Lay-up Mandrels

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#1

So I’m looking for a bit of help here, I’ve been making these tubes lately and would like to speed up the lamination process. It’s a two sided tool, I laminate both sides with the prepreg, marry the two halves, then fish in a tie-in strip to tie the laminate together. All in all it usually takes about six hours per tube. I should also mention they are bladder molded.

The tubes are about 13" long, 1.75" tall and 1.5" wide, with a 0.1" wall. Parting line is on the construction line.

The idea I have been toying with is to make some silicone mandrels. Layup on them, freeze, remove mandrel, place “preform” in the tool, you get the idea. I’ve never done anything like that before so I guess I’m looking for input ideas or feedback. Thanks for any help.

#2

I should start by saying that I have not tried this myself but… If you can lay up prepreg directly onto a silicone mandrel then place it inside the tooling, clamp it, then cook it the CTE of the silicone may be enough to consolidate the prepreg against the tooling surface. The more you heat it the greater the pressure and the better the surface finish.

#3

That’s What I’d ideally like to do, but my resin system begins to cure to quickly before the silicone gets a chance to expand.

#4

Here’s what I would do.

Make a silicone mandrel with the proper outside geometry and a Ø.75 hole through the center.

Mount this mandrel on a Ø.75 bar for layup.

Lay up the material on the outside of the mandrel.

Clamp the mold on the outside of the mandrel.

Remove the Ø.75 bar.

Install a Ø.8125 OD bar where the other bar was. This may be best accomplished by having the end of the smaller thread into the larger bar and using the smaller to guide the larger in. A tapered section on the larger bar will be advantageous.

The change in ID will stretch the mandrel and compact the laminate.

EDIT

I had a chance to think about what I wrote. I will make the following additions if it wasn’t clear.

The above process will only apply enough pressure to aid in the initial compaction. Heat will also need to be supplied to expand the silicone. Perhaps the Ø.8125 OD bar could have a heated core, expanding the silicone before the laminate.

-Joel

#5

Without trying to highjack, can someone give me a link with what product to use for a silicone mandrel.
Bill

#6

I’ve used the silicone RTV from this site.

http://www.freemansupply.com/MoldMakingSilicone.htm

We used the silicone to make some removable cores for co-cured stringers on a fuselage test section. To make the cores, we used tooling wax sheets on the part mold to build up the right thickness offset for our laminate.

#7

Prog: www.polytek.com

Cleuk: If you already have the 2 female molds, and need a better way to lay up, look into SmartTooling, or reshapable mandrels. You can also use a secondary mandrel, with a silicone coating, or inner bladder. Lay up on that (which would also let you make a real tube, not 2 halves bonded together), insert into female mold. Remove mandrel leaving the bladder, and pressurize the bladder.

#8

Hmm interesting, I’ll have to read into that. I’ll need to figure out how to post pictures to show you my current setup.

#9

Responding to Progetto,
Bill, I’ve been looking into this too. Not sure what is making its way to oz, but Parlee always used aircast 3700 an Airtech product. Silicones Inc has “P-50” and “P-60” that have similar properties and the same cte. Also smooth-on(also in Easton, Pa) has the mold-max series.
Hope that helps.
-M