Molding and Casting Simple Parts : The Simple Way

  • hdr2
Things you will need:
  • Liquid Latex Rubber (usually found in art stores)
  • Brush (disposable oil brush is good)
  • Glass Surface (so the latex wont stick)
  • Gauze Bandage
  • Hot Glue Gun and Stick
  • A lot of Patience (yes I mean lot’s of it!)
  • Step 1 - Prep
    Set your cast master piece on the glass surface. It would help to make it more stable by applying a little white glue underneath so it won’t move during the initial application of latex over it. In this project I used a simple shaped M4 Sherman’s front transmission bolt rack, which has a flat back. Most liquid latex rubber can be purchased from art stores; the common brands you may find are Mold Builder by Castin’Craft or Mold Craft by Burma Rubber Company. Both brands usually sell the latex in 1-liter plastic containers and that will be enough to make a lot of simple molds.
    Step 2 – Latex Layering
    When the piece is stable on the glass, apply the initial coat of latex over it with the brush, making sure you fill any small spots and crevices with latex. Then thoroughly cover the piece extending at least an inch away on all sides.
    Make sure you apply enough yet a thin coat all over, the biggest mistake you could make here is apply it too thick. What will happen is the top surface of the latex will dry quickly but the inner coating wont!
    After the application, leave it alone for at least 24 hours to dry - IMPORTANT!

    Make sure you wash your brush thoroughly with water after each use; this is done simply by running hot water from the tap until the latex is all gone from the bristles.

    After drying, the top surface may feel a little bit tacky, but it should look more yellowish-clear in color. The good thing about using a glass surface for a base is that you will be able to look and check the status of your piece underneath. Image four shows both top view and bottom view (through the glass base). Now you can go ahead and apply another thin coat using the same motion and amount, but this time try to extend another half an inch at the edges, do this once every day for at least the next 4 days!
    Step 3 – Gauze Layer
    After a few coats (after a few days!) you can optionally add a piece of gauze on top of the mold then add another layer of latex to mix it in. This gauze will make the mold stronger and tear resistant. Image 5 shows gauze in a layer.
    Step 4 – De Molding
    After a few more coats (and a few more days) you may now remove the mold from the glass surface by using a flat blade or even a plastic spatula and carefully pull it away, image 6. Inspect the bottom part of the mold making sure the first coat from a few days ago has actually dried over your master piece. Remove it carefully from the mold and inspect it for any miss-matches or bubble holes, image 7. Trim the edges too so its not too flimsy, image 8.
    Step 5 – Casting
    Now you’re ready to cast!
    Heat up the glue gun with stick, then with one hand open up the mold a little wider as you apply the melted plastic glue inside the mold. This will ensure that it will flow evenly to all the crevices in the mold and will also prevent making bubbles, image 9. I usually apply more plastic glue than the mold can handle therefore making sure I have completely filled it up, image 10. The melted plastic glue is also clear in color so it’s hard to see if your have put enough or not.
    Step 6 – Removal and Cleanup
    The glue normally takes a few minutes to dry and become solid. I usually leave it in the mold for at least an hour just to make sure it takes its shape. Then after removing your new cast from the mold, remember that plastic glue is softer and flimsier than polystyrene model plastic (almost like toy soldier plastic). So it takes a little time to trim off the excess flash properly, but then there you go!

    After a few days I’ve got myself an extra Sherman transmission bolt rack. The biggest thing on this project is patience as I have mentioned above. This particular molding project started on the 17th and ending on the 26th of the month- almost ten days! And again remember this is LATEX, not RTV Rubber so you can't pour out a whole lot of it over your piece at one time cause it wont dry at all if applied thick, In the case of RTV which is more expensive, it dries faster no matter how thick it’s poured over because of the catalyst or hardener added to it. The bottom line is with latex you get to have your simple shapes duplicated cheaply!
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    About the Author

    About Rei (muttley)

    I started building armor and aircraft models when I was 9. The passion for the hobby was on and off depending on my other interests in life - like serving in the military, working overseas, getting married and all that stuff. I decided to go back and polish on my building skills once again, probabl...


    slick little way of doing things isn't it..
    NOV 13, 2007 - 04:23 AM
    I've used the Latex mould method for years and there are a few things you have to watch out for: The latex hardens by drying and not curing (RTVs cure) so shrinkage is _always_ a problem. Adding the gauze is a good way to limit shrinkage, so don't forget it. If there's no gauze, the mould will end up 2-4% smaller than the original and, for size-critical items, they just won't fit after you pour them. Don't be tempted to lay down a lot of latex, hoping to reduce the number of layers needed to make the mould. Thicker material actually takes longer to dry than many layers of thin material. Whenever you are finished with the mould and remove the master, let the mould sit and continue to dry on the interior surfaces for at least another day or two. Longer if you can spare the time. The latex can be used for moulding more normal Polyeurethane resins (Alumilite and the like), just make sure you spray the mould with mould release before each and every pour. Unlike RTV, resin will stick permanently to latex moulds without a release. Latex moulds are based on natural materials, that means over time, they will deteriorate, even if you're not using them. They will continue to shrink for some time after hardening and may actually start to rot if left in a mildew/fungous friendly environment. Bottom line is that they are not stable and shouldn't be expected to last long. Still, they are useful and inexpensive moulds, so use them as you need them. Paul
    NOV 13, 2007 - 09:07 AM
    Very useful feature as I bought some latex on impulse recently while shopping for paintbrushes
    NOV 13, 2007 - 12:17 PM
    Very Cool Technique. Thanks for the info.
    APR 20, 2008 - 05:44 AM
    This is what I don't get about many modelers: they spend thousands on kits and aftermarket accessories and then cheap out when it comes to glue, paint, brushes, airbrushes and casting supplies. Come on, why waste an inordinate amount of time trying to brush yukky and stinky latex in every nook and cranny of your master, wait days for it to fully dry and then end up with sub-standard (and possibly under-scale, due to shrinkage) parts made of (ugh) glue, when for a mere $30 you can get enough RTV silicon rubber and Alumilite to easily and "professionally" cast your small parts for months, if not an entire year?
    APR 20, 2008 - 08:41 AM
    t34-85, First of all I dont spend thousands on kits and aftermarkets, in fact I dont even use aftermarkets on my models, I improvise or "scratchbuild" which is actually an older practice in modeling before aftermarkets came out. Sorry to say but, every model hobbyist has every right on how to spend their money to get the job done the way they want it- and no that is not "cheaping out" - its being practical and at times creative. I put up this article simply to offer others an old technique that they can try out if they want to, and not to tell them this is the only way to go and push them to it. It is an OPTION should you want to accept it or not. Lastly, a jar of latex from the art store is about $5.00, still around $25 less than your Alumilite and this jar can last longer and make more simple molds, which is what this article is pointing out, making simple molds for simple parts. You wanna spend $30 to cast a simple flat 2 inch piece of a 1/35 scale Sherman then by all means go for it!
    APR 29, 2012 - 11:38 AM
    Hey Scott thanks for the idear, Been looking at makeing molds for some time now might play around with it a see what we come up with
    MAY 05, 2012 - 11:25 AM
    Easier to use Alumilite RTV Molding rubber to make the master mold, then Alumilite 2 part plastic to duplicate.
    JAN 07, 2015 - 06:27 PM
    ^^Ditto^^ I don't see the point in this time consuming and imprecise method. I'll stick with RTV rubber and 2-part casting resins.
    JAN 07, 2015 - 09:53 PM
    ^^Ditto^^ I don't see the point in this time consuming and imprecise method. I'll stick with RTV rubber and 2-part casting resins. [/quote] Well the point is there are those who don't want to spend whatever amount the Alumilite package goes for these days only to use it for a few small parts. In my case, I'd be paying upwards of 50 euros for a similar package that would need to be used within a month or two, not going to happen unless I'm planning on casting a lot of stuff. Additionally, the latex makes a pretty good masking liquid and can be used to make torn tarps and so. Just because you find it pointless, doesn't make it so for others. You could just as easily have said why bother messing about with casting? Just pay someone a hundred bucks to do it for you. Kimmo
    JAN 07, 2015 - 10:58 PM