Vacuum forming

[EatMyShortsRacing]

Folks,

I have seen the vacuum bags being used for carbon composite moulding. What about a vacuum chamber like converting an old refrigerator to being an oven / vacuum chamber for moulding boats... is this better than the bagging?

EMS Racing Shorts across America!!! LOL

 

[TimD]

Craig,

The bag is used to pull the cloth tight onto the surface of whatever you are laminating, and when used with some type of drawing media, reducing the amount of required resin.. I'm not sure how you would substitute a fridge to to this. Maybe I just don't fully understand your idea.....

There are other methods for moulding composites like compression moulding etc. - it depends on what sort of finished item you are trying to achieve. Vac bagging produces very light items as it gets rid of unnessesary resin, but is not suitable for some shapes. Compression moulding can produce very, very strong objects. ( I want to try making hardware outta this method - Stronger / lighter than Alloys)

 

[EatMyShortsRacing]

Tim,

I am lookign at doing Hydro tubs. I was considering the fridge idea to include heat lamps so you could form the hulls under vacuum and heat. Wouldnt the general vacuum get rid of excess resin anyway?

EMS Racing last in a long line of bachelors

 

[Ian Inverarity]

Craig,

I talked to a friend about vac bagging, he used an old fridge compressor to create a vacuum, but in a bag. The vacuum uses air pressure to force out air in the layup and compact it all into the mould for sharp corners etc. Just evacuating a fridge to low pressure would cause the trapped air to bubble out, but would not compact the layup. The excess resin is removed in vacuum bagging by using various special cloths over the layup to absorb excess resin squeezed out.

You can do a system with positive pressure as well as heat, and build your own autoclave like they do in F1, but that is proabaly a lot harder and more expensive.

Ian.

 

[EatMyShortsRacing]

ok the next question....

what level of vacuum is required for effective bagging?

 

[TimD]

Craig,

For hydro tubs, I would do them as a split mould, Vac bag the two separate split moulds - use a peel ply on the inside as the mould would put a good surface on the outside of the tub. Leave them under vaccuum for the cure time of the resin. Then remove them from the moulds and join the two halves together adding bulkheads at the same time. Heat would only be needed in cold weather unless you are using a resin that needs heat curing...

Vac bagging will get rid of some of the resin but not as much as you can get away with in order to make it super light and still keepthe strength up.

Have a read of MikeP's posts on building boats from CF. Some really cool stuff in there. Lots of pic's and links to tech sites.

 

[Ian Inverarity]

Craig,

As much as you can get! Higher levels of vacuum are only a problem if you are forming over soft cores (like foam wings for gliders) as long as the mold is reasonably strong. Of course about 1 bar is the maximum pressure you can get from a vacuum, hence the positive pressure systems used by those who can afford it to get even more pressure.

Ian

 

[Ian Inverarity]

Also I agree with Tim about the construction, but I would have the top and sides in one mould and a simple flat bottom in the other. This would give you nice sharp edges on the bottom, but you can make a nice aerodynamic top!

Ian.

 

[TimD]

If you go the fridge pump then you might also want to get a vac reservior - an old LP gas bottle of fire extinguisher will do the job.

Put a vacc guage in the line and use a vacc advance diaphram from a dizzy to adjust the pump cutout to adjust vaccuum levels. 7 p.s.i is fine for white foam / laminating and up to 1 bar (approx. 14p.s.i) for anything else.

 

[TimD]

Also I agree with Tim about the construction, but I would have the top and sides in one mould and a simple flat bottom in the other. This would give you nice sharp edges on the bottom, but you can make a nice aerodynamic top!

Ian.

Are sharp edges on the bottom that critical? I thought that was just a limitation of using sheet ply.

 

[Ian Inverarity]

Tim,

I am not positive, but I would figure that sharp edges on the bottom would be better for breaking the flow of water away and providing lift. With rounded surfaces the water can climb up the sides and cause drag. Which is one of the reasons you try to make all your sponson edges sharp.

My old aluminium struts for monos had a bit of a funny shape. Round on the bottom 180 degrees, but stepped in above that to stop the water climbing up too far. Grease residue proved that this works well. But I cant do that with my thin-wall SS struts anymore!

Ian

 

[waterbug]

Hay guys, This may help some of you. Build you a wood or metal box to size. {heat resistant} Make you two frames for the top, like flat picture frames, over lap corners so they are flat.drill holes and put wingnuts and bolts around it to clamp plastic. The bottom part of box needs to be 1''to 2'' deep with peg board recessed to be flush with wood frame.put some kind of seal around top and bottom faces{plastic frame and bottom box lip} take a 16 gal. shop vaccum and hook to bottom face of box.[p.v.c. fitting] Clamp plastic in frame, heat in oven or with heat gun until plyable, turn on vaccum, close lid, instant vaccum forming! Make forum to mold out of wood or plaster, or auto body filler. canopys and sponsons should be a breeze! R/C MODLER magazine sells a book with info & diagrams. Us airplane guys have been doing this for years. Vaccum bagging you can use shop vaccum for vaccum sorce. This might not help every one but will help some!