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Flexible Thermoplastic elastomers are plastic
injection molded by Rebling Plastics for a multitude
of applications. The ability of these thermoplastic
materials to be processed via conventional plastic
injection molding equipment, as well as their
inherently short cycle times compared to cure
times for thermoset rubbers, have provided engineers
and designers with a lower cost alternative when
specifying flexible plastic materials. In addition,
there are standard compounds available that provide
a choice of mechanical properties, hardness, chemical
resistance and flexibility not readily available
with thermoset rubbers.
Rebling Plastics molds thermoplastic elastomers
for components such as cable strain-reliefs, over-molding
of handles or other components, aircraft helmet
applications, and medical devices.
Properties of Thermoplastic
Elastomers
Thermoplastic elastomers are available in durometers
typically ranging from 30 to 95 on the Shore A
hardness scale depending on the class of material
selected.
These flexible materials, commonly referred to
as TPRs or TPEs, typically belong to four classes
of thermoplastics: the olefinics; polyurethanes;
polyester copolymers and styrenic block copolymers.
PVC is another option for flexible thermoplastic
materials.
Two types of Styrenic elastomers are available
for plastic injection molding. The Styrene-Butadiene-Styrene
(SBS) block and the Styrene-Ethylene/Butylene-Styrene
(SEBS) block. Trade names for these materials
include KRATON® D and KRATON® G respectively. The
SEBS block elastomers have higher temperature
resistance and can withstand prolonged outdoor
exposure while the SBS block should be limited
to indoor applications. Both types are resistant
to water, acids and bases, but both are also attacked
by solvents. Both types can be molded in a variety
of colors.
Polyurethanes are known for their excellent abrasion
resistance and load carrying capability. Polyester,
polyether and polycaprolactone based urethane
grades work well with the plastic injection molding
process. The polyester types exhibit better mechanical
properties while the polyether types have improved
low temperature properties and resistance to hydrolysis.
The polycaprolactone group offers improved hydrolysis
resistance compared to the polyester based urethanes
while offering similar mechanical properties.
All of the urethanes can be molded in a wide
selection of colors and are available in many
grades for specific applications including flame
retardant grades.
The Olefinics possess the lowest density of all
the thermoplastic elastomers. DuPont's Engage® is an example of this material which is available
in clear and colored grades. Olefinics offer excellent
flexibility at temperatures approaching -60 °F.
Polyester copolymers provide for flexibility
and fatigue strength over a broad temperature
range. DuPont's Hytrel® is an example of this type
elastomer. This group is generally higher in price
than the olefinics and styrene based elastomers.
Designing Guidelines for Thermoplastic
Elastomers
Rebling Plastics design engineers
have extensive experience in optimizing plastic
part designs for functionality, cosmetics, cost
and processing. As with other thermoplastic materials,
maintaining uniform wall thickness throughout
the part is essential in producing an aesthetically
pleasing molding. Ribs and gussets should be used
to core out thick sections. The rib thickness
should not exceed 50% of the intersecting wall
thickness. Bosses incorporated in the design can
also result in sink marks if not properly designed.
To minimize and eliminate these issues, Rebling
Plastics engineering staff should be consulted
prior to issuing a final design. This design assistance
is provided free of charge.
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