Silicone vacuum bags

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#1

Hey all,

I’m looking to develop some reusable silicone vacuum bags for several of my pre-preg parts that have proven to be quite popular, so I am interested in speeding up production by using silicone bags.

The problem I have, is that all of these tools are small multi-piece moulds. All of them are currently being envelope bagged, and so it isn’t an issue. However if I were to switch to silicone bags, is there a way to tackle that problem reliably? I know I could use bagging tape or cauking silicone etc, but in all honesty I don’t find those methods consistent enough to warrant incorporating them.

I can post photo’s of the moulds if you like, but as a simple description - one is a pipe, another is a simple four-sided box, etc.

#2

You can get silicone bags,or the raw sheet of silicone but you can also get a sort of silicone zip seal to put on the siiicone

#3

I’ll be making the silicone bags myself, I can get Rhodorsil 3428 cheaply enough through my second job and it works very well.

I’m sure I won’t be able to get those silicone zip seals locally, we don’t have anything good in Aus… So it will probably need to be a solution that I can make myself.

#4

I have been making silicone vacuum bags recently and had to work through a bunch of issues. Most of the problems were related to the biggest strength of silicone - it doesn’t stick to anything except silicone and glass…

I used the brush-on silicone vacuum bagging product from Smooth-on. They have two. The other is a spray-on silicone but I went with the brush-on platinum cure product for cost reasons.

The first issue is sealing the bags. When I researched it online, most of the solutions seemed beyond my skill and patience so I developed my own method.

Instead of using tape to seal the edges, I used more of the brush on silicone to make the bag all one piece with the exception of one side to allow me to insert the mold.

To seal the remaining side for the vacuum, I glued on a regular reusable bag seal which I cut off a Home Depot vacuum bag. You would obviously use something rated for high temp to seal it. Whatever you use today will probably work.

I used Sil-poxy to glue the bag seal to the silicone bag. This is the only product I found that worked for this purpose. It’s not cheap but none of the silicone based glues I tried were even close to being effective at adhering to silicone reliably.

I also use the sil-poxy to stick the vacuum bag connector to the bag and seal the edges. This isn’t 100% necessary but I like to eliminate the leak potential where possible.

All that worked just like regular vacuum bags but with the benefit of not needing the release film etc. also, because the bag was made in the shape of the mold cavity, it just goes right in without any pleating and all the other tedious parts associated with bagging complex shapes.

I also work with two-part closed molds (mostly) and I am still working on a reliable process to do it all as one step. I have decided that, for my mold design, it is most likely to work if I use the silicone vacuum bagging product to make an inflatable bladder instead of a bag.

The process of making the bladder (which I haven’t yet finished) should be broadly the same as making the custom bag except I intend to add a one-way valve instead of the vacuum bag connector.

Based on my experiments with the bagging silicone and sil-poxy, I am fairly confident the bladder process will work once. I am not 100% sure how reliably it will release from my part for a second and third use though. It’s not that hard to tear by mistake on a sharp edge…

For simple shapes like a pipe / tube or a square box, I am positive a silicone bladder would work and release cleanly. Silicone usually has fairly good heat resistance too. I haven’t checked how much heat the bagging silicone can take but I have had some of my others in the oven at 300-400f without issues. I cast tin and pewter in mold max 60 molds.

It is easy and cheap to buy silicone vacuum tubes too. Thinking out loud, maybe one could be built right in to a silicone bladder and stick reliably without additional glues.

To be honest, I am not sure I would buy the bagging silicone again for regular open mold bagging. I was able to achieve my desired outcome just as reliably using regular silicone inserts in between the part and a regular vacuum bag.

For a simple straight pipe / tube shape, maybe you could use a more rigid silicone rubber (like Mold Max 60) to make a male core insert. You could then wrap the prepreg around this semi rigid silicone cylinder and then insert that into the two-part female mold for curing.

I have read about companies using compressed air to reliably remove the silicone from inside the pipe after curing. I have used a variation of this myself to make cf tubes. The difference with what I did is that I wrapped the outside of the tube with smooth release film instead of using a female mold as I was only interested in the internal diameter of the finished tube.

Smooth-on sells trial size pots of their bagging silicone for less than $30. It is worth giving it a try.

#5

Thanks Zebra. I’ll have a look into the seals you mentioned.

My objects aren’t actually super simple, there is a bit of complexity to them. Just the simplest description is a pipe and a box. I have considered bladders for the pipe, but it isn’t something I have done before and I don’t really want to mess around too much with these parts. They are working perfectly at the moment, so I just want to see if I can simply speed up the process a touch without too much experimenting and wasting materials. Will get into bladders etc later and may switch these over then if I can make it work reliably, but in the meantime I’d like to stick to vacuum.

Really I just need to find a way to either make a silicone bag that encases the entire mould and can seal reliably; or I need a way to make a regular silicone bag and just seal the barriers of the mould on the back reliably. Either way, it needs to be reliable otherwise there is no point switching from regular vacuum bagging film.

#6

These are the parts I would like to make silicone bags for. As you can see, nothing too complex, but enough complexity to warrant a reusable pre-formed bag.

You can also see I’m currently getting a perfect finish on these parts straight from the mould (none of these are clear coated, that’s raw pre-preg), which is why I don’t want to mess with the process too much.

#7

I use a lot of silicone for bags. In 2015, it was about $105,000 worth.

We have a big variety of molds that we use silicone bags for. They range in size from about 2’ x 2’ x .5" up to almost 8’ x 5’ x 2’. They can be unwieldy to handle. It’s not so much the weight (although the larger ones can be heavy), it’s the wet noodle factor. We’ve built in lifting points on the bags and use a jib crane with a spreader bar apparatus to set them in place.

We started this operation about 4.5 years ago with LRTM molds. About 1 year into the operation, a new plant manager was brought in. He thought silicone bags were the way to go. Even though I disagreed for a variety of reasons, being the good soldier that I am, I did my best to implement them. He’s gone and I’m trying to get us back to LRTM molds. Let’s just say I’m not the biggest fan of silicone. I think it has it’s place but, for us, it’s a black hole. Corporate has finally recognized that.

I can give a little generic advice on this stuff. The first thing I would do is ask the vendor about compatibility with your resin. From what I understand, platinum cure is better in all respects compared to other cure formulas but it needs to work with your resin.

In general, epoxy is tougher on the bag than poly or vinyl ester. We primarily use polyester on the products we build and after a few parts, the bag doesn’t release very easily. We’ve resorted to using a slurry wax on the bag to improve process time and bag longevity. We are cycling up to three times a shift so we are probably tougher on molds than most.

Be careful when pulling the bag. If there is a little snag or sharp edge, the bag can tear. Since it’s being stretched when you are pulling it, it can open up like you pulled on a zipper faster than you can let off. You can patch the bag but that’s sometimes limited as well by the shape of the bag. Patching a corner is much harder than patching a flat.

Speaking of patching, our supplier (Prairie Technologies - Sworl) says you can go over cured virgin material with new material up to a year later and it will stick. In practice, I haven’t found that to be the case. A day or two is okay but after that adhesion is questionable. And never try to bond Sworl to Sworl if you’ve used the bag with styrene based resins. Styrene gets into the silicone and totally screws up the cure. For bonding, just use the cheapest one part silicone caulk you can get (look for acetoxysilane in the ingredients).

Even though it’s stretchy, it isn’t a miracle material. The bag needs to conform fairly close to the part shape or bridging can be a problem. You can get away with excess material (just like you can when using bagging film) but you’ll get some pretty big resin trails on the part. The silicone won’t completely conform to the surface like bagging film will where you have wrinkles. You should build an offset to represent the part thickness.

Another thing that I’ve found useful is to install a polyester mesh in areas that need reinforcement. This adds to the expense as you’ll use a bit more silicone but I think it’s worth it in some areas of high stress. Here’s a link to the poly mesh: http://www.joann.com/utility-fabric-cargo-netting/10173359.html?promo=20top&gclid=CPzv7sWpzcwCFcQjgQodjRwCfw

That’s about all I can think of off the top of my head. I’m sure there are other things I haven’t put in. If you have any specific questions that this little diatribe of mine didn’t cover, please feel free to post them up.

#8

How come you had silicone vacuum bags that weren’t in the shape of the mold? Were you using a generic bag shape for lots of different mold patterns? The silicone bagging product I have is designed to be brushed into the mold cavity to make the bag, so it is always in the shape of the mold.

BTW, the issues with resin compatibility with platinum cure silicone don’t usually apply to epoxy. I’ve had one or two urethane resins that warned against using in platinum cure silicone molds, or not to use without mold release but never an epoxy. Do you use some epoxy resins which have issues curing in platinum silicone? If so, are you able to share which brand so I can avoid a potential issue?

You are so right about how sometimes adding more silicone later won’t work. It usually works for me if it is the same type of silicone and I clean the surface with rubbing alcohol first but I have definitely had some that don’t stick to other cured silicones.

I have never successfully repaired a torn silicone mold anyway. Once they break, I usually cut them up and use the chunks of silicone as keys for the next mold.

I think silicone has a place in bagging but I am not convinced it is best for every scenario. I know that I would always be reluctant to mess with a process that worked unless there was a meaningful benefit.

I prefer using a semi rigid silicone insert between the part and regular bagging film. It makes any vacuum bag reusable with the right seal and is less prone to tearing as the silicone doesn’t stretch around sharp mold corners etc.

I think it’s strength is for complex parts that take a long time to bag (because of all the pleats) and possibly with some larger parts to save on all the costs for release films and breathers etc.

When the minimum cost was over $1,000 it wasn’t worth testing imo. For $25 or $30, it is worth spending 10 minutes on a little test to see if it is for you. If not, it has lots of other uses. I used some of my leftover batch to make a non-stick surface for my work table to stop it getting covered in epoxy.

I also used some to add a non-stick coating to a plug so I could quickly make multiple rigid molds without spending the time applying wax and pva.

#9

See my comments in blue above.

#10

Guys thank you for your thoughts and replies.

I should point out that I am not new to silicone vacuum bags, I have done it before with Rhodorsil 3428 so I know the process and I know the materials all work very well. Epoxy does definitely degrade the silicone much faster, so I do intend to still use a release film. My inexperience is that I’ve never done a split mould before, all the silicone bags I’ve made have been for one piece moulds where I sealed the bags by simply having a second fibreglass flange section that I bolted or clamped down on top of the silicone on the mould flange. This was extremely quick and reliable, worked every time. So I just wanted to see if people had any experience with making it work on a multi-part tool.

As for messing with the process, I don’t believe that I would be messing with it too much. Keep in mind that these parts are quite small, but all use triple their dimensions in vacuum bagging film. Considering I could make a silicone bag for roughly what it costs me in bagging film for 15 pulls, and I am currently producing 20-30 of each of these products in a year, I don’t find cost a prohibitive factor. The main benefit would be in the time saved by skipping the bagging film, if each part takes me ten minutes to bag then that’s 3-5 hours in a year that I’m spending bagging each of these parts. At the moment there’s 4 of them, so that’s potentially 20 hours of time spent bagging parts all year. That doesn’t sound like a lot, but I would rather spend those hours elsewhere if I can, so if I can get the bagging process down to 2 minutes then I would be ecstatic!

#11

I have been running with silicone bladders for a while, it was a bit of a struggle to get the process down and have a repeatable product as well as a bladder system that was sturdy enough. The biggest improvement was incorporating internal intensifiers, this really applies excess pressure in tight corners where air pockets form and the laminate has a difficult time conforming into a tight corner. This was one of my first attempts, the new bladders are slightly different and are smaller than the mold to accommodate the thickness of the laminate.

#12

I’ve actually followed your progress on that front Compositeman, and to be honest it is a large part of what has put me off bladders. If I was developing a new product that I hadn’t already perfected with vacuum bagging film, then I may be willing to go through the trial and error process to get a workable bladder. But I don’t want to change process completely and mess around with all the troubles that come with it.

Honestly, I’m perfectly happy with vacuum bags, I don’t have a problem with continuing the way I’m going, it works. I just wanted to find out if I could speed up the process at all and save myself some time, but it’s looking like nobody has done what I’m trying to do either. So I think I’ll persist with regular bagging film for now.

#13

What about doing your tube part with a full solid silicone core/intensifier? I have done parts like this and couldn’t be easier or simpler.

Layup the moulds and close, drop in the silicone solid and retain the ends so that it can’t squeeze out, then cook. Lots of pressure too.

For your shape you could probably do the solid core as a two piece core to help with pulling it out.

#14

Yeh that’s something else I’ve considered, but I’m not entirely sure the fibreglass mould would be up to it. Its an 8mm thick mould, so it’s bloody solid, but I don’t want to risk breaking it for the sake of trying another process.

It would also use a hell of a lot of silicone, and even with employee discount the 3428 is damn expensive stuff for those volumes.

#15

Hi Martin.

Sorry to be off topic , but those parts are great , are they 261 pre preg ??

Cheers

Tim

#16

Thanks Tim, yes correct, they are all VTF261.

#17

For large parts where the volume of silicone required would be too expensive (which is almost everything I do), you don’t use 100% silicone. In fact, pure silicone is not always ideal for this purpose anyway because most lack the rigidity to be optimal.

The process I use is somewhat similar to the hard-shell silicone molds people make for parts that are too large for box molds. I.e. You apply a thinner layer of silicone, add silicone keys and then support it from the back with something rigid. It can be supported with almost any resin, epoxy putty, fiberglass, carbon fiber etc or a combination.

After a lot of expensive trial and error, I settled on a surface layer of the thixotropic silicone from Smooth-on, then a layer of mold max 60 with mold max 60 keys. Mold max 60 is harder so it prevents the keys showing through on the surface layer. I then support it with a brush on plastic layer plus carbon fiber (to stop longer parts fro mm snapping.

This process lets me get away with using $25 instead $125- $200 worth of silicone.

The hard part is deciding how much (if any) to adjust for part thickness. The silicone has some flex and you want it to be a tight fit but not too tight. One day I’ll figure out how to calculate this part without trial and error…

#18

You lost me at this bit… Haha.

I’ll mess around with all this stuff when I’m developing a new product that vacuum isn’t ideal for. For all these parts I’m currently doing, vacuum is the most practical process to use so I’ll stick with it.

I may have come up with a solution for sealing the back of the moulds though, so I’ll update this thread with how I get on.

#19

I have had a few questions about the bladders I showed in the pic, and how it is removed. The bladder is slightly smaller than the laminate, I calculated .040" (1.02 mm), so the bladder is a perfect fit after layup. The bladder is inserted after layup and is vacuumed out slightly to collapse, mold closed and pressure to the bladder applied. When the part is cured, the bladder is vacuum collapsed before the de-molding process, and it easily slips out of the shell with little effort. and can be reused many times.