Plastic parts in kitchenware and toys are generally massproduced with a molding process. But mass-producing complicated plastic microcomponents is another matter entirely.
"Molds for producing large objects are usually composed of two or more pieces that fit together," says University of Maryland Chemistry Professor John Fourkas. "That makes it possible to create components with complicated shapes that include features such as holes the dust guard on a computer keyboard, for example. But when you try to use the same procedure to create microscopic objects, it becomes problematic to align different parts of the molds and there are other problems as well."
For mass-producing plastic parts that are smaller than the diameter of a human hair, the team modified a technique known as microtransfer molding. In that process, an elastic substance called PDMS (a component of bathtub caulk) is cured over an object laying on a surface to form a mold. The hardened mold is then removed and used to create copies.
"The problem with microtransfer molding comes when the original object contains closed loops," says Fourkas. "Imagine you want to mass produce a microscopic version of the Golden Gate Bridge. The bridge is anchored to the surface at its towers, forming a closed loop. Once the PDMS has been cured, the original bridge model will be stuck inside it."
Up to now, the closed-loop problem has been addressed by molding in layers. "This layer-by-layer technique can only mold a limited range of structures, and it requires precise alignment of each mold," says Fourkas. "We realized we could use a property of PDMS that is usually viewed as a problem, which is that it sticks to itself."
The team created a thin wall of PDMS in the original structures, effectively removing any closed loops. "For instance, on the Golden Gate, we would create a thin wall underneath the entire length of the bridge model. That would make it possible to remove the mold from the original object," says Fourkas. Then, once the mold is free, the wall region in the mold can be closed off by gentle pressure, making it possible to create copies of the bridge that do not contain a wall.
"This represents an important step towards the mass production of micromachines made from plastic," Fourkas claims.