Apple is developing ways to improve the durability and usefulness of ceramic as a housing for smart phones and watches.
In a patent application published today, Apple discloses a method of insert molding a ceramic housing. Thermoplastic is injected into the mold, giving the part added crack resistance and the ability to incorporate design features not possible with ceramic alone. The advantage of comolding is that the parts adhere without use of adhesives.
The concept of insert molding ceramic, of course, is not unique, but the Apple patent application details features specific to an electronic device. None of the information is technically groundbreaking, but reveals Apple’s thinking on future devices.
Apple likes the aesthetics, strength, scratch resistance and optical properties of ceramic as shown in its smart-looking Apple watch.
But there are drawbacks. “For example, small retaining features for coupling housing components together (e.g., clips, arms, detents, grooves) may be relatively simple to mold into a plastic piece, but may be difficult or impossible to form out of glasses and ceramics,” the patent application states.
Another problem is that ceramic can be brittle under stress.
The patent application states: “Moreover, the flowing of polymer material into imperfections and/or irregularities in the ceramic component (e.g., microcracks, discontinuities, or the like) may increase the overall strength of the ceramic and polymer part. In particular, the polymer material may reduce the stress concentrations that occur at or near such features, thus reducing the likelihood that the ceramic material will crack, shatter, or otherwise break under stress.”
One powerful way to overcome ceramic’s tendency to break under stress is to use a comolding of carbon-fiber reinforced plastic—the same reinforcement used in the fuselage and wings of Boeing’s 787 Dreamliner aircraft. The Boeing composite uses a thermoset binder. Apple said it may also use a thermoset, but that would require a slower forming process.
The application also states that in some instances it may be desirable to use a transparent plastic (such as acrylic) to show the inner workings of the device. There are also provisions to mold in body-tracking sensors.
For most purposes, the most likely plastic Apple would use for comolding ceramic would by polyamide (invented by DuPont as nylon), a strong engineering thermoplastic widely used in cars.