Page 194 - Engineering Plastics Handbook

Thermoplastic Polyetherimide (PEI) 167

at least 2 to 3 h, although longer drying times or higher temperatures
[but at least 15°C (27°F) below the T g ] rarely cause problems.
When one is injection-molding PEI, care should be taken to keep tem-
peratures high. This will yield parts with little molded-in stress and pro-
vide articles with better mechanical properties, especially in terms of
impact. With transparent parts, polarized light can be used to determine
the degree of stress [25]. Such techniques can be employed as nonde-
structive quality tests. Often, oil-heated molds are employed to prepare
low-stress PEI parts. However, recent work has shown that, under some
conditions, water-heated (cooled) molds can also be used to make stress-
free parts [26].
While PEI resins are melt-processable, they still produce fairly stiff
melts compared with other thermoplastics (largely due to their high
). They also generally exhibit good adhesion to metal. To open the melt-
T g
processing window for injection molding further, use of a mold release
agent is recommended. Mold release grades of PEI—available in either
transparent or opaque versions [27]—allow for fuller packing of the
part, while still providing good release from the steel tool. In most
instances, the milder processing achieved by using the mold release
agent allows for production of parts with less molded-in stress and
better practical impact strength. Surprisingly, in many cases the release
agent does not have detrimental effects on subsequent bonding and
metallization operations. Of course, in any application this assumption
needs to be verified by part testing under actual assembly and end-use
conditions.

Compounded Products
PEI resin can be compounded with a number of different ingredients to
make a family of thermoplastic materials. One of the most useful of these
additives is glass [28].
Combinations of the thermoplastic polyetherimides with 10 to 40 wt%
fiberglass reinforcement are very effective for increasing strength and
modulus, as shown in Table 8.3. Heat resistance is slightly increased too.
The increase in stiffness is shown in Fig. 8.3, which compares modulus
versus temperature for an unfilled BPADA-MPD-based PEI resin and
blends with 10 and 30% short-chopped fiberglass.
Addition of glass improves both tensile and flexural strength, while also
increasing impact strength. PEI’s dimensional stability, which is very good
in the unfilled (neat) state, is even better with added fiberglass. As shown
in Fig. 8.4, a 40% fiberglass-filled PEI blend has a coefficient of linear
thermal expansion (CLTE) in the flow direction that is lower than that of
aluminum: 15 versus 23 µm/(m⋅°C). Unfilled PEI resins have a higher
CLTE of about 50 to 60 µm/(m⋅°C).