This Instructable shows how to modify and use a pressure paint tank for bubble-free resin casts. The first part shows the modification of a paint tank and the following section depicts the casting process. I assume you know how to make a mold and cast material (here is a great Instructable on making a mold and video on making a simple block mold like I'm using below).
A professional 5-gallon pressure chamber costs upwards of $700, not including the compressor. A modified paint tank should cost under $120, and of course you will still need an air compressor.
You will need:
- Pressure Paint tank
- Air compressor with Air hose
- 1/4" Compressor Coupler
- 1/4" Female Compressor Plug
- 1/4" npt cap
- 3/8" npsm (fine thread) cap
- Thread Seal Tape
in addition to a mold, casting material, and a release agent.
The main image shows two casts of the same material (Smooth On's popular 325 urethane plastic), one that was pressurized and another that was not. I used the exact same mold and process for each cast except for pressurizing the first cast. Hopefully this image is enough to convince you that pressurizing is ESSENTIAL if you are serious about casting resin/urethane plastic.
Note 1: After conducting more research and talking to a number of techs, it it imperative that the mold used with the pressure chamber have its silicone vacuum-degassed. If this is not the case you will get "measles," as tiny air bubbles will become exposed when pressurized (essentially ruining the mold). Again - the ultimate way to remove air bubbles is to first vacuum-degass the silicone used for the mold, let that fully cure, and then vacuum-degass the resin (casting material) after mixing, pour vacuumed material into a mold, and then put mold into pressure chamber (as a few people have also pointed out in the comments below). If you're wondering why you can't put the mold in a vacuum chamber, its because the material expands before contracting so it would spill out of mold. Also if you're wondering why you need the pressure chamber, its because you'll likely induce additional air bubbles when pouring. Again, the ultimate way to remove air bubbles is to combine both processes. This Instructable covers the second part of that process.
Note: Some good mixing techniques include keeping the stir stick on bottom of container, and pouring "high" for silicones (also know as. "ribbon technique") or "low" for resins (discussed later) to minimize air bubbles in both the mold and the cast. These are certainly worth practicing, but will not achieve results that come close to a pressure cast.
Step 1: Set up compressor, acquire a pressure tank, and read manuals
Once you have an air compressor with an air hose and attached 1/4" female quick-connect coupler you'll want to acquire a pressure paint tank. I've been using the 2 & 1/2 Gallon Central Pneumatic Pressure Paint Tank from Harbor Freight. The price is advertised as $100 online, but I bought mine from a local store during a holiday sale for $80. I don't think its too critical which brand you choose. Perhaps more important is the size to make sure it can accomodate your mold(s).
Step 2: Convert the pressure paint tank into a pressure chamber for casting
NOTE: for steps 2 & 3, I recommend bringing your paint tank to a local hardware store and testing caps on the appropriate connectors/outlets to make absolutely sure you are purchasing the right parts.
(1) Wrap thread seal tap around connector to left side of regulator and attach a 1/4" female plug using wrench of your choice (I think its easier to use an adjustable wrench but this will most likely scratch the cap. I don't think this really matters because I don't plan on ever removing the cap, but if you're concerned about this, then use a constrictor strap wrench).
(2) Wrap thread seal tap around connector to right side of regulator and attach the 1/4" npt cap using wrench of your choice.
(3) Wrap thread seal tap around paint outlet and attach a 3/8" npsm fine thread cap (different threading than previous cap) using wrench of your choice.
(4) On the underside of the lid, there is a metal tube. Take a hacksaw and simply cut this off. In the conventional usage of the tank, this is the draw for the paint, but since we're closing off the paint outlet and using the chamber for something else entirely, we'll want to create maximum space inside of the chamber.
Step 3: Using the Pressurizer: Prepare mold, pour material, put in chamber, and pressurize
Now that the paint tank is converted into a bona fide pressure chamber, its time to prepare your mold for casting. I will be using Smooth-Cast 325 (this process is also essential for the Crystal Clear series as well), so there may be some steps that are specific to this material. Always read material data sheets and consult with technicians during your purchase for further details. If you need to brush up on mold-making terminology, here is a good resource.
Before we start pouring, I think its important to discuss other potential problems, as a lot can go wrong in the process and result in not only lost time, but serious money. First, if you are using silicone as your mold material, you should know that most RTV silicone kits need to be vacuum degassed in order to reduce surface air bubbles on the interior of the mold (here's a video on the process, note the poor pouring technique at 1:57 - in order to further reduce air bubbles, technicians at Smooth-On recommend that you pour high, not directly on top of your objects, and only in one spot as shown in this video at 1:12). Artifacts in the mold cannot be fixed by pressurizing.
Professional vacuum degassing chambers costs upwards of $200, but thankfully there is an Instructable on making a DIY vacuum apparatus for about $20. There are other tricks that people use to prevent air bubbles, such as putting the silicone in a large ziplock bag, mixing, cutting a corner off and pouring. I guess its worth a try, but if you're pouring a mold that needs more volume, I think the DIY vacuum apparatus is the way to go.
That all being said, there are a few products that de-air themselves, though they're not perfect. One such product is Mold Star, and of course you'll want to practice the techniques shown in the product video. I have not seen other products that de-air themselves but am sure they exist. Unfortunately there is no resin that pressurizes itself, hence the need for this Instructable.
Ok moving on. Once you have a mold ready to go and are working in a well-ventilated area, put on gloves, glasses, and a mask before spraying release in the mold. Let sit for about 15 minutes, or however long your release recommends. Mann's Ease Release 200 is the recommended complimentary product for Smooth-Cast 325.
Next, mix part B first (clearly labeled on product). Keep stir stick on bottom of container and be sure to scrape sides too. If you are adding dyes or any additives such as a UV resistant curative, Smooth-cast requires it to be mixed during this step.
After part B is thoroughly mixed, add an equal amount of part A (Smooth-cast 325 has an easy mixing ratio of 1:1). At this point you'll need to work fast, as the working time ("pot-life") of 325 is about 3 minutes. If this is too quick for your application, you can also try 326 or 327 which have working-times of 7 and 20 minutes, respectively. Be sure to have everything set up before adding part A (i.e. compressor and pressurizer ready to go). When part A is added, remember to mix gently as this material has a low viscosity and can splash around. Its also unnecessary to mix vigorously like you would with silicone. I mix for about 45 seconds, and always time myself with a stopwatch.
Once the material is mixed thoroughly, "pour low" and in one spot, which helps prevent the introduction of additional air bubbles (i.e. bring mixing container as close to the mold as possible, then pour and hold steady). After your pour, you should notice some air bubbles on the surface, but since you're about to pressurize the cast, you don't have to worry about them! (Note: pouring low is much more critical when NOT using a pressure chamber...in any case it never hurts to practice good technique).
Next, place the mold in the tank. I use a circular piece of wood to make sure the mold will not fall over, as the bottom of my paint tank is concave. Once the mold is inside be careful not to move the tank, as the mold could topple and spill (learn from my mistake and be conscious once mold is in tank). If it happens, don't try to clean up the material - wait until it cures and then pop it out. If you're worried about this happening, spray some release in the tank beforehand so clean up will be easy.
The following step is a recommended practice from the paint tank manual. The manufacturer recommends that the opposite lid screws be tightened at the same time when sealing the lid. I tighten these screws in tandem one full-turn at a time before alternating to the opposite pair, which ensures the lid is sealed equally at all points. If not sealed properly (i.e. too tight in one place or not enough in another spot), you will hear the sound of air leaking and the optimal pressure will not be reached. I think its wise to practice pressurizing the chamber once or twice before pouring a mold to double check that all caps and lid are secure.
Once lid is sealed tightly, take the end of the air hose, peal back cover and then attach to plug, as shown in images. Then turn on compressor and monitor pressure - the compressor will reach between 20-30 PSI before the paint tank's gauge moves. Once the paint tank's gauge reaches between 45-50 PSI, turn the compressor off. You should not hear air leaking, and the pressure should remain constant.
Leave the mold in the paint tank for the duration of its demold time, which is the earliest recommended time that a cast can be removed from a mold.
Step 4: Demold and Examine cast
After the demold time is up, release the compression coupler from the plug. I recommend wearing ear plugs because it is LOUD. Then unscrew lid and pop cast out of mold.
While pressurizing helps achieve superior results, its the sum of all good practices & techniques that will ultimately produce the best work. My pressurized crystal looks great compared to the unpressurized cast, however, if you look closely, there are some artifacts from not degassing (I used Mold Star 15 and degassing is still recommended, which I didn't do) and pressurizing too high (above 50 PSI), which resulted in "the measles" - tiny holes that damaged the mold. Initially I followed the technical bulletin for the Reynolds Advanced Materials pressure chamber (which recommended pressurizing at 60 PSI) and later talked to technicians who advised me not to exceed 50 PSI. At that point, however, my mold was ruined. Hopefully yours will not be. Since this first experience I have not damaged other molds pressurizing at 50 PSI, and wanted to show what can happen if you exceed this threshold.
I wish you luck in your casting endeavors and feel free to post comments if you need additional advising. I will do my best to answer your questions though I am by no means an expert, and have had a lot of help along the way in my journey as an object fabricator.