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Downsized Speed Sensor Gets Rugged Encapsulation...for Less

Lucas Control Systems has met the challenge of protecting the delicate circuitry of electronic wheel speed sensors through the use of DuPont thermoplastic encapsulation technology. Mounted either on a fixed member close to a car’s wheel bearing seal or on another rotating part fitted with magnets, the sensor uses either the Hall or magneto resistive effect to determine wheel speed and generate digital signals for antilock braking and other control systems.

Zytel® nylon 612 meets requirements for slow, low pressure mold filling at moderate temperatures, long-term compatibility with circuit components and resistance to damage by thermal cycling, moisture, road debris and salt, and automotive fluids.

   

The sensor is much smaller than coil-type, variable-reluctance sensors. Its internal electronic assembly consists of an IC chip, a capacitor, and possibly a diode welded to a lead frame. The lead assembly is encapsulated by placing it in an injection mold equipped with retractable support pins and then overmolding it with glass reinforced Zytel® nylon 612 resin. This encapsulated part may, in turn, be overmolded again.

Precise positioning of the IC chip is crucial to accurate speed sensing. Due to Zytel® nylon 612’s suitability for slow, low-pressure mold fill and slow crystallization at melt temperatures typically used in molding engineering thermoplastics, Lucas can avoid displacement or damage to the sensor’s delicate internal parts.

In Lucas’s grueling tests, the sensors functioned well with no moisture leaks in temperature cycling from -40 to +150°C (-40 to +302°F) followed by immersion in salt water and automotive fluids. In addition, the tough encapsulation of Zytel® nylon resists impact damage by road debris.

Manufacturing cost is up to 15 percent lower than that of using traditional methods, according to Lucas, and a DuPont technical specialist provided valuable assistance to Lucas engineers in resin selection, tooling and molding for the single-step encapsulation.

To learn more about any of these designs or to discuss your application needs, please contact us.