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Making masters larger


#1

besides making new models altogether, does anyone have any methods
to enlarge silver or brass masters, for casting, like painting them
with something thick, and then making the mold?? I make different
bails for different pieces, and if i need to cast them they are
sometimes either visually or mechanically, slightly too small. I
have no problem working on the casting wax, if i could get a
larger(1-10%) workpiece back, even if less defined than the
original. I obviously haven’t explored making molds or casting yet!

Also, how workable/carvable is the silicone rubber inside the mold,
so as to make parts of pieces larger, for rework? thankyou! dp


#2
    besides making new models altogether,, does anyone have any
methods to enlarge silver or brass masters, for casting, like
painting them with something thick, and then making the mold?? I
make different 

You may be able to do Silastic E RTV Silicone Rubber Kit put out by
the Dow Corning Corp. Midland, MI 48686-0994, USA (517)469-6000 –
basically about 1 lb base and 0.1 lb curing agent (Rio has the
Silastic J which seems to take a lot longer to cure, but remains
workable longer, but I haven’t worked with Silastic E since 2000 so
it could simply be their new line with little difference). You have
your frame for the mould, with a little cone for where the molten
metal will be channelled, use one of the wax wires you can pick up,
lay out four corners with other pieces of wax or what have on hand.
The wax piece, plastic piece, metal work, whatever it is that you
are trying to make a mould of gets heated onto the wax wire, or
crazy glue to a piece of wire proper if it’s, too, heavy a piece
(this is called brass sprue formers and rods in Rio Grande) . . .
mix the curing agent and base, pour out into mold until model is
half covered. Take out the corner holders, a light amount of
petroleum jelly then lay out the rest, let cure and take apart the
moulding frame and pull apart the mold, remove you original.
Because of it’s room temperature vulcanizing there is no heat
involved, nor any expensive vulcanizers. You could most likely add
careful layers to your piece before making the mold. Making things
smaller is easier with the Reduc-It Mold Compound (Also from Rio),
you pour it into your mold and it will shrink to 50% volume – it’s
a positive casting material.

    Also, how workable/carvable is the silicone rubber inside the
mold, so as to make parts of pieces larger, for rework? thankyou!
dp 

the vulcanized stuff that I’ve worked with is hard as all heck, the
RTV stuff would take to a knife fairly well, you might be able to
get the results you need from working it. My carving blows so I
couldn’t do it, but with patience and a sharp knife I think it is
possible. Also you chop up RTV molds you don’t need anymore and
fill up a lot of the lost space in new molds with those, they’ll
integrate pretty much fully into the mold so you inject wax or what
have you.

Good luck.
David Woolley
Fredericton, New Brunswick, Canada


#3

dp, I once had to make a ring that I was working on a bit thicker in
cross-section. Being the somewhat lazy person that I am, I tried to
think of a way to “fake it”, rather than doing it over again. What I
came up with worked very well, and I have used it since with
success.

I took some pink Vigor Sheet Wax and put it between two sheets of
silicone coated paper, the kind that is found on the back of self
adhesive labels. Next, I ran it through a rolling mill until I got a
sheet that was 4 to 5 tenths of a millimeter in thickness. Next, I
applied this to the outside of the ring and had a silicone mold made
from it. The resulting casting from the built-up model was
reasonably close to the original. The time required to redefine the
detail was much less than doing it over again.

It is better if you heat the wax slightly (body temperature) before
working with it. Once it is on the piece, it will adhere better if
you heat the piece to around 100 to 110 degrees F or so.

Of course, this technique is better suited for pieces with minimal
detail, so I can’t say if this will work well on the pieces that you
have mentioned.

Hope this helps…
-Tom Murray


#4
   besides making new models altogether,, does anyone have any
methods to enlarge silver or brass masters, for casting, like
painting them with something thick, and then making the mold 

if you have access to plating baths you could electroform metal over
existing models to grow them up slightly. the model can be reworked
after it’s run in the tank to restore detail. the tendency of
electroforming is to deposit material on the high spots to a greater
degree than the dips. another method would be to split the bail,
open it enough to work with and solder a thin plate of material to
the back of the piece to enlarge it enough to account for shrinkage.
with silver the shrinkage is about ten percent in weight, physically
about 7 to 8%. by back soldering you reduce the amount of area
you’ll need to clean up and redetail.

Also, how workable/carvable is the silicone rubber inside the mold,
so as to make parts of pieces larger, for rework. its been my
experience that you can get fair to horrible results carving or
grinding rubber molds. if you must use a fine burr (one with mega
fine teeth the more the better) and a really fast handpiece. i
prefer 12,000 -15,000 rpm and up. most important is to stabilize
your hands on the bench and use a light brushing stroke. it’s gonna
make a mess so do it away from anything you want to keep clean. but
again the best results you can expect is a rough job. hope this
helps.

Talk to you later Dave Otto


#5

besides making new models altogether, does anyone have anymethods to
enlarge silver or brass masters, for casting, likepainting them with
something thick, and then making the mold?? What I have used in the
past, and works rather well, except for fine detail is to take a bowl
and melt some paraffin wax (gulf wax) and dip your model into it.
(quickly repeating this process builds the model up more) Then make a
RTV mold of the model. There will bea clean up process since the wax
builds up all areas.I have never tried, but I suppose the paraffin
wax could be painted oncertain surfaces as well.

Contentment should be the hallmark of a man or woman who has put all
of their affairs in the hands of God


#6

Hello , This is slightly off topic , but may be of some use . If you
have made a rubber model that has [ or is ] thin spots or areas that
will not fill properly … Use a hot tool and burn [melt] the
rubber from the back of the mold . By melting rubber from the mold
you are enlarging the cavity and thus getting a thicker wax . We
really like using our giles waxer as it gets plenty hot enough to
melt the rubber and with fine enough tips to give pretty good
control .

Mark Clodius


#7

I do not have any experience with these product(s); but, have a look
at this URL

http://industrialpolymers.com/water_abs_expanding_urethanes.html

…and let the rest of us know how it goes…


#8

Hi, I posted this link a while ago but here it is again …

http://industrialpolymers.com/water_abs_expanding_urethanes.html

This substance can be “injected” into an existing mould (like you
would inject wax) but when it has set it will expand by up to 60% by
just adding water. I would assume you could then create a new mould
from the expanded master.

I think they also do a substance call “Hydrospan 400” that can
reduce by 40% using the same method, but it is not mentioned on the
site …

I have not tried any of these (as I don’t do casting) but they seem
to be very interesting.

Cheers Paul Townsend
www.beaujangles.com