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KOPA®-PA6 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℃(167~185)will reduce water content to 0.1% or lower.
Selection of Injection Molding Machine
For molding KOPA®-PA6, 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 KOPA®-PA6 has such low viscosity when melted, compared with other resins, this may cause reverse flow of molten polymer. Therefore, the injection molding machine must be equipped with a screw head having a reverse-flow blocking valve. It is also advisable to use an intermittent nozzle to prevent nozzle drool.
Temperatures of Cylinder and Nozzle
Resins are melted by the heater located inside the cylinder and 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 suitable for melting down resins, and the nozzle area is set a temperature 10℃ lower than the melting zone, thus preventing nozzle drool. If the temperature of the melting zone is too high, thermal degradation and substance volatility can occur, causing bubbles or silver streaks in 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.
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 increases due to faster cooling, but cosmetic and mechanical properties are degraded due 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 KOPA®-PA6 is a crystalline polymer, and crystallization varies based on cooling speed.(Normally, the mold temperature of KOPA®-PA6 should be set at 60~80℃) If the mold temperature is uneven over melting zones, product warpage can occur.
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 injection pressure is required.
|Injection Speed||1st step||Low||Low||Low||Middle||Middle|
KOPA®-PA6 has very low melt viscosity and is highly sensitive to temperature changes. Generally, a temperature of 15~60℃ above the melt point should be maintained. Consideration should also be given to the fact that KOPA®-PA6 is highly water-absorbent and its residual monomer is easily melted down. Being affected by moisture, extrusion molded parts may contain bubbles and result in degradation of mechanical properties, and the lowered melt viscosity caused by the residual monomer, which adhere to the die mouth or its surroundings and the part itself after being extruded from die in a sublimate. This may produce defective molded products. Therefore, moisture content must be controlled at 0.1% or lower, the residual time inside the extruder must be minimized, and a die without dead space should be used
<Example> Extrusion molding for Monofilament
|Diameter of Monofilament||0.4 ~ 0.6 mm|
|Spinneret Size||2,0 ~ 3.6 mm in dia. X 15 ~ 20 holes|
|#1(Feeding)||200 ~ 230℃|
|#2(Melting)||250 ~ 270℃|
|#3(Metering)||230 ~ 260℃|
|Die Temperature||230 ~ 250℃|
|Quenching Water Temperature||7 ~ 8℃|
|1st Godet Roll||15 ~ 2 m/min|
|2nd Godet Roll||60 ~ 80 m/min|
|3rd Godet Roll||80 ~ 100 m/min|
|4th Godet Roll||75 ~ 90 m/min|
|Stretch Ratio 1||1: 4.0 ~ 5.0|
|Hot Water Batch Temperature||90 ~ 95℃|
|Hot Air Temperature||200 ~ 250℃|