Week 13 - Molding and Casting

This week we have the following tasks to complete:

• review the safety data sheets for each of your molding and casting materials
• make and compare test casts with each of them
• compare mold making processes
• design a mold around the process you'll be using
• produce it with a smooth surface finish that does not show the production process toolpath
• use it to cast parts
• optional: use more then two mold parts

Group Assignment

The weekly group assignment can be accessed here.

Safety Review Summary

We reviewed the datasheets and safety instructions for the materials. In general, the use of personal protective equipment (PPE) is required.
The resin is significantly more hazardous than the silicone. In both cases, I wear gloves, a respirator, and safety glasses.
If any material comes into contact with your skin, remove it immediately and rinse the affected area with running water. If irritation occurs or any unusual reaction develops, consult a physician. The same applies to eye contact.

To test the different casting materials we used one of the TU Ilmenau logo molds and a mold of our unofficial fablab mascot "Fabi".

Material Comparison

In the lab we have two different casting materials available: Smooth Cast 305 and Smooth Cast 326. Firstly we tried Smooth cast 326 because we have a few minutes more time to work with it and to get a better feeling for the material and workflow we decide it is better to eliminate a little bit the time factor. After we got some practice we then go to the other material. After the first parts should be cured we recognized that we make a small mistake with the mixing ratio because we mistakenly mix them 1:1 by weight but the mixing ration of 1:1 are the ration by volume. For the Smooth Cast 305 we make sure that we mix them correctly corresponding to the ration by weight.
The wrong mixing ration ended up in some flexure for the cast parts.

Following you see the visual differences of the two materials.

Design

I want to achieve small chocolate bars with the logo of the students club I'm part of to give them to the other members. Our Logo looks like that:

To reassemble the two colors of the logo I want to use ruby - and dark chocolate. To cast two different materials I saw two options:
use two molds in the first one cast one color and than put the cured parts in a second mold to cast the other color on top. There than was the question which color should be on top of the other.
use one mold and just pour a little bit of chocolate in the deepest parts of the mold let it cure and that add the second chocolate.

To decide which color should be on top the following questions about the castability came in my mind:
How well can the chocolate flow in to the deeper areas of the mold?
In which contour (red or black) is it easier to cast material in?

In this week I used Fusion 360 to design my part because of its easy import of .svg files.

I rotated the logo around 45 degrees to maximize the size of the logo. The positive mold is cut so deep because than the silicone have space to rise while it is in the vacuum chamber.

The milling process depends on three different operations.
1. Adaptive Clearing which removes most of the material.

With the following settings:

1. 2D Pocket removes the last remaining material in the corner of the outer form.

With the following settings:

1. 2D Pocket whit a smaller endmill to mill all the details from the logo.

With the following settings:

My first experiment is with one mold and two castings. The stock material that I could use for this test is 49,3 x 44 x 18 mm so my positive mold for the final silicone mold needs to fit in there. This are for now the size limitation. For the thickness of the part my guideline is a typical chocolate bar with a thickness of around 8 mm. So the overall thickness will be 8 mm and the emboss logo will be 1 mm, because i thought that is a good value between its to flat that its annoying to cast chocolate in it and its to thick that the more delicate structure will broke while transporting the chocolate bars.

Before casting the final mold with Sorta Clear 37, Ferdi recommend to try out the positive milled mold with some cheaper silicon. I did this casting together with Jakob because it is helpful you are working with some nasty stuff which can really easy result in chaos and a big mess to have someone around who can help you or hand some tools and materials. I began with filling up the milled mold with some water and weighted the water in the molds to got an idea how much silicone I need it. After it I calculate the right amount of material from component A and B to make sure that the mixing ration is correct and that I have as much silicone as I needed probably a little bit more to make sure that the mold will be complete. While mixing the two components together make sure that a homogeny mass is produced. Its important to mix everything also the material that is on the walls or the bottom of the cup because every unevenly mixed silicone will not cure probably. After the substance is mixed together we put them in a vacuum chamber which helps to eliminated trapped air in the silicone because the air bubbles will cooked out of the silicone. In the next step I slowly cast the silicone into the milled molds. In this process there are two things to take on consideration. First cast the silicone into the lowest point of the mold and second let it flow is lamina as possible over the same area and don't move. After casting we repeat the process in the vacuum chamber. A little bit of shaking or vibration can help to let the bubbles burst at the top. While the silicone is in the vacuum chamber it will increase its volume. From time to time it can triple its volume so its important to have some kind of boundary around the mold otherwise the silicone will run out the mold, produce a mess and the mold will be not usable because the needed material is not in the right position anymore. The shaking or also a slightly variation in the pressure, produced by slightly open and then close the valve will help to burst the bubbles and let the silicone decrease its volume again. The final step is to let the silicone cure on a even surface. For the hole process it is good to have a timer so you have a feeling how much time is left to work with the silicone.

After a few hours (depend on the used silicone) the silicone is cured and can be carefully removed from the milled mold. To make the removal a little bit easier I just pull the silicone slightly away from one wall and then use compressed air to let the silicon mold raise it self out of the milled mold. The mold looks great so the next step would be to cast them again out of Sorta Clear 37.

I was really confident that it will work again but it didn't. While mixing the first time silicone I felt like I mixed the silicone a little bit ti aggressive which results in more trapped air, so this time I mixed everything slightly more careful, which was bad because after we did everything similar to the first time, we realized after the curing time was theoretical over that most parts are slimy and sticky because they was not completely cured. Also while we put our mixing cup into the vacuum chamber for the first time we decided to shake it, which was a terrible idea because the cup fall over and a the silicone leaked into the vacuum chamber which was some kind of mess. We could also notice some difference between the different mold so we thought that the mixing was insufficient. Before the other cast there mold out of Sorta Clear 37 we could notice that after round about 8 hours more the molts finally cured completely, so we didn't have to cast them again.

While my own mold is still in the process of curing Ferdi and I did a test chocolate casting in a mold Ferdi produced a few hours earlier. We began by setup a water bath in a pot with a sous vide device, which allows us to regulated the temperature really precisely. For temper the chocolate we orientate us on the Fabacademy page from Gerhard Mattisen a previous student from Ferdi in Kamp Lintfort. This was a great reference but we messed it up because we didn't read the instruction on the chocolate package that we used. We started with a temperature of 45 °C until the chocolate is completely melted and than we set the device to 29 °C and waited really long until the water reached this temperature because the sous vide stick can only heat and circulate the water. So we mostly used it to circulate the water and to measure the temperature. To make the process a little bit faster we carefully added some cold water to our pot, which speed up the process a little bit but it still took over an 1 hour. At this point there is still room for improvement. One possible solution to speed up the process would be to use two water bath one thats at 45 °C and one that is at 29 °C or possible a little bit higher so the cool down process works faster. After the chocolate reached the 29 °C you could easily heat up the water until 35 °C. in both steps you should wait some time until the chocolate and the water around have the same temperature. Alternative I will do it next time like I work with this chocolate at home let it slowly melt than add some more chocolate to cool down the whole chocolate and that heated t up again until you not feel any different between the chocolate and your body temperature. To try this out I took a small amount of chocolate and put it on my lip because they're quiet sensitive.

In the next step we did a mistake, because we removed the bag where our chocolate was melted and cast it in to the mold and let it cure at room temperature because if you cool down chocolate to much it will destroy its structure. The package says that you shouldn't cast it at 29 °C instead you need to heated it up again until 35 °C than cast it and let it cure at 10 °C. I realized that we make a mistake because 3 hours after we cast it the chocolate was still viscous and not cured like I thought it would. After this realization we put the chocolate in to the fridge for 15 minutes which mostly fixed our mistake, accept that it was a little bit to long so the chocolate got to cold. Next time I will try 10 minutes that could be the sweet spot but I have try it out.

What I learn this week

• liquid plastic and there components are really fluidity.
• casting is a similar mess than sla printing and if you only have small parts its less work if you can cast them simultaneously because the work to do it for one part is similar to do it with 5 parts.
• Reading correctly and everything make things a lot easier.

What I want to improve next week

Design Files

bc-logo (Fusion360)
bc-logo (Inventor)

To create this page, I used ChatGPT to check my syntax and grammar.

Copyright 2025 < Benedikt Feit > - Creative Commons Attribution Non Commercial

Source code hosted at gitlab.fabcloud.org