Ice Crystals: Food-Safe Molds From a 3D Printed Object

This Instructable covers the process of making a food-safe mold from a 3D printed object. I will provide two methods for making an ice mold (one being a tray), and some of the steps can be mixed and matched. My chosen forms are large and small crystals - I thought a large crystal would not only look awesome/appropriate in a glass with a high-priced beverage, but also would take longer to melt.

One additional note: for best results with large objects I recommend designing your object for use with a particular glass, the reason being that your object will float with the addition of liquid UNLESS it fits perfectly snug into the glass (of course it will eventually melt, but will look totally awesome for a short time :).

You will need:
(1) A 3D printed object
(2) Food safe material for mold. I used Smooth-On's Smooth-Sil 940 as well as 1/8" Polyethylene sheets from Tap Plastics
(3) Release Agent
(4) Plastic Container & Stir Stick
(5) Glue Gun
(6) Nitrile or plastic gloves that ARE NOT latex
(7) Scale (Smooth-Sil 940 uses a material ratio of 10A:1B)
(8) Sandpaper or Sanding block (grades 180, 220, & 400)
(9) Clean surface


For intermediary objects (explained later):
(1) Cheaper silicone than Smooth-Sil, such as Mold Star (which de-airs itself)
(2) Smooth-Cast 325
(3) Plastic Container & stir stick in addition to relevant items mentioned above
Recommended: Pressure tank and compressor

If you decide to make a tray as opposed to a mold you will need access to a vacuum forming machine. Alternatively, you can build one: check out this great Instructable.


<sub>Note 1: Thanks to HexCorp in Tarzana for letting me use your space and 3D printer, a great makerspace is emerging! Thank you to UC Santa Cruz for continued sponsorship as a research associate and owning a vacuum forming machine.

Note 2: I am in no way affiliated with Smooth On, Autodesk, Makerbot or any other company mentioned in this Instructable and receive no compensation for the publication of this Instructable by either of the aforementioned companies.</sub>

Everyone I know is jazzed about the idea of 3D printing, but there's nothing quite like seeing your first object created before your eyes.

There are several free CAD software programs out there, and I have to say my favorite is Autodesk's Inventor Fusion (free). Its more stable than 123D Design (which crashes all the time). If you are just starting out, SketchUp is probably your best bet as there seems to be the largest community support and volume of free instructional videos.

Once you have modeled your form, export as .STL which can be uploaded directly to a service that will print for you (like Shapeways) or imported into Makerbot's Replicator software.

If you want to use my crystal, feel free to download the .STL attached.

I had access to a first generation Makerbot via an up-and-coming makerspace in the San Fernando Valley (HexCorp), and it was my first time witnessing the creation of my own designs.

Since there are a number of great tutorials on using this machine, I don't think there's a need for me to explain the whole process. Essentially all you have to do is load your material (I used PLA) into the extruder head, heat the pad in the software's control panel, open your .STL and postition/size it, generate the GCODE, and press print.

One thing I learned from this process is the concept of infill, which in my case, was totally excessive. Basically this setting controls the amount of material that fills the inside of your printed object. This is one of two concepts (the other being "shells" or exterior layers) that determine the weight, structural integrity, and perhaps most important for the hobbyist, price, of your object. If your object is just for display, Makerbot recommends no more than 10% infill, and 80% for objects that will see heavy use. Read more at their 3D Printing Concepts page. I used 35% and for molding, could have actually used 10% and backfilled with a cheaper material. Nevertheless, the big crystal looks awesome.

Before digging into this section, I want to address one characteristic of 3D printed objects (especially lower-resolution ones) which will have to be dealt with: the Z-lines (or ridges). I've seen a few Instructables (like this one) that discuss cleaning up 3D prints for prototying (in some cases using a spray filler before sanding), and while these methods definitely work, I will take two approaches - one that will involve sanding without a filler material, and the other which will include an extra step - making an intermediary object and cleaning it up before making a second mold.

For the first mold, I will use the large crystal from the previous section. Although I would have liked to use filler like the other Instructables, I decided that because my object had too many angles that needed to be preserved, I did not want to damage the original object by oversanding and filling. Perhaps it could be done, but I think a great way to eliminate the ridges is to make an intermediary object, subtract/add material as needed and then vacuum form it (shown later). In any case, sanding was fine for my proof of concept and eliminated the most pronounced ridges, though some are visible in my final cast. Be sure to sand the top of the crystal until its flat so that silicone does not seep underneath when you make your mold. If it does, this can be remedied as the material can be cut away, but its best to avoid this potential problem. Start with the most-course sandpaper (180) and take your time (just remember how long it took for your object to print!), and progress to medium grit before finishing with a fine grit (400).

After sanding I recommend using a dampened rag to remove particles rather than putting object underneath running water. In addition you may want to spray the object with a compressor so that stray particles do not find themselves into your mold. When your object is dry, find a clean surface that you can glue to (I don't recommend wood. I used a styrene sheet but you could even use foam core or strong poster paper) and hot glue the bottom of the object to the surface. Don't worry about the undercut caused by inverting the object because the silicone is flexible enough to ensure that the object/cast can be released. For more complex molds, look into a two-part or multi-part mold, OR use silicone with a lower durometer.

Next you'll need to create a mold shell, and in the interest of saving material, its best if you don't use a box. For my small crystal, a 4 oz. dixie cup was suffice, and for the large crystal I cut down a 16 oz mixing cup and hot glued the parting line to ensure that there wouldn't be a leak. If you're wondering why I bothered, its because shrinking the diameter of the shell results in a significant material savings, as you do not need more than 1/2" wall of silicone between any point of the object and shell. Note, you can actually save more material by making a glove mold, but this is a more time-consuming and difficult process (note 2: the previous link is the "trick" way to do a glove mold, but PlastiPaste can be substituted for cheap plaster). In my case, the material saved wasn't worth it (as I only had a weekend to work on this project), considering it would have been a few ounces at most.

When the glue has dried, spray some release agent (I used Mann 200) inside the shells, making sure to hold the can 6" away from the object. Let dry for approximately 20 minutes before proceeding with next step (also refer to your data sheet too for recommended release agents). Wear gloves that DO NOT have latex, as latex inhibits the curing of silicone.

Its time to mix your material and pour. Keep in mind that we're making two different molds for two different purposes. One important note: generally its best to vacuum degas silicone before pouring. Unfortunately I don't have a vacuum degasser and haven't had time to build a DIY one. However, I have to say that while vacuum degassing is ESSENTIAL if your final cast is made of silicone, in my experience with simple block molds, the air bubbles rise to the surface and don't affect the mold cavity.

Mold 1: Since I dealt with Mold 1 first in the last section, I'll keep things orderly. Many of Smooth-On's products (my chosen manufacturer of mold-making products) have a ratio of 1A:1B, meaning that you use equal amounts of parts A & B, so measuring is easy. With Smooth-Sil 940, a high-quality food grade silicone, the ratio is 10A:1B, so you'll need a gram scale to accurately weigh each part before mixing. I'll walk you through this process.

First, put an empty mixing container on the scale. You'll notice it has a value (of course). Now, press the TARE key so that the scale resets to 0, meaning that you're taking into account the weight of the mixing container and weighing only the material itself. This is especially important if you are using mixing cups of different sizes, which makes sense given the fact that you'll only need a little bit of part B (a 4 oz. dixie cup works well, in fact, I always keep them on hand). Now pour however much material you think you'll need. Generally I eye it out, through you can calculate the exact amount if you figure out the volume of the space inside the mold shell minus the volume of your object. However, this can be complicated, and if you ever mess up and don't mix enough material, you can always mix a little more and pour on top of your previous layer of silicone because it sticks to itself! Note, a good habit to get into is wiping the container with a rag. You'll thank yourself when you handle the container without gloves and don't get the nasty stuff on your hands (if you do, Denatured Alcohol on a rag helps to get it off).

Next, repeat the same process with part B. Since its 10:1 ratio, it should be pretty easy to figure out (i.e. if you weighed 300 grams of part A, you need 30 grams of part B). Don't forget to mix part B by itself (as recommended by the datasheet) before combining with part A. When mixing, keep stir stick at bottom of container at all times and don't forget to scrape sides. The less you lift up your stir stick, the better, as this induces air bubbles. This is much more critical if you are not using a vacuum degasser.

Now the real trick to reduce air bubbles is in your pouring technique. Pour from high up (like in image 6 in this sequence) and NOT directly on top of the object. Rather, let the material rise up and over the object. Take your time and keep your arm steady. As you pour you'll notice that as the material falls on top of itself, it resembles a ribbon (hence the name "ribbon technique"). Don't stop until you're sure the material is at least 1/2" above the object. Let sit for 24 hours at room temperature on a level surface. While the material may seem hard enough after 12 hours, be patient and let it sit for 24. Afterwards cut away any silicone covering the object with a scalpel or razor knife and demold (or gently pull it out). This mold is done.

Mold 2:
This one is much easier. I'm using a different silicone (MoldStar 30) because it is cheaper than Smooth-Sil which I save for food molds only, and supposedly de-airs itself (though degassing is always recommended). Plus, it has a ratio of 1A:1B so no scale is required! Simply mix equal parts of A and B together (mixing part B first), and then pour using same techniques discussed in previous section. Cure time is a mere 6 hours.

This section only pertains to the second mold process, making an intermediary cast and vacuum forming an ice tray. This is absolutely NOT a complete guide to vacuum forming, rather just an introduction - processes will depend on machine selection and material.

Here we go, now that your mold is ready for casting, we'll use Smooth-Cast 325 (which is cheap considering alternatives and easy to work with) to cast your intermediary objects. After mixing, spraying release, and pouring material, I recommend pressure casting. If you have never heard of this, I recommend checking out my Instructable on how to make a DIY pressurizer. I believe this step to be essential - hopefully images 2 & 3 of this sequence are enough to convince you. If you're thinking "why does it matter if there are bubbles on the inside?" keep in mind that they are on the outside too! The previous link will walk you through the entire process of setting up the equipment, prepping your mold, and pouring the material.

Once you have your cast object, you can subtract or add material accordingly without fear of messing up your original object. At this point you can either make a food-safe mold with Smooth-Sil when you deem your object perfect, or follow the next steps for making a tray.

Making a tray:
First, build a vacuum forming machine or get access to one, and acquire some 1/8" or thinner/thicker polyethylene sheets depending on your object (of course you'll have to do some tests to see what works best for your application). Keep in mind that polyethylene sheets are certified food-safe plastic for thermoforming (2x check this with your supplier to make sure yours are too) and while many food-safe products are made of styrene, my supplier (TAP Plastics) informed me that their product is most likely food-safe but couldn't verify it. Also make sure you're working in a well-ventilated room.

Next, make a few casts, as many as you want for your tray. At this point you can sand/fill each one individually or make another more perfect mold and produce casts from it. Keep in mind that the Z-lines will be much less pronounced with thicker polyethylene sheets.

Now for the vacuum forming: first put objects on platform (its best if you put them on a thin block so they're level, which I didn't do here) and then lower the platform with lever. Next, put your sheet over the opening, lower the frame/brace, and tighten brackets to ensure its locked it place. Then its time to turn on the machine and let it warm up before pulling the heating pad over the sheet to heat it up and make it malleable. Do some tests to find the optimal heating time. One trick is to look between the crack of the frame to see the moment it begins to droop. Don't wait to long or it may become goopy and unsuitable.

When you think its ready, hit the vacuum switch and then pull the lever to raise the platform with the object. Let it cool fully before removing. After you think its cooled down (consult data sheets and manuals for your specific application), while the vacuum is still on, pull the release lever to aide in releasing the objects. Another trick to remove objects if they are really stuck is to put the sheet in the freezer so the plastic contracts. Don't force it or you could crack the sheet. A tool may help but be sure you don't damage the mold cavity.

This is the easy and fun part - pour some water into the form, put in the freezer and then pour yourself a beverage!

As suggested in the first comment below, make your ice crystal clear using this Instructable!