Ive had very good success with vacuum casting

By Anita Gordon,2014-06-17 01:00
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Ive had very good success with vacuum casting ...

    Advanced Casting

    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

    d) Toxicity

    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

    the box.

    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

    risk it.

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

    boiling now.

    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

    degaussed RTV.

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

    16 hours.

    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


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