I have just completed my first effort at vacuum bagging. I made a carbon fiber keel fin and am pleased with the result. Balsa core, two vertical uni-directional lays and one 5.8 oz cloth on the diagonal. Weighs 2.5 oz rough. It’s posible I was pulling too much vacuum and compressed the balsa a bit. Next time a little less vacuum I think. Now comes the question. The object of vacuum bagging is light weight, which I acheived, but the surface is rough from the weave of the cloth. I could sand most of them away but that would be removing carbon and strength. What do I use to fill the weave marks without adding too much weight back on? Bondo?
You might try the technique used by the model airplane guys for their foam core wings: http://www.pilotsguide.com/rc/vacbag.shtml
It involves using a wrap of mylar film & will produce a mirror smooth finish if done properly. I’ve been using it succesfully on wingmasts for my rc ice/land boats(http://www.iceboat.org/RCBoats/rc%20boats.htm)
I usually draw a vacuum of 10-12 inches of hg.
Did you build your vacuum baggin unit? if so how or do you have a link to a good site on building them.
The method I used involved a perforated film and an absorbant wrap to soak up the excess resin for lighter parts. This makes it impossible to see the part while bagging. If I have to use filler the weight lost in babbing is gained in finishing. I was also using 29" vacuum which I beleive crushed the balsa a bit.I will correct this next time. I think next time I will use your method.
I went to my local appliance repair shop, helped him pack a fridge out to a truck and he gave me a compresser. He says they send lots of them to the dump. I got a bleed valve from Can. Tire for $8 (in the plumbing section-needle valve)and some vacuum hose from the junk in my garage. I got a piece of tubing (that would fit in the hose) and drilled a bunch of little(30 or 40 thou)holes in it. I made a sleeve of heavy cloth to fit over the tube. This stops the plastic from sucking into the holes. From there it’s just follow the info on Bill’s link above or plunk “vacuum bagging” into Google and follow the links. There is lots of info out there. I used window putty to seal the edges. Worked good and is re-usable.
not quite sure what ‘perforated film’ or ‘absorbant wrap’ is all about, but i am assuming you did not use peel ply…is this correct?
my 2 cents…next time use peel ply and you will have some sanding ability without removing cloth.
Answering a question with a question. What is peel ply?
Edit- I looked up peel-ply on google and most sites say peel ply is to leave a rough surface so subsequent layers will have some tooth. i want a smooth surface. What am I missing here.
quick description…peel ply is a fabric that has a silk like appearance (i actually use a taffeta that is cheap and easily available). it actually performs in many ways:
- as you said, leaves a rough surface for better bonding properties (although you still should sand some before lamination)
- it draws tight to the surface allowing compression of the laminates, therefore making a stronger structure
- it helps to squeeze excess resins out of the laminates
so just think of what silk looks like under a microscope and picture that in reverse on your carbon piece. it leaves a texture of very small high and low spots…sand the highs off down to the lows and stop. you will be able to smooth your piece and never actually sand any of yoiur structures fibers. of course, should you want to fair even further, i suggest a light coat of epoxy with low-density filler added…a very fine coat will add very little weight when sanded and complete (bondo is bad news…could give you some horror stories but i wont)
one other thing that you should consider, when making up a multi-layer laminated piece, it is best(*) to start with a bi-directional piece before using uni-directional. a few reasons for this:
- bi has the ability to form around abstract shapes better than uni. it provides a great ‘laminating base’
- bi is also stronger in multiple directions and therefore provides your structure with excellent strength properties from the start. think of multiple layers of carbon as a sandwich, and bi-directional as the bread (no open faced sandos!!!)
- structural analysis dependent, but you may find a need for less uni in your layup schedule
on another note, i recently made up some sheave boxes for my full size boat. i used two layers of bi on the exterior, not because the extra strength was needed, but because with two you can sand smooth and even remove some of the outermost layer and yet still not loose the true strength of the exterior layer of bi (wish there was an easier way of saying that, but i’m sure you get the point).
carbon construction is so very simple and really rewarding (it can actually be loads of fun), so no matter what, enjoy yourself…i hope that this is helpful[:-spin]
iall of my carbon knowledge was learned from an industry professional. while i am in no way an expert, i do have a fair amount of practical knowledge as well as professional tutelage. take my 2 cents for what you deem them to be worth[8)][/i]
The perforated film you used i assime is the proper stuff for vaccing??(a mylar or nylon film with pinholes every 15-20mm squared) if so you should not have the need to use peel ply to get a smooth surface. you should be able to laminate and ontop of it put the perf film then a breather cloth and finnaly your bag. if this is what you did than your problem is pulling too much vac. most resins can handle up to 25inHG of vacuum, but if you pull too much it will starve the cloth and suck the film into the pattern of the cloth, when you debag you can notice the weave. try it again (or next time) check your resin specs and vac to it’s recomendations. the epoxy we use at work has a limit of half an atmosphere (15inHG of vacuum) so be careful. Lighter is not always better if you starve the fin there isn’t enough resin to support the fibres and you will have a very flexable part. on the flipside too much resin and it’ll be stiff but heavy and brittle.
I’ve vacced stuff with the perf film on the laminate and they come out close to perfect you just have to give it a very light wet sand to remove the litle spikes of resin where it went through the perforations.
boat builder 8 yrs and even i don’t know half of it!
I think I sucked too hard. I’m going to sand it smooth and put on another lay of cloth at 10 to 15 in. Yes I am using the proper film allthough I think that “Ziplok” vegetable keeper bags would make a suitable substitute in a pinch. They are poly (won’t stick) and have roughly the same perforations. And they are more readily available in small towns. It took two weeks to order in my stuff.
O.K. Lay number two is out. I sanded the original and put another lay on using perforated film,breather cloth and bag. Maximum 9 in. vacuum for 6 or 7 hours out in the sun. I still have the weave of the cloth showing. The surface of the fin looks like there is no resin other than it being shiny and stuck really well. There was resin soaked into the breather so there was enough resin on it. It now weighs 3.25 oz which is .75 oz gain so there is that much more cloth and resin. I’m stumped! If I have to drop the vacuum any more there is hardly any point in vacuum bagging. I’m doing something wrong, HELP!!
One thing that we do at work is let the resin go off till its like treacle consistency before putting the vac on. Leave a sample of the resin you have used on the bench so you can see when it gets to that point. The other thing, use peel ply!
Luff 'em & leave 'em.
As you say, the idea of vacuum bagging is to try to soak out the excess resin in a layup. The peel ply is designed to let the resin saok out under the pressure of the bag. The “diapers” soak up that excess resin. This method really only produces acceptable surface finish when it is used with a female mold (you are pulling the resin out of the unfinished surface). It will not produce accepatble surface finish when you use it with ply over core construction. You also run the risk of the cloth not bonding to the core because you are pulling the resin out rather than letting it soak in…
Bill K’s method is probably pretty good and will only require a small amount of finish work at the leading and trailing edges. But you will not be soaking out the excess resin with this method, so you will need to be sure to keep your resin usage under control. This is particularly hard with Carbon because it does not like to wet out readily. You should plan on thinning out your resin to help it flow and you may even want to heat it slightly with a hair dryer to help it soak.
If you really want a nice surface finish without a bunch of hand work after the part has cured, you should probably consider building a set of clamshell female molds. You can build these out of fiberglass, resin and wood fairly easily. Then you lay up the skin in each mold with a vacuum bag process. Once those have cured, you can sand them dowm to the parting plane, put the two halves together with glue and fill them with a two part liquid foam core. I’ve done this with much bigger sized wings and it works great. Doing it with something as small and thin as a kell fin might be a bit tricky…
Here’s a couple links where you can get information and supplies:
http://www.acp-composites.com/ACP-CAT.HTM (Scroll down to the vacuum bagging supplies)
Don, you’re not really doing anything wrong. It sounds to me like you got a perfect pull. Excess resin doesn’t add anything to the strength of your keel other than holding the layers together. The problem as I can see it, is that your last layer of glass is 5.8 oz glass?? If that’s right then you’ve got a big heavy weave to fill. I would buy a roll of 1/2 oz glass and use it for your final layer of glass as it gives you a smoother finer weave to fill. Maybe next time go carbon uni-directional then 6 oz glass (or 2 layers of 3 oz glass) and then 1/2 oz glass.
If it’s too late and you want to keep the keel that you laid up then go ahead and take your epoxy and mix it with micro balloons and fill in the weave and sand it out and you’ll have a beautiful keel.
I talked to the rep from Industrial Formulations(the resin I am using)and he doesn’t think I’m doing anything wrong BUT- He says vacuum bagging is normally done with prepreg which I think is thicker resin. He thinks that for what I am doing that vacuum bagging is not the way to go. Vac bagging pulls the resin out and leaves pin-holes which makes finishing difficult(just ask me!) He said a good bubble free hand lay-up would suit the situation much better. He said if I want a nice finish I should lay up a thin layer of glass on a waxed sheet of window glass and when it sets to epoxy the balsa core,wrap the sheet of glass (shiny side out)on it and then vac bag strickly as a clamping method. This is a method that glider builders use for their wings. I’m getting a lot of imformation here and I’m wondering if a combination of stuff might work. I was thinking balsa core, unidirectional lay, 5.8 oz cloth on the bias, 1 oz cloth for finishing and possibly peel-ply. Lay it all up and let it get like treacle(I’ve never used that word before) and then only apply the vacuum long enough to suck out the air and level everything out. Then turn the vacuum pump off and let it set. Does that sound feasable? I think I will try something like that next time. Comments please.
Sounds good but I would leave the pump on or at least leave it under vacuum. You don’t want any air bubbles between your laminates (layers). If you turn your vacuum off, there is a chance that your layers may delaminate. Having sucked a lot of glider wings for the last 15 years, I’m usually trying to minimize the resin so I usually slip the wing into the bag as soon as I can before the resin starts to set. And hope that as much resin sucks out as possible.
The mylar envelope is a great way to get a nice glassy smooth surface.
Pre-preg is just resin coated fiberglass (or carbon fiber) that has to be oven baked to trigger the catalyst.
“hand layups” ie dry cloth and a pot of resin braught together by hand benifit greatly from vacuum bagging because your voids (which might be minute and imposible to see especially with carbon or aramid(kevlar) but are unavoidable in handlay) get vacced out creating a stronger layup. pre-preg must be vacced but most professional boat builders vac hand layups because you can use less cloth, have a thinner layup, lighter, stronger part/repair and better bonding to cores also. At work if we have any repairs to a cored boat there is no question we vac it because it forces resin into the core giving you a better bond and less chances of delamination.
Make sure your guage that reads your vacuum is ported into the bag (or inside)and not connected to your pump or connected to your hoses. reason for this is you could have leak in the join of the hose leading to the guage so it will read incorrectly. or your vac supply hose could be blocked so you’d have plenty of vac beetween the bag and pump but none in the bag. The guage could also be blocked good idea to check the guage first to see it’s reading correctly.
Really got me stumped how you still have weave pattern coming through i’ve done parts in the same manner and come out with a polished carbon look no problems and NO peel ply. Make sure your wetting out the carbon enough otherwise the resin could be “floating” on the surface and when you pull vac the floating resin will pull through into the breather cloth and give you the apearance everthing is fine but your cloth will be dry and give you that dreaded weave. we use a wetout board (old ply covered in wax paper) put the carbon (or cloth) on it and pour the resin ontop of the cloth then squeegie the resin over it repeatadly, pick up the cloth turn it over and repeat. Then wet out the area your bonding to and finnaly apply your “pre-pregged” cloth,perf film,breather fabric,bag, blah blah. one more point if your using an epoxy like west systems they are VERY thick and can take a long time to soak into the cloth so use a laminating resin (or warm a thicker resin or add a flow medium but NOT thinners) to get your cloth nice, wet and sticky before you apply it to the job. not sure what Industrial Formulations resin is like as we dont have it down here that i know of.
if its too hard to wet out first make sure the fin is COVERED in resin before you put the dry cloth onto it and then add more resin and squegee again and again to be sure your wet out before you think about vaccing. but dont be discouraged about vaccing parts, once you get the swing of it you’ll never go back to handlay-novac laminating.
you know what they say third time lucky!
How much vacuum do you pull?
I’m pretty confident of my gear.With the gauge conected directly to the pump it reads 29" so that seems good And on my first pull I started to crush the balsa core so I know the vacuum is making it to the bag.
When I’m laying this up I paint the part with resin, apply the cloth and then work a little more resin in from the outside so it seems saturated. When it is all finished setting the breather fabric has a bunch of little spots of resin that don’t quite touch each other. The spots of resin vary from 5 to 10 mm. I am using an old piece of flannel sheet for breather. This is becoming a challenge even though I’m not convinced anymore that is is necessary. I will keep trying and if I can pull I nice smooth part that doesn’t need filling maybe my opinion will change. It is a technique it would like to become confident with though.
if 29 " curshed your balsa core it should have flattend your carbon weive out also. what carbon uni are you using??? if it is to heavy it will show up the weive. its hard to get away with not using a faring mix vac to resin manafuctures guide and lay up cloth in right order then vac then just fair and paint a lot stronger than non vaced:)
Never hold your farts in.
They travel up your spine, into your brain,
and that’s where sh*+y ideas come from.
normally pull around 20inHG but depends on the part and resin.
got any photo’s of the part??? how thick is your resin? honey or motor oil?? and what is the shedule (cloth type, weave, weight, layers, organisation of layers) difficult to say without knowing all. a failsafe way out is to modify your shedule by doubling the layers and halfing the wieght of your cloth. half wieght cloth will have a finer weave which is more likely to be filled. finer weave cloth is less likely to be a dry layup because it soaks up resin easier and because you apply more resin beetween each layer your much less likely to have any problems.