Moisture has a significant effect on nylons in processing: heating to molding temperatures while wet (>0.2%) will result in hydrolytic degradation and a significant loss of physical properties. Recommended mold temperature can vary by type, grade and Consult the material supplier's product specification sheet to determine the best.This allows betterĬontrol of the temperature and permits mold half temperature differentials when Provide individual zone controllers for each half of the mold.A moldįlow analysis will highlight areas where improvement can be made. The distanceīetween channels and proximity to the cavities must also be considered. This allows for turbulent media flow and maximizes heat transfer. Be sure the proper channel diameter is provided during the mold design phase.The mold design process should consider the following:
Material that is not processed after 20 min stands a greater chance of absorbing moisture as it cools.Ī circulating fluid-type mold temperature control system is recommended when processing most types of nylon. Keep only a 20-min supply of nylon at the machine. Use a just-in-time loader on the machine to transfer the dry nylon from the dryer hopper to the processing machine. This reduces the chance of the nylon picking up moisture again. This eliminates over-drying.ĭry air conveying is recommended for central drying. Some dryers monitor the weight of the material being used AND the temperature of the material. drying hopper and a 15 cu-ft/min blower with two or more desiccant towers or rotating desiccant beds. If you are processing 15 lb./hr, you need a 60-lb. This eliminates over-dryingĭesiccant dryers require a 4-hr residence time.
The depth of the resin tray should not exceed 1 in. An air oven with recirculated, dehumidified air is recommended. Oven, or dehumidifying dryers are recommended with oven dryer temperatures set at 180 F for 12 hr to sufficiently dry the material. Some nylons can be used without drying if they are not exposed to humidity (i.e., nylon shipped in closed drums or gaylords). Nylons must be molded with low moisture content to avoid splay and possible brittleness due to degradation. Proper drying of nylon materials requires an awareness and control of the essential variables that affect the drying rate and the ultimate dryness. Froth (the foaming reaction of water in nylon.Nozzle pops (air and water popping when purging the screw and barrel).Nozzle drool (an uncontrolled flow of material out of the injection nozzle).Splay (silver streaks or flaws on parts).The drying time required depends on particle size, dew point and temperature. This diffusion rate is temperature dependent and takes time. The moisture absorbed by pellets and regrind must diffuse to the surface before being removed. Glass reinforcements reduce the effects of moisture even more.
Over-drying has the opposite effect, causing brittleness.Īddition of fillers reduces the effect of moisture by reducing both the volume of nylon polymer in the mixture and the space available to moisture molecules. Moisture has a plasticizing effect on nylons that increases flexibility and impact resistance. Under atmospheric conditions, virgin nylons absorb moisture slowly, with regrind and larger pellets absorbing faster than smaller pellets. The rate of absorption depends on temperature and humidity. The American Plastics Council reported that, in 1998, more than 1 billion of nylon were sold in North America and sales are expected to grow, increasing 6 to 10% over the next few years That's a lot of nylon that will require drying.Īll nylons absorb moisture from their immediate environment and eventually reach a level equal with the relative humidity of the atmosphere. Nylons are used in a number of applications and offer a combination of strength, stiffness, chemical and impact resistance and good wear and friction characteristics. While all polymers absorb some amount of moisture, none are as affected as much as nylons. Moisture absorption of nylon has been a source of study for many years.
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