Vacuum Bagging Basics

I must preface this entry with a citation for a homespun vacuum source that saved my bacon in this exercise. DrCrash has a great solution using nothing more than a hardware store bicycle pump for this project, as there was no way I'd encumber my little apartment with an unwieldy electrical unit.

Vacuum bagging is a commonly used technique in composites, and for good reason. When done correctly, your part will have a nice, uniformly distributed compression about the surface of the layup. This in turn helps to minimize any voids (e.g. bubbles and wrinkles) in the buildup of layers. Also, when used with a porous substrate made of peel-ply and breather-cloth as we'll see, it assists in having the optimal ratio of resin to reinforcement (e.g fiberglass, or carbon fiber, or Kevlar) by squeezing out the excess.

The project is quite simple for illustrative purposes. I'm making a small container from a mold of a nice tea container lid. I'll go over the steps to make this part, but the vacuum bagging phase is what this topic is about, so we'll be delving into that a bit more thoroughly.

First, a few suppliers that I've had quite good success with:
Fibreglast dot com - Lots of supplies for the intrepid.
Aerospace Composite Products - They also had the check valve and bag attachment and 1/4" neoprene tubing used in DrCrash's vacuum tool, and very personable customer service.
Tap Plastics While they don't have much for vacuum bagging, it's still a worth while source to mention with all their instructional material and from my experience, good staff.
USPlastics Is also well known supplier.
Both TapPlastics and Fiberglast have some very good instructional material that is invaluable for someone learning about new techniques.

For the hardware store bike pump vacuum:
Check Valve Only allows air to pass one way in the vacuum line. The one available at ACP appears to be for an aquarium (brand name tetra), but it works okay.
1/4" neoprene tubing Acts as the vacuum line.
Tubing clamp holds the air out of the line once the vacuum has been set
Bag Connector securely attaches the hose to the bag.

ACP has all the parts that I used to make the pump.

For the bag:

Bagging Plastic The stuff I'm using is made of stretchy and resilient plastic, and it comes in 60" wide rolls, that you can purchase by the yard from the suppliers that I mention.
Bagging tape is a ribbon of pliable and sticky material that's coiled up on itself such that one side is covered with removable wax paper. You could probably use window caulking, too.
Peel-Ply is a plastic fabric that wont stick to your layup but allows resin to seep through.
Breather Cloth Is a high-loft material placed on the outside of the Peel Ply to absorb the resin and allow the vacuum to be passed uniformly over the surface of the layup.
Knife or Scissors The bagging material is kind of hard to cut; I've found a good rotary cutter works best.
Masking Tape Holds the peelply and breather cloth together when wrapping the layup. Also good for holding down the vacuum bag plastic when making the bag
Straight edge for measuring and will help hold the bag down when sealing the edges.
Cutting mat also handy. I got this one made by Fiskars, but I found that the rotary cutter can slice through the top layer :(

We're going to be making an envelope big enough to wrap around our part. There'll have to be extra room to allow for the peel ply and the breather cloth. I usually cut out a rectangular piece big enough to fold in half to then be sealed on the three open sides.

Here's where having a nice graduated cutting mat can be helpful.

Apply a small bit of masking tape to the far corners of the bag. Keep tension with your straight edge while you pull the opposite side over and overlap with the bottom. Find the point where they meet and mark on the straight edge with a pen.

Now that you have the tape applied, use the straight edge to keep a bit of tension on the plastic and apply some more masking tape in the other two corners of your envelope. Try to get it nice and tight so that there are no wrinkles running crosswise on this bottom layer.

Cut two pieces of the vacuum tape such that they'll extend along both edges of one side of your rectangular bagging plastic, starting from the fold. Apply both pieces of tape such that the inside edges are as closely lined up to the fold line as possible.

Now, Peel back a bit of the two side tapes and run another piece along the outside edge. This will be where you insert your layup so the backing will remain while we seal the other sides. Make sure to overlap the two sides to ensure that there is a fully sealed line.

Now begin to peel some of the tape backing on one side and carefully apply the folded side of bagging plastic over, all the while keeping tension from the upper left corner in this picture.

You don't see my right hand in this picture because I'm holding the camera, but if that weren't the case, I'd be applying a little tension from the upper left to the bottom right corner on the lower right corner of free plastic. This helps ensure that we have fairly symmetrical halves facing one another.

OK, I've got the first edge sealed, so now it's time to seal the other one just like the first. Here's a shot of how I keep tension on the upper right corner. If my left hand were free instead of holding the camera, you'd see an index finger carefully mating the plastic with the bagging tape up there.

This tension is important to maintain as we'll see in the next photo: You want to make sure that there is the same length of plastic on the free side as there is of tape on the affixing one. More? Ok, you might need to introduce a little more of the tape to take up that slack. Less? Hopefully you can stretch the mating plastic enough to match everything up.

OK, both sides sealed. Now it's time to place the bag-tube adapter for the vacuum line. First cut a small line in the bag by pinching a small amount of plastic between your fingers and cutting it with sharp scissors.

Then Insert the adapter piece, thread the gasket and nut onto the assembly and tighten.

There You GO!!! A bag ready for some vacuuming!!

OK, I've got a very simple female mold that's going to be the layup for this instructable in the first picture. It's just a lid from a special tea cannister, treated with PVA mold release that I brushed on and let air dry. A female mold is also known as a negative mold: The interior surface of the mold becomes the exterior surface of your final part.

I then cut the glass and peel ply for this layup in a loose approximation of the required amount to fully coat the interior of the mold. Notice the axial relief cuts made so that the material can lay up smoothly.

I then set aside an a generous amount of breather cloth to soak up the excess resin and allow the pressure to be applied uniformly. Looks like about two layers of breather cloth for this layup should be okay.

I then collected all the pieces onto my work table so that they'd be handy. You want everything in place before mixing the resin!

Here's the goopy part with all the fiberglass applied. I tinted the resin black for this project.

Now over the fiberglass goes the layer of peel-ply. I always go "shiny side down" with it, figuring it sticks less. Not sure it matters. Notice how the relief cuts allow the fabric to lay down a bit more uniformly than if none were there.

Carefully place the part into the bag. Very important to have a spare piece of breather cloth running from the bag connector port to the piece so that it can get a draw of air when the surrounding plastic is squeezing tight.

OK! Carefully peel back the tape backing a few inches at a time and press down on the bag to seal everything in. Attach your pump and begin sucking the air out. Once vacuumed to a satisfactory level, pinch the tube shut with your clamp and review the assembly for any signs of a leak. You'll know if this is happening by the tightness of the bag against the part. It should be quite tight.

Now go to the next step to show some more variations on how to troubleshoot leaks.

DrCrash Shared some good ideas on testing the system for leaks, as well as splicing a clever vacuum reservoir into the system. At some point I'll make one to illustrate, but for now, just text:

You can use the vacuum pump itself as a crude vacuum gauge, to tell whether your bag is leaking. Just stop up the end of the hose, and pull on the pump, and feel the resistance to "calibrate" your judgment of the vacuum level. Assuming the pump is working right, the resistance will top out at about 5/6 of a full throw.

If you feel noticeably less resistance after pumping the bag down, you have a leak or significant outgassing. If it's loosing air, carefully look around your bag assembly for any possible wrinkles in the plastic which would let air by. I've sometimes found that by carefully stretching the seal along the length of the tape where a wrinkle has occurred can help get the tape to adhere to the wrinkled plastic.

You can also enhance the setup with a vacuum reservoir, so that the vacuum level drops more slowly when you have a very tiny leakage, or outgassing from your resin.

The vacuum reservoir can just be a glass gallon jug, if you put it in something like a sturdy trash can in case it implodes. (Not likely just due to vacuum, but if you accidentally drop it or bang something against it, it could implode with a bang, and send glass flying on the rebound.)

That will maintain the vacuum level in the face of moderate outgassing and very tiny leaks. If you come back every now and then and pump until you feel the usual resistance, you can "top off" the vacuum. (If the resistance has dropped much, you need to be doing it more often.)

The usual way to add a vacuum reservoir is to just use a tee between the pump and the bag, and connect the reservoir "off to the side". I got a rubber stopper and a double-ended hose barb (hose butt splice) for my (apple juice) jug, drilled a hole in the stopper, and shoved the barb in. (I think the rubber stopper was about $3 and the nylon 1/4" ID hose butt splice was about a dollar, both at Lowe's.)

Another way, which doesn't require the extra plumbing, is to make the bag oversized and put the vacuum reservoir right IN the bag. For example, you could use a quart mason jar with a few holes punched in the lid, and some breather material taped over the holes to keep them from getting blocked. (Wrap the mason jar in duct tape to lessen the risk if it gets broken somehow. It shouldn't break due to the vacuum, but you never know.)

The idea there is that the volume of air outside the layup but inside the bag is normally very small. Putting the vacuum reservoir on on or in the bag greatly increases the internal volume, so that any small amount of gas weakens the vacuum within that volume much less.

Any kind of reservoir will soak up minor outgassing, and maintain your vacuum level longer.
It will also make it easier to judge the vacuum level using the pump resistance. If the internal volume of your setup is very small, you'll raise the vacuum level a lot with one stroke of the pump, and might not notice the low resistance at the beginning of the stroke. With a quart of volume, and especially a gallon, you'll notice the weakened but increasing resistance over a few strokes.

One of my concerns about simple vacuum bagging setups like the Roarockit kit is how long they actually maintain the vacuum. I'm wondering if the roarockit kit loses most of its vacuum almost immediately, because any little bit of outgassing pollutes tiny volume within the bag. For vacuum pressing wood laminates, one of the sources of gas is just the air in the wood. Wood is very porous and dry wood contains a lot of air. (It's so porous that that air may mostly flow out immediately, while you're pumping, but I'm not sure more doesn't seep out over the next few seconds or minutes.)

For resins like most polyester fiberglassing resins, there's a fair bit of solven that evaporates out fairly quickly---in the case of polyester, it's styrene monomer. Liquids generally expand tremendously when they boil or evaporate, so that may be significant, too.

Without a reservoir or a continuously operating pump, I'm wondering if you only get one good squeeze at the beginning, and then your vacuum level rapidly drops due to outgassing. (That would not be obvious from just looking at the bag---a tiny amount of gas would be enough to lose most of your vacuum, but the bag would still look tight.)

One good squeeze at the beginning may be be good enough for many purposes, to squeeze out excess resin, but holding reasonably high vacuum until the resin is set would be better.