If you're in the New York City area, I highly recommend going to The Compleate Sculptor for supplies and technical advice. They helped me a lot.
These instructions will work for a 3D printed object of any type. The figures used/pictured here were printed on a ZCorp powder-based printer. These steps will also work for plastic-based printer.
plastic mixing containers (1 quart)
1 inch chip brush
wide rubber bands
long bolt of any size with matching nut
small piece of wood (approx 1 x 5 inches)
foam core board
small digital scale
mixing sticks (usually found at paint stores)
sandpaper (400, 600, 800, 1000, 2000, 4000, 8000, 12000 grit) - yes, you need all of them
Plasteline sculpting clay
Mold Silicone (if you're new to this, start with Smooth-On Mold Star 15 SLOW which has a 4 hr. cure time. When/if you're more comfortable, try Mold Star 16 FAST which has a 30 minute cure time)
Mold Release (I used Mann 200)
Casting Silicone (in my case, Sorta-Clear 40)
Pigment (I chose Silc Pic Pigment, black)
60 mL syringes with catheter tip (optional).
Step 1: Sand Your 3D Printed Objects.
In most cases, you're going to want to accurately reproduce your shape while minimizing any surface anomalies. For that reason, you're going to have to do some sanding. But don't worry - you only have to do this to the original and it will pay off in the end.
1.) Sand the object you'd like to duplicate until it is as smooth as possible. Start with a rough grit paper to insure that you are getting past any bumps in the surface, then work your way up incrementally by 200 until you reach 1000 at which point you can proceed by steps of 2000. Wet sanding is recommended.
Why do I have to use so many grades of sandpaper?
To get a super smooth surface, you need to use a very very high grade sandpaper. If you jump too many grades as you work your way up, you risk leaving textural differences in your surface. In other words, if you scrub your object with some 400 and then skip to 1200, there is no way you will file down every little groove you just blasted into it with the 400. By working your way up gradually, you are actually saving yourself time because it's easier to correctly sand a 400 grit surface with 800 paper than it is with super fine 1200+ sandpaper.
In this case, I want extremely smooth rubber objects, so the more sanding the better.
2.) If the object is dull and you would like a shiny final object, finish with shellac and resand with higher grit paper to make sure all surfaces are smooth and shiny. In my case, I just used an off-the-shelf spray shellac. I applied a light coating, sanded the surface with the top two levels of sandpaper, and repeated the process one more time.
* PHOTO EXPLANATION *
1 - Objects fresh out of the printer. These were printed with black powder. If you're printing with powder, make sure you've infused them as recommended by your printer's manufacturer.
2 - Midway through the sanding process, the objects loose some of their color. That's normal.
3 - Objects have been painted black with spray paint because I wanted them to be shown along with the final rubber versions. This added some extra sanding. Then they were shellacked as described above.
Step 2: Surround Your Object With Plastalina
1.) Imagine a line running down the length of your object. If you were to split your object in half along this line, you would have two mirrored halves of your object. You want to embed the object in Plastalina up to that line so that one of the mirrored sides is in the clay and the other is exposed. Try to make the top surface of the Plastalina as smooth and perpendicular to the side of the object as possible. Use the popsicle stick to refine the edges around the object so they are flush.
2.) Trim the Plastalina at each of the four sides of the object to give straight edges (use a metal ruler, knife, or popsicle sticks).
3.) Cut pieces of foam core to fit these sides and rubber band the foam core around the Plastalina so it forms a box. One of the sides of your piece should be flush with or come in contact with one of the sides of foam core. That's important because you will need an entry hold into which you'll pour the silicone. If there isn't an obvious surface to make flush with one side of the foam core, embed a dowel rod (1/2") going from an edge of the piece to a foam core wall.
4.) The corners of the box might not look totally sealed - that's okay. To seal the edges inside the box, make skinny snakes of Plastalina and use it to seal all of the edges around the piece where it touches the foam core, and the corners of the box.
5.) Lastly, take a small cylindrical shape like the end of a marker or a dowel rod and make at least 2 impressions in the clay approx 1/4" deep. These will serve as keys later when you have two halves of the mold and need to line them up exactly.
Step 3: Pouring the First Half of the Mold
1.) Open your mold silicone containers (Mold Star 15 or 16 as recommended above) and mix well with mixing stick. Pour the necessary amounts of mold silicone into the plastic mixing cup and mix thoroughly - don't forget to scrape the sides and bottom of the cup.
2.) Pour slowly from a good height (2-4 feet, this reduces air bubbles) until the silicone surpasses the highest point of the object by about 1/2". A small note, you don't have to mix all the silicone you're going to need all at once. You can mix and pour, then mix some more and pour. This gets harder if you're using the Mold Star 16 because it starts to set within 6 minutes. With the Mold Star 15, you have 60 minutes.
3.) Lightly tap the sides of the mold box for about 5 minutes to release any bubbles. Wait for the silicone to reach its cure time, then take off the foam core, and remove the Plastalina from the bottom of the object. You should have a good looking, solid mold with your object still in it. Leave it there. Save the foam core and the Plastalina.
Step 4: Pour the Second Half of the Mold
2.) Put the foam core back around the mold and secure it with rubber bands. Reseal the interior sides and corners of the box with the snakes of Plastalina.
3.) In a ventilated and dust-free space, spray the object and mold (the whole interior of the box) with the release agent. It should be a light but even coat. Take the chip brush and lightly brush all surfaces in the box to make sure the release agent is covering everything well. Recoat it with another round of release agent and then let sit for 30 minutes. (The 30 minutes is really important. If you don't let it set, the casting silicone will fuse to the mold and you'll have to start over).
Why didn't I have to do this for the first half of the mold?
The release agent is only used when silicone is coming in contact with other silicone (it fuses together when that happens).
4.) Mix and pour the second half of the mold exactly as before. When the cure time is up, remove all foam core and clay and open the mold - you can now remove your object and marvel at your beautiful mold.
Step 5: Prepare to Cast!
The drill will be used to create a vibration during the first few minutes after you've poured your casting material into the mold. This vibration will help release all air bubbles trapped in the mold. Basically, it's just an off-center weight at the end of a rod attached to your drill.
1.) Take the small piece of wood and drill a hold just a hair smaller than your bolt diameter. The hole should be offset from center by at least 1/2", and doesn't have to be exact.
2.) Thread the bold through and secure it between the bolt head and nut. Tighten as much as possible so that the nut bites into the wood ensuring a tight grip. Then put the bolt into your drill head and tighten.
3.) Test it out by pressing the trigger of the drill while it's in contact with your work table. Do you feel a steady gentle vibration? It should be obvious, but not so strong that objects are starting to move around on the desk.
4.) Re-coat both sides of the mold with the release agent and wait 30 minutes.
5.) While you're waiting, preapare your casting silicone. In this case, the Sorta-Clear 40 is measured by weight when mixing the base and catalyst so I get the scale ready, set out my buckets of silicone and get the pigment ready with a couple of popsicle sticks. Things are going to move fast once you start mixing this stuff together, so look over the instructions for your material a few times and make sure you understand the order of steps necessary to properly measure your ingredients.
6.) Don't get nervous. You've already made your molds and you can't really mess them up (unless you forget the release agent). You can mess up the first casting and still try again.
7.) When they're dry, put the molds back together and rubber band them so they're sealed, but not so tight that you're disfiguring the shape. Use a small level and popsicle sticks to make the mold level (see the video in the next step).
Step 6: The Final Step - Pour That Rubber!
2.) Weigh out the amount of base (Part A if there are two parts) casting silicone you need into a plastic mixing cup. Add a little bit of pigment (a little goes a LONG way) using the popsicle stick and mix thoroughly. Put the cup back on the scale and figure out how much catalyst you will need to add. (For example, if the mixture is 10:1, you're going to need to increase the weight of that cup by 10% of what you already have. Move the decimal point in your head, add it to what is already there, and you have your target weight).
3.) Add the catalyst (Part B if there are two parts) using the scale and mix thoroughly. This stuff has a set time of 60 minutes, so there's no rush.
Optional: I used a syringe to pour the rubber into my molds because it gave me a little more control. See the link on the first page for my recommendation of what kind to use. If you have a buddy handy, use two syringes and have them fill up one syringe while you're pouring the other. If not, don't sweat it. You have plenty of time.
4.) Slowly pour the silicone into the molds. Don't worry about pouring from too high up. It's more important to get it in there smoothly. We'll take care of the bubbles with the drill. Save a little silicone in case you need it during the drill phase.
5.) Continue until you fill the mold just past the top. Level the top out by scraping a popsicle stick lengthwise along the top of the mold.
6.) Break out your drill contraption and put it close to the mold and pull the trigger. The wooden piece will most likely require that you hold the drill upright with the bottom of the handle touching the table, if you need to build a rig, do so before you get to this step. Now sit there for at least 20 minutes with the drill running. Bubbles will rise up during this time. Pop 'em with a toothpick. If a large bubble comes up, you may notice that after you pop it, your silicone level has gone down. Just pour a little more in there.
After about 20 minutes, you can relax and you're done!
Come back when the silicone has reached its cure time, open up the molds and hopefully you've got a rubber duplicate of your original object.