How do I skim coat rtv without getting bubbles?

[crystaldot]

Hi,
I have not had much success with bombs away method as I am still getting bubbles on items such as roses where there are deep cuts and corners, as I am getting bubbles. Yesterday I tried a skim coat of rtv, I did not let the skim coat dry as I then poured on the bombs away to fill the rest of the mould. I found less bubbles with the skim coat by in undercuts of the small shapes I am molding I still found bubbles. My question is should I skim coat and let it dry first before filling the mold with the bombs away, or should I skim coat then do the bombs away straight away. As I still got some bubbles even when I did a skim coat, where the designs when around sharp corners (I am molding small pieces with lost of detail) should I buy a vacuum chamber, if yes should I just vacuum it first before pouring (my items are poured in to very shallow pots, so do not have room for expansion of rtv) will that be enough or will I still get bubbles when a design has a sharp corner or crevice of a petal etc.
Would really appreciate any thoughts on this please, as I am trying to avoid paying out for a vacuum chamber.
thanks so much
Crystaldot

 

[Nwerke]

If you will be doing a lot of work and getting paid for it then vacuum chambers and/or pressure pots will pay for themselves. Vaccing silicone won't prevent bubbles happening in undercuts, it just reduces issues from air being mixed into the silicone as you catalyse it. If this is simply hobby stuff then it may be overkill.

Maybe try doing a couple of skim coats and do let them dry. Brush the silicone into the undercuts using a small paintbrush (not a good one!); if you still get bubbles you should be able to pop them manually.

There are also thixotropic additives which make normal silicones brushable, they get 'stickier' and will stay on a vertical surface. Probably not quite what you want here though.

 

[crystaldot]

Hi Nwerke,
thanks so much for your reply.
I am going to be making rtv food safe silicone molds for cake decorating as a business. As mentioned, I did put a skim coat on yesterday without letting it dry, so on your advice I will now let it dry. That method yesterday sorted out the bubbles I was getting on the design even with the bombs away method but it did not sort out the bubbles in areas where the objects went sharply around corners or deep cuts in the rose designs. I could not see the silicone when I painted it on in the deep cuts in the rose, or underneath a balloon that had a small bow on the end that was slight raised at the corners, as I could not see underneath it. What would you suggest for the undercuts as above, as I still got bubbles in these areas even with painting on a layer as some of these very small items have very deep cuts in them. I even poked the cuts with a cocktail stick but could not see any bubbles, so felt happy to pour on the rest of the rtv to fill the pot.
If I do buy a vacuum chamber would it be best to put these little pots in after pouring as well (if I can build up a cuff around the pot). Time will be a factor as well as its going to be a buisness, but really just want to get it right, so not bubbles on the deep cuts of small designs.
thanks so much
Crystaldot

 

[Nwerke]

Any time. Given that the moulds themselves are the product then I expect you would get best results from degassing the silicone (in a tallish container!) before pouring the moulds; you can also then pop the mould back into the chamber and degas again after pouring to remove bubbles trapped in undercuts. This might be faster than manually painting silicone onto every item. Not necessarily a must, you understand, just a time saver.

Another option is to *thin* your silicone. You can add a percentage of silicone oil to reduce viscosity, within limits. I've never tried it though.

Before going too far, are you confident the moulds will work OK with frosting? Deep undercuts might be tough on sugar.

 

[crystaldot]

Hi Nwerke,
thanks again for your reply, it sound like good advice.
I am going to see about building a cuff around my posts for the second de gassing, as I really feel that buying a chamber is going to best in the long run.
I am trying all of the moulds out with fondant that has a thickener added to it, so the fondant is stiffer, which is better for using with molds. Any shapes that do not work well with the fondant I will not sell to customers.
I will try another skim coat today and let is dry as you suggested, also pay more attention to the undercuts but feel a vacuum chamber is the answer plus a second go in the chamber. I will also look in to the silicone oil, thanks for that as I did not know about it.
thanks so much for your help, its much appreciated.
Best regards from Crystaldot

 

[Nwerke]

My pleasure. Take your time, others may have better advice, I've got no experience with cake stuff at all. (My mother in law is an amazing decorator though, what she can do with icing sugar is mindblowing.)

WRT the oil, check the data sheets for your product to be sure it is compatible, and what percentages can be used.

 

[crystaldot]

Hi Nwerke,

sounds great!

I will check out the data on the oil to see if its suitable for food safe silicone and if it is, what amount I should be using.

thanks so much again for all of your help
Best regards from Crystaldot

 

[Sigma-LS]

you can also then pop the mould back into the chamber and degas again after pouring to remove bubbles trapped in undercuts.

Would this work with acrylic masters embedded in clay? My gut reaction is that the expanding air trapped under the objects would pop them loose, especially as they are already less dense than the silicone.

 

[Propsjonnyb]

If you can't afford a vac chamber , then the next best thing I have used for over twenty years is a camera vibration plate normally used in photographic studios , they are available for amateur photographers as well so it does offer a less expensive alternative the vibrations make the bubbles rise to the surface , I let my molds sit for about fifteen minutes on them before removing then to my curing rack it remove almost all bubbles form the cast surface - it will not get all out but it does make for less filling and sanding afterwards

 

[crystaldot]

Hi
thanks so much for your replies.
so far I have used a foot spa (without water) to vibrate the bubbles but it is still not helping with the undercuts.
I have had a real problem with silicone going underneath the item I am molding. I have stuck it down on clay which works the first time but the clay then hardens, so I need to find something else. It has been suggested to use wax but as I will be making a lot of items every day, its the time factor. Does anyone have any other suggestions please on sticking the item down. Its small items approx 1/2 to 3 inches, on a stainless steel tins.

thanks so much
best regards Crystaldot

 

[Nwerke]

Sigma, it would depend on the expansion path for the air. If the air is between object and clay then probably yes...but you should ideally clay your pieces up in such a way that that doesn't happen. If you're trapping air between clay and item, I'd hazard a guess that your seam line is going to be in the wrong place anyway?

CD, yeh, vibration is probably not enough with small very undercut items. When we talk about clay in this context we're usually talking about oil-based clay, not water-based clay that dries out. Find yourself some Kleen Klay or other similar product intended for mouldmaking.

 

[Ray22]

Hi
thanks so much for your replies.
so far I have used a foot spa (without water) to vibrate the bubbles but it is still not helping with the undercuts.
I have had a real problem with silicone going underneath the item I am molding. I have stuck it down on clay which works the first time but the clay then hardens, so I need to find something else. It has been suggested to use wax but as I will be making a lot of items every day, its the time factor. Does anyone have any other suggestions please on sticking the item down. Its small items approx 1/2 to 3 inches, on a stainless steel tins.

thanks so much
best regards Crystaldot

As for your bubble issues buy a silicone that degasses itself like Smooth On's Mold Star 15 the bubble rise to the top no need for a vac chamber and it's 1:1 easy mix, as for sticking your items down try Elmers White glue the stuff you used as a kid in school once dried it will hold your object down to mold and then pry it off and peel the Elmers off without destroying your pattern I use it all the time.

 

[WolframCreative]

you can also then pop the mould back into the chamber and degas again after pouring to remove bubbles trapped in undercuts. This might be faster than manually painting silicone onto every item. Not necessarily a must, you understand, just a time saver.

Why are you suggesting this technique? Have you tried this?

This could draw all the material out of the mold (material expands under pressure), and possibly damage the mold itself by putting it under pressure if there is any trapped air inside it (stone, silicone, fiberglass). I did technical assistance for mold making for 6 years for one of the biggest suppliers out there, I would never suggest vacuum degassing a mold with material in it!

 

[merch]

If you don't want to use vacuum to degass silicone or resin - you could try a spinning rig - I've done a quick mock up with a tub from the chippy and string as it is easier to show in a picture. If you pour the mould as normal then get spin your stainless pot inside a heavy duty PVC tub the centrifugal force squeezes the wet silicone into the gaps and the bubbles get pushed out.
I know it looks daft but the I rig made for casting small parts for a garage kit is still going strong today and not a single bubble in tiny resin parts. Try it with water first till you have got the hang of it - if the PVC tub has a screw on lid you won't get silicone every where if you catch the rig on something (wear some goggles!). Spin the same speed and direction as a skipping rope (in circles at 90 degrees to the ground) - you don't need to go fast 2 or 3 revs a second is plenty - spin it gently for a couple of minutes then a fast 10 second burst at the end. I know this sounds like a wind up but in the old garage kit days we used to spin up resin moulds in 5 supermarket plastic carrier bags each one inside the other - the nylon strings and pots were my refinement - had a resin blow out one day when all the bags ripped and the back yard got covered in isopon!.
View attachment 151953

 

[Sigma-LS]

If you don't want to use vacuum to degass silicone or resin - you could try a spinning rig - ....
View attachment 151953

This right here is why I come here. I would give this a shot if I hadn't just finished building my vac/pressure setup. Fantastic low tech solution.