Too much vacuum crushes white foam - need regulator or other technique  
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All Forums >> RC Airplanes >> Composites Fabrication And Repair >> Too much vacuum crushes white foam - need regulator or other technique
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Too much vacuum crushes white foam - need regulator or ... - 6/13/2003 5:55:46 AM   
DMcQuinn



Posts: 1588
Joined: 1/9/2002
From: Chesterfield, MO, USA
Status: offline 		This is what happens when there is too much vacuum. The foam collapsed into the hole that I had drilled for the servo wires. This is a 35% Cap wing made of 1Lb white foam covered with 1/16 balsa. I had prepared the wing core by drilling out a hole for the phenolic tube and another hole for the servo sires to run through. I installed the phenolic before the balsa skin and I put plywood support at the end of the phenolic.
There are two problems with this I think. First, you should not cut the servo wire hole until the skins are attached. Secondly, I think I have way too much vacuum, because the entire wing is compressed even in areas away from the access hole.
The phenolic tube seems to have survived the vacuum process, so I guess it is OK to install it first (you really need to in order to get the end support installed.
I am using a vacuum pump acquired for free that has no gauge or regulator. I have successly covered smaller sample pieces without problem, but obviously for this large wing I need less vacuum.
Is there any simple technique to reduce pressure (like putting a pin-hole in the vacuum line)? CST has a gauge/regulator for $63.
In the picture, the blue stripes are masking tape strips I use to hold the balsa together. You can see the indentation and wrinkles of the over-compressed wing. The thickness of the wing at its thickest point is about 1/2 inch less than it is supposed to be.

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Too much vacuum crushes white foam - need regulator or ... - 6/13/2003 6:15:52 AM   
A5fly


 

Posts: 36
Joined: 5/29/2003
From: Stillwater, OK,
Status: offline 		I can sympathize. One of the first white foam parts I made, I not only used too much vacuum, I used too much temp in the oven. Imagine what cottage cheese would look like if you covered it in carbon and you have the idea...

You can make a low cost vacuum reducer with a simple 2-way air hose valve. You can do it one of two ways: First, put it in-line with the hose that goes from the vacuum to the bag. Closing the valve reduces pressure. The other way that I prefer) is to use it as a bleed system. Put a "T" fitting after the pump. One line will go to the bag, the other line will go to the 2-way valve. Opening the valve will reduce the vacuum pressure.

Hope that helps.

(in reply to DMcQuinn)
       Post #: 2

Too much vacuum crushes white foam - need regulator or ... - 6/13/2003 9:28:55 AM   
probligo



Posts: 354
Joined: 5/13/2002
From: opononi, NEW ZEALAND
Status: offline 		I have seen two (#8 wire) versions of this...

First - used a vaccuum advance mechanism off an old carby to operate a microswitch, which turned the pump on and off. He had a small scale pre-calibrated using a guage (borrowed) to create a scale for adjusting the vacc level.

Second used a small bowl (metal) with input and output tubes as an oil trap. The top of the oil trap was a neoprene sheet (thickness unknown but I imagine fairly substantial). The arm to a microswitch was set in the middle of the neoprene to "measure" the vacc level and operate the pump. Total movement of the neoprene was about 5mm when operating.

(in reply to DMcQuinn)
       Post #: 3

Too much vacuum crushes white foam - need regulator or ... - 6/13/2003 10:12:25 PM   
Mluvara



Posts: 1429
Joined: 1/14/2002
From: San Jose, CA, USA
Status: offline 		Usually, you don't want more than 6" of Hg on white foam.

I built a vacuum bagger that has a reservoir and a switch that is selectable for its amount of pull between 0 and the max pump rating. Attached here is the switch that I use and have found at local surplus stores for $5-10. It's part # is E1S-H-VAC and is made by Barksdale, witha 0 to 30" Hg range. The switch is simply connected to the reservoir, and inserted in the circuit to the pump so it turns the pump on and off as needed.

Info:
http://www.barksdale.com/products/pressure/e1s_e1sh_vac.html
http://www2.barksdale.com/draw/3395.pdf
http://www.barksdale.com/products/pressure/PDF_Press/E1S_pg12.pdf

You'll need either a one way valve or a solenoid that activates so that from the reservoir to the vacuum bag is sealed. Otherwise, it will leak out the pump.

Michael

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_____________________________

Michael Luvara
RCATS - http://www.rcatsystems.com
Luvara Air Racing - http://www.LuvaraAirRacing.com

(in reply to DMcQuinn)
       Post #: 4

Too much vacuum crushes white foam - need regulator or ... - 6/14/2003 9:38:41 AM   
A5fly


 

Posts: 36
Joined: 5/29/2003
From: Stillwater, OK,
Status: offline 		If your pump is a type that can run continuously (such as the diaphragm types), you won't need a reservior or a regulator. Just the simple bleed will do nicely, and it is the lowest cost option.

(in reply to DMcQuinn)
       Post #: 5

Too much vacuum crushes white foam - need regulator or ... - 6/15/2003 6:22:38 AM   
canadianjosh


 

Posts: 253
Joined: 3/2/2003
From: victoria, BC, CANADA
Status: offline 		I ran into this problem when i first built my vaccum bagger. I have a simple solution. Put a brass T fitting inline in the tube that goes from the vaccum to the bag, on the open end of this put on some more tubing, then install a fine thread screw gate type valve, this will alow you to bleed different amounts and even to turn off the bleed. works great for me.

(in reply to DMcQuinn)
       Post #: 6

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