Scratch-built 1/96 Oscar II

[KevinMC]

G'morning Steve,

Thanks for your concern,

G'morning Steve,

Thanks for your concern, but I think I should be okay. One of the reasons I went with the Smooth-On system is that Craig has a long history with it and the only things he's ever had issues with were sulphured clay and really sappy, old wood. (I've had no issue with the RTV not kicking off on my wooden stir sticks though.) I did specifically ask before beginning about not curing over 'red putty' or CA (which I know the BJB product has issues with) but he's had no problem with the Smooth-on products. I guess everybody's RTV formulation is just a little bit different...

I troweled down coats 3 and 4 onto the hull and sail molds last night. The Rebound 25 in it's natural state, as I used it for the second layer, has a consistency a little thicker than liquid honey. Mixed with just 1% (by weight) of the thixotropic additive, it turns to peanut-butter. No kidding, I couldn't have put this stuff down with a paint brush if I tried! Being as thick as it was, I used an old credit card to smear down coat number 3, and once down on the hull it didn't sag a bit. Four hours later I came back and smeared down coat number 4 using the same process, and here is the result.



The only thing that remains to do now is to add some keying features on the RTV and then I can get going on the hardback.

[pirate]

Kevin,
Great idea with the "old"

Kevin,
Great idea with the "old" credit cards. Did you get all those "old" credit cards out of the wife's purse? That would be a good way to get rid of them.

Lookin' good. What are you going to use for the hard back? I've used plaster with my sculpture work, but it's very heavy for such a large mold.

Pete

[wayne frey]

You da man Kevin! I

You da man Kevin! I am getting excited. Did Dave give you a turnaround guesstimate on the metal cast stuff?



Edited By Wayne Frey on 1144168492

[KevinMC]

Pete- Now there's a money

Pete- Now there's a money saving idea! As an aside I've been using old business cards for most of the spreading I've been doing so far, but for the RTV with thixotropic additive anything less than a plastic card would be useless as a spreader.

As heavy as it will be, I'd planned to use plaster and plaster impregnated burlap to build up the hardback. (But I'll also work in a plywood spine/support frame so the plaster doesn't shatter under it's own weight.)

Wayne- I'm embarassed to admit it, but I still haven't gotten them away to him. <Kevin hangs head in shame.> I'll be expressing shipping them down as soon as I can get to a postal outlet while they're still open!

[pirate]

Kevin,

Whenever I made a plaster

Kevin,

Whenever I made a plaster backing for my sculpture molds, I use hemp shreds mixed all within the plaster. KInd of works like glass with resin, gives it some rigidity. I guess that's what the burlap is for as well. The wood spine is a great idea. It'll need something like that, but won't plywood get soggy and lose any of its integrity in the plaster? You could use a grained piece of wood, like a portion of 2x4 and wrap hemp strands up into and around it. That would really hold on.

Can't wait to see the final casting. Then I'll have to sick my Seawolf and Virginia on it if you come to the Sub regatta this year. Just don't name it Kursk, or it's a goner.

Pete

[anonymous]

Very good Kevin. that's good

Very good Kevin. that's good to hear.

I use Ultracal30. Much stronger than plaster by far. Takes longer to set up too giving you more working time. Burlap, fiber, it all works.

And yes you can make a wooden crutch that will also allow to sit on it's own legs sort to speak keeping it nice and level to work with. In addition it will cradle the two halves if you make the mother mold in two pieces. I just lay plastic rap over the mother model and make the cradle against it so it can be removed but always holds the mold pieces together accurately.

steve

[raalst]

Kevin,

marvellous work. and

Kevin,

marvellous work. and thanks for taking the time to
inspire the rest of us.

would it be an idea to check out the pharmacy for
that stuff they wrap your leg into after you had too much
fun with skiing ?

this stuff certainly does not shatter, and is made to be as
light as possible.

I don't know what burlap is though, might be the same
thing I am thinking of.

[koeze]

Burlap is what we Dutchies

Burlap is what we Dutchies call Jute.

Erik Jan

[KevinMC]

Pete- Yes, that's exactly what

Pete- Yes, that's exactly what the burlap is for. Although a plywood spine may go a bit 'gimpy' as the plaster is drying, its stiffness should return once all is dry. (And since I won't be demolding until it is dry...)

I am working to attend the Sub Regatta this year- it's a bit of a haul for me, but I want to come play with everyone! Of course I have a lot of building in front of me yet, but I believe I can do it. Assuming I am there, I'll be happy to come "play" with your Seawolf and Virginia- It'll be good practise for when I go to play with Ben C. and his Seawolf ]that stuff they wrap your leg into after you had too much fun with skiing[/quote]
Burlap does not have that much elasticity to it. Think of the heavyest glass cloth you've even seen, but woven from fabric instead of glass strands...

[anonymous]

Kevin,

I get it here. http://burmanindustries.com/

Kevin,

I get it here. http://burmanindustries.com/ I drive over and pick it up but if you do a search I bet you can find it in your area as it's just a building material.

Steve

[KevinMC]

Sorry folks- technical difficulties prevented

Sorry folks- technical difficulties prevented me from posting these last week...

Per recommendations, I put a fourth coat of Rebound on the mold, and took care to ensure that the surface had no large protrusions or crevices that would lock into the mother mold.

Next, I deliberately put some features around the periphery of the mold that would lock into the mother mold (in a controlled way). These features were built up from really thick RTV and should hold the RTV component into the mother mold like a dome-snap. I also put some non-locking features along the top of the molds to help center the RTV tool in the mother mold.







As a final note to anyone wishing to use this technique, I've been told that my keying features would lock things in better had they been aligned longitudinally instead of radially. (ie having their long dimension running lengthwise insted of across the side of the hull.)

[novagator]

She is coming along nicely!

She is coming along nicely!

[KevinMC]

Thanks John,

The real moment of

Thanks John,

The real moment of accomplishment for me will be when I peel back the RTV from my first hull and oogle the results. So close!

[KevinMC]

Yikes, I'm getting behind again...

Plaster

Yikes, I'm getting behind again...

Plaster time! But before we get there, I should point out that it's also worth while making sure that the RTV doesn't stick to the plaster at all. To do this I thinned some Vaseline with turpentine in a stainless mixing bowl until it flowed freely. Then I brushed the mixure onto the RTV and parting board using a foam brush.



Now on to the plastering. I would have liked to use Ultracal for this, but I couldn't find any promptly so I reverted to using actual plaster. I'm going to apologize up front that the photos just "jump to the end", the fact is that the plaster set up so fast I just didn't have time to pick up the camera!

Here's all the stuff needed, and the completed hull and sail shells.



To make the above I went through three 4lbs buckets of plaster and about two yards of coarse burlap. Not to be confused with the thin stuff found in the gardening store, this stuff has some good body to it and was found at a local fabric shop. Boy did I feel out of place in there! The final shell thickness is just over 1/4".



Laying it all down was quite easy- just mix up the plaster and smear it down. Go get your burlap damp in the sink so it will take the plaster more easily. Mix up another batch of plaster, drop the burlap in the bucket and lay it down on the hull.

One of the features to note on the shell is the integrated stand to prevent this sucker from rolling on me when I lay up the hull. This is nothing more than a suitably cut piece of 1/4" plywood. The other feature to note is the wooden "spine", made from a slice off a 2X4. (Pine I guess?) This should provide a bit more strength to the plaster shell.



Another shot of the spine. Not captured in this photo ('cause I didn't add it until later) are some additional burlap strips that I ran over the spine and down onto the shell. I found that the pine didn't bond well to the plaster, so I wanted to lock it in with some additional material.



Here's a detail shot of the sail shell. This was allowed to set up for a full 24 hours before moving, but then...



Demold time! I lifted the shell from the base and found that the RTV and master came with it. No matter though, with a tug on the RTV glove the whole inner portion slid right out. Next, the RTV was peeled back from the sail master leaving it completely free of any damage. Now I'm ready to make sails!

[pirate]

Kevin, may be too late

Kevin, may be too late but, if I were you I'd encase that wood spine in a wrapping of some of that burlap and plaster before removing the shell so it won't wiggle loose on you. As well, any movement of that piece of wood while removing or laying glass could send cracks out throughout the entire piece, causing it to crumble. I've had molds do that on me.

Pete