[SPEEDRE]

I'd like some comments about using a regular vacuum cleaner, or small shop vac for resin casting. I don't plan on doing a lot of it, but it would be nice not to have air bubbles and voids in my castings. Generally resin casters put the newly poured molds in a vacuum chamber to expend the air, I want a D.I.Y. version. I thought to use a valve of some sort to apply the vacuum by diverting the suction to a lexan enclosure purpose built, from .375" lexan. for medium vacuum so I can see whats happening. What can you add to this idea. All comments are welcome. Thanks.

[RICHARD ZASTROW]

I may be demonstrating my chemical ignorance here. Do the "air bubbles" contain vapors that could "fog" your Lexan surface(s)?

Dick Z

[ger21]

Build a vacuum pump from Joewoodworker.com and you'll get much better results. I don't think the vac will work good enough.

[rowbare]

I don't think you will get enough vacuum from a vacuum cleaner. It seems to me that need something like 28 in hg or better to deaerate though I don't remember where I got that figure from. A cheap HF vacuum pump will do the job though.

Freeman Supplies had a good library of videos. At least one of them deals with vacuum degassing.

bob

[brian257]

I did some small cast acrylic lenses a few years ago and it was important to vacuum degass to keep bubbles out of them. The size was small so I was able to do a very effective job degassing with a 10cc syringe. I mixed the resin then sucked up about 1cc in the syringe. Then I put my finger over the tip and pulled the plunger all the way out. Not sure what vacuum that creates, but it foamed up the mixed resin and got all the bubbles out.

Also keep in mind that you want to vacuum degass resins then presurize the resin and mold after it is poured. You can pressurize them in a pressure pot and standard compressor. If you do just the pressurization and skip the degassing it will be better than doing nothing at all.

[RomanLini]

Vacuum degassing needs much higher vacuum than a shopvac. It also needs a higher vac for more viscous liquids, and to remove smaller bubbles.

Even a single stage vac piston pump is probably not enough you need a dual stage 2 piston pump or oil vane pump. There are charts on the net showing how high a vacuum you need to remove bubbles of X size and various viscosities.

[SPEEDRE]

Yep, It looks like it is not going to work. Need at least 28 in hg.

[wildwestpat]

Suggest you think about using a the compressor from an old freezer or car air con. If even that is too much $ then this link will do the job for small qualities of resin Chapter 6: Aerodynamics: Homemade vacuum pump and water pump a bicycle pump with the washer reversed is too short to pull a satisfactory vacuum.

The problem with the domestic vacuum cleaner is that it uses the air flow to cool its motor and if you run it without letting it \draw air it will burn out. Industrial vaccum dust extractors have the dust flow and motor cooling as separate functions and these will work is the pulled vacuum is enough but the noise is another matter.

By the way it helps to degas if the resin is warmed and arrangements made to add and mix the catalyst under reduced pressure. I did this for polyester resin casting for the kids when they were having problems with bubbles in their models made with commercial rubber moulds intended for plaster. Plaster proved too weak hence the resin!

Regards - Pat

[SPEEDRE]

Thank you this is cool, I have all this stuff on hand. This is sooo simple man thanks.

[SPEEDRE]

Re The Aerodynamics Homemade Vacumm Pump link. It is based on 1.00 dia. PVC pipe pulling a max 26 in. of mercury with a three foot stroke. What if I changed the dis. of pipe to 2.50". and shortened the stroke to 20 in., could I achieve a max of 32 in. of mercury?. Basically increasing the dia. by two and halving the stroke to minimize the size of the hand pump. I'm not too hot with math calculations, what do you think.

[ger21]

29.9" is a perfect vacuum, so you won't get to 32".

I doubt changing the sizes will make any difference, but I'm no expert.

[keebler303]

changing the size of the bore changes how hard you have to strain to work the pump. At peak vacuum you will have close to 15psi of atmosphere pushing against you on a 2.5" diameter circle. That will mean close to 75 lbs of push or pull to achieve "full" vacuum. A 1" diameter would be less than 15 lbs of force. How big are your arms?

Matt

[SPEEDRE]

Just trying to make it smaller and compact.

[keebler303]

A shorter stroke also makes your maximum attainable vacuum lower because you have some "dead volume" in the pump. The air space around the end of the plunger and up to the o ring means that you always start with that much air, regardless of how much you pump it. If the stroke was say 1/2", you would never get any vacuum at all.

Best case is a really long plunger so that the dead volume represents a small portion of the total displacement of the pump.

Matt

[SPEEDRE]

O.K. Then how could I build a manual pump capable of 30 in. hg with a short stroke, approximately, using a check valve to control the flow of air/vacuum and PVC pipe components? Any ideas. Want to keep it small but powerfull.

[keebler303]

Minimize dead volume. This equates to combustion chamber volume in an engine. The more dead space you have the less effective each stroke will be.
Use a good check valve, perhaps a spring loaded ball bearing on a viton seat?

What is your idea of short stroke and why does it have to be short stroke if you are working it by hand?

Matt

[SPEEDRE]

O.K. What if I have an 18" stroke, can I achieve 28" hg.? I want to make it compact, and cheap, so I don't take up too much space in my Man Cave, don't want to garner the ire of my spouse! I may only have a chamber that is at max six inches in dia. and maybe, if I can find the right stuff, six in. in height. Do these measurements make sense for this idea?.

[keebler303]

You need to use the Ideal gas law. Wikipedia will provide some background.

For your use it states:

(P1*V1)/T1 = (P2*V2)/T2

P = pressure
V = volume
T = temperature

We will assume for now (possibly incorrectly) that the temperature does not change so it simplifies to:

P1*V1 = P2*V2

Lets the pressure is 10 to start and the volume is 1. If we increase the volume to 10 (by drawing the piston out of the cylinder in the pump) then the pressure will now be 1.

I'll draw a picture and post some more detail in a bit.

Matt

[keebler303]

See attached picture.

Lets assume the ideal case where the vacuum vessel has zero volume.

When the piston is retracted, the volume is the dead volume only and it is at atmospheric pressure. Then you extend the piston to draw a vacuum in the pump. Your volume is now the swept volume plus the dead volume. knowing these volumes, you can calculate what the pressure will be when the pump is extended. When you retract the piston for a second stroke, no air will exit the pump because the air inside will still be at atmospheric pressure when you retract the piston fully. This means that the maximum attainable vacuum in absolute pressure would be like this:

dead volume * atmospheric pressure = (dead volume + swept volume) * lowest possible pressure

lets put some numbers to it:

dead volume is .5 in^3
swept volume is 36 in^3
atmospheric pressure (STP) is 29.9 in Hg

.5 * 29.9 = 36.5 * Pressure

Solving for pressure gives .399 in Hg absolute pressure. This would correspond to about 29.5 in of vacuum. Pretty darn good.

Notice that dead volume will ruin your chances. Lets try 4 in^3 dead volume

4*29.9 = 40 * Pressure

Now the Best pressure we can get is about 3 in Hg or 27 in of vacuum.

These calculations are ideal and could drop by half or more if the pump is not well constructed or has leaks.

To maximize the chance for success, you want the swept volume to be as large as feasible. This is generally done with a longer stroke as a large bore requires significant force to operate.

Assuming you could achieve 28 in vacuum in a 6 in bore, it would require 400 lbs of force to pull the piston out of the cylinder. I am guessing that may be a bit much for you. You can work the calculations and see what is feasible. A 2in bore would need only 46 lbs of pull at full vacuum but you would have to make the cylinder longer to get a larger swept volume.

Clear as mud?

Matt

[SPEEDRE]

O.K. So it looks like I will stick to self gassing mediums for my really small projects. Thank you all for your thoughts and comments.