Molding and Casting¶

This week I created a silicone mold from an FDM 3D print to make a fishing sinker out of concrete.

I also once again teamed up with Kerstin and Lars, the other students from HRW FabLab, to compare some of our first tests and gather the safety datasheets for our materials.

You can find our page right here.

Design and Printing¶

To start off, a big thank you to my brother, who came up with the idea in the first place.

Not only is he really into fishing, but he also studies civil engineering and knows a thing or two about how to work with concrete.

After we discussed the idea, he quickly whipped up this technical drawing for me.

So I got to work in Autodesk Fusion, which I covered extensively in previous assignments such as the CAD week, 3D week and CNC week.

I followed my brother’s design pretty closely but thought it would be best to slightly undershoot the calculated volume, since I planned on using filler primer to get rid of the layer lines and other artifacts from the printing process.

I split the sinker in half and added back panels as well as indexing features to later be able to align the two halves of the mold.

I personally didn’t really like the shape all that much, so I created a second version with a slightly less boring shape.

Just as a sanity check I also subtracted my geometry from a solid block to simulate the molds.

With that done and looking good, I sent my positives off to printing.

You can find both of them in the Download section at the bottom of this page.

Making the Molds¶

After about 2 hours I had my parts ready to go and began building some boxes out of carboard.

For now they are only held together by duct tape but I’ll seal all of the edges with hot glue once I finished postprocessing the printed parts to prevent the liquid silicone from leaking.

Speaking of post processing: I sprayed my prints with filler primer.

While the primer was drying I got my silicone stuff ready.

According to the datasheet, the material I am using is pretty safe, doesn’t need any ventilation and cures in about an hour.

The only things I needed were some goggles and gloves.

I mixed 15g of each component while trying not to trap any air in the mixture.

Back on the filler primer side, about 4 coats did the trick so I got to sanding for which I used regular 240 grit sandpaper.

I really didn’t want to breathe in any of the dust, so I went outside and wore a respirator while sanding.

It took way longer than I expected so when I came back, the test pour had already hardened.

With the positives finished and placed in their carboard cases, I got worried about potential leaks.

I figured that hot glue would make too much of a mess so I went with Play-Doh instead.

It didn’t really stick to the walls all that well but it still worked.

Everything was relatively well sealed off so I sprayed the boxes with some mold-release and whipped up a new silicone mixture.

I must have taken too long during mixing so when I started filling up the first box, it already started solidifying.

Hoping this mess of a pour would still work in the end, I had to mix up another batch for the other boxes.

This time I kept an eye on the clock and was eventually left with some pretty decently looking pours.

The second batch solidified really well but the first one was quite sticky at the top, so I gathered the last few drops of silicone, mixed it up and hoped it would bond well enough to cover my failure.

Unmolding everything showed that it clearly didn’t.

The process also kind of killed my neat boxes and artistic Play-Doh accents but oh well…

At least the second batch looks good enough to use and the indexing features work as intended.

Casting a Sinker out of Mortar¶

With the molds done, I gathered some supplies for the next steps.

First I started bending some 0.9mm 304 stainless steel piano wire into a shape that would sit well in my molds and hold on to the concrete, leaving a loop at the end and working in a swivel in the process.

Next I gave the molds a light coating of vegetable oil as a release agent.

Then I got everything ready to mix some mortar and clamped the halves of the mold between two pieces of wood.

This mortar is way finer than regular construction concrete and thus easier to pour into the mold.

It is surprisingly simple to work with, you just add water to the powder in a flexible container until you feel like you have a pourable consistency.

Unfortunately I added a little too much water, which is why I had to wait for the mortar to settle at the bottom in order to soak up the excess water with some paper towels.