Vacuum bagging fins question

[DM1975]

I have fins cut from 1/8” ply that I’m wonting to fiberglass using a Foodsaver. I’ve already profiled the fins on both sides and was wondering what I should do to prevent any warping under pressure? I’ve watched John Coker’s video on laminating fins but I’m not sure what to use to back the fins with to avoid warping since they are profiled on both sides. Would it be ok to bag them without something backing them?

 

[JimJarvis50]

I have fins cut from 1/8” ply that I’m wonting to fiberglass using a Foodsaver. I’ve already profiled the fins on both sides and was wondering what I should do to prevent any warping under pressure? I’ve watched John Coker’s video on laminating fins but I’m not sure what to use to back the fins with to avoid warping since they are profiled on both sides. Would it be ok to bag them without something backing them?

My favorate method for laminating fins is to use a couple of tiles to provide pressure on both sides. Ideally, you want to have the tiles extend past the fins by at least an inch but not much more than that. If you then use relatively thick bagging material, it won't get pulled between the tiles far enough to contact the edges of the fins (i.e., don't use stretchy bagging material for this). If there are areas of the tiles that are not supported, then you can make some supports from your fin material and a little fiberglass so that the tiles won't crack. Both sides of the fin need to be laminated at the same time. Doing one side and then the other will result in a warped fin. With this approach, your profiling of the fin won't matter.

Jim

 

[DM1975]

Thanks. A couple questions if you don’t mind. Are the supports basically just pillars supporting the edges of the tile? Also, what holds the glass to the fin along where the airfoil taper back?

 

[JimJarvis50]

Thanks. A couple questions if you don’t mind. Are the supports basically just pillars supporting the edges of the tile? Also, what holds the glass to the fin along where the airfoil taper back?

Yes, just pillars. I'm not sure what a foodsaver will do with respect to vacuum, but I know that with a high vacuum, the tiles can be broken. I had an unsupported part of a tile - maybe four inches? - and the vacuum cracked that. Like a gun shot.

This method just works for flat fins. When I do it, I start with a flat fin, laminating the entire fin, and then sand in the profile I want (removing the lamination from the profiled area). I envisioned doing essentially the same thing for your fins. Laminate the flat part and sand off anything that covers or sticks to the profiled section. If your fins have a large portion of their area that is profiled, then you probably need a different method. I still think you will want a method that does both sides at once, as I am aware of a number of examples where people tried one side at a time and failed. It's sort of a rule of thumb not to do that.

By the way, I typically cover the fiberglass with a layer of peel ply and one layer of breather. I also cover the tiles themselves with a layer of parchment paper. I would then do tip to tip over the laminated fins. If you aren't following up with the tip to tip, then you might use perforated film instead of peel ply to get a smoother surface.

Jim

 

[DM1975]

Thanks. I’m thinking that maybe if I double up the absorber that under pressure it’ll be enough to compress the cloth to the fins.

 

[DM1975]

I used a couple of different layers of used bleeder cloth under new cloth to help with the profiled fins. Seems to have worked well.

 

[Dan Griffing]

I'm not sure what a foodsaver will do with respect to vacuum

I was doing a dry run test of my FoodSaver with an 18" x 4" OD Apogee cardboard airframe section, and the vacuum was almost to the point of sealing when it caused the middle 2/3 of the airframe tube to collapse with a loud "BAM"!

So it does pull a really good vacuum as our freezer full of big game meat will also attest to.

 

[JohnCoker]

I use a FoodSaver for a couple of different steps making a fin in this video:

 

[rharshberger]

A really strong vacuum is not necessary about 18-20" is good enough for most of our rocketry purposes, I just took apart a Foodsaver today so when I get a chance I will put a gauge to the pump to see what kind of vacuum it produces.

 

[TonyL]

I use small granite surface plates as cauls. They are cheaper than you might think, and they go on sale now and then. I do not put them inside the bag so that they stay clean. For a plain laminate sheet one does not necessarily need vacuum if the sheet is being pressed between plates [standard practice in industry].

For the tube issue, one should not have open volumes inside a vacuum bag as they will draw resin, not to mention the collapse noted. A food-saver does not pull all that much vacuum, so that is a good illustration of what can go wrong. Wet layup and a tensioned peel ply may serve better. Tubes large enough can have an inner bag to support the inside wall, but pleats are always an issue to work out.

hope this helps,

Tony

 

[stantonjtroy]

For the tube issue, one should not have open volumes inside a vacuum bag as they will draw resin, not to mention the collapse noted. A food-saver does not pull all that much vacuum, so that is a good illustration of what can go wrong. Wet layup and a tensioned peel ply may serve better. Tubes large enough can have an inner bag to support the inside wall, but pleats are always an issue to work out.

hope this helps,

Tony

Though not Vac Bagging, Heat shrink tube is very effective for tubes ( and some other parts). I use it frequiently and he price is good. https://www.sollercomposites.com/ShrinkTubing.html

 

[Banzai88]

Though not Vac Bagging, Heat shrink tube is very effective for tubes ( and some other parts). I use it frequiently and he price is good. https://www.sollercomposites.com/ShrinkTubing.html

Do you use couplers inside the tubes to support it while the resin cures?

 

[stantonjtroy]

Do you use couplers inside the tubes to support it while the resin cures?

No. I find it provides enough compression on the glass/carbon to get a smooth, relatively bubble free, finish without crushing the tube. If laying up over a mandrel, have the layup/Shrink tube stop about an inch or two short of the ends of the mandrel. Of note, you don't seal off the ends as you would on a vac bag. I typically run it out about 2" past either end of the layup. Then hit it as evenly as you can with a heat gun. The heat shrinks the tube down nicely as well as slightly lowering the viscosity of the resin/epoxy, enhancing the wet out, and kick starts the cure. Excess resin gets squeezed out the ends so be prepared the collect the mess. Once the resin/epoxy has cured, slice the shrink tube lengthwise with a sharp blade and peel off. Minimal sanding/sealing is required and no blush. They also make a shrink tape (1"-2" width) that works the same way but you wrap it spirally around the tube with about a 25% overlap. The tape can (in some cases) bleed excess resin/epoxy through the overlap. Makes for a bit more of a mess. The tape does leave fine spiral lines that need to be sanded out (that's why I like he tubing) but they are minimal. Also of note; You'll want to use a good Laminating Resin/epoxy as opposed to one formulated more for bonding. FWIW

 

[Dan Griffing]

I use a FoodSaver for a couple of different steps making a fin in this video:

Here’s my first carbon fiber lamination for a 1/4” Apogee Peregrine fin using a modified version of the method John Coker shows in his video. In addition to using a Food Saver for vacuum packing, I pressed the bag assembly between two flat boards with ‘C’ clamps for extra pressure to keep the fin flat while the epoxy cured.