Is speed necessary? (pressure / vacuum)

[Squib]

I have done some casting before, all with no degassing and no pressure and the inevitable bubbles resulted in maybe 1 of 6 casts being usable.

I am now considering getting a pressure pot and dual purposing it for vacuum and pressure. Everything I have read to date (not that long) has seemed to indicate that you want speed when doing the vacuum and then again the pressurisation.

Now my questions are:
Is this only because of the curing time of the resin used? - I can understand this if one is doing larger scale castings and you want to be done with it asap.
Would a longer cure time resin allow for a slower vacuum to be created and then a slower buildup of pressure?
Or is there some chemical reaction in the cure process that is sped up to a noticeable degree due to the vacuum / pressure?
If you had to pick only one, either vacuum or pressure, which is the better option?

[mangozac]

Hi Squib, welcome to RA!

Indeed, the primary reason for fast vacuum/pressurisation is due to short pot life resins used for production volumes. That said, I do believe that the speed of change in pressure also helps with the dislodgement of bubbles to a small degree. Ultimately all but the smallest compressors (or those without a tank) are going to give you acceptable pressurisation times. It's the vacuum pump where speed costs more.

To pick only one I would go for pressure. You will need to ensure that the parts are very well vented, and have separate gates (where resin enters) and vents (where air leaves). Ideally the gates should be on the bottom of the part. It's harder to do single part split moulds like this.

[Squib]

Thank you very much for you help. It is nice to get a clear answer without people trying to make it sound complicated.

[mangozac]

No problem, glad to help!

[Dr Mathias]

With my limited experience in pressure casting, I've found that even 30 seconds (7 minute pot life, poured and in the pot within five minutes) can make a difference removing bubbles in the cast.

[mangozac]

Indeed, the rated pot life is generally longer than the actual pot life achieved. I would always try to get the vacuum and/or pressure process completed within at most 75% of the rated pot life.

[Squib]

Thanks for the advice and thoughts. I went out and got a pot and compressor and was so excited that simply could not wait....... I used one of my old moulds, the ones that cured out in the open..... Guess what?

A near perfect cast of something that I have never been able to get more than a half recognisable bit of resin that breaks in the narrow bits if it falls onto the desk. The foot powder on the mould definately also helps, even without using pressure.

[Dr Mathias]

Good news. I felt the same way when I started using vacuum and pressure

[mangozac]

Congratulations Squib! Always great to hear a new member having some success (especially after forking out for some equipment)!