Carbon arm guards

General Composites Discussions Composites Talk
#1

Hi i’m new here and to the composites world in general. I started up a new company making arm guards for ski racing and so far i have gotten my product to be pretty good, but its not quite perfect and I have a couple questions

  1. Are there any tricks to reducing/eliminating bubbles in the surface in a time efficient manner without vaccum bagging? (i use male and female mold clamped together.

  2. The parts i’m making need to take considerable impact repeativly for at least a year or two. Currently i have been using 2 layers of 12k carbon and that seems to be working but it still can break. I’m developing a new product that need to withstand much more force. I made a test piece using 3k kevlar and carbon in this order:

kevlar
kevlar
carbon
carbon
kevlar
carbon
carbon

and i have not been able to even get hairline fractures with a baseball bat. The downside is that so many layers of 3k kevlar and carbon is quite a bit more expensive than 2 layers of 12. Does anyone have experience in this field they would be willing to share?

thanks!
-PK

#2

with out vacuum u need high pressure from presses.

#3

I’d start by leaving off the two inside layers of carbon.

It might help if you described what you are doing in a bit more detail or showed some photos.

#4

The bubbles i am getting are little pin holes between the weave. They aren’t the end of the world, most people don’t notice them but i would like to create a more porfessional looking product. I can post some pictures later if needed

The arm guards i make are basically just the shape of the outer halves of your forearms. i have steel female molds with fiberglass male counterparts that i use with spring clamps to sandwich together. It squeezes out a bit of epoxy but i think i am not using enough pressure as hojo said. Perhaps my epoxy to carbon ratio is not good enough. I am hesitant to get into vaccum bagging because the process is much more expensive and appears more time consuming as well, but i will if it produces a better quality product

I hope this helps explain where i’m at

#5

just vacuum alone wouldnt help u since it is only 14.7 PSI, unless you are to vacuum infuse you need much higher pressure.

#6

Why would a vacuum only put down 14.7 psi? Isn’t the point to get more than 1 atm of pressure?

Are you putting epoxy in the mold first, then laying the first sheet and wetting it out?

#7

atmospheric pressure varies, average at sea level is 14.7 psi. In certain weather conditions it can actually be higher. if at higher elevations or poor weather it will be lower. everyday it will change slightly based on the weather. If you pull a near perfect vacuum you will acheive close to whatever the atmospheric pressure is in your area on your part.

#8

Kayakdude, Yes i do. I put a layer of epoxy in the molds then use a heat gun to get rid of the bubbles. then i lay in a sheet of carbon and wet it out then continue to add layers and wet them out as well.

What about impact resistance? any ideas on the most effective combination of carbon and kevlar for that?

#9

found your stuff on facebook… pretty cool product. Looks like a great product to use a silicone backing tool. The tool would be made to contact initialy in the center and deform/close toward the edges as pressure is applied. Air moves more easily than resin, the progressive closure would work fantastic. Set up a pneumatic press, add a little heat and you could cycle these parts very quickly!!

Your kevlar question is interesting. In my experience, impact sufficent to fail carbon in a catastrophic fashion will fail the kevlar as well. As the carbon fails it creates a huge concentration of stress that takes the kevlar to failure as well. Once in matrix, kevlar does not impress me and it’s a royal pain to trim. I does well in a “titanic” scraping situation like a canoe going over a rock, but in pure inpact, I wouldn’t bother other than marketing.

I see you’re in washington… have you discovered QCM in kent?, they have some nice compression molding epoxies at very reasonable prices. It’s been a while since I was doing biz with them but they were great to work with.

#10

2cents: look for an impact resistant resin, aka toughened epoxy. Sometimes they have plastic or whatever else inside the resin to add fracture and brittle resistance. It would be good for an impact product.
I personally would like to have a layer (thin, or maybe even a veil) of kevlar/vectran on the body side of the guard, incase it does break…i don’t want carbon splinters in my arm :slight_smile:

intrepid: I just did a study of using vectran and carbon faces in a foam sandwich for impact/puncture resistance. The vectran DID help alot for spreading the forces out from the carbon. Yes, it did fail, but it took so much more energy to break the surface than carbon alone.

#11

I really like the sound of a silicon press! Would i make that like a normal silicon mold? if so i hear silicon molds don’t have a very long life, maybe 15 uses?

I have not heard of QMC, i’ll have to check them out. Currently i am still using expoy from tap plastics, i tried working with an epoxy company but the epoxy they recommended didnt quite work out not to mention very hard to work with and corrosive. The tap stuff is like elmers glue comparativly and so far has been working quite well. But for my new products i need more impact so i’ll have to take a look for something better

Another competing brand uses kevlar in theirs and they have a reputation for being practically indestructable. they must also have a pretty good epoxy too. interesting detail tho they do not press theirs in any way, the weave texture stays on the outside of the guards

thank you both for your help!

#12

I was paying about 20-30% of the cost of “hobby” epoxies in 5 gal pails from QCM.

I have no idea how you did you initial tooling, but if you have cad files, scale your cavity -10% along the part (x & y) and +10%(z if you’re looking at a top view) against your part, extend the silcone past the part about an inch and put a nice big radus on the edge. Make the back side flat to press against. Use a press platen that is bigger than the silicone so it doesn’t get cut by the edges of the press. Close the tool relativly slowly, a minute or 2, to allow the resin and air to move. With the right resin and some heat, you can get 15 minute cycle times pretty easily.

You’ll end up with a very neat part on the back side with no excess resin and the front will be as nice as your tool.

Would be fun to play with…

#13

ok that makes sense. I do my molds by hand, i don’t have any CAD machinery. i could probably just eyeball the shape for the silicon and make separate molds for it.

And 15 minute cycle? do you mean 15 minutes until i can pop it out of the mold or 15 minutes to put together? right now my epoxy takes 2.5 hours @ 85 F

Also sent any email to QCM and got a quick reply. they seem pretty good. Thanks for the recommendation!

#14

These products lend themselves for more high-tech production techniques, such as RTM. If economics allow it, see if you can source an epoxy compatible RTM machine (second hand). refurbish it (usually just replacing some seals and NRVs, occasionally a hose) and you will have big fun.

For resins: Get into the higher end, rubber toughened resins, or if you can apply heat (80 degrees C or more) opt for an epoxy anhydrate resin.

Another thing you might try, although I never tried with carbon, is compression molding. Just pour in your resin you need, preferably in an X shaped pattern, then place your fibers (floating in the resin) and close the mould. Apply more pressure over time, allowing the resin to flow.

Works wonders with polyester, in fairly large structures. (say 3ft x 3ft).

You might need to change flange design to prevent resin runout before the whole part is saturated, but it is with a try.

#15

RTM sounds awesome but unfortunantly a little pricey as well.

I would definitly be interested in learning more about epoxies as i am still fairly new to composites

#16

the toe adjustment arm on my compensating arm will not come off
i think it is due to a bent bracket where the toe adjustment arm goes…

how to fix this?