Introduction: Bubble-Free Resin Casts With Modified Paint Tank
This instructable shows how to modify and use a pressure paint tank for pressure casting. This is mainly useful for achieving bubble-free transparent or translucent resin/urethane casting effects.
The first step involves modification of a low-cost paint tank and the following section depicts the pressure casting process. I assume you have a mold (here is a great instructable on making a mold and video on making a simple block mold like I'm using below) and have basic knowledge of casting.
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
- Hacksaw
- Wrench
in addition to the mold (ideally free of air-bubbles), casting material, and a release agent.
Note: the gallery 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.
Step 1: Set Up Compressor, Acquire a Pressure Tank, and Read Manuals
First things first, if you have never operated an air compressor, understand that it is a very noisy machine that can be dangerous, especially in combination with a pressure chamber. Read safety manuals before proceeding to make sure you understand how to safely operate the machine. If you are purchasing an air compressor just for use with the pressure tank, a good & relatively inexpensive one is "the pancake" (you'll need a starter kit too). Don't forget to drain it after each usage to ensure its maximum lifespan.
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
Four things need to be done to convert the pressure paint tank into a pressure chamber for casting. For the first three steps that involve attaching a plug or connector, you will need to use thread seal tape. Make sure that all three of these plug/connectors are tightly screwed on.
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.

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74 Comments
2 years ago on Step 2
The metal tube inside the tank unscrews so no hacksaw is needed.
3 years ago
Very glad I found you! Was about to start converting our tank & walk through has helped immensely. Thank you!! :)
Reply 3 years ago
Awesome, glad you found it useful and thanks for the feedback!
4 years ago
Great set-up, nice instructions. I bought everything at Harbor Freight as well. A few thoughts:
1) 1/2" cap for the paint outlet - easy to figure out by bringing the pressure tank lid in.
2) the tube unscrews easily, so no need to use a hacksaw. Just have one person hold the lid and the other unscrews the tube.
3) Buy the starter kit before getting the tape and hose connector, as they come with it. It is much cheaper than buying pieces individually.
4) the ridge at the bottom of the tank, just above where it goes concave, is 9 3/8" across. Re-measure to make sure, then you can have your hardware shop cut a piece of plywood for you and you can screw a smaller piece of wood to the bottom of the plywood to rest on the bottom of the tank.
Reply 4 years ago
Thanks for your thoughts, I'm sure this will be useful for others going forward! Glad it worked out for you!
Question 4 years ago on Step 4
I have successfully made a pressure chamber as described worked perfect the first time at 3 bar then the second time I had troubles getting the pressure chamber pressure up w the compressor cutting out when chamber pressure showed 1 bar but compressor showing 3. So I depressurised retried having turned the regulator knob anticlockwise- no luck. So I degassed and turned regulator knob clockwise- tank got to 2.5 bar then pressure stopped rising so I bailed out leaving tank to sit at that pressure. Any ideas why this has happened and is it the regulator causing problem?
Answer 4 years ago
Hi there! I'm not following - not sure what you mean by "1 bar, 3 bar" etc. Can you please describe what step in the instructable where you're encountering a problem?
5 years ago
this is very cool and innovative!
5 years ago
thanks for your succinct instruction complimented by such detailed & extremely vital information!
5 years ago
Am having trouble with getting cast to work bubble free. Using moldstar 30 with epoxacast 690 with harbor freight paint tank. Degassed moldstar with one stage vacuum pump three times each for full minute followed by rapid pressurization. Once mold was set used same degass process on mix of epocasast before pouring into box mold. Tank is set on side so pour outlets face upward. Final process was placing mold with resin into tank, degassing again before pressurizing to 40psi for 24 hours. Resin cast is being mixed based on weight. Only retains full pressure for few hours however. tank is heated with terrarium heaters which maintains temperature at 80. Is there step I'm missing? material is close to quality and strength I'm looking for but is still bubbley.
Reply 5 years ago
Thanks for your comment. I'm not really understanding or following your process. I'm unfamiliar with the concept of rapid pressurization of your mold agent after degassing or degassing the casting agent before pouring into mold. The process outlined in this instructable doesn't get into degassing - given a mold (that has been de-gassed when it was made, or using an agent like moldstar that effectively self de-gasses), simply pour your casting material and put the mold into a pressure chamber for the duration of the set time and you should be good. I'm not too familiar with epoxacast material, maybe ask the manufacturer for some tips as well. I'd be curious to know your results. Hope this helps and good luck!
Reply 5 years ago
With rapid pressurization meant that mold would be mixed and put into pressure tank and vacumn degassed. Rather than simply turning vacumn off and allowing mold to slowly equalize to 0 psi; I would open valve to rapidly pressurize chamber instead. Not sure if this helps release more bubbles or if degassing for ten minutes or one minute is recommended.
Reply 5 years ago
Am doing vacuum on moldstar mold and epoxcast cast. am getting bubbles primarily on horizontal surfaces. Pressure chamber seems to be removing bubbles from volume but not surfaces. Have been using mold release spray. would using talcum powder be better option for final cast? or better yet might it work with none at all?
Reply 5 years ago
Do you mean the mold itself or the molding agent?
5 years ago on Step 4
WOW! I cannot thank you enough for your succinct instruction complimented by such detailed & extremely vital information!
Everything I needed to know was provided in your instructable. Thank you so much for taking the time to put all this information together (esp the links!) and sharing it with us.
Reply 5 years ago
Thanks so much for your feedback, glad you found it useful! I'd love to see what you've made :)
5 years ago
what about vibration? can't vibration cause the bubbles to move to the surface? i know with urethane you typically have a very limited pot life, but i was just wondering if anyone puts there filled molds on a "vibrating table" (i think there is name for these)???
Reply 5 years ago
I have tried it and the technique doesn't seem to work for resin casts and it actually causes bubbles in silicone (although it works well for plaster). I haven't seen any results close to casts that have been pressurized.
10 years ago on Introduction
Another source of bubbles in the cast is the water from air. I try to do casts where bubbles could be critical over the fall/winter time when humidity goes down to below 45%, and preferably even lower than that. And of course a combination of vacuuming the components and pressurizing the cast is what works best - neither seem to solve the problem completely just by itself.
As far as vacuuming, I usually only vacuum part B, mostly because I often need to add color or mica and those are very potent sources of bubble, especially mica. If you're going for metallic look and need to add mica, there is no way you can cast anything without vacuuming your part B first - your cast will look like Spongebob Square Pants.
I could not grasp from other comments here, are there people here that vacuum the mix of A and B ? Some of the comment sound like this is what happens and I just cannot wrap my head around how's that possible unless you're using a 30+ minute resin like CrystalClear. I usually work with 5-10 min SmoothCast325 - seems to leave you no time to do both vac and pressure. Still, if someone does that on a quick curing mix, I'd love to know how!
Reply 6 years ago
I'm just getting started with resin casting and I'm one of those people who try and read everything before I actually get hands on, and I just wanted to say thank you for this tip as it seems like it will solve a headache before I even run into it!