Hey guys,
I have been thinking about mold making and saw a post about making a mold of the part without flanges as a guide for cutting once the part is complete.
It got me thinking, couldn’t you make a mold of the part with no flanges and then vacuum bag the whole mold? The only issue I see is getting the part out of the mold might be hard if the surface coat runs under the part. But you could just cut away the excess.
Does anyone do this? Thoughts?
You can do this, it’s called envelope bagging. I’ve personally used it for vacuum bagging and I’ve seen it work with vacuum infusion as well. Things that annoying about envelope bagging though is sealing the bag, and also the increased use of bagging material.
Benefits though are no flanges needed!
I may give this a try tonight! I wanted to make another mold because the ones I have made so far have not has the epoxy degassed.
I am also considering trying to build flanges. So far I have just layer the part up with clay. I don’t know which is better, but I know the clay leaves a really rough surface and I am afraid it might be letting in air or something like that.
The guy who worked before me made the moulds that way. The designer who designes the parts for the CNC millers designs them that way. I hate them 
Envelope bagging isn’t that bad, but resin gets everywhere, you have no proper surface to apply force on for releasing, You have to be carefull with your layup, you have to pay a lot of attention trimming your moulds, you have to take care of the edges, wich are the most tender, you have no surface to glue frames on, no stiffener around your mould, etc. In my eyes this method is bad craftmanship.
I do infusions by envelope bagging the whole mold.
If your part isnt too big, the easiest way to do this is to buy the vacuum bagging film that comes in tubes instea of sheets. All you have to do then is to worry about sealing the ends. If you are doing everything at room temp, you can use the reusable bag seals that they sell as well.
Be careful at the edges of your mold if you are bagging without a flange though.
Any fabric that sticks out beyond the edge of the mold can fold over the edge and then get stuck. Ive ruined a mold because of that before.
I personally still would recommend that you make a mold with flanges though. If you are going to infuse in the future you will need some sort of a flange.
As for using the mold as a cutting guide I would recommend against that since you have a chance taht you will damage the mold. If you want a guide to trim the part, I would just make a separate guide to use for trimming
Ok, it sounds like I should just continue to build the molds like I have been, or start making flanges and not putting them in a box. I want to do infusion so I better stick with some sort of flange.
DDCompound has been recommending double bagging if I cannot get a perfect vacuum. I am going to try that, but I may also try just envelope bagging an entire mold with flanges.
I work with both flanged and unflanged molds. Both have advantages and disadvantages. The nice thing is that the trim line becomes part of the layup. If you really need a trim line though you can just scribe one into the mold surface. Then every part out of the mold will have the trim line on it. If its hard to see you can just wipe some talc onto the surface and the scribe line will stand out. Trimming against mold only really works for unassisted hand layups. Here the fabric and be trimmed along the mold edge once the resin gels.
The biggest disadvantage to a mold without a flange is a reduction is stability. The flange can do wonders in keeping the mold in the right shape.
I prefer envelope bagging over just sealing along the flange. For me it’s cheaper because no tacky tape is needed. I use tube bags and seal the ends with a large impulse sealer. I often will reuse the bag multiple times. I start by making it longer than it needs to be and just trim one end to open it. The next use only requires me to seal one end. I stop using the bag when it’s too short to use. On some processes I use end clips to the seal the bag. I’ve been known to use the same bag over 30 times. Some molds are obviously to large or awkward to slide into an envelope bag.
Adam
Where can you get the premade bags?
Google search “poly tubing” or “polyethylene tubing”. It comes on rolls. The tube is continuous from the start of the roll to the end. You just cut to length and seal the ends. You can use an impulse sealer, which is the fastest way, or tacky tape, or seal it with a 40 watt soldering iron with parchment paper on top. You can get it in various thicknesses and widths up to 48". I use to 2 mil for one shot, and 4 mil for multi-use.
Adam
Nylon film is also available as tubing. Airtech calls it “lay flat tubing” or LFT.
And I second what Johannes said. No flanges, no fun. Especially in infusion or prepregs.
For hand layup it is OK, and you can cut the part once the resin gelled. But for infusion I like a wide, smooth flange.