loading
Picture of faux concrete ultra-lightweight pot
Maybe you've seen those pots in the stores that look very heavy but are in fact very easy to lift?  I think this is a great idea, however their prices are still unreasonably high. 

Here's one way to make a large lightweight pot that could pass for being made of concrete on casual inspection. 











 

 
 
Remove these adsRemove these ads by Signing Up

Step 1: Why bother?

Picture of why bother?
I wanted to plant some things and went to get my favorite large pots from the backyard, but look what happened to them!  I pondered how to fix them and decided I would just try to make something new.

Well sure, it's a lot less effort to just go buy a pot, but I always wondered if I could mold things out of spray foam insulation, and after a few tries, I found out that I could.

It remains to be seen how long it will hold up to the elements and whether plants will tolerate being in a pot made of polyurethane foam.

Step 2: Warnings, disclaimers

Picture of warnings, disclaimers
cugogglesign.jpg
This project uses spray foam insulation which carries warnings about flammability and hazardous vapours.  If you try this be sure and read the label and observe the cautions listed.

Likewise, spray paint is employed at the final step and that too has its risks to be mindful of.

Wear a vapour mask, safety glasses and gloves and wear clothes you don't care about if they get anything on them.

The spray foam insulation material is extremely sticky, if you get it in your hair, you pretty much have to cut off your hair to get it out, so if you have long hair, you might want to pull it back out of the way.  Same thing for clothes, if you get it on your clothes, your best bet is to let it cure completely, about 24 hours and then you stand a chance of pulling it off the fabric.  Don't attempt to wash it out right away, you will only succeed in embedding it into the fibers and then it simply will never come out. 

If you get it on your skin, you can wait for it to cure.  They say that acetone can be used to remove it, though the times it has happened to me, I just put up with it till it cures and then you can just peel it off for the most part.


Note that foamcore does not require much pressure to cut if the blade is sharp.  Always use a straight edge when cutting to help protect your fingers, don't try to freehand it.

Step 3: Materials

Picture of materials
bigpot2.cutting.jpg
Img_3155.jpg
Img_3157.jpg
Img_3159.jpg
Img_3153.jpg
three sheets of foam core to be the walls of the mold:   $1.00/sheet Dollar Tree                 
box cutter
t-square
ruler or straight edge
pen/pencil

two cans of Great Stuff Spray Foam Insulation or similar:  $3.00 - $3.50 /can
spray paint in some color                                 $3.50  - $5.00 /can

duct tape

a big cutting surface, such as a large board

disposable gloves
vapor mask
safety glasses

cutting plans (attached)




Step 4: Cut out the mold pieces

Using the cutting plans (or the .mtc files attached), cut out the 11 pieces indicated. 

Four outer side panels
four inner side panels
Outer bottom
inner bottom
top frame

I used a craft cutter to cut templates out first, and then just traced those onto the foam core before cutting them.



Step 5: Tape mold pieces together

Picture of tape mold pieces together
Img_3189.jpg
Img_3190.jpg
Img_3194.jpg
Lay out four trapezoids side by side as pictured and tape them together where their edges meet.

Apply tape to one of the outer edges, fold it in and then fold in the trapezoid on the opposite side on top of the exposed tape to complete the main shape.



Insert the bottom square and tape it into place.  Mine isn't quite a perfect fit, but it is enough to ensure that the overall shape is generally square.


Turn the shape over and retape the bottom on.  Close up any open seams with tape.

Repeat the same steps for the other trapezoidal shape.

 

Step 6: Leave room for a drainage hole

Picture of leave room for a drainage hole
Img_3219.jpg
cut out six equal small squares of foamcore and tape those together, then tape them to the inside bottom of the outer box shape.

Step 7: Fill the mold with spray foam

Picture of fill the mold with spray foam
Img_3229.jpg
Img_3365.jpg
Img_3337.jpg
eunjung.jpg
Put on the gloves and vapor mask and the crappy old clothes at this point.  Put your hair out of the way and try not to brush your hair out of your face if you have spray foam stuff on your hands, or you'll have it in your hair and that will be tough to get out and you might have to get a severely uneven haircut or something, but you will definitely have a lot of trouble getting it out of your hair.  Definitely don't try to comb it out or you will have a permanent bouffant

User Cambenora adds:  do this outside, with no flames anywhere nearby!

Begin by shaking the spray foam as indicated on the packaging.  Also, be sure to check the expiration date on the bottom of the can.  The first time I tried this, I used some that had been expired for six months, so it didn't expand very much and I had to start over.

Spray some in around the drainage hole shape, then place the inner trapezoidal box shape (actually called a "frustrum") on top of that, inside the larger frustrum shape.

Fill in the remaining void with spray foam, but leave room for expansion, otherwise you'll probably have a seam failure somewhere.

If the spray foam is new and within its use by date, you can probably expect it to expand at least 50% of the volume you put in, so if you fill the container 2/3 full with the stuff, hopefully you won't be overfilling it when it is done expanding.  However you don't want to underfill it or you will have voids in it which you would have to try to fill in later.

Insert the inner frustrum if you haven't already and then center it using the top frame shape.

Tape everything together good with duct tape, reinforce all the seams and then set the whole thing aside for at least 48 hours.

The package tells you that it fully cures within 24 hours, but I found it takes longer, possibly because the stuff does not get as much air exposure in the mold as it would usually.

Step 8: Tear away the mold

After two to three days, I spent about an hour tearing the foamcore mold away from the finished shape.  I broke a nail or two as well.

I probably ought to figure out some sort of mold release step to make this easier...  

Step 9: Paint it!

Picture of paint it!
Img_3402.jpg
Img_3404.jpg
IMG_3406.JPG
Img_3405.jpg
Img_3399.jpg
Remember the vapor mask, safety goggles along with the old clothes...

Unlike styrofoam, this foam takes spray paint well and doesn't deform from it. 

I painted these with some gray and brown outdoor grade spray paint to give them a look something like stone.

Step 10: Progress report

Picture of progress report
Img_3417.jpg
Okay, I've let the paint dry for a few days and now I've planted some things in them.

Geraniums are nigh unkillable, so that will be the canary plant.  I usually kill lobelias first, I added some of those anyway.  And some other random plants.

One thing I did notice was that the drainage hole was too large, so I added a sheet of plastic netting at the bottom to keep the soil from falling out.

I watered thoroughly and did notice a tiny bit of seepage here and there, but nothing major.

I'll try to remember to post another progress report in a week.



Ninzerbean2 years ago
One other thing I want to add is when it is really hot outside, say in the 80's, the foam drips and drips - and it gets on everything and yes I got it in my hair and had to cut it out.
foobear (author)  Ninzerbean2 years ago
yup! been there!
PS1182 years ago
I'd like to do this, but I'd want to do several matching ones for all around my deck.

The problem is I don't really want to buy a jillion cans of "great stuff". Is there a way to get this foam stuff more cheaply -- like by the gallon or something?
foobear (author)  PS1182 years ago
Not sure about that, but if you find a good deal somewhere, let us know
It comes as a two part mix, you can google it under casting with expandable foam, lots of youtube videos.
saturnengr2 years ago
I found this to be a great idea for duplicating a hard-to-find pot for my wife, but I also found myself wondering if I could use it for other tasks. It seems that, properly painted and protected, that this would make a nice method for forming other solutions also. The foam core seems to be used both to provide a smooth boundary for the foam as well as the strength to resist the expansion of the foam, forcing it evenly into the spaces between the form. But I believe you're destroying the form each time.

I was hoping you could comment on a way to reuse the forms. Perhaps a wooden form covered with wax paper or even plastic (you can get big, cheap rolls of plastic), allowing the wax paper/plastic to be destroyed when you remove the forms, but leaving the wooden forms for use again.

In my case, I was hoping to make about a half a dozen of the same mold, which would be a small trough-like planter about two feet long, but only about 6 inches or so high. I can easily make a mold out of wood from hobby store then cover the wood in wax paper/plastic and make about one or so copies every day. In this manner, each copy would be nearly identical.

In any case, thank you for the wonderful idea.
foobear (author)  saturnengr2 years ago
You're welcome. I have heard that this type of foam will not stick to plastic trash bags, so if you can devise a method to line your mold with that type of plastic, you might have a reusable mold there... Good luck and let us know how it turns out. Thanks for your comments!
jlyvers7432 years ago
This is a fantastic idea and your instructable is very clear and easy to follow. Have you filled any of these with dirt and watered the plant, to see if it can withhold the weight? Thank you for posting.
foobear (author)  jlyvers7432 years ago
See the last step. I planted them today and I will post pictures of their progress over time
foobear (author)  jlyvers7432 years ago
Thank you! I was making sure all the paint was dry first. I'll probably try that tomorrow and post a pic.
Cambenora2 years ago
Excellent 'ible, RaisedByRobots!

I think polyurethane foam is a fantastic material, but you're right, there are some precautions that should be taken, particularly the flammability. Take it from one who has found himself knee deep in a pool of flame - do this outside, with no flames anywhere nearby!

For the curing time, before inserting the inner frustrum, try spraying the foam with water, using one of those misting sprayers you use for ferns and such. Just use a little water. This will speed up the curing time. It may make it foam a little more as well. Reacting with the moisture in the air is what makes it behave the way it does.

Incidentally, for those playing at home, make sure the foam you get is polyurethane. There are water based acrylic foams on the market which, while intended as a non-toxic, water based alternative to the polyurethane, actually behave in a completely different way, and would not be suitable for this particular task. Once dried, (it doesn't "cure" as such) it has the consistency of sponge cake.
foobear (author)  Cambenora2 years ago
Interesting, I may try the water spraying next time. I'll also be looking out for that water based one.. Does it have a name? thankx!
The water based one is Selleys Space Invader. That's an Australian product, so I don't know whether it would be available in your part of the world. But there's sure to be an equivalent.
BTW I'm not criticizing Space Invader (in case any Selleys people are reading ;-). It's advantages are: non-toxic, non-flammable and easy to clean up using water. You'll probably find these things written on the can. That's how you can tell you got the non-polyurethane one.

jim_lewis12 years ago
it's a nice effect and good result, but I suspect you'll find issues with water/soil working its way through the sides.
However you could of course just put the soil in a plant pot inside this pot to avoid this issue.
If you were so inclined you could also embed LED lights, (or anything really) into the pot prior to forming
foobear (author)  jim_lewis12 years ago
Sure, it still needs to stand the test of time. I'm more concerned about sunlight and exposure wearing it down too. Solar powered led night lights is a great idea too.