Has anyone tried...

[SukiSolo]

Compression moulding is pretty good but you need matched tools, usually in aluminium. My car has compression moulded bonnet and rear hatch/tail made out of it. Had quite a few bits made for a medical unit a while back, you get consistent reliable quality. Tough as hell too. One irony was there were a couple of prototypes I made out of epoxied marine ply!. Of course sprayed up in 2k Polyester and 2k acrylic, looked exctly like the real thing, company name in relief too.

Groper has it right on vacuum, even a fraction of 1 bar will pull stuff down onto/into a mould/former. More than enough for veneer or composite layups. In fact it is better to go with the least pressure possible so the mould/former is not distorted or even collapsed by it. As long as the shape is held and on layups the air drawn out of the laminate, that is normally enough for good quality parts.

 

[redreuben]

I've wondered if there is any benefit to use a bit of air pressure to push the resin into a vacuum infusion ?

 

[groper]

I've wondered if there is any benefit to use a bit of air pressure to push the resin into a vacuum infusion ?

Nope, as this will simply inflate the bag as the resin enters and the bag will go slack, resin will pool under the bag as there is unequal pressure inside and outside the bag.

Back to the original question, I'd love to hear an explanation as to why you think the OPs idea will not work. Provided the shape is single curvature, there should be no wrinkles to worry about... I'm confident my idea will work also, perhaps I should do a small mock up in the shed to show you... Shouldnt take long if I keep it small... Will a 2m length of round bilge hull satisfy the naysayers? I might even coin the phrase "gravity molding"

 

[Fanie]

I've wondered if there is any benefit to use a bit of air pressure to push the resin into a vacuum infusion ?

I think it will work very well, if you pressure the resin vessel it will pump the resin in faster and the flow process will be expedited. You stop the inflow before air gets in in any case.

You can even dunk the calculated resin in the bag and then vacuum it out on small parts.

 

[Mr Efficiency]

Nope, as this will simply inflate the bag as the resin enters and the bag will go slack, resin will pool under the bag as there is unequal pressure inside and outside the bag.

Back to the original question, I'd love to hear an explanation as to why you think the OPs idea will not work. Provided the shape is single curvature, there should be no wrinkles to worry about... I'm confident my idea will work also, perhaps I should do a small mock up in the shed to show you... Shouldnt take long if I keep it small... Will a 2m length of round bilge hull satisfy the naysayers? I might even coin the phrase "gravity molding"

This could become the Eighth Wonder of the World.

"The Hanging Boats of Bathtubdom"

 

[groper]

Ok, i didnt have enough scrap material to do a very large peice.... im not going to waste a bunch of expensive material just to prove a point.

One thing became clear early on - just glass alone will not stay fair once the vacuum is pulled, there needs to be some sort of core material which has a little bit of inherrant stiffness just to resist a little bit of bag pull once the vacuum is applied. If not, the glass will wrinkle a bit. So i had a small peice of 4mm PVC foam handy, put it in the middle between 2 layers of glass to make a sandwich structure, and voila! The peice stays nice and smooth and fair with the vacuum applied.


Pics - The 100um film is suspended between 2 steel tubes to act as edge battens.

The glass is layed in and the core was placed;

Top layer of glass placed on the foam;

Another peice of plastic over the top to close it, sealed with tacky tape, and Vacuum is applied;

The peice stays as fair as the material you choose as a core - PVC foam is great in this regard.., this shows the outside surface of the potential hull, nice and smooth;

Only thing left to do would be to add some feed lines and infuse it - but again, im not going to waste any resin for the sake of it - i know it will work based on my experiences. It not really any different to envelop bagging a part - which is done all over the world all day everyday. The only thing different is using some battens and gravity to hold the shape instead of a mold. Provided the lengths between battens are kept to the designed separations for the intended curvature, and the core material is cut to the same circumferential diameter of the hanging plastic, it simply must assume a fair curve by default...

Other possibilities include twisted developable panels and partial cones etc. If the lengths and radii are worked out prior, and the plastic cut and fastened to these lengths, the plastic will stay wrinkle free. Small wrinkles wont worry the surface finish anyway, these just tear off with the peel ply. Large winkles would pull the panel out of fair...

 

[Fanie]

From experience I can safely predict you are going to have a LOT of inconsistencies. The dry material will seem to look ok, but when wetted out and curing the glass is going to start warping. Even the vacuum process needs a guide for the glass. Also the resin weight is going to alter the shape.

Do go ahead and waste the resin to prove it.

 

[Mr Efficiency]

It might work if you want a constant section, but that is not a boat. And making a mould for a constant section is dead easy anyway, so what do you gain by this ?

 

[groper]

Fanie, your completely wrong about pressure feeding a bagged infusion setup, your also wrong about the warping. Have you ever infused anything? I know the answer is no, so how can you justify such an informed opinion? Ever heard of very low exotherm resins? Or very low shrink resins? From here, its not upto me to prove anything, if you dont beleive it, then you can prove it aint so, to yourself, enjoying blissful ignorance.

Mr E, your always the eternal pessimist... What can you potentially gain? A very fast to build, very cheap mold for a start...

Thicker foam could be used, and it would work even better because of its higher inherent stiffness. The curvature could be done with strategically placed kerf cuts and letting it hang... like below but inside the envelope bag rather than a forma;

Heres a rudder i made via envelope bagging around some thicker foam (50mm of airex), its as fair as the foam it wrapped and the entire exterior laminate was done in a single shot without warping -5 layers of 750gsm triax;

And the same rudder below after the flash was trimmed around the perimeter;

Everything stays fair because the vacuum pressure (atm) is identical all around the part, on all sides... so there is no pressure to pull or push anything away from how it will naturally settle. The only issue is bag tension as the voids under the bag shrink and conform to whatever it is you have inside the bag. A very small amount of stiffness and careful laying of the bag is all thats required... as the bag pulls tight, you have time to hold the vacuum lightly (~5-10% vac) and pull out any large wrinkles or bag tension problems, before pulling it down to 100% vac.

 

[Mr Efficiency]

Mate, you are pretty handy around composites, but I doubt this one is gonna catch on.

 

[groper]

Hey, im not saying you could build the QE2 with this technique, but some small boat stuff would be a breeze...

 

[Mr Efficiency]

Anyway, this has morphed from hanging curtains to using foam as an integral former, not the same thing. And I want to know how you get a boat shape, and not just a trough.

 

[groper]

Have to make it in 3 parts... bow cone or twisted lowers upto a chine.

Mid body as shown.

Stern could be a another cone resting on an angled plank to flatten the bottom of the cone for a higher prismatic.

Bond or tape all 3 sections together and you end up with a slender displacement hull...

Like i said before, provided all shapes are develop able, then there's no problem and all the same shapes we see on ply , flat panel and not rolled metal boats will be possible. Only the compound shapes wouldn't work...

 

[Fanie]

Fanie, your completely wrong about pressure feeding a bagged infusion setup, your also wrong about the warping.

You're cheating because you use a form for that rudder. You didn't hang the shape up somewhere and vacuum formed it.

I have mine cut from polystyrene, paint it a few layers water based paint to prevent the resin from attacking it and it can then be glassed over. Instead of supplying the resin from a bucket, you can prepare the bag and place the rudder in the bag, dunk the resin in, seal it and vacuum the excess resin out.

It would be exactly the same as pressurizing the resin into the bag faster. The only thing you have to check for is that the resin doesn't flow to one side in the bag so you have an inconsistent flow on one side and the other side is dry. What I do is make connections in different locations, if it looks like the vacuum draws too much to one side, I connect the second vac pump to the next outlet and seal the first off.

I use the higher density polystyrene because the low density is soft and deforms a bit under pressure.

 

[groper]

What form can you see for the rudder Fanie? The blade is suspended in mid air, hanging off the side of a table by the rudder post! Here maybe this will help;