Once you get to a certain level of scale modeling, you’ll start making parts that need to be produced in multiples and, most likely, are not commercially available. This is easily dealt with by making a master part and then reproducing it with a mold and resin casting liquid. Resin casting is considered an advanced technique, but you will be surprised that it is fairly easy if you use the proper materials and techniques. Here’s how I made several 1/3-scale spark plugs for a resin-cast rotary engine.
The original engine casting is available from Nick Ziroli Plans (www.ziroliplans.com), and just like all other parts of a scale model, the finer details are what make it look realistic. So the first step is to produce a master part to cast. Of course, if you want to save time and effort, the Evolution 1/4-32 spark plugs ($13) for the Evolution gas engines from Horizon Hobby are perfect for a 1/3-scale part.
At the very least, it can be used as a guide for making your own less expensive copies.
So I started with an old burned out O.S. #8 glow plug and turned down the unneeded threads and I drilled out the coil. This is easy to do with any small shop machine lathe.
I then used a piece of acrylic plastic rod (from Michaels Craft Store), and I turned the top isolator to shape. I used a jeweler’s file and 400 grit sandpaper to smooth it and then I flipped it around in the lathe chuck and then drilled out its base with a (.093) 3/32-inch drill bit so it would fit over the top of the glow plug.
With a couple drops of medium ZAP CA glue, I glued the isolator to the glow plug. Looks pretty convincing to me. Now it’s time to make a silicone mold of the master part.
Everything needed for making resin cast parts and silicone molds is readily available online. Check on www.Amazon.com. I prefer to use Amazing Casting Resin (www.moldputty.com), and Alumilite HS3 Silicone molding material. If you prefer to make a 2-part mold, you also need the mold release spray. For this simple technique, we’ll make a single piece mold, since the HS3 is very strong and can really be stretched to release your casted part after it cures.
The Base of the silicone is thick but pourable and it has to be mixed with its catalyst. You mix it 1 part catalyst to 10 parts base (by volume or weight,) with the included measuring/mixing cups.
I use a digital gram scale to make the ratios precise. so for 28 grams (1 oz. of base) you would then pour in 2.8g of catalyst. The base is white and the catalyst is pink. Mix it very well so that there are no swirls of color in the mixture.
To make my mold making container, I simply use a drop of thick ZAP to tack glue the master to the center of one of my mixing cups. This makes for a quick and easy setup. You could also use smooth sheet plastic and a base and glue a box around your master.
Once your mold mixture is completely mixed through, gently pour it into the container. Do this with a steady stream poured in from high above. This helps eliminate bubbles in the silicone as it flows around and over your master.
The silicone takes about 18 to 24 hours to fully cure, but you have about an hour before it starts to setup. Here’s a tip for saving the relatively expensive materials. Should you not mix enough silicone to cover your master completely, (you should have at least a 1/4 inch over the top of it,) you can insert objects into the container to raise the level of the silicone. AA and AAA batteries are ideal for this. But make sure what ever you use, that it does not come in contact with your master piece. Now set the mixture aside, on a level surface and let the silicone cure fully.
So here you see the master part, which is easily removed from the mold after the mold has been removed from the container. Notice the container and the master are perfectly clean. Silicone only sticks to more silicone and to nothing else.
Even the batteries come out easily.
The casting resin is a much less critical mix. Both parts are mixed together 50:50 and you have about a minute or two to work with it. Use the included mixing cups and sticks and mix together for 30 seconds. Again mix until there are no visible swirls in the mixture.
Here the resin has been mixed together and is ready to pour into the opening in the top of the silicone mold. Notice I re inserted the batteries to ensure the mold isn’t deformed before pouring in the resin.
Very quickly, the resin with start to turn white as it cures. An unusual aspect of casting resin is that larger batches of it cure more quickly than smaller amounts. So, the left over material in the mixing cup will fire off in about 2 minutes. And the material in the mold will take somewhat longer to cure. This is because the chemical reaction is a thermal event, and the mixture gets hot to cause the resin to cure. The less resin, the longer it takes to cure.
Here you see the resin is still clear. Time in mold, 2 minutes.
After about 3 and a half minutes you see the center of the casting is starting to turn white.
Here it is after 5 minutes. It is still not opaque but getting close.
After about 15 minutes the casting is complete white and as you can see, if you stretch the mold, it easily separates from the inside of the mold. I use a pencil to tap the top of the casting while it is in the mold to see when it is hard. if the casting is still soft, you will leave slight indents on the outer surface of the casting. In about 20 minutes the part is hard enough to pull out of the mold.
(Tech Tip) Because heat is needed to cause the resin to cure, you can speed the process by preheating the mold. This can be done by placing you mold in a 175 degree heated oven for 10 minutes. Then the resin casted part will cure more quickly.
So here is the resin casted part next to the master part. As you can see it has exactly same smooth surface as the original part, straight out of the mold. After about a half hour, the resin casted part is full hardened and you can drill it, cut and sand off the waste material and paint it. ZAP CA glue works great for gluing the unpainted resin casted surfaces.
So here is the original resin casted part I made using the actual Evolution 1/4-32 spark plug as a master. It was reproduced using a 2-piece mold.
Here’s the painted and detailed rotary engine with four spark plugs and ignition wires added. Subtle detailing is what scale RC is all about.
Two piece molds are a little more involved to make but the basic techniques is the same. For making this type of mold, I recommend HS2 silicone molding material. It is slightly less stretchy and is more suited to this type of mold.
The technique requires that you make a molding box, (Lego Blocks work great!), and then adding a layer of modeling clay in the bottom of the box, about 1/4 inch thick. You then press the master part into the clay so that half of it is exposed.
Then use the back of a paint brush and poke indents into the clay next to the part. These form keying nubs that help align the finished mold halves. After this you pour the first half of the silicone molding material into the box to cover the master part. Once the silicone has fully cured, you flip the mold box over, and remove the clay. Clean the master piece from any scrape clay and then replace it in the mold box. Now spray on a health coat or two of mold release agent over the part and the silicone. Make sure to completely cover the face of the silicone mold or the second half will stick to it sealing your part inside.
Then mix and pour in the second half of the mold and let cure overnight. Separate the mold halves, remove the master and then with a sharp X-Acto blade, cut in a molding channel so you can pour in the resin. To make the part, tape the two parts together and place on a level surface with molding channel opening at the top. Mix and pour in your casting resin and the rest of the technique is the same.
So there you are, Resin Casting made, easy.