The technique put forward by Apple describes melting a portion of a plastic component onto a metallic surface, so that the melted plastic integrates into one or more surface irregularities.
In this way, the plastic part will be ultrasonically bonded to the one having a metallic surface.
The method of ultrasonic welding can be more effective than the one based on adhesives.
However, ultrasonic bonding can prove difficult due to the use of materials with two different melting points, like metal and plastic.
“Any desire to substitute metal parts in for plastic parts in a design where welding is the mode of attachment has been traditionally impossible,” the application states. “This serves to limit the abilities of designers with respect to the materials that can be used in a particular design, such as for the housing and internal features or parts of an electronic device having complex internal features.”
In the case of compact portable devices like the iPhone, Apple’s solution is to build metal components with surface irregularities that are “adapted to accept the flow of melted plastic” in order to allow a plastic component to be ultrasonically bonded to a metal one.
“Also, the plastic hardening step occurs while the melted plastic is inside at least one of the surface irregularities, thereby attaching the plastic part to the metallic part,” the document says.
The iPhone and the iPad are products credited by the application as devices that the technique could be applied to in the future.