1st timer worries with clamshell mold, hollow tube, and prepreg

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#1

Hi all, first post here, I’ve been snooping around and while I feel there’s more I could learn from more research, I’d end up researching forever…

I am preparing to set up a prototyping operation so I can investigate product performance of various materials, angles, and resins. Product is a nearly round, straight tube, about 0.040" thick wall. I’ll make prototypes at 3’ long. I need to control OD and certain axial features hence the clamshell mold.

Mold will be made of 7075-T651 Aluminum. It will have heaters and a ramp/soak controller. Materials will be epoxy prepregged sleeves of CF, aramid, and fiberglass and hybrids.

Bladder will be a simple 2 mil poly sleeve. I intend to seal the end with my bag sealer. I’m working on way to seal the supply end so I can vent trapped air. Bladder may stay in the part when done, it’s not critical.

Questions and concerns:

  1. Should I make the mold set so it’s open at each end or closed at one end? There is no need for the product to have a finished closed end. Just thinking about sealing the ‘far’ end.

  2. Shrinkage: I can’t find out much about epoxy resin shrinkage except ‘not much.’ Right now I have it drawn with a 1.5% shrinkage factor…sound OK?

  3. Trapped air…really worrying me. Are vent holes necessary? Here’s my idea:
    a. pre-assemble my sleeve/bladder partly over a long bottle brush,
    b. wet it with resin before I lay it in the mold,
    c. loosely clamp the mold,
    d. gently pull bottle brush towards the open end, to burp out any trapped air and press the sleeve tighter to the mold,
    e. tighten mold, pressurize, and begin heat cycle.

???

  1. Thoughts on use of bleeder material between bladder and sleeve if the bladder is to remain in the part?

  2. thoughts on whether vacuum could be helpful here, or would it just add complexity?

Looking back, this is long…I thank you in advance for your help,

Haywire

#2

Wonder if I didn’t describe my needs, or am I asking too many questions?

#3

See my thoughts in RED below:

Have you seen my wet, juicy, and bladdery thread: http://www.compositescentral.com/showthread.php?t=7007

Hope that helps some.

#4

Sleeves perform pretty poorly in tube structures unless the tubes primary purpose is to handle torsion forces. For bending you need lots on 0* and some on 90* to boost the hoop strength to offset buckling.

I actually need a fair bit of flexibility and some hoop strength. I have several varieties and diameters (to stretch or shorten) of sleeves and I’'ll be experimenting with the best blends for my needs. I have the goal pretty well mapped out and testing rigs constructed. I also could add some unidirectional fabric in either/both 0° and 90° to really control behavior in hoop and flex.

If the ends of your mold are open the bladder will pop at very low pressures (3 psi) especially if the temps are elevated. The bladder must be supported by the mold in every location.

Yep, thanks - I’d certainly seal off each end before inflating but I’m pondering whether there’s a good reason to keep the far end open for bubble removal.
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2. Shrinkage: …I have it drawn with a 1.5% shrinkage factor…

The part will not shrink that much.
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I’'ll have it made assuming 0% shrinkage - thanks.
[i]
3. Trapped air…really worrying me. Are vent holes necessary? Here’s my idea:
…(refer to original post)

You will end up with lots of pinholes…The air trapped between the bladder and the layup will migrate out quite easily.
[/i][COLOR=Blue]
I was hoping you’d say 'great idea, you’re brilliant, the bottle brush will push/pull the air out each end" (assuming the far end remains open at this point). But, sigh, no.
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I prefer to do the layup into the mold in a void free fashion, close the mold, and then insert the bladder…You want to use juicy prepregs and allow the bladder to drive out the excess resin. If the prepreg is too dry you will end up with lots of voids.

[COLOR=Blue]OK…so I have to think of how to keep air from between the bladder and the prepreg. A seam along the length is not going to be acceptable for the part needs. I understand the ‘juicy prepreg’ concept so I thought I’d superjuice the standard prepreg with some additional resin, and in doing so create a better finish with hopefully no fabric bleedthough…but I’m really guessing and projecting and won’t know for sure until the molds are done and I’m actually in the game…[/COLOR]

Have you seen my wet, juicy, and bladdery thread: http://www.compositescentral.com/showthread.php?t=7007
Oh yeah, several times before this post. Outstanding.[/quote]

Hope that helps some.
A ton, and please accept my sincere thanks.