Mold and cast an object

For this assignment, students were asked to model an object, produce and replicate it by molding and casting tecniques. As a napolitan architect and a lego lover, I decide to produce a special brick for Lego, reproducing the pyramids on the Palazzo San Severino facade, in the center of Naples. I thought about a brick or something which can be replied several times to underline this great advantage of molding/casting tecnique.

Since the brick should work with Lego, I took all the measures I needed from the standard bricks.

Here are some sketches about the shape and the milling issues. Also, I had to figure out how may sides should have my brick: 2.

To understand better the process I also followed this tutorial.

After levelling my wax block, I loaded the stl on the pc connected to the Modela MDX-40 and I have imported it into software SRP Player Software.

Here the setting for level the wax block, before starting milling process. I used ClickMill as software and a 5mm square tool.

After importing the stl file into SRP Player Software, I start to set the machine for this work, checking the bounding box and defining the correct orientation.

Then, I set the tools which I used for milling (roughcut ): 2mm square.

The roughtcut should be something like this

Then I set the machine for the finishing milling, using first a ball d1.5mm, then a Roland SF2mm.

This is the outcome after the roughtcut

After completing this step, I changed the tool for roughcut with a ball d1.5x2.5 tool for finishing process. Unfortunatelly, due to its conical shape, the tool was hitting some part in the wax and not workinf properly. So, I changed the tool with a SF2mm, with cylindrical shape.

This is the outcome after finishing cut.

Silicon Rubber

At this point, I prepared the silicon rubber for mixing. The one I used is this one, the MM810 from ACC silicones. After reading the Technical Sheet, I used a scale for weigh both components: the rubber base on one side and the catalyst on the other side, whose weight must be 5%.

As showed in the following picture, the result has been pretty good.

Polyurethane Resin

Let's produce the positive piece by preparing and pouring some Polyurethane Resin. I used the Sintafoam, from Prochima. Here is the Technical Sheet Instead of measure the weight, I have measured the volume of each one, which ratio must be 50%-50%. After mixing for couple of minutes, I have poured the resin in the mold, with the help of a syringe. It took only 15 minutes to dry enough the resin. Here are some photos.

First results are not very satisfying, since there are some bubbles of air which were trapped while pouring. After some tries I understood that I have to move a bit the mold and to be stable while pouring the resin.

Final results are here:

GYPSUM

I also tried with some gypsum I had at home, called Gesso Alabastrino (alabaster gypsum), like this one. It is quite cheap and not toxic like the resin. It just needs water to work. Usually is 0.8 liter of water for 1kg of gypsum. It is very important to add the gypsum to the water very slow, letting a few amount of gypsum be wet before adding more. After some couple of minutes of resting, I started to mix the gypsum/water mix until I got a uniform and consistent texture. Because of its viscosity, it was impossible for me to pour the gypsum into the closed mold, so I poured it into each half mold and then I put them together, waiting for 1 hour. The result is quite good and very natural but more fragile than the resin pieces.