Degassing vs pressurisation – HUH?!

[plaything]

Hi all

As a relative newby to the resin game, I'm still blundering around – can anyone tell me the difference (in practical terms) between degassing & pressurisation?

I have a degassing system that's worked fine for silicone, but I've had less luck so far using it to pull resin casts. The resin I'm using is supposed to have a pot-life of 7-8 mins, though seems to be less (I think I may need to shake the filler a bit more thoroughly), but I've been advised that pot life of +1 hour might be more suitable, though I'm not sure I'm keen on the demould times with that route.

I'm wondering if my vacuum pump can also be used to pressurise (given the correct chamber) – am assuming my degassing chamber would not be suitable for reverse engineering… Would be nice not to have to fork out for even more kit…

Any advice would be greatly appreciated!

[Blindhorizon]

Degassing sucks the air out of the chamber causing the (silicone or resin) to expand and release the air stuck with in the liquid when it's mixed. Pressurization add's air to your chamber increasing the pressure and shrinking the air caught inside the (silicone or resin). Some people do both of these steps, some people will have their mold prepped for pouring and mix the resin, then degass the resin pour it into the mold and then put it under pressure. This by my understanding will create near flawless casts, as for me i only use pressure and i get very happy results on my end. I don't own a vaccuum pump or degassing chamber. I do however know your pressure pot can be used for casting and degassing. You use the standard pressure pot lid for pressure casting, and when degassing you can use a clear lid (make sure it's rated to use with vacuuming you don't want it to implode, you can do a search on these forums to find the proper material and thickness) with a vacuum gauge hook ups and a rubber gasket ontop of the pressure pot. There is a bunch of information that can be gleaned off this website as well to fill any holes i have left, i hope i have helped you with your questions.

Blind-

[mangozac]

Blindhorizon has summarised pretty well.

Vacuum removes air pockets and helps distribute resin into the mould. Pressure will take care of any tiny bubbles left over after vacuuming. For perfect casting you need to use a combination of both techniques, but unfortunately your vacuum degassing system is unlikely to support pressure.

Making moulds only requires a vacuum chamber for degassing the silicone (although, again, using vacuum and pressure for moulding has its advantages).

A resin with a longer pot life won't help, and just stretches out your demould time. Some people have had success with the addition of an anti-foaming agent to their resin (available from Bentley Chemicals in the UK) and vacuum only, but it can be temperamental.

For best results you will need to purchase a compressor and pressure chamber. The pressure chamber can be used for both vacuum and pressure (and it's best if you do for the resin, since it saves a lot of time rather than having to transfer the moulds from the vacuum chamber to the pressure pot).

Does that answer your question?

[johncast]

Hi Plaything

Sorry to hear you are having problems with the casting .

The 3rd issue I have found with the resin casting and Degassing and pressure, is the shape and volume of the items you are doing, then comes the way the items are positioned in the mould you make. Some items are good and cast well in vacuum (Large items)and need no pressure , some ( mostly very fine items) need pressure only. Then there are the items that do well with both. If time allows with the quick set resins.

I've had problems with several items and had to redo moulds by changing the way the item is positioned in the mould and sprued, and in some cases vented. it is part of the resin casting art and science to try to predict the difficulties one is going to have with a particular model part and finding the best way to arrange the mould and which way to cast it either with pressure, vacuum or both. This all gets a little easier with experience.

So if casting of a part continues to give bad results in either vacuum and /or pressure look at the design of the mould , position of the part and spruing and venting.


Johncast

[plaything]

Guys, just seen all your replies to my post, thanks a million – answers my question perfectly

One last query: do you know if there is a pump that can handle both vacuuming and pressurisation, or is this a two-pump / two chamber job?

[mangozac]

Technically you can use a compressor to generate pressure and then feed that through a venturi vacuum device to generate a vacuum, but it's not the most efficient way to go. Best to just get a proper vacuum pump.

You can use a pressure chamber for both vacuum and pressure, but a vacuum chamber is only useful for vacuum. I have a dual-use setup on my pressure chamber.

[plaything]

[mangozac]

You'll still need a compressor as well as your vacuum pump.

This thread describes my previous setup (it has changed a little since, but the basic premise is the same):

[plaything]

[mangozac]

No that's a vacuum pump. You need a compressor for pressure.

[plaything]

[mangozac]

On the plus side, compressors are generally far cheaper than vacuum pumps

Make sure you get a compressor with at least a 20L tank. Airbrush compressors and the like are not suitable.

[plaything]

[mangozac]

I'm not sure what you mean. I'm referring to a compressor with an inbuilt 20L tank. Something like this:

[plaything]