Cobett's Monograph on Sodium Silicate and Other Core Making Methods

Discussion in 'Sand Casting' started by Melterskelter, Jun 27, 2021.

  1. Melterskelter

    Melterskelter Gold Banner Member

    I decided to post this article up as the link to it I had previously posted is dead. I was worried the article might be lost and had contacted Tom a few days ago. Then I finally thought of using the "Wayback" app and quickly found the article. I do not want it to disappear again! So here it is. I (probably a lot of us) really feel a debt to Tom for putting together such a succinct and accurate summary.

    The article pasted below was captured first captured by Wayback in 2015. I do not know when it was first written/published.

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    Sodium Silicate for Core Making by Tom Cobett
    Hi All!

    It has been a while since I have posted anything, but, I could not pass up a few
    comments on core making and the use of Sodium Silicate. You guys are in luck,
    since this happens to be my personal specialty. I actually have patents on using
    Sodium Silicate to make sand cores and molds!

    Let's talk about Sodium Silicate first. Sodium Silicate is made by melting
    Silica Sand and Soda Ash together in a furnace. The resulting glass can be
    dissolved in water, steam and much pressure, to make liquid Sodium Silicate.
    This is one of the oldest and cheapest industrial chemicals in the world.

    When you ask for "Sodium Silicate" (a.k.a. Water Glass) you may get just about
    anything. It's a bit like asking for "Motor Oil." The two things that describe
    any Sodium Silicate are the Weight Ratio and the Solids Content. The Weight
    Ratio is the ratio of the Silica to the Sodium Oxide. The most common Sodium
    Silicates are all made with a Weight Ratio of 3.22 parts of Silica to 1 part of
    Sodium Oxide. If you just ask for Liquid Sodium Silicate, this is most likely
    what you will get, a "3.22 Ratio Silicate." In bulk tankers, I buy it for about
    $.06/lb. It weighs in at about 11.6 lb. per gallon and has the viscosity of
    Olive Oil.

    This 3.22 Ratio Sodium Silicate usually has a solids content of about 36 to 38%.
    The balance is water. Treat this liquid like VERY strong industrial detergent.
    It has a pH of about 13.0 and stings like a bitch of you get it in your eyes or
    into a cut on your skin. Otherwise, it is completely water soluble.

    The largest manufacturer in the world is PQ Corporation. They sell this basic
    product as "N" Grade Liquid Sodium Silicate. You may be able to buy a 5 gallon
    pail of this stuff for about $50.00. Check the website www.pqcorp.com

    This is not the best choice for making sand cores and molds, since it does not
    provide very good strength. The preferred product would be a liquid sodium
    silicate with a weight ratio of about 2.4 to 2.6. It will cost slightly more,
    but, will make make much better cores. PQ sells a 2.4 Ratio liquid sodium
    silicate as "RU" Grade. It comes as a very thick liquid. You can add about 15%
    by weight water to get the solids content down to about 38 to 40%.

    Sodium Silicate can go from liquid to solid by simple de-hydration (baking), by
    passing Carbon Dioxide THROUGH it, or by reacting it with a liquid ester, like
    Propylene Carbonate or Glycerol Triacetate or Ethylene Glycol Diacetate. If you
    use the straight Sodium Silicate, the collapsability of the sand core won't be
    very good. Sodium Silicate does not burn out, so you will need to add something
    organic. The simplest thing to use is Sugar. Start by adding about 10-12% by
    weight of the liquid Sodium Silicate. It will mix in nicely, and stay as a
    stable liquid.

    You could also add something as simple as 0.25 to 0.5% by weight of the sand of
    wheat flour, starch, molasses, ground charcoal, or even kaolin clay to get
    better breakdown. The kaolin clay will provide some green strength which might
    make it possible to strip the core without gassing it, and then let it dry in an
    oven. You don't need to get it much hotter than 225�F.

    How much to use? If you use a clean, dry sand that has a grain fineness of
    between 60 and 90, I would start with a Sodium Silicate addition of 4.0% by
    weight of the sand. You can mix this with a simple kitchen mixer in a plastic
    mixing bowl. Mix for about a minute or two at high speed. Once the sand is
    mixed, put a lid on the bowl, or put the sand into a plastic bag to keep it from
    de-hydrating. The covered sand should stay good for MONTHS, so long as it does
    not dry out.

    If you want to use Carbon Dioxide Gas for hardening the core, you must allow the
    gas to pass through the entire sand mass. Simple exposure to the surface of the
    sand won't give you any appreciable hardening. You can get small bottles of CO2
    from the guys who service soda pop machines. YOU MUST USE A REGULATOR TO CONTOL
    THE PRESSURE OF THE CARBON DIOXIDE! The soda machine guys will sell you an used
    one for a few bucks. Generally, you don't need much more than 10 - 15 p.s.i. for
    the gas.

    You can use a nail or a wire to make a vent in the sand to pass the gas through
    it. If you were using Carbon Dioxide at 10 p.s.i. and you want to cure 1 lb. of
    sand, it will only take about 5-10 seconds of gassing. This is enough strength
    to get the core out of the core box. After you get the core removed from the
    form, just let it dehydrate overnight in a dry place. Or, you can bake it out in
    a warm oven or even a Microwave. (Microwave works REALLY well, just be carefull,
    since the Sodium Silicate attracts MW and the sand is likely to get extremely
    hot in just a minute or two.)

    If you opt to use the ester hardener, you need to add it to the sand mix at
    about 10% by weight of the liquid Sodium Silicate. Just keep in mind that once
    it is mixed together, you will have only a certain amount of time before the
    sand mix begins to harden. We call this "Work Time." You need to have all the
    sand in place before the end of Work Time. Then you need to wait until the sand
    is so hard that you cannot put a nail or knife into it. We call this point
    "Strip Time."

    If you use PQ "RU" Grade Silicate watered down to 40% solids, and then add
    Propylene Carbonate as the ester hardener, you will have about 10 minutes of
    Work Time and a 20 minute Strip Time at room temperature. If the sand is hotter,
    it goes faster.

    If you are casting Aluminum, you can get by without adding any organic additives
    to the sand mix, Just dunk the finished casting in water and the cores will
    flush out in a few minutes. If you are pouring Brass, Bronze or Iron, you will
    fuse the Sodium Silicate core into something that behaves like concrete, unless
    you have added some additional organic material.

    You can also make incredible cores with a two part epoxy binder. I would not use
    more than a total of 1.5 to 2.0% total epoxy binder and hardener on a 60 to 90
    Grain Fineness sand. Let it harden completely before you take it out of the box.
    Dust the box with baby powder before you ram the sand into it. This will help it
    to release from the core box.

    If you can get Corn, Potato, Rice or Tapioca Starch, you can make very nice
    cores by mixing the starch at about 2% and water at about 1% into the sand. You
    can harden the core by baking or microwave oven. These work really well for
    Aluminum castings, and they fall out of the hole!

    You can also use Linseed Oil or Tung Oil at about 1.0 to 2.0%, with a little
    Kaolin Clay and water as the binder. Strip the core green and then bake it at
    350-400�F for 10 minutes per inch of thickness. We call this "Baked Oilsand."

    You can also make great cores by boosting up the strength of greensand with more
    clay, corn starch and water. Let them dry out before you use them.

    I have been making sand molds and cores for over 35 years. there is no mystery
    to how this is done. There are several binders that can be used. Sodium Silicate
    is cheap and safe. Starch is a little more difficult to use, but shakes out
    better. Epoxy and other organic adhesives are expensive, but work extremely
    well. I can remember making water jacket cores for this guy who was building a
    12 cylinder Jaguar engine. That was a tough one. The epoxy worked really well.

    On many occasions, I started with a block of cured coresand, and then ground
    away the extra sand to get the shape that I needed. Recently, I have done this
    with a 3-axis milling machine. If you want to go this route, make a really
    strong core mix first. I use a grinding stone (like from a Dremel) instead of a
    milling cutter and the results are very nice.

    If you want to know more, just ask.

    Tom Cobett
    Cleveland, Ohio

    "In Pyro Veritas"

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    Denis
     
  2. MortyNTenon

    MortyNTenon Copper

    Does anyone have a suggestion on what specific type of two part epoxy binder to use with iron castings? There are probably thousands of two part epoxy formulas available. There was a member here that was using Magikast 70D two part resin with aluminum. Its a Polyol/MDI system that makes nice strong cores but burns out too fast for iron temps though. I see Melterskelter is working on this in the Motorcycle intake thread but I'm posting here hoping that Tom Cobett might share the finer details we're missing.
     
  3. Melterskelter

    Melterskelter Gold Banner Member

  4. rocco

    rocco Silver

  5. MortyNTenon

    MortyNTenon Copper

    I'm trying to find what specific brand/type anyone has used. With all of the different types of epoxies available, one probably works better than the others. Melterskelter lists West System 105 but hasn't poured with it yet. Magikast 70D works with aluminum but not iron. There's already plenty of variables in casting, just thought the knowledge base here could help narrow down the generic term epoxy.
     

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