Cure characteristics are typically reflected using what is known as a “cure curve.” As shown in Table 17, a cure curve is essentially “torque versus time (at a given temperature).” The torque value is a direct indication of the sample’s shear modulus (resistance to shearing deformation). A number of processing characteristics can also be read, including the minimum pressure needed to make the material flow properly into the mold cavity, scorch time (prior to vulcanization), optimum cure time (typically 85 to 95% of maximum cure), and maximum cure (prior to over cure). Keeping the initial cure slightly below the maximum helps avoid over cure by allowing leeway for any necessary post cure (controlled continuation of vulcanization to finish cure, drive off byproducts, and stabilize) or inadvertent after cure (uncontrolled continuation of vulcanization after heat is removed).

Though specific vulcanization questions can be answered via a cure curve, rheometers also help molders address more general concerns about processibility and consistency. No matter what the cure curve says, “optimum” cure time is a matter of economics and logistics. There is no “universal” cure time for a given compound. A batch of rubber may have different cure times if given to different manufacturers, depending on their capabilities. Ultimately, the cure time for a specific molded part is based upon the design of that part.

The old adage about time being money is especially true when it comes to cycle time (the time between a given point in one molding cycle and the same point in the next cycle; e.g. loading of raw stock, through molding and unloading of finished parts, then to reloading; see Figure 46). Generally speaking, the longer the cycle time, the more expensive the process and the more costly the part. As a cost-cutting measure, manufacturers may increase mold temperature to decrease cure time. A 20° F boost can cut cure time in half, but this is not always advantageous. Sometimes the ratio of the time the mold is open (for unloading and reloading) to the time the mold is closed and in the press allows the mold temperature to dip below what is needed for full vulcanization. Partially-vulcanized, unusable parts can result.

Again, consistency among different batches of the same material is always a concern. The cure curve can serve as a “fingerprint” for a given batch of rubber. By comparing cure curves, it is possible to see if the properties present in one batch are present in another. Because wasted processing can be costly in terms of both time and money, compounding errors are much more economically spotted in batch testing than in subsequent stages of quality control, such as vulcanizate (cured rubber compound) testing.

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