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Vacuum bagging molds

Freemasm

Well Known Member
Composites. Learning there's yet another thing that I suck at. I can get decent results but it's taking probably ~ 3X the time that a skilled person would take.

Sorry if this has been discussed. I could not find a thread based on my assumed keywords.

Could someone point out some good resources for vacuum bagging molds?

I would also like any info on cheaper/repurposed/etc. equipment and materials to do this. I'd rather not go too crazy expense wise as I don't see myself doing much of this. Your help would be appreciated.
 
What sort of molds do you want?

Some are easy to make out of foam or clay. Larger ones take a bit more effort.
 
Some are easy to make out of foam or clay. Larger ones take a bit more effort.

Guess I wasn't clear. Typical for me. I'm making the molds to fit my purposes. Getting clean inside/outside corners and similar is the challenge. Looking to learn good techniques with some hopefully budget equipment.
 
Molds can be easy or hard. Pick the easiest you can get away with. Two extremes pictured below. The wheelpant mold consumed hundreds of hours of my time, but now I can crank out shells with relative ease. The bent tube elbow was used just last night. I simply stretched a piece of plastic over the heel of the bend (as a release medium) and laid up a few plies of glass and now I have a nice transition for some ducting I'm doing.

Vacuum pumps can be old refrigerator compressors or commercial stuff. I dont do anything much bigger than wheelpants so an old jobsite diaphragm paint compressor works fine for me.

You will need a bunch of perforated breather sheet (from Spruce, among others), and some kind of bleeder cloth (probably cheap by the bolt form some commercial source, but Spruce has it too.

Bagging material is up for debate, but I almost exclusively use heavy duty black plastic trash bags - the so called "contractor duty" yard bags.

Best bet is to just start doing it and dont be afraid to screw something up. Some lessons learned is that any mold needs to have reverse draft - very difficult to get a part to release from something with parallel walls. The deeper the part, the worse it is. Also, really sharp corners are tricky. If you see my mold for the wheelpants below, there is a generous transition radius onto the flat base.
 

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For simple "at home" work I use...

A vacuum pump from some unknown machine dumped at my buddy's junkyard

Pink foam for forms, epoxy shelled or with a ply of glass for surface durability

HD garbage bags, big furniture storage bags, or 4 mil plastic sheet

Seal with packing tape (small stuff) or yellow tacky tape (larger bags)

Scrap dacron polyester aircraft fabric for peel ply

Cheap polyester flannel from the fabric store for bleeder ply
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3d Printing and large zip lock bags

I've had great success using 3D printed molds with vacuum bagging. The upfront cost of a 3D printer can be a bit high and there is a bit of a learning curve with using the printer, but once you get the hang of it you can make a precision mild for very little cost. The epoxy doesn't stick to PLA printing plastics. Also for small parts I like to use these large zip lock bags from amazon. They are tough and very easy to quickly seal. https://www.amazon.com/Extra-Strong-Storage-Gallon-Freezer/dp/B071X78B92/ref=sr_1_32?crid=15YU762YSU3B5&keywords=large+ziploc+bags+5+gallon&qid=1668458889&sprefix=%2Caps%2C55&sr=8-32
 
Some Pics

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All the comments here are great and should help you to get good results. I can add a few additional tidbits and other ideas.

Vacuum pumps: A diaphragm airbrush compressor can be configured to be a vacuum pump for small projects. It won't pull a great vacuum, but its good enough for many applications.

In our shop we have two vacuum pumps. One is a cheap surplus medical vacuum pump that I got on ebay for less than $50 a long time ago. It is just one step up from the airbrush compressor approach. It is fitted with a vacuum gauge and a needle valve bleed port to allow us to regulate vacuum, and a small surplus tank (it was a Freon tank for charging air conditioners) to act as a reservoir.

A vacuum regulator (needle valve) is needed if you make simple molds from beaded polystyrene foam, or expanded polystyrene foam, since those foams have compression strength low enough that too much vacuum will compress/distort the mold. We regulate the vacuum to about 12--14 in. hg.

The reservoir is optional, but it allows you to get small to medium sized bags sucked down fast, which can be helpful to keep all the layup in proper position, to find leaks quicker, etc.

Our other vacuum pump is a large high-capacity pump plumbed into a 30 gallon tank for a reservoir. We use this system for big layups like wing skins, fuselage skins, etc.

Consumables:
Peel ply: If you get polyester interfacing material from a yardage store, try and select a cloth that has a good sheen to it. fabrics with a dull finish are much harder to peel off.

Perf-ply: at one time we made our own perf ply by getting ultra-light drop-cloth material (1/2 mil polyethylene) and drilling 1/16" holes in it when it was all folded up into a small patch so just a few dozen holes would cover a whole sheet. But this is a ridiculous waste of time -- real perf ply is not THAT expensive, and we often can re-use it.

Bleeder: polyester quilt batting works pretty well. polyester flannel will work fine, you probably want to use several layers.

Bags: It is helpful to use clear bags so you can see what's going on inside. For small parts, we have 18" square bags that are easily closed with a special bag-closing clamp, not sure what else to call it. It has a nylon rod about 1/4" diameter, and a mating sheath that goes about 3/4 the way around the circumference of the rod. you sandwich the open end of the bag between the rod and the sheath and snap them together.

For big layups you have to seal a sheet of plastic down to a table. There is special stretchy vacuum bagging film for this if you have compound curves, its called Stretchalon. It is kind of pricey, so you may just want to stick to polyethylene drop cloths, 1 or 2 mil thick. To get a good seal around the perimeter of the bag, taping it down with packing tape or 2-sided tape will give mediocre results but may get you by. Or, buy the special sealant tape. It is about 3/8" wide, 1/8" thick, super-sticky urethane rubbery material that comes as a roll with a paper backing like 2-sided tape. We have found it best to put the sealant tape on the bag first (do this in advance of doing the layup), and then seal it down to the table. Warning, it won't stick where there is any wet epoxy, so mask off the area of the table where you plan to stick the bag down. When you are ready to stick the bag down, clean the perimeter area with acetone, peel up the masking tape and stick the sealant tape down. It is also handy to build in some "darts", like expansion joints to help keep the bag "baggy" as you work around the perimeter. If it starts getting too tight, just put in another dart. (hope that makes sense)

To easily connect the vacuum hose to the bag, you can buy a special re-usable tap. It has a portion that you put inside the bag, then cut a small slit and put the external piece through the slit and it seals against a gasket when you turn it a 1/4 turn, locking it together with the part that went inside.

Lay-up tricks: If you have inside corners that you want really crisp and sharp, start with a thickened resin paste in the corners. Depending on application, this might be mostly microballoons, or cotton-flock and Cabosil, or even something like bondo. Then lay up your cloth over that. The vacuum can push the cloth down pretty well into the corner, but probably not all the way into a sharp edge, so the filler paste will form that for you.
For external corners, vacuum bagging should give you nice crisp edges. Depending on the details of the shape, there can sometimes be a problem where the bag will actually pull the cloth up off the mold surface at a corner and form a kind of tag of cloth. This can be really frustrating and the solution is very case-dependent.
 
Bleeder: polyester quilt batting works pretty well. polyester flannel will work fine, you probably want to use several layers.

Photo illustrates what Steve said about using several layers of flannel. Depending on how wet the layups are, it's possible to squeeze a lot of epoxy into the bleeder ply. The shiny areas are fully saturated bleeder ply flannel. I think this was two layers thick.

For sure you don't want it sucking up into your vacuum pump, so at minimum, stack up multiple layers where the hose will attach.
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I use a 1 inch diameter suction cup (the ones used to hang things on windows and mirrors) as the bag interface. I have learned not to place this thing anywhere on a part that needs a good surface finish because it sometimes leaves a little mound of resin right at the cup location. Place the vacuum off the part itself so the bag lays down flat across the entire part.
 
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