Cracks in Lexan covers

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Thud 54963

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I have been stabilizing numerous batches of blanks for about the last 6 months. Initially I was using a converted 5 inch wide by 7 inch tall pot for a vacuum pot. As my need for more capacity grew I moved to a 11 inch wide to 12 inch tall pot. On both of these setups I installed a vacuum gauge in the center and valve assembly about 2 inches from the center of a piece of ½ inch thick Lexan plastic. A piece of 1/8 inch thick butyl rubber served as a gasket.

My process is to heat the wood for stabilizing for 36 hours in a toaster oven at 225 degrees, then cool the wood to room temp. The wood is then placed in CJ stabilizer and vacuum drawn to 27 in/Hg for another 36 hours or until bubbles are no longer visible. I then release the vacuum and allow the wood to soak for 24 hours. Finally I heat the wood to 225 for a couple of hours until dry.

The smaller pot worked flawlessly. But since I have moved up to the larger setup I have had nothing but problems with the Lexan cover. Two covers have developed spider like fractures in the plastic emanating where the hardware exits the top. The first cover this happened on the 2nd run. The second cover fractured on the first run. I didn't mention that the hardware is cushioned with butyl rubber gaskets on top and bottom.

I'm sure the fractures are caused by bowing of the Lexan toward the vacuum. I am guessing that I shouldn't use the fractured covers. Any suggestions anyone has to eliminate this problem would be most helpful. Thanks in advance.
 
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Dhop1218

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The short answer here would be to use a thicker lid. How much thicker is a little trickier, and probably mostly guesswork.
when you increased the diameter to 11inches the area that the vacuum was acting upon increased a lot. This means that the amount of force acting upon the lid has increased also, even though the pressure reading is the same.

To calculate the new thickness of lid required we would need to know a lot more about the plastic as I believe lexan is a brand name for some types of polycarbonate.
Hopefully someone else running that size of pot will be able to chime in with what has worked for them.

Also for what it's worth the middle of the lid is probably not the best place to mount the hardware as this is where the maximum stress would occur.

If it were meI'd double the thickness because it's better to be safe than break more lids and spend more money replacing them! This could be complete overkill though!



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Thud 54963

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Duncan, thanks for the reply. I agree about the hardware position. Thought that might be a problem. I also wondered about the thickness. Is 1 inch enough? Also, is lexan the right plastic to use?


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More4dan

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The cracks will start where there are stress risers, any sharp edges. Threads are classic areas for starting cracks in overloaded materials. The center of the lid would be the least stressed area of the lid with the worst being at the edge of the pot. Moving the threaded hole to the side won't help the situation. The lid is likely too thin. The amount a force on the lid has gone up almost 5 times that of the smaller pot and the bending moment has gone up over 10 times with the larger lid. It would be better if you could glue in a threaded nipple to the lid instead of dirrectly tapping the plastic. It would reduce the chance of starting a crack. Someone else will have to chime in on the thickness required.

Danny
 

PatrickR

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if you are using a metal vessel, move the connector to the side of it instead of through the top. Also switching to glass instead of the plastic is good. Check with a glass company to see if they have a scrap of .5" plate.
 

Dhop1218

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I am confused now about the bending moment of the lid! I am assuming that this is a typical vac pot where the edges aren't clamped though.
Either way it would be better to have a lid strong enough that the positions or methods of attaching the gauges etc, doesn't make a difference.
I would have thought that 1" would be enough. Reading around the web and various how tos on YouTube, people seem to be using at least 1" polycarbonate for pots around 12" in diameter and it has worked for them.


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Terredax

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1" should be thick enough. That is what is used on some commercial vessels with capability to 29.9" Hg of vacuum.
Of course it's relatively hard to source, and for a 12"x12"x1" piece, it costs about $250.
Acrylics are easily found in thick dimensions, however, I wouldn't use acrylic.
 

More4dan

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A wireless camera with a flashlight might be cheaper than a clear lid for watching bubbles.

Or you could just slowly ramp up the vacuum to keep from foaming over without being able to watch it.

Danny


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RobS

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My TurnTex chamber is made of clear pvc.

Mcmaster has 12X12x1" clear pvc for $62
https://www.mcmaster.com/#pvc/=1ca7zqn

When ever I used my Lexan Bell, I ended up using a thick walled glass mug, with a bar running across the top to support the center of the lexan bell. However, even doing that, the lexan always cracked, because the injection molded it with the gate in the middle.
 
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Terredax

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My TurnTex chamber is made of clear pvc.

Mcmaster has 12X12x1" clear pvc for $62
https://www.mcmaster.com/#pvc/=1ca7zqn

When ever I used my Lexan Bell, I ended up using a thick walled glass mug, with a bar running across the top to support the center of the lexan bell. However, even doing that, the lexan always cracked, because the injection molded it with the gate in the middle.

A flat sheet of PVC will have flex. It is a soft material.
PVC tube has a higher strength due to the shape.

McMaster does have polycarbonate and the standard 1x12x12 is $156.92. The high strength is $247.91.
 

Thud 54963

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Thank you one and all for your advice. The cost of of 1 inch Lexan took my breath away, just like my wife does. I didn't think other products would offer the durability or the safety. I opted for another solution I hope will work.

I picked up a foot of schedule 40 PVC 6 inch diameter. I am going to cement on a lid on one end and go back to the old lid and see if that works out. If it doesn't work out I'll have to resort to a commercial product.

Thanks.


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magpens

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Would it be practical and wise to place a post in the middle of the vacuum vessel ... a post that would lightly touch the end Lexan end plate ... and perhaps glue thin rubber on the end of the post ?

I agree that your pressure gauge and other hardware should not be in the center of the Lexan.
 
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