Proper predrying is the key to ensuring quality in products made of engineering thermoplastics like nylon, polycarbonate and PET. Considering how costly these materials are, it is easy to see why it is so important to have good data about what is actually happening in the drying hopper and to use that information to gain tight control over that process.
Among the tools to help determine how effectively drying is occurring is a proprietary device that monitors drying. It can be used to measure temperature at several different levels in the drying hopper, documenting whether or not conditions are right for proper drying.
This article will examine why this information is so important, how the device works and how it can be used to take the guesswork out of the drying process, providing a higher level of control over the quality of plastic products.
As soon as an engineering polymer is manufactured and exposed to the atmosphere, it begins to absorb moisture. That is why they are called hygroscopic. If the moisture is not removed from the resin, it can cause processing problems, cosmetic defects and structural and physical weakness due to hydrolytic degradation.
Plastic pellets do not dry instantaneously. It takes time to raise the temperature of the pellets so that moisture can migrate to the surface. Resin manufacturers have defined how long this process takes for their particular resin types and grades. Remember, however, that effective drying time is the time the pellets are exposed to optimum drying conditions. Any time that the polymer spends in the drying hopper at anything less than the recommended temperature and dewpoint cannot be considered drying time, and you run the risk of insufficiently drying the material.
As long as the drying process is under control, you can be reasonably assured that the material being delivered to the molding machine or extruder has been properly conditioned and good parts will result (figure 1). With a process as complex as drying, however, how can you be sure it is under control? Almost more important, what are the consequences if the process should go out of control?
What Happens in the Drying Hopper?
If the material you are drying requires four hours under proper drying conditions, and you are processing 100 pounds (45 kg) per hour, then you need to have at least 400 pounds in the drying hopper at all times. Assume for a moment that a kink develops in the air hose delivering hot dry air to the hopper so that the airflow is reduced by 25 percent. A temperature sensor at the hopper inlet shows that the air temperature is at the correct setpoint — let’s say 250°F (121°C) — so there appears to be no problem. However, the reduced volume of air being delivered to the material in the hopper is now only sufficient to heat a portion of the material in the hopper to the proper drying temperature. Gradually, the temperature of the material in the upper portion of the hopper drops to 225°F (107°C) and finally to 200°F (93°C). The resin is spending only two-and-a-half hours at the prescribed drying temperature of 250°F (121°C). Several hours after the problem first occurred, incompletely dried material reaches the processing machine and the quality of the finished product begins to deteriorate (figure 2).
Suppose that the operator or quality-control inspector recognizes that there is a problem with the parts after about 30 minutes of production. The machine is shut down, the kink in the hose is discovered and damage control begins. Half an hour’s worth of production is suspect and must be scrapped. The 400 pounds of material in the hopper also is suspect and must be drained from the hopper. This material is quarantined and may need to be scrapped. Then it takes several more hours to refill the hopper and restart the dryer. Now, the fresh material needs to be dried for four hours so more time is lost. Finally, after a full shift or perhaps longer, it is possible to restart production. The damage is done, however. Hours of downtime have accumulated and hundreds or even thousands of dollars in raw material and finished product have been scrapped — all because the temperature reading on the dryer itself could not tell the whole story. It could not describe the conditions actually existing in the drying hopper.
The scenario just described is just one of many that can cause improper drying and cost you time and money. Other problems are shown in table 1.
The really unfortunate aspect to all these conditions is that they do not need to become problems. They can all be prevented by using a drying monitor device that can be added to even the simplest dryer controls.
The key to its successful performance lies in an in-hopper temperature probe that measures the material temperature profile at various points (up to six of them) from the top of the hopper to the bottom. These temperature readings confirm that every pound of resin is held at the proper temperature for the time specified by the material producer. If a heating element burns out or airflow is disrupted, the drying monitor device will recognize the change in conditions inside the hopper and sound an alarm so that the fault condition can be corrected long before product quality is affected. If loading equipment fails or the material supply runs out, if incoming material is too wet or too cold, even if the hopper or the dryer is the wrong size, the drying monitor will generate a warning long before the first bad part is made.
The drying monitor also can give processors the information they need to adjust airflow so that just enough heat energy is directed to the material for effective drying. The resin is neither overdried nor underdried and energy costs are held in check.
Considering everything that is invested in producing a saleable part made of an engineering thermoplastic, it is foolish to risk the scrap, downtime and energy waste that result when a problem occurs in the dryer. A relatively small one-time investment in dryer control technology — like the drying monitoring technology — can go a long way toward achieving the maximum return on the investment made in valuable raw materials.