post curing out of mould ?

awallin · Post by **awallin** » 31 Mar 2006, 10:16

Hi group, a question for experienced builders:

I have a new mould which I do not want to mess up.

In the past I have destroyed a number of moulds by temp-cycling i.e. repeated heating of them while post curing
(the mould surface slowly develops irregularities etc. when you do this for 10-15 hulls...)

With the new mould I do not want this to happen so I plan on leaving it 'cold' (room temp) all the time.

So my question is, once a part (hull, deck) is released from the mould, can post curing at a higher temp (6 hours at +60 C for the resin we have) be done out of the mould ?
The concern is that the laminate might get soft and distort out of shape ?
do I need a separate 'rubbish' mould to place the hull in for post-curing ?

All epoxies I have come across seem to benefit greatly from post-curing at high temp, but maybe there are ones that can be cured fully at room temp also ? These usually have quickly acting hardeners(15-25min) so I would need to work quite quickly - hard to do with a complex part...

Anders

Steve Landeau · Post by **Steve Landeau** » 31 Mar 2006, 19:04

Hi, Anders.
I'm experienced, but would not consider myself an expert. I have been lightly trained in aerospace composites , so I have a bit more knowledge than the average home builder.
From what I have learned, your molds generally must be able to withstand your post curing temp. Yes, it is highly probable that your hulls will distort if post cured without the support of your molds. If you wish to post cure at 60C, then your molds need to be cured at a higher temp than that, or at the very least the same temp.
For the products we are building, you should be able to get by with post curing your molds even just slightly higher (10-20C).
Are your molds made of epoxy or poly/vinylester?

awallin · Post by **awallin** » 31 Mar 2006, 21:39

Hi Steve, thanks for your comments.

The mould is polyester ("isoftal", I am told this kind shrinks less when curing than normal polyester)

The boats will be made with epoxy ofcourse.

The 6 hours at 60 C recommendation is for an epoxy(SP ampreg 2? ) with a very slow hardener, several hours pot-life. Maybe I should move to a faster hardener, the fast one has higher mechanical spec without post-curing and would probably be OK with post-cure at only 30-40C as you suggest.

I am trying not to repeat past mistakes as I am just now putting in several hours of sanding and polishing on the mould surface... finished the whole thing with 600grit today, 800,1000 and 1200 to follow and then two grades polishing compound - all by hand as I have not found a machine suitable for the curved mould surface.

While on the subject of building, I am planning to use three layers of 165g/m2 twill-weave glassfiber. Newest idea is to use two pieces of cloth for the hull(bow+stern) overlapping at the keel/mast area so we get 6 layers there, should be plenty of reinforcement. The three layers should probably not all be oriented the same way, so maybe 0/90 + 45/45 + 0/90 ?(or the other way around 45+0+45)

Three layers is a bit overkill but extra rigidity won't hurt...

What are you guys using ?

Anders

spaldi01 · Post by **spaldi01** » 10 Apr 2006, 09:19

What are you guys using

Hi Anders I would not consider myself an expert builder but I have recently made a hull using 2 layers of 165g/m2 cloth with both layers at 90 degrees. I do not post cure other than puting the hull and mould in a warm (35-40) cupboard for a few days and the hull appears stiff enough

I do however build my boats with a central bulkhead to which the mast slot, fin and shroud plates are fitted. This stiffens things up greatly.

awallin · Post by **awallin** » 10 Apr 2006, 12:52

spaldi01 wrote:
What are you guys using
Hi Anders I would not consider myself an expert builder but I have recently made a hull using 2 layers of 165g/m2 cloth with both layers at 90 degrees. I do not post cure other than puting the hull and mould in a warm (35-40) cupboard for a few days and the hull appears stiff enough
I do however build my boats with a central bulkhead to which the mast slot, fin and shroud plates are fitted. This stiffens things up greatly.

Thanks for the input.

I've now spent about 4-5 evenings sanding and polishing the mould so I really don't want to mess it up by heating it too hot. Several people told me that lower temperatures, maybe 30-40 degrees, for an extended period of time, up to two weeks, should do the trick.

I will try orienting one layer 0/90 and the other 45/45 to see what effect it has.

AW

Steve Landeau · Post by **Steve Landeau** » 10 Apr 2006, 17:28

Anders, are you using S glass or E glass?

awallin · Post by **awallin** » 10 Apr 2006, 20:12

Steve Landeau wrote:Anders, are you using S glass or E glass?

hmm, the older boats we have are from the cheaper, less stiff, and more common grade.

I think the latest roll I got is the slightly stiffer stuff. It has blue yarns weaved in at 100mm intervals or so.

Do you remember how much difference there is in stiffness in theory ? What about in practice ? is it noticeable ?

Just got the deck plugs milled yesterday (see www.anderswallin.net). They need to be sanded and then painted and sanded again. Then a mould can be made.

AW