Alternatives to vacuum bagging wet layup...and why.

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#1

To qualify this post, I am not necessarily trying to avoid vacuum bagging, but rather I want to explore alternatives to achieve a similar result.

For this to be logical, the alternative process needs to be nearly as effective as vacuum bagging the wet layup, depending on the criteria. It should also hopefully provide a solution to the usual issues with vacuum bagging.

I welcome other ideas and theories, but allow me to start with my own: sand forming and sand compression.

Using a sand typically used in stone pavers, such as Sakrete, one could (theoretically) form a mold using the wet sand (which hardens when dry), spray in PVA or similar, lay in the fabric, lay a sheet of flexible plastic, then cover with more wet paver sand to achieve a compression molding effect.

Thoughts?

#2

Why does the second layer of sand need to be wet?

Also, vacuum creates a huge amount of force, you are hardly able to replicate that with sand.
Laminating on a piece of loose plastic is difficult at least.
Shifting numerous lbs of sand is something I Always try and prevent. (keep in mind my garden sinks about 1 - 1 1/2" yearly. My house is on poles, and does not sink. Every 5 years I need to do “some landscaping” (you are all invited)

#3

I suppose the second layer of sand doesn’t need to be wet, so no.

That is some interesting points you make, so with that said, how about using a single solid mold, followed by compression via silicone sheet covered with sand? Perhaps if the sand were in a closed box, with a lid that could be clamped down on top to force the sand down into and around the part? This would prevent the need for vac bag, the silicone sheet would flex around the part, and the need for multiple molds on a curved 3D part would be unnecessary.

#4

You can also apply pressure with a water bag, or even compressed air.

#5

What if you were to build the mold out of poured concrete with raised sides? You could seal the mold, spray pva, lay up your composite, cover with plastic, then fill it up with water. The higher the raised sides, the more pressure created. Siphon or pump the water out after its cured.

#6

You would need a 9,9something metre high watercolumn to get the pressure you can reach with a full vacuum. Not very practical for the most of us :wink:

#7

On the how: yes, there are plenty of ways. But indeed, for a USD 250 vacuum setup I would not create too much problems.

On the water thing: I believe it was Plastech that developed a system to use relativey cheap (thin) tooling, placed in a water bath, then pressurised, to make boats. The advantage was that the cheap tools could be made in multiple series, so filling the mould with glass for an RTM session was not taking up the complete production facility (mother mould / water bath and RTM room.)

Did a search: Plastech MIT system, or Gunmar VEC system

http://www.boatingbusiness.com/news101/industry-news/mit_matches_vec

#8

One of the problems with using solid weights like sand is that there could be a large variation in the pressure applied to the flat and vertical mould surfaces.

#9

Super interesting ideas!

I feel I failed to make a few points about the “why’s” I want alternatives to vac bagging. 1) Ease of applying compression to small one-off parts. 2) The assumption that perhaps “full” atmospheric pressure on a part isn’t necessary to achieve a quality part. 3) No concern of air leak issues, but will require a high quality seal to prevent sand/water from destroying the part (which is a downside, as an air leak won’t necessarily cause total part failure).

I also thought of the poured concrete idea, possibly enhanced by way of glass fillers (just found something called “quikwall” http://www.homedepot.com/p/t/100318462?catalogId=10053&langId=-1&keyword=fiberglass+cement&storeId=10051&N=5yc1v&R=100318462#.Um4OiHCkrDU) so I’m glad I’m not the only one thinking this way.

#10

If anything, you want a really heavy mix. Maybe a polyester resin filled with steel dust.

But even then, it wouldnt create as much pressure as a vacuum can. And full atmospheric pressure is just enough to get a quality part. Sometimes its not enough.

Although you could get your part, vac bag it, then drag it to the bottom of a deep pool.
Youll get some pretty good pressure then. You only need to go down 1 metre to get 1 bar. Go down 10 and youll get 2 bar. Thats additional to the 1 bar of atmospheric pressure.

#11

1m to get 1 bar??? Even with heavy water you will not get 1 bar at 1m. Every 1m you will get 1 bar! So 1m=0.1bar

#12

Sorry I got that wrong, its not additional to the atmospheric pressure, its including it. You get an extra bar 10 metres down.

#13

Ok, so I attempted a part, which is curing as we speak.

I wrapped a foam spherical shape with carbon (which sucks to do, by the way), wrapped it in saran wrap (shipping wrap on a roll), then dropped it into a pvc pressure vessel, and then put it in the freezer.

A couple things to note:

  1. I am assuming that it wont cure, but I am assuming that the resin will gel.

  2. When water reaches zero centigrade, it expands approx 9% to form ice. This is the pressure I wanted to compress the part.

  3. I plan on pulling the part from the freezer, keeping it in the pressure vessel, and letting it cure at room temp. I will do this after 24 hrs, which is at 2000 tonight. I will let it cure for another 12 hours thereafter, so 0800 tomorrow morning… which won’t actually happen until around 1200-1300, so no big deal.

  4. When I wrapped the part in the saran wrap, I am unsure if I inadvertently trapped some of the plastic under a “dog ear” that may have lifted while wrapping it.

Pics to come tomorrow sometime.

#14

Why not just bag it?

Sent from my SPH-L710 using Tapatalk 2

#15

Because the status quo is boring.

UPDATE: The part was a complete and total failure! But…a few of the simultaneous experiments were a total success!

I will update with the writeup soon I promise :slight_smile: