By Joseph ‘Sparky’ Suttie
The Tools to practice with:
1. A vacuum pump that can get you to 28” Mercury
2. A pressure pot (a paint pot will do)
3. Compressor that can produce up to 40 psi
4. RTV (Room Temperature Vulcanizing) Rubber
5. Two Part Resin compatible with your rubber
1. A vacuum pump that can get you to at least 28” of Mercury
This might be the most critical item, combined with your pressure pot. It
is used primarily for making the mold but is used in an advance casting
process referred to as vacuum casting. (Later on this). I have purchased a
‘refurbished’ vacuum pump off e-bay from an estore selling them with a buy now option, cost was ~$230 after shipping. The pump came in the original
manufacturer’s box with enough oil to fill the pump. There is no vacuum
gauge on the model I got. I found a vacuum gauge for around ~$20 dollars
after researching them from various auto parts stores online. Higher-end
gauges are available from other sources, but mine shows me a good relative
measurement. When vacuuming down a pot, I usually go through three stages:
Pressure is less than ambient, the needle is pulling down off the neutral point,
while I set the gasket. Then it sweeps down and over. This can be heard in
the change in pitch of the vacuum pump. Then the needle/pressure gets to
where it’s drawing out the last of the air in the tank. Then the pump starts to
pull the pot to near vacuum and the needle finishes by bottoming out the
2. A pressure pot (a paint pot will do ~$70)
I now have two paint pots from Harbor Freight. I use one as a pressure
pot the other is set up in a stand a fellow club member made for me and is
used for vacuuming molds and pressure casting. These pots are 2 ? gallon
pots, with 10 & ? diameters. They aren’t very big once you start casting things like tank hulls, or space station docking slips but they will get you to the
point where those are the things your making. A paint pot is rated for up to 80
psi, you don’t need even 50 psi and working with less pressure is safer than
more. Also since air pockets in the molds will collapse, distorting the mold
(and its copies) you want to use as little air pressure as needed just incase the
mold’s not perfect. (Always check the pressure release value before each use
resin/rubber can find its way anywhere.)
The paint pot’s gasket is not the greatest. They intend for you to really
clamp it down. I removed the gasket from mine and mixed some Oomoo,
pouring it into the grove to replace the old gasket. This was suggested on the
Starshipmodeler forums. I didn’t have a vacuum pump at the time so that seal
has long since died, it was so full of micro bubbles that it shredded when I
started using the lid to vacuum molds, like a perforated paper model’s parts
page. A seal of Mold Max 30 has since filled the lid and looks to last a long
time. The old gasket is used with the plexi glass lid in vacuuming. I can align it and press the plexi down until the vacuum takes. A gasket will be made eventually to replace this as well, so that I’m not using my fingers to align and
hold the gasket in place.
a) Brass fittings and hoses. You will need various connectors and hoses
to hook up the pressure pot, vacuum pump, and compressor. These can nickel and dime you to death (adjust cliché for inflation). Take a day at the home supply place and get your system fitted. Don’t forget release valves. I removed most of the hard ware on the paint pots, replacing them with air-line quick connects and a lever valve.
b) The vacuum chamber, a paint pot and it’s lid is going to frustrate you.
I’ve had rubber explosively decompress on me every other time I used the paint pot lid. The effects of Murphy’s Law get worse if you can’t see the thing that might go wrong. Now getting some lexan and making a clear lid for the vacuum chamber isn’t error free either! Get a full face grinding shield at Harbor Freight while your there (I got some goggles too). You don’t want to know why, but one of my molds has some dark brown dry material in the outer corner.
3. Compressor that can produce up to 40 psi.
I got a ‘big’ compressor from, you guessed it Harbor Freight. At the time
we were building a big space station out of PVC and the J-57 saucer. I had a feeling that we would need plenty of air pressure. This unit has a pressure switch that lets it monitor the pressure of its 5 gallon tank. When it gets too low it kicks on and fills the tank to 100 psi than shuts off. You can fill the paint pots up with 40 psi from this several times before it will need to re-pressurize itself (it has a nice regulator on board).
4. RTV (Room Temperature Vulcanizing) Rubber. There are many kinds of liquid rubber. Some mix by measuring out the volume of the two parts. Some by weight. The important aspects of the RTV are:
a) Pot life
b) [Mixed] Viscosity
c) Tensile strength
a) Pot life: Like mixing two part resins, the time you have to work with the rubber before it begins to solidify is called it’s pot life. When just starting out you want to go with a nice long pot life. This gives you time to mix, degauss, pour, and maybe degauss again.
b) [Mixed] Viscosity. This is a measurement of how thick the mixed rubber is during it’s pot life. A thicker rubber doesn’t necessary mean it’s cured strength will be higher. It does mean that you will whip in more air while mixing and trap more air in the master while pouring the rubber over the master.
c) Tensile strength. There are usually several other items on an RTV’s spec
sheet that I lump into this category. It lets you know how stiff, strong, and
tear resistant the mold will be after it cures. This impacts several things:
i. How long the mold will last (how many copies can you pull, ‘pulls’ you
get before the copies start to look sloppy).
ii. How thick will the mold need to be. If a part is thin you still need a
certain amount of rubber around it so that say stacking the mold on
other items in the pressure pot won’t distort the mold and thus the copy.
The stiffer the mold the less rubber you will need to use simply to
reinforce the area of the mold actually in contact with the liquid resin.
You can supplement the mold by making a ‘mother’ (later on that).
iii. Under cuts. If your master has a lot of intricate groves and lips the mold
will need to handle having the part pulled out. You can bend the mold
while ‘pulling’ the casting, peeling it off so to speak, but the lips and
protrusions formed in the mold can be thin, the stronger the rubber the
longer these will hold the shape you intended.
c) Toxicity. Slightly less important but certainly notable. You should wear
latex/vinyl gloves when working with liquid rubbers and resins. But I always
manage to get some on my arms or hands, after the gloves get to much goop
on them. So take note of the specs on them.
5. Two Part Resin compatible with your rubber. This seems like a ‘no brainer’.
But I have found that some rubbers reacted to my clearcasting resin, leaving
the mold and copies, sticky. Some of the parts never fully cured, like a sticky
Jujube or Jolly-Rancher candy. The molds were recovered by first cleaning
them with dish soap and warm water, then running a batch of ‘regular’ resin
through. Also there are some rubbers used in casting cement/plaster. ‘Model’
resin will adhere to these molds amazingly well. Similar considerations to pot
life apply to picking your resin. I have not used tensile strength to direct my
decisions on which resin to get for modeling. I did get some high strength
stuff for another project, which needed to be tape-able/machinable and handle
temperature. There is also some concern with picking your mold release, when
using a liquid spray kind, go with the one suggested by the maker of the RTV
you used. I did have problems with using Smooth-on’s Man Release 800 and
Smooth-on’s clearcast. It took forever to cure and pulled sticky/gooy.
Mold it up:
1. Prep the part
2. Build a box
3. Bury the Body
4. Mix some RTV
5. Let the air out
6. Drown the Part
7. World turned upside down, uncovering the evidence
8. Drown it again!
9. Fossilization? Give me resin!
1. Prep the part:
There are considerations for the part/model you are going to mold. Mastering for casting is a topic unto itself. But as mentioned in the section
regarding choosing your RTV, one of them is under cuts and other intricate
details. First of all make sure the part/model is sealed. Some builders will
prime their model, others will ‘prime’ it with Mr. Srufacer, the finer grained
version (1000 I think). This will seal the gaps between parts, even photo etch
which has been glued down and doesn’t appear to have a gap. Rubber, under
a vacuum high enough to cause it to boil at room temperature, will replace the
air between the photo-etch and the part.
When pulling the master from the mold you may find it tears the detail area out, or pops of a part. If the part popped off, you can use a sharp hobby
blade to trim the offending rubber lip by catching it between the piece left in
the mold and slicing it away. However, not all detail pieces may come off. If
you look into the new mold and the details look ‘fuzzy’ you have rubber flaps
that formed when the rubber was sucked in behind the details that where
‘skinned’ on. This can be bad when you go to make your first cast. If the
resin gets between the flaps and mold, you will get a clean copy. But when
you go to pull it, there’s a chance some of the flaps will be stuck in the resin. When they tear, the rip my run into the surface of the mold where real detail is
reproduced. Copies there after will show this defect, they may be useable but
the effort that went into the master is lost.
The other likely effect is that resin was unable to get into these nooks and crannies. The mold will not be damaged in this area put the copies will be
pitted with little bubbles and loss of surface detail.
A little work with the part/model will help the mold turn out many good copies. After sealing the part/model you may still have some undercuts and
details that you think won’t survive enough pulls to make it worth your while to
mold. Note these areas and begin formulating a dividing line. When we get to
the step where (3. Bury the body) we pour our first RTV we can relieve the
under cut by making the mold a two part mold with the halves splitting right
along the difficult detail.
2. Build a box:
Ok so you have your part/model prepped. Now we have to build a box to contain the rubber around the part. The RTV spec or data sheets should
suggest how much more area you should leave around the part to get a stable
mold of it. Smooth-on suggests a ? inch for their rubbers (Mold max and
Oomoo). I use legos/mega-blocks to make my molds. Using a [trimmed] base
plate (make sure it will fit in your pressure pot) I leave two rows of tabs
between the edge of the part and the lego wall. You need ? inch around the
top/bottom of the part too! The tallest part of the model will need to be ?
inch bellow the surface of the RTV after it levels.
This is not all the height the box will need however. If you degauss the rubber in the mold outright expect to double the height of the box! I’m not
kidding. When I mix 300 grams of Mold max 30 in a 2 ? pint painter’s bucket,
the mixture will try to overflow when the vacuum gets down. (Later on how to
coax the air out with out letting the rubber over the lip of the container.) I
usually leave ? to ? inch extra, or about three lego bricks more to the top of
Note: on the size of the box/bas plate assembly: We added a stand (as mentioned above) so we could use the pot side ways. This allows me to have
molds that are degaussed the long way. This was added after I had been
vacuuming molds. It can be added later.
If the part to be molded is semi flat say engine nozzles, I use flat lego pieces, the kind used for making sidewalks or ‘smooth’ surfaces in the lego kit.
These raise the part from what will become the surface of mold, providing a
little step. Then just use double stick masking tape to hold the part down.
This helps if you’re filling the molds to the ‘brim’ and then squishing the other
mold half down. If you decide to make a half copy of your part, this step will
allow you to fill the mold to the edge of the detail and then some, with out the
resin spilling over the mold’s edge (carrying the mold from mixing table to pot
without spilling gets tricky when you have several molds to get into the
pressure chamber before their resin cures).
A little forethought, or hindsight:
Now you need to think about how the resin will be filling the mold. If you intend to split the mold in half you may want to build in some reservoirs.
This will be a section of the wall that protrudes into the mold, but doesn’t extend all the way to the top. Orient it so it will be above the point of the
part/model that you plan to have a pour spout/vent coming off of (a little
forethought can save you in cleanup here too). You may want to have a
separate reservoir that will be the vent/spout. This will allow you to fill the
model from one reservoir. As you squirt in resin with a pipette, you can watch
the other reservoir to see when resin begins to flow out. The size of the
reservoir also depends on whether you will be trying the vacuum casting
technique. With this technique the reservoir may need to be able hold as
much resin as the mold will, and leave room for the resin to bubble up with out
spilling over too much.
Seal its fate:
Now if you have some RTV which is not as viscous as Oomoo (even the Mold Max 30 runs on me) you may want to seal the box. You can use the Mold
Builder material found at local craft stores. You’ll know it when you find it, the
smell of ammonia is strong. This liquid latex works by thin applications, as it
has to dry (all the ammonia has to evaporate). It’s perfect to skin the legos
and the seam at the base plate. Rubber my also be drawn into the lego bricks
as the air is evacuated, legos being mostly hollow. This is wasteful of the
rubber, but these scraps can be used in the next mold, as filler (later on that).
You will need to let the box sit over night (or under a fan) for the liquid latex to
dry (don’t’ put it on to thick). Note that sometimes I build the box half way up
so I can more easily set the part/model clay assembly into the bottom of the
mold. This complicates the sealing procedure so I usually skip it and recycle
any scraps of rubber formed from box leaks.
3. Bury the Body:
Now is the time to re-consider your part/model’s potential problem
undercut details. If the part is shaped such that it will not release from a
single mold you have some options:
a. Make the mold and slice it so that during de-molding you fold back
the mold and pop out the part. As this sounds it is hard on the
b. Burry half the part in a suitable clay (Alumilite’s Synthetic Modeling
Clay (Kleen clay) hasn’t given me any problems).
I use the clay for just about everything now, to fill voids in a master so I
don’t have to use epoxy putty or other material. It’s good to have in your tool
Your mold halves will need some physical feature that allows you to; line
them up, hold the mold in place as you rubber band it together, and keep the
halves aligned while on the table top/in the pressure pot. Since the part only
needs to be buried you will have the lego tabs that mark your ? extra mold
area to work as marker pins. Now is also the time to lay in some tubing or
other strip materials that will form the vents and/or pour spouts. These places
are where resin will be squirted in and where air and then the excess resin will
escape. Not all molds may need these. Molds were one half protrudes into the
other, a displacing mold half, can just squish out the excess resin (small
engine nozzles for example). Connect up the vents/pour spots to any
reservoirs you built into the box. Some mold maker’s choose to cut these in
after the process, its’ up to you which you’re more handy with, the hobby knife
or the wire clippers. (You can use just about anything for the vent/spouts
electronics hook up wire is handy in my shop.)
You may also add some pegs, or bricks in an unused clay area or on one
of the base plate tabs. This will give you a more positive orientation of the two
mold halves. I you forget or notice that you need one after this half is poured,
you can cut one in before pouring the second half.
Now you bury half the part in clay using some sculpting tools to keep the
clay neatly up against the part/model. (If there is an area of high detail you
may tape it over so you don’t’ need to spend so much time cleaning the clay
out of it) You may be doing this right on the base plate of the box if you did a
partial box build. If not, then when you set the assembly into the box, check
all the points around the part/model to make sure the clay didn’t pull away.
Now finish off the box wall if you didn’t already. Ok time to get some dinner,
What it’s cold already? Wow that took some time. . .
4. Mix some RTV:
If you are using a rubber like Oomoo you can mix the parts A & B by
volume, a cup of one and a cup of another. Or you might be mixing by weight.
Now I started my test pack of Mold Max 30 using a Chef’s Mate scale from
Target. And it came out fine! I also bought a scale from American Science
and Surplus for ~$50, it can weight out more then the Chef’s Mate, but then I
have to degauss that much more (you’ll see what a mess this can be later)!
What ever your rubber, you’re going to need some basic supplies:
a. Newspaper/paper towels
b. [Disposable] measuring containers/scale
c. Disposable Gloves
d. Mixing utensils
e. The RTV
a. Newspaper/paper towels:
Line your worktable (and the floor, you’ll know if you need to). I put
down two to three squares of a paper towel, doubled up. These last a long
time, (resin is usually the culprit that ends its tour of service). You will want 2
pieces/sheets of towel, one to wipe the lid and mouth of the part A container,
one for part B’s. This helps preserve the seal and prevents the lid from becoming ‘glued’ on, should the two parts get mixed on the threads of a
container you will not know until next you use it. You might break them loose
with out destroying the bottle or lid next time but rubber is precious so don’t
b. [Disposable] measuring containers/scale
The disposable containers may or may not be needed. I can add in the
catalyst (part B) for Mold Max 30 by hand with a pipette. I start with my
painter’s bucket and 300 or 600 grams of the Part A weighed out and on the scale. Then I use the side of the pipette to pour in the Part B adding the last 2
or 3 grams with the pipette’s normal action. Since so little part B is added and
it is so watery.
For equal part volume mixes, get some cups, with have as few ridges as
possible, you’re going to be scraping out the Parts by hand.
Make sure the bucket you plan to mix the 2 Parts can handle at least
twice the volume of RTV you’ll have after mixing.
c. Disposable Gloves. This stuff gets everywhere. Also where something you’d like to have end up looking like you’re a Mad Model maker when wearing.
d. Mixing utensils
I have scrap brass strip about 1/8 inch thick and as deep as the main
mixing container. A plastic artist’s mixing spatula. And the main gun: a variable speed electric drill with an old flat blade wood drill bit with the tip
ground/filled off. I tried the squirrel cage mixer, bad things happened, not
right off but they did.
To figure out how much you will need to mix up, I’ve heard some neat
tricks. Of coarse I eye ball it. I mix so little at a time to keep from fighting
with the degaussing procedure that I end up mixing another batch to cover the
last little bit of the part. This is time consuming and more than once I’ve had
to go back and add another layer to a part of the mold that was too thin (parts
cast came out warped inwards due to the rubber bands). The one technique
I’ve tried was the rice method. Get some rice and fill the box/part assembly
then pour off the rice into a container. Divide the rice into equal containers,
this is how much of parts A & B you will need. Works for the ‘by weight’ RTVs
too, since the catalyst is such a small volume just measure out enough Part A
to replace all the rice.
The container you mix the two parts into will be recyclable, rubber left in
it after you pour it into the mold will peal off post cure.
Mix-it, Mix-it good:
I’ve seen Oomoo come out in so many different shades. Part A is a thick pink goo, part B is the muddy water consistency. Spec sheets say, mix for 3
minuets, scrapping the sides thoroughly. If you pour the Part B into the bigger
mixing container first it will be easier to ensure the thick pink stuff doesn’t get
trapped in the bottom of the bucket. I mix with the electric mixer, then scrape
the blade with a spatula, set the drill a side, and make sure the: sides, bottom
and bottom ring, are scraped of whatever part is hiding out there in
concentration. If there wasn’t’ too much un mixed, I mix it around by hand.
5. Let the air out
Now you get to watch the RTV Boil.
Close your relief valve on the vacuum assembly.
Place the bucket in the paint pot.
Set the plexi glass lid in place with the gasket aligned.
Start the compressor.
WATCH the RTV, as the pump gets most of the air out of the pot, the mixture will start to rise looking like foam. If it nears the top of the bucket to-
fast/ to-close open the relief valve, letting the mixture fall back down, then
close the valve. After some time you will see the foam/rising action change as
though a giant air pocket (or spore bulb we are sci-fi ers) inside has burst.
Then you will see great air pockets opening up and bursting. The RTV is
Wait a few seconds.
Turn off the compressor.
Relieve the air pressure. You should see the RTV surface grow smooth (Mold Max 30). Remove the plexi glass lid and gasket and retrieve your
6. Drown the Part
You hear it everywhere, ‘pour the rubber into the lowest area of the box letting it flow over the part on its own’. Rubber doesn’t cooperate all the time,
it’s thick and wants to pile up and spill over while you wait for it to seek the
other end of the box. Not only that but after all that work an air bubble on
some brilliant detail part will seriously ruin your day. So I pour the rubber,
add any recycled shredded rubber bits on top, pushing them into the mold and
then degauss again.
The more recycled rubber you use the more you need to degauss the
whole mess. Air pockets that form in the mold now, not the surface bubbles
that from in pouring resin, will kill the mold in pressure casting. The voids may
hold up for a bit, then collapse rendering the mold recycling fodder.
Degauss as before, accept you won’t need to worry so much about the mixture expanding so much. Still keep a hand on the relief valve and an eye
on the [lego] box.
7. World turned upside down, uncovering the evidence
Your RTV will suggest how long to let it sit before handling the mold. Mold Max 30 specs say it’s full tensile strength is reached 7 days after mixing.
I don’t want to use cure accelerators but you can remove parts if need be after
If you’re making a two part mold things are a little easier. Now’s when the lego box pays off. You can pull the base plate away from the bottom half
of the mold, leaving the box intact. Remove any rubber which was sucked
under the clay/flat panels. Some needle nose pliers can help pluck out any
keys that you added. The sculpting tools can help pry up the clay. If the mold
sat a long time the clay may need to be carefully scrapped out of grooves and
Now add to the box, you may also add to the reservoirs at this time. Once the box has been build up you need to apply something to keep the other
half of the mold’s RTV from bonding to the ‘old’ half. RTV will bond to RTV.
That’s why recycled RTV shards can be used, they seal in completely. I use
petroleum jelly thinned with naptha (I found it at the home improvement
store). This is not the stuff the Star Gate it made out of, it’s in the paint
supplies isle. Use a brush you don’t care about but save it somewhere, I keep
mine until I loose them or I need the space clean (it stays oily). Try to keep it
thin, and use cotton swaps to clean any of it off the master, it makes a nasty
looking surface defect if left on.
By the time you get the next batch mixed the jelly will be ‘dry.’ Don’t let it sit over night. I had a mold half strongly resist separating after letting the
first half sit over night with its jelly coating.
8. Drown it again!
Well you’re an RTV mixing machine now:
Degauss and let it cure.
9. Fossilization? Give me resin!
Now that you have a mold, and the master returned to your friends
prized, rare kit. You ready to fill the space in that mold with resin. You have
picked a resin that will meet your needs and match your skills. I still use long
pot life resin. I have lots of little molds to work extra resin into after a mixing
a) Mold release.
I have tried talcum powder from Alumilite. It doesn’t ‘stick’ well to the
Mold Max or Oomoo rubbers. I’m comparing this to using some of the
metalizer powders. These give the resin parts a shinny or dull metal finish,
there’s even copper. These are inherently good mold releasers and work well
for it. I have used Man Release 800 as well, this works great. It also helps
with slipping a mold into and out of it’s resin mother. Apply this now and wait
as directed by the release’s spec sheet. Usually wait 5 to 10 minuets.
b) Mix-it, mix-it good! Again?
Yes, resin needs to be mixed even more thoroughly than rubber. I use
the little paint mixers the Badger Rep was selling at the last Wonderfest. True
these can whip in a lot of little air bubbles. Even using a pipette to mix, blows
in air bubbles though. I find that practice allows me to easily stir the resin
with the paint mixer without adding to many air bubbles. When you begin to
add coloring, you will enjoy the thoroughness the paint mixer provides to mix
the color quickly. I have not begun adding metal to my resins but the one
experiment I did required the paint mixer to even get the process started
before the resin started thickening up. The air bubbles that cause problems
are found in the pockets formed in the part’s undercuts, and the small bubbles
that form in little crevasses and niches of the mold’s details as you fill the mold.
Once the resin has been mixed use a pipette to fill areas of the mold
[areas that look air bubbles might form or get trapped once the mold halves
are mated and stood up on end. This is optional and depends on the size and
type of part. As you create pools of resin look for air bubbles stuck in detail
areas, a jet of resin from the pipette can be used to dislodge these.
If your mold has no reservoirs, pour or vent spouts, you can mix up
some resin and dump it in. Depending on how much of the other mold
protrudes into the half you fill, you may not need to top off the part, use more
when in doubt, or you might end up with a bubble at the lip of an otherwise
good cast. Be ready to toss that paper towel or newspaper lining.
Once the mold halves have been fitted together, carefully rubber-band
them together. Not too tight you don’t want to distort the mold.
If you used the fill and squish method, put it in the pressures pot, dump
any extra resin in those old open face molds you have and if they were vacuum
molded, set the in the pot too.
If you have vent and reservoir style molds: Setup the mold so the
reservoirs are ready to be filled. Fill your reservoir. If you made a pour spout
and a vent spout inject the resin with the pipette until you see the resin
flowing up out of the vent. Be sure while squeezing the pipette you AVOID the
temptation to squeeze every last bit of resin out each time, this blows a big air
bubble in to your mold trapped between an other wise solid bank of resin. If