<p>Yes, there's a secret. You need to use a different solvent if you want no bubbles (but be careful what you wish for).</p>
<p><strong>What the bubbles are</strong></p>
<p>If you were breaking down the Styrofoam and using it immediately, some of the bubbles could be the original air. As you discovered, there's still a tiny amount of trapped air initially, which dissipates if you leave the goo in the acetone for a few days. As an example, say 99% of the air is released initially. If the Styrofoam is 95% air, that means you still have roughly 1 part air for every 5 parts polystyrene, which would be a lot of bubbles. But since you're letting all the air dissipate, that isn't the source of your bubbles.</p>
<p>When you mix polystyrene and a solvent, you can think of the volume of material as roughly the volume of the plastic plus the volume of the solvent. If polystyrene dissolves in that solvent, the distribution of solvent will stay somewhat uniform throughout the material, especially if the material is not too thick. So the material will shrink as the solvent evaporates.</p>
<p>Polystyrene doesn't dissolve in acetone. If it did, you wouldn't get the goo, it would go into solution and the solvent would become a uniform syrupy liquid. With polystyrene, acetone acts somewhat like water does in clay, it allows the plastic molecules to slide around. That makes it soft, and it loses its structure (and the trapped air).</p>
<p>If the goo sits for a long time in acetone undisturbed, the polystyrene settles and compacts a little. After a few weeks, the tightly packed polystyrene goes from translucent to clear, and stabilizes like that. At that point, it isn't gooey or sticky, it's like stiff, rubbery dough. The reason it's like dough instead of plastic is that there is still a lot of acetone in it.</p>
<p>When you make a casting, as the polystyrene solidifies, the trapped acetone collects in pockets. It eventually migrates out, leaving the pockets behind. That's why it doesn't shrink when it hardens. The bubbles will give you an idea of how much of the gel was acetone.</p>
<p>If you don't want shrinkage, the bubbles are your friend (although it leaves the plastic brittle).</p>
<p><strong>&quot;Non-bubble&quot; solvent</strong></p>
<p>If you don't want bubbles, you need to use a solvent that polystyrene actually dissolves in. Xylene is one, and can be found in paint departments. Limonene (pure) is another, and can be found online. It's much safer than Xylene (and smells like oranges), but also much more expensive.</p>
<p>There are a few ramifications to using an actual polystyrene solvent:</p>
<ul>
<li><p>If you have plenty of time (days), you can get enough Styrofoam to dissolve to build the concentration to a dense, gel-like consistency that will still self-level in a mold.</p>
</li>
<li><p>While making the gel, when the concentration of plastic gets high and the gel gets dense, the air trapped in the Styrofoam will have a hard time getting out of the gel when the Styrofoam breaks down. You may need to let it sit in a sealed container for weeks to get rid of the bubbles.</p>
</li>
<li><p>If you keep the mixture no denser than a thick syrup, like honey, the air will get out in a reasonable time. But the material will have a high percentage of solvent and will shrink a lot when it hardens (and the finished plastic will come at the cost of more solvent).</p>
</li>
<li><p>You don't get a nice-to-handle material. Anything between a syrup and soft gel consistency will be sticky and messy to handle. When it gets very dense, it becomes rubbery and loses most of its stickiness.</p>
</li>
<li><p>With these solvents, the plastic doesn't skin over like it does with acetone. The entire material gets denser and shrinks (although eventually, the surface will harden while the inside is still a bit pliable).</p>
</li>
<li><p>The more solvent it contains, the longer it will take for the plastic to harden. Even a thin casting starting with a dense gel can take months to harden throughout. Gel made from acetone hardens much faster (weeks rather than months).</p>
</li>
<li><p>The gel will stick to many kinds of plastic that you might use for a mold until it is completely dry and hard at the mold surface, which can take half a year or more just for something thin (unless you use mold release).</p>
<p>This stickiness encourages people to use the goo as a general purpose adhesive. It will work for plastics that dissolve in the solvent. But for most other surfaces, it will easily separate once it completely hardens (unless it penetrates the surface and has some mechanical connection).</p>
</li>
<li><p>It can take years for all of the solvent to migrate out. Even after it feels hard and solid (months), there is still some solvent distributed throughout. If you warm the plastic, it will soften and flow again. If you were to cast something and let it harden for months and then leave it in a hot car for hours, you might find that it has slumped.</p>
<p>I've actually used this as an advantage. I made a big batch of xylene gel long ago and kept it sealed in a plastic paint container. Over the years, enough xylene found its way out that it hardened into a tough rubbery material. I cut out chunks of it and warmed it, which turned it back to a dough consistency that could be pressed into a mold.</p>
<p><a href="https://www.instructables.com/Recycling-Styrofoam-EPS-Into-Castable-Styrene-Plas/" rel="nofollow noreferrer">Recycling Styrofoam (EPS) Into Useable Castable Styrene Plastic at Home</a> describes that author's experiments. They came up with a similar solution as their best way to mold recycled Styrofoam. They dissolve the Styrofoam in limonene and then dry it to <a href="https://en.wikipedia.org/wiki/Gummy_bear" rel="nofollow noreferrer">&quot;gummy bear&quot;</a> consistency. They put chunks in a mold and heat it in a toaster oven. It melts and flows to fill the mold, and they bake it for a while to drive off some of the solvent. When it cools, they have a solid plastic casting.</p>
</li>
</ul>
<p><strong>Helpful Hints</strong></p>
<ul>
<li><p>With an actual polystyrene solvent, you can speed hardening by leaving it in a hot place for a few days (xylene boils at 281°F, limonene at 349°F, so if you bake it at too high a temperature, the solvent could create bubbles).</p>
<p>Acetone boils at 133°F, so if you warm the acetone goo much above the temperature of hot water from your faucet, you might get even more bubbles. But you could experiment with keeping the mold warm below that temperature to see if that drives out acetone to reduce the bubbles.</p>
</li>
<li><p>The limonene method described in the link is a nice solution, but it will be very expensive &quot;free&quot; plastic. You can reduce the amount of limonene required by substituting time. Do the Styrofoam dissolving in a sealable, see-through jar. Keep adding Styrofoam to the gel until the Styrofoam doesn't quickly break down. Seal the jar and leave it for a few hours or overnight. If the Styrofoam has all dissolved, repeat with a little more Styrofoam until you get a gel that still has tiny remnants of Styrofoam the next day.</p>
<p>If you go too far and have chunks of Styrofoam that won't dissolve, add a small amount of limonene and let the sealed jar sit overnight. If necessary, repeat until all of it breaks down except for some tiny remnants.</p>
<p>Let the sealed jar sit in a bath of very hot water until the Styrofoam remnants disappear and all of the air bubbles have migrated out (likely many hours to a day). That will give you much denser gel to start with, getting more plastic out of the limonene. You will still need to let limonene evaporate to gummy bear consistency if you want to minimize shrinkage and produce a hard plastic.</p>
</li>
<li><p>Because of the inherent problems (bubbles or shrinkage and ridiculous hardening times), polystyrene gel can be more useful as a binder for filler material if you don't need it to be clear. If the gel is just filling the voids in packed filler, the mixture won't shrink when it hardens. With significantly less gel, plus filler for structure, it will also harden faster. You can use almost any dry, cheap material as filler, which saves most of the plastic needed (and the cost of the solvent). The mixture will be like sticky clay that you press into the mold.</p>
<p>In this form, it will become hard enough to handle sooner (but you may need mold release to easily and cleanly demold it before it is completely hard throughout). If you use a filler that doesn't slump, you can demold it when it's hard enough to handle without damage, and put it on an elevated screen to let it dry from both sides (warming it will speed the drying more).</p>
</li>
<li><p>When the plastic doesn't need to be clear (e.g., it will get painted), and you don't want to mess with blending filler, acetone makes sense. The whole process is much faster, it is easier to handle, and you don't get the shrinkage.</p>
</li>
<li><p>Be aware that polystyrene, at least made this way, breaks down in the stong UV of sunlight. Anything you make from the gel won't survive outdoors long term.</p>
</li>
</ul>

CC BY-SA 4.0
2023-03-06T23:08:30.710Z
This is really useful. Thanks so much! Just for clarification, the linked article mentions a number of other, mainly petroleum-based, solvents that dissolve Styrofoam. All but limonene were eliminated in the article for various reasons. Is there a reason you included xylene in the answer and not any others?
Dolly
CC BY-SA 4.0
2023-03-06T23:27:49.000Z
I've tried all the solvents in the article, plus a couple of others. The article is right about the problems most of them have. Limonene is the nicest to work with, but way more expensive than the others. I sometimes create the gel in bulk for big projects, so want a solvent that is readily available locally in gallon cans. Acetone and xylene do the best job and have the least drawbacks of the other solvents. I use acetone when bubbles don't matter, and xylene when they do. Xylene is also faster drying than limonene, and works better for making glue for plastics and certain other uses.
fixer1234
CC BY-SA 4.0
2023-03-02T19:06:04.460Z
2023-03-03T17:38:48.803Z
2023-03-03T17:38:48.803Z
fixer1234
<p>I'm trying to do low-budget plastic casting. The process is pretty simple. Put Styrofoam in acetone. It dissolves into some goo. Stick the goo in a mold and wait for it to harden.</p>
<p>When I try it, the goo goes into the mold looking good. But after it hardens, it's full of bubbles. It isn't clear where the bubbles are coming from.</p>
<p>The air in the Styrofoam bubbles off as the Styrofoam dissolves and shrinks to a tiny amount of goo. To make sure all the air is out, I've even let the goo sit in acetone for days. It goes from white goo to a uniform, translucent gel, which I take to indicate that the last bit of air is gone.</p>
<p>When it hardens (which can take weeks for even a thin casting), it's clear and hasn't noticeably shrunk. But now it's full of bubbles that make up a substantial amount of the volume.</p>
<p>Is there some secret to casting with this polystyrene goo to get a solid, bubble-free result?</p>

<plastic><casting><material-making><styrofoam>
How to avoid bubbles in castings using dissolved Styrofoam?
Dolly
# How to avoid bubbles in castings using dissolved Styrofoam?

(Asked by: username_0 on 2023-03-02)

I'm trying to do low-budget plastic casting. The process is pretty simple. Put Styrofoam in acetone. It dissolves into some goo. Stick the goo in a mold and wait for it to harden.

When I try it, the goo goes into the mold looking good. But after it hardens, it's full of bubbles. It isn't clear where the bubbles are coming from.

The air in the Styrofoam bubbles off as the Styrofoam dissolves and shrinks to a tiny amount of goo. To make sure all the air is out, I've even let the goo sit in acetone for days. It goes from white goo to a uniform, translucent gel, which I take to indicate that the last bit of air is gone.

When it hardens (which can take weeks for even a thin casting), it's clear and hasn't noticeably shrunk. But now it's full of bubbles that make up a substantial amount of the volume.

Is there some secret to casting with this polystyrene goo to get a solid, bubble-free result?

**Tags:** plastic, casting, material-making, styrofoam


---

## 1 answers

### Answer by username_1 on 2023-03-02. Score: 3 | Accepted answer

Yes, there's a secret. You need to use a different solvent if you want no bubbles (but be careful what you wish for).

**What the bubbles are**

If you were breaking down the Styrofoam and using it immediately, some of the bubbles could be the original air. As you discovered, there's still a tiny amount of trapped air initially, which dissipates if you leave the goo in the acetone for a few days. As an example, say 99% of the air is released initially. If the Styrofoam is 95% air, that means you still have roughly 1 part air for every 5 parts polystyrene, which would be a lot of bubbles. But since you're letting all the air dissipate, that isn't the source of your bubbles.

When you mix polystyrene and a solvent, you can think of the volume of material as roughly the volume of the plastic plus the volume of the solvent. If polystyrene dissolves in that solvent, the distribution of solvent will stay somewhat uniform throughout the material, especially if the material is not too thick. So the material will shrink as the solvent evaporates.

Polystyrene doesn't dissolve in acetone. If it did, you wouldn't get the goo, it would go into solution and the solvent would become a uniform syrupy liquid. With polystyrene, acetone acts somewhat like water does in clay, it allows the plastic molecules to slide around. That makes it soft, and it loses its structure (and the trapped air).

If the goo sits for a long time in acetone undisturbed, the polystyrene settles and compacts a little. After a few weeks, the tightly packed polystyrene goes from translucent to clear, and stabilizes like that. At that point, it isn't gooey or sticky, it's like stiff, rubbery dough. The reason it's like dough instead of plastic is that there is still a lot of acetone in it.

When you make a casting, as the polystyrene solidifies, the trapped acetone collects in pockets. It eventually migrates out, leaving the pockets behind. That's why it doesn't shrink when it hardens. The bubbles will give you an idea of how much of the gel was acetone.

If you don't want shrinkage, the bubbles are your friend (although it leaves the plastic brittle).

**"Non-bubble" solvent**

If you don't want bubbles, you need to use a solvent that polystyrene actually dissolves in. Xylene is one, and can be found in paint departments. Limonene (pure) is another, and can be found online. It's much safer than Xylene (and smells like oranges), but also much more expensive.

There are a few ramifications to using an actual polystyrene solvent:

* If you have plenty of time (days), you can get enough Styrofoam to dissolve to build the concentration to a dense, gel-like consistency that will still self-level in a mold.
* While making the gel, when the concentration of plastic gets high and the gel gets dense, the air trapped in the Styrofoam will have a hard time getting out of the gel when the Styrofoam breaks down. You may need to let it sit in a sealed container for weeks to get rid of the bubbles.
* If you keep the mixture no denser than a thick syrup, like honey, the air will get out in a reasonable time. But the material will have a high percentage of solvent and will shrink a lot when it hardens (and the finished plastic will come at the cost of more solvent).
* You don't get a nice-to-handle material. Anything between a syrup and soft gel consistency will be sticky and messy to handle. When it gets very dense, it becomes rubbery and loses most of its stickiness.
* With these solvents, the plastic doesn't skin over like it does with acetone. The entire material gets denser and shrinks (although eventually, the surface will harden while the inside is still a bit pliable).
* The more solvent it contains, the longer it will take for the plastic to harden. Even a thin casting starting with a dense gel can take months to harden throughout. Gel made from acetone hardens much faster (weeks rather than months).
* The gel will stick to many kinds of plastic that you might use for a mold until it is completely dry and hard at the mold surface, which can take half a year or more just for something thin (unless you use mold release).

  This stickiness encourages people to use the goo as a general purpose adhesive. It will work for plastics that dissolve in the solvent. But for most other surfaces, it will easily separate once it completely hardens (unless it penetrates the surface and has some mechanical connection).
* It can take years for all of the solvent to migrate out. Even after it feels hard and solid (months), there is still some solvent distributed throughout. If you warm the plastic, it will soften and flow again. If you were to cast something and let it harden for months and then leave it in a hot car for hours, you might find that it has slumped.

  I've actually used this as an advantage. I made a big batch of xylene gel long ago and kept it sealed in a plastic paint container. Over the years, enough xylene found its way out that it hardened into a tough rubbery material. I cut out chunks of it and warmed it, which turned it back to a dough consistency that could be pressed into a mold.

  [Recycling Styrofoam (EPS) Into Useable Castable Styrene Plastic at Home](https://www.instructables.com/Recycling-Styrofoam-EPS-Into-Castable-Styrene-Plas/) describes that author's experiments. They came up with a similar solution as their best way to mold recycled Styrofoam. They dissolve the Styrofoam in limonene and then dry it to ["gummy bear"](https://en.wikipedia.org/wiki/Gummy_bear) consistency. They put chunks in a mold and heat it in a toaster oven. It melts and flows to fill the mold, and they bake it for a while to drive off some of the solvent. When it cools, they have a solid plastic casting.

**Helpful Hints**

* With an actual polystyrene solvent, you can speed hardening by leaving it in a hot place for a few days (xylene boils at 281°F, limonene at 349°F, so if you bake it at too high a temperature, the solvent could create bubbles).

  Acetone boils at 133°F, so if you warm the acetone goo much above the temperature of hot water from your faucet, you might get even more bubbles. But you could experiment with keeping the mold warm below that temperature to see if that drives out acetone to reduce the bubbles.
* The limonene method described in the link is a nice solution, but it will be very expensive "free" plastic. You can reduce the amount of limonene required by substituting time. Do the Styrofoam dissolving in a sealable, see-through jar. Keep adding Styrofoam to the gel until the Styrofoam doesn't quickly break down. Seal the jar and leave it for a few hours or overnight. If the Styrofoam has all dissolved, repeat with a little more Styrofoam until you get a gel that still has tiny remnants of Styrofoam the next day.

  If you go too far and have chunks of Styrofoam that won't dissolve, add a small amount of limonene and let the sealed jar sit overnight. If necessary, repeat until all of it breaks down except for some tiny remnants.

  Let the sealed jar sit in a bath of very hot water until the Styrofoam remnants disappear and all of the air bubbles have migrated out (likely many hours to a day). That will give you much denser gel to start with, getting more plastic out of the limonene. You will still need to let limonene evaporate to gummy bear consistency if you want to minimize shrinkage and produce a hard plastic.
* Because of the inherent problems (bubbles or shrinkage and ridiculous hardening times), polystyrene gel can be more useful as a binder for filler material if you don't need it to be clear. If the gel is just filling the voids in packed filler, the mixture won't shrink when it hardens. With significantly less gel, plus filler for structure, it will also harden faster. You can use almost any dry, cheap material as filler, which saves most of the plastic needed (and the cost of the solvent). The mixture will be like sticky clay that you press into the mold.

  In this form, it will become hard enough to handle sooner (but you may need mold release to easily and cleanly demold it before it is completely hard throughout). If you use a filler that doesn't slump, you can demold it when it's hard enough to handle without damage, and put it on an elevated screen to let it dry from both sides (warming it will speed the drying more).
* When the plastic doesn't need to be clear (e.g., it will get painted), and you don't want to mess with blending filler, acetone makes sense. The whole process is much faster, it is easier to handle, and you don't get the shrinkage.
* Be aware that polystyrene, at least made this way, breaks down in the stong UV of sunlight. Anything you make from the gel won't survive outdoors long term.

> **Comments:**
> **username_0**: This is really useful. Thanks so much! Just for clarification, the linked article mentions a number of other, mainly petroleum-based, solvents that dissolve Styrofoam. All but limonene were eliminated in the article for various reasons. Is there a reason you included xylene in the answer and not any others?  
> **username_1**: I've tried all the solvents in the article, plus a couple of others. The article is right about the problems most of them have. Limonene is the nicest to work with, but way more expensive than the others. I sometimes create the gel in bulk for big projects, so want a solvent that is readily available locally in gallon cans. Acetone and xylene do the best job and have the least drawbacks of the other solvents. I use acetone when bubbles don't matter, and xylene when they do. Xylene is also faster drying than limonene, and works better for making glue for plastics and certain other uses.  

---

