Molding and Casting




The 3D Print

For this week, I wanted to gain an understanding on the thought process and the reactions to be able to mold & cast moving forward comfortably. I decided to make a paper weight as my first object. I simply designed it on Rhino and printed it on an Ultimaker 2+ using White PLA, 2.85mm. The materials to be used are: Oomoo 30 silicone rubber and Smooth-cast 326 liquid plastic.

The Steps

Step 1:

The first thing to do once the object is printed is to set it up in a container to pour the silicone rubber into. I hot glued my object onto a foamcore piece making sure there are no spaces underneath. Then, I cut a piece of a plastic water bottle and superglued it around the object on the same foamcore keeping a good 5-10cm above the end of the 3D printed object (it will be the base). Once it is glued, I used a piece of non-hardening clay and sealed off any section of the "wall" that could have let the silicone seep through.

Step 2: Prepping

Once everything is glued well and dried, I sprayed the enclosure completely with Smooth-On Mold Release to make sure that once the silicone mold has cured I can easily remove it without having leaks or tearing parts apart. You don't necessarily need a lot of the mold release, but you do however need to make sure that the spray gets into all the niches of the object so make sure to rotate the enclosure well why you are spraying.

Some Instructions on using the Mold Release:


Step 3: Measuring & Mixing

The next step is to mix the liquid plastic parts A and B separately and then together before pouring them into the enclosure where the reaction will take place over a period of time.
Oomoo 30 is an easy to use tin cure silicone rubber compound that features a convenient one-to-one by volume mix ratio, it has low viscosity for easy mixing and pouring, and cures at room temperature with negligible shrinkage.

Measuring & Mixing

Once you open the containers A and B, use a long wooden stick and mix each one separately in their own containers. Once they are stirring smoothly, you can pour equal amounts of A and B in a cup before pouring them in one larger cup. Once they are poured into the same cup, you will need to mix it thouroughly for approximately 3 minutes. Make sure that you scrape the bottom and sides to make sure nothing is stuck.



How do I know that I am done mixing? When you see that the mixture is uniform in color, with no colors streaks.


Step 4: Pouring

Pour the mixture in a single spot at the lowest point of the enclosure. Make sure to pour from a distance and keeping the poured string uniform and thin to minimize any entrapped air. Keeping pouring until the liquid is a minimum of 1.3cm over the highest point of the model surface.

Step 5: Curing

At this stage, you will need to take a look at the Technical Overview Manual. Looking at the manual we can see a section called "Pot Life" and another called "Cure Time".



Pot Life is the amount of time a specific mass of mixed resin and hardener remains a liquid at a particular temperature, which means the maximum time we have to pour it out before it hardens at room temperature. Cure Life is how long the mixture needs to fully harden and cool down. In our case, Oomoo 30, the curing time is 6 hours. Keep it at room temperature throughout its curing time.

Step 6: Releasing The Mold

Once your 6 hours are complete, you can release the mold. To release the mode, I cut the plastic lightly and removed it and since we sprayed the mold release the plastic should come off very easily. Then, using an olfa knife I cut zig-zags into the mold. I started the cuts where the top of the model was (glued to the bottom of the container) and stopped the cuts around 5cm away from the closed bottom edge of the mold. Try to keep the cuts as clean as possible as you will need to make sure its closed tight when pouring in the model.

Once the mold is released, realign the cuts to make it look seamless and then use a thick rubber band to seal it shut. You will need to make sure that the lines align perfectly so that when you pour in the model it comes our exactly like intended.


Step 7: Using the Mold | Pouring the Model

I decided to use Clear Liquid Plastic for the model because I wanted to test out how mixing in sawdust would look like. To prepare for pouring, seal the mold, and then use the Mold Release one more on the inside of the mold. Make sure to rotate the mold to reach all the sides. Once the mold is ready, you will need to measure and mix the liquid plastic. Smooth-Cast 326 also uses a 1:1 approach and so the steps to mix are the exact same as the rubber mold.

Once the mixture is mixed in well which you will know when it becomes clear, pour the mixture into the mold the same way as earlier, pour into one spot at the lowest point and allow it to fill in all directions on its own as well as keeping it a consistent thread to minimize the air entrapment.

BE CAREFUL NOT TO TOUCH IT DURING THE CURING PROCESS AS IT GETS VERY HOT For this material, the curing process is 60 minutes and might take a little longer to cool down to room temperature. Once it has been cured and cooled, remove the rubber band from around the mold and slowly peel off the mold. Since we have sprayed the mold release, this process should be very easy and should not cause any tears.




Final Thoughts

Now that I understand the process and its limitations I am considering making a collection of home decor elements for my sister's new home! The part I enjoyed the most was using the sawdust to add texture and pigment to the liquid plastic, it did mostly sink to the bottom because of its density and I did learn that the more foreign liquids or objects in it the higher the level of failure but it did make me want to test these limits further. Another point that I would like to test for future reference, is making a hollow mold.

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