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“Innovations in Cure Meter and Mooney Viscometer Technology” : Page 4
It should be realized that deflection corrections assume a linear relationship between stress (torque) and strain (arc). This appears essentially true for the ODR at strains from 0 to ± 1° of arc and from 0 ± 0.5° for the MDR. Obviously these broad guidelines are compound sensitive. This lower arc requirement is a reflection of the thinner sample used in the MDR, which results in greater shear strain for each degree of arc.
Furthermore, the nonlinearity of the arc vs. torque relationship is even greater for the viscous region of the rheometer cure curve. Clearly, operation at the lowest arc consistent with the electromechanical restrictions of the test instrument is highly desirable.
There is another reason for operating at the lowest possible strain: the potential for slippage between the test sample and rotor/die is reduced (i.e., the “dirty die/rotor” effect) [17]. This slippage is highly hysteretic and it causes S'' to increase as cure proceeds and torque increases.
Sample Removl Problem
The heated rotor feature and correcting for deflections resolved many of the key limitations of the ODR. However, one vexing problem that has plagued ODR users from the outset is the difficulty in removing the sample from the embedded rotor.
Often the sample cannot be removed without removing the rotor with the concomitant loss in testing time and heat loss. Although the heated rotor satisfies the heat loss concern, it does nothing to negate the operational frustrations of sample removal.
Moving Die Rheomemter (MDR)
The MDR overcomes this annoying problem. As previously mentioned, this device has no rotor and shears the sample between two heated dies. Thus, the sample is easily removed and the temperature is well controlled.
However, MDR type machines are typically difficult to disassemble for the required routine maintenance and die surface cleaning to ensure that there is no sample slippage (dirty die/rotor effect) [17].
Therefore, it would be highly desirable to capture the positive features of the MDR but with reduced mechanical complexity and without the inherent additional cost associated with the conventional MDR design.
ODR — MDR Design Concept
Having developed directly heated rotor temperature control, we envisioned a hybrid ODR with the performance of a conventional MDR. This concept is depicted in Figure 6, the ODR configured as an MDR (US Patent 5,526,693 was issued June 18, 1996 other patents pending).
It consists of a heated rotor with an elastomeric seal between the bottom of the rotor and the bottom of the lower die cavity. The seal extends from the edge of the rotor to the inside wall of the die. This prevents the sample from flowing under the rotor, and contains it in the upper die cavity.
The contribution of the seal to torque is small and readily corrected. Thus, the heated rotor actually becomes a moving heated die, i.e., a virtual MDR. The thin sample rapidly achieves testing temperatures, the sample is easily removed from the test chamber, and the simplicity of the design provides for easy maintenance.
ODR — MDR Cure Curve Comparisons
Figure 7 shows a test comparing the XDR® configured as both a conventional ODR (unheated rotor) and the virtual MDR. Note that the cure time is reduced due to the small sample size and the homogeneous temperature of the sample resulting from the heated “die”.
A comparison of results obtained with all three of the ODR configurations described above are shown in Figure 8. Note, that the MDR generated data is nearly identical to the curve obtained with a modified ODR (heated rotor).
This is not surprising, since with the heated rotor the top and bottom test specimen are surrounded by heated / controlled elements in a manner similar to the MDR, i.e, the top and bottom of the test specimen may be visualized as two stacked MDR specimens with a heated rotor captured between the two specimens.
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Copyright © 2006 CCSi, Inc. • All Rights Reserved • Published February, 2006
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