Basic requirement for transmission
The extruder of the present invention is characterized primarily in that at least those pin bodies associated with the entry region of the single screw extruder are provided with inserts which are incorporated into an external electrical monitoring circuit, are insulated relative to the pin bodies, and comprise a material of which the electrical resistance changes as a result of deformations. Alternatively, instead of electrical conductors, and with essentially the same effect, photo conductive filaments can be provided which are also incorporated into an external monitoring system, and the optical conductivity of which changes as a result of deformations. The inserts are expediently guided from the outer ends of the pin bodies, i.e. those ends remote from the housing, to the inner end portions which project into the interior of the housing. Each of the inserts may be accommodated in a central blind hole which is accommodated in a pin body and is open toward the outer end thereof. With a view toward the desired direct connectability to the monitoring circuit, it is advisable that the inserts, pursuant to a specific further feature of the present invention, contain an electrical or optical conductor which passes through in the form of a two-wire loop. Light conductive filaments offer no great selection with regard to their material as relates to the basic requirement for optical transmission. However, in the case of an electrically conductive element, it is advantageous if, pursuant to an important further feature of the present invention, the inserts contain a conductor which is made of a material which is considerably more brittle than metal, such as graphite or compound materials of particles; the conductor may be embedded in a glass, ceramic, or other electrically insulating synthetic material mass.
The present invention makes it possible reliably to recognize the occurrence of failure or breakage of pins at the time that they originate and to take preventive measures promptly prior to the occurence of severe secondary or consequential damages, or to already switch off the twin screw extruder drive as a result of pronounced deformations of the pins so as to prevent the breakage of the damaged pins altogether. Even if the strength of the pins themselves is not affected, the present invention does, however, produce the conditions for avoiding, according to all experience, the essentially more extensively feared secondary or consequential damages, and to limit the extent of necessary repairs to the mere replacement of the damaged pin body.
The electrical or optical pulse triggered in the event of damage can be converted with known means into an audible and/or visible warning signal, or can be utilized directly to shut off the drive of the sheet production extruder. The present invention is not restricted to any particular transmission means, circuit element, and control means, and is similarly not limited to a preferred monitoring system. Rather, it is within the scope of the present invention selectively to make use of electrical, photo electrical, inductive, capacitive, or possibly some other transmission means, depending upon the suitability and the existing conditions. Under certain circumstances, it may also be possible to use compressible or non-compressible pressure media.