Basis of Establishing Tire Cure

Long vulcanisation times are required for thick sections like tire
shoulder.
To obtain uniform vulcanisation throughout thick-sectioned articles,
the accelerator systems are adjusted in such a way that the cure
rate of the least heated portion is fastest whereas those close to the
heat source is of the slowest cure rate.
In a typical truck tire, the PLC is found to be at the geometrical
center of the tire shoulder. This point is referred to as the “Tread
Mass”.
Another critical location in the tire is at the fabric-rubber interface
(tire shoulder) known as “Under tread”.

A "cure equivalent" is defined as one minute of curing time at a
constant reference temperature, usually 287°F.
The PLC is so referred to because it is the critical point at which the
desired number of cure equivalents is to be delivered. When neither
more nor less cure equivalents than optimum are delivered to the
PLC, the article is said to have had a "perfect cure".
Methods of computing the number of cure equivalents to be
delivered, or determining the location of the PLC are known as
Thermocouple study & Blow point study respectively. Blow point also
gives an indication of cure safety

Blow Point Study

“ Blow point is the time of internal pressure release in the press at
which there is just a trace, or no evidence of under cure porosity
detectable up on tire sectioning ”.
?A tire is said to "blow" when its state of cure is not sufficient to
prevent entrapped or evolved gases in expanding the rubber matrix.
?When sufficiently cured, even if the optimum number of cure equivalents is
not delivered to the PLC, most of the entrapped gases have escaped through
vents in the mold, the matrix of rubber is thoroughly reinforced by virtue of the
cross linking of polymer chains, and releasing the pressure does not produce any
visual evidence of porosity in the cured article.

Curing Time of a Tire

Curing time = State of cure definition + Safety factor + Pressure
release time
Typically, cure time for a truck tire is Blow point (time) + 5 minutes
Safety Factor
•Deviations in compound (reaction time)
•Deviations in tire thickness
•Deviations in temperatures
•Steam temperature
•Hot water temperature
•Mould temperature
•Deviations in bladder thickness

The point of time at which there is no porosity in the PLC
( point of least cure) is defined as the technical cure time
The remaining ‘heating’ will take place while ‘cooling’ the tire in
post cure inflation units.
Thus in practice, when cure cycles are developed, the Heat
engineer is to ensure that at the end of the post cure inflation,
various parts of the tire attain optimum cure - be it a small tire or a
gigantic one like Off-the-road tire.

Thermocouple Study

Thermocouple studies investigate the rate at which heat is furnished to the
tire from inside & outside.
Temperature profile at critical locations like Tread Mass, Under Tread, Tread
Surface are obtained by using T type thermocouples (Twisted wires of Copper-
Constantan).
Temperatures are measured at fixed frequency, cure equivalents calculated
using Arrhenius eq. and compared against target cure equivalents at each
Location
The cure equivalents of cool spot (at drain) must be 15% state of cure
corresponding to the compound rheograph, and 90% state of cure after cool
down.