Silicon thermal expansion pressure

General Composites Discussions Composites Talk
#1

Hello to everyone.
I want to create an aluminium mold for the construction of a prepreg saddle. I want to lay the prepreg on the mold then put a silicone piece that will have the morph of the hole engraved by the cnc on the aluminium .Cap the top and screw it with a piece of metal and then let the thermal expansion of the cured silicone to apply pressure to the walls of the piece.

  1. How does it sound as a theory to you ?
  2. Is there a way i can calculate the pressure that will be applied at the prepreg with the expansion of the silicone ? I can calculate how much the silicone would like to expand , but i dont know the result of not letting it to expand measured in bar ?
    3)Does the hardness of silicon rubber used effect the pressure effect on the prepreg ?
#2

I recently made some parts just like this. They are the end male moulding shown below. Laid up the pre preg preg in two halves and closed the moulds. Then dropped in my silicone plug and placed a piece of fibreglass plate over the top of the silicone plug and secured it there with wire while cooking.

#3

How do you know the pressure applied in every spot of your piece ? By the picture i would guess around 2 cm expansion along the tube and less than 1 cm to the walls.
Do you just place the piece of silicon inside or is it air pressured ??
It looks like good finish though … bravo friend

#4

The silicone plug is a very good fit to begin with. About 1mm clearance to the laminate.

For this I made a dummy part with sufficient layers to replicate the part thickness, prepared the inside of it to be smooth and then cast my silicone plug (about 2.5kg silicone). My silicone was Shore A 40 hardness.

I got some bridging in the end corners and elsewhere but not much. These are still good usable parts. Not bad for a 9 ply laminate with no debulking.

#5
  1. no theory, it’s a pretty common technique. I’ve used it much with great success
  2. not sure on the pressure, one of the engineers did some calculations at one point but you can’t get a definitive answer since it’s dependent on the geometry of the part and temp. It’s a lot though…
  3. the silicone will greatly effect the rate of expansion.

THis method can have some issues, but once you get everything dialed in, it makes some great parts. The parts i’ve mad don’t get a debulk and the layup isn’t exactly perfect but, the parts always come out looking immaculate.