Nylon Processing

Nylene Nylon Does Not Require Drying Before Use

Nylene Nylon is packaged in completely moisture-proof containers. The moisture content of the resin is less than 0.1 % as received by the user, and it is not necessary (or even advisable) to dry the resin before processing. The elimination of the pre-drying step, of course, is of considerable economic importance in terms of total production cost.

Figure 1 illustrates the rate of moisture absorption of Nylene Nylon, in granular form, exposed at 65% relative humidity and room temperature. It shows that, in addition to being delivered dry and ready for immediate processing, Nylene Nylon can be exposed to air for a certain length of time without adverse effects on its molding or extrusion characteristics. The recommended maximum moisture level for optimum processability is .3%. From Figure 1, this level is reached after about six hours of exposure.

Nylene Nylon is produced under exacting, controlled conditions and the uniform granule size (1/a inch cylinders) of the material is added reason for its low and evenly distributed moisture content. Irregular granular compositions are more susceptible to moisture. The smaller particles present in such compositions pick up moisture much faster than the larger particles and cause moisture to be present in irregular amounts throughout.

When a container of Nylene Nylon is partially processed and then stored for later use, it should be carefully sealed. For the storage period, the ” u-PRESS-it” type of Nylene Nylon container is recommended, but any clean moisture proof container with a sealable top may be used.

If Moisture Is Absorbed Before Processing

If Nylene Nylon is exposed to ai r for some reason (the container may not have been properly sealed during storage) and the moisture content becomes greater than the maximum .3% level recommended for best results, the resin may be dried in trays, not exceeding one inch in depth, in a circulating air oven at approximately 175°F. Oven temperatures much higher than 175°F. may cause gradual surface oxidation and discoloration of the granules. A maximum of 60 grains of water per pound of dry air is recommended as the highest humidity for the drying oven. An atmosphere containing more than 60 grains per pound may substantially increase required drying time. Regardless of oven temperature, the greater the percentage of water in the oven, the slower will be the drying time. If the moisture absorbed by the granules is much greater than .5%, vacuum drying is necessary. Drying by this method requires a vacuum of 2 to 5 millimeters of mercury.

Drying may take anywhere from 3 to 15 hours, depending on the amount of moisture absorbed. The time required can be rapidly determined by trial and error. There are analytical ways to determine the moisture level of Nylene Nylon, and more information on these methods may be obtained on request, from your Nylene representative. A relatively simple visual method for determining moisture level, is described below.

Visual Determination of Moisture Level

The visual test is a good indication of moisture content. It consists of placing several granules between glass plates on a hot plate, or similar heat source, maintained at a constant temperature approximately 50°F. above, the melting point of Nylene Nylon (melting point is about 425°F.).

The tendency of Nylene Nylon to form bubbles under such conditions increases appreciably with moisture content. A dry resin, suitable for processing, will have few bubbles. If moisture content is excessive, a number of bubbles will be present, and the material will require drying.

Splay marks or silver streaks (microscopic bubbles appearing in the product) may be caused by moisture in the material. They may also be caused by degradation due to high heater temperatures. From this we can conclude that if splay marks are present and if the visual test indicates no moisture, it usually means that heater temperatures are too high.

If Moisture Is Absorbed During Processing

While the amount of moisture absorption of Nylene Nylon in actual processing is ow it has a definite effect, both on product properties and on cert!3in processin9 characteristics. Control of moisture absorption in the hopper, then, may influence the uniformity and consistency of the end products. Therefore, this section discusses the absorption of moisture by Nylene Nylon (and polyamides generally) while it is in the hopper and the effect of hermetic sealing of the hopper. The study was conducted in two phases, using two widely different sets of machine conditions:

1. An injection molder with spiral mold.
2. An extruder fitted with crosshead die for small diameter tubing.
   
   

In accordance with standard practice, a relatively low viscosity Nylene nylon was used in the injection molding evaluation and a higher viscosity Nylene Nylon was used for the extrusion phase. The resins were basically similar in all other material characteristics.

Injection Molding Evaluation

The machine used for the injection molding evaluation was fully automatic. It was operated (1) with a loose hopper cover, permitting the resin to contact air and (2) with a hopper cover hermetically sealed with rubber gaskets and clamps to prevent air _contact. In_ the second case dry nitrogen was introduced into the base of the hopper to eliminate air and decrease the possibility of moisture absorption. Figure 2 illustrates the hopper area set-up for the two operations.

A spiral mold was used, and machine adjustments were identical. Evenly spaced marks molded into the spirals were used to determine spiral length (a point at the sprue being zero). In both cases, the hopper was filled with enough

material for about eight hours of continuous molding. Machine conditions (hopper temperature, humidity of surrounding air, cylinder temperature,  mold temperature, injection pressure, cycle, etc.) were kept constant.