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This guide will cover making molds and casts of the Pinky St series of toy figures. While it's not an exhaustive guide, I do try to cover the process for someone who has never done molding or castings before. Long ago when Pinky-Street.com still existed, they had a forum where a number of people had provided tutorials on doing this same process. As far as I can tell, those tutorials are lost to time, but they kickstarted my own interest in casting and molding, so I thought I would write my own to give back!For making the molds for the pinkies, I use SmoothOn's OOMOO silicone. If you don't have a pressure pot, OOMOO 30 is great for providing a lot of time for the bubbles to work themselves out of the mold, but you basically have to let it cure over night. It doesn't damage the parts, and it doesn't require a lot of precision pouring.
For making the cast parts, the original forum thread talked about using SmoothCast 325 as having a similar consistency. I used that originally, and it works, but I will note it comes out transparent, which makes painting it a pain, and it's both a harder/stiffer material, and a little bit lighter than the normal Pinky material. I haven't been a big fan of it, so instead I've been working with SmoothCast 65D, which is nice because it's a softer material that cuts a bit more like the original pinky plastic. It's also an opaque white color by default. It is still definitely lighter and a bit stiffer than original pinky parts though.
The core materials you need are:
Some optional bonus materials you might want:
Making the mold itself is actually where I'll spend most of my time, since a good mold setup makes casting the parts pretty trivial. There's a couple components to this, and some of them are optional, but can be helpful once you've seen how casting and molding works.
First and foremost you need a box to put your Pinky part in and pour the silicone into. This box needs to be non-porous so it doesn't absorb the liquid silicone, and it needs to be sealed on the bottom and sides so the silicone doesn't leak out.
The simplest method of making a mold box is packing tape and cardboard. You wrap the cardboard in the packing tape to make it non-porous, and then tape the sealed cardboard into whatever shape mold box you want. This was how I did many of my initial molds. You could also use something like posterboard or acrylic and hot glue to make a box. Or really any materials.
Most of my molds are small, but I make a lot of them, so I actually picked up some legos as dedicated mold box legos. They are gross as heck as this point, but the precision fit of legos means they don't leak, and you get molds that can have very consistent, uniform sizes. This is important because generally you want to keep your mold box for stability when you cast parts into the mold.
This step is optional, but does help make for nicer cast parts! Take a look at the legs in this mold box and then imagine I fill the box with silicone. What part of the legs doesn't get covered by silicone? Yep, the bottoms of the feet. This should leave you with two major questions 1) How do I fill the mold with the cast plastic? and 2) How do I get the original part out?
The first questions is simple: you just pour into the openings left by the bottoms of the feet, but this wont make for a particularly nice finish there, which is manageable, but not ideal. Thus the value of a pour spout, which is an intentional spot to pour into that minimizes how much clean up you need to do in post.
The second question is also simple, but not ideal: you take a hobby knife and cut open the silicone mold (carefully, so as not to cut the part) to get it out. This is unideal because you could cut the original part, but it's also not ideal because now you have smooth-ish flat planes that you have to try and fit together precisely. This is where keying comes into play. If you pour your molds in two parts, then you can key the sides, so they will fit together very precisely and consistently.
To add both pour spouts and keys, you can use polymer clay as a filler to define the shape of the silicone. Do note that you should limit the amount of time and surface area that polymer clay touches your Pinky part!! It doesn't seem to react with the raw Pinky plastic, but it will discolor and mark the paint on the Pinky. It's not really an issue for pour spouts which touch such small areas, but for keying, you should use a thermoplastic or other medium that wont harm the Pinky instead.
If you want more indepth details, SmoothOn has a pretty (overly) indepth video presentation about this topic here.
Now comes the fun part! Once you combine part A and part B, you'll have limited working time, so having everything pre-set can be really helpful. You'll want to first stir each part really well to make sure it hasn't separated in the bottle, and then pour out equal amoutns of each part into one of your disposable cups.
At this point, stir thoroughly, scraping the sides of the container periodically until it's well mixed and a uniform color. It should be a nice purple color if you're using OOMOO 30. At this point it's recommended to pour it into another cup (while not scraping the sides at all) in order to prevent accidentally pouring in any unmixed silicone that wont cure. (I am bad at doing this step, but the real key is just to be careful not to introduce any unmixed material from the sides of the cup into your mold box.)
With only the well mixed silicone, you'll slowly pour it into the lowest point in the mold box. This allows it to self level as it fills, and helps mitigate air entrapment. Once the box is filled and the part is fully covered, you're pretty much good to go.
Maybe give it a couple gentle taps to help agitate any bubbles to help them come to the surface... And let it cure! You'll want to let it cure in a warm dry place. ~72F with low humidity, is the recommendation from SmoothOn's material data sheet, so don't leave it outside or put it somewhere cold or it wont cure properly.
Once the mold is fully cured, it's time to remove the master part. This is where things like mold release and two-part keyed molds can start simplifying the process, but if you just put the part in and filled it, then you gotta cut open your mold with a hobby knife.
Do this very slowly! There's no rush, and you don't want to accidentally harm your Pinky, yourself, or the mold box. You'll want to cut it open in a way that it can be removed pretty easily, and then just take small cuts, holding the silicone open as you go. You'll see it stretch, and you'll know you've reached the part when a little hole forms. Then just carefully cut along that with zero pressure, and the silicone should tear without the knife even touching your Pinky part.
So, bubbles and trapped air are your main enemy for any cast part. This can lead to blank spots and voids in your part, or if you're doing injection molding, you can end up with burn marks too. There are several methods to mitigate bubbles and trapped air and some are simpler than others.
The simplest method is simply being intentional and careful in your stirring to avoid folding air into the liquid plastic. Your part orientation also plays a role, because pouring into the lowest point in your mold can prevent air getting trapped.
The next method is incredibly powerful, but complex, and that's venting your molds. SmoothOn has a pretty (overly) indepth video presentation about this topic here. That said, venting is all about preventing air entrapment at the high points in your part. So as your pour plastic into the mold, the air gets pushed up, and if it doesn't have anywhere to go then it will get trapped at the high points in the mold.
The most expensive methods are vaccuum degassing and pressure pots. Vaccuum degassing pulls a vaccuum to pull all the air out of your plastic before you pour it into the mold. Pressure pots put pressure in and squeeze all the bubbles down flat while the plastic cures.