Researchers at MIT have created a new fabrication technique to quickly create thin, rubbery shells – involving drizzling of a liquid polymer over dome-shaped molds and spheres, for example ping pong balls.
The objective of the experiment is to predict the mechanical response of many types of shells such as small pharmaceutical capsules, large airplane, rocket bodies etc., with the help of only a few key variables – the size of the mold and the polymer’s density.
It was found that the shell’s final thickness does not depend on the volume of liquid or the height from which it is poured onto the mold.
The research is supported by the National Science Foundation. The experiment was inspired by chocolatiers making chocolate shells like bonbons by simply pouring chocolate into molds, then inverting the molds to let excess chocolate drain out – to create smooth and even chocolate shells of relatively uniform thickness.
Similarly, a liquid polymer after being drizzled over the molds was allowed to cure for about 15 minutes. The resulting shell was then peeled off the mold and it turned out to be smooth and virtually free of noticeable defects. It was then cut in half to find a nearly uniform thickness throughout.
The coating dynamics were systematically characterized in each of their experiments, including – the physical properties of the polymer, the size of the mold, how fast the fluid flows down a mold, and the time it takes for the polymer to cure.
Based on these data, the researchers developed a simple formula to estimate the final thickness of a shell, which essentially equals the square root of the fluid’s viscosity, times the mold’s radius, divided by the curing time of the polymer, times the polymer’s density and the acceleration of gravity as the polymer flows down the mold.
The formula boils down to the following relationships: The larger a mold’s radius, the longer it takes for fluid to flow to the bottom, resulting in a thicker shell; the longer the curing time, the faster the fluid will drain to the bottom, creating a thinner shell.
Professor Pedro Reis from MIT said “This is a really simple, robust, rapid prototyping technique, and we’ve established design principles together with a predictive framework that characterizes the fabrication of thin shells. I think that will be powerful. We’re revisiting an old topic with new eyes.”
Reis also added that he hoped that the group’s theory will reinvigorate studies in shell mechanics, which was significant during 1950s and 60s.
At the end, the researchers also developed a numerical and analytical model to further test their simple mathematical formula. This gave way to explore more complex experimental configurations that are not easily feasible in the lab.
Image and Video Credits: MIT