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Given below are a few commonly asked questions.
(Q): Why does polycarbonate housing appear brittle?
(A): There are two major causes for embrittlement in plastic materials.
The first is contamination. Few materials can be mixed together at
the press without causing a wide variety of problems. The second cause
is molecular weight reduction. The property that benefits the most
from higher molecular weight is impact strength. Any time a polymer
is heated and subjected to the stresses of melt processing, there
is the potential for these polymer chains to break. If enough of the
chains break, the molecular weight is reduced, and beyond certain
point, we begin to see a decline in toughness. The polymer chains
that make up polycarbonate will break up rapidly if the material contains
more than .02 percent moisture when heated to processing temperatures.
(Q): What is the use of MFI?
(A): In diagnosing, processing changes while the melt flow test
has its limitations, it can be very useful in measuring the degree
to which a material has been altered by its processing. Reground moulded
parts can be melted and the viscosity can be tested. This result can
be compared to the value for the raw material. As a rule of thumb
that works for almost any unfilled material, the melt flow rate of
pellets should not increase by more than 30 percent during processing.
If it exceeds this limit, then either heat or moisture (if it is a
moisture-sensitive material) has degraded the material and performance
problems are more likely to occur.
Managing regrind for maximum quality.
(Q): Most moulders use regrind in a continuous loop. Virgin resin
is blended with a percentage of regrind and fed back into the machine,
and then virgin resin is blended with a percentage of regrind and
fed back into the machine. Is this familiar?
(A): After each pass through the machine, the amount of first pass
material that remains in the regrind becomes less and less, but it
is never completely removed unless the entire regrind system is purged
and restarted. Key performance properties of the materials can be
maintained in a continuous loop.
(Q): What effect does this continuous looping have on the real-world
processing environment?
(A): It is reasonable to imagine that even a few parts per million
of an incompatible material can make a virgin material unless if a
material is subject to hydrolysis. Processing even a very small amount
of wet material and introducing it into the continuous loop have a
bad effect on parts after the hydrolysis occurs.
Courtesy: Mr. P.M. Jariwala, Kolsite Industries
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