Page 37 - Plastics News August 2018

FEATURES

hydrolysis is essential. Proper control over melt temperature, upper limit, what some in the industry might refer to as
residence time, and—where appropriate—moisture content “overdried.” In many instances, there is outright denial that
ensures that the molecular weight of the polymer will be WKH GHFLVLRQV PDGH RQ WKH PROGLQJ ÁRRU FDQ LQÁXHQFH WKH
properly maintained as the material undergoes conversion integrity of the polymer at a molecular level.
from pellets to parts.Part of the problem is that product At the lower melt temperature, parts with a good retention
made of degraded material can often look just as good, of average molecular weight were produced from all three
can be made at the same cycle time, and, when critical moisture levels. Even the wet material yielded parts with
dimensions are measured, the parts are to print. Unless no excessive reduction in molecular weight and there
some type of mechanical performance test is performed by was no evidence of the cosmetic defects that are often
the processor, changes in molecular weight will go unnoticed associated with incompletely dried material. However, at
until something fails after the product has moved farther the higher melt temperature, only the very dry material
down the supply chain. In a worst-case scenario, the part
gave acceptable results. The material dried to a marginally
LV LQ XVH E\ WKH ÀQDO FXVWRPHU ZKHQ LW IDLOV 7\SLFDOO\ WKH “safe” level produced parts that exhibited a slightly higher-
observed response is a brittle failure under stresses that than-recommended change in molecular weight, while the
are consistent with normal use.
wet material produced the type of change that had been
In a proper failure analysis on anything made of a polymer, a REVHUYHG LQ WKH ÀHOG IDLOXUH
molecular-weight determination should be performed, and Once again, all parts had an acceptable appearance.
ideally this result should be compared with that of the raw Dimensional checks across all the experimental groups
material used to make the part. If this test reveals that the
VKRZHG QR VWDWLVWLFDOO\ VLJQLÀFDQW YDULDWLRQV (YHQ VRPH
molecular weight has been reduced to an excessive degree, of the mechanical tests showed a negligible change in
the process parameters should immediately become the performance. However, when tests were performed that
focus of the investigation.
involved an impact loading, the problems with the parts
In some instances, it is necessary to demonstrate to the molded from the degraded material became more evident.
processor that orchestrated changes in the process variables Having been through an event such as this changes a
mentioned above can turn the molecular-weight problem on
processor’s focus—or at least it should. New attention is
DQG RII :KHQ WKHVH H[SHULPHQWV DUH SHUIRUPHG WKH UHVXOWV given to the process conditions and there is a heightened
are often surprising because they reveal an interaction DZDUHQHVV WKDW JRRG SDUWV FDQQRW VLPSO\ EH GHÀQHG E\
between these process conditions.
appearance and dimensional compliance. The condition of
Recently we performed such an exercise on a highly glass- the material from which the parts are made is important,
ÀEHU UHLQIRUFHG Q\ORQ $ IDLOHG SDUW UHWXUQHG IURP WKH WKRXJK LW FDQQRW DOZD\V EH YHULÀHG ZLWK WKH WHFKQLTXHV WKDW
ÀHOG H[KLELWHG H[FHVVLYH UHGXFWLRQ LQ DYHUDJH PROHFXODU are typically employed in a molding facility.
weight. A review of product that had not yet been assembled For processors who have never made the connection between
showed that the problem was greater than just an errant
their process conditions and the health of the polymer in the
VWDUW XS SDUW WKH PROHFXODU ZHLJKW RI WKH SURGXFW YDULHG molded part, their role in that outcome remains a mystery.
widely from marginal to considerably worse than what had
In many instances, there is outright denial that the decisions
EHHQ UHWXUQHG IURP WKH ÀHOG
PDGH RQ WKH PROGLQJ ÁRRU FDQ LQÁXHQFH WKH LQWHJULW\ RI WKH
In response, we conducted a relatively simple experiment: polymer at a molecular level. This is despite the fact that
:H PROGHG SDUWV DW WZR GLIIHUHQW PHOW WHPSHUDWXUHV we have known about the relationship between molecular
) & DQG ) & ERWK DUH ZLWKLQ WKH OLPLWV weight and plastics performance for almost 100 years.
of the suggested processing conditions. At each of these Molecular weight is just one of the material characteristics
temperatures we produced parts from material that was LQÁXHQFHG E\ WKH SURFHVV FRQGLWLRQV 1H[W WLPH ZH ZLOO ORRN
above the maximum recommended moisture content, at another one: crystallinity
material that was dried to just below that upper limit, and Mike Sepe is an independent, global materials and processing
material that was dried to a level considerably below the consultant.
Courtesy: Plastics Technology

37 August 2018 Plastics News