Page 52 - Plastics News September 2018

FEATURES

F\FOH WLPH LV JRYHUQHG WR D VLJQLÀFDQW H[WHQW E\ WKH UDWH Another benefit of higher crystallinity that is often
at which the material achieves a certain modulus. This overlooked pertains to the way that nylons interact with
modulus, in turn, is related to the number and size of moisture. As nylon parts absorb moisture they lose strength
the crystals that form. The faster the crystals form, the and modulus. They also exhibit changes in dimensions. As
faster the part can be demolded. The graph shows that nylon parts take up moisture they swell. But the moisture
at some point, reducing the temperature of the mold, is only absorbed by the amorphous regions of the part.
probably the most common tactic that processors use to Therefore, the more crystalline the material in the part,
reduce cycle time, can be counterproductive. Even more the smaller the effect of the absorbed moisture will be
important, reducing the mold temperature will reduce on critical dimensions. Since cooling rate is a critical
the degree of crystallinity that is achieved within the factor in determining the degree of crystallinity in a
molded part. This is where the performance element of semi-crystalline polymer, the mold temperature selected
the discussion comes in. by the processor will be important to part performance.

:H FKRRVH VHPL FU\VWDOOLQH SRO\PHUV RYHU DPRUSKRXV Part geometry will, of course, be an important aspect of
polymers for a variety of reasons. Among these are WKLV SLFWXUH 7KH ZDOO WKLFNQHVV RI WKH SDUW LV D VLJQLÀFDQW
improved chemical resistance, environmental stress crack element in determining cooling time, and it will always be
resistance, and fatigue resistance. In addition, a higher the case that material that is in direct contact with the
degree of crystallinity is associated with higher strength mold wall will cool more rapidly than material in the center
and stiffness. of the wall. This means that the degree of crystallinity in
any molded part will vary with location. But part geometry
If the polymer is not given the opportunity to crystallize,
these properties will be compromised. There are is a constant. Mold temperature is a variable that is set at
other benefits the discretion of the processor. Often, the selection of a
of increased mold temperature is made without any awareness of its
crystallinity. importance to the performance of the part. Higher mold
One of these temperatures will typically promote a higher degree of
is the degree crystallinity if the mold temperature is somewhere on the
of modulus left side of the curve in the graph on p. 36. But raising
retention the mold temperature is usually considered to bring with
above the glass it an automatic penalty in cycle time. For some materials
transition. this may be true. Polyethylene has a Tg well below room
One of the key temperature.
benefits of Studies show that the peak crystallization rate for
semi-crystalline polyethylene is also below room temperature. For
polymers is that materials like this, colder is better and there is likely
they possess no downside to part performance. But with higher
XVHIXO VROLG VWDWH SURSHUWLHV DERYH WKHLU 7J :KLOH performance engineering polymers, where the Tg is well
amorphous polymers essentially soften and become useless above room temperature, lower mold temperatures can
as load-bearing materials above their Tg, semi-crystalline introduce a wide range of problems that may not be
PDWHULDOV HYHQ ZKHQ XQÀOOHG ZLOO UHWDLQ RI WKHLU apparent until the part is in use.
PRGXOXV )RU H[DPSOH DQ XQÀOOHG 3%7 SRO\HVWHU ZLWK D In our next article we will look at some examples of the
room-temperature modulus of 340,000 psi (2340 MPa), will unintended consequences of attempting to improve
still have a modulus of 48,000 psi (330 MPa) at 100 C (212 productivity by reducing mold temperature.
) 7KLV UHWHQWLRQ FDQ EH LPSURYHG E\ LQFRUSRUDWLQJ ÀOOHUV
VXFK DV JODVV ÀEHUV LQWR WKH PDWHULDO %XW LI WKH PDWHULDO Mike Sepe is an independent, global materials and
fails to achieve its intended degree of crystallinity, the processing consultant
expected modulus retention at elevated temperatures Courtesy: Plastics Technology.
will be reduced. This can have implications for creep
resistance, as we will show in a later article.

Plastics News September 2018 52