Silicone rubber is available in many variants of different chemical and mechanical properties. I use an additive cross-linked precison casting silicone with a working life of 25 minutes and a demolding time of 2 hours. The silicone is mixed from two components using a mixing ratio of 10:1 and cures to a translucent light blue color. It is highly elastic, has a good mechanical stability and is suited for molding of complex master models even if they show indentations.

Modulor direct link: Modasil EH 10:1

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Because the finished prop was supposed to contain an electric circuit with a super bright red LED and five green sequencing lights, the mold had to be done in a way so that the later resin casting is hollow. This could only be achieved by doing two resin halves which were then put together. This circumstance didn’t make the task of molding the prototype any easier…

Here’s again the finished master model just like it was about to be duplicated. To reveal small imperfections, I already applied some coats of primer and then repaired whatever didn’t seem right.

Since both halves of the cast resin parts should be as symmetrical as possible, I marked the middle of the body using a pencil. This helped me to accurately shape the clay bed later.

Next I had to make sure the work surface was leveled. This is checked best using a spirit level. If the work surface is not leveled now, the seam line of the two resin halves will not be straight later and they won’t fit together.

The next step is the most difficult and time consuming part of making a two part mold: The master model has to be half embedded in clay to create the first half of the mold. The above mentioned marking on the body of the prototype now helped me.

Be sure not to use just some material for this but non-reactive clay to avoid inhibiting the chemical reaction of the two silicone rubber components. To ensure that, I use a clay that doesn’t contain any sulfates and is oil-based. It works very well together with the additive cross-linked precision casting silicone I use.

The master model has be mounted evenly on the work surface. I achieved that by sticking chunks of clay under it to stabilize it:

Next up is the task of shaping the clay bed. I started by putting material close around the prototype body and then worked my way away from it.

Now the mold box had to be created. It determines the final size of the mold. Most people use styrene or wood to construct this box and seal the corners with clay but I found that LEGO bricks work very well because they can be re-used every time to create boxes of different sizes and the buildup of the box is fast and easy..

Now comes the tricky part: The important thing is that exactly half of the model has to be embedded in the clay. This just alleviates the buildup of the kit later. Also very important is the fact that the clay needs to be aligned exactly to the body. There mustn’t appear any gaps between the model and the clay. This could be repaired later, but it’s easier to do it right here. So, precise modeling work is in order here!

Of course, the entire inside of the mold box has to be filled with clay. As you can see here, I first did this only roughly and then did the fine tuning later to get the surface smooth and to close all the gaps between prototype and clay.

Still quite some work to do here!

The next picture shows a pretty good progress on the clay bed. To do the shaping I use a putty knife which makes it easy to reach tricky areas, for example corners.

Remeber to wear gloves! Otherwise the clay will get stuck under your fingernails which is quite uncomfortable. Furthermore, the clay will get dirty rather quickly if it is exposed directly to your fingers even if you washed your hands before!

Here’s the result after a lot of work: The surface is now very smooth and there are no more gaps anywhere.

Next I used a pen shaped tool to create several small indentations in the clay around the model. They will later become pins as the silicone flows into them and cures. The other mold half will then again show these troughs and both halves will be perfectly aligned to each other.

This is important for creating solid castings by pouring the resin into the mold while both halves are closed. Even though this was not the type of casting I wanted to do, I nevertheless chose to implement this step because it’s not possible to add it later.

After I had enlarged the mold box up to the desired height, I was ready to pour the mixed silicone rubber into it:

The deepest areas have to be filled with rubber first to avoid air bubbles. For the same reason you should pour a thin stream so that bubbles cannot occur in the first place and those created by the mixing of the silicone components can break.

When the prototype is completely covered with silicone, you need to add at least one more centimeter in height to make sure the “base” of the mold is thick enough.

After the silicone has cured, the entire mold can be turned around, …

… the LEGO bricks can be dismantled and the clay can be removed. Be careful not to move the master model in any way. Note the above mentioned pins in the silicone.

Remaining clay residue has to be removed.

Most silicones are self-seperating, which means that the master model won’t get stuck to the rubber so it can easily be demolded. This however isn’t the case when silicone is poured onto silicone. In this case, the two rubber parts would bond together and become one piece.

So, to avoid enclosing the prototype into a solid block of rubber, I had to apply a release agent to the first mold half. I used vaseline for this, which I brushed on carefully. Don’t be stingy with the release agent! The more you apply, the easier the demolding process will be. Remember to mask the master model off to avoid blemishing it with brush marks which would be transferred to the new mold half.

After I had removed the masking tape, I repeated the above pictured step of pouring the silicone into the mold, waited until it had cured, demolded the prototype and disassembled the LEGO frame. Here’s the result:

The silicone mold is now finished and ready to be used for resin casting. However, if I had used this mold as it is to cast parts, they would have been solid just like the master model. This however, wasn’t my goal. I wanted to create two resin shells in which I could install electronics. So, I had to do a little more work on this mold to make that happen.

In order to get hollow resin shells I had to find a way to displace the resin that I would pour into the mold halves. The first step to achieve this was to use clay again to shape “walls” which had exactly the thickness that I wanted for the resin pieces later. Everything that I made out of clay here, would become resin later.

Note the recess for the LEDs!

Next, I placed wooden slats with screws in them over the mold. The screw heads had the purpose to attach themselves to the silicone rubber I was about to pour into the clay lined mold. This way I would have a comfortable handle to place the inner silicone displacement piece inside the mold later.

Here’s the result of this rather complex mold making process: two two-part mold halves (front side and back side, only one half is pictured) which produce two resin shells that fit exactly together (well, more or less since there’s always some excess resin that needs to be sanded off).

The development of the two most important parts for the ODN Scanner kit is thus complete and the buildup of the hero prop can now begin!

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With every building project, there’s one essential question: What material am I gonna use? Well, in prop building there’s most of the time only one answer to this question and that is “styrene“. Styrene is nothing more than sheets of plastic which is available in several thicknesses. It is also used in other model building efforts, for example architectural models.

So, the first step of building the prototype was to cut out all of the needed pieces from styrene (I used styrene with a thickness of 1 mm). After I had determined the shape of these parts by drawing a blueprint, I printed them out and glued them onto the sheet of styrene using spray glue. Next up was cutting the pieces which I did using a sharp precision knife. After scorching the sheet two or three times, they can easily be broken off along a straight edge.

After that it’s no problem to remove the paper from the styrene. If it doesn’t come off easily, simply put it in luke warm water for a few minutes. Then, the edges of the styrene parts had to be sanded lightly to create clean edges. The opening which would later become the display window proved to be especially difficult. A lot of sanding work was necessary to achieve really straight lines and rounded corners. Very useful in this step was sanding under running water which requires a special kind of sandpaper but results in very little dust and a very smooth edge.

Here are all 24 seperate pieces which I marked with the respective codes to know which one belonged where.

To see if these pieces could fit together to result in the object of my wishes, I did a test fit and glued them together using removable masking tape:

Styrene is best glued together with a special plastic glue which joins the parts permanently:

I filled the inside of this open box with putty to gain additional material for the later sanding process of the corners and edges. I wanted to prevent creating a hole in the body which I would not be able to close again.

After I had closed the body, I sanded the hole object roughly to remove all of the sharp edges and corners.

Next up was the handle. Here I also did a test fit first to determine how I would have to glue the pieces together. Because the handle would have been too thin consisting of 1 mm styrene, I glued two identically shaped parts together to make it thicker.

To perfectly fit the handle onto the main body, I actually attached the respective parts to it while glueing them together.

As you can see, the edges of the handle were very raw. Because of this I again had to use putty and a lot of time to give the handle a sleek shape and a smooth surface. The complex form obviously didn’t make this easy for me. However, the time was well spent as you can see here:

Fits like a glove! However, the main body being curved and the inside of the handle being straight, small holes were visible in between which again had to be filled and sanded smooth.

After that, the complete body had to be sanded very carefully to make every surface smooth and to achieve the desired degree of roundness on all edges and corners. Because silicone rubber transfers each and every imperfection to the mold very rigorously, the master model has be as perfect as the final kit or prop is supposed to be. Mistakes made here are being carried all the way through to the final product and then have to be corrected on each casted part anew. So, a lot of work can be saved here by thinking everything through and doing things right.

I here fabricated one set of greeblies, which are normally being put on as the last step in the buildup of a prop, to see what the final look of the prop would be:

In the same way I did the handle, I also made the scanner array on the top of the prop above the recessed display area. This way I could simply plunge it on and then attach it.

Even if it doesn’t look that way, the construction of such a prop from scratch using the methods shown here is a lot of work! Sanding is by far the most time consuming task because you always spot another small blemish which has to be repaired or areas you are not satisfied with. Also, the whole process requires a lot of patience because it just takes very long if you want to do it right. Anyhow, the effort is well worth it especially when you see yourself making progress and getting the project somewhere.

So far, so good. The master model is finished. Next is Part II in which I describe how I made the silicone mold.

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