Marco Antonio Sardo

Introduction

Mold and casting, and 3D cnc, this will be the topic of this week.

-- Mold and casting explanation, 3D milling --

The project I decided to create for this week is inspired by a Lego Brick:

Specifically I'm going to create a simplified form of a LEGO that, as the original, will be able to fit with its copie in order to make contructions with them. The idea is to make it out of chocolate hence I will use food-safe silicon.

The steps needed for this assignment are the following:

LISTA

• Create a 3D model of the object you want to produce
• decide the casting material
• Obtain the G-COde through specific sw, in my case Modela
• Choose the appropriate mill-bits
• Mill the mold
• Clean and dry the mold before using it
• Carefully read the substance datasheet to be used for casting
• Generally mix 2 ingredients (in my case Part A and B of Food-Safe Siicon)
• If you have time, degas the mixture
• Pour the mixture into the mold trying to avoid creating air bubbles
• Wait for the cure time of the compound
• Eventually Post-Processing

I will explain all the steps in detail on this page:

Once I chose the object to reproduce, I modeled it in Fusion360, he model consists of the upper and lower halves of the LEGO brick, also you can see 4 hemispheres in the four corners of eath half of the mold, they will be useful to align the two halves of the silicone cast when you will then pour the melted chocolate.

Also, it is important to create openings, one to be able to pour the chocolate and one at least 3mm in diameter to let the air go out to avoid air bubbles in the final casting.

Modeling

Made the model, to get the G-Code we used the sw Modela, that is specific to the CNC SRM-20 we have in our Lab.

The sw is very straightforward to use, once you've chosen which bit-mills to use for roughing and finishing you can see the simulation of the result.

In my case, as you can see from the image below, the semispheres for alignment were slightly big for the types of mill-bits I chose for milling.

And the simulation in fact showed me that I would not have had a perfect finishing.

To solve the problem I had two options:

• 1) Add another type of mill-bit for finishing, hence increasing the count of milling to 3 (1 roughing and 2 finishing)
• 2) Change the model by reducing the size of the hemispheres

Obviously I chose point 2, for simplicity.

Below the simulation Video of the fixed 3d model:

Milling

Once you have the G-Code you go to milling, specifically I used a 1/8 inch flat tip and a 3/32 inch rounded tip, as shown in the pic.

And here is the result of the milling:

Preparing the Casting

To prepare the casting, you must first calculate how much silicone you need.

After having carefully washed and dried the mold, we proceed to measure the volume of water needed.

In my case the volume of water needed was about 200 ml.

For the food-safe silicone Easyl 940-FDA (Datasheet) I need to mix two substances in one ratio 1: 1.

I will therefore need 100ml of substance A and the same amount of substance B.

For greater precision it is advisable to use a scale and have a reference in WEIGHT compared to VOLUME. I then poured 100ml of substance A into the container upon a scale, I measured the weight and therefore I poured the same weight of substance B.

Once this is done, I proceed to mix, carefully considering the edges and the bottom of the container so that every part of the two substances is mixed.

At this point, according to the datasheet, you have about 30 minutes before the compound starts curing, hence I have time to degass it.

To do this, I took the degassing machine we have in the laboratory. It's simply a cylindrical container of thick plastic, which rests on a rubber surface and a lid with a pressure gauge and valves installed, one of which is directly connected to the aspirator.

I putted the cointeiner with the mix inside the degassing machine (be sure that the the volume level of the mixture does not exceed half of the container, because during degassing you expect an increase in volume which could spill the whole thing out.

Close the lid, activate the aspirator and press the lid firmly so as to start vacuum packing. Sometimes I noticed that there were leaks from the bottom as well that preventing the vacuum, so it is also useful to fold the bottom rubber to ensure that there is air leaks from the outside.

I then proceeded to degassing until I see no more bubbles (or very few), the process would generally last no more than 15 minutes.

Having done this, I then very carefully poured the mixture into the mold, in order to prevent other air bubbles from reforming.

I filled the mold and then left it overnight so that it cured.

Here you can see the cast complitely cured.

Post production

The following day it was ready for a post-processing, that is to baked it in oven for about 1 hour at 110 ° C.

I repeated the same process 3 times, thus getting 3 Molds to test simultaneously 3 types of chocolate.

The fun part

I repeated the same process 3 times, thus getting 3 Molds to test simultaneously 3 types of chocolate.

We therefore first boiled the molds to disinfect them.

We then melted the chocolate one at a time and put the mixture through a sac-a-poche.

We then put the three molds in the freezer for 30 minutes, and this is the result.

Unfortunately, the third cast didn't come out complete, but it was found that the bricks fit together very well.

The next step will be to use caramel and insert an LED to create a structure of chocolate and bright caramel. I hope I have the time to be able to realize in the future. Stay Tuned.