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