Effect of preheat temperature on the location of the minimum state of cure in a molded component

Abstract

ABSTRACT:The distribution of state of cure in a molded rubber component is usually nonuniform due to the slow heat transfer processes involved in the molding-cooling cycle. The minimum in the distribution usually occurs in the center of the rubber section. For large components, one method of reducing molding time is to preheat the uncured rubber prior to molding, thus carrying out a portion of the required heat transfer in a more favorable geometry than the molded shape. In this work, computation of the state of cure distribution in preheat-molding-cooling cycles for one-dimensional rectangular geometry shows that as the preheat temperature is increased, the locus of the minimum state of cure will move outward from the center toward the surface of the component. This has important implications for the range of cure states existing in an article and calls into question the convention of removing samples for state of cure tests from the center of the component. The variables considered in this work are the preheat temperature, the component thickness, and the target state of cure of the rubber compound. A design curve is developed whereby the preheat temperature to optimize the distribution of the cure can be established.

Simulation of the effect of variation in the cooling cycle on the state of cure in a rubber component

“…In addition, if the preheat temperature is not very high, the minimum in the state of cure profile is in the center of the component. 8 Since the temperature profile during heating in the mold decreases from the surface toward the center, more cure accumulates near the surface as the press cure proceeds. As the convection coefficient is increased, the press cure time must increase to balance the decrease in accumulated cure attained in the center of the component from the cooling cycle.…”

“…The effects of preheat temperature (T preheat ), component thickness (h), and target cure (Q target ) on the location of minimum cure (X min ), the variation in state of cure within the component, and the press cure time (t press ) have been investigated previously. 8 The main conclusions were that the location of the minimum in the state of cure distribution moves from the center toward the surface as the preheat temperature is increased and that it is possible to choose a preheat temperature that will confer the minimum average state of cure or minimum variation in state of cure profile. A common assumption in the determination of an appropriate cure cycle by analytical or experimental means is that the location of the minimum cure occurs in the ''center'' of the object.…”

“…This is a rational assumption and is often the case. It has been shown 9 that if a sufficiently high preheat temperature is used, the minimum in the profile of state of cure in a 1D rectangular geometry is not in the geometric center of the component. This occurs due to the combined effect of the outside-in temperature gradients that occur during conduction heating in the mold cavity and the inside-out temperature gradients that occur during convection cooling after removal from the mold.…”

Simulation of the Distribution of State of Cure for Multi-stage Curing Processes

“…The effects of preheat temperature (T preheat ), component thickness (h), and target cure [7] (Q target ) as well as the effects of the convective heat transfer coefficient during cooling (h cool ) and the temperature sensitivity of the cure kinetics [8] (represented by the activation energy Ea/R) on the distribution of state of cure have been investigated previously. The main findings were that the location of the minimum in the state of cure distribution moves from the center toward the surface as the preheat temperature is increased, it is possible to choose a preheat temperature that will confer the minimum average state of cure or minimum variation in the state of cure profile, ambient air temperature affects the state of cure profile significantly for low values of convection coefficient, and the required press cure time is essentially independent of the convection coefficient for h cool 420 W/m 2 Á8C.…”

“…This is a rational assumption and is often the case. It has been shown [7] that, under sufficiently high preheat temperature conditions, the minimum in the profile of state of cure in a simple infinite slab geometry is not in the geometric center of the component. The effect of the elastomer activation energy (Ea/R) and the convective heat transfer coefficient (h conv ) present during cooling on the state of cure distribution has also been investigated [8].…”