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Postby raalst » Thu May 17, 2007 11:12 am

thanks for the post mylo !

I am slowly getting to this same point.

I wonder about one thing. Kevin used "gooey"
RTV on his Oscar. Why don't we use that
technique for small molds as well ?

- clay in the box,
- master in the clay
- then paste the gooey RTV as a layer over the
clay and master
- top off with gypsum or another less expensive
filler material
- then do the other side of the mold the same way.

Seems to me this procedure will avoid/limit the
risk of leaks, allow for slower, more controlled
applying the RTV, and save RTV. maybe it even helps with the bubbles, because you paint it on (I expect you can see what happens and fix it using this method)

I intend to try this with my tower structure
simply to exercise for the large hull I need to do
afterwards.

keep us posted, I will too !
Regards,

Ronald van Aalst

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Postby Mylo » Thu May 17, 2007 11:42 am

Ronald,

Excellent question. I understand what you mean and that might work well too. I intend on experimenting with some other molding processess for my tower as well, due to it's size. I'd need a gal of RTV for that baby alone.

I went with the method that I did simply due to the fact that I was trying to copy Kevin's techniques as close as possible....to avoid....problems. For the smaller molds, I feel that going with all RTV is likely the way to go. The final mold has a lot of strength, is durable, plyable (ease of part removal), has nice flat sides for placing a hardback next to during casting, and generally works very well. Like anything else in this build, if I had to build molds all over again, I could do it quite quickly (relatively) and with a lot of success, without wasting much RTV at all. I wouldn't hesitate to use the same technique over again for THIS SIZE of mold. Big molds.....different question. RTV is way too expensive to use as "filler". On that note, I've seen guys put "stuff" in the molds to do just that, ...fill up space, so that less RTV is needed. This might be an idea.

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Postby KevinMc » Thu May 17, 2007 1:19 pm

Hi Mylo,

Hey, this is great progress!

With regards to putting "filler" in the RTV, I'd be very careful. IIRC, Pete (Pirate) did that when making his SeaWolf (?) molds and found that the filler wound up negatively affecting his molds to the point that he had to re-pour them. (Pete, please confirm!)

If I may offer a tip for making your mold boxes: a hot glue gun is a very worth while investment. Once the box pieces are cut out (for those of you who prefer building up boxes) you can make a completely sealed box in a matter of minutes by applying a continuous bead of glue along the seams of an assembled box. The best part is once your mold has cured, you can peel the beads of glue away and partially disassemble the box on the clay side of a 2-part mold allowing you to extract the clay while leaving the master stuck in the first half of the mold. By doing this you will prevent RTV from seeping in between the master and the first half of the mold while you're casting the second half.

Fantastic work!
Kevin McLeod - Oscar II driver
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Postby Mylo » Thu May 17, 2007 2:33 pm

Kevin,

"Great Progress" .... ?? ... I feel I'm behind schedule with the VERY LONG delay I had not only getting materials, ..but with other interruptions in my life. I HATE it when I can't get at my sub. :) I've got a long way to go before I have anything to put in the lake.

Didn't know that about the "filler". ....even if it's rumour, that's good enough for me. That's another lesson I learned in the mold making venture is....sometimes cutting corners costs you nothing but time and money. A confirmation on that would be nice though.

Hot glue gun....of COURSE !! ... I have one sitting on my shelf. *slaps self in head*. I finally ended up using my air nailer with carpenter's glue, clamping all together (as per the pic) to make my good boxes. They were really good boxes when I was done, but definately permanent, plus I had to let everything set overnight. I like that "semi - permanent" idea with the hot glue, as well as the sealing effect. On a positive side, I have the boxes for future projects, the MDF material to construct them is cheap enough. I was going to mention, I forgot to line one of my boxes with packing tape on one of my molds. It didn't create any problems at all with the mold or getting the molds out of the box. In fact, the mold was easier to get out of the box as well as the clay. Whether it will have some sort of effect on the durability / life of the mold has yet to be determined, but I can't see it.

There is considerable difference in mold craftsmanship from my first one, to my last one, ....with the latter ones not only being better molds, but taking less time to make. That's the way she goes.


Mylo
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Postby Sub culture » Thu May 17, 2007 2:36 pm

A technique I've recently experimented with, is using candle wax for small moulds.

This won't work for parts with undercuts, but it does mould fine detail, the material is very cheap plus the wax is reusable.

The other benefit is that the wax is self releasing, so you have the benefit of no mould preparation.

I've found this is especially useful when making lots of small parts for a master. Final parts can be moulded in RTV if you wish.

To use this technique, I melt the wax in small vessel, spray some water onto the master, and pour the wax over. Once the wax has solidified, it should release without much effort, and you can then cast up with whatever resin you choose.

For one off bits for a master, I use polyester putty/filler. This is cheap, sets in no time and doesn't require pressure casting.

Andy
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Postby KevinMc » Thu May 17, 2007 2:58 pm

Andy- Wax?!?! :shock: :shock: :shock: Cool!!!! 8) 8) 8)
(On that note, I wonder if you could make a 2-part mold if you "froze" the first half for long enough before attempting to pour the second half.)

Mylo- Indeed, I said great progress. The measure of progress is more than what you can physically bring to the table... Clearly you're learning a great deal as you go, which (in my mind) is a show of great progress.
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Postby Mylo » Thu May 17, 2007 3:11 pm

Andy,

With this wax approach, I'm assuming you still make your mold boxes, press your master into clay, and then pour the wax over top, just as you would RTV, or is that what you're talking about Kevin ? Does the water sprayed over top act as some sort of release, or does it just cool the wax quicker ? Great idea for doing a bunch of little things. Any pics ?

Kevin,

Ah yes, ....in terms of expanding my knowledge base, for sure, FANTASTIC progress. Every inch of it the hard way it seems. Anyone in doubt of that, check out my "Hours to date". ....and that's only the half of it.....I've also contributed quite generously to the local land fill.

Mylo
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Postby Sub culture » Thu May 17, 2007 4:35 pm

Haven't taken any pictures. I will take some next time I make up a part this way.

I haven't made any two part moulds with wax, but have given it some thought. I think if one half was coated with shellac, this will stop the wax from bonding to itself. Either that, or use a very hard wax for one half, and a softer wax for the bottom half.

I think RTV may still be the way to go for two part moulds, but there are a lot of things that can be moulded with a single ended mould.

You need to make a containment for the wax. this can be anything you like, strip of metal, card etc. Then just spray with water and pour in the wax same as RTV.

The water helps the release.

Another wax I've messed with is 'green' wax used for lost wax/investment castings.

This is peculiar stuff that can be worked with hand heat (goes like a sticky clay) but goes rock hard once left to cool down.

With this wax you can effectively 'knead' it over your shape you wish to cast.

The downside with this wax, is that the exotherm from the resin tends to burn the wax impression out dulling the detail. I find about five or six pulls are all you can get. Then you just re impression the mould.

One nice thing about the wax method, is it's very quick (perhaps a little dirty too).

Another thing I've been considering using for moulds with large undercuts is jelly.

Gelatine was used by stucco plasterers in the days before RTV rubber was available for casting intricate shapes. As a hard case plaster of paris was used to support the soft gelatine mould.

Now if you mix a very firm jelly mix and pour that over the mould, then whack some plaster over as a hard case, I reckon that would be good for a couple of hull pulls.

How does the price of jelly compare with RTV?

Andy
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Postby Mylo » Thu May 17, 2007 5:03 pm

Andy,

Some great out of the (jelly) box thinking.


....plus the obvious, if you screw up a jelly mold, ...serve it for dinner. :)


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Dental casting techniques

Postby TMSmalley » Thu May 17, 2007 8:04 pm

My Dad was a dentist 1940s- 80s and I remember him carving dental appliances from hard wax and making a mould from a plaster-like product - dental stone.

He would then melt the wax out, leaving a beautifully detailed mould. Then he could make multiple silver casts from the mould. Sometimes he would put the mould in a certrifuge to force the silver down into all the tiny crevices.

I wonder if that technique would work for our hobby? I might have some of the details wrong - I was more interested in schmoozing his cute dental assistant than the ins and outs of casting inlays.

Dr. Art or Dr. Diane might have some ideas. . .
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Postby Mylo » Thu May 17, 2007 8:33 pm

:)
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Postby Art Broder » Thu May 17, 2007 10:19 pm

Tim,
I only made one sub part from wax, and then had my dental lab invest it in stone and cast up the part in heat cured dental acrylic. It was the base for my L.A. sail , to convert it into a waterproof housing for the wireless camera in the sail. I also made the bow planes for that model by pouring dental repair acrylic into a hand shaped form. That acrylic comes in a powder that mixes with a liquid monomer and self cures. Also, I constantly use my dental lab drill for more precise drilling than my Dremel can accomplish. Other than that, and using dental impression material to help recreate small parts, I can't think of much else in the dental realm for subs, unless you want to count my extraction skills for removing difficult to remove parts!
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Postby TMSmalley » Thu May 17, 2007 10:28 pm

Could the stone be used for moulds to cast planes, guns etc? Should be cheaper than silicone.
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Postby Sub culture » Fri May 18, 2007 4:47 am

The 'stone' would be sacrificial for lost wax/investment casting. i.e. you break it apart to get the casting out.

For casting resin, you would need to prep the mould with a release agent. This could be something as simple as wiping a bit of wax around the mould.

I've used tool plaster (pure alpha) for casting simple objects, and it works very well indeed.

All the same, if I was making something with fine detail, i.e. lots of rivets etc. then rubber is still the preferred material for moulding.
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Postby Mylo » Sat May 19, 2007 1:03 am

I have found the rubber RTV material easy to work with, durable, flexible, captures the details, convenient, and effective.

....BUT


Expensive at around $130/gal.


I personally am willing to pay the price for RTV for small/med size molds to enjoy the benefits I mention. Some further experimentation is in order for me to come up with a reasonably affordable method for the bigger molds.

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