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 Post subject: More on bubbles.
PostPosted: Tue Mar 18, 2003 9:00 am 
Has anyone casting from silicone moulds had problems with bubbles in the silicone? This is a different problem to bubbles in the resin itself.

If I have a horizontal plane on the under side of the piece I am casting, I get loads of bubbles collect there. These bubbles have thin "skins" over them, so I can use the mould but after I use it a couple of times the skins break and the mould is useless.

Things I have tried to eliminate the problem include:-

Tilting pieces to avoid these horizontal planes. This works great but the angle need is quite large, this usually means creating undercuts elsewhere on the piece.

Lowering the piece into the wet silicone slowly and at an angle after having waited an hour or so for bubbles to float out. This has been the most effective but still not a total cure.

Starting with the piece suspended in the mould box, then adding the silicone very slowly in one corner and allowing it to flow around the piece. Ok, but not as good as above.

"Painting" the underside of the piece with silicone before lowering it into the main bulk of silicone. Doesn't make any difference!

A fiddly solution that works is to throw out the bottom half of the mould when it breaks then use the top (never has bubbles) as a bottom and remake the other half. This works but I would prefer a cheaper, right-first-time method!

Anything else I should try? Anyone overcome this problem?

Max


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 Post subject: Re: More on bubbles.
PostPosted: Tue Mar 18, 2003 9:25 am 
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Joined: Wed Nov 24, 1999 12:18 pm
Location: Palerang Shire, NSW, Australia
The last thing you mentioned makes the point the best. Notice that when you throw out the bottom half and start again, that you are actually molding upside down?

That is your answer. Some silicones have more trouble getting rid of bubbles than others but regardless of that, you should be molding upside down whenever possible if there's a chance that bubbles can collect.

Here's a couple of tips:

1) Mold upside down. This is never impractical or impossible. You should consider using plasticine as a base and make sure the plasticine hugs flush against the surface basically acting like a fake lower half of a mold. You can make patterns or holes in the top open areas of the plasticine too if you want to make keys. Obviously, remove the plasticine while still in the cured silicone base, lubricate and make the top half.

2) When the silicone is poured into a mold area, theres nothing wrong with running a soft tool into the mix and rubbing the sides of the object to dislodge any stubborn bubbles.


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 Post subject: Re: More on bubbles.
PostPosted: Tue Mar 18, 2003 10:10 am 
Plasticine. Thats the answer! I thought it might contaminate the silicone or cause a reaction, but I guess not. Thanks, Wayne.

Max.


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 Post subject: Re: More on bubbles.
PostPosted: Tue Mar 18, 2003 11:26 am 
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Joined: Tue Oct 09, 2001 6:14 am
Location: Orange County, CA, USA
My solution has been to stick the puzzle piece being molded to to the bottom of the mold to avoid air bubbles as well. I guess I've been lucky so far though not having to make two piece molds.


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 Post subject: Re: More on bubbles.
PostPosted: Tue Mar 18, 2003 11:56 am 
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Joined: Tue Feb 11, 2003 8:18 pm
Location: Melbourne, Australia
Jim, I'm curious as to what twisty puzzle pieces could ever be made from only one mold?

And to everyone else, is there a good web site to look at (not necessarily puzzle related) before getting into plastic molding? Having not done it before, there's lots I don't understand.

Thanks,
Rob.


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 Post subject: Re: More on bubbles.
PostPosted: Tue Mar 18, 2003 11:08 pm 
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Joined: Tue Oct 09, 2001 6:14 am
Location: Orange County, CA, USA
I molded the pieces I needed for a Mental Block using the 1 piece mold method. Actually I had pretty good success doing it this way. The molds are holding up pretty well but the main problem was air bubbles in the casting itself.


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 Post subject: Re: More on bubbles.
PostPosted: Wed Mar 19, 2003 3:47 am 
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Joined: Wed Nov 24, 1999 12:18 pm
Location: Palerang Shire, NSW, Australia
I used to use single part molds for ease but found that the molds ruin easily because of the difficuly getting the cast out of the mold. This is due to the complex bumps and undercuts of the original pieces.

There is areally good guide that has been posted here several times about molding creatures. Try a search through the archives.


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 Post subject: Re: More on bubbles.
PostPosted: Wed Mar 19, 2003 11:26 am 
I have made moulds by "sticking" the piece to some plasticard with release wax, building a lego box and pouring the silicone.

This works well and it is easy to close the mould! But I was getting resin between the card and the thin silicone that moulds the rounded edges of a piece. I could make the pieces square edged and sand down after casting, but I am trying to get pieces that are usable straight out the mould.

Max


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 Post subject: Re: More on bubbles.
PostPosted: Wed Mar 19, 2003 11:26 am 
I have made moulds by "sticking" the piece to some plasticard with release wax, building a lego box and pouring the silicone.

This works well and it is easy to close the mould! But I was getting resin between the card and the thin silicone that moulds the rounded edges of a piece. I could make the pieces square edged and sand down after casting, but I am trying to get pieces that are usable straight out the mould.

Max


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 Post subject: Re: More on bubbles.
PostPosted: Wed Mar 19, 2003 12:42 pm 
Bubbles in the resin isn't too much of a problem for me. I discard about half of my pieces for this reason. If you are willing to accept 50% success, its ok. I wanted a first right time solution for the moulds because they take 24 hrs to go off. I don't mind throwing out half the pieces because they only take 30 mins.

I throw out any piece with bubbles that would be visible after the piece is stickered. Small bubbles under stickers and ones not visible from the outside of the puzzle I fill with super glue. I use a pin to put a tiny blob in the hole, give it a few seconds to become tacky, then dab with it with my finger to flatten it. When it goes off a single "wipe" over 1200 grit wet-and-dry will smooth them (much more might reveal more holes). After polishing on a cotton sheet which has polishing compound on, these repairs are hardly visible.

To reduce bubbles in the first place, I use every hint that's been listed on the forum!

Everything has to be perfectly clean, dust makes loads of bubbles. So I don't cast and sand in the same room.
I use a Sika pure wax release agent, I think silicone spray creates bubbles at the surface. I also let the wax dry for a few mins, it sort of self levels and goes very shinny.
I add pigment well before mixing the two parts, to allow bubbles to float out.
I use a very low viscosity resin, bubbles float quicker!
I use disposable coffee stirrers to mix the resins.
I mix with a slow figure 8 in film canisters.
I let it stand for 30 sec - 1 min before pouring (risky).
I pour a very thin thread of resin into the mould (cut a tiny "V" in the lip of the canister), pouring down a vertical flat surface.
I pull bubbles into the middle of the pieces, away from external faces.
I very slightly over fill the moulds.

After a few practice tries I now get the 50% that I find acceptable, might get better but I doubt it. Timing is the hardest thing. How much stirring is enough? How long to wait before pouring? How long to spend fiddling before closing? I am using a 4 - 4.5 min resin!! It actually takes more like 6 mins to gel. Fast resin means I can de-mould after half an hour.

I use Dow Corning Silastic 3481 silicone with Silastic 81 catalyst. I have some S81-VF catalyst, this will make the silicone set in 90 mins, haven't tried it yet. Will now I know to make my moulds up side down.

Sika Biresin G27 LR polyurethane because it is very low viscosity (70 mPas (cPs)),fast curing and minimal shrinkage, <0.2%.

Sika Release Wax 818 and Sika BU black pigment (I have a 6 colour set, but haven't used them yet).

In the UK, all this can be bought from Ambercomposites (.co.uk) thay are very helpful, giving loads of advice. They have a network of reps, they will even deliver to your door for free!

Sorry for boring people ;)

Max


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 Post subject: Re: More on bubbles.
PostPosted: Thu Mar 20, 2003 3:47 am 
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Joined: Thu Jan 24, 2002 1:10 am
Location: Toronto, Canada
Here's the "creature molding" link Wayne mentioned. I had it bookmarked:

http://home.houston.rr.com/dpstudios/workshoppages/molding_casting.htm

Sandy


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 Post subject: Re: More on bubbles.
PostPosted: Thu Mar 20, 2003 12:42 pm 
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Joined: Tue Feb 11, 2003 8:18 pm
Location: Melbourne, Australia
So even using every trick of the trade, you still only get about 50% usable pieces? Do others agree with that? Sounds a bit disappointing to me.

Are any resins designed to produce less bubbles?

Couldn't you fill bubble holes with black milliput and sand and polish to fix most pieces?


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 Post subject: Re: More on bubbles.
PostPosted: Thu Mar 20, 2003 8:57 pm 
Got some plasticine today and have used it to make a mould with silicone, worked well

BUT I also did a little experiment. I made a simple one piece mould using just plasticine and NO SILICONE!

I placed a part on plasticard, sprayed with silicone spray, pushed a big blob of hand warm plasticine onto the piece, let it cool by the window and then gently twisted it off the card. The spray allows the part to slip easily from the plasicine! I then cast into the mould and closed with card as usual. I works very well!

Uses are limited. Obviously the mould can only be used once and can have no undercuts. This is restrictive, but the piece I was casting came from stereolithography, I wanted a duplicate to sand a little before making a proper mould. I never want to start sanding expensive prototype parts. It would be a lot of hassle to make a silicone mould to cast a single piece. This is a very FAST and CHEAP way to acurately reproduce such parts.

I would recommend this if you only need to make a few copies of a simple piece. Temperature might also become an issue with larger pieces.

Its like the way you see people copy keys in old movies ;)

Max


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 Post subject: Centrifuge
PostPosted: Thu Mar 20, 2003 10:14 pm 
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Joined: Thu Dec 16, 1999 9:31 am
Location: Kaunas, Lithuania
I've heard that the dentists use centrifuge while making plastic parts of prostheses. It isn't difficult to create significant centrifugal force without any equipment. I can't try this, since I still can't get necessary materials here in Lithuania to mold plastic pieces. Anybody tried?

Juozas Granskas


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 Post subject: Re: Centrifuge
PostPosted: Fri Mar 21, 2003 10:14 pm 
I have had an idea about bubbles! I am going to do a few experiments and see how it turns out. If it works it should be a total cure!!!

I was thinking that there are two way bubbles are created. First the mixing introduces micro bubbles which gather and create bigger bubbles during the cure. The other way is that air gets trapped in the moulds during the pour/close.

What I am going to do is this. Put the mixed resin in a vacuum to de-gas then after closing the mould apply pressure.

I have a foot pump which can create both pressure (pumping tires) and vacuums (remove air from air beds). I am going to drill a hole in an old pressure cooker and fit a tire valve. I can then pour the mixed resin in to the two halfs of a mould, place it in the pressure cooker and suck out the air. I wont be able to get a total vacuum, but it should be near enough. I think this will get rid of almost all the micro bubbles and trapped bubbles. I will then open the pressure cooker, close the moulds, close the cooker again and apply pressure (I should be able to get about 60-80 psi easily, the gauge on the pump goes up to 80 psi), then use a clamp on the pumps hose to hold that pressure for the whole cure. This should have the effect of crushing any small bubbles which remained after the vacuum or were created by closing the mould. Remember air is compressible, liquids aren't!

It might turn out that the foot pump can't move enough air quick enough. If this is the case I will try one of those mini electric compressors, for inflating/deflating air beds, that plugs into a cigarette lighter socket. If that fails I will try a hand operated bilge pump used on boats.


Max


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 Post subject: Re: Centrifuge
PostPosted: Sat Mar 22, 2003 10:43 pm 
Doh, I just figured out why this works. Centrifuge makes mass want to move outwards, so bubbles (space) move inwards.

Should work, but would need to make sure the mould was closed very tight and would need to spin it in each of the 3 axes for a while.

Very clever idea ;)

If you want materials I can send them to you. I would have to post resins and catalist in different boxes because of the fire safety rules.

Max


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 Post subject: Re: Centrifuge
PostPosted: Sun Mar 23, 2003 10:43 pm 
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Joined: Thu Dec 16, 1999 9:31 am
Location: Kaunas, Lithuania
Generally speaking, bubles must move towards the rotation axis. If the axis doesn't cross the mold, bubles "prefer" one side. You don't need 3 spinning axes. I think, there are always one side, which could be "sacrificed".
Do you live in UK? Your assistance getting materials would be very helpfull.

Juozas


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