Thread: clear casting epoxy scales

Can I heat silicon or PU pouring rubber to remove bubbles?

I have tried putting the mixtures in a glass jar with a metal cap that has a valve screwed in and suck out the air with an inversed soccerball/bicycle pump. This does create a decent vacuum but not strong enough to get rid off all the tiny bubbles.

Originally Posted by Piet

Can I heat silicon or PU pouring rubber to remove bubbles?

I have tried putting the mixtures in a glass jar with a metal cap that has a valve screwed in and suck out the air with an inversed soccerball/bicycle pump. This does create a decent vacuum but not strong enough to get rid off all the tiny bubbles.

I haven't tried that. I do get preey good results with placing the item to make the mould from on a small piece of flat glass and buiding up a dam around it with plasticene or small glass slips with the modelling clay holding them down and sealing the corners, then brushing a thin cot of the silicon onto the item to be moulded.

The set time is long enough to let you brush any bubbles that you can see out of the way before gently pouring the rest of the silicon into the chamber.

If any bubbles have formed, the holes in the hardened silicon mould can be plugged with modelling clay, or the projections can be sanded off the finished cast item.

Regards,
Neil

Originally Posted by avatar1999

Instead of making your own silicone molds, why not use one that's already made for you?

This may not be the best example but...

*click picture for amazon link*


Made to withstand high temps, and reusable. Mix up your resin, pour enough in the bottom to make a sheet, swirling in whatever colors you want and let it set. Should be easily removable...as long as the epoxy won't fuse to the silicone (I don't have any experience in this dept.)

Just a thought tho

Allright !! You'd need quite some blade to get a decent balance with those handles !!

Originally Posted by Neil Miller

I have made a fair number of scales using this method, using acrylic, polyester, polyurethane, epoxy and urethane resins and pourable (aka "RTV") silicone to make the initial moulds with. If you don't use silicone you may have to use a release agent on the mould to get it to separate. Silicone is easy though - a 50:50 mix and a breeze to use. Use existing scales hot-glued to a base, or make your own that you wish to replicate, build a dam around them, and pour on the silicon (brush the scales with the silicon using a soft brush first so you can see if and where bubbles might form - gives you a fighting chance of shifting them before you pour the rest of the silicon on).

Bubbles in the resin are always a problem - if the resin sets slowly enough you can paint the first "layer" on with a soft paintbrush - this helps to avoid bubbles on the underside (the side you will see if you are pouring it into a mould). It's not foolproof, though, by any means. Using a chamber that can be de-pressurised is quite a reliable method to drive out bubbles, but costly. some people, as mentioned above, use a propane torch to make the bubbles migrate to the surface.

From my experience (and it is just that - my experience, yours may differ) the polyester resins are too brittle with hardly any give in them - unless you use additives like glass fibre strands. Then you have fibre-glass! If you must use it, don't get the laminating type.

The polyurethane resins are are more flexible and are the ones I use. They only have a pot life of around 5 mins and to get it to gel properly you have to be spot-on with the mixing: that means mixing a comparatively large amount to minimise the mixing errors. That means having a lot of moulds ready. It also limits the colour runs. A certain amount of heat is necessary else it will not set - some people put it in a box with a low-wattage light bulb in it to keep the temperature up.

The epoxies are even more pliable, but very slow setting, which may help if you want to add splashes and swirls of colour.

Urethane resins are quite flexible - perhaps the most flexible - but can be made to be rigid. They smell a lot though, and the smell lingers on.

Some uncured acrylic resins are quite toxic. Some require special equipment (ovens, pressure chambers), but some are easy to use - the clear-cast type. I haven't used them to any great extent, so cant really comment much on them.

Using special fillers gives a variety of results and solves some problems. all resins are exothermic and generate heat from the core, leading to shrinkage. fillers can be used to minimise the shrinkage.

Mixing in the correct proportions is of primary importance. Too little catalyst and it will never set. A little tiny bit under and a bit of heat may help, maybe not. Way too much and the whole lot will get really hot. It can begin to smoke and then catch on fire - I have seen it happen in boat yards! Ever seen a boatyard making fibreglass hulls go up in flames? - I have - it's like a whole town of firework parties all in one place!

Regards,
Neil

I was looking up info on doing this just the other day (mainly watching videos on urethanes and molding). Unfortunately, it didnt really seem very approachable if you're just planning on doing a couple scales every so often, as your post seems to confirm.

When pouring up resin in the dental field, I use a vibrator to get out bubbles. By placing your mold on top of the vibrator and turning it on before pouring, you get the bubbles to come to the surface where you're working. http://www.henryschein.com/us-en/Sea...yWord=vibrator