Page 20 - Plastics News September 2020

but we know that the melt density of an amorphous temperature slightly higher than the temperature at
polymer is approximately 10% lower than that of the which they were created. Consider PEEK again. A
solid material. Without the constraints of the mold and common annealing routine for PEEK is a 2-hr exposure
the ability to apply pressure while the material is to 200 C (392 F). This temperature is well above the Tg
cooling, our experience of the way materials shrink of the polymer and will effectively provide that last
would be very different. little bit of crystallinity that we could not obtain from
Annealing takes place in an unrestrained environment. the molding process.
Therefore, the dimensional changes the part will But consider a part that has been molded in an
exhibit during annealing will be much greater than essentially an amorphous state. When annealed at 200
expected from our experience as processors. In C, assuming the part is still in a form that is
addition, a part molded at a relatively low mold recognizable, it will consist of crystals that melt at
temperature will contain a much higher degree of approximately 220 C (428 F). This is a long way from the
retained orientation in its structure. The material that typical melting point of 343 C (649 F) that we expect
forms the surface will have a substantially different from the polymer. It is also below even the quoted
structure than the material in the core, and during the continuous-use temperature for the material, which on
annealing process the molded-in stress will tend to the UL yellow card can be as high as 260 C (500 F).
relax. This frequently results in warpage. The Therefore, while it is technically possible to forgo the
dimensional changes that the part will exhibit during
elevated mold temperatures normally associated with
annealing will be much greater than expected from our best practices for processing high-performance semi-
experience as processors crystalline materials, it is very risky and likely to yield
The final problem with “annealing in” most or all of the p a r t s t h a t d r a m a t i c a l l y u n d e r p e r f o r m
crystalline structure goes back to something we expectations—assuming that they emerge from the
mentioned in our last article: Crystals formed in the annealing process looking anything like the drawing. In
solid state through annealing are not as large or as our next column we will extend the annealing
perfect as crystals formed from the melt. It is known discussion to its application in crosslinked materials.
that crystals that are “annealed in” will melt at a

AIPMA created a Helpline for the Members and Industry

during lockdown

The template was circulated widely in the Industry to
provide services and bridge between members and GOI
with regard to manufacturers facing problems for
transporting essential goods etc. we received 350+
requests from units which were communicated to

concerned department.
Coordinated with DPIIT, DCPC and respective State
Governments.

Solution to most of the requests made on our Helpline. This initiative was widely
appreciated by the Industry

September 2020 20 Plastics News