hotter. I had 25 hg in on a 41 gal tank, followed by 15cfm pump; I was not able to get it to draw down.
Any thoughts, do you normally get sag when a sheet that size and thick is ready?
Or perhaps I need to recheck for seals on platen, etc.
Any help is appreciated, as each trial and error is $15 a pop Shocked
Thanks
Mark
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Fredo
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Joined: 21 Mar 2007
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Location: Kingdom of Nye Area 51

PostPosted: Tue Jul 12, 2011 10:07 pm    Post subject: Reply with quote

Read the posts by CrazyFool on his foray into the sign making game. He used 1/8 in. white translucent acrylic, and ran into just about as many problems as one can. He also over came the problems encountered, and finished his projects when many less inspired DIYers would have quit. Seeing what problems he had may help you avoid a few...........Fredo
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IMMark
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Posts: 189
Location: Columbus, Ohio

PostPosted: Wed Jul 13, 2011 8:14 am    Post subject: Reply with quote

Thank you Fredo
I do remember reading that thread and will have another go at it.
I think that my problem was, I did not let the acrylic get soft enough.
I really got no sag at all and when I used gloved hands to try to help it around the mold, it just didn't feel soft at all. I have reread some data sheets, and found some temps as high as 360 for 1/8", I only went to 310.
I am going to give it another go.
Thanks
Mark
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spektr
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Joined: 07 Jan 2008
Posts: 425

PostPosted: Wed Jul 13, 2011 11:45 am    Post subject: Reply with quote

Just thinking out loud..... Why arent you drape forming it?
Also, I use 375F as a molding starting point on my clear acrylic
stuff. Sneak up on it, and remember that for prototyping, you can reheat the sheet and flatten it enough to play with it twice.....
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kayaker43
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PostPosted: Thu Jul 14, 2011 7:58 am    Post subject: Reply with quote

You just aren't hot enough yet. Forget temp guns or timers,.. sheet sag is the only good indicator.

Infra-red temp guns are pretty much a joke unless you calibrate for the emissivity factor of different surfaces. See this video for a demo.

http://www.youtube.com/watch?v=QHszoA5Cy1I

Temp guns are great for comparing two nearby spots on the same material to see which is hotter or after getting successful pulls you can record the number for later. Just make sure to record the plastic, surface gloss and color for the next time. White Styrene won't read the same as black abs so don't put too much faith in the actual numbers.

You can put 10 objects in an oven at 250f and they will all read different. some people use the scotch tape trick to even out the emmissivity values of different surfaces but you still need to know the value for tape to get real numbers.

They are also useless for measuring element temps. A temp gun averages evertything inside quite a large sample circle, not just under the laser dot.

Laser temp guns are however great for playing with the cat Laughing
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spektr
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Joined: 07 Jan 2008
Posts: 425

PostPosted: Thu Jul 14, 2011 8:40 am    Post subject: Reply with quote

Doug, we agree in a roundabout way.... I find digital/IR temp guns usefull as an ANALOG DEVICE. There may or may not be correlation between the temp displayed on the gun and the REAL temp, But I find a high degree of correlation between the readings sheet to sheet on the readout. This means that I might see a number on my display, know this material is workable at this NUMBER and have a repeatable way to control the process. I really have no way of measuring ACTUAL sheet temps in process. I use the IR readings as NUMBERS to define process windows. Too cold is XXX Too Hot is YYY. These numbers have no real relavence beyond my processes to those in the real world, BUT they are exceptionally usefull if you understand their limits. Saying they are a joke might be a bit harsh to me.....

Scott.
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kayaker43
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PostPosted: Thu Jul 14, 2011 9:46 am    Post subject: Reply with quote

Your right, that was a bit harsh. I'm a little sour on them because I bought a high end gun thinking it would be great. With all the limitations a cheap Harbor freight gun would have been just as useful. They are best used as a refrence if you disregard the actual number or if you just want a ballpark number.

I did have an issue with two different batches of Styrene though. One had a fairly high gloss and the next came with an egshell or matte surface. This will affect the reading even though they are the same plastic. Most sheets I can get around here come with one side glossier than the other.

When you take those differences into account along with the plastic formulations from batch to batch, an IR gun has limited usefulness. Sheet sag factors out all the variables and indicates formability best. When I keep notes, that's what I write down.

I once came a cross a thick book showing sheet plastic formulations for extruders. Under ABS there were 450 different recipes of the same 3 basic ingredients along with the associated temps which varied greatly. ABS is the worst in this regard, some distributors have a reliable supply of fresh sheets from the same vendor. Others have multiple vendors, some of which use wide spec recycled content. Its all about who has the best price on the day they order.

Funny story,.. Some deptartment at NASA used a Proto-Form to make clear nose cones for prototype heat seeking missles. They took a machine that cost $1300 to build and added $11000.00 worth of automation to it. They were determined to automate the forming process and write a mil-spec so they could farm out to contractors and get consistent parts. They tried measuring exposure time, then IR temps, then optical clarity from an oblique angle, then contact probes. They finally settled on sheet sag but not the easy way with optical beams, they had to use sonic proximity sensors. It would kill those guys to try simple first.

They bothered me for months then I stopped hearing from them. A year later I got a call from a contractor hired to make the parts. NASA gave him the machine but he disabled all the techno gadgets and was using it manually with great results Rolling Eyes Rolling Eyes
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Also other plans books and videos for people who like to build things
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jdougn
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Joined: 18 Mar 2009
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Location: Louisville KY area

PostPosted: Thu Jul 14, 2011 2:44 pm    Post subject: Reply with quote

Kayaker43, thanks for sharing the NASA story. It would've been interesting to have been a fly-on-the-wall while they tried all the automation.

I guess I use both the IR temp gun and sag. For sure, on the black ABS that I use, surface temps read with a IR gun vary from batch to batch and surface finish to surface finish. This was very apparent when some sheets needed "deglossing". The amount of sag compared to the temp the gun was giving was very different. IR temps give me a close ball park and reference points. The actual sag fine tunes when the plastic drops.
just my .o2,
DougN
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