Bladder moulding

[carbongr]

what is the best material for the bladder? Is it silicone, latex or something else?
We are making a tube of about 45mm diameter (but not constant along its length) and about 1800mm long. We CNCed the mould in aluminium (total 8 pieces with a 0.3mm gap between them when fully closed) and placed the heating elements. Curing temperature is 180-200 C. Required pressure inside the blade 5 bars.
What should be the wall thickness of the bladder?
Also, what is the best way to secure the air fitting to the bladder? Are there any special for this purpose fittings available?
Thanks in advance

 

[Compositepro]

At those temperature silicone is generally called for although latex may work for one use. Check-out:

http://www.piercanusa.com/Compositeproducts.htm

You might also consider trapped rubber molding where the high thermal expansion of a solid rubber mandrel in a closed steel mold can generate very high pressure.

You have to custom design the air attachment to a tubular bladder. I've used a taper at the end of a female cylindrical mold cavity. A matching tapered plug fits inside the bladder which gets pinched betweem the two tapers. A flange on the plug allows screws to clamp the two tapers together.

 

[carbongr]

Hi Compositepro,
Thank you one more time for the advices and help.
My first thought was about pressure moulding by the high difference of the thermal expansion of silicone and aluminium, but after a few calculations I found out that in this diameter the total expansion will not be enough to get the desired pressure on the laminate.
However, I didn't think of a solid silicone male mould, but one with a hole inside in order to insert a tube (around 7-8mm diameter, made of brass), so I can get the heating element inside (diameter of 6.5mm). I had this idea for the following reasons:
-Not to burn the silicone (as the temp of the heating element is by far more that 200 C),
-To extract the silicone from the part easier (first pull the brass tube and then the silicone mould),
-If i do not use this tube, i have to heat the mould first. The heat will go through the laminate and then to the silicone. By the time it gets to the silicone, i am afraid the epoxy will cure (or get very close to cure).

Of cource i could use steam to heat the brass tube and the silicone mould (through a much smaller diameter hole (i.e. 3mm), but then again the total expansion will not be enough. By the way, i am using a silicone with a coefficient of thermal expansion 250*10^(-6) cm/cm*C. The average internal diameter of the part is about 45mm and the length 1800mm. Are my calculations correct?

Regarding the air attachment, I have designed a very similar assembly to what you describe. I have just have to wait until I test it.

 

[Compositepro]

A heater in the rubber is the correct approach. If you have any cavity in the rubber, like a steam channel, then it basically becomes a thick bladder. In the trapped rubber process, it is the volumetric expansion that is important. This is three times the linear CTE. Restraining the rubber expansion in the x and y direction causes all expansion to occur in the Z direction. John Deutch who founded D-Aircraft Products in Anaheim, CA. wrote a good handbook on it. They are now a tiny division of Cytec. If you contact them you may be able to get a copy.