Page 32 - Plastics News October 2019

FEATURES

One of the most notable early failures of PC came in the the molded part. This arises in part because of the more
small-appliance industry when the material was molded rapid development of the frozen layer as the material
into parts that routinely handled hot water. The short-term ÁRZV LQWR WKH PROG 7KLV FDQ UHVXOW LQ ÁRZ OLQHV WKDW DUH
effects of this application environment were negligible. YLVXDO HYLGHQFH RI LPSHGHG ÁRZ ,W DOVR SURGXFHV D KLJKHU
But over time with repeated exposure the material began degree of retained orientation in the more rapidly cooled
to embrittle and crack. The same mechanism, known as layers at the exterior surface. Orientation is useful for
hydrolysis, will take place very rapidly if the polymer is promoting shear thinning and reducing the viscosity of the
SURFHVVHG ZLWKRXW ÀUVW SURSHUO\ GU\LQJ WKH UHVLQ :KDW PDWHULDO DV LW ÁRZV LQWR WKH FDYLW\ %XW LI WRR PXFK RI WKH
occurs over hundred of hours in the solid state takes place RULHQWDWLRQ LV UHWDLQHG LQ WKH ÀQDO SDUW WKH SURSHUWLHV
far more rapidly when excess moisture in the pellets is become anisotropic, maximizing strength and stiffness in
exposed to processing temperatures of 536-608 F (280-320 one preferred direction but creating a weaker condition
C)for several minutes. The resulting polymer degradation in other directions.
reduces the ductility of the polymer. An even larger contributor to internal stress is the
But an often-overlooked factor in optimizing the ductility difference in cooling rate between the layer of material
of PC is the rate at which the polymer is cooled in the that is in direct contact with the mold surface and the
mold. Many years ago, I had a client that perennially interior material. Plastics are poor conductors of heat.
complained about its brittle PC parts. The client blamed Therefore, the interior layers in a part cool more slowly
this on the resin manufacturer initially, and when the than the surface layers. Running a low mold temperature
FOLHQW GLG ÀQDOO\ WXUQ LWV DWWHQWLRQ LQZDUG LW IRFXVHG exaggerates this difference in cooling rate. Variations in
RQ GU\LQJ DV WKH NH\ WR LPSURYLQJ SHUIRUPDQFH :KLOH the cooling rate in the different layers of material that
this was good, it did not solve the problem. The missing make up a part produce differences in shrinkage. This
piece was the mold temperature. This processor that SUREOHP EHFRPHV PDJQLÀHG DV WKH ZDOOV JHW WKLFNHU 7KLV
molded parts for its own product line, routinely ran its shows up as a phenomenon known as critical thickness, a
molds at 110-120 F (43-49 C) in order to minimize cycle property particularly well documented in polycarbonate.
times. Several times, it was demonstrated that when This behavior is observed as a very large reduction in
the parts were produced in molds that ran at 180-190 F notched Izod impact resistance as the wall thickness of
(82-88 C) the parts displayed excellent toughness. This the test specimens increases.
was done with no penalty in cycle time. But old habits All this supports the argument that hotter molds produce
can be hard to break, and soon after the changes were parts with better properties, even in an amorphous
made, technicians within the plant intervened, turning polymer like PC where there is no expectation that a
the mold temperatures back down and reintroducing the crystal structure has time to form. But what is the optimal
brittle condition. mold temperature? In our next segment we will cite case
Lower mold temperatures and the associated faster studies where mold temperature was used to improve
cooling rates produce higher levels of internal stress in performance as a way of answering that question.

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Plastics News October 2019 32