Vacuum bagging a silicone mold?

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#1

Hello!

I have been doing custom FRP for 15yrs, however this is out of my realm of expertise… So I need some help.

I was wondering if it is possible to make a silicone mold from a set of steps for a customer (That’s the easy part)…, then when I go to make the part I would Spray my gelcoat on the inside of the mold (female) then instead of using our chopper gun to lay up the FRP (and getting air voids). I was wondering about using a Silicone bag system to infuse the resin into the dry CSM? I have made many RTM Lite molds but never have combined a “soft” mold aka Silicone (with a mother mold of course) with a Silicon inner skin to infuse the Resin?

I am sure this is possible but has anyone done this? Any suggestions?

#2

CSM does not allow for resin flow.

#3

I tried infusing triaxial fiberglass ( uni 45x uni 45opposite, csm all stiched together) the 600cps resin infused on a flat surface only 7 inches before the resin stopped flowing.

Maybe he can use flow medium like Enkafusion or Airtech netting?

#4

The media is not the point in this question… But since you brought it up I will be using “rovicore” it has a layer of glass on either side of a flow media. I want to know if you can vaccume a silione mold on top of another silicone mold (assuming the mother mold is stable.

#5

It would need supporting structures, and the silicone will likely get pushed into the weave and make resin flow impossible at the surface.

#6

I dont know exactly wht you are doing…but the mold itself…if silicone…would conform oddly under vacuum. In otherwords, it would not stay the same shape. I’ve that that problem before. You might even have issues of the mold pulling into the fabric…

#7

Was a flow media used? Sounds like the major mistake was using CSM with the biaxial. Did the resin stop flowing because it gelled?

Using two flexible silicone molds will be just like using a heavy duty envelope bag. The molds will almost certainly not hold their shape when vacuum is drawn.

#8

Silicone has been used before in a similar way to RTM Lite, its called closed cavity bag moulding or CCBM. Cool stuff, comes out of a tube in a caulking gun to then be spread out, but after drying is like a flexible bag that never needs waxing or PVA unlike fibreglass membranes in RTM Lite.

#9

what if you cast/poured/spread the silicone on the stairs let it cure. After that you make a backup structure with plaster/hemp or glass/resin. I believe this is called a glove mold and I’ve seen it done for many smaller parts for props and movie gag stuff like guns. Or else forgo the silicone and just use glass? As for the bag, you can make an reusable vacuum bag, RVB, It’s fairly pricey so i’d only do it if were going to be making a fairly good number of parts. Of course if you mold one step and then make a dozen or more, it could be useful. Otherwise just use regular bag material. The RVB is especially nice because you can make the bag seal without the use of sealant tape and you can also build in channels for resin flow, intake ports, and vacuum ports. It’s a bit of setup, but again its great for making large runs of parts.

#10

Google smooth-on. They deal in silicone vacuum bagging systems. They have some truly amazing uses. Look them up on youtube also for some very helpful silicone uses and step by step on how to use their products.

It is pricey but you can reuse it over and over instead of buying your bagging material, peel ply ect.

#11

I know it’s been a few years since this thread was created, and a little more than a year since the last post, but I wanted to give a bit of information so that others can find it useful.

Vacuum bagging onto a rubber tool is difficult but the results all depend on the durometer of the rubber. A low durometer, up to 45A, will distort under vacuum pressure. A higher durometer, over 55A, will hold up much better. 60A hardness or higher will likely not distort.

I choose the hardness of my silicone molds based on the detail of the part being molded. If there is a lot of negative draft, I need a lower durometer silicone because the elasticity is much higher, so its resistance to tear is higher than a high-durometer silicone. If I need to remove the mold from the part like a wet sock by turning it inside out, a lower durometer rubber is needed because it is harder to do with a high durometer rubber. How the silicone is removed from the part, and the amount of detail on the part being created, should be the first thing to consider when choosing the hardness of the silicone to use for the mold.

If a lower durometer silicone is needed but you still want to vacuum bag or infuse the fiber reinforcement, adjust the vacuum pressure to about 6 inches. Six to eight inches of mercury is all that is needed to infuse a Light RTM part in the cavity of the mold system when the B mold is a shell laminate. If the mold has no negative draft, create a smooth caul plate so that the pressure distribution is equalized across the bagged surface, and infuse at the lower pressure.

Also, consider the type of silicone you want to use. RTV silicone is softer and does not hold up as well as Platinum cure silicones but platinum silicones are very sensitive to contamination, so an inhibitor is needed.

Lastly, I have had students use silicone molds for vacuum bagging and the parts turned out great because we followed the principles listed above. I also know of a company that makes faux rock fiberglass window wells using the VIM process with a high durometer silicone A mold and a silicone bag system, and the surfaces are not distorted, also because the principles listed are used.

Hopefully this helps someone.