Pressure Casting Questions

[Bunny]

Hi


ive read the guides and have done a few casts myself but now im looking to improve my molds and castings. The most cost effective way at the moment seems to go with a pressure pot casting then at a later date buy a vaccum pump to use on a pressure pot.

Im looking at buying a painters pressure pot and a air compressor (2.5 hp) in order to do my casting.
Though i do have some questions in regards to the pressure casting methods.

As there is no viewing port, does this make a difference when casting?

How long should pressure be applied to the resin? The entire time it takes to cure or just a few minutes?

When using a split part mold and putting pressure upto 40 to 50 psi is there not a risk of the molds splitting from the increased pressure upon the resin? If so how is this stopped?

Also when putting your silcone mold into a pressure pot in order to remove the bubbles when making the master mold, does the mold box not also suffer from the same problem? Again if so hows this stopped?

Thanks Max

[mangozac]

Hey Max I'm just about to head to bed but I'll see if I can give you some quick replies:

1. A viewport isn't necessary for anything pressure related. It's nice for when vacuuming though...

2. My resin has about a 20 min curing time (it's solidified but still softish). I leave it under pressure for at least 30 mins. Any sooner and the small bubbles around the top can deform the still soft resin as they expand at room pressure.

3. All moulds are going to have at least one pour hole and optionally some vents. These allow the pressure on the inside and outside of the mould to be equal and preventing any warping. The pressure will only cause the air bubbles in the resin to compress.

4. I think you're confused as to how the pressure works. So long as the mould box has no airtight voids in it there's no danger of implosion. Remember though that if your model has cavities inside it there is a risk of implosion there too unless you take steps to avoid it.

Am I making sense or should I get some sleep?

[Bunny]

yes that does!


Though for a mold box im using lego in square shape, mostly doing bases that should be fine?

Also is a 2.5 hp compressor enough?

[mangozac]

Lego has always been my choice of mould box. I once read somebody whinge that all their silicone leaked out through the cracks between the bricks but they must have been using some super thin silicone or something. My silicone seeps into the cracks a little so when you remove the lego you have to clean off all of these little dags.

Here's a WIP pic from making one of my first moulds:


2.5HP should be just fine. The one I'm using now is about that.

[Bunny]

ahhh looks good! Ive learned alot espically about polyuerthane resins! Avoid them! They go off after a month and get large amount of air bubbles in them if left!


Ah gd old lego. Btw silly question but what do you use for the base of the mold box?

[mangozac]

Polyurethane resins are really the only option when you're doing detailed model parts that need to fit together - other types will exhibit a lot more shrinkage, which is a bad thing. They can be susceptible to air bubbles, but that's why we use the pressure pot. And the shelf life is much longer than a month - after opening a container of one of the polyurethane parts I wouldn't want to take more than 6 months to use it. The bottom line is that there are heaps of different brands and types available - you just need to find the most suitable for your application and technique.

The base of the moulds is just a bit of scrap plastic. A sheet of styrene will do but something more rigid is preferred.

[geomod]

Hi Guys,

Just a quick thanks to mangozac as these tutorials have guided me into pressure casting and I'm amased I havnt taken the plunge before.

LEGO is excellent for use as a mould box, I hadnt used it before but now swear by it. It makes great square, almost leak free boxes and can be expanded up or down as needed.

The soulution for the base that I came up with is a piece of plastic cut to the size of the pressure pot base with a pair of grips cut in the sides. This gives me a clear limit to the size of my mould in relation to the pressure pot. It also allows me to quickly put the master into the pot once I have painted it with Pinkysil (my silicone of choice).

Cheers,

Geomod

[paulson games]

Legos work great, best source for them is to order the bricks from lego online as most of the kits in the stores don't have many of the long straight pieces. I ordered up a bunch of them ranging from 1x2 all the way up to 1x16, I absolutely love using them for the boxes.

One thing you may notice is that the legos will loosen up over time so if you make regular use of a certain mold pattern you can always glue the pieces together to make them a bit more permanant. I also sometimes tape the outer face of the walls to help hold everything together tight. If you tape the molds make sure it stays under pressure the entire time because air traps inside the lego part and if it's epressurized before the silicon fully cures it can burb air out and cause cavities in the silicon. (The tape keeps the air from escaping outwards as easily and instead it goes into the silicon) However it won't be an issue if you leave it under pressure the entire cure time.


For sealing up the underside I cut pieces of sheet styrene to the correct size and then scotch tape them to the bottom. Most of the boxes that I use are a fairly uniform shape and size so I can reuse the stryrene pieces over and over. A good source of cheap styrene is buying "for sale" signs at the hardware store, I've found that the local home depot carries it much cheaper than the hobby shop. For a thicker styrene source you can also use the huge sheets of lighting panel plastic.

Most of the paint pots converted into a pressure pot have a round bottom to fix this I picked up a cheap baking pan that was a close fit, I had to bend the lip upwards with a pair of pliers so it'd fit but it give me a nice smooth level surface for placing molds on and the non stick coating helps if you have overspill as the resin flakes right off. I placed a bunch of sand in a bag under the pan that way it filled up most of the excess volume, with less physical volume to fill the pot pressurizes much faster and the air compressor does less work.

When I make my molds I keep them under pressure the entire cure time as the silicon is susceptable to bubbles. I've switched to using Smooth-On as their stuff is very good on price and the cure time on their silicon is just 70 minutes, although I usually let mine sit for a full 2 hours just to be safe.

Normally my resin takes about 30 minutes, they state that it's a 10 minute cure time and a 3 minutes pot life so in theory it should be finished in 15 min total. The problem is that their estimate is only good for large pieces which can heat up and cure faster, small thin parts take considerably more time as they don't generate as much heat. I've found that almost all of the parts I make take about half an hour.

I let the resin stay under pressure for at least 10 minutes as it's relatively solid at that point and can be removed without any left over the bubbles expanding. So figure out what your average cure time for the resin willbe and it should be ok to remove at about half of that time. Every brand of resin will have differant properties so what works for one brand may not carry through on others so it's always a process of experimenting.

I let mine stay under pressure for 10 minutes and then stand in the molds for another 20 before I remove the rubber bands.

It's also important not to let the resin sit in your molds too long, I forgot and left some parts in the mold overnight and the next morning it was like they were superglued in place and it tore the molds which were quite new, even with mold release it'll eventually bind up so make sure to always remove them within an hour or so of casting at the latest.

[mangozac]