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. 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.

Moisture has a plasticizing effect on nylons that increases flexibility and impact resistance. Over-drying has the opposite effect, causing brittleness.

Addition 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. Glass reinforcements reduce the effects of moisture even more.

The moisture absorbed by pellets and regrind must diffuse to the surface before being removed. This diffusion rate is temperature dependent and takes time. The drying time required depends on particle size, dew point and temperature.

Signs of wet nylon include:

• Splay (silver streaks or flaws on parts)
• Nozzle drool (an uncontrolled flow of material out of the injection nozzle)
• Nozzle pops (air and water popping when purging the screw and barrel)
• Froth (the foaming reaction of water in nylon

Nylons must be molded with low moisture content to avoid splay and possible brittleness due to degradation. Some nylons can be used without drying if they are not exposed to humidity (i.e., nylon shipped in closed drums or gaylords).

Oven, or dehumidifying dryers are recommended with oven dryer temperatures set at 180 F for 12 hr to sufficiently dry the material. An air oven with recirculated, dehumidified air is recommended. The depth of the resin tray should not exceed 1 in.

Desiccant dryers require a 4-hr residence time. If you are processing 15 lb./hr, you need a 60-lb. drying hopper and a 15 cu-ft/min blower with two or more desiccant towers or rotating desiccant beds.

Some dryers monitor the weight of the material being used AND the temperature of the material. This eliminates over-drying.

Dry air conveying is recommended for central drying. This reduces the chance of the nylon picking up moisture again. Use a just-in-time loader on the machine to transfer the dry nylon from the dryer hopper to the processing machine. Keep only a 20-min supply of nylon at the machine. Material that is not processed after 20 min stands a greater chance of absorbing moisture as it cools.

A circulating fluid-type mold temperature control system is recommended when processing most types of nylon. The mold design process should consider the following:

• Be sure the proper channel diameter is provided during the mold design phase.
This allows for turbulent media flow and maximizes heat transfer. The distance
between channels and proximity to the cavities must also be considered. A mold
flow analysis will highlight areas where improvement can be made.

• Provide individual zone controllers for each half of the mold. This allows better
control of the temperature and permits mold half temperature differentials when
shrinkage control is required.

• Consult the material supplier's product specification sheet to determine the best
mold settings. Recommended mold temperature can vary by type, grade and
supplier.

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. Hydrolytic degradation is a chemical reaction that occurs at high temperatures with some polymers in the presence of water. It causes primary bonds in the molecular chains to be severed, thus reducing molecular weight.

Over drying (>0.08%) water will remove the plasticizing effect of the water molecules and make the resin viscous and hard to flow.


How to eliminate over-drying

There are some dryers on the market that not only monitor the weight of material being used per hour out of the drying hopper, but also monitor the temperature of the material. If no material is being used out of a particular drying hopper, the dryer will turn down the heat to slow any material degradation.


Some final advice

Check incoming moisture levels of nylon before accepting it from the vendor. In high humidity areas, make sure the moisture content of the pellet coming out of the dryer is less than 0.15% before processing it.

Also, check and record the moisture levels in the nylon on every shift. Do not depend so much on checking the dew point. This is a measure of the relative humidity in the dryer; the lower the dew point, the less residual moisture will be in the pellet for a given drying time.