Bladder Mold

General Composites Discussions Composites Talk
#15

I think wyowindworks, correct me if I’m wrong, You’re saying that the ‘bleed’ ports are for the air to escape and to avoid void entrapment?

If so then would this bleeder do the same? If I’m using prepreg and I have bleed ports this means my part is losing some resin and thus creating a higher fiber ratio. Which may be good if intended but, what if my resin ratio is already low and I don’t want to lose any more resin.

I’ve had the same idea as Trackdawson in using a bleeder instead of vents. I could see some issues with keeping everything in place and even… overlaps might cause an imprint but, maybe it doesn’t matter since it’s on the inside.

As for too much pressure!!!? Bring it on! Though the issue I run into is having a mold that can take it. The more compaction the merrier right?

#16

If you do a laminate debulk and no resin is absorbed or evacuated then your laminate is almost guaranteed to be porous. If you are trying to control the Vf by how much resin is the prepreg then you have have insure that you are getting a compaction level that can guaranteee little to no porosity. It’s more reliable and consistent to use a juicier layup and control the Vf through the proces with excess resin porting… At least that is my experience.

The compaction pressure is part of the process that dictates the Vf. Vf can only be increased by pressing the fibers closer together. If the mold isn’t vented then compaction cannot occur regardless of the pressure. For the fibers to get closer together then air or resin must leave. If you use less resin but don’t get adequate compaction then the laminate will have a higher degree of porosity. The impact strength of carbon fiber can decrease dramatically in laminates with Vf greater than 60 percent or laminates that are porous.

I discussed the issue with breeders and high pressure in my previous post. The bleeder must be vented which has it’s own issues.

#17

Not sure what Vf is? Could someone explain. Thanks

#18

Fiber volume fraction. This tells you how much of the total volume of the laminate is fiber and how much of the volume is resin.

#19

I have an idea. Not sure if this is something that can be done but it looks like it would be plausible. This other part I am making is a Tee shaped part. If this would work it could be done on all my parts. What do you all think of using a solid silicone mandrel. The Silicone mandrel will be smaller by the thickness of my carbon fiber layers. Lay my pre-preg on top of the Silicone mandrel then close up the mold and cure it. Haven’t seen anything like this (not to say it hasn’t been done) but it looks like it could make a nice looking part. Would love input from everyone! Thanks a million!!!

#20

good thinking

It’s definitely not a new technique. We just made a part here at my work using this technique, came out awesome. I also made a bridge for a student competition using this technique. The compaction is ridiculous… I had Boeing engineers asking how i made it and whether it was autocalaved.

This is one of my favorite techniques. It gives such good compaction. You do need a very strong mold to contain all the force of the silicone expanding. I know a good silicone that is easy to work with and has a hight CTE for this application if you need a recommendation. Also it’s good to imbed a piece of rope to help in removing the mandrel after curing.

I’m not sure exactly how to calculate the compaction in PSI, maybe some one else can give some equation for it?

You can also pour the silicone and use something to displace some of the volume if you want to control the compaction. What I mean is if you have a part with varying shape, the thicker bits will get more expansion and thus more compaction than the thinner bits.

Nice drawing by the way. Though I don’t thik you’ll need locators for the silicone mandrel. You could also cast the longer piece so it has a small recess for the longer T piece to fit in. The metal alignment stuff you show could pose more difficulty than a purely silicone piece. If you did want to use location pins, just cast the recesses into the silicone pieces, no need for sleeves or screws.

Of course this technique require making a mold for the silicone piece. You could either machine something, or 3d print the pieces, pour a mold, then cast the silicone pieces. You can also make the silicone with an an inner and outer piece, if necessary to help with it’s removal after curing.

You’re planning on doing a metal mold? You can do a two piece mold and then either layup right on the silicone, or you can lay up on the mold and then join them together. I’d just go right on the silicone. It’s pretty high durometer and would be easy to work with.

There’s a lot of considerations to make things fit, but very doable.

#21 
  1. Using this (silicone mandrel technique), if my CF layer is approx. .072 thick how much gap do I need between the mold and mandrel?

  2. What is High CTE?

  3. Does the silicone expand after it is heated?  If so I could make the (mandrel) even smaller than the thickness of the material (so you can get the mold closed without pinching/wrinkling?

  4. FWI, Pre-preg & aluminum mold.

  5. When I put the mold together, does the fibers bunch up at the parting line or other places?  

  6. Is the (silicone mandrel technique) forgiving?  Meaning do the layers of CF have to be perfect in the layup?

  7. Reason for the locators is to hold Center Lines for assembly of the final part.
  8. How do you hold the two silicone pieces together for layup?
  9. Using this technique, can I reuse this mandrel over and over again?

The layup will me similar to that “smart tooling” mandrel I showed earlier in the thread. Could you PM me the info on the silicone?

#22

Many questions! Good!

  1. Gap? I would only really have a gap to allow for some variation in the thickness of your layup schedule. IF you do have some gap, you’ll be able to add material to increase the strength later. Otherwise a tight fit is good and will give good compaction. By tight fitting, I mean the size of the mold cavity must account for the mandrel and the carbon. As we had discussed in the thread, having vents is necessary to allow air/resin to escape.

2)CTE is Coefficient of Thermal Expansion. Or the rate of expansion of a material. Every material has it. There are silicone that have a higher value than most materials and thus will expand more as the temperature increases than the mold does or the carbon and thus provides compaction.

  1. Per 2, yes the mandrel will expand and create the compaction for a good part. You can make it smaller but you must have some space for the mandrel covered in carbon to fit in the mold.

4)FWI?

  1. Forgiving… well perfect is best. Why would you cure a part if it’s not right? But I suppose if there are wrinkles they will be crushed down but of course the laminate will be thicker there and will have an error so technically would be a failed part. If you just are careful and plan the layup well and fit everything nicely it should be ok.

  2. I understand keeping everything aligned is very important if you plan on laying up on the mandrel. Though you could either use pins like your drawing shows, which is probably fairly easy, or you could mold one half to fit inside the other for location.

  3. this plays into 7. If the one piece actually fits inisde the other, then the friction of sticking it in should hold it in place. Or use the pins molded in as you suggested. You probably won’t need threads, I would think friction would be good.

  4. Yup! Reusable. Over time silicone does degrade. The hotter you cook your part the more it would degrade. Also after you cast the mandrel, you should post cure it for the silicone to develop it’s maximum mechanical properties. Though the life span should be fairly good, it will lose it’s ability to expand and become more brittle as it ages.

Yah, I’ll find the silicone info in a bit.

#23

SO here’s the silicone I’ve used and continue to use

http://www.bjbenterprises.com/silicones/platinum-based/tc-5045-a-b-1/

It’s pretty nice stuff. Easy to work with and haven’t had any issues with it not curing. As always, never use latex anything when dealing with silicone and keep things free of oils and grease. This stuff pours nicely and have done great pours with no degassing and very little to no bubbles. The CTE is 5 x 10^-4 (0.0005) which is good compared to say a molding silicone with 16.5 x 10^-5 (0.000165)

#24

How will you get the silicone out of the part?

#25

Regarding the High CTE. It expands with heat, but then does it contract back to the original size after cooled? Would removing it be pretty easy?

The silicone doesn’t stick to the carbon fiber and it has some slack to remove it?

Sounds interesting

#26

Yah CTE is the rate of expansion as heat increases/decreases. It goes both directions, so if you were to heat it would expand and cool it would contract.

Epoxy doesn’t stick to silicone or much else except for more silicone. SO you could pour silicone cure it, and then later repair or add more silicone to it.

To get the silicone out you pull it! When you pour the mandrel, inlay a piece of rope into it so you can tug on the rope and stretch the silicone slightly shrinking the diameter and making it pull out easier. Does take a bit of force but as long as there is no major negative draft it comes out.

In Ducks case, he’ll be doing a two part mandrel so one piece will come out and allow the other to be removed. it’s a bit tricky in forming the molds for the mandrels, but I think it’s doable.

#27

Is there a problem with silicon migration? My parts need to be bonded to other parts once finished. I’d hate for silicone to somehow hurt this bond.

#28

I don’t believe so with Platinum silicon. There might be with other grades.

#29

no, there is no migration… the silicone is cured at RT then post cured to something like 450 or whatever the spec says. There are some silicones that are ‘wet’ and exude an oily like substance, but these are for casting things like concrete. The silicone I’m talking about it platinum cure, composite aerospace grade material. Used all the time in composites applications. It self releases, holds it’s shape, provides excellent compaction, and has a good life span. The parts i’ve made/make are secondarily bonded using adhesives. Of course you have to follow bonding procedures after cure, normal stuff.

#30

Fantastic! That is the answer I was looking for :slight_smile:

Seems like I will be ordering from BJB again soon.

#31

Oh definitely. They mix up some good stuff there. Sadly now I live far away… i used to work around the corner from them so I could stop there on my way home from work.

Also you should call them and get a sales tech on the phone if you have questions. They would be able to give you good info on the various chemicals they stock.

#32

What kind of core material are they using? Pretty neat process to watch. It looks like silicone but they melt it out after curing.

[ame=“http://www.youtube.com/watch?v=N8oTbr0cHFM”]Time factory and production virtual visit at twohubs.com - YouTube[/ame]

#33

There are latex mold making liquids that can be purchased and allow you to paint on layer by layer for adequate buildup.

Good thread by the way, great info

#34

They use a wax which withstands a certain temperature, but melts when heated a higher temperatures.

The cores are injection molded, same process like Investment cating for the cores.