Hello and welcome to another Easy Composites video tutorial.
We're going to be just taking a quick look at how easy it is to degas RTV silicone rubbers
and casting resins using our complete vacuum degassing system.
Degassing is a highly effective process for removing trapped air from within mixed materials
such as RTV silicone rubbers polyurethane casting resins infusion epoxies; all sorts
of materials where air can become entrapped when the two parts of a material are mixed
together and if we have a look at this example here, which is an RTV silicone rubber which
has been mixed with its catalyst and allowed to cure at room temperature without any degassing
and as we can see we've got lots of trapped air within the silicone, particularly on this
example this has not been mixed in a way to reduce the air entrapments and we can see
the effect quite clearly.
But compare that to this which is exactly the same material, same silicone, but this
has been degassed in this chamber and then allowed to cure we can see that this is completely
free of any air entrapment whatsoever, so this would have a far improved tear strength
over the undegassed version and also an improved surface finish as well.
Other materials that we might want to degas, this is a polyurethane, water clear polyurethane.
Now in this example again this has been mixed, part a and part b together, and then allowed
to cure without any degassing.
Now we have to look a little closer so if I hold this still you can see very fine air
bubbles trapped within the material and then we compare that to this one, where we've got
a perfectly clear material, no trapped air bubbles whatsoever.
A degassing chamber is very commonly used for degassing resins once they've already
been poured into a mould, particularly moulds with fine surface detail, and especially if
they've got undercuts and the reason can be ably demonstrated using this nitrile glove.
If we tie a knot in it, not trying to get any air particularly trapped within the glove
itself but inevitably there'll be a small amount of air in the glove.
Tie a knot drop it into the degassing chamber.
We pull a vacuum on this now, what happens immediately as soon as the air is evacuated
from this chamber, so we've got very low pressure in the chamber, the tiny amount of air that's
inside the glove expands hugely so it's volume increased massively.
Now we've sped this footage up slightly, this actually took roundabout two minutes but it
does give you a really good demonstration of why we degas and what the whole principle
of it is which is to expand the air to a bigger volume.
So as soon as we let the air back into the chamber, so the degassing chamber becomes
at normal atmospheric pressure the it crushed that air back down to its original size.
I'll try and explain why we might want to do this by means of a diagram on the board.
So if I just draw a random shape that we're trying to make a casting of.
I've got a couple of details that I've drawn here at different scales.
So this is a large significant part of the moulding and it's a potential, or it is certainly
an undercut which is likely to have resin not flow into it properly resulting in a large
air cavity which means a significant part of the casting of a part would be missing
if we didn't do something about this.
Now, when we've got something at this scale, we would generally have a vent hole that we
would have put in at the time when we made the mould using a drinking straw or a dowel
or some other means so that this whole can breath, and that means that as the resin is
poured into the mould it can carry on rising up into this area because this won't become
trapped air, because the air can escape as the resin fills it.
Now what we can't do is have vent holes to every single fine surface detail, a nose,
a chin, tiny leaves on a casting, it just isnt pratical you'd have vent holes everywhere
and you'd be forever finishing the part.
So that's where the degassing chamber comes in and it really is the only way to tackle
this problem of fine undercuts, such as this once we've demonstrated or I'm trying to demonstrate
here.
So what we would do is pour the resin into the pot and then load the whole part in the
mould into the degassing chamber.
When we draw a vacuum on the whole part then trapped air that's been caused where the resin
has filled into these fine surface details would expand significantly just like it did
with the glove that we showed earlier.
Now when it does so those larger air bubbles will come out and rise to the top you'll still
be left with a cavity but there'll be really nothing in that apart from a very minute amount
of air that's expanded significantly.
When we put the pressure back on by venting the degassing chamber to atmosphere then the
size of those air pockets, or the size of the air will contract hugely so would take
up a much smaller volume just like the glove did when it shrank back down.
When this happens it sucks resin back up into these cavities so you'll be left with almost
negligible, in fact probably negligible, you probably wouldn't see it on the finished part,
and the resin would be fully occupying what was originally a cavity, and so by doing the
degassing process with the resin already in the mould we can ensure all these fine surface
details and undercuts are filled with resin properly on the finished parts.
The complete vacuum degassing system available from easy composites is as the name suggests
a full turn key solution for degassing.
When you buy the product you have a choice between this large vacuum pump and this smaller
one.
The way to decide which pump is right for you is simply to look at the materials you
expect to be degassing.
If you're working with highly reactive materials, like fast cast polyurethanes or water clear
polyurethanes and these tend to have quite a short pot life so you need to degas them
as quick as possible so that you can get on with pouring them before then start to cure.
If that's the case and you're working with those materials then the larger pump because
that will empty this chamber in about thirty seconds.
Now silicon rubbers tend to have much longer pot lives so if you're only considering and
only want to be able to degas less reactive materials, like the silicones then this small
pump will pull down this chamber in a roundabout three minutes.
Now for a typical silicone rubber that's plenty of time to degas it fully and then use it
before it's anywhere near the end of its pot life, so if you're only working with slow
materials the small pump's fine if you're working with a mixture of those materials
or generally fast then go with the large pump.
Setting up the system is as easy as can be.
It would be the same whether you were using the large or the small pump, so we'll pop
the small one away and we'll demonstrate the system using the large pump.
So taking the silicone hose that's included in the kit push one end onto the pump and
the other end onto the degassing chamber itself.
If you're using the system for the first time like we are here today then it's very important
to remove this dust cap from the vacuum pump and also ensure that the pump has got it's
all filled up to the correct point the oil filler point is here on the vacuum pump.
So with that done we're ready to go and I can mix up some silicone to show you the degassing
process.
It's the high viscosity of the silicone itself which makes it inevitable that we're going
to mix in air at the same time we're trying to mix in the catalyst.
Now it is so essential that the catalyst is thoroughly dispersed that we don't concern
ourselves too much with the air entrapment that will be happening while we're doing it,
particularly as the catalysts tend to be a very low viscosity material so they'll always
sit on top of the silicone rubber but that's not a problem and we'll just carry on mixing
and let the degassing chamber do its work.
Now if we just get close up on this mix silicone we can see that is it absolutely full of trapped
air bubbles within the rubber itself so this will be an interesting comparison to take
a look at that and compare it when it comes out of the degassing chamber.
With the silicone inside, put the lid on the chamber and that's just a push fit onto this
silicone seal.
So looking at the valves that we've got here, this is the vent valve which lets air back
into the chamber so we want to ensure that's closed.
So a valve is closed when it's at right angles to the alignment of the hose or the fitting,
so that's closed.
We've got the valve that connects the chamber to the vacuum pump we want that to be in the
open position.
We've got a valve on the vacuum pump we'll leave that closed for now, we'll switch on
the vacuum pump, as soon as we do we can open this valve which will start pulling air out
of the chamber and then immediately we're going to see the silicone start to degas and
this vacuum gage registering very quickly the vacuum level we've got.
So let's do that, vacuum pump on.
Open the valve.
So you can see the needle start to rise very quickly and then if we have a look inside
you can see that very quickly the silicone starts to expand and froth and foam as it
begins to degas.
So we've sped this footage up slightly, we're roundabout the minute mark here and this will
contain to expand until we get to a point which we call the self collapse this particularly
happens with silicones where the walls ahve got so big that it will then collapse in on
itself.
So we wait for that to happen, this is the self collapse coming up now, roundabout the
two minute mark and then once that's happened we can switch off the vacuum pump and then
let the air back into the chamber and that's when you see the true extent of the degassing
effect.
Now it's depressurised we can remove the lid easily.
Then inside of course we have our perfectly degassed silicone.
You can see a marked contrast here if you look closely at the silicone the difference
between this completely clear rubber and the aerated silicone that went into the degassing
chamber.
So what we're doing here is pouring that degassed silicone around a pot as though we were making
a silicone mould and then quite importantly we're performing a second degassing step where
having poured the silicone around the pot, load it back into the degassing chamber, we've
sped this footage up considerably because there's much less silicone and much less air
entrapped in the silicone at this second stage so again.
So again having degassed it for roundabout five minutes this time let the air back into
the chamber and this could then be allowed to cure fully having done so we can demould
the parts so take the original part out and because of that original degassing and the
second degassing procedure we're left with a perfect silicone mould.
I hope you've enjoyed our brief introduction to vacuum degassing.
The exact same process that we've used today on the RTV silicone rubber can also be applied
to a wide range of material such as polyurethanes, epoxys in fact just about any material where
you get air entrapment following mixing.
The system that we've used today is our complete vacuum degassing system which is available
from our website easycomposites.co.uk.
Thank you.