Turning a compressor into a vacuum pump?

[choc-ice]

I've had some success casting resin under pressure, I bought a compressor and after some fine tuning of the process it seems reliable to get excellent quality parts out with no voids.

Now I need to make some silicone moulds, I've done a few but I can't seem to get rid of the air bubbles and of course they don't show up until I've done the first cast. Degassing the silicone is the way forwards, but after spending my cash on a compressor I'd rather not fork out another £100+ on a vacuum pump.

Has anyone used something like a venturi from a compressor to make a vacuum?

[Nigel Heath]

A compressor from an old fridge will pull a sufficient vacuum to degass silicone resin. I once made a set up from a glass demijohn with the bottom cut off and a thick turned aluminium base plus a silicone rubber gasket. Its amazing how much gas comes out of mixed resin.

Nigel

[Fazer]

Hi Choc ice,

I vacc my silicone by using the suction pipes on the compressor.

By connecting the suction pipes to the same pressure vessel (also replacing the pressure vessel lid with a sheet of thick plexiglass & rubber sheet) it can double as a vacuum unit. I have been using mine now for a few years with no adverse effects on the compressor. (BTW, I am an engineer, so a bit of experience before you ask! )

Also remember that when you vac the silicone it will double in size as you vacc it so make sure your container can hold the volume...I know from experience

Mark D

Edited May 21, 2013 by Fazer

[hacker]

Hi Choc ice,

I vacc my silicone by using the suction pipes on the compressor.

By connecting the suction pipes to the same pressure vessel (also replacing the pressure vessel lid with a sheet of thick plexiglass & rubber sheet) it can double as a vacuum unit. I have been using mine now for a few years with no adverse effects on the compressor. (BTW, I am an engineer, so a bit of experience before you ask! )

Also remember that when you vac the silicone it will double in size as you vacc it so make sure your container can hold the volume...I know from experience

Mark D

l have heard of this. l am looking for alternatives to purchasing a vac pump. l would be interested in how the suction pipes are put together.

[Nigel Heath]

In my experience this will foam up to three or four times the original liquid volume during degassing, so as Mark D says use a big container or there will be tears.

[Caerbannog]

How much vac do you need to degas? -1bar??

I am currently experimenting with a simple DIY vac champer for resin castings: An old old 5l pressure cooker makes the chamber: I shut the valve and added a pipe to the cooker which is connected with a two-way handpump. A gas valve is installed in the pipie which I close when I sucked out enough (?) air.

I have no clue how low the negative pressure is but when I open the in line valve you can clearly hear the air going back into the pressure cooker. The cooker is sturdy enough to widthstand my handpump - all previous chamber attemps failed impressively :-D

[kev67]

Depends how big your Silicone mould is but I put something that vibrates (naughty not that type of thing) next to the mould for about half an hour to draw some of the air bubbles to the surface

[Fazer]

gentlemen, I will take a couple of photos of my set up soon & post the pictures.

[Fazer]

Right Gents, here is my original set up using a fridge motor & using the suction pipe to de-gas. The orange air brush tube is connected to the single pipe on the fridge motor which gave a good suction.

My second set up is this connected to my twin suction pipes on the air brush compressor which I have now been using for a few years.

sorry, got a bit cut up in editoring but the orange pipe is connected to the 'Y' piece from the 2 pipes. I don't use a vac gauge & judge it purely through watching the silicone until it subsides, never let me down yet...

To pressure cast, I have a metal lid which I bolt on & reverse the orange pipe to the airbrush outlet & pressure up to 60PSI. BTW, The pot is made by me at an engineering workshop & the pot is pressure tested to 150PSI, don't want any accidents at that pressure...

Mark D

Edited May 22, 2013 by Fazer

[Caerbannog]

Do you only degass the silicone and resin before the casting?

I only once tried to cast with vaccuum: I poured the resin in the mold and placed it in my vac chamber. After I sucked the air out as good as possible I left the vacuum until the resin set completely. But the resin looked like a sponge... Thereafter I only kept the vacuum for about a minute or two and then let the air back in. No problems with my resin becoming a sponge after that. Is this a comon problem or just my resin???

[Fazer]

Do you only degass the silicone and resin before the casting?

I only once tried to cast with vaccuum: I poured the resin in the mold and placed it in my vac chamber. After I sucked the air out as good as possible I left the vacuum until the resin set completely. But the resin looked like a sponge... Thereafter I only kept the vacuum for about a minute or two and then let the air back in. No problems with my resin becoming a sponge after that. Is this a comon problem or just my resin???

I don't usually vacc the resin as I make sure the mixing is gentle & without frothing. The pressure casting 'compress' what air bubbles are left to a point where they are not a problem. I have used some old resin which is liable to 'sponging' but a short 'vacuum' as you mention took care if it.

Mark D

[Caerbannog]

Hi Mark,

I thought the negative pressure will suck the air bubbles out of the resin/mold so resin gets in every tiny place of the mold. How you describe it I should make the short vac first to take care of this and after that set the whole thing under positive pressure to keep possible gasing bubbles to a minimum? Interesting - I will try tonight :-)

Cheers,

Rene

[Fazer]

Hi Mark,

I thought the negative pressure will suck the air bubbles out of the resin/mold so resin gets in every tiny place of the mold. How you describe it I should make the short vac first to take care of this and after that set the whole thing under positive pressure to keep possible gasing bubbles to a minimum? Interesting - I will try tonight :-)

Cheers,

Rene

Make sure you do it quick as that bloody resin goes off prety sharpist!!

[Caerbannog]

Ha! I poured some resin yesterday: First I used vacuum to degas the resin and mold for about a minute and then let the resin cure under pressure. This made a huge improvement over previous castings. Thanks for this tip.

Btw: Does anybody here uses mold release agents for the silicone molds to give them a longer life? If so what do you use and how do you clean the resin parts afterwards? (sorry that this is quite off topic...)

Cheers,

Rene

[Fazer]

Hi rene, I have used it occasionally but the silicone moulds usually last for a good 20-30 castings by which time I have enough of what I require

I use something like acetone (nail polish remover) or carburetor cleaner as Cleaning off fluid

Mark D

[choc-ice]

Thanks, everyone! I don't get a reminder that replies have been made so haven't checked back since I posted...

I'll have a look on my compressor to see if I can use the suction port onto a vacuum container, I already use the compressor for pressure casting the resin with very good results so hopefully it'll be a cheap fix for the vacuum. If it works I'll get a new lid made up from 10-12mm clear plastic, it's always nice to see the air bubbles being pulled out!

Should I just vacuum the silicone after mixing, or do it while the mould is curing? The slight worry is that I don't get much of a cure time, it's only a few minutes if I mix the silicone as per the ratios on the instruction sheet.

[hacker]

one school of thought for me was to find a small compressor, the type you would use in your car to pump up your tire if there was no other means available. I been looking for a AC/DC version in which l can place the pump inside the chamber and doing the appropriate modification to make it work

sort of like this version but l am open to other options too

[CarLos]

If you have one of those big and noisy compressors it's even easier to use it as a vac source: just link your chamber to the head, where the air filter is screwed. You'll get more than 25 in Hg in less than 30 s.

Carlos

[Fazer]

Should I just vacuum the silicone after mixing, or do it while the mould is curing? The slight worry is that I don't get much of a cure time, it's only a few minutes if I mix the silicone as per the ratios on the instruction sheet.

I always vacc my silicone when I have mixed it then pour over the item to mould. This prevents any chance of air being trapped under any detail or cavity. I then re-vacc again as a safeguard...I haven't lost a mould since...

mark D

[heloman1]

One way to ensure longevity of your moulds it to take care of them, especially if you only want to cast a few parts and then come back on a later occasion and cast more.

Don't allow the resin to stand in the mould for a prolonged time (longer than the set up time), always wash your moulds in hot soapy water once the casting session is completed. Then dry and store your moulds (an airduster works well) on a rack in a warm cabinet (a box with a 40watt globe/lamp inside) with any openings facing downwards. The latter allows any volutiles in the rubber from the resin to escape.

Vacuum of 4+ bar should be sufficient to de-gas the rubber. I've heard of a venturi device which attaches to a garden hose and pulls an adequate vacuum, which might be the cheapest alternative but is a waste of water!

For those who cannot afford a vacuum pump the old method of slowly trickling the rubber over a flat piece of palstic or Perspex held at an angle in the mouldbox works well and the potlife of the rubber will allow you plenty of time to pore the mix.

To extend the pot life of the resin, keep it at a cool temperature in a fridge of on a concrete floor. The resin being exothermic generates it's own heat, so the colder the resin the langer the pot life.

Colin

[Fazer]

One way to ensure longevity of your moulds it to take care of them, especially if you only want to cast a few parts and then come back on a later occasion and cast more.

Don't allow the resin to stand in the mould for a prolonged time (longer than the set up time), always wash your moulds in hot soapy water once the casting session is completed. Then dry and store your moulds (an airduster works well) on a rack in a warm cabinet (a box with a 40watt globe/lamp inside) with any openings facing downwards. The latter allows any volutiles in the rubber from the resin to escape.

Vacuum of 4+ bar should be sufficient to de-gas the rubber. I've heard of a venturi device which attaches to a garden hose and pulls an adequate vacuum, which might be the cheapest alternative but is a waste of water!

For those who cannot afford a vacuum pump the old method of slowly trickling the rubber over a flat piece of palstic or Perspex held at an angle in the mouldbox works well and the potlife of the rubber will allow you plenty of time to pore the mix.

To extend the pot life of the resin, keep it at a cool temperature in a fridge of on a concrete floor. The resin being exothermic generates it's own heat, so the colder the resin the langer the pot life.

Colin

Top advice Colin, I also rotate my resin tins slowley after a storage time to remix the resin & activator as they tend to 'settle' & you can get some thickening of the fluids at the bottom of the tins. But slowley rotating them remixes the liquids.

Mark D

[choc-ice]

Progress…. I took the cover off my compressor and it looks a lot like CarLos’ pictures. So I’m just trying to get all the fittings I need, Machine Mart today couldn’t help so I’ll try somewhere different tomorrow.

One thing that I’ve read a few times on the internet (that well known reliable source) is that if you want to cast resin under pressure then you should cast your silicone under pressure too. I was planning to vacuum the silicone then pressure the resin, exactly as Fazer describes.

[intolight]

hello Brit Modelers,

I'm trying to build a vacuum chamber from an old airbrush compressor by switching the hose to the threaded intake. It creates a vacuum but will not degass the silicone or clear casting material. (pic below) Any ideas on my design fail? Is their simply not enough suction (i have a large air compressor but the intake is not a threaded hole, it's a large plate) or do i not have a tight enough seal?

Any advice would be greatly appreciated.

new here and i see this thread is a bit old. So hoping to connect.

thanks !!!

grgeory

[Fazer]

Hi Greg, I think your threaded intake will not draw enough volume of air to create a good vacuum. Diaphragm pumps like yours are good for medium pressures but you need a good draw from a fridge type motor or compresser to work!

(Good to resurrect old threads for interest!)

Mark D

[choc-ice]

If it's any help I failed on this idea. I got all the fittings and connected it together but it just wouldn't pull enough vacuum to degass the silicone properly. I got around it by casting the silicone under pressure instead and that works perfectly, the compressor can cope with it easily and there are no bubbles at all in my mould.