Equipment and advice will definitely help, but in the end, good management will make a difference. #Best Practices

A well-maintained high-quality resin drying system enables you to precisely control four variables: 

 • Temperature is the cause of the movement of moisture, and it is very important to choose the ideal temperature for drying a particular resin. If you have any questions about the ideal drying temperature, resin manufacturers can publish online material-specific processing guidelines that apply to almost all materials.

 • The time is determined by the size of the drying hopper. These hoppers are specifically designed to allow mass flow or first-in first-out movement of materials. The term "residence time" refers to the length of time that a certain amount of resin—whether it is a single batch or each part of a continuous stream—is exposed to appropriate drying conditions. When all other factors are well controlled, maintaining a consistent residence time will cause the resin to dry all the time.

 • Airflow is what the dryer uses to transfer heat to the particles and remove moisture from the resin. Different dryer manufacturers may have different recommendations on how much dry air flow is necessary. Generally, Conair recommends at least 1/2 cfm/lb of material, which represents the “connected” air flow rate, which takes into account the “back pressure” of the closed drying system. Of course, there are many factors that affect the air flow, such as the relationship between the bulk density of particles and the recycled material, the volume of the drying hopper, and the cleanliness of the filter.

 • Dew point is a measurement of air moisture saturation based on temperature. Large amounts of air with very low dew points (such as -20, -40, or -60 F) have very low saturation points, so they are very, very dry. However, the low dew point air heated to hundreds of degrees during the drying process will produce an ultra-dry airflow, which has a huge ability to capture moisture and take it away. Therefore, a dryer that can maintain a consistently low dew point creates an environment that is not affected by environmental weather, temperature or humidity changes. This is very useful for dry consistency.

However, as every processor, manager, and operator knows, precise control of these four factors is just the beginning. There are more variables that need to be controlled, including some internal and external variables in the drying process, which will ultimately determine the quality and consistency of the resin drying operation. The following tips explain how to more consistently identify and manage these less understood variables and obtain better drying results.  

As mentioned earlier, the resin processing guide lists the finish line of the drying process: the ideal temperature and moisture content of the resin to be processed. But the starting line of the process is up to you. It is determined by three factors: material type, exposure time and environmental conditions. The graph below represents nylon resin and shows the rate at which the moisture-absorbing material absorbs moisture when exposed to a humid environment. At 75% relative humidity (RH), nylon like this can rise from 2000 ppm to 7500 ppm in a single shift (see Figure 1). This is a huge change in drying requirements.

Figure 1 Resin manufacturers usually specify the moisture absorption properties of resins in their material processing guidelines. This graph shows how nylon exposed to 75% relative humidity has increased from 2.5% moisture by weight to nearly 8% moisture in just 8 hours.

Of course, if you are using polycarbonate or ABS, exposure to ambient humidity will be more tolerant. But the point is the same: it makes sense to store the resin to minimize exposure to environmental conditions and maintain a relatively constant temperature.

Here are some ways to reduce the initial moisture content:

 • Store resin in a central location away from the dock door to avoid continuous changes in temperature and humidity levels. Let it adapt to the environment without interference.

 • Open only the resin you plan to use, one bag or container at a time.

 • Seal unused materials in bags or boxes.

 • Consider using foil-lined Gaylord boxes to obtain more unused quantities.

By adopting the practice of maintaining a consistent moisture content in all resins you use, you will minimize large fluctuations in drying time and conditions, and improve the consistency of resin processing and the quality of the final product. You will avoid a common pitfall—the temptation to "expand" dryer operations, equipment, and energy use to deal with extreme initial moisture levels caused by improper resin handling.  

Obviously, you must determine the size of the dryer according to your maximum requirements, but your minimum throughput should not be less than 50% of the maximum throughput. When the capacity is less than 50%, the airflow, temperature curve and mass flow rate will all be affected. In other words, using an oversized hopper or drying a small amount of resin will increase the residence time, because the resin below the full load will be exposed to the full load of heat and airflow. The result is that the risk of excessive drying or thermal damage is much higher (see Figure 2).

You must determine the size of the dryer according to your maximum requirements, but your minimum throughput should not be less than 50% of the maximum throughput.

Processors also use more regrind, which affects drying performance. Most notably, compared to smaller, denser resin particles, the lower bulk density of recycled materials usually requires the use of larger hoppers to maintain the same drying throughput. For example, a dryer with a hopper that can handle 1,000 lbs/hr of particles cannot handle the same rate of recycled material at all. Moreover, as the percentage of recycled material increases, the drying residence time should decrease because the total weight of the material is lighter.

Figure 2 The size of the drying hopper should meet the maximum throughput requirement, but the minimum throughput should not be less than 50% of the maximum. Therefore, the very small hopper load on the right side may be over-dried and damaged.

Consider the difference between feathers (recycled material) and pebbles (particles): to capture feathers of the same weight as pebbles or stones, a larger container is required. In addition, due to the shape of the flakes, the performance of dry air flow and moisture removal in the recycled material is different. Therefore, if you encounter problems in handling recycled materials-or encounter processing problems that may be caused by moisture-please contact your drying equipment supplier to help determine the appropriate dryer and hopper size for your application .

Another problem is to match the dryer with the hopper. We recommend using a dryer/hopper combination to deliver at least ½ cfm/lb of material at a speed of 20 to 40 feet per minute. Again, these numbers are based on the "connected" system. This is important because not all equipment suppliers rate their systems in the same way.

Another thing: it can be tempting to use existing equipment, perhaps by matching a spare dryer with a spare hopper, even if they do not match. Suppose you have a dryer that can reach 1,000 pounds per hour, but your hopper can only run at 200 pounds per hour. The capacity of the dryer is more than sufficient, but this combination can cause problems.

For example, the dryer will discharge air (and heat) suitable for a full load of resin in a large hopper of appropriate size (for example, 1/2 cfm/lb at a rate of 20-40 feet/minute). If the air flow is pushed through a smaller diameter hopper, the speed of the air will increase sharply. If too much airflow starts to fluidize the material, it will be difficult to keep the hopper loaded properly because the material will want to be discharged through the hopper outlet along with the airflow. On the contrary, due to the lack of proper air flow and insufficient drying energy, installing a small dryer on a very large hopper will produce poor results.  

According to the resin manufacturer's regulations, each material has an ideal drying temperature. Although some materials (such as RPET) can withstand "low and slow" drying-longer residence time at lower temperatures-other materials (such as nylon) may never reach the target moisture content unless exposed to specified In the same way, due to the risk of over-drying or thermal damage to the material, trying to use a temperature higher than the recommended temperature to accelerate drying is also problematic.

Take nylon as an example. Because their moisture content can become very high, as high as 8-9%, it seems appropriate to increase the drying temperature to reduce the drying time. However, the increased heat can cause thermal damage-brittleness, yellowing, or other color changes (see Figure 3). As mentioned in Tip 1, if you can limit the initial moisture content of materials through smart material handling practices, your situation will be much better. Doing so is the best way to ensure that you can dry the resin consistently without the risk of working outside of the materials recommended by the manufacturer.

Due to the risk of excessive drying or heat damage to the material, it is problematic to try to use temperatures higher than recommended to accelerate drying.

Figure 3 Four samples of the same nylon: The sample on the left maintains its natural color, indicating normal drying, while the sample on the right shows excessive drying, which leads to increased heat damage and yellowing.

In the continuous drying process, achieving a consistent residence time for all materials depends on keeping the hopper level uniform and maintaining continuous flow: the finished material flowing out should equal the new material entering. Because the material level in the hopper is related to the residence time, any reduction in the hopper level reduces the residence time and leads to inconsistent drying. Also, unless you design to lower the hopper level to limit the residence time, this may become a problem.

In other words: do not load continuous systems in batches. If you allow the hopper to dry material within 4 hours to run halfway before it is full, the first part of the newly loaded material will only have a residence time of 2 hours. It will be insufficiently dried, leading to possible production problems.

Do not load continuous systems in batches.

To obtain a continuous material flow and consistent residence time, use an integrated electric hopper loader or a central conveyor system. Although manually loading smaller systems seems to be an easy way to solve this problem, it is very inconsistent and requires constant monitoring. Another good use of the technology is to use the monitoring and control functions of the new dryer:

 • Use a level sensor to keep the material overflowing and make sure you get a good fill.

 • Use hopper temperature probes, such as Conair Drying Monitor, to ensure uniform temperature distribution, reflecting uniform air flow and material inflow from the bottom to the top of the hopper. If someone tries to load a large amount of "cold" material in batches at once, a sensor like this will detect temperature changes and issue a "low temperature" alarm, allowing you to make adjustments to prevent insufficient drying.

 • Use temperature suppression technology that can detect the increase in return air temperature and automatically reduce the dryer temperature to a lower standby level to slow down the drying speed or prevent excessive drying.

In general, you should limit the amount of material to be processed outside the dryer to approximately 15-20 minutes of supply. This means that, for example, it is not a good practice to dry the material and then load it into an undried machine hopper that can hold 1 to 2 hours of inventory. This will expose the material to temperature loss and moisture regain, and expose your process to changes in production quality.

Limiting machine-side material inventory is a good way to maintain process consistency.

You can adjust this rule according to the type of material and environmental conditions; but in general, limiting machine-side material inventory is a good way to maintain process consistency.  

The desiccant dryer has three key filters: process air, aftercooler, and regeneration (see Figure 4). Regular monitoring and cleaning of these three can keep the drying airflow running smoothly, eliminate system and desiccant pollution, and prevent overheating. At the same time, regular inspection of the dryer hose and connection for kinks, breaks or delamination can prevent process leakage, energy loss and poor drying results.

Figure 4 The location of the three types of filters on a typical mobile desiccant dryer. All air filters should be checked and cleaned regularly to ensure consistent drying temperature and airflow.

Moisture analyzers are not cheap. But when you (or everyone you ask for help) work hard to determine the cause of a process or quality problem, the guesswork, effort, and production losses that occur will not occur. You want to know: Is the problem caused by material supply, drying, or some factors in the machine or mold? It is worthwhile to own a moisture analyzer because it can provide quick and reliable answers to questions about moisture.

If the analyzer check shows that the machine has material moisture problems, you can go back and test the initial moisture level of the material before it enters the drying process. If these initial levels are abnormally high, you can isolate suspicious materials and correct material storage practices. If the initial moisture is normal, you can troubleshoot the drying equipment, knowing that it may be the source of the problem.

To obtain the maximum accuracy of the analyzer, use consistent sampling techniques:

 • Use the same container that can be sealed (ie a glass jar with a lid);

 • Fill the sample container completely, leaving no air space;

 • Test two samples of each material to reduce variability.

The success of resin drying and plastic processing lies in achieving and maintaining consistency. The above techniques provide seven fully validated and actionable methods to eliminate common mistakes and perform more efficient, consistent and productive resin drying operations. As you can see, the success of drying is not only related to the quality, functionality, dew point or alarm settings of the equipment you use. It is also related to what you do by identifying and managing material properties, limiting the impact of the surrounding environment, and investing in automation and work aids to eliminate variables and make more informed decisions. There is no doubt that good equipment and timely supplier advice are very important for drying success. But in the final analysis, good management can have an impact.  

About the author: Anthony “AJ” Zambanini is the Drying Product Manager of Conair Group, responsible for the continuous development and innovation of the Conair dryer product line. Before joining Conair in 2014, Zabanini was a former customer responsible for product management, plant engineering and capital improvement. He graduated from Pennsylvania State University in 2010 with a degree in mechanical engineering with a minor in technical sales. Contact: 724-584-5558; azambanini@conairgroup.com; conairgroup.com.

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