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View Full Version : Need Help! Demolding: Mother molds vs poured silicone vs ??



speakparrot
08-10-2011, 02:29 PM
Hey guys, I'm in the process of making a silicone mold of my large lap tray prototype. I've been getting some advice from the guys over at Smooth-On but unfortunately I've gone through 2 trial and errors ($$) and could really use some advice as I'm out of ideas and not keen on spending more money on guesses.

The problem: Demolding my brush-on silicone is pie of course because it's super flexible. The issue is the mother mold. Previously I've tried a rigid foam and a paint-on plastic—Because they're such ridged material, and the tray is flat on the bottom, there is nothing to get leverage on to separate the two. (Pulling them appart does not work because the tray has so many crevices [see attached photos] and the mother mold material gets in there so snug)

My question: Does anyone have a recommendation for how to circumvent this demolding issue? I was thinking of biting the bullet and paying the $2000+ to make a poured silicone mold—My thinking is that the silicone will be strong enough to keep the form of the tray, but flexible enough to "peel off" for demolding.

Second train of thought was maybe there might be a soft foam I could use that again would be sturdy enough to hold up the walls of my paint-on silicone mold, but squishy enough to grab it and pull it out of the crevices of the tray.

If anyone has advice on these ideas, or a completely different idea, I would REALLY appreciate it! I've been banging my head against a brick wall for far too long and it's driving me crazy!

Photo #1 - prototype
Photo #2 - paint-on mold over prototype
Photo #3 - paint-on mold without a mother mold
Photo #4 - the prototype/model in the mold, nothing to get a hold of to wiggle it out of the mold
Photo #5 - need to have something that allows me to get my hands under the mold to extract the model

Thanks again guys!

mezalick
08-10-2011, 03:01 PM
try a good sized suction cup on the flat bottom side.
Something like the suction cups they use in the glass industry.
Michael

79TigerPilot
08-10-2011, 03:46 PM
Ok, I think I see what your up against -- the brush on compound is hard to get off and doesn't hold it shape after beign removed. I think you need to do what the bronze sculputre folks do.

After you paint your master with the rubber molding compound (and before you remove it from the master) dab on a bunch of nibs of the molding compound to the outside sruface of the molding compound. The nibs need to be firmly attached and part of the molding compound. Not sure exactly how to describe them but something like the nibs on this "dryer fluff ball" only possibly longer and not near as dense.

Amazon.com: Dryer Magic/Dryer Fluff Balls Dryer Balls- SET OF 2: Home & Garden

Now, after applying a mold release, add a thick backup coat of fiberglass or plaster of paris. Now yo have a two layer mold, top surface is rubber and below that is the plaster or fiberglass for strength. The nibs hold the rubber on the plaster or fiberglass.

I would use air or water forced between the master and the rubber to demold the master. I have seen this done removing a row boat from a mold and it works well. In the sculputre molds I hae seen the rubber mold (usually silicone) also comes loose from the backup plaster. Most likely due to the fine details on the master (with no draft) that holds the silicone better than the nibs in the plaster. But once it is all demolded the silicone rubber molde is put back on the plaster with the nibs locating everyting. I guess the nibs should be fromed with some draft and not too long so they can be removed from the plaster with ease.

I think with your large flat sections that is going to be the only way to keep the mold holding its shape. You could alos just make the rubber mold much much thicker.

Craig








Hey guys, I'm in the process of making a silicone mold of my large lap tray prototype. I've been getting some advice from the guys over at Smooth-On but unfortunately I've gone through 2 trial and errors ($$) and could really use some advice as I'm out of ideas and not keen on spending more money on guesses.

The problem: Demolding my brush-on silicone is pie of course because it's super flexible. The issue is the mother mold. Previously I've tried a rigid foam and a paint-on plastic—Because they're such ridged material, and the tray is flat on the bottom, there is nothing to get leverage on to separate the two. (Pulling them appart does not work because the tray has so many crevices [see attached photos] and the mother mold material gets in there so snug)

My question: Does anyone have a recommendation for how to circumvent this demolding issue? I was thinking of biting the bullet and paying the $2000+ to make a poured silicone mold—My thinking is that the silicone will be strong enough to keep the form of the tray, but flexible enough to "peel off" for demolding.

Second train of thought was maybe there might be a soft foam I could use that again would be sturdy enough to hold up the walls of my paint-on silicone mold, but squishy enough to grab it and pull it out of the crevices of the tray.

If anyone has advice on these ideas, or a completely different idea, I would REALLY appreciate it! I've been banging my head against a brick wall for far too long and it's driving me crazy!

Photo #1 - prototype
Photo #2 - paint-on mold over prototype
Photo #3 - paint-on mold without a mother mold
Photo #4 - the prototype/model in the mold, nothing to get a hold of to wiggle it out of the mold
Photo #5 - need to have something that allows me to get my hands under the mold to extract the model

Thanks again guys!

RomanLini
08-12-2011, 02:12 AM
...
My question: Does anyone have a recommendation for how to circumvent this demolding issue? I was thinking of biting the bullet and paying the $2000+ to make a poured silicone mold—My thinking is that the silicone will be strong enough to keep the form of the tray, but flexible enough to "peel off" for demolding.
...

Solid silicone mold will run about $50 a kg for materials and you should be easily able to make a mold out of 3kg of silicone, costing around $150.

Be aware that with any type of silicone mold you might only get 10 to 25 products cast out of the the mold before the mold is trashed.

A big part of the problem looks to be your master, it has no draft angle and appears to not be surface-finished with any smooth coating etc.