pressure casting problem

[thedreadking]

Hi All,

I am new so please be gentle

I am having some problems casting models even under pressure!!

The model always appears to fail in the same places towards the top of the model, ie where you pour the resin into the mold.It is in an area where there is no way for the air to escape out on the mold, however i would expect the air to shrink and force the resin into place? but i have dreaded bubbles

I am casting at 60 psi and the bubbles are not shrinking as I would expect, am I doing something wrong??

cheers all and thanks for the forum its full of excellent advice.

[Munkey Joe]

im new here too BUT I can already see it coming....

post up some pics. its worth a thousand words. This will allow everyone to see whats going on better.
Everyone likes to look at pics!!



NOW on to your problem.... I dont know alot about casting under pressure BUT if your getting distortion under pressure it must mean one of two things logically...

1) under pressure your molds distort thus giving you a perfect copy of an imperfect mold. did you pour the mold under pressure? are you casting at that same pressure? have you ever gotten a pretty good cast from the mold under pressure or not? if you did what was the conditions you made your best cast?

2) pressurizing gas may not have anyplace to go. Even though you "crush" a bubble down to as small as it will go it still should take up SOME space. you might look into making a "relief spur" where the miscast forms to allow a place for air to compact better OR to suck more resin to that area of the mold. this might allow you to have better casts with only moderate cleanup.

Now be advised these are BOTH hypothesized fixes, but saying it might help fix the problem. if you have tried these already then just ignore me and allow an expert to answer...

[mangozac]

MJ is pretty on the mark. My guess is that it's number 2.

There are effectively two mechanisms of air that conspire to ruin a cast: air bubbles and air pockets. The pressure casting will take care of the air bubbles, but air pockets can only be dealt with by using adequate vents which allow the larger volumes of air to escape the mould.

But, yes, pictures would be much better in helping us help you

[thedreadking]

Thanks for the advice chaps, munkey Joe your a star..but Im sure I know one of those women from a night out on the town:oops:

I agree after reading the info that it must be pockets of air trapped, the casts have fine detail on the so I know the resin is getting there.

I was under the impression that all the air in the mold would be compressed down to a very fine dot, however it shows my lack of knowledge on the matter

Do you suggest degassing the resin prior to pouring? or just drop the lot into a vacuum chamber and degass the resin in the mold?

I have both fast setting and slow(fast) setting resin from tomps if that helps?

I have founbd that if you degass the fast setting by the time its degassed its like syrup already, so stick to the slow setting giving an extra few minutes of working time.

help and knowledge is always appreciated chaps

cheers

[mangozac]

The problem is that even degassing there's probably still nowhere for the air to go. It will probably help, but just not enough, but if you have the vacuum pump it's worth a try. Indeed the slower curing resin is a must if vacuuming. Remember though - during degassing the resin expands temporarily and if you don't have a reservoir built into the mould then it will overflow out over the top

[thedreadking]

Hi Chaps,

Success!!!!

What I did (following all your advice) is:

1. degass the resin

2. pour resin into the mold and again degass to remove the trapped air in the mold

3. remove and pressure the mold upto 50psi

After leaving for 30 mins remove from your pressure pot and you have a perfect result ever time.

Thanks a lot guys

Best wishes & Merry Xmas

[Munkey Joe]

AWESOME!!!! Very glad you got good casts!

Your pics are still not loading though....

[mangozac]

Glad to hear! It's a bit of a pain doing two degassing steps though - in the future try to build an overflow reservoir into your mould so that you don't need to do the first degassing step

[thedreadking]

Hi Chaps,

Will take all your advice onboard and develop the skills required to be a better 'resin addict'

'munkey joe' think you have seen enough pictures in this thread

cheers all & merry xmas