Introduction: Soft Drink Mixer Caps

More fizz! No Mess!

Making your own carbonated drinks at home can be a fun and/or convenient way to get your soft drink/pop/soda fix. There are numerous different home carbonation appliances available on the market that use high pressure CO2 cylinders to carbonate water before you mix in some kind of flavouring syrup. One problem with these systems is that you have to remove the bottle from the carbonator to add the syrup and doing so inevitably makes much of that CO2 rapidly come out of solution. Firstly, that wastes some of that precious CO2 that you just forced into the bottle. Secondly, it can lead to a big mess if the bottle fizzes over before you can get the lid on. This instructable will show you how to make your own 3D printed and sealed caps with their own reservoir built in to avoid those problems above. Keep more of the gas in your drink until you're ready to drink it and avoid the mess that sometimes happens when adding the syrup.


You'll need the following tools and materials:

  • 3D printed mixer cap
  • 3D printed bottle stub (optional)
  • Silicone O-Ring (roughly 1 3/16" ID and 1/8" thick) I used the #14 O-Ring from this set
  • Pourable Epoxy Resin, preferably food safe. I used Art Resin but any resin with about 30 or more minutes of working time should do.
  • A small container for mixing the resin
  • Some bamboo skewers or similar sticks
  • Container to catch excess resin draining from the cap
  • Vegetable oil or other food-safe release agent
  • A small bowl
  • A plastic bag
  • Plastic, paper or some other material to act as a drop sheet to catch wayward resin

Step 1: Set Up Your Work Area

Clear yourself a comfortable work area. Lay out your plastic or paper drop sheet and all of your tools and materials so that they are easily and readily accessible.

Step 2: Mix Your Resin

Read the directions for your resin. Pour out the appropriate proportions of the resin and hardener into your plastic container and mix well. For my particular brand of resin, this required equal parts of resin and hardener out and then vigorously mixing them for 4+ minutes to ensure they are well mixed. You'll need enough to coat the entire inner surface of the cap with a thin layer of resin. Don't skimp. It's probably more than you think. Something like about 2 tablespoons worth of resin is probably a good amount. You'll end up wasting some of it, but it's better to have too much than barely enough. You need to ensure full coverage inside the cap to make sure you have a good, gas-tight seal and spreading it around inside the cap if you have barely enough will be challenging. You'll probably also end up with a lot of bubbles. Don't worry too much about those.

Step 3: Pour Resin Into Cap

Pour your resin into the reservoir of the cap. Again, don't skimp. Tilt the cap and roll it around to get your large blob of resin to slowly flow over and coat the entire inner surface of the cap. If necessary, you can use one of your skewers to help move it around to cover everywhere, although in the main part of the reservoir it's generally better to simply ensure you have plenty of resin in the cap so it just naturally flows over and covers the inner surface as you tilt and roll the cap around. As you get the reservoir portion all covered and start moving resin down into the collar/neck, this is a good place to start using a skewer to spread the resin. Be very careful not to get any resin into the threads or the outside surface of the collar.

If you do get any resin on the threads or on the outside of the collar, wipe away quickly and as best you can with some paper towels.

When done place the cap inverted (ie. collar/opening facing up) and allow to sit for 5 minutes to allow resin to drain down into the top of the reservoir.. This is to ensure that you get a good coating there where the 3D printer was bridging and close up any gaps. The bridged area of the print may be more prone to gaps, holes and crevices which are difficult to clean and could harbour bacteria and other nasties. Ensuring that a good amount of resin flows into here should help fill that in and ensure a flatter, easier to clean surface.

Step 4: Allow Excess Resin to Drain

Place a couple of skewers over your resin container just wider than the collar and rest the cap on the skewers. You will almost certainly have more resin in there than you need, so this will allow excess resin to drain from the cap.

Allow the cap to sit and drain for 10 minutes.

Step 5: Seal Outside of Cap

Pour some of your remaining resin onto the top of the cap. Again, don't skimp. You probably need more than you think.

Place the plastic bag over your hand like a glove or mitt. Carefully pick up the cap with your other hand by the gnurled section at the bottom and place your bagged hand over the smooth portion of the cap with a snug but light grip. Slowly rotate the cap inside your bagged hand so as to spread a thin layer of resin over the top and sides of the cap all the way down to the top of the gnurled section. You shouldn't need more than that and keeping the gnurled section of the cap dry gives you a safe area you can use to hold the cap without getting resin all over your hands.

Step 6: Put Small Amount of Resin on Top of O-Ring

Using a skewer, dip it in the remaining resin and carefully put a thin layer of resin on the top side of the O-Ring, only. Here you should skimp. You don't want too much resin here. This is possibly not entirely necessary, but the intention here is mostly to ensure a good seal and seating of the O-Ring between the cap and the bottle. You want just enough to sit under the O-ring and fill any irregularities in the cap where the O-ring lies, but you dont' want so much that the excess resin may ooze around and glue the bottle or stub to the cap or gum up your threads.

Step 7: Place O-Ring Down Into Cap

Place your cap down on it's top with the thread and opening facing upwards toward you. Carefully place your O-ring over the outside of the collar with the resin side down. Be very careful to avoid getting resin on your threads. Carefully push the O-ring down with your skewer until it's fully seated into the cap.

Step 8: Coat Bottle Stub With Release Agent

Pour some of your vegetable oil or other release agent into a small bowl. Take your bottle stub and dip it in your oil. Roll it around to ensure that all surfaces are well covered with oil. This is to help ensure that the resin does not stick to your bottle stub.

Step 9: Firmly Screw Bottle Stub Into Cap

Firmly screw the bottle stub into the cap. This is to firmly seat the O-ring into the cap and ensure that the resin that is there cures into a nice, smooth, flat, form-fitting surface for the O-ring to seal against. To some extent, that resin may also hold the O-ring in, although resin doesn't normally stick very well to silicone. Your O-ring may fall out after some use, so best to make sure that the resin cures into a good flat surface to ensure that the O-ring securely seals against both the cap and the bottle when in use.

Step 10: Place Cap Over Container to Continue Draining and Curing

Place a couple of skewers over a container just wider than the collar and place your cap on the skewers to sit and cure. Check your particular resin's curing time, but typical resins probably need about 24 hours to cure to a nice hard surface. At this point you can unscrew the stubs. You probably need to wait something like 72 hours for a full cure. You can handle the caps once it cures hard, but I would suggest waiting the extra time for a full cure before using them to make drinks, just in case they're still off-gassing or any of the resin may leach into your drink before fully cured.

Step 11: Optional Alternatives

The bottle stubs are essentially a throw-away component that you print only for use during the sealing process. If you'd rather not waste filament or time on printing this, there are two alternatives you can try.

  1. Don't use any resin on the O-ring. Depending on your printer, filament and slicer settings, you may have a good enough seal already without seating the O-ring in a little resin. If you want to try this, just stop after sealing the outside of the cap and place it on sticks over a drainage container to cure. Even if you do this, you can try the other alternative if you have trouble getting a good enough seal first time around.
  2. Use a bottle to seat the O-ring properly in the resin. If you're going to try this option, you will want to put the O-ring in after the resin on the inside of the cap has cured - typically at least 24 hours after the initial sealing. If using this method, wait until the resin on the interior and exterior has cured hard, then mix up a smaller amount of resin and coat the O-ring in the same way as described. Coat the threads and top of the bottle in vegetable oil and screw it in. Place the bottle upside down on the bottle cap for at least 24 hours while the O-ring resin cures.