Pressure Casting & Compressors

qc · Post by qc » Wed Jan 02, 2008 9:16 am

I'm intending on pressure casting pieces in the near future. I'll be picking up the 2.5 gal pressure pot from Harbor Freight shortly. I was intending on using my airbrush compressor (iwata sprintjet). I see that it can supply the pressure I need, but I'm worried about volume. Not sure the flowrate of my compressor will provide adequate pressure in enough time for a regular or slow cast resin.

Does anybody else here pressure cast with their airbrush compressor? with only a 2.5 gallon vessel? and not have any problems?

Many thanks.

modelnutz · Post by **modelnutz** » Wed Jan 02, 2008 10:00 am

I started casting years ago with an airbrush comp. but I burnt it out after only a few months.
What you really need is a decent sized paintshop style compressor.
Consider getting a small contractor's portable comp. ( I got mine at Home Depot...~ $300.00...been going strong for 3 years now)
You want the type that requires oil....very important!)
Be sure to get a moisture trap as well....water and urethane do not play well together.

Stay away from the oil-less diaphram types....they just can't take the continuous duty.

2 more cents from....
Modelnutz

qc · Post by qc » Wed Jan 02, 2008 10:12 am

M'nutz,

How much did YOU use that airbrush compressor? Just wondering if you were mass producing kits and pushing that compressor every night? once a month...etc? And considering that the resin should set up quickly (20 min) compared to how long i may use the airbrush compressor continuously to paint (hour plus). I do realize we are talking very different pressures from these two activities....just posing the question.

Just curious if i can skim by on my little iwata...cause putting down another $300 isn't pretty considering I need to get a vacuum pump also (~$100).

Then again, can i use my (yet purchased) vacuum pump "backwards"? Sorry for the ignorance (since i've never used one, yet am going to buy one), but are vacuum pumps able to "switch" from vacuum to pressurizing? Maybe some do and some don't?

Many thanx (again)

modelnutz · Post by **modelnutz** » Wed Jan 02, 2008 12:18 pm

How much did I ues it?

Oh, pretty much every day...4-6 hours a day.

Yea, I guess you could say that I was doing production mass casting (with a spraybrush comp.....

)
Guess I learned the hard way

The pressure you're trying to achieve may not be the issue...the volume of the pressure pot may be a bigger issue when using the smaller comp's.
If your comp has to run for a long time to pump the pot to your working pressure...you run the risk of overheating.
I've heard of folks placing things ( bricks, blocks etc.) into the pot to "eat up" some of the volume so the pump dosn't have to work as hard. Myself, I'm filling the small pots to capasity every time I cast
(well, most of the time)

As for using a vacum pump as a comp....I couldn't answer that one..sorry

Good luck, and keep asking questions...I'm sure you'll find the answers here!
M'Nutz

Dr. Snuts, M.D. · Post by **Dr. Snuts, M.D.** » Sat Jan 05, 2008 11:27 am

Vacuum pump? I'm curious how that works in pressure casting...is the idea to suck all the bubbles out instead of compressing them to an invisible size?

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Sat Jan 05, 2008 12:59 pm

I have a couple molds for railing stancions that have a large resivoir on the top. I will oftent pour half the resin into the mixing cup and put that in to the vacuum chamber. Once it "boils" out, I mix the second part and put it under vacuum and again let it "boil" out.

I fill the resevoir with resin and burp the mold. Then it goes into the vacuum. This draws all the trapped air out of the pieces, I pull and release the vacuum several times, then put the mold under pressure to cure.

If all I did was pull a vacuum on an open face mold and then set it on the work bench to cure,... The deaired resin would be sitting at a near equivilant of 15psi..

modelnutz · Post by **modelnutz** » Sat Jan 05, 2008 2:03 pm

I use a vacume chamber to de-gas my silicone before I pour a mold.
This removes most of the air in the silicone so that when I cast under pressure ( in the pressure pot ) I will not get distortions in the casting due to air bubbles just under the mold surface.

So...vacume for the silicone...pressure for the urethane

This is my process...other's may vary....

M'Nutz

Sparky · Post by **Sparky** » Thu Jan 24, 2008 10:27 am

$300 for another compressor?

try $99 for a 5 gallon one. For the pressure pots a regular oil filled compressor is fine. If you need to use it for painting be sure and get an oil trap for that line (you can use coper pipe for an air distribution system.

qc · Post by qc » Thu Jan 24, 2008 2:42 pm

since i wrote this...
i've figured another way to ensure the "volume in time" concern.
i.e....the problem of a compressor being able to provide enough pressure, but not provide adequate flowrate

use an auxillery in-stage tank.

i happen to have a 10 gallon air tank (from when i first started airbrushing - before i bought my iwata). i'm using a 2.5 gallon pressure pot.

when i'm ready to cast (or this even works backwards for pulling a vacuum on rtv), i'll simply fill the tank to 50 psi (or whatever the math is) even if it takes 1/2 hour....then when ready to cast, turn a valve (into the pressure pot) and the pressures between the pot and tank will equilize around 35-40 psi (haven't done the math yet, but should be easy).

it doesn't work as easy with RTV since there is an absolute bottom (30" Hg)...but i can vacuum the tank, release into the pot .....that should equalize to about 24". and just keep the vac pump going. not near the need for instant environment due to rtv not setting up as quickly.

this all makes sense to me, but i haven't read this method anywhere. can anybody find any mistakes in this method? (hope not)

Sparky · Post by **Sparky** » Thu Jan 24, 2008 2:59 pm

This is common, especially for folks doing vacu-forming. There are several DIY diagrams showing 4 tank vacuum systems. It is more critical in that application as your set up time is the time for the vac material to loose its heat. High impact ABS is usually what they are forming.

My compressor is basically a 5 gallon tank with a compressor and a pressure activated 110 VAC switch. Oh and the regulator. you should be able to put a regulator on the other side of the tank so you can set the pressure to the pot. You might have to switch that out if you use the same tank for the vacuum system.

What tank do you have and where did you get it. I tried to convert one of the fill and go tanks but the connector was goofy. I couldn't find anything to let me hook a regular line to it (1/4" npt)

qc · Post by qc » Thu Jan 24, 2008 5:36 pm

i think its this tank..or something very similar
http://www.lowes.com/lowes/lkn?action=p ... 80-W-1010A

are you saying, that once i rip off the brass head off that cat that the threads are typical for 1/4 NPT or something?
i contemplated that a little...but doubted....maybe i should run home and double check?

good idea on putting on (another regulator)...and think i could keep it on during vacuuming, just leave it wide open, no?

i may (against modelnutz) advise just pick up a $50 campbell hausfeld 5gallon/compressor at wally world tho. i'm not going to be a casting factory. can't see dishing out $300 right now.

on the second thought...
i don't even need to rip the current fitting off that tank.
i'll just add a T off the 1/4" NPT currently....and add a ball valve between the tank and the pot. don't see why that wouldn't work.

thoughts?

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Thu Jan 24, 2008 6:23 pm

I only use one pressure regulator off of the compressor.
The pots all need the same pressure, and the pressure is set there.

On each pot, there is a pressure gauge. This way I can verify that
each pot has the right pressure, and isn't dropping. The guage is
also useful to see that a pot is depressurized before attempting
to open one. Also handy on a "T" fitting is a pressure release valve.
I have one that will pop at 60 psi, and another that is adjustable to as
low as 10 psi... I keep it set at 50psi max. This way if the regulator should
get bumped, or I use the air compressor to inflate a bicycle tire up to 70 psi, the release valves will pop if I forget to reset the regulator down to
45psi or less.

DX-SFX · Post by **DX-SFX** » Fri Jan 25, 2008 5:34 am

Doesn't pressure casting just reduce the bubbles in size or does it lose them altogether? Aren't you left with small but high pressure bubbles in the casting when it comes out the pressure pot?

Sparky · Post by **Sparky** » Fri Jan 25, 2008 8:46 am

Umi_Ryuzuki wrote:-snip- This way if the regulator should
get bumped, or I use the air compressor to inflate a bicycle tire up to 70 psi, the release valves will pop if I forget to reset the regulator down to
45psi or less.

And she's a cyclist

The tank you show from, lowes, that was the kind I got. I couldn't change out the presta valve on it easily. I did eventually find something that was the thread but I haven't been able to figure out what hte thread was, and I need the bolt to fill the whole on the paint pot. At one time I had used a pipe threader to re thread the head on portable air tank, but the threads weren't strong, they leaked and the tank can't be pressurized any more. Actually I used it to give a casting demo twice. Filled it to its limit (100 psi maybe) and it was able to fill the paint pot to 40 psi (i bled off some extra pressure).

While casting the resin will try to form air bubbles by keeping it pressurized those bubbles won't form. Also bubbles stuck/formed on the surface of the mold will collapse under pressure and then break free to rise to the 'top' of the mold, their buoyancy is derived from volume and pressure, which is why a tank of compressed air must be weighted down to allow a scuba diver to dive.

The issue 'arises' if you have a large pocket of air and then collapse it down. It will still be at the top of the mold so when you peel off the backing the high pressure air will just bleed out, but the part will already have cast collapsed.

Mold rubber can take that abuse to some extent. In fact when casting you have to maker sure your molds are properly supported or you get parts that look warped or bent. These aren't warped they are formed with that curve or bend or dent. Something one can use when trying to cast multiple sets of panels where only one set needs to fit a curve.

qc · Post by qc » Sat Jan 26, 2008 8:58 am

(i don't have the tank beside me this weekend, but from memory)...the tank i have has a hard presta vavle (no line) on one end, and a 1/4" npt at the other end (no line)...with a pull-release "whistle" valve. seems like i remember hooking up an trigger air whose to clean.

i figure with that configuration, i can just put a "T" on the 1/4" NPT and be good to go.

(crossing my fingers, thats the configuration), if not, then looks like i may be in for the same work you put on yours, spark

i appreciate all this help guys and gals (never new "umi" was a girl...just fig'd the avatar was a fave 'toon or something)

Sparky · Post by **Sparky** » Sat Jan 26, 2008 1:57 pm

That's a great tank you have there, a T should work fine. I'll have to keep an eye out for a sale on those.

DX-SFX · Post by **DX-SFX** » Sun Jan 27, 2008 6:44 pm

While casting the resin will try to form air bubbles by keeping it pressurized those bubbles won't form. Also bubbles stuck/formed on the surface of the mold will collapse under pressure and then break free to rise to the 'top' of the mold, their buoyancy is derived from volume and pressure, which is why a tank of compressed air must be weighted down to allow a scuba diver to dive.

Whoa, hang on there. I don't think that can be correct. The larger the void (regardless of pressure) the more it wants to rise because the resin is heavier than the void. Bubbles are more bouyant in a vaccuum chamber when at their largest than in a pressure pot at their smallest. That's how a vacuum degasser works to get the bubbles out. Also the bubbles form from small quantites of air whisked in as a result of the mixing. If your resin is forming bubbles on it's own as it sets, it's getting old and wants throwing out. Casting resin is the same resin as the expanding polyurethane foam they use in bouyancy compartments in boats but it has an agent in it to stop it foaming. If your resin starts to foam, typically after it's been opened for a long time and has been in contact with the atmosphere, it's because the non foaming agent has broken down. That's why it has such a short shelf life once it's been opened.

A pressure pot will make bubbles smaller but that will disincline them to rise.

If you can mix your resin and hardner while under vacuum, any tiny bubbles remaining in the resin will all but disappear once you release the vacuum.

Sparky · Post by **Sparky** » Mon Jan 28, 2008 8:52 am

I have heard of both techniques for de-airing RTV, vacuum and pressure.

One thing to note though about vacuuming resin or rubber. Many fluids will boil at room temperature under the level of vacuum a good vacuum pump can produce. For example mold max 30 will 'continue to bubble' for ever under my vacuum pump. there's no air in there it is just plain boiling.

Same problem with smooth-on 310 I use. In fact I had a cast last night that I vacuum degassed to deep on the vacuum and it left me lots of bubbles in the cast The small amount of resin in the pour spout foamed up terribly. It was put under pressure after the vacuum but apparently the 40 psi wasn't enough to clear all the bubbles up at the top of the cast. I suppose it is possible if I continued to vacuum it it might stop bubbling, but by then I'd loose 1/3 or 1/4 of the resin in the mold, from foam over flow.

Also the faster set resins get real hot, hot enough to cook an egg if you are casting a thick part. Case in point I had a big anti grav tank hull, I poured a big batch of resin in it. the 320 stuff is clear unless you tent it. I could see there weren't bubbles in the heart of the casting, some at the surface. After it cured the guy that mastered it took the casting to re mill it. He found a huge cluster of bubbles right under the turret (center of the mold bottom). These bubbles had to have formed after the resin had started to kick they would've floated to the top other wise. I believe the bubbles were from boiling heat the resin created during the cure process, I mean resin will bubble if you boil it right?

Also with bubbles int he mold, they will stick in surface detail quite readily. The most prevalent of this was with some surface mount capacitors I molded. Unfortunately the way these are made is that the top of the cap is slightly larger then that base, the base of the metal can is pinched/rolled in circular clamp as the cap is sealed. The leads are then laced through a square plastic mounting piece. When this is molded the pinched/plastic bottom side end's face up. this shape is ideal for holding a big air pocket, even when I used a pipette to blow resin in to the pocket, there tended to be large amount of air retained. This then collapsed distorting nearby parts as well as the cap casting. These were a real pain cause they damaged nearby castings, I could've lived with them not casting but messing up nearby parts was a real pain. they ate up to much time in the pouring process just to get enough air out so they wouldn't mess up nearby parts.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Mon Jan 28, 2008 2:12 pm

If your resin is foaming over the top of the cup,
you are not using a big enough cup.

When my first vacuum pump began to fail, it would pull
just enough vacuum to make the resin or RTV to foam.
Then it would leave the foam and just hang...

When the pump got replaced, the vacuum chamber would
blow clear past foam to boil, once that occured, I knew
there would be no issues.

By stopping the vacuum degass at foam, you are actually creating
an air bubble issue rather than preventing it. If your material foams
to the top of the cup, crack the release valve for an instant, and drop
the foam. Then allow it to boil to the top again. Doing this several times
is better than just stopping because the foam has reached the top of
your cup or bucket. The materials need to settle to a full vacuum "boil".
Otherwise it is not truly being de-aired.

In regards to trapped air in tight places of a mold.
Those issues can usually be worked out by setting your
master at odd angles prior to molding. If you have trapped
air pockets after making a mold, note which side of the mold,
and at what angle the air can be released. This will allow you to
fill and tip the mold to burp that area prior to completely filling
the mold, and placing it in pressure pot

qc · Post by qc » Tue Jan 29, 2008 2:07 pm

i wanted to say thanks to all of you for helping with these questions...

however, i've stumbled onto a couple related issues:

1) are there special airlines that i should use for pulling vacuum? i'm afraid the ones i'm used to http://www.harborfreight.com/cpi/ctaf/D ... mber=54075 may collapse under a vacuum. am i crazy, or how i know which line to use?

2) i got the standard starter Harbor Freight 2.5 gal paint pot for my first pressure pot. i don't have a plexi see-thru top, so i'll be boiling the RTV "blind". what is the recipe for this action? : how long (once i'm up around 28") does it need to be held there, and how much expansion (boiling volume-to-dead volume) can i expect?

3) the "paint pipe" in the aforementioned pot is in the way (of the chamber space). i want to remove it but it's a grizzly to undo. what shall i do or what have you guys done? worst case i can see grinding it off (with elec wheel grinder) and just capping the "outside" (ABOVE the lid) end.

thanx again.

Sparky · Post by **Sparky** » Tue Jan 29, 2008 3:15 pm

3) paint pickup.

Ya the first one I got came off no problem (plumber's wrench), the second one snapped off leaving the threads in there. The last to i got one came off and the other I torqued so hard I manged to damage the seal that the fitting it set into uses. The 2 newer pots all have through lid connections which use a regular rubber seal. It leaks so I hate chaining to use that lid, as the compressor kicks on quite often to keep 40 psi

2)I think the formula is 3 times more volume. I always have to burp my RTV since it would foam over if didn't, just cause using a large tube would require me to scrap and scope a lot more, which introduces air bubbles all over again. The 2./5 gallon paint pots limit your height, a narrow cup would require less scraping and scooping. . .

1) vacuum hoses. Other than collapsing I have notice that the time it took to get to the deep vacuum was faster with bigger tubing. I had 1/4 in nylon line (same kind that the fridge uses to get water for the ice/water maker). Then I went to 3/4 inch big soft nylon tube the one with the built in webbing would be more collapse resistant but i wasn't sure if it would work and it cost more.. Part of the tube into the pump is ridged gas line just to keep the soft nylon from crimping right at the vacuum connection point.
http://www.kc6sye.com/techmages_10_30_05.html

DX-SFX · Post by **DX-SFX** » Tue Jan 29, 2008 9:59 pm

The vacuum pipe that most of the bigger companies use is transparent with a spring/wire coil inside to stop it collapsing.

Sparky · Post by **Sparky** » Wed Jan 30, 2008 11:16 am

That's what I was looking at, the stuff with the white mesh embedded in it? If I had to make a new or rebuild the old setup, that's what I'd get this time around.

qc:
I can probably get you some acrylic sheet. thick stuff that was given to me by another guy in our club, he got it from a clean out of the storage attic at his work. I can even cut it now. The trick will be making a gasket for it. The one I made worked for a long time but it's no longer secure on the acrylic top and is a PITA to place on horizontally mounted paint pots.

I'll leave it to you to think up a way to make the gasket, cause the method I used sucked, if you get somethign that works good please make a little how to page with photos and what not, so I can take another whack at making a seal. I need to be able to degass the gribbly molds (made in the horizontal pot).

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Wed Jan 30, 2008 1:38 pm

ACE hardware sells 1/8 thick, 12" wide rubber sheet by the roll.
If that fits your pot, cut a hole in the middle set it on the pot, and
set your acrylic over that. I had an piece of 1/16" thick cork that
I used as a gasket for several years.

My vacuum hose is a 7/8" id Tygon hose. The wall thickness is probably
1/4". I don't think it would collapse unless I drove the car over it..

qc · Post by qc » Tue Feb 05, 2008 12:34 pm

geez Umi, that tygon stuff is expensive....did you have it willed to you?

even tho it looks neat and helpful, i'm wanting to keep things simple and easy and not use the plexi window top when degassing.

is there a recipe for how long you pull the vacuuum...how many times you burp it, etc...while being blind to it? if i recall, feel like i read ody'slipways mentioning his blind degassing directions ...or something. anybody here?

i may take you up on getting that plexi from you sparky...but would need it cut. PM me and hit me with all the details if you would, please?

umi, how did the cork gasket work? would assume the porosity of the cork would not lend itself to being airtight? however i will take a look at my Ace hardware (my local one sucks...but know of others)...what area of the store was this sheet rubber?

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Tue Feb 05, 2008 4:48 pm

qc wrote:geez Umi, that tygon stuff is expensive....did you have it willed to you?

...

umi, how did the cork gasket work? would assume the porosity of the cork would not lend itself to being airtight? however i will take a look at my Ace hardware (my local one sucks...but know of others)...what area of the store was this sheet rubber?

Essentially, The tygon tubing and the first RV8 ATEX Rotary Vane Vacuum Pump were just items that people I know had sitting around collecting dust. Kind of sad really... poor things. Same goes for the pot.

As for the cork, I had some 1/8" sheet left over from an architectrural project. When the pump started to fail, and couldn't run the large pot, I built a lid for the smaller pressure pot and threw a cork ring between the rim of the pot and the lid. Worked perfect.
just like wine bottle corks keep the wine in....

The pump couldn't evac the large pot, but it was able to keep up with the smaller one. At least till this year. Considering rebuilding it as a back up.

The new pumps landed in a driveway after they had caused trouble at a warehouse. Someone tipped them on their side to look at the carriages, and oil spilled out the top. Apparently this had happened before, so the owner wanted them out of the building. They got dumped on a friends driveway, and then delivered to me.

One pump was labeled "column", and the other "gun"... Probably an Electron microscope component.

I wouldn't run your Vacuum chamber blind, I think you would find too much of your materials on the bottom of the pot. 1/2" thick plexi/perspex should be fine if you have a 10" diameter or less sized pot. Just work slowly when creating the port or you are liable to crack the piece.
Two minutes is pretty typical of a quick vacuum run for me.

The rubber sheet was in the back of the store with "sheet/roll" material. Probably a plumbing item.
They sell six inch squares of it in blister packs, but you will want the large sheet.

DX-SFX · Post by **DX-SFX** » Tue Feb 05, 2008 4:48 pm

There is no formula for degassing. It depends on the viscosity of the liquid and the quantity. A perspex top is almost mandatory as you need to see if your cup is about to overflow or if all the bubbling has ceased. I'd hate to say it was impossible to do without a window of some sort but I guarantee you'll be cursing and swearing that you didn't have one when you actually start doing it particularly with unfamiliar materials. Trust me, it's worth the effort and will save you far more time than it takes to arrange.

qc · Post by qc » Wed Feb 06, 2008 8:57 am

alright, alright...

i'm going to make some calls....and price a clear lid.

what should i be asking for?

how thick minimum?
and which type of plastic...plexi? anything else? (assuming diff materials may require diff thicknesses)

many thanks

forgot...
everybody has been cool about giving options on the vacuum hoses...but does anybody know if the yellow spring hose i linked to (from harbor freight) would be satisfactory for pulling 29" hg or would it collapse. please explain if your response is experience, calced science or opion.

thx again

DX-SFX · Post by **DX-SFX** » Wed Feb 06, 2008 1:49 pm

How thick is based on the diameter. A twelve inch diameter degassing chamber with a high vacuum capability typically has a minimum of a 3/4" Acrylic (Perspex/Plexiglas) lid. Be very cautious about going thinner. You might get away with it BUT it gives no warning if it's going to break and if it does, make sure you're no where near it. It goes with a hell of a bang. Err on the side of caution every time.

Umi_Ryuzuki · Post by **Umi_Ryuzuki** » Wed Feb 06, 2008 2:12 pm

DX-SFX wrote:How thick is based on the diameter. A twelve inch diameter degassing chamber with a high vacuum capability typically has a minimum of a 3/4" Acrylic (Perspex/Plexiglas) lid. Be very cautious about going thinner. You might get away with it BUT it gives no warning if it's going to break and if it does, make sure you're no where near it. It goes with a hell of a bang. Err on the side of caution every time.

Having experienced this

, I have to agree... 3/4" is best minimum...

Do expect a price around $80.
Nothing compared to loss due to physical injury.