Make Your Own DIY Concrete Planter Mold For A Triangle Planter With Copper Accents.
So you want to learn how to make a triangle concrete planter mold? Well, this is a fairly easy way to do it and the beautiful thing is that the material is inexpensive and you can create just about any shape- though curves can be a bit more challenging.
I’ve had the design of this planter in mind to make for a long time. Unfortunately, my goal was to make it a really dark grey, almost black with a greenish tint because I thought it would set off the copper accents beautifully, but I wasn’t quite successful at achieving that.
I still am happy with the way it turned out though. It’s amazing how nice the combination of metal and concrete can look, especially when it’s a nice warm color like the copper is. There’s just so much you can do with cement crafts!
Well, the bottom line is that if you can’t find exactly the right size or shape for your molds, don’t be afraid to make your own concrete molds!
Some of the links on this page have been provided as a convenience for finding materials. These links may also be affiliate links. As an Amazon Associate I earn from qualifying purchases, at no extra cost to you. For each project, I do lots of tests and if a material or tool doesn’t work, I won’t list it. Click here to read my full disclosure policy.
Quick Links In Post
DIY Difficulty Level | Moderate
The reason for the moderate rating is because there’s a lot of careful measuring and cutting of multiple pieces. That part feels tedious and you do need to measure accurately so everything lines up.
Materials For How To Make A Triangle Concrete Planter Mold
- Cement, Rapid set
- Disposable bowl
- Disposable cup
- Durable nitrile gloves- I like these because they are so durable, you can rinse and reuse and they end up costing less in the long run
- Safety glasses
- Safety mask
- Black & Green pigment
- Copper embossing metal sheet
- Utility knife/ box cutter
- X-acto knife
- T-square- (with a corked back) -is ideal
- Acetate sheet .005 or .003 and at least 12” long- I used one that was 40×24”- longer makes it a little easier.
- Corrugated plastic sheet– ideally a minimum of 32” long, but must be at least 12”
- Packing Tape
- Blue tape
- *Foam board scrap or foam core piece- optional
- Drinking straw
- Smooth mat or board to use as the base
- Spray adhesive
- Hot glue gun
- Sharpie marker
- 2-Part epoxy
- Craft sticks
- #220 grit sanding sponge
Optional or May Not Be Necessary- good to have on standby
- Pliers- for demolding- may not be necessary
- Putty scraper- for demolding- may not be necessary
I use the foam to help stabilize the straws that I use to create drainage holes. This way I don’t need to hot glue them in place, which makes them easier to remove.
Tutorial Steps For How To Make A Triangle Concrete Planter Mold
The corrugated plastic is going to become the mold material to create your triangular shape. Corrugated plastic is a great option for creating molds since it is flexible, easily cut and inexpensive. The drawback is that it has ridges that will show up in concrete. So to hide the ridges and keep it smooth, I use it in combination with acetate sheets to it. This keeps it smooth as glass.
However, if you don’t want your planter to be glossy, just lightly sand it with a 220 grit sanding sponge and it will remove the sheen.
Step 1 | Cut The Corrugated Plastic To Size
The first step for how to make a concrete planter mold is to cut out the corrugated plastic to the appropriate sizes. Start by using the T-square, mark a length of 32” x 3” wide.
Be sure you orient the corrugated plastic board so that the length is against the “grain”. You want the corrugated lines short, not long- this makes it very easy to score and bend instead of having to cut every piece separately and then taping together.
Now with the box cutter and T-square, cut the corrugated plastic to size.
Next, measure and mark one wall of the triangle at 11 ¾”, then the second wall also at 11 ¾” and the last one at 8 ½”.
You are going to fold these corrugated plastic legs- or what will be the walls. To do this, just score it by cutting through only one side, not all the way through both sides. This is where you will bend the plastic.
Now mark and cut the inner mold. The overall length will be 24 ½” and 2 ½” wide. Score and cut one side so that the first two walls will be 9”, and the last wall will be 6 ½”.
Using blue tape, temporarily tape the inner mold on the outside corner where the walls meet. Check to make sure your triangles fit and have about a ½ to ¾” of space between the walls.
If you like modern planters, then you may also want to check out the tutorial for the Modern DIY Concrete Planters. For planters that are little more organic looking, there’s the Cement Balloon Planters.
Step 2 | Create A Lid For The Inner Cement Planter Mold
You need to make a top for the inner mold. Trace the inner mold over a piece of the corrugated plastic. You want the lid to fit inside so that it doesn’t overlap the inner planter mold- which will create undercuts. The undercuts will cause the top to get stuck on the bottom of the planter and make it very difficult to release it from the mold.
If it does get stuck it’s okay, but not ideal if you plan to plant succulents- just compensate by adding better draining soil.
Check to make sure the lid fits properly. Make sure you place it inside the triangle in the same position that you traced it. It should fit fine and if so, mark each wall with letters -A, B and C and then mark the corresponding letters on the lid so that when you go to glue it, it doesn’t get flipped around and then not fit properly.
Step 3 | Make The Drainage Hole Molds For The Planter Mold
Now cut 2 pieces of straw, each a couple of inches long. These will be for the planter’s drainage.
Next, cut a piece of foam that is long enough to cover the locations where your drainage holes will be. Use hot glue to secure it to the underside of the top- meaning, glue it to the side that doesn’t have the letters.
Take an X-acto knife and cut two circles into the plastic and the foam. These circles should be just wide enough to fit the straws. Carve down into the foam a bit and test that they are large enough, by pushing the straws in.
You want them to go in about ½ to ¾ of an inch. Once you have the straws fitting properly, you can remove them for the time being.
Now go ahead and carefully glue the lid inside the smaller triangle. Be sure you have your walls properly matched with the top by double checking that your letters are lining up. Line the corner edges of the lid with hot glue and then insert it into the triangle. In the photo above, I didn’t get as close to the edges as I should have.
Next, cut the packing tape into thinner strips and cover the open corrugated edges because these will be facing up when you cast the cement. This will help the mold release more easily once it’s cured.
Step 4 | Cut The Acetate Sheet Barrier
Take the acetate sheet and cut it to the same sizes that you cut the corrugated plastic triangle walls. It’s easier if you cut the whole length first, like you did with the plastic, so measure, then cut one piece 32” x 3” wide and another piece 24 ½” x 2 ½” wide.
Cut these to the length of each plastic wall, with two pieces at 11 ¾” and one at 8 ½”. And for the smaller triangle, two at 9” and one at 6 ½”.
Using spray adhesive, attach the acetate sheets to cover the inside walls of the corrugated plastic mold.
Step 5 | Create The Inset For The Copper Inlay
You will use acetate to create the inlay for the copper, but since the copper is thicker than the acetate, you will need to double it up. Cut two strips at ¼” wide by 23 ½” long (which is the 11 ¾” times two), and one at 17” (8 ½” times two).
Since the spray adhesive dries quickly, you should spray and fold one strip at a time. Take the first strip, spray your adhesive and then fold it over and line it up, then press together. Now repeat for the two other strips.
Next, you’ll decide how far from the top to have your inlay. I had mine at ⅝” from the top. Keep in mind, what will end up being the top, is the side that faces down on the mat, mark the distance from the side that faces the mat.
Once the strip locations are marked and you’re sure they, then attach them to each matching wall length with spray adhesive. You may need to trim the length slightly after gluing- to compensate for where the walls fold.
Step 6 | Secure The Molds To The Base
Use packing tape to close both triangles.
Now use hot glue to secure the open end of the inner mold to the base. Be sure to glue it from the inside edge of the mold, not the outside.
Step 7 | Glue The Outer Mold
Center the outer mold over the inner mold and glue it to the base. This one you will glue from the outside where it meets the base.
Now insert the straws back into the drainage holes.
Step 8 | Mix Cement And Cast The Mold
I recommend placing a rigid board underneath the mold, like a piece of scrap plywood. This will make it easier to vibrate and also transport it, if necessary.
Start by mixing the concrete to a thick milkshake consistency- which is pourable, but not runny. Then add the pigments. My goal was to make this planter a blackish/green color, but I didn’t use enough of either color and it came out lighter than intended. So keep in mind, it will cure lighter than the color of the wet cement mix.
Ensure the mixture is smooth and without lumps, then you can pour the cement into the mold. Once the mold is filled, shake, tap and vibrate until the air bubbles release.
Though the cure time is typically one hour with this rapid set cement, I recommend letting the planter cure overnight since this is larger and fairly thick.
Step 9 | Release The Planter From Your Concrete Mold
First, remove the straws, then using the box cutter, slice the corners of the planter open and pull it apart. You may need to cut through the hot glue that is attaching the mold to the base. A paint scraper may be helpful to gently push the plastic away from the cement. Take care to not scratch the cement while doing this.
The inner mold is usually a little more testy when trying to remove it. Just push the sides in toward each other and use the box cutter to cut off the excess walls as you go. The lid should come out with the walls.
Step 10 | Sand And Clean The Planter
Since the Cement All is quite hard and not the easiest to sand- it will be easier if you sand the rough edges as soon as you have demolded the cement planter. After sanding, wipe it down with a dry rag to remove dust before moving on to the next step.
Step 11 | Attach The Metal Inlay
Trim the copper embossing sheet to ¼” wide and to the length of each wall.
Use blue tape to mask the areas surrounding the inlay so you don’t accidentally get glue on them. Now mix up the 2-part epoxy with a craft stick and apply it to the copper strip. The epoxy cures quickly, so I recommend doing this one strip at a time.
Now stick it in place and press it and hold. After a couple of minutes, use a clean craft stick to press down on the copper and smooth it down the length of the strip. These will be a bit slippery -especially at the ends, so use blue tape on the copper ends to secure the corners in place.
It’s okay if some of the epoxy gets on the copper- you can remove this later. Repeat for the other two copper strips. Let the epoxy cure for 30 minutes.
Final Step | Clean Up The Copper Inlay For The Cement Planter
Go ahead and remove the tape. If there is epoxy residue on the copper, use acetone and a q-tip to rub it off.
And now you are finished with your completely homemade, made from scratch, DIY cement planter!
Don’t forget to pin it!