Both tests, as you describe them, are completely arbitrary. The resulting gel time is in no sense "real", but rather an arbitrarily defined endpoint that meets the needs of some application. If you want the rheometer test to match the sunshine test, you will need to run samples with a range of reactivities by both tests, and see if there is a correlation. If there is a straight line correlation, or close to it, then you can use that calibration to convert rheometer gel time to sunshine gel time.

There may not be a correlation. The sunshine test depends on a lot of things that are not necessarily related to cure speed, such as adhesion to the spindle and the way the bulk resin reacts to shear. The geometry of the rheometer is quite different from the sunshine instrument, but the behavior of the layer of resin between the cone and plate should be less prone to artifacts not related to cure. If there is not a correlation, then you may find adjusting the test parameters on the rheometer will help. It is possible, however, that the rheometer and the sunshine are simply not testing the same property of the material, and that there is no correlation. Then you will need to look closely at the application to see if the rheometer is able to make useful predictions. You might even find it to be more accurate than the sunshine.

Maxing out the torque on the rheometer may not be a very reproducible signal for gelation. It would be better to signal gelation when the G’ curve or G* curve reaches a particular set point. An alternate signal for gelation on the rheometer is the point where the G’ curve crosses the G” curve. This is also arbitrary, but it is at least a definition that should be repeatable across instruments and is less dependent on artifacts like geometry or adhesion. The disadvantage of the cross point definition is that it occurs early in the cure process. Often real world properties are better predicted by measurements done later in the cure process. In the end you must determine which test makes the most useful predictions about cure.

Thank you for your response Theodore Nevin Morrison. It was helpful. The rheometer and software that I'm using doesn't allow me to measure G', G'', or G*. I'm able to do constant shear stress, constant shear rate, and there's a mode called "soft solids," which allows for testing creep and shear modulus. I'm just trying to make the best of what I have.

"Constant shear stress" and "Constant shear rate" should be settings that tell the instrument how to run the test. They are inputs, but what are the outputs? The instrument must have some sort of readout. Some form of G (shear modulus) or E (Young's modulus) is usually available as an output of a rheometer. If you have a really nice instrument (i.e. the spindle can oscillate ) you can do dynamic measurements and there will be separate outputs for storage modulus (G' ) and loss modulus (G" ) and for tan(delta) (G"/G' ). Even if the only output is viscosity, I would pick some arbitrary value to signal gelation, rather than wait for the instrument to max out. What kind of instrument are you using?

Theodore Nevin Morrison It's a Brookfield R/S CPS+ with Rheo3000 software. Besides torque, you can see outputs of viscosity, deflection angle, deformation, density, kinematic viscosity, and delta deflection angle. I've been trying a constant shear rate experiment with a 50 mm cone spindle at 10 micron gap. I'm isothermally measuring torque over time. It exponentially increases over time and is programmed to stop at the instrument's maximum torque.