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“Innovations in Cure Meter and Mooney Viscometer Technology” : Page 3
ODR with a Heated Rotor
However, it was found that a heater and temperature control sensor could be inserted into the head of the rotor by passage through the central shaft and up the shank of the rotor as shown in Figure 3. (U.S. Patent Number 5,526,693 was issued June 18, 1996, other patents pending).
By virtue of the availability of the new generation of digital PID controllers, it was found that the temperature of the rotor could be precisely maintained within a very narrow range. The temperature also recovered very rapidly when a sample was inserted into the curemeter. Even more surprising was the observation that heating the rotor from the center provided the same temperature over the entire rotor head.
The heating element and sensor are attached to the central shaft at all times and the rotor, with the sensor orifice through the central axis of the shank, is simply threaded over the heater, Figure 3.
The effect of heating the rotor is dramatic. As can be clearly seen in Figure 4, the temperature of the sample rapidly reached the actual die temperature. As a result, the measured time for completion of cure is shorter (compare the solid and dashed S' curves) and the test more accurately reflects the curing characteristics of the stock at that set isothermal temperature.
With the XDR® (in the ODR heated rotor mode), the rotor now provides heat to the sample rather than acting as a heat sink, as was the case with the conventional ODR.
This rapid heat recovery combined with a homogeneous temperature environment as shown in Figure 4, substantially improves reproducibility while shortening testing times and providing more nearly true isothermal data. It should also be pointed out that the heated rotor will substantially negate the adverse effect of 'loading time' on variability of results, a characteristic previously observed with the ODR [15].
Consequently, the results variability of the XDR® in the ODR heated rotor mode should approach that of the MDR. Furthermore, the heated rotor makes this improvement available to the user while providing the option to operate under conventional ODR (non heated rotor) conditions for correlation with historical data.
Deflection Corrections
In a conventional ODR, the system deflects under stress causing the strain (arc) to decrease as the torque, due to sample curing, increases [16]. These deflections occur primarily in three areas:
(1) deformation of the mechanical portion of the machine,
(2) deflection of the torque measuring transducer, and
(3) the twisting of the rotor shaft (potentially the most damaging).
Combined, these deflections lead to a significant error in both the measurement of cure times and torque, particularly in high modulus vulcanizates.
A value proportional to the modulus can be calculated by using the following formula:
Therefore, by determining the spring constant of the system, a cure curve can be generated that accurately reflects the true torque divided by the strain actually imposed upon the sample, i.e., a virtual modulus. The proposed design eliminates the need for correcting for deflections that occur in the mechanical portion of the machine and the torque sensor.
This is accomplished by measuring the actual strain at the central shaft. The signal, calibrated in degrees, is processed by the computer to correct for these deflections. Therefore, the experimental torque values can be normalized to the selected arc, even though the strain is changing throughout the test.
It is obvious that the twist of the rotor shaft can contribute much more substantially to the loss of strain as the torque increases. Typically the spring constant for a conventional ODR rotor is 0.002 degree per in/lbf or higher, depending on the length of its shank prior to connection to the central shaft.
Even more importantly, the cure curve without correction could yield misleading and inaccurate cure characteristics. To avoid these problems, the XDR® computerized data processing system provides corrected information based upon the spring constant of the rotor.
The impact of the twisting of the rotor and instrument deflection is shown in Figure 5 on a hard rubber formulation. Note the tendency of the uncorrected cure curves to “flat—top”. Of course, the stiffer the cured compound the greater the “flat—topping” and the larger the error in torque and curing characteristics.
An interesting, but dangerous, corollary of “flat—topping” is the “apparent” improvement in precision that one observes upon repetitive testing in these situations. The improvement is only an artifact of lower test sensitivity, misinformation for a uninformed operator. It is clear from Figure 5, that although T2 is only minimally effected, T90 is severely shortened and Lmax is greatly reduced.
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Copyright © 2006 CCSi, Inc. • All Rights Reserved • Published February, 2006
Corporate Consulting, Service & Instruments, Incorporated
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