Introduction: Bubble-Free Resin Casts With Modified Paint Tank

Picture of 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

Picture of 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

Picture of 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

Picture of 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 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

Picture of 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.


rajimon (author)2017-11-12

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)???

thelostspore (author)rajimon2017-11-12

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.

elabz (author)2013-02-17

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!

Cat Magic (author)elabz2017-02-20

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!

thelostspore (author)elabz2013-02-17

yeah, I'm wondering too how people can vac & pressurize resin at same time, though it could be possible with 326 or 327.

as for the degassing, I've never heard of vacuuming only one part. sounds like your application is pretty specific though, but I still think that you'll induce more air bubbles if you mix by hand after degassing.

elabz (author)thelostspore2013-02-17

Part A is less viscous (seems to me, gotta check the tech sheet though) and bubbles seem to come up much faster than in part B.  I just shake the Part A can well in advance of casting and just leave it sit, bubbles seem to come up by themselves and are mostly gone in 30 mins or so. Part B can sit with bubbles for hours. I guess they will eventually come up, too, but it's just easier logistically to help them by vacuuming.

Anyhow, the main reason I vacuum Part B is because there's usually much work that needs to be done with it - mixing paints and sometimes mica - both of which leaves plenty of bubbles and mica in particular is extremely bubbly.  If you do need mica, you have to work quickly before it starts separating to the bottom, so you just cannot wait for the bubbles.

I guess you can say the application is pretty specific though - together with my wife we are casting parts of fantasy BJD art dolls (samples here)  and colors are very important. There are no parts that don't have at least one of two different paints and mica is used often for parts that need metallic or pearly shine, such as mermaid tails and such.

By the way +1 on vouching for Smooth-On, those guys are great, most especially in art casting / mold making, which is what I'm more familiar with. Their resins are very UV resistant, too - important for pieces that you're going to be displaying in bright light.


thelostspore (author)elabz2013-02-17

Cool stuff, thanks for sharing, it's always nice to see what others are doing with these materials.

Just for people who may not be aware, not all smooth on resins are uv resistant. Ex. Crystal clear is but 325 series is not. However there is a one part curative called "sun devil" that can make 325 uv resistant.

chadroth (author)2017-02-05

Hey guys I have got a revolutionary vacuum/pressure tank that you got to check out

UncleCory (author)2014-10-06

where did you get the mold? I need that so bad!

eruger (author)UncleCory2016-09-05

If you can't find or afford the crystal you want, you could also make it using plaster. Pour a block, rough-cut it with a junky saw, finish the flats by wet-sanding with sandpaper glued to a board. (you'll go through some sandpaper doing it, which is one reason why you probably don't want to use a sander with its expensive belts) You can fill any holes with more plaster. Wash all the dust off, let it dry completely, wax it good with paste wax and buff. You're good to go, and the wax should act well as a release.

thelostspore (author)UncleCory2014-10-06

I made was pretty easy. Just got a quartz crystal from a local hippie shop, glued it to acrylic base, built up a wall around it, applied release, and used SmoothOn's easy-to-use, 1:1 ration Mold Star silicone. There are some good instructables on mold-making...I've been drafting one too for a long time...stay tuned!

Maniacy made it! (author)2016-07-18

Thanks, mate.

AndrewB52 (author)2016-06-26

Is it possible to use the Venturi device to pull a vacuum in the pressure it strong enough? Can enough vacuum be pulled?

denisvanb (author)2016-05-25

Hi all !

I build aircrafts models in 1/72 scale and I use to cast resin parts, sometimes full kits of rare aircrafts. I use pressure casting, and I always pressure cast silicon molds.

I would say the molds have to be pressure cast, I first tried pressure resin casting with non de-bubbled molds and I found the resin was forced in the sub-surface air bubbles of the molds, and the parts were covered with pustules...

I also tested vacuum de bubbling, the main problem is the need of a vacuum pump, plus a compressor... Vacuum de-bubbling is faster, as pressure degassing of the silicone molds requires to keep the molds under pressure during the 12 hours or more of their curing...


araiza444 (author)2016-05-10

Hi! Can the mold also be pressure casted or does it have to be vacuum degassed? I would like to make good quality casts (and compatible molds) but would like to stay away from having to purchasing both a pressure tank and a complete a vacuum degassing system.

Thanks : )

thelostspore (author)araiza4442016-05-17

If I understand your question, you are asking if you can pressure cast the material of the molding agent during the mold-making process as an alternative to vacuum degassing. The simple answer is NO.

If you are using silicone as your molding agent, it MUST be vacuum degassed before the mold material is poured or else you will get measles on your casts and your mold will be damaged if you pressure cast.

Stay tuned for another instructable that will show how to use this tank as your degass chamber (I've been working on it for a while :)

ChrisC419 (author)thelostspore2016-05-19

You are in fact wrong on this point. RTV silicone can be pressure cast when making your mould. We do this all the time. You simply pour your moulding material into you mould box over the master and stick whole thing in the pressure pot.

thelostspore (author)ChrisC4192016-05-21

Do you have any evidence or results to support your claim? Just because you do it in your shop doesn't mean you're following a best practice.

After years of working with these materials and talking to technicians at companies like Smooth-On, I've learned that the present tried-and-true best practice for removing air-bubbles in silicone is vacuum de-gassing. That being said, there are many low-tech ways to reduce air-bubbles during silicone mold-making, and pressure casting can be a component in one of those workflows.

After talking with some more technicians this week, I've realized that my original comment is a bit extreme - if you are not de-gassing silicone, you can pressure cast in combination with other techniques such as the ribbon pour (and perhaps use certain products that help to reduce air-bubbles), however, I'm told this will only coalesce, not eliminate bubbles. And you will still have the measles problem when you subsequently pressure cast with plastic inside of the mold. When I said "you MUST vacuum de-gas.." what I really meant was - if you intend to pressure cast plastic inside of a silicone mold (which this instructable covers), you will damage your mold unless you de-gas the silicone when making the mold. I personally have used similar techniques to avoid de-gassing but have ruined every single mold I have subsequently subject to pressure casting. This is my experience, but if you have evidence that proves this to be untrue, I'd love to see your results.

araiza444 (author)2016-05-10

By the way! I really appreciate your instructable on this topic. I was not aware that pressure casting was such a successful process.

thelostspore (author)araiza4442016-05-17

Thanks !! :)

ChrisM41 (author)2015-06-09

Is it possible to fill in any bubbles in the rtv mould?

thelostspore (author)ChrisM412015-06-10

I'm not exactly sure what you mean - where are the bubbles? Do you mean on the exterior of the mold? Feel free to upload a picture.

ChrisM41 (author)2015-06-09

Could I use my wax injector as a pressure chamber? Not sure how much pressure I can pump it up to but I'm guessing off the cuff its about 35/40 psi. Also how long would I leave it in there? I'm not an engineer and the set up you describe is meaningless to me...Thanks in advance

thelostspore (author)ChrisM412015-06-10

The setup described here is meant to be as straightforward as possible - you don't need to be an `engineer` to modify a pressure paint tank (but you will need to know how to operate a compressor, which doesn't require a degree...sounds like you already know how to do this).

I'm not familiar with wax injectors but if the chamber can be pressurized (~35 PSI), you might be able to use it. For best results, try leaving the mold in there for the duration of the casting agent's cure time. Experiment and let us know what you find!

FayP (author)2015-05-14

I don't see a lever that turns off the air once it gets into the pressure pot. Where is it? and another question, do you leave the compressor running the whole time the it is connected to the pressure pot? I'm not getting it.....Thank you

thelostspore (author)FayP2015-05-15

The regulator is the mechanism that allows you to dial in the desired PSI (this is the metal knob underneath the gauge). Your compressor will also have its own dial (it'll need to be set to something higher than the desired PSI of the chamber). The compressor will stop doing its thing once the desired pressure is established. Use your regulator(s) to dial in on the target pressure. Hope this helps.

hyacinthlove (author)2014-01-25

i have been looking in local hardware stores as well as online but i can't seem to find the 3/8" npsm fine thread cap. do you know if there is a different name for this part or can you suggest somewhere to find it? thanks for the awesome tutorial!

I had to do a bit of research to dig up the specific name for that part after I found it, as it was found loose in a bin with no packaging information. Turns out the paint outlet threading is not compatible with the standard 1/4" npt compressor couplers and plugs, which made it a little more difficult to locate. It was probably in the automotive section where I found it (Harbor Freight of North Hollywood), but if you're looking in local hardware stores you might want to try the plumbing department.

Robot Lover (author)2013-02-15

My dad has been using resin for half his life and has always used vacuum in an old pressure cooker. It works perfectly. Although, pressure does work as well, I just find that vacuum works better.

steveastrouk (author)Robot Lover2013-02-17

The highest pressure you can achieve is 14 PSI in a vacuum, you are limited only by the safe rating of your tank the other way. Me, I'd degass with vac, and THEN squeeze it.

Robot Lover (author)steveastrouk2013-11-13

I just recently got into the mold making work a little bit more. Typically what we do (depending on the material's pot life) is mix the compound and then set it in our vacuum pot. Then we draw a vacuum on the pot and this will make the air bubbles turn microscopic, and I mean so small that they basically don't exist (unless you have a microscope). So we would use this process to de-gas silicone for molds. If we are casting a part we might put it in the vacuum chamber to cure to get out any air bubbles. If I remember, I will take some pictures of this silicone mold we made that was de-gassed with the vacuum pot.

thelostspore (author)Robot Lover2013-02-15

Cool, it'd be great to see the results, and A/B it with a pressure cast. Have you done this? Maybe make an Instructable on it! I'd like to see the gear he's using.

chadisrhad (author)2013-10-26

trying to find the right air compressor was looking online for a quiet one need it for a 5 gal tank will this one work?

thelostspore (author)chadisrhad2013-10-27

looks like it'd work but it'd probably be overkill (this process only needs 35 psi)

cameraeyes (author)2013-04-29

I just bought this same unit and was stumped by the little hole built into the regulator next to the T-handle. It's purpose-built, leaks 100% of the time, and I decided to thread it for a #4 machine screw and epoxy one in. Anyone else see/change this?
For that matter, my safety valve was busted right out of the box—popped at about 5 psi. When my local hardware store didn't have a replacement, I bought a stiffer spring to put in it and although it doesn't pop at 60 psi as it should, it doesn't take much more to set it off.
I should say: Don't modify your pot! And I did this Saturday and the epoxy is still curing; if anyone sees giant red flags here please reply. Ha. Thanks.

thelostspore (author)cameraeyes2013-05-05

The giant red flag is that you didn't return your busted unit, and it seems that you aren't using the regulator correctly. It sounds like what you are doing is potentially dangerous, I would advise against epoxying ANYTHING on the unit.

cameraeyes (author)thelostspore2013-05-06

Yeah. Absolutely. I would normally send it back, of course, but it's a time-sensitive project at work; had to have prototypes out asap...
FWIW the little hole I plugged up seems fine. Still have no idea why there'd be a built-in constant leak (right across from the safety valve, no less). I wear goggles and tightly control the pressure before leaving it for the rest of the 30-minute pot life of the material.

stephenfitton (author)2013-04-14

Question,I haven't tried it . But would an Ultrasound water bath around the mould encourage the bubbles come to the surface like they do when vibrating concrete.
Thanks for the Indestructible.

vibration techniques don't work for resin & silicone used for concrete and plaster

Edgar (author)2013-02-19

Many Inventors will use this, for countless Gizmos, thanks! .D
Went to my Blog:

thelostspore (author)Edgar2013-02-19

Awesome, thanks!

Edgar (author)thelostspore2013-02-19


thelostspore (author)2013-02-19

Thanks to everyone for posting their comments, I've learned a lot too! I've amended the first part of the Instructable based on the information people have provided in addition to some more research I've conducted.

dangre (author)2013-02-19

Here are a couple samples using Alumilite Clear. I used vacuum only after mixing. The optical distortion is caused by the shape of the mix cup I used as a mold.

dangre (author)2013-02-18

I do a lot of casting and the way we do it is to vacuum the resin after mixing (you can also vacuum each before hand if you think it has a lot of bubbles). You need to pull at least 26 in hg to be really effective. Then pour the resin into your mold like pouring a good beer to lesson the addition of bubbles. You can mix and vacuum in less than a couple minutes. You want to get rid of as much suspended air as possible.  The resin looks like it is boiling under vacuum until all the air is removed and then it goes as clear as glass.   Pressure casting will drive the bubbles to a smaller size but they are still there if not removed with vacuum first. The ultimate is to do both vacuum and then pressure. If your part or mold is porous or may contain moisture you may get bubbles that emit from your part or mold. Dry your part and mold thoroughly. I use a food dehydrator for encapsulating food or insects. I have great success with Alumilite clear resin and it looks like crystal with NO bubbles using vacuum alone. They have a bunch of great instructional videos. Dan

iceng (author)2013-02-17

Back when I did fine casting of a 45 plastic record, it was important to avoid the tiniest bubble protrusion. 

The vinyl record makes music by vibrating a stylus but I didn't want pops from micro bubbles.
The whole casting was put in a windowed vacuum chamber and pulled successively harder
vacuum re pressure and vacuum again to avoid over frothing the back of the new epoxy record
until no more bubbling occurred.

Then slowly release to normal pressure for the duration of the epoxy cure.  It worked very well.
I still have it.


Tomdf (author)2013-02-15

I knew about degassing silicon but I'd never heard of pressurizing the cast. The results really speak for themselves, I just can't believe it! Thanks for the (well written) info :D

What exactly does pressurizing do? Does it shrink all of the bubbles into oblivion, or does it bring them to the surface?

thelostspore (author)Tomdf2013-02-15

I'm not exactly sure but my thinking is that the pressure forces the bubbles through a mold's sprues. If there are no sprues and only an air vent, mold measles are more likely to occur, unless you're using a block mold with an open face (because there is plenty of room for air to escape). I don't think the air bubbles "dissolve" though, but rather are expelled through force.

elabz (author)thelostspore2013-02-17

I'd say the pressure works by keeping the bubbles small - they don't really go anywhere, you just can't see them. It cannot be "forcing bubbles out" simply because not all molds are made to allow for an easy evacuation of bubbles and some are made just plain wrong (sprue and vents in wrong places) yet even those would benefit from pressurizing, at least as far as the amount of bubbles inside the material is concerned (as opposed to bubbles  ontop of material). That's why vacuuming works well - it actually removes the bubbles, not suppresses them.

Tomdf (author)thelostspore2013-02-17

Cool, thanks for the response. I'm learning a lot of useful stuff here.

About This Instructable



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