Describes the definition, electric resistance values and materials of conductive plastic bearings.
Plastic is originally an insulating material, but can be made conductive by changing a resistance value with a conductive filler. Today, many conductive plastic materials are marketed according to applications.
Take information devices as an example. Along with recent digitalization of copying machines and printers, there has been an increasing demand for conductive plastic component parts, ranging from prevention of electrification to aggressive signal transmission.
Bearing parts are not an exception. An electrification preventive function is required for the paper feed section, transcription section, etc. For a sliding material, however, a contact state changes over time because of friction. Accordingly, it has a slightly different concept from general structural members.
Surface resistivity and volume resistivity are used as guides for the electrical characteristics of plastic materials (JIS K 6911). These are measurements conducted on a sample contained in mercury. For injection molded parts, however, filler segregation occurs because a plastic flow condition in the dies differs depending on the gate and product shape, resulting in a non-uniform resistance value from one measuring spot to another. There are an "apparent area of contact" and a "true area of contact" in contact of a solid body. In actual use, the "true area of contact" has an effect on conductivity. Accordingly, the JIS measuring methods are not always appropriate; the measured values by those methods may completely differ from the resistance values in actual use.
Our catalog also lists the surface resistivity and volume resistivity. Eventually, however, it is strongly recommended to measure with an actual shape for use and in an actual combination for use.
The graph in Fig. 2 shows a guide for resistance values when a sleeve-shaped bearing is combined with a shaft. As mentioned above, the resistance value differs depending on the actual product shape. Measurement has been conducted on many shapes, but note that Fig. 2 does not always apply to the shape which has no measurement experience.
Fig. 2 Electrical Characteristics and Guide for Resistance Values (Not Specifications)
Resistance value measuring conditions
| Item | Conditions |
|---|---|
| Test sample size (mm) | φ10×φ14×L10 straight bushing |
| Shaft | SUM＋Ni plating |
| Vertical load | 5N |
| Holding time | 30sec |
| Measuring instrument | Portable tester |
Shown as an example is a selection method when a thermoplastic conductive sliding material is applied.
Fig. Example of Selecting the Conductive Plastic Sliding Material
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