I trust a boat builder or aerospace engineer will step in soon with further detail, but in the interim, from wikipedia:
quote
A vacuum bag is a bag made of strong rubber-coated fabric, open at one end, and used to bond or laminate materials. The item to be laminated is placed inside the bag, the open end is sealed, and air is drawn out with a vacuum pump. As a result, uniform pressure approaching one atmosphere is applied to the surfaces of the object inside the bag, holding parts together while the adhesive cures. The entire bag may be placed in a temperature-controlled oven, oil bath or water bath and gently heated to accelerate curing.
Laminating of flat objects can be performed more efficiently in a heated laminating press, but when the objects are curved or have irregular shapes, a vacuum bag works better.
end quote
In addition to holding parts together it also ensures more uniform resin penetration.
I'm pretty sure composite materials are always vacuum-bagged before being autoclaved but vacuum-bagging can also be done stand-alone.
I doubt that the only difference is the use of an autoclave in the production process. The autoclave probably allows for different construction and fiber layering etc.
Bob Miller ramiller@dogstardreams.com
Stelph <stelph@hotmail.com> wrote in news:1178873578.488461.56590@o5g2000hsb.googlegroups.com:
Just out of curiosity, I noticed today that Stampfi in Switzerland
have brought out a new design of single called the ST1, the main
difference being it is made in an autoclave rather than the X1 and S1
composite boats which appear to be made in vacum bags
Have I gotten the currency conversion right? Is the Autoclave Staempfli single actually 12,000 US dollars?
This is about $2,000 more than Empacher is asking for one of its single racing shells. Can anyone explain what justifies the cost? Is it in the material? Or the labor?
They are boat heated to 130 degrees centigrade but the Autoclave boats
are put under 6 bar of pressure and the composite boats are,
supposidly, in a vacumm. Does the autoclave process produce a stifffer/
tougher boat?
Out of interest, the only other autoclave boat manufacturer I have
found are Lola-Aylings, the other top manufactures fo carbon boats
(e.g. Fillipi and Empacher) seem to use the composite method..
I thought autoclaves used high temperature, high pressure, and steam to sterilize (sterilise?) things like surgical instruments. Surely an autoclave big enough and hot enough for a boat would be a very expensive pressure cooker to build and run. Do the composite materials in a racing shell take well to the temperatures in an autoclave? Or is my understanding of an autoclave too narrow? Thanks to the Molecular Biologists present and past for an 'injection' of humour (Ben, stay away with that needle - oh, wait, there's 15,000 km between us). Walter
They are boat heated to 130 degrees centigrade but the Autoclave boats
are put under 6 bar of pressure and the composite boats are,
supposidly, in a vacumm. Does the autoclave process produce a stifffer/
tougher boat?
Out of interest, the only other autoclave boat manufacturer I have
found are Lola-Aylings, the other top manufactures fo carbon boats
(e.g. Fillipi and Empacher) seem to use the composite method..
I thought autoclaves used high temperature, high pressure, and steam to
sterilize (sterilise?) things like surgical instruments. Surely an
autoclave big enough and hot enough for a boat would be a very expensive
pressure cooker to build and run. Do the composite materials in a
racing shell take well to the temperatures in an autoclave? Or is my
understanding of an autoclave too narrow?
Thanks to the Molecular Biologists present and past for an 'injection'
of humour (Ben, stay away with that needle - oh, wait, there's 15,000 km
between us).
Walter
Further to this - how do they account for the thermal expansion/contraction when the boat is taken out of the oven? Carbon fibre IIRC has a very anisotropic thermal expansion and I would have thought this would pose a problem.
On May 11, 4:52 pm, Stelph <ste...@hotmail.com> wrote:
Does the autoclave process produce a stifffer/tougher boat?
My understanding is that using an autoclave or 'oven' is more about
quality control and speed of production.
If you can control the curing time and make it consistent (with a
consistent temperature and pressure) then every boat that you make
using that set up will be very similar. It is possible to acheive good
results without it, but it requires more skill and some luck with the
weather. If the weather is not ideal, then your curing times will
alter.
Vacuum bagging aids the even distribution of the resin and a good
bond. The pressure in an autoclave is helping to achieve the same
things.
I know that Sykes and Prime (Geelong AUS) both have ovens that are
similar to those used in the car painting business. Lads at Sykes
wouldn't tell me the temperature they used, but said you could walk in
while it was happening and be ok for a bit (so I'm guessing not hotter
than 60deg C). KIRS (NZ) used to have a slogan that said "we don't
just make 'em, we bake 'em".
I suspect that Carl will probably correct us all very soon!
Why would I do that?
To make an effective laminate requires a curable resin, controllable pressure & an appropriate (in time & temperature) curing cycle.
The bonding resin can come in a number of forms: 1. Liquid, which you mix up from 2 or more reactive components, plus fillers as appropriate to modify the physical characteristics. The resulting resin may cure well over time at room temperature but normally requires heat to achieve acceptable cure rates and optimum properties. 2. Resin film - a part-cured, tacky material, impregnated by the supplier into a lightweight "carrier" fabric. This normally has to be stored in a freezer to prevent premature cure, but even so has a limited shelf life. It will only cure satisfactorily if heated to & held at a stipulated temperature (or made to follow a stipulated temperature/time cycle) which allows it to liquefy, flow & properly wet the materials to be bonded. 3. Pre-impregnated fabric - really a variant on 2 above, in which the bonding resin has been infused into the reinforcing fabric by the supplier.
When you make a laminate, especially a large one like a boat or part of an aircraft, you need to have an adequately long "open time" before pressure must be applied & the heating cycle begun. The resin must remain curable (not have started to "go off") during this period
In nearly every case, the pressure required to ensure intimate contact & conformity to the mould is achieved by applying vacuum. At sea level, atmospheric pressure is ~1kgf/cm^2 or 14.7lbf/in^2, & when you consider applying that much pressure over the entire surface of a boat that's an awful lot of force (1kgf/cm^2 = 10 tonnes/sq m). Yet simply by enclosing your laminate & its mould in a flexible, impermeable bag & sucking out the air with a vacuum pump, you can easily achieve >90% of that pressure as atmospheric pressure outside presses against the reduced pressure inside. In this case, of course, it owes nothing to fluid-dynamic lift ;)
Sometimes even such a pressure difference is insufficient to achieve good compaction of the laminate, or too much vacuum may cause a resin component to boil off. In such cases, you may use less vacuum & put the whole moulding into a pressure chamber. Pressure chambers are costly &, unless properly designed & maintained, can be dangerous (compressed air, even if only at 1 atmosphere above normal, can do a lot of harm if a vessel ruptures) so are subject to inspection regimes which increase costs
Then you need to apply heat. This either requires internal (steam, thermal fluid or electrical) heating of the mould - a very economical & effective process, but making for costly tooling - or external heating in the form of a heated enclosure or oven.
If your pressure vessel is also your oven, then it is called an autoclave. The same term applies to biological sterilising devices as to laminate curing pressure chambers & to any other heated pressure-processing chamber.
Each method of lamination can produce top-quality laminates, & the choice is based on a mix of tooling & production economics, the characteristics of the materials you wish to bond, the qualities you require in your laminate & a host of other factors. Thus, where very long open times are required for the lay-up of a laminate (take a large yacht or aircraft wing) then the ability to us very slow-curing pre-preg systems which require very high temperatures to effect cure will lead to an autoclaving process. On the other hand, if you need to use particular types of resin formulation for reasons of aesthetics, yet can work fast enough to close the laminate within an appropriate time, then liquid resins with fillers & other additives may be the only answer.
Of course, there are many other ways to use these & other materials to achieve fine laminates for particular uses, but that's for another time.
As ever, it's not just the process but also the quality of design, workmanship & materials which go together to determine the quality of the end result.
HTH - Carl
-- Carl Douglas Racing Shells - Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK Email: carl@carldouglas.co.uk Tel: +44(0)1784-456344 Fax: -466550 URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
Most of the worlds Boat builders manufacture their boats using an oven
cure process which involves the use of either "wet-lay" or "pre-preg"
material and normally with a honeycomb or foam core. These boats are
cured in a vacuum bag with around 30psi of pressure as that is about
as much pressure as you can get with this process.
I think that was a slip, Boyd, & that you meant ~30" of mercury, which is all you can get under total vacuum when operating your shop at normal atmospheric pressure
Cheers - Carl -- Carl Douglas Racing Shells - Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK Email: carl@carldouglas.co.uk Tel: +44(0)1784-456344 Fax: -466550 URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
Very interesting comments so far, many thansk to everyone who took the
time to reply!
As well as this discussion about autoclaves another thing I find
interesting about boat manufacture is the stiffness of the hulls, id
always thought that the stiffer the hull, the better the boat! (up to
a limit of course, extremly stiff boats would shatter if there was any
impact and I think i read somewhere that they would be very difficult
to sit)
One way i always tested the stiffness of a boat is to gently squeeze
the hull of the boat, for example near the bows, to see if theres any
give! Now hulls which have the nomex honeycomb (like empacher or
swift) have no give at all,but ive noticed that boats like Hudsen and
Fluiddesign are quite squidgy at the bows. Now i would have though
that this squidyness would mean the water would slightly deform the
boat as it passes through the water, resulting in lost energy and so
lost speed? But as both makes are thought to be very quick boats im
wondering where my logic has fallen down....
any help?
There are no meaningful data, nor theory, on which to base the popular assumption that stiffer is better. Just to test that assumption, if you will: would it be better for your boat to somewhat conform to a swell or to be so stiff that it in effect does not flex in the slightest (of course that's never going to be possible, but just suppose)? That said, all boatbuilders accept that clients do judge boats by longitudinal stiffness so they build them as stiff as they can.
Nor is panel stiffness (squidgyness) in any meaningful sense related to longitudinal stiffness (boat bendability) at the stress levels encountered in rowing shells. Provided a boat is designed such that it in use it nowhere approaches local compressive stress levels which could induce incipient panel buckling (Euler sorted out the maths for that), it already has sufficient panel stiffness to resist its main service loads.
Interestingly, the most common mode of initial failure of certain makes of honeycomb-cored shells is by compressive buckling - the hull or deck structure on the compression side during a bending load case (e.g. a glancing impact or running up the bank) starts to buckle (wrinkle) across each hollow cell of the honeycomb, depriving those cells of compressive stiffness along the axis of the boat & thus initiating collapse in the most compressed areas which spreads rapidly, allowing increased bending on the tension side, local skin rupture & final fracture.
Also interesting is the fact that a hull with a more squidgy characteristic can, if properly designed, be much more resistant to perforation on impact, since it is able to yield, absorb the impact energy & spring back. A stiffer panel with no more material in it concentrates the impact very locally & is usually much easier to puncture as a result.
Finally, water pressure on the surface of a shell is so slight as to have no meaningful influence on hull shape for normal shells of all types of construction.
HTH Carl
-- Carl Douglas Racing Shells - Fine Small-Boats/AeRoWing low-drag Riggers/Advanced Accessories Write: The Boathouse, Timsway, Chertsey Lane, Staines TW18 3JY, UK Email: carl@carldouglas.co.uk Tel: +44(0)1784-456344 Fax: -466550 URLs: www.carldouglas.co.uk (boats) & www.aerowing.co.uk (riggers)
There are no meaningful data, nor theory, on which to base the popular
assumption that stiffer is better.
Carl,
I wish you had told me this when I was in Staines last summer and persuaded you to grab the bow of one of your shells while I held on the stern. Then we just kind of lifted the shell up and down.
Sandy asked what we were doing, and you smiled and said, "Charles is just testing the shell for stiffness."
Now I feel like the proper fool. But what's new there?
I will, however, offer one comment. Of all the boats I have ever tested for stiffness, none has been stiffer than the Carl Douglas we held in our hands that Sunday morning.
I have a Staempfli ST1 - I am at the upper end of the weight range for the ST1 (the heavy weight will come out next February)
The unique benefit is not the stiffness but the flexibility on impact. After the recent storms our lake was full of debris. I hit a semi-submerged log head on. Boat was sitting on the log - had a hard time to get off the log.
Boat had NO damage - not even a scratch.
No try to do that in an Empacher.
That is why the ST1 is more expensive than the Empacher - lower maintenance.
My ST1 is regularly used by several inexperienced rowers - no damage yet.
