Getting Started with Molding and Casting

Getting Started
with
Molding and Casting
by Denis Bohm

hy would I want to mold and cast my own parts? Good question! Here are some of the things that motivated me to try it out:

You can make multiple "chips" of model pieces and try out different paint schemes on them. This let's you see all the paint schemes at the same time and you don't need to strip the part between tests.
You may want to modify part of a kit. You can cast the part that you want to modify and instead of modifying the original you can modify the cast. You can try a number of different modifications. If you don't like any of the modifications then you can still use the original part.
When sculpting your own items, like a base, you can cast the part when it is done so that you have a robust final part. Sculpey remains fairly soft and bendable even after hardened and is still too fragile to use as a final part.
If you are sculpting your own items you can make casts at various stages so that you won't lose much work if you make a mistake or damage the fragile hardened Sculpey.
By doing it yourself you can see how the different types of artifacts (seam lines, air bubbles, warping, etc) can occur. You can also see how long it takes and how much it costs. These things can bring a better appreciation of the reasons behind the quality and price of garage kits.
And as if that isn't enough, it's pretty fun to do it!

So What Are We Going To Do?

To keep things simple I'm going to use a very simple base for the master part. This means that we can make a one part mold. Remember this article is for beginners!

The base that we will use was made for Horizons Dr. Doom. He can't stand by himself because he isn't properly balanced. Horizon includes a thin black rectangle of plastic for a base, not very inspiring. This new base is just a set of four large stone blocks like you might find making up the floor in a castle where Dr. Doom hangs out. It was sculpted out of Sculpey and then baked in the oven to harden. Scupley is weak and the surface is still fairly soft even when hardened, so I wanted to cast the part to have a solid base for Dr. Doom. Hey, he deserves the best!

Before you start on a project like this one you need to make sure you have all of the materials that you will need. Here is what I used for this project: cardboard, tape, scissors, two plastic tubs, two craft sticks, Experts Choice two part mold material, and Synair two part resin. That's it!

The sequence of steps to mold and cast a part is pretty simple:

Make a box to hold your master part to be molded
Mix and pour the mold material
Pull the master from the mold
Mix and pour the casting material
Pull the cast from the mold

Of course, the details of how to do each step can be pretty complicated. Let's not waste any more time, now we'll jump into the details!

Make a Box

For this project I decided to make the mold box using cardboard and tape. It's cheap, easy to work with, and does the job. When laying out the box, make sure you leave enough room around the part so that the mold will be thick enough to keep its shape. You can simply create a rectangular box that will hold your part.

If your part is large and irregularly shaped then you may want to add some filler to take up some of the unused space. It's worth the effort to add the filler because the mold material is fairly expensive. Danny Soracco has a good tip: recycle old molds by cutting them up into parts and using them for filler.

When your box is built you can set your part in it and move on to mixing and pouring the mold material. Since we are using a base, we want to make sure that the box is on a level surface so that the mold will be level when it is done and we can easily pour the casting material into it without having to worry about leveling the mold.

Mix and Pour the Mold Material

There are some really quick hardening mold materials available. These are great if you are making casts in the middle of a project or only want to cast a couple of parts. For a final part where you want to make a lot of casts you would use a different type of mold material.

I used Experts Choice Quick N Easy A&B Low Viscosity Mold Putty in this case. It cures in 20 minutes, is good for a small number of casts, and doesn't require the use of any release agents. It is very easy to use, simply mix equal amounts of part A (black) and part B (white) in one of the plastic tubs. Stir slowly so that you don't introduce air bubbles. When the mixture turns a uniform gray you are ready to pour.

Pour the mixture slowly and let it run into all of the cracks. It's kind of like pouring syrup on pancakes. Take your time and let it spread all over so that air won't get trapped anywhere. Then you just need to wait 20 minutes until the material has hardened.

Pull the Master

Since we are molding a base we end up with a one part mold that is really easy to use. Cut the tape around the edges of the box with an X-acto knife and then peel away the bottom and sides. Now you should be able to see the bottom of the part. Bend the outer parts of the mold away from the part to loosen them up. Then just bend back one side of the mold and start to work the part out of the mold. Don't bend the mold too far or the mold material may crack. Next I trimmed away some of the flash around the top of the mold (which is the bottom of the base). Now I was ready to cast some new parts!

Mix and Pour the Casting Material

I used Synair Por-A-Kast resin to cast my bases. This is a two part resin consiting of Curative (clear) and Isocyanate (brown). When the two parts are mixed a chemical reaction takes place that heats the resin and causes it to cure. Thicker areas in your cast will cure more quickly because they have more material and thus generate more heat. Thin parts will cure less quickly.

Por-A-Kast is easy to use, just mix equal amounts of Curative and Isocyanate. Stir slowly for 30 seconds so that you don't introduce any air bubbles. Now slowly pour the mixture into the mold. Again, take your time and let it spread all over so that air will not get trapped anywhere. But don't take too long because you only have a couple of minutes before the mixture will begin to thicken. Then step back and watch it "kick"! It should get cloudy then turn white. It gets pretty hot when it kicks, so be careful.

If you will be pouring more than one cast you might want to use two measuring cups: one for part A and one for part B. Then pour both parts into the plastic tub to mix them. Since you never mix the two parts in the measuring cups you can reuse the measuring cups over and over without having to clean them.

Pull the Cast

In a matter of minutes the part should be hard enough and cool enough to pull from the mold. To pull the cast follow the same method as used to remove the part. First loosen up the cast by pulling back each edge of the mold. Then bend back one side of the mold and work the part out of the mold.

Consume Beer and Doughnuts

And there you have it, a casting of your original part. Time to celebrate by consuming some beer and doughnuts!

Caveats

Wow that went smooth! A perfect part the first time, cool! Um, er, well, maybe I edited things up so that it would be easy to read. I did have a couple of problems.

The first problem was I didn't have enough mold mixture to cover my part and fill my mold box! So I had to quickly grab some filler and try to stick it in the big gaps. Next time I'll make sure to do a better job estimating the amount that will be needed. I'll also make sure I have some filler on hand before mixing and pouring.

The second problem was when I went to cast the first part I noticed that the mold wasn't level! So I had to take a minute to find something to stick under one part of the mold to level it out. Then I poured in the rest of the resin. At this point the resin was already starting to thicken. This resulted in some air bubbles just below the surface near the top of the mold. On the second cast I made sure that the mold was level before I mixed the resin and the part came out perfect!

Just Do It

There are lots of uses for reproducing parts for kit builders as well as sculptors. With the great materials that are out there it is easy and fun. Molding and casting should be a skill in every modelers bag of tricks!

Warning Will Robinson!

OK, kids: don't try this without an adult helping out. Both the molding and casting materials can be dangerous.

The Experts-Choice Mold Putty label warns:

Do not take internally. Keep out of reach of children.

And the Por-A-Kast label warns:

As a saftey precaution, all Por-A-Kast systems should be used in a well-ventilated area and skin and eye contact should be avoided.

Thanks Guys!

I'd like to thank Randy Mason from Bare-Metal Foil & Hobby Co. for encouraging me in this adventure and for supplying the molding and casting materials as well as some useful advice. Also, thanks to Danny Soracco from Dimensional Designs for showing me his workshop and sharing some great tips.

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