First of all, I want to point out that these materials can be cast in molds that are not 3D printed, with the foremost of those being wood and silicone. With wood you can even partially trap parts of the form within the concrete as long as there is a pathway to let fumes escape when burning it out. However, since this site is geared toward explaining the 3D printing process we have been using for our research, I will focus on that that option.
Basic Subtraction Method
The most “simple” way of approaching this mold-making process is to start with a positive form, enlarge it slightly is all directions, and then subtract the original form from the enlarged form. In practice that process can range in complexity depending on the form, and on how well suited a particular 3D modelling program is for this process. The easiest way I have found uses a free Autodesk program called Meshmixer (here is a set of instructions for that). However there are many ways using many different programs. When performing that process, one needs to arrange the form in an orientation that will allow the concrete mix to be poured in with a level surface, which usually means flipping it upside-down and leaving the bottom open. It also usually requires printing a structure that keeps the form stable until the concrete sets. In the process I use with Meshmixer, I find that I need to add those structures on before I subtract out the interior.
More Complex Methods
Because these molds are being created virtually, they can potentially be “constructed” in ways that do not follow the logic of traditional mold making processes. The basic subtraction method, though it uses ways of joining and removing parts of the form that could not be done that way with physical material, could still be said to be based on the “lost wax” process. And it makes sense to adapt approaches that have a long history of success when using new technology. However the ability to create form digitally also opens up the possibility of thinking of mold-making in different ways — perhaps in ways where there mold is directly constructed without having made a positive first, or using some combination of addition and subtraction of form that would not be feasible with other methods. My own research in this area has led me to the realm of algorithmic design using Grasshopper3D which is within Rhinoceros3D. That is a platform with a huge amount of potential, but with a substantial learning curve. That program makes it possible to create forms out of a set of interrelated instructions and dimensions, such that once a particular “definition” is set up, it can create a range of form variations, or it can be used to precisely fit a form to a specific situation or other form. For most, that program is not the best place to start though.
Design Issues Related to 3D Printing
However you create your mold form, in creating it, you will need to consider the issue of support structures. Those familiar with FDM printing know that such forms need support when they have an angle more shallow than about 45 degrees. Lower angles will not interfere with this process if the supports are on the outside of the form, but if they are within the form, they will get in the way of the concrete. Those who have access to a printer with dual extrusion, set up with dissolvable filament, will not need to worry about that issue. But for many of us who do not, it is necessary to plan your design accordingly. This means that the best forms to start with will be ones that are more vertically aligned. Another issue to look out for is having bottleneck areas in your form or very thin areas in general, as the concrete may not be able to pass through sufficiently to cast the parts beyond that section.