The control of the variables and the selection of the appropriate technology depend on the physical understanding of the process and the material to be processed.
The presence of moisture in the materials usually generates appearance problems, sometimes making processing difficult. It may even lead to degradation of the resin, since the steam generated at high temperatures fractures the chains and modifies the properties, mainly in those materials that are produced by poly-condensation. Lack of drying can be evidenced by defects such as sparks, stains, discoloration, or in some cases loss of mechanical properties or variations in viscosity.
The materials are dried to avoid the problems generated by processing them in the presence of moisture. However, it is common to confuse the concepts of preheating and drying (dehumidification).
Materials that due to their chemical nature are classified as polar must be subjected to a dehumidification process, since they manage to incorporate humidity from the air inside their molecular structure. These include polyamides, PET, ABS, PC, PBT, and PMMA. These materials are called hygroscopic.
For their part, non-polar materials, such as PE, PP, PS or PVC, will only have moisture on their surface, so the pre-heating process is sufficient to remove it before processing.
In this article, from now on, when we talk about drying we will refer to the dehumidification process.
Variables to Consider During Drying
The drying process has four main variables that must be monitored and controlled, in order to achieve the correct consistency and the positive impact that the transformation process requires (whether we are talking about injection, extrusion, injection blow molding or extrusion blow molding) .
Drying temperature as a measure of the heat applied to the material to release moisture is the main variable. The heat generates movement of the molecules to such an extent that the moisture is released from the polymer chain.
The dew point temperature is the temperature at which the humidity in the air condenses and falls, and is dependent on the vapor pressure; a low vapor pressure around the material favors the migration of moisture from the material into the outside air. A stable and low dew point temperature reflects the ideal condition for the material drying process. Dew point temperatures close to -20 ° C are sufficient to dry most hygroscopic thermoplastic materials. Lower dew point temperatures involve a significant amount of energy and are not reflected in significant increases in drying speed.
The drying process does not take place instantaneously, it takes time. First to reach the temperature at which the water molecules start to move freely. Then, it takes time for the diffusion process to take place through the material. The diffusion speed will depend on the material and also on its internal humidity level; as it decreases, so will the rate of diffusion.
The dry air flow (with a low dew point) will be responsible for removing moisture from the drying hopper. The flow must be that necessary to ensure that the temperature inside the hopper remains stable according to the recommendation for the required time. If the flow is not required, the temperature profile may decrease and drying may be less efficient.
It is very important to find the ideal combination of the aforementioned variables, in such a way that the humidity conditions suggested for the process by the resin manufacturers are achieved. Also so that excessive energy consumption or even damage to the material (such as degradation or caking) is not generated.
Energy Saving Strategies
It is key to raise the temperature of the material only to the required value with the least amount of energy possible, keep that temperature stable, guarantee the regeneration of the drying material and thus use the least amount of energy necessary to remove moisture from the material.
Decreasing the heat losses in the system through insulation and regulating the air flows with speed variation in the fans will allow the least amount of energy possible for heating the resin. It is important that the least amount of energy is returned to the dryer and that it remains in the storage hopper using it effectively, so monitoring and controlling the hopper air outlet temperature is valuable.
The regeneration of the desiccant material is the process through which this material releases the humidity that trapped the material to be processed; This stage consumes an important part of the energy of the process, so it is also relevant to do it in the optimal way to keep consumption under control.
Ensuring a consistent drying process is essential. This is accomplished by keeping residual moisture levels within the limits set by resin manufacturers. The stable drying temperature is also evidence of a well-done drying, as well as maintaining the good operating conditions of the dryer over time (in heating elements, air ducts, filters, fans and drying material). Frequent monitoring of the status of these items is very important.
Warm Up vs. Dehumidification
In a non-hygroscopic material, the humidity from the condensation remains on the surface of the pellet and because there is no affinity between the water and the non-polar polymer, it is sufficient to subject it to pre-heating, because it is not necessary to remove moisture from the inside of the material. In these cases, a sufficient stream of hot air is ideal.
In hygroscopic materials, due to the affinity between water and polar resins, moisture is incorporated into the polymer structure and cannot be removed only with the flow of hot air. It will be necessary to generate a hot and dry air environment, in such a way that the vapor pressure that surrounds the granule is less than the internal one and thus the humidity migrates from the inside out. Achieving the exposure of the pellet long enough will allow it to be brought to the necessary humidity condition to avoid defects, as well as variations in the process and in the material properties.
• Maguire, in its ULTRA line of extremely low energy consumption, has one of the most interesting offers on the market. Using vacuum, this line consumes a tenth of the energy of a modern desiccant dehumidifier, saving thousands of dollars a year.
• Conair offers its high performance, industry ready 4.0 Carrousel Plus D that controls dew point, drying temperature and air flow. 75% of the wheel is used for drying the process air, while the rest of the area is in regeneration with high temperature and cooling.
• Wittman Battenfeld with its Drymax - Aton line also delivers lower than standard energy consumption results.
• Piovan recently launched the GenesysNext for PET preforms. It is distinguished by its self-adaptive technology, optimized for the treatment of recycled PET. You can handle the process automatically, autonomously adjusting all drying variables even against variations in humidity conditions.
• Moretto with its XD family offers a flexible dehumidification system for technical polymers. It is innovative because it is not the dryer that dehumidifies the polymers, but the polymers that give the dryer all the inputs for dehumidification. System management is not based on the traditional material / temperature ratio. Scheduling requires only two parameters: type of material and production per hour.
• Motan with its patented ETA plus technology, promises superior energy savings, achieving high level drying.
Article originally written and then translated for us by Tecnologia del Plastico.