3D printing is a novel method to print molds for cement items. While the internet is full of craft items that are made with silicone molding methods, I wanted to see if I could skip the mold making process and produce molds directly with 3D printing.
After watching some videos on mold making, I noticed that all of the content creators were using ridged materials to make the masters for the silicone mold. But I also noticed that the molds were flexible enough to peel back. Casting and dealing with silicon molding material is messy and expensive, and I saw that I could use Ninjaflex TPU to directly print a flexible mold.
With this in mind I first designed some basic candle holders in TinkerCAD. In Cura 2.6.25 there is an advanced mode that will automatically generate a mold form for any master you open. When I tested this mode I was not convinced that it would be strong enough to hold the pressures of casting cement.The mold mode also closed off the top of the mold, making it difficult to use without serious cutting on the mold.
I went back to TinkerCAD to make a sturdy mold from the master. This gave me access to the top of the mold to pour cement and a stable base for the crown shaped casting.
Because this was an experiment, I did not want to spend a lot of money exploring and be left with a bag of cement I could not use afterword. I went onto a local Facebook group, and asked if anyone had a bag of cement they would part with. Within an hour I got a direct message from someone local. A handshake and quick car ride latter, I walked away with a free bag of Quickcreete cement.
This cement I got for free had a catch. It is general purpose and designed for pouring sidewalks and post holes. The aggregate in it is too big to use for small projects, so I would have to sieve the coarse gravel out.
Using a wire kitchen sieve, I poured the cement mix through it, collecting the fine cement and sand over a newspaper. I sieved out about 2 cups of mix at a time for these castings.
Preparing the castings
To prepare the molds for casing, I first applied a liberal coating of cooking spray on the inside of the molds. In a latter casting I would also cut slits in the side of the mold, then seal the sides closed with clear packing tape. This way the molds would better peel off the cast cement.
Mixing and pouring the cement
I tried to keep the mess for this to a minimum. I mixed about ½ cup of the processed fine cement into a small sandwich bag, then enough warm water to bring the cement to a creamy peanut butter consistency. Once the mix was ready, I could use the bag to carefully pour the cement into the prepared molds.
To reinforce the casings, I pressed in some picture nails into the poured cement.
For any extra cement, I poured it into small cups and pressed an eye bolt into the mix. I suspended the eye bolt with a nail. I can use the small castings for table cloth weights, fishing weights, or painting stands.
After the pouring and nails, I placed all the molds into a tub and shook the tub to vibrate the bubbles out of the casting. In the future I’ll get an orbital sander to really shake the casting. After this I brought the castings inside so they would dry at room temperature.
1st Cement Casting
For this first set, I waited 24 hrs before trying to un mold them. After checking the cement directions again, I should have waited 48 hours.
The results of the first attempt were mixed. I did break one casting, but even that I could glue back together. In the first set of molds I did not cut any slits, so it was difficult to remove, and I lost some details in removing the cast.
I did notice that I could see the layers from the mold in the casting, which is a great detail.
The other mold came out better, but cutting the mold to open it did also damage the casing as well. While the shape did come out ok, leaving the cement to cure for another day would have helped.
Another material that is infrequently used in 3D printing is PVA. This is a water soluble material that is used mainly for dissolved supports. In my filaments experiments I found that it took a thin sample of PVA 3 – 6 months to completely dissolve outdoors.
I was thinking that as long as I don’t apply hot water to this while casing, this may make a mold material that I can smooth, and can cast cement in it as well. Later I would simply dissolve the mold off the casting.
After revising the design for my candle holder in TinkerCAD, I printed up a batch of PVA molds. Pouring the cement and letting it set, I quickly noticed a couple issues.
First the PVA mold started bulging in the center, and a couple castings cracked on the surface as they dried. After 3 days of curing I removed the molds by cutting them off. PVA is easy to physically remove, which I did after I realized how sticky PVA was as I tried to dissolve it.
The result was 3 badly cracked and gooey rocks. All 3 of the PVA castings cracked beyond repair, and separated in the center. It looks like the PVA shrunk unevenly as the cement cured, and the uneven stress bifurcated the casting in half in all of them.
2d Attempt Mold in Ninjaflex
For the second set of molds made with ninjaflex, I cut slit along the side to make
un molding easier. This time I also let the cement cure for 3 days before pulling them. The extra curing time made all the difference. I was able to cleanly remove the molds from all 4 cast. The detail from the 0.4 mm layer resolution on the molding could also be seen on the casting.
Lessons from Printing Molds
The general purpose cement that I used was ok for this application, but sieving it was a hassle. If you want to do this on your own, explore using countertop cement or plaster for this application. Mixing the cement in sandwich bags did keep the mess down. Time wise a project like this will take 4-6 hours to print the molds, 1-2 hours to mix and pour the cement, and 2-4 days to dry.
PVA was not a good molding material for this application. It warped and caused the casting to dry unevenly. Future experiments may make a stable mold in PVA, but overall PVA is too messy for my taste.
With some practice, Ninjaflex can make a great molding material. You can print your molds directly without going through the hassle of making a silicone mold. Once cleaned and prepared again, the molds are reusable.
This process is a great way to make craft items for casting in cement, plaster or resin. In looking online at etsy, I found other craft products made with cast cement. You can use this process to easily cast your own holiday products.
For future improvements, a finer layer resolution or applying filler may help smooth out the mold. A finer cement would also be better for small castings like this. In other videos they use countertop mix for their projects, and in future experiments I’ll have to get a bag to test.