Bladder Mold

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#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.