Composite doubts

General Composites Discussions Composites Talk
#1

Hi Guys

I have just started a new hobby of rc planes,helis and copters. And now plan to build some of my own.

Here are a couple of doubts I have:

  1. Prepregs already impregnated with resin right? Vacuum bagging and curing them won’t require any more resin to be applied?

2)We can cut into prepregs(before curing), can’t we? Say make slots in them, shape them to our needs, make them into circular plates etc. Can we drill holes them? Has anyone tried making small holes in uncured prepregs, something that can generally be achieved by drilling only

3)For general (non-prepreg) vacuum bagging how does one estimate laminate thickness and and volume fraction of fibres( I researched a lot online. Have not got any real answer. Best answer was from Fibreglast. For a particular farbric,resin and manufacturing/curing combination, there are estimates for laminate thickness and Vf. Is that how it is supposed to be done? by experience?)

  1. Making round hollow tubes. The best (and cheap) DIY method? Just vacuum bagging will still give a seam line, the line where the vacuum bag rolls over and meets the other end. Would adding a flexible half cut pipe on that end of the tube improve finish?(between the vacuum bag and the breather cloth to not be indirect contact with the actual fabric)

  2. What vacuum pump should I go for? How do we size (cfm or volume flow rate)? I understand better the relative pressure drop with respect to atmospheric pressure, more the pressure acting on the part. Higher the cfm, more quickly vacuum can be achieved. However, a local manufacturer told me that a smaller pump(low cfm) with high vacuum pressure rating will achieve lesser vacuum for a large part. Any scientific reasoning behind that (unless the larger the part the more chances there are of leakages?).

  3. Dry vs wet vacuum pumps

7)Safety, what masks to wear? How to dispose off waster epoxies and fibres.

Apologies if any or all of these are covered in other threads.

Thanks in advance:)

#2

Correct, althoug some application may require some more resin content, ordered by customer specification in the fibre or by applying a glue film (resin film) by the customer him/herself.

2)We can cut into prepregs(before curing), can’t we? Say make slots in them, shape them to our needs, make them into circular plates etc. Can we drill holes them? Has anyone tried making small holes in uncured prepregs, something that can generally be achieved by drilling only

Yes, prepregs can be cut with knives and scissors.
What do you need those holes for? drilling that small after cure is easier. but prepregs can be cut very accurately.

3)For general (non-prepreg) vacuum bagging how does one estimate laminate thickness and and volume fraction of fibres( I researched a lot online. Have not got any real answer. Best answer was from Fibreglast. For a particular farbric,resin and manufacturing/curing combination, there are estimates for laminate thickness and Vf. Is that how it is supposed to be done? by experience?)

cloth thickness by seller documentation. Vf mostly guestimate, experience, and calculating the cloth weight, leftover resin (in the tols as well) and cut off scraps.

  1. Making round hollow tubes. The best (and cheap) DIY method? Just vacuum bagging will still give a seam line, the line where the vacuum bag rolls over and meets the other end. Would adding a flexible half cut pipe on that end of the tube improve finish?(between the vacuum bag and the breather cloth to not be indirect contact with the actual fabric)

The bag won’t be of much importance with finisch quality.
Probably laminate over a aluminium tube, and switch the peelply for compoflex 400 or similar for better surface. only sand the seam

  1. What vacuum pump should I go for? How do we size (cfm or volume flow rate)? I understand better the relative pressure drop with respect to atmospheric pressure, more the pressure acting on the part. Higher the cfm, more quickly vacuum can be achieved. However, a local manufacturer told me that a smaller pump(low cfm) with high vacuum pressure rating will achieve lesser vacuum for a large part. Any scientific reasoning behind that (unless the larger the part the more chances there are of leakages?).

What are you going to use it for?
smaller pumps will build up their pressure, but can’t handle large leaks. bigger flow is easier. For prepregs and infusion use one with can reach at least 96% vacuum. The bigger your products, the bigger your pump(s).

  1. Dry vs wet vacuum pumps

Both have their pro’s and con’s. My own pump is dry. No oil to leak, replace, get in the filter and fill the building with smoke, etc. But oil lubricated pumps are mostly better capable of reaching better vacuum, and probably will last a bit longer too. (have no real experience yet to support, my dry pump keeps on going)

7)Safety, what masks to wear? How to dispose off waster epoxies and fibres.

see your docs. I usually wear one with organic filters, if regular ventilation isn’t is sufficient. For dust just normal dust masks. Wear good gloves and skin protection! wash off with a good soap if it comes on your skin. don’t use acetone, it will desolve and thin the epoxy, make it even faster penetrable trough your skin.
Dry fibres in the normal trash, prepregs after being heated in the normal trash, cured epoxys too, uncured as chemical waste. can be different with other regulations though.

#3

just a little more info you are getting from a newbie FYI

why bother making the tubes when you can just buy them for cheap off ebay
http://www.ebay.com/itm/Carbon-Fiber-Tube-3K-16mm-Diameter-330mm-Long-for-Quadcopter-Multicoptor-4-Pcs-/191359293326?pt=Radio_Control_Parts_Accessories&hash=item2c8de7178e

the vacuum pump I own is a habor freight one made for vacuuming out car ac lines. I think it was around $80 after 20% off coupon. not sure if it will work well, but it is pretty cheap and I’m guessing fine for small stuff.

#4

I use the same harbor freight pump and it has been working great

#5

Thanks a lot for the reply

#6

@aluminiumweld

Firstly, I am in India. The shipping would kills me

Secondly, Where’s the fun? :wink: I was looking for CF tubes in local hobby stores. Everything is out of stock

Can you send me a link for specifications of the pump? I can compare with the ones locally available.

#7

@susho, I replied to your comment, it’s stuck in moderation, not sure why, thanks for the advice. If my reply does not come up soon I ll retype it

#8

@Sushi
“Yes, prepregs can be cut with knives and scissors.
What do you need those holes for? drilling that small after cure is easier. but prepregs can be cut very accurately.”

Most parts in RC models I see need holes (of about 2mm diameter). How can such small holes be cut in prepregs before curing? I hate machining composites, it gets too messy, plus I don’t have the right equipment.

“The bag won’t be of much importance with finisch quality.
Probably laminate over a aluminium tube, and switch the peelply for compoflex 400 or similar for better surface. only sand the seam”

I think I have an idea for it, will post it here if it works out.

“What are you going to use it for?
smaller pumps will build up their pressure, but can’t handle large leaks. bigger flow is easier. For prepregs and infusion use one with can reach at least 96% vacuum. The bigger your products, the bigger your pump(s).”

Arms and rods of RC planes not more than 2meters (at max).
I have found a good deal for a 3.5cfm, 15 microns vacuum pump, locally. That should be enough I guess.

For Safety, well I live in a very polluted unregulated city. There are not many regulations for waste disposal. Things are often burned if there are to be gotten rid off. I would prefer to dispose my wastes as one would in US/Europe.

Also, I have breathing problems, want to make sure, I am well protected against epoxies (and so are others in my surroundings).

Looking forward to your reply.