How to create a "bridge" in a tube?

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#1

Hi,

I plan to create a tube out of UD-prepregs which is similar to the picture attached - AND I have to put at least one bridge into the tube to improve stiffness.

ad picture:

75 mm long (3 inch) and 25mm wide (1 inch)

wall thickness 1 mm

created out of UD prepregs with vacuum bagging (I guess!?) in an autoclave

My question is:
Does anyone know how to build such a tube including the bridge!? :confused:
(same procedure: vacuum bagging; UD prepreg -> thatā€™s not a big dealā€¦)

Sorry for my english - itā€™s not my first languageā€¦

#2

silicone mandrels

http://www.compositesworld.com/articles/high-speed-press-cure-for-high-speed-racers

#3

deleted because of a mistake of mine - sorry!

#4

There are several techniques. Silicone mandrels in one but it requires significant heat and stiff heat resistant molds to get the compaction. The other is to take low density cnc hotwire cut foam mandrels. A vacuum bag is placed over the mandrel and low vacuum is applied. The layup is applied to the mandrel and the parts are placed into the mold. The bags are then pressurized with around 15 psi to compact the laminate. If the mold is large it has to be pretty strong. After cure, high vacuum is applied to the bags to crush the low density foam mandrels. They are then pulled out one end. If the molds are not capable of containing pressure then vacuum can be used in place of inflation. The entire mold is placed in and envelope bag and he inner mandrel bags are ported to the atmosphere. With this technique the parts can be produced at room temperature, any post-curing is done without the pressure to protect the molds, and the mold doesnā€™t have to be as strong.

You can also lay the fabric in the molds and just use the bagged mandrels to create the seams and I-members.

#5

Iā€™m not sure if it would work for the shape of the part in your picture, but there is a system used quite a bit in cycle frames to make them stiffer. Itā€™s called isogrid by Vyatek - www.vyatek.com/vyatek_/tech/isogrid.aspx

Itā€™s also used on/in tennis rackets.

#6

looking at the photo it looks like they inserted a moulded piece for the shear web, you can see on the left side the shear web is very flat and there is what appears to be a seam line where this insert ended. they would of used 3 bags, one for each chamber and another to enclose them to bag it, actually a bit complicated, especially if it were to taper or be very long. A better way would be to use a bladder as suggested and preforms. The bladder would always place the shear web correctly unlike the moulded piece that could deviate from position

#7

For me this seems to be the ā€œeasiestā€ way to create such a part in my small workshop so far: enough 2 compound silicone is also in stock :wink:

What Iā€™m really wondering about is how I should be able to remove the mandrel out of the tube: if I do not have any draw on the tube (about 80cm/ 31 inches) I think itā€™s kind of hard to remove the silicone mandrel even if itā€™s wrapped with vacuum foilā€¦ :confused:
ā€¦maybe seal the foil, lubricate it inside, put the mandrel in, seal it completely so that the mandrel comes out of the bag due to lubrication followed by the bag - but that seems to be a little unprofessionalā€¦

By the way: Many thanks for your effort everybody so far! :slight_smile:

#8

1: called a shear web, or spar.
2: you can make the I-beam type spar, and bond it two halves of the airfoil shape
3: you can CNC metal mandrels, and lay up as needed, then bag. Pulling mandrel out will not be easy, but normal procedure. Strap onto the tube, strap onto other side of mandrel sticking out, pull with hydraulics.

#9

1: called a shear web, or spar.
2: you can make the I-beam type spar, and bond it two halves of the airfoil shape
3: you can CNC metal mandrels, and lay up as needed, then bag. Pulling mandrel out will not be easy, but normal procedure. Strap onto the tube, strap onto other side of mandrel sticking out, pull with hydraulics.

#10

If you go with metal mandrels, heating while curing, and cool when hardened, for an easier release :wink:

#11

Ohh - never heard of dimension changes when heating up/cooling down! :smiley:

Iā€™ll definitely go for silicone or a 3D printed mandrel as I do ā€œonlyā€ have a 3d printer available - no mill or something elseā€¦

#12

The whole point of using silicone is that the cte is so massively high. You can create some amazing pressures with it, I recommend curing in a heated platen press to help keep the mold closed. You will create an under-sized mandrel (to account for laminate thickness and some fudge factor). Having draft or draw will make removal easier. But if you get silicone that will elongate as you pull on it, It will effectively ā€œneck downā€ and pull away from the part walls. Cast in some rope loops with knots to aid in removal.

#13

Excellent idea -thanks! Why not rope with beads as you have it on sun-blinds! :smiley:

As I had not used everything when using the silicone I always use for different purposes a few days ago I just tested this flabby disc (15mm high) and how it behaves under temperature:
24Ā°C -> 52,8mm diameter; 120Ā°C -> 54,0mm

ā€¦not too much I guess as it has only 13 ShoreA - pretty much the material you have for s**toys :rolleyes:

Unfortunately I do not have a press for the mold except standard clampsā€¦

#14

Silicon madrels require very rigid molds that can remain stable at the elevated temps.