Hello,

I've recently decided to make some resin casts, I've read the hirst arts guide and a few others on making 2 part moulds for resin casting

As far as I can tell, the 'embed the object in modelling clay' part is somewhat redundant.....

surely you can suspend the model from your mould box, and pour in silicone until it reaches the desired point (halfway up the model or thereabouts), add keyways, wait for the silicone to cure, then brush on some mould release and pour the other half?

I would appreciate if someone more knowledgeable than me could explain why the modelling clay stage is necessary (especially since the hirst arts guy used a pourable compound in his 'make a 2-part mould of a torch' example)

thanks

_________________
Just some guy

My hobby/painting threads

Army Forge List Co-ordinator

The only problem I can think of is that the rubber won't have gravity to help force air bubbles out. With it contacting the model from the bottom up I would think that the air bubbles would stay put rather than be forced out by the weight of the rubber.

_________________
Dave

Blog

NetEA Tournament Pack Website

Squats 2019-10-17

You can suspend it yes, you wouldn't even need to put mould release and pour in 2 parts as you could just slice it apart into 2 parts once made ( risk of damaging a fragile master if you slice it apart though).

If you do it suspended for 2 part pour you have to have a flat mould plane. If you embed in plasticine you can vary the height of the mould plane at different parts of the mould around the mini, giving you a few more options when casting more complex shapes. Also makes it easier to match up mould lines if you are making a new mould from a previously cast piece (eg you ruined your master first time around, so remaking mould from a cast).

_________________
AFK with real life, still checking PMs

Well I wasn't planning on cutting the mould as I don't want to risk damage to my master

I was planning to use the 'slow-high' pour technique to remove the worst of the air bubbles, I'm sure a good few taps would be good to remove bubbles, I've also heard things about people setting the mould boxes on a running washing machine to vibrate bubbles out....

_________________
Just some guy

My hobby/painting threads

Army Forge List Co-ordinator

I did the 'suspend the part to avoid the modeling clay' experiment, but it fails.

The problem is that the silicone creates a meniscus where it touches the part to be molded. This is a problem because it creates a thin lip on the mold that is prone to tearing once the silicone sets. Also, the meniscus can actually creep around some details, resulting in the bottom half of the pour becoming the bottom half plus 1-2mm.

By the way, in a related experiment, I have found that one-sided molds are not appropriate for metal casting. Metal tends to have a convex meniscus, making it nearly impossible to get a planar surface at the pour side of the mold.

ahhh that does make sense..... better go raid my son's plasticene then.....

_________________
Just some guy

My hobby/painting threads

Army Forge List Co-ordinator

Is this for a nid-dinner locomotive?

_________________
www.epic-uk.co.uk
NetEA NetERC Human Lists Chair
NetEA Chaos + Black Legion Champion

I do the suspension method.

I'd strongly recommend investing in vacuum & pressure equipment for mouldmaking and casting respectively.

_________________
Currently doing a plastic scenery kickstarter

eventually yes, although it's for something else right now

I'm currently slowly translating my concept drawings to 3D, I would share the drawings, but they are pretty crappy, I've gone for a 'non military vehicle adapted for military service' type look, so lots of added on armour and weapons, I'm quite chuffed with the concept so far...



Funds are a bit low at the moment, although I could probably build something.... there is a nice bell jar sitting around ready to be chucked at work right now, I might appropriate it for making a vacuum rig....

I assume you use the pressure equipment to pressurise the resin itself, or do you put the mould into a pressurised chamber?

I have ideas for both methods.....

_________________
Just some guy

My hobby/painting threads

Army Forge List Co-ordinator

QFT. Also, the slow pour may remove air bubbles from the silicon, but the rising silicon will still trap air.

So I now have my moulds made, and am awaiting delivery of my resin, while I'm aware with good mould release agent you can get many pulls from a mould, is it worth bunging the mould into the oven to further vulcanise the rubber and toughen it up for more pulls, or is this not worth doing?

_________________
Just some guy

My hobby/painting threads

Army Forge List Co-ordinator

I pour the resin into the mould, then pressurise it before the resin sets in my pressure chamber.

_________________
Currently doing a plastic scenery kickstarter

yeah I'd managed to find some stuff on that thanks, I've got my pressure pot and bike pump(!!) ready to go, I've tested it and can easily get it to over 5 bar in under 30 seconds, plus it's damn good exercise!

_________________
Just some guy

My hobby/painting threads

Army Forge List Co-ordinator

5 bar isn't really that much - a bit over 20 psi. Most tutorials I've seen reccomend 30-50psi/7.5-12.5 bar pressures for pressure casting.

_________________
SG

Ghost's Paint Blog, where everything goes that isn't something else.

Wrong calculation on my part..... Pretty embarassing considering I work with high pressure kit all day...... I can get it to 40psi no problem

_________________
Just some guy

My hobby/painting threads

Army Forge List Co-ordinator