Page 26 - Plastics News June - 2020

the polymer declines rapidly over a very narrow Often parts are removed from the oven as soon as the
temperature range of 140-155 C (284-311 F). prescribed annealing time is completed. The parts cool
rapidly from the annealing temperature to room
Recommendations of an appropriate annealing
temperature, undoing all the work that was done by the
temperature for polycarbonate vary between 121 C (250
annealing process.
F) and 135 C (275 F). These temperatures are close to the

Tg but remain below the onset of the rapid decline in A good guideline is a cooling rate no faster than 45° F/hr
modulus in order to prevent deformation of the parts. until the parts have reached a temperature of 140-149 F.
The objective is to use a temperature as close to this
The ultimate test of the efficacy of an annealing process
onset as possible without producing part distortion or an
is a solvent stress-crack evaluation. For each polymer
excessive degree of dimensional change. This will
there is a chemical or a mixture of chemicals that will
depend somewhat on part geometry and the level of
target a certain threshold of internal stress. Often this
support that can be provided to areas that tend to be
approach involves a mixture of two substances. One acts
most susceptible to distortion, such as areas around
as the inert ingredient while the other is the active
gates.
ingredient that promotes stress cracking. By changing
The second important parameter is the annealing time. the ratio of these two constituents in the mixture, the
This will depend upon the part thickness. Plastics are targeted threshold stress can be adjusted so that the
relatively poor conductors of heat, and the part must be stress in the part can be measured precisely.
allowed to come to a uniform temperature throughout.
ABS, for example, uses a mixture of an acetate such as
Typical recommendations are a minimum of 30 min once
ethyl acetate and an alcohol such as ethanol. Higher
the parts have achieved the desired temperature, plus 5
concentrations of the acetate required to induce stress
min/mm (0.040 in.) of wall thickness. For parts with
cracking correlate to lower internal stresses in the part.
sections thicker than 6 mm (0.250 in) the best results are
The same approach is used in polycarbonate. However,
obtained by doubling this time. Failure to provide
with polycarbonate the mixture is one of n-propanol and
sufficient time to reach and maintain a uniform
toluene. Parts are immersed in the mixture for a
temperature for an appropriate amount of time may
prescribed amount of time, removed and rinsed, and
actually produce an increase in the level of internal
then evaluated for cracking. The location of any
stress.
observed cracking helps to identify areas of the part that
Perhaps the most important condition associated with are susceptible to the formation of elevated levels of
annealing is the rate of temperature change, stress.
particularly the rate of change that occurs during the
An alternative approach uses a single reagent and the
cooling process. Ideally, the parts should be heated from
immersion time required to produce stress cracking is
room temperature to the annealing temperature at a
related to the internal stress in the part. As an example,

rate of no more than 50° C/hr (90° F/hr). But it is the

polycarbonate can be tested using propylene carbonate.
cooldown portion of the annealing process that has the
The level of internal stress in the part is a function of the
greatest influence on the result. Here again, specific
time that the part is immersed in the fluid. With either
recommendations vary.
method, an effective annealing process will produce a
However, a good guideline is a cooling rate no faster than notable reduction in the measured threshold stress.
25 C/hr (45 F/hr) until the parts have reached a
Annealing of semi-crystalline polymers is done for a
temperature of 60-65 C (140-149 F). Some parts may
completely different reason. In our next segment we will
need to be cooled at a rate as slow as 5° C/hr (9° F/hr).

discuss this process and the guidelines for getting most
The most common mistake that results in an
out of annealing this class of polymers.
unsatisfactory annealing outcome is cooling too rapidly.
June 2020 26 Plastics News