Putty : Material compositions for variable density 3D printing
In nature the density of various materials is varied in a interesting fashion. The chemical composition of the material remains largely the same and the density varies due to the structure and the porosity of the material. This is clearly seen in the structure of the human bone and corals.
Through this project i have worked on a material composition which changes density in a similar fashion. The materials for the experiment were chosen on the basis of availability, cost effectiveness and the feasibility of using them with extruder based mechanisms. The focus was more on the material compositions than on the 3D printing mechanisms.
The following picture shows a sample of “Plaster of Paris” with varying densities.
The composition of the mixture involved the following components : Plaster of Paris (Gypsum) , Calcium Bicarbonate (Ca(HCO3)2) Acetone (C3H6O) Hydrogen Peroxide (H2O2) Water (H2O)
The main active ingredients in this mixture that are responsible for the change in density are the Carbonate and the Hydrogen Peroxide. The varying densities were achieved by varying the proportion of Bicarbonate in the mixture and then reacting it with a suitable amount of Hydrogen Peroxide.
The acetone is used in the mixture so as to aid the homogenous mixing of the Plaster and the Bicarbonate (using water might release the Carbon Dioxide prematurely)
Bicarbonate + Peroxide = Water+ CO2 +Salt
How it works
As the Hydrogen Peroxide reacts with the Bicarbonate. It forms Carbon Dioxide gas, this gas creates the pores and pockets in the material. Hence more gas being released translates to a more porous material, and thus has a lower density than the solid material. The viscosity of the Plaster mixture traps the gas in the bulk of the material (though the gas formed close to the surfaces escapes to form visible cracks and pores)
The image below shows the reaction taking place in the samples after the peroxide has been added
There are still various parameters that have not been tested such as compressive strength, actual density of the samples etc. I plan to make better samples in a more controlled environment to make these measurements in the near future. For now I have included the shots of the change in density with increasing Bicarbonate proportion.
The percentages indicate the percentage of Bicarbonate by weight in the mixture.