Resin Addict Forum

but as you said above you did what you did with what you have and it works! so job well done imo!

slightly off topic but in regards to making mold boxes for pressure casting. Do these have to be reinforced at all? Id be using foam core along with tape and clay to make the mould box. I only wonder as its adding external and internal force to the box?

So long as there is equal pressure everywhere then there is no difference on materials such as the mould box. Foam core will be fine, as the porosity of the material will prevent warping.

Clay will be fine too, however you need to ensure that there are no large air bubbles in the clay or spots that can't otherwise vent, otherwise the pressure could cause them to implode and thus force silicone into those areas.

ah cool, to give you an example here is how i do my mold boxes





I had thought about using lego, but i hear you get quite abit of wastage without gluing them.

I'm a big fan of old pizza boxes for the mould formwork and PVA glued together to ensure no leaks.

Foamcore is good stuff, but why the big spaces between the bases?

ah the gaps where because it was my first mold and was really going off abit of reading i had found on making them, ive learnt my lesson now

But a mold box like that should be able to hold up to pressure casting hopefully.

I have to say I agree with Zac the casting I have done in this manner works a treat as long as you ensure the bricks are tightly fitted togther. LEGO is definately the way forward

Legos have been working great for me as well.

You will want to use the "pick a brick" function for their webstore so you can order the ones you need directly from lego that way you're not buying tons of sets to get random bricks.

http://shop.lego.com/pab/

I use the 1x4 bricks up tthe 1x16 to make all my mold boxes.

Also one suggestion do not tape up the outside of the lego box as it'll trap excess air inside the parts which can cause bubbles along the interior wall of the legos if you depressurize it before the siliocn has set 100%. You can place tape over the bottom row of bricks just try not to tape over the crack between the brick layers.

You'll ocassionally get some silicon leak inside the brick ocassionally just pull it out with a sculpting tool and it'll be fine.

LEGO mold boxes:
I had a few molds where the RTV seeped under the LEGO mold box. My solution was to use Bluetack/ poster adhesive as a sealant , just run a bead around the inside edge of the blocks and press down.

Pressurizing molds while curing:
I've heard that vacuuming a mold is the preferred method. Pressure does not work as well on RTV as it does on resin. If you have bubbles near the part surface and you pressurize while curing the RTV you just compress the bubbles. The result is a mold that works great if you always cast at the same pressure as the mold was cured at. If the pressure is lower you can get dimples on the surface and if higher you can get bumps. If you can vacuum the poured mold then let cure at atmospheric pressure it should remove any bubbles in the mold and you can cast at any (or no) pressure.

I've been scouring the forum here, reading everyone's info about their pressure casting setups and I'd like some advice. Take a look at this:



I'm looking at the first model, the 2.5 gal. This looks as though it's all ready to go, no modifications needed. Am I reading that correctly?

What do you guys think about this as opposed to going the other route and doing my own mods to a cheaper pot? Keep in mind that I am probably less mechanically inclined than all of you.

If I go with the above item, any suggestions on a suitable compressor?

If you've got the money and don't feel mechanically confident enough and don't plan to be using it both for dual pressure and vacuum work then it's the way to go. The price actually isn't too bad (although what about shipping?).

I mention the vacuum work because if you do that you will need to mod it anyway and if you're going to be doing mods you might as well just mod the paint pressure pot from the start. I wish I had the time to do a proper tutorial on the modification and its costs...

Thanks Zac, I already have a vacuum system set up and working.

As for a compressor, just look for something that can get around 50-60 PSI, correct?

Oh sorry I forgot the compressor question

All compressors will go well above the 80PSI that the pressure pot is rated to, so compressor pressure isn't a big issue. You need to ensure that the tank on the compressor is large enough - from my experience you want a tank that is at least double the capacity of your pressure pot. My pressure pot is 10L and the tank on my compressor is 24L.

You shouldn't be spending much over $125 on the compressor (for a cheap one to get started with anyway).

All of you wondering about how to make the lego leak free-

vaseline, smeared light onto the inside of the mold box walls, seals it beatifully. I also use vaseline to "glue" strips of wood to the edges of lego walls when Im making a box with two lego sides and two wood sides (easier for larger molds.

On topic-

My set up is a 25L air compressor with two outlet valves, each hooked up to a 10l paint pressure pot, I used 1/4 BSP blanking plus to seal up the paint outlets and removed the internal paint pipes with a hacksaw...

Two pots means I can be setting one up while the other cures, allowing me to continuously be emptying and filling molds.

AH