Fin and Flat Sheet Stock Composite Construction

This page talks about making flat sheet composite stock suitable for fins, centering rings, and other rocket components. In particular, this is about vacuum bagging techniques. This is largely based on experience gained but starting with the excellent and highly recommended. Vacuum Bagging Techniques (Tape 3) by Dave Triano of Shadow Aero.

This vacuum bagging technique makes excellent quality stock - but is high in consumables as well as set up time. The results are worth it though!

It's also not required but a curing oven is very helpful to accellerate the curing time.

A flat platen is required to ensure that the stock you make is flat. If you simply set up sheet stock inside a bag you're more than likely to end up with and oddly shaped and most importantly non flat product.

While a platen can be fully inserted inside a bag, it's easier and more convenient if the platen is the bottom surface, in effect, the bottom of the bag. This does place a requirement that the top of the platen be a suitably flat, smooth, release surface.

Construction

The easiest way to do this is to affix a sheet of glass to a suitably sized perfectly flat sheet of plywood. Affix the glass with  a tight grid of silicone rubber beads and protect the edges by banding the edge to the top of the glass surface with wood trim.

Release Agent

The last part of the trick is ensuring the composite will release from the glass - and that is where a suitable release compound is required. I'll discuss it later, but I'm currently using PVA which is easy to apply and gives a good result. It does require drying time and must be washed from the composite with water but for my non production speed needs this is not a problem. And it's much less expensive than PFTE or other spray release agents.

Cleaning

Prepare the platen by first cleaning it up. Particularly if it's been used before there will be release agent and sealing tape. The release agent comes off primarily with the sheet stock, tape, and the rest of the layup,  but there will be little bits and strips left. Use a flat safety razor (always a fresh one - don't be cheap!) and scrape the glass clean. Then polish it and buf it with a good glass cleaner.

Clean up your wood edging. I use low tack painters tape and replace it when torn or soiled. Do this now.

Preparation

After a good cleaning, touch up the wood edging with fresh painters tape. Then apply the release agent.

I use PVA release agent. It's a light viscosity (similar to cooking oil) fluid and is easily brushed on. I pour a suitable amount into a disposable paper cup and then apply with a 2" disposable chip brush.

Apply generously and brush out with the chip brush. Pull them out lengthwise and overlap them widthwise as you work down the piece. If you're pulling it to thinly it'll pull out with circular voids and that is very apparent. Simply apply more and brush out. It's very easy to get the hang if it, particulaly if you're not trying to pull it too thinly. Ideally you get a good cost with a single application. It's easiet to apply that way and it will flow out into a beautifully flat smooth release surface with no lines.

Allow to dry thoroughly - 4 hours or more depending on humidity. It's moderately fragile at this point so take care to protect it. I also don't recommend preparing  it substantially in advance of using it as it's difficult to suitably protect it.

This process is high in consumables. Basically the release, tape, bagging film, breather, release film, peel ply, nearly everything except the resulting product is disposed of after the product is cured. Fortunately most of these consumables are relatively inexpensive. Still, for the work involved, it's nice to do as big of a piece or a number of pieces as possible.

Platen release agent

Peel Ply

Release Film

Release Film is used against the carbon fiber (or fiberglass) if Peel Ply is not used. Remember that not all consumables will release, so the Peel Play and/or Release film must go against the fabric to ensure you can extract your finished work piece.

Breather Bleeder

Bagging Film

Edge sealing tape provides the outer seal. Note it is applied directly over the PVA on the outer edge. It's not necessary or desired to apply it directly agains the glass. Applied over the PVA it will come off easily when it needs to and also ensures the entire platen is protected by the PVA and therefore easily cleaned up for re-use.

Plan your work. Your Peel Ply, Relase Film, and fabric should be somewhat larger than the wood (if not a truly composite only layup).. Your breather bleeder should cover well into the vacuum attachment area. Your bagging film should substantially overlap your work piece. This in particular along the edge that will have the vacuum port as it will (likely) have a built up breather area (epoxy trap) that will take up film. it's cheap; be generous.

Cut each piece as required and set aside in your work area for easy access. Be particularly careful with your fabric, particularly Carbon Fiber as it's prone to losing weaves.

Apply your sealing tape along the outside edge of your platen. fully on the PVA'd glass. Do not pull up if it's not entirely straight as it will pull up the PVA and you won't have a seal in that area. Apply by  rolling it out off the roll as you apply it while carefully guiding it relatively straight along your edge. Don't worry if it wanders a bit inside or outside as long as there's a good bead on the glass (1/4" or so) to seal.

Epoxy Dam

Apply your sealing tape along the edge of your release agent prepared platen.. Do NOT remove the top release off the tape. It will protect it from epoxy dribbles as well as keep it from catching (and being contaminated by) your various fabrics and consumables.

If you're not using Peel Ply then brush in epoxy directly onto the PVA in the area of the layup. Or if you'd like, apply Peel Ply first. Note that the peel ply will absorb some epoxy so you'll need to account for that.

Apply Peel Ply in the areas you need it. For bulk stock, cover the entire platen in the area the wood (and /or fabric) will lay.

Lay in your fabric. Line this up as carefully as you can first as it'll stick some to the epoxy and will be somewhat difficult to adjust much. Start along one edge and work that edge down, smoothing and keeping the edge smooth and straight as you lay it out.

Work in epoxy. Use the rolling brush technique (smooging it in by rolling the brush not pulling it). Once it's mostly worked in you can carefully brush it around a bit - but be careful not to pull the fabric much. Use a bit of excess as your wood or substrate will absorb some as well.

Apply the wood or additional layers of fabric. If fabric, work in more epoxy.

Apply Epoxy now to the top of the substrate (plywood, foam, whatever). It should be wet  and shiny to ensure the wood has absorbed most of what it needs to ensure there is no epoxy starvation in the laiminate.

Apply the next layer of fabric. Apply as with the first to ensure a smooth, straight, wrinkle free application. Allow some overlay over the edge to be trimmed off later

Apply epoxy again and work into the fabric. It should be somewhat wet, particularly if peel ply will be used, to ensure there is no starvation. Remember that most excess epoxy will be pulled into the breather bleeder layer.

Apply Peel Ply in the areas needed. This is somewhat porous but not very much so (depending) so you may need to work out large epoxy bubbles to ensure they don't remain in the product.

Apply Release Film over the remainder of the fabric, particularly in the area that peel ply isn't covering it.

Apply breather bleeder layer over the Peel Ply / Release Film.

Apply the bagging film. Lay it out with your overlap and smooth it out as much as possible. Start with one corner and one edge. Pull the tape release back slowly while smoothing in the bagging film. Start and work slowly to ensure no wrinkles. Wrinkles here are vacuum leaks!

Now work down another edge; usually from the first corner. Proceed as before with particular attention to keeping the film square so it won't have diagonal wrinkles when the opposite edge is sealed. Wrinkles on the top may telegraph into the top surface of your product.

Now work down the opposite edge. From the 2nd corner. (You're making a U starting both vertical legs from the bottom corners. Gently pull out wrinkles to keep the film consistent as you work towards the top. It needn't be tight; just no gross excess. In fact you want enough slack to later slip in the breather attachment and it's breather padding.

Don't try to pull up any flim - you'll likely pull the tape off of the PVA thus generating a vacuum leak! We'll deal with leaks and wrinkles later.

Now, along the top we want to be generous. Make a cut and affix your breather tool and under that slip in your breather bleeder epoxy trap (if necessary).

Starting from the far corner, start working your bagging film down against the tape.as before. Stop 4-6 inches away from the final corner. Now cut that length plus some of tape and apply it to the bagging film in the ares that will seal down. Now apply this bead of tape against the bottom tape. This trick allows a full, proper seal without leaky wrinkles. A picture will be good for this, but consider you're getting tape against no wrinkle surface - platen or bagging film, and using the tape to seal itself against what is now a wrinkle in the tape that will seal to itself.

I use a 6 CFM Robinair pump. It'll pull down to 30" and is strong enough to tolerate small leaks from a corner or wrinkle. My Robinair has no resivour at this point so runs continuously. For this, it really likes a fan, and a fan goes a long way toward preventing oil misting. For oil misting I also bought 30' or vacuuum hose to allow placement of the pump outside of the garage.

I'm planning on adding a resivour with a vacuum switch, say from 20" to allow the Robinair a break to stop until the bag leaks down enough to turn it on again.

There's a great trick of using a surplus refrigerator compressor as a pump and this should be able to run continuously without needing a fan.

But misting aside, the Robinair is a great and powerfull pump. And available for $100 or so at pawn shops if not eBay.

I use my temperature controlled curing oven and cure my stock at 140-170F overnight. I have fittings that allow me to pass vacuum through the door and maintain vacuum on the piece in the oven without propping the door open.

Tear off all your consumables. Seriously, you're pretty much done. The top layer may need some encouragement to come off the product. Plastic or wooden wedges (trowels for example) are usefuly to pop and edge and pop the top layer off. Use the trowel or wedge also to pop the product off the glass platen being careful of your glass surface. Toss away all the consumables you so tediously measured and cut!

Trim up your piece on a saw or dremel. particulary if Carbon Fiber, the edges, particularly from the overlap, will be quite sharp, and carbon fiber slivers are no fun!

Don't forget that the PVA release agent you applied to the glass will come off onto your work piece. If you used Peel Ply it'll be stuck to that, otherwise you'll need to wash the PVA off your finished piece. Note that the PVA isn't visible as such. If anything it'll make your piece just look shinier and prettier. So do it now before you forget.

Clean up

Clean up your Platen if you haven't already done so and put it away carefully - or prepare for the next piece.

As personal notes, my platen is about 24 x 28 and allows me to lay up an 18x24" piece of stock. I usually use about 350 gm of  epoxy for this layup with 5.7 oz CF, 1/8" ply, 5.7 ox CF in 18x24" size.

Use Mr Fiberglass Slow Epoxy or AeroPoxy