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Cutting mold locks


#1

When I make molds, I normally use vulcanized silicone rubber and
metal mold locks, and open them via the powder separation method and
I haven’t had to cut a mold open in about two years. But I am
currently making a bunch of molds from plastic models that can’t
handle heat. I used Silastic RTV. The first batch of molds turned out
well, but these RTV things are slippery inside. They could benefit
from some sort of mold lock.

The way I was taught to cut molds involves cutting a zig-zag line
down the side of the mold and following line that as the mold is
opened. But this still allows these slippery molds to side-slip a bit
when squeezed for wax injection. I bought some old molds that were
cut with beatiful, neat square locks carved right into them. Is there
a book or video somewhere that shows how to do this?

Kathy Johnson
Feathered Gems Jewelry
http://www.fgemz.com


#2

Hi Kathy,

Before I started making heated rubber molds I made RTV molds. I was
shown how to make RTV molds that did not require cutting by a fellow
that made RTV molds for various projects not jewelry related. The
steps are as follows:

  1. Lay down a layer of modeling clay. Cut the edges to square them
    up.

  2. Build a wood mold frame around the clay.

  3. Press the model half way into the clay.

  4. Take a conical shape tool about inch in diameter and press it
    into the clay about inches deep at various locations. These
    depressions will form the locking keys when the mold is finished. The
    tip of the tool should not be sharp pointed.

  5. Pour RTV onto the clay and model and let cure.

  6. Once the first half of the mold is cured removed the frame and
    clay.

  7. Turn the mold over and place the wood frame around it.

  8. Now here is where some of the chemical oriented Orchid members
    can be helpful. I coated the mold with some liquid which I can not
    remember the name of. It prevented the new RTV from sticking to the
    first RTV

  9. Pour RTV onto the first half of the mold.

10 Once the second pour of RTV is cured the two half could be
separated without cutting. Some internal cutting of the mold may be
required depending on the shape of the original model.

I can illustrate the process on my blog if anyone is interested.

Lee Epperson


#3

Kathy,

Making square locks at the corners of the moulds was the way I was
taught a few years ago - not that I’ve cut many moulds since then. I
think you will find that it is a matter of practise running the
scalpel in a straight line around the mould and at the corners
digging the blade in at a sharp angle to form the two outer sides of
the square followed by the top side and the two inner sides.
Continue the slightly jagged cuts towards the master but try to cut
along an edge rather than down the side of the master. The locks if
done properly will be all that is needed to secure the mould parts
and careful cutting of a straight edge near the master will give a
better finish without having to remove a lot of the casting. All the
best and try to keep all your fingers (a cut resistant glove may
help),

Roger


#4

Hi Kathy,

After graduate school, I did a number of post-graduate years in the
School of Hard Knocks, including 3 years as the designer /
model-maker / moldmaker / superviser / trainer at a gold jewelry
factory (casting and etching). I don’t know about a video, but I have
two thoughts;

  1. Modify the zig-zag method, so that on the narrow sides, which are
    allowing slipping, the zig zags go up and down more tightly/often
    than usual. Also, cut that zig zag line more deeply than usual—go
    around one or two more times, deepening the zig zags before opening
    up the rest of the mold.

  2. I think you can figure out how to cut the square locks, since you
    can already do the other kind. Let me see if I can talk you through
    it:

  3. Cut a line all around the mold, along the mid-line.

  4. Each corner lock is cut one at a time.

  5. pull open one corner, and cut up two sides, then the top, and
    then back down to cut the other two sides. That’s one!

  6. Cut the other 4 corners.

  7. Finish cutting, as usual.

I hope this helps!

Cynthia Eid
http://www.cynthiaeid.com


#5
I bought some old molds that were cut with beatiful, neat square
locks carved right into them. Is there a book or video somewhere
that shows how to do this? 

Check out
http://www.donivanandmaggiora.com/info/RubberMolds/RubberMolds.htm

Johns technique is so close to mine that it is eerie. The only real
difference is that 2 of my squares go in the opposite direction (all
corners cut with the same mold orientation and knife movements)

Much easier than a zig-zag cut and you get more usable volume in the
mold. I too initially learned the z-z method but a few months sitting
next to a full time mold cutter sure changed my style.

Jeff
Demand Designs
Analog/Digital Modelling & Goldsmithing
http://www.gmavt.net/~jdemand


#6
I bought some old molds that were cut with beatiful, neat square
locks carved right into them. Is there a book or video somewhere
that shows how to do this? 

Kathy, I’m not pretending to be a great mold cutter - in fact I hate
it, and only do it as a chore when I have to. But I wrote an article
for an art college class, which is heRe:

http://www.donivanandmaggiora.com/info/RubberMolds/RubberMolds.html

Castaldo also has some things: http://www.castaldo.com/ It’s not
difficult, just a little trick with the scalpel. It’s also a lot
faster than the zig zag method, I think.

http://www.donivanandmaggiora.com


#7
Now here is where some of the chemical oriented Orchid members
can be helpful. I coated the mold with some liquid which I can not
remember the name of. It prevented the new RTV from sticking to
the first RTV 

Oooooh, I would SO love to know what that mystery liquid is! All the
plastic models I’m molding are flat, and this method would work
perfectly with them. Does anyone know what would be used to keep
liquid RTV from sticking to cured RTV?

Kathy Johnson
Feathered Gems Jewelry
http://www.fgemz.com


#8

i have seen legos being used to build up the frame instead of wood
when making RTV molds. thought this was a neat idea a lot less
trouble then having to cut wood and easy to customize size. plus its
fun playing with legos!

meghan


#9

Kathy -

I did a lot of RTV molding my last year of undergrad. Let me see if
I can describe how I was shown to do it so that you actually
understand me, lol…

Get a piece of tubing (I used brass, probably a bit more than 1/4
inch in diameter) and sharpen one end with a file around the outside
edge. After your first half of the RTV mold cures, use your tubing
(with the sharpened end down) and push it into the mold where you
want your “keys” - push straight down, then rotate in a circular
motion to flare the ends (this helps it to lock better), then pull
straight up to pull the tube of RTV mold out. After you make all of
your “key” holes, paint the RTV mold release over your mold like
usual, but make sure it gets down into the holes REALLY WELL. Poar
the second side of your mold like usual. When it cures, you will have
nice, locking keys!

Hopefully this makes sense - let me know if you need clarification.

Jen
http://www.jmwjewelry.com


#10

Thank you to everyone who replied with advice and directions on how
to cut mold locks. I tried the technique with the next batch of RTV
molds I cut open this afternoon. The results aren’t pretty, but they
hold the mold halves in place. With more practice, I’m sure I can get
them to turn out better. I guess I just needed to see pictures of how
it’s done–I couldn’t seem to visualize the process just by looking
at the finished molds I had purchased.

Now if I had only remembered to drill holes for jump rings in last
night’s batch of models before I encased them in the RTV material…
oh well, it’ll provide more mold cutting practice when I re-mold
them after drilling the holes, right? [sigh…it’s only money…bigger
sigh]

Kathy Johnson
Feathered Gems Jewelry
http://www.fgemz.com


#11
i have seen legos being used to build up the frame instead of wood
when making RTV molds. thought this was a neat idea a lot less
trouble then having to cut wood and easy to customize size. plus
its fun playing with legos! 

And foam mounting board works great when put together in a frame
using a hot glue gun.

ff


#12
I would SO love to know what that mystery liquid is! All the
plastic models I'm molding are flat, and this method would work
perfectly with them. Does anyone know what would be used to keep
liquid RTV from sticking to cured RTV? 

A lot depends on what the RTV is i.e. urethane or silicon, as to
what works best. Try one these…Vaseline (could be diluted w/
kerosene or mineral spirits), a good grade of paste floor wax
(Johnsons – that’ll be easy to remember – can also be diluted),
Nutragena cream, and a pricey mold separation cream from Smooth-on.
Brush or Q-Tip on the surface to separate. Powdered mica makes an
excellent parting agent for defining the parting line for vulcanized
no cut molds.

Experiment before you pour up an irreversible mold that “goes
south”.

ff


#13

And… of course I inadvertently left out the least expensive and
easiest to find RTV wall material…a good heavy carboard box cut
to the 4 side dimensions needed and hot glued together. Down and
dirty…

ff


#14

Okay, from reading more, it seems that you are pouring the RTV mold
entirely in one piece and then cutting it apart? You are making it SO
much harder on yourself! Some of you have talked about clay and
similar things, but I was taught to simply take some very thin
cardboard or very stiff, thick paper (Iused things like sketchbook
covers; I always have something like that laying around) to make my
box to form my mold inside of. Much cheaper and easier I think. I
would figure out how big I needed it to be, and then cut out a strip
that was the height I would need (for the thickness of the final
mold, but plus some to give me some leeway) and the length that
equaled the total of all sides (or two strips if the mold was too big
for me to make that way), score where the corners would be, then tape
the two cut ends together, and I’d have a rectangle that I would then
tape to a backing piece (just make sure your tape covers all gaps).

My actual method for pouring the molds is similar to what is on
Micro-Mark’s website here:

http://micromark.com/html_pages/instructions/82083i/part_mold.html

I don’t suspend the item like they suggest, though - my professor
taught me to pour some RTV mold into the bottom first, let it set,
and use that to keep the item from sinking all the way to the bottom.
Then I pour the rest of the half of the mold and place the item in
that. The rest is basically the same, tho, with the addition of how I
already described making the “keys” in a previous post.

As far as the liquid I use to keep the two halves from sticking, I
actually got that from Micro Mark (I ordered all of my RTV mold
supplies through them) - it’s called “Rubber to Rubber Mold Release”.

I used this method to make all of the cast fossils in my fossil
series on my website (see the link in my signature).

Jen
http://www.jmwjewelry.com