Moulding Silicon

[dai sensei]

My elcheapo silicon moulds are starting to wear and need repair. Rather than fixing the old ones, which is possible, I thought I would make some good ones with propper pourable silicon.

So I just ordered some Barnes 1.1kg RTV2 M4642 moulding silicon to make some more at $76, not wanting to spend too much at this stage, and ensure it was the right silicon. I thought 1.1kg would be OK as my old ones took just under 4 of the standard 300g tubes.

It arrived today and it is a relatively small tub, probably not enough for even one of my old moulds. Seems heavier than the tube silicon.

I don't suppose this stuff expands does it?

I can use it for smaller shaped moulds, but making more from the tubes starts looking good again. Afterall, I can probably make 6 moulds for what it is going to cost to make one out of this stuff :- I can also correct what I did wrong to ensure my new ones last longer (take my time and ensure form is solid).

Any ideas appreciated.

[Big Shed]

Neil, you don't necessarily need moulds made from silicone.

I have quite a few moulds from HDPE cutting boards, including single oversize ones for "worthless wood".

For $76 you can buy a lot of cutting boards in the cheap shop (chicken feed really!)

I also use the cutting boards to make moulds for block casting where I can cast a block 4 blanks wide, then cut them on the bandsaw. Very handy for my camo blanks, as well as ribbon casting and swirls.

I have 2 silicone moulds for 4 individual blanks in each moulds, bought them in the US and as I was bringing over a flat rate box, the postage didn't cost me anything extra. They cost me $25 each and at the time I worked out buying the silicone resin here would have cost me a lot more.

[tea lady]

You don't have to have the whole thickness of the mold as silicon. If you want the smooth surface from the silicon you could just face a plaster mold, or even wood.:cool: You have little bumps on the wood or plaster base to hold the silicon in place.

[BoomerangInfo]

No I'm afraid it doesn't expand (if it's similar to the stuff I have, anyway).

After using mine for a while, I'm finding that it's rather difficult to do the colour swirling I usually do in individual moulds anyway, so I'm also going back to my big block mould for normal colour mixing, and using the silicon one for special pours. Mind you, I waste more resin using the block mould, although with some tweaking of the sizes this could be fixed.

My biggest problem with the cutting board moulds is that I can't get the joins water tight, so I've had to cover the edges with melted wax to get a leakproof seal.

The silicon is very expensive in small quantities. It does become more economical in larger containers, but unless you can afford several hundred dollars just for the silicon, it's difficult to justify.

the big advantage I see down the track is being able to actually make custom shapes with the silicon to make special pours (assuming you can make the masters as well that is). i've had some ideas for grids and other types of things that would just be too hard to do in anything but a pourable moulding material.

Russell.

[tea lady]

My biggest problem with the cutting board moulds is that I can't get the joins water tight, so I've had to cover the edges with melted wax to get a leakproof seal.

roll little sausages of plastercine and push into the seems. Smooth with fingers or a shaped stick or a shaped eraser end of a pencil. I'de use clay in a plaster mould, but if you can't have water in the mold you can use plastercine. (Get it from a toy shop. Can get from a mold materials shop too, but really exy. ) And its reusable. :cool: (Maybe you could make the whole mould out of plastercine? Don't know how it would go in a preassure pot though.:doh: )

[Big Shed]

My biggest problem with the cutting board moulds is that I can't get the joins water tight, so I've had to cover the edges with melted wax to get a leakproof seal.

The silicon is very expensive in small quantities. It does become more economical in larger containers, but unless you can afford several hundred dollars just for the silicon, it's difficult to justify.




Russell, I find that is only a problem on the first pour, if you sand the edges before assembling the pieces in to a mould it is not a big problem Once the leaking resin cures the mould is sealed, unless you have to disassemble it to get the cast out, which I haven't had to do yet.

I have also had some good succes with conduit and 15mm PVC plumbing pipe, it only takes 50gr for a blank. Tend to only use these for the single colour blanks though.

[beer is good]

Forgive my ignorance, but what do you use moulds for?? Why do you re-use them? :?:?:?

[dai sensei]

Thanks for all the comments, all good ideas to think about.

In reply:

Ok silicon doesn't expand out of tub - dam, I thought so, wishful thinking I guess:-

I'm casting worthless wood, pine cones and banksia, so using cutting boards and then assembling/dissassembling all the time is too much mucking about :rolleyes:

I have already ordered some small resin saver moulds from NewLondon. I am after something for a bigger mould for the casting of multiple blanks similar to the ones I already have.

Filling some areas with plaster is a good idea, if there was large areas of solid silicon, but would make mould more rigid. Plastercine would be better for a filler, provided it was surrounded in silicon. No good for the entire mould, or filling holes as it would come out, crack and fall out . My current moulds only have walls 10mm thick and flexible to enable easy removal of the blanks, so not sure if any filler could be used.

I am currently doing at least 2 casts x 2 moulds (ie 2 fit in pressure pot) per week, but I have done up to 4x2. Each mould does 4 normal and 3 half sized blanks :2tsup:.

Thanks again

[NewLondon88]

Filling some areas with plaster is a good idea, if there was large areas of solid silicon, but would make mould more rigid. Plastercine would be better for a filler, provided it was surrounded in silicon. No good for the entire mould, or filling holes as it would come out, crack and fall out . My current moulds only have walls 10mm thick and flexible to enable easy removal of the blanks, so not sure if any filler could be used.

If you have any 'reject' molds (the ones that didn't come out right? leftover cured silicone)
this can be chopped up and used as filler. I have molds that I don't use any more because
they deformed or didn't come out right (made too thin or mold box leaked, leftover cured
buts in the mixing bowl etc.) and these can be chopped up and used in the mix. I'm
pretty sure that a caulking mold could be cut up and used too, as long as it is cured.

I'm not sure what plastercine is, but I know that when I made my mold boxes I used
silicone caulking to seal the seams of the mold box. The seam bonded to the newly
poured silicone and became part of the mold, just a different color.

I also used a hot glue gun to seal the seams, but found I liked the caulking better.
When you smooth out the seams, cool caulking feels better than hot glue on skin :p

When I made caulking molds, I squirted the caulking out into a bowl of water with a
few drops of dish detergent (do you call that washing up liquid?) and then worked the
silicone into a soft putty while under water. That made it easy to work, the soap will
help the caulking not stick to your hands, and the consistency of the putty is good
for working into the seams. It molds to your blanks nicely, too.

ok, that's all I've got on that account. I've been buying the silicone on 5 gallon batches
and found a marine supply with a better price than what I had been paying. I'm not
sure what they use it for in the marine industry, but this is about $350 shipped for the
5 gallons. Might be worth a few emails to see if others might want to split a shipment
with you.. it's easy enough to decant into smaller buckets..

[BoomerangInfo]

I have also had some good succes with conduit and 15mm PVC plumbing pipe, it only takes 50gr for a blank. Tend to only use these for the single colour blanks though.

Yeah I use these too occassionally, but like the silicon mould, not much use for swirling. They're great for getting an extra blank, even if only a Sierra, if I've measured out too much resin though :)

Russell.

[BoomerangInfo]

I'm casting worthless wood, pine cones and banksia, so using cutting boards and then assembling/dissassembling all the time is too much mucking about :rolleyes:

I'm not sure why you think you have to disassemble the mould all the time. I simply turn mine upside down, give it a tap on the bench, and the whole block falls out. I'm usually good to go straight away again. I did my Banksia's this way. Biggest pain was holding them all down to stop them floating in the resin.

Russell.

[dai sensei]

If you have any 'reject' molds this can be chopped up and used as filler

Now that's a good idea


I've been buying the silicone on 5 gallon batches
and found a marine supply with a better price than what I had been paying. I'm not
sure what they use it for in the marine industry, but this is about $350 shipped for the
5 gallons. Might be worth a few emails to see if others might want to split a shipment
with you.. it's easy enough to decant into smaller buckets..

Hmm, it is costing $350 for 5.5kgs (about 1.5 gals) here :o

Perhaps have to look into getting some from US. What is the product name etc you are talking about from the marine indusrty, perhaps I can look into the same stuff here.


I'm not sure why you think you have to disassemble the mould all the time. I simply turn mine upside down, give it a tap on the bench, and the whole block falls out. I'm usually good to go straight away again. I did my Banksia's this way. Biggest pain was holding them all down to stop them floating in the resin.

Russell.

I assumed you would need to dissassemble to get them out. If you just need to tap, this starts looking like a real option :2tsup: Are you using any release?

With the silicon moulds I can easily wedge the banksia ww etc against the ends to hold them in place. I assume they slip against the cutting boards. If so, how do you hold them in?

[BoomerangInfo]

I assumed you would need to dissassemble to get them out. If you just need to tap, this starts looking like a real option :2tsup: Are you using any release?

With the silicon moulds I can easily wedge the banksia ww etc against the ends to hold them in place. I assume they slip against the cutting boards. If so, how do you hold them in?

No release, although since I used wax as a gap sealer, that acts slightly as a release, but generally the cutting board material is release enough. What adds to the release action though, is the fact the resin shrinks as it cures, and therefore comes away from the edges anyway.

As for holding down, it's an issue. I stuck some sticks of wood across the top to hold them down, then either rubber band the mould (not good due to making the bottom uneven) or just put more weights on top of the sticks. I don't consider any of these ideal, so if anyone else has better suggestions, let us know.

Russell.

[NewLondon88]

Hmm, it is costing $350 for 5.5kgs (about 1.5 gals) here :o

Perhaps have to look into getting some from US. What is the product name etc you are talking about from the marine indusrty, perhaps I can look into the same stuff here.

Ouch!

I doesn't have a brand name on it, but I get it from jgreer.com in California. I can only
imagine the shipping to Australia on 5 gallons.. :oo:
But I'm sure something similar must be available somewhere more local..
If you got your molds, that is the silicone I'm using now.

[dai sensei]

Thanks for the info, I can at least look up the specs etc on their site.

Moulds not here yet, but sounds like they are on their way :2tsup: I'll have to wait till I get back from Fiji, but probably better that way, otherwise I might get distracted from packing :U