THERMAL INSERTION

In thermal 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 heat from the thermal tip travels through the insert to the interface of the metal and plastic. Heat generated by the thermal tip causes the plastic to melt, and as the thermal tip advances, the insert is imbedded into the component. The molten plastic flows into the serrations, flutes, or undercuts of the insert, and when the heat terminates, the plastic resolidifies and the insert is securely encapsulated in place.

Inserts that can be installed by thermal insertion 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 thermal tip (see illustration). 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. 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.To maintain an accurate depth of insertion, the total distance the thermal tip travels should be limited either mechanically by a positive stop, electrically by a lower-limit linear encoder, or both.

THERMAL STAKING

In thermal staking, also referred to as “heading or riveting”, the controlled flow of the molten plastic is used to capture or retain another component, usually of a different material.

Thermal 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. The thermal tip contacts the boss and creates localized heat. As the boss melts, the light pressure from the horn forms a head to a shape determined by the horn tip configuration. When the heat is terminated, the plastic material solidifies and the dissimilar materials are fastened together.

It is the progressive melting of the plastic boss under continuous light pressure that forms the head. When staking, low pressure rather than high pressure is usually recommended.

Thermal 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 integrity of a thermal staked assembly depends greatly upon the geometric relationship between the boss and the thermal tip. Proper design will produce optimum strength with minimum flash.

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.

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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.

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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.

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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.

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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.

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Hollow Stake

Bosses with an OD in excess of 5/32 inch (4 mm) should be made hollow. Staking a hollow boss produces a large, strong head without having to melt a large amount of material. Also, the hollow stake avoids sink marks on the opposite side of the component and enables the parts to be reassembled with self-tapping screws should repair and disassembly become necessary.

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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. Knurled tips are available in a variety of fine, medium and coarse configurations.

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