hello everyone. im new to the board and im excited to finally find a place where there appears to be an abundance of composite knowledge. i got away from composites (primarily carbon/ bagging) years ago because resources like this board, as far as i know , did not exist and the whole experience was, for the most part, frustrating.
i need advice on my current project. i intend to manufacture carbon/epoxy/cored rotor blades for RC helicopters. the demensions are 24X2.25X.3. a syntactic epoxy cast foam core wrapped in 1-2 plys of carbon in a closed metal mold using RTM or some derivitive of RTM is what i beleive to be the best foundation for the process. in there lies my first question. should i use vaacum to draw the resin in to the mold, pump it in, or use some combination of both. any input would be greatly appreciated.
I have seen a couple manufacturers that used presses and metal molds to make rowing padels and other small carbon parts. they would lay up both halfs with the carbon reinforcement and press it around a preformed piece of core. The molds were heated. one manufacturer did wet layup, while the other did prepreg, they both had A class finish out of the mold, and looked and performed amazingly. with such a small part as yours, and since you will have a set of metal molds this might be a better route then RTM or VARTM, and much simplier. I think either way it will take some experimenting and tweaking to get it right
hojo,
The compression molding using wet layup sounds intriguing. That would eliminate all the plumbing associated with RTM and potentially curb the cost to manufacture slightly. It would seem some air entrapment would be possible if you just slapped the mold halves together though. Can you shed any more light on the process you witnessed?
Certaintly, the process I saw was at a chinese factory I was working with before. they used heated aluminum molds to compress the laminate around preformed foam core that had a fiberglass sleave around it. they would wet lay the carbon on the mold halves, place the core in, place the top mold on then put it into a hydraulic press, heated the molds, and pulled out a nice finished part, just needed to be trimmed up slightly, occassionally they would get air trapped in it, but for the most part it was very air-free. these were kayak paddles… the other manufacturer I have seen was in NH, they used prepregs, and to tell you the truth I am uncertain about there core process, maybe the same, maybe casted in after? but prepreg pressed parts are absolutly amazing… some of the best parts I have seen… if you are going to put the money in to nice tooling, then it will be well worth the extra in prepregs, they do pay for themself long term
For small parts cold press moulding would probably work fine, as if you needed to use heated tooling may be as costly as RTM process?
Might need to warm resin a little to get good results, but main problem I can see is that in low volumes, process time would mean it would be very difficult to make any money out of something like this?
You can definatly use braided sleeves for this, then you will not have a seam along the edges of the blade, which I would expect to be bad.
You can do that, and either VATRM it, or in a closed mold, either VATRM or RTM. You would have to test what kind of resin, and pressures you need if you use pressure infusion.
Using VARTM wouldn’t work as well with a standard bag. You might have to use a reuseable bag, or find some way to get the top part of the blade to be smooth. Possibly using a hard shore rubber mold top, and a metal/composite bottom.
i was planing on 2 symetrical mold halves (the blades are symetrical) to form a closed mold when processing. so no bagging.
Ok, if that will be your set plans, I would say a good start is using braided sleeves (you can even get kevlar in there for safety…incase the blade breaks, the kevlar will hold it together), and then either VARTM it, or play with various pressures. I’ll try to ask around, and see what presures are normal. I think I heard 80psi once.
Hmm…Maybe clear acrylic blocks would work well as molds. Then you can see when it is finished infusing!
On small parts like this wet lay up, and cold press moulding will probably work just fine. There doesnt seem to be a lot of need for pressure or vacuum to make these sort of parts, as long as resin is thin enough to allow the process to work properly.
Riff
i was thinking on the same lines as you. i actually have prepared some .5 thick clear lexan for the first molds to see the resin flow. i was thinking kevlar too for 1 ply. most manufacturers use a wire inside to keep them from coming apart but they still explode. also gonna use the lexan to evaluate the compression molding approach. i appreciate you asking about the pressures. the braided sleeves im sure would be best but im hell bent on using 4X4 twill and will try to make it work
Classic,
the rtm looks appealing because it may be less messy. both process have there pros and cons. looks like its time to get in the shop and start working.
Hojo,
were the compression molds you have seen have some type of vents to release excess resin and air?
they vented out the side of the mold, along the seam
KISS is something that has to be a real help in any composites work! A very simple production method, which might be of interest to anyone looking at small scale production for retail?
If you look into using plastic molds that are machined out… try the clear acrylic plastic… that will help keep material cost down some compared to polycarbonate plastic. Also the more you polish out the molds the better view you will have of what’s going on inside the mold halves.
I would think you could sand the outsides, and inside of the mold with 320 grit, 400, 600, 800, 1000, 1500, then use automotive paint rubbing compound like Farcella brand or 3M brand, then switch over to a very fine polishing compound to finish it up. It could look just about crystal clear when finished.
Classic i think the reason Riff said a heated mold is due to the prepreg needing to be heated to cure…but heating plastic…i’m not sure how high of a temp acrylic or polycarbonate plastic can withstand before it changes shape. Wet lay up probably is better if you use a plastic mold. On the other hand for a few hundred dollars you could have aluminum molds machined.
i was planning on doing the RTM at room temp, then corking it off then into the oven at a couple hundred deg F to cure or heating the molds. i have a sheet of the lexan (polycarbonate) so that doesnt cost me anything and it will only be used to visualize the flow. it is clear so i will only have to sand and buff the machine marks out. once that is done then i will machine the metal molds,one set for the core and one set for the complete part, probably aluminum beacause its easy to machine and polish .im hopeing the CTE wont be a problem at 200 deg. fortunatley i have a cnc so it only costs time and material. now if i can just rap up the last few details with the aerodynamisist i can move on to the troubleshoot/trial and error phase
Any resin will attack PC (polycarbonate) or PMMA (arcylic) plastic…
You would want HDPE or LDPE as those plastics will self release from any resin matrix!
will a release film and/or waxing be an adequate barrier against this?
what do you have for a cnc?
PVA will keep the chemicals in the resin from coming in contact with the mold surface if doing more than a few tests a lacquer or urethane clear coat waxed could be easier . If you use only epoxy resins they shouldn’t have any reaction with plastics. There could be exceptions though, so always test.
haas vf2/3axis, vf3/4axis, vf6/5axis vertical machining centers. sl-20 cnc lathe
Awesome BustedBlade, our machine shop has a VF-2 ( 2006 model) a VF-3( 1998 model ), a VF-0 (2000 model), HAAS Mini Mill. I am currently running some screw parts on our SL-20 lathe. We also have an SL-30 the boss bought last year. Unfortunately I don’t get to do very much of my own stuff in the shop. I am going to try and squeeze the boss to allow me to make one aluminum mold sometime soon though… hopefully before Christmas.
HDPE would be super nice for molds, not sure how much it would cost for a bar of it.
aluminum molds are very nice but you must becareful with the surface because it scratches easy. and also if you do go with meatal molds i would highly suggest switching over to a semi perment sealer and release agent, especially if your going with elevated temerature, and if you go with high temp you should just switch over to prepregs and kiss infusion good bye