Three-Dimensional Prototypes

Like other rapid-prototyping processes, Contour Crafting is a way to realize a three-dimensional object designed with a Computer Aided Design (CAD) system. Most existing prototyping processes work by first having computers analyze a CAD representation into a stack of flat, thin sections. A cone, for example, is visualized as a stack of disks of constantly increasing size; a hollow cylinder as a stack of identically sized rings.

Then, various techniques are used to create these visualized layers, one by one. Some rapid-prototype systems use ultraviolet light or lasers to harden soft plastic or plastic powder layer by layer onto rising platforms; some use a computer-controlled laser to cut glued-together layers of special fiber paper, one layer at a time. One technique, called "three-dimensional printing," uses an ink-jet-printer-like nozzle to spray layers of glue on plastic powder, one layer at a time. All of these layering techniques share a common problem - rough edges. If the layers are thick, the side edges of the resulting shape show laddering effects on what should be smooth surfaces, so that what should be a cone looks instead like a stack of discrete, differently-sized disks. The effect can be minimized by making the layers thinner; but doing so makes the rapid prototyping much slower, because it takes many more layers to create a finished product. Making even a relatively small part can take hours or even days with existing machines. Since such machines cost $100,000 or more, the time factor sharply increases the cost of parts.

Even when cost is not a factor, all existing systems have physical limits on the size of the objects that can be created. The maximum size is about one cubic meter - a box 39 inches on a side.

Breaking the Boundaries

Khoshnevis' Contour Crafting system breaks those boundaries. It improves on an existing technique in which a computer-controlled extrusion nozzle squirts out plastic to build up layers, somewhat like the way a cake decorator squeezes out patterns.

Khoshnevis has added to this setup a pair of movable, flat control surfaces that he calls "trowels" - just above and to the side of the nozzle. The trowel movements, shaping the material coming out of the nozzle before it sets, are controlled by the computer. In the Khoshnevis system, the nozzle and trowel arrangement creates the object's outside walls as a thin but strong shell. A separate pouring mechanism fills in solid objects by adding material in bulk, layer by layer. The researcher said he got the idea when smoothing plaster on his house.

The inventor acknowledges that programming the Contour Crafting system is more complex than with existing rapid-prototyping systems. Such systems need only describe the fabrication of the object using three control parameters - the three dimensions of space, represented as the x, y, and z axes of a CAD representation.

Contour Crafting programming needs to specify not just the three spatial coordinates, but three more control parameters - the desired orientation of the two trowels, plus a flow-rate for the extrusion nozzle. "These are not trivial problems," Khoshnevis said, "but they are solvable, and the solutions are becoming easier as we gain experience."