A printer fabricates an object from a computerized model using a fused filament fabrication process and a build material. One or more energy directors such as ridges are formed in an exposed surface of the deposited build material to provide regions of high, localized contact force that can improve interlayer bonding between successive layers of the build material. An ultrasonic vibrator can also usefully be incorporated into the printer to apply additional energy along these energy directors during deposition of a subsequent layer.