Before machining of the molds can be started, we must select
the plastics to be used for each part. There are a great many
plastics to choose from that can be used for injection molding.
We first discuss some considerations and general properties of
the resins we are considering using, and follow that with a comparison
of properties and characteristcis of the several plastics we've
zeroed in on as top choices for the radome itself.
There are two main options for the base plate and ring parts:
" use the same plastic as we use for the dome, and
" use one of the other resins listed below, or else a higher
flow 'nylon' resin such as a type 66, with 14% glass reinforcement.
(see footnote #1)
Previous plastics used in various radome-type GPS antenna covers
include lexan, acrylic, foams, ABS, Xenoy, and a variety of other
polycarbonates and alloys. Fiberglass covers, and other inventive
materials such as coated fabrics have been used in operational
or prototyping, as well as plastics. We recognize from these experiences
that opacity or low transmissivity of the GPS signal can occur
from things like metallic additives, used in fiberglass and plastics
either as a fire retardents or colorant. We also recognize that
some of the materials used in the past are insuffienctly resistant
to strong impacts, and feel that for the new injection-molded
dome, the plastic selected must be as strong or nearly as strong
as the lexan thermo-formed dome prototypes we had made in late
1997. Key parameters for evaluating this are 'notched impact'
and 'tensile elongation' values that we list below for Lexan,
Xenoy, and Geloy. The material must also resist degradation from
weather and ultra-violet (UV) radiation over a long time interval
(5-10 years) as well as possible. Whereas polycarbonates have
long molecular structures that tend to break down into shorter
molecules with time, UV stabilizing additives are available to
slow this degradation. Other plastics, such as polycarbonate-PBT
alloys or ASA-PVC alloys considered here, are inherently less
susceptible to UV degradation. Longer polymers are stronger but
less resistant to UV degradation, so there is a trade-off between
weather resistance and strength (see simple table below).
Please Contact Us to discuss your lexan machining
needs. You design it, we build it.
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