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.
Step 1: Ingredients
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 :(
Step 2: Cut the Plastic
Here's where having a nice graduated cutting mat can be helpful.
Step 3: Secure Corners, Find Fold Line
Step 4: Secure Other Corners
Step 5: Lay Down the Sealing Tape
Step 6: Apply Final Sealing Edge
Step 7: Sealing the First Side
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.
Step 8: Seal the Other Side
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.
Step 9: Placing the Bag-Tube Connector.
Then Insert the adapter piece, thread the gasket and nut onto the assembly and tighten.
Step 10: BAM!
Step 11: Peel Ply, Breather Cloth, Setup
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!
Step 12: Layup
Step 13: Affix the Peel Ply
Step 14: Into the Bag
Step 15: Seal and Vacuum!
Now go to the next step to show some more variations on how to troubleshoot leaks.
Step 16: Checking for and Dealing With Leaks
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.