ULTRASONIC INSERTION

In ultrasonic insertion a metal insert is placed in a cored or drilled hole which is slightly smaller than the insert. This hole provides a certain degree of interference and also serves to guide the insert into place. The vibrating, ultrasonic horn contacts the insert, and the ultrasonic vibrations travel through the insert to the interface of the metal and plastic. Heat generated by the insert vibrating against the plastic causes the plastic to melt, and as the horn advances, the insert is imbedded into the component. The molten plastic flows into the serrations, flutes, or undercuts of the insert, and when the vibrations terminate, the plastic resolidifies and the insert is securely encapsulated in place. In ultrasonic insertion a slow horn approach, allowing the horn to develop a homogeneous melt phase, is preferable to “pressing” the insert. Ultrasonic insertion provides the high-performance strength values of a molded-in insert while retaining all of the advantages of post-molded installation. Inserts can be ultrasonically installed in most thermoplastics. Some of the advantages of ultrasonic inserting over other methods include rapid installation, minimal residual stresses in the component following insertion, elimination of potential mold damage, reduced mold fabrication costs and increased productivity as a result of reduced mold cycle times.

In some applications multiple inserts can be imbedded simultaneously with special horns increasing productivity and further reducing assembly and manufacturing costs. Ultrasonic insertion is not restricted to standard-type, threaded inserts. Inserts that can be installed ultrasonically include a variety of bushings, terminals, ferrules, hubs, pivots, retainers, feed-through fittings, fasteners, hinge plates, binding posts, handle-locating pins and decorative attachments.

Typically, the plastic component is fixtured and the insert is driven in place by the horn. (See illustration.) However, in some cases, the part configuration might prohibit insert contact by the horn, and the horn is made to contact the plastic component instead of the insert. The functional characteristics or requirements of an application actually determine the insert and hold configuration. In all cases a sufficient volume of plastic must be displaced to fill the undercuts, flutes, knurls, threads and/or contoured areas of the insert. Care should be exercised in selecting the proper inserts. Inserts are designed for maximum pull-out strength requirements while inserts with vertical grooves or knurls are usually recommended for high torque retention. Regarding the hold configuration or insert selection, the recommendations provided by the insert manufacturer should always be observed. Because an insertion horn contacts the metallic insert, it is subjected to some wear and is usually made of hardened steel or titanium. For low-volume applications titanium horns with replaceable tips can be utilized. Ideally, the diameter of the horn should be twice the diameter of the insert. To prevent a “jack-out” condition, the top of the seated insert should be flush or slightly above the surface of the part. Rigid fixturing should be placed directly under the insert. In most instances it is necessary to initiate ultrasonic vibrations prior to horn contact with the insert. To maintain an accurate depth of insertion, the total distance the horn travels should be limited either mechanically by a positive stop, electrically by a lower-limit linear encoder, or both.

CAUTION: When inserting, do not use weld time in excess of 1 1/2 seconds.

ULTRASONIC STAKING

In ultrasonic staking, also referred to as ultrasonic “heading or riveting”, the controlled flow of the molten plastic is used to capture or retain another component, usually of a different material. Ultrasonic staking provides an alternative to welding when the two parts consist of dissimilar materials which cannot be welded or when simple mechanical retention of one part relative to another is adequate (i.e. as opposed to molecular bonding). The most commonly used application involves the attachment of metal to plastic. A hole in the metal part receives a premolded plastic boss. Vibrating at high frequency, the horn tip contacts the boss and through friction creates localized heat. As the boss melts due to frictional heat, the light pressure from the horn forms a head to a shape determined by the horn tip configuration. When the vibrations stop, the plastic material solidifies and the dissimilar materials are fastened together. Unlike ultrasonic plastics welding, staking requires that out-of-phase vibrations be generated between the horn and the plastic surfaces. Therefore, a light, initial contact pressure is a requirement for out-of-phase vibratory activity within the limited contact area. It is the progressive melting of the plastic boss under continuously but light pressure that forms the head. When staking, low pressure rather than high pressure is usually recommended. Ultrasonic staking should be considered when the parts to be assembled are still in the design stage. Several configurations for boss/cavity design are available, each with specific features and advantages. Their selection is determined by such factors as type of plastic, part geometry, assembly requirements, machining and molding capabilities, and cosmetic appearance. The principle of staking is the same for each; the area of initial contact between the horn and the boss is kept to a minimum in order to concentrate the energy and produce a rapid melt. The integrity of an ultrasonically staked assembly depends greatly upon the geometric relationship between the boss and the horn cavity. Proper design will produce optimum strength with minimum flash. Whenever possible, the bosses should be designed with an undercut radius at the base to prevent fracturing or melting. Holes in the mating parts should be radiused or at least deburred. Long bosses should be avoided, but if used, they should be tapered from the base to the top. The boss should be properly located and rigidly supported from below to ensure that the energy will be dissipated at the horn/boss interface rather than exiting the entire plastic assembly and fixture. Best staking results are obtained when the ultrasonic vibrations are started before the horn contacts the boss. This prevents “cold forming” and allows for the gradual reforming of the boss. The pretriggering of the ultrasonic vibrations is normally accomplished using a pretrigger switch.

Flared Stake (high profile)
The high profile flared stake satisfies the requirements of most applications. This stake is recommended for bosses with an OD of 1/16 inch (1.6 mm) or larger and is ideally suited for low-density, nonabrasive amorphous plastics.

Flared Stake (low profile) The low profile flared stake satisfies the requirements of most applications. This stake is recommended for bosses with an OD of 1/16 inch (1.6 mm) or larger and is ideally suited for low-density, nonabrasive amorphous plastics.

Spherical Stake (high profile)
The high profile spherical stake is preferred for bosses with an OD less than 1/16 inch (1.6 mm) and is recommended for rigid crystalline plastics with sharp, highly defined melting temperatures for plastics with abrasive fillers and for materials that degrade easily.

Spherical Stake (low profile)
The low profile spherical stake is preferred for bosses with an OD less than 1/16 inch (1.6 mm) and is recommended for rigid crystalline plastics with sharp, highly defined melting temperatures for plastics with abrasive fillers and for materials that degrade easily.

Flush Stake
The flush stake is used for applications requiring a flush surface. The flush stake requires that the retained piece has sufficient thickness for a chamfer or counterbore.

Knurled Stake
The knurled stake is used in applications where appearance and strength are not critical. Since alignment is not an important consideration, the knurled stake is ideally suited for high-volume production and is often recommended for use with a hand-held, ultrasonic welder. Knurled tips are available in a variety of fine, medium and coarse configurations.

Spot Welding Spot welding is used to join to compatible plastic materials together.