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Processing of KOPA6 |
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1. Pre-Drying
KOPA6 has a high water-absorption rate. When resin absorbs water
it causes hydrolysis during the injection molding process, resulting
in degradation of properties and unsatisfactory molded parts.
Therefore, pre-drying is essential prior to injection molding.
Drying equipment may include a hot-air dryer or a dehumidified dryer,
the latter being more efficient.Normally, a dehumidified dryer operated
4 to 6 hours at a temperature of 75-85℃will reduce water content to
0.1% or lower.
2. Injection Molding
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2-1 Selection of Injection Molding Machine
For molding KOPA6, a screw-type injection molding machine works
best, with a minimum 20 L/D screw and a compression ratio of 3-4,
and a chrome-plated nitrogenous steel base.
As nylon 6 has such low viscosity when melted, compared with
otherresins, this may cause reverse flow of molten polymer.
Therefore, the injection moldingmachine must be equipped with a
screw head having a reverse-flowblocking valve. It is also advisable
to use an intermittent nozzle to prevent nozzle drool.
2-2 Temperatures of Cylinder and Nozzle
Resins are melted by the heater located inside the cylinderand by
the share stress generated between the screw and cylinder barrel.
Generally, the bottom side of a hopper is sufficiently cool for smooth
transportation of pellets. The melting zone is set at a temperature
suitablefor melting down resins, and the nozzle area is set a
temperature 10℃ lowerthan the melting zone, thus preventing nozzle
drool.
If the temperature of the melting zone is too high, thermal degradation
andsubstance volatility can occur, causing bubbles or silver streaks
in the the produced part.
Conversely, if the temperature of the melting zone is too low, the
flowability of the resin is reduced. This necessitates excessive
injection pressure, thus degrading the properties of the produced part.
2-3 Mould Temperature
Mold temperature affects product appearance and the accuracy
of product dimensions, and has a direct effect on productivity due
to shortened cycles. If the mold temperature is too low, productivity
increasesdue to faster cooling, but cosmetic and mechanical
properties are degradeddue to reduced material flow.
It is essential to set the proper mold temperature necessary to
maintain properties as required by the particular product specification,
because KOPA6 is a crystalline polymer, and crystallization varies
based on cooling speed.(Normally, the mold temperature of KOPA6
should be set at 60~80℃.)
If the mold temperature is uneven over melting zones, product
warpage can occur.
2-4 Injection pressure
Molten polymer flows through the mold cavity under the force of
injection pressure, and a considerable amount of pressure is lost as
material flows through the narrow sprue, runner, gate, etc.
Normally, the pressure applied to the cavity may require about
300~500kg/㎠ per projected area of the produced part.
If injection pressure is too great, residual stress is increased due to
over-charge that can cause horizontal strength degradation. If the
pressure is too low, it can cause short shots, shrinkage, and
bubbling.Also, in order to compensate for the 15~25% volume
decrease incurred during material cool-off, a holding pressure of
30~80% of the initial injectionpressure is required.
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