Pulling a vacuum for mold materials

[pnolan48]

I bought a 2-gal pressure pot from Harbor Freight, and a 6 CFM vacuum pump online. Some observations I'll pass along:

At first, with smaller molds taking about 0.25 liters, I was mixing the mold materials first, then pulling a vacuum for about fifteen minutes. The difference in the molds was noticeable, as the vacuumed molds were nearly clear amber, while the unvacuumed molds were a cloudy brown. But I did notice that, after fifteen minutes together, the mix was considerably thicker when pouring.

Some of my latest pours were one liter, or half a liter of each part. I was afraid that if I mixed them together before vacuuming, they would be far too thick to pour. So I made a little container to hold each cup level (the pressure pot, or in my case the vacuum port, has a spherical bottom) and drew them down for fifteen minutes. Then I mixed them thoroughly in a Tupperware container, and drew the mix down for another five minutes. The pour was nearly perfect, and the first hull casting, which was nearly 0.8 liters, was better than I have ever achieved. I learned that, when mixing that much casting materials, I had to work very quickly, and pour the mix more quickly than usual!

I've made much smaller molds since, for parts like boat cranes, capstans, barrels, etc. With these smaller amounts, I vacuum down the two parts for fifteen minutes, mix them, then vacuum the mix down for about eight minutes. Comparing earlier molds for a 21-foot RIB (Rigid Inflatable Boat), the latest molds are simply generations ahead.

Anyone else like to share their molding/casting experience?

[Philip H]

No techniques, but let me know when you have the RIBs ready for sale . . . they will mak enice brown water patrol boats for MSO Baton Rouge . . .

[C855B]

'Scuse my n00b-ness to casting, but after drawing down the pre- and post-mixed ingredients, you put the poured molds under a vacuum, too, right?

[diezmon]

I messed around with trying to vacuum too.. then discovered it works just as well, and is easier, to just pressurize during molding and casting.   I put my mold in the pot, pressurize to about 45psi, which makes all bubbles disappear.. then pressurize when making parts too. 

works like  a champ.. and no expanding mess..

[haasmarc]

diezmon,

Be carefull with pressure if your master is hollow!  Don't ask how I know.

[DKS]

I've found vacuum completely unnecessary for moldmaking/casting as well.

After mixing and pouring the mold materials, I place it in a pressure pot and pressurize it to ~80-90 lbs.

When the mold is done, I mix and pour the resin into the mold, then place it in the same pressure pot and pressurize it to ~80-90 lbs.

That's it. No vacuuming. No mold material prep. No resin prep. No dribbling the mold material from a high point, no special mixing techniques, no mold release, nothing. Just mix, pour, pressurize, repeat.

Also, I do not use pressure pots from Harbor Freight. The pots are OK-ish, but the clamps will fail pretty quickly.

[pnolan48]

Hmmmm.

So, when making small molds, and therefore parts, it's easier to pressurize to 50-80 lbs. I'll do that in the future. I not having problems with smaller parts, unless I've designed the mold with built-in air bubbles (one learns not to do that).

When making 17-inch hulls, or 20-inch barges, I'd have to build a pressure container long and wide enough to hold the molding box. The mold materials would be no problem, as they set in 16 hours or so, so pressure could build up gradually. But mixing materials for casting, closing the container, then pumping it up to 50-80 lbs would require very fast mixing, closing, and pumping up, I think, even with slower setting materials. When I'm up to 0.8 liters of casting material, it begins to set up quickly, and gets pretty hot. I'd guess that five minute materials set up in two minutes or less. I guess I'll look at some of the much slower setting casting materials.

I'm also not sure I can build a safe pressure container that large (with larger, "plugged" hulls to come), or afford a compressor large enough to pump it up. So pumping "down" the mold materials before and after mixing might still be the best I can do, and I've been pleased with the result.

The Harbor Freight pressure pot works very well as a vacuum pot. I hardly use the clamps at all, as once the vacuum starts, they are not needed.

As always, thanks for your responses. I value them very much.

[peteski]

I do some simple 1-piece mold castings such as:






As you can see, those are tiny molds.  I have a dual-stage GAST diaphragm vacuum pump and I made my own vacuum pot from a pressure cooker. I use 1" thick piece of clear Lexan for the cover (so I can see what is happening inside the pot.)  That makes things much easier.  The gasket is cut from a piece of rubber roofing material.

I also bought the Harbor Freight pressure pot (for pressure casting) but I haven't had a chance to start experiencing (damn spell checkers) experimenting with it.   DKS mentions that he pressurizes it up to 80-90 psi. That seems unnecessary high. All the info I've seen about pressure casting mentions that 30-50 psi is plenty high for good results.  I don't have the specs handy but I don't even think that the Habor Freight's pressure pot is rated that high for its maximum pressure.

[diezmon]

diezmon,

Be carefull with pressure if your master is hollow!  Don't ask how I know.

LOL..  bummer!

Yep, I always allow for air vents.

[diezmon]

I've found vacuum completely unnecessary for moldmaking/casting as well.

After mixing and pouring the mold materials, I place it in a pressure pot and pressurize it to ~80-90 lbs.

When the mold is done, I mix and pour the resin into the mold, then place it in the same pressure pot and pressurize it to ~80-90 lbs.

That's it. No vacuuming. No mold material prep. No resin prep. No dribbling the mold material from a high point, no special mixing techniques, no mold release, nothing. Just mix, pour, pressurize, repeat.

Also, I do not use pressure pots from Harbor Freight. The pots are OK-ish, but the clamps will fail pretty quickly.

wowza.. 80-90?  I've never needed more than around 40-50.   80 or 90 scares me LOL   Then again, you're probably doing micro parts and the flaws show up more?   

I have a pressure pot from Sears.. got at a garage sale new, and right out of the box the clamps weren't great.  I ended up putting in a bike valve and I leave my compressor attached, set to 45.

All this mold talk is getting me in the mood to make some stuff

[pnolan48]

Does pulling a vacuum also remove moisture from the materials? I don't foresee that much of a problem, as I work in an air-conditioned house, and go through material very quickly. But I do have small quantities of mold materials hanging around in gallon jugs, which I mix up for the small parts. Some of that is getting up to four months old. The water vapor coming out of the vacuum chamber is fairly steady for a while, until it tapers down.

[peteski]

Vacuum pump removes *ALL* the gases in the vacuum chamber.  That includes humidity in the air. Think about it: vacuum is "no air" and "no whatever is in the air".  Like water vapor. Vacuum also lowers the boiling temperatures and certain liquids will evaporate much quicker in the vacuum.

[DKS]

Does pulling a vacuum also remove moisture from the materials? I don't foresee that much of a problem, as I work in an air-conditioned house, and go through material very quickly. But I do have small quantities of mold materials hanging around in gallon jugs, which I mix up for the small parts. Some of that is getting up to four months old. The water vapor coming out of the vacuum chamber is fairly steady for a while, until it tapers down.

Get yourself a can of Xtend-it if you're keeping large containers of material around for appreciable amounts of time.

[pnolan48]

Sorry, there was a typo. I meant "Doesn't pulling a vacuum . . ." as a rhetorical question. Unfortunately, my vision defects are making it hard to proof what I write, so I miss some obvious things. Besides, my brain has been cooked with illusional forumitis by inhaling too many posts for too many years.

I've gone through six gallons of molding materials and six gallons of casting materials in the past five months. If I had bought the materials from the same supplier, with the same chemical make-up, I'd have no problem with the three or five ounces left in the containers, as I could just mix them in with the next batch. On the Internet, low-cost suppliers come and go, and I'm always looking for better materials and methods, so I have twelve gallon bottles, each with small amounts of material, and I'm a Yankee who hates to waste anything. I solved the problem last night and today by making a whole lot of small casting boxes for small part masters that I've made over the years but ignored.

I'd like any thoughts on better methods to mold and cast this 10.6" hull. The superstructure is cast in three parts: forward house, pilot house on top, rear house. The hull is molded with integral bulwarks, as applying them afterwards is a real PITA.



I'm also known to pull a leg or tweak a beard now and then.