Plastic production processes are a set of operations that require extremely precise and dynamic control. One of the most important factors in these processes is controlling the moisture content of the material and introducing the plastic material in granule form into the production process with the correct moisture values. Moisture can cause serious problems in plastic production, reducing product quality and decreasing production efficiency. At NKT - Humidity Control Technologies, in this article we will focus on the strategies used to combat moisture in the plastic industry and the importance of these methods.
Plastic materials absorb moisture from their environment according to their chemical structure. Materials such as Nylon, ABS, and PC are classified as hygroscopic plastics that absorb moisture into their internal structure, while plastics such as PE/PP/PS retain moisture on their external surfaces. Therefore, the drying methods for each plastic differ before they are introduced into process machines. Particularly in injection and blow molding processes, the moisture content of the material directly affects the surface quality and durability of the product.
Condensation Type Mechanical Dehumidifiers
Plastic materials in granule form are included in the transportation process in containers as bulk or in big-bag sacks after production. Especially in hot and humid climate conditions, the moisture of the air trapped inside the container along with the loaded materials condenses due to day/night temperature differences and transforms into liquid form inside the container. The condensed moisture in the container environment settles on the surfaces of plastic granules as liquid. This condensation occurring inside the container is called "container rain." To eliminate the condensation occurring inside the container and the liquid dripping onto the products, containers with integrated dehumidifiers are used, as well as single-use silica gel materials in granule form that can be loaded together with the products during transportation.
Plastics in granule form that arrive at factories for processing are pumped into central silos located inside or outside the factory via bulk tankers. During this process, outdoor air is sent into the central silos along with the granule material. To ensure that plastic raw materials are sent with dry air from raw material transfer stations such as bulk tankers into the silos, dehumidifiers are used at the bulk tanker suction point. The dehumidifier, equivalent to the suction air flow rate of the bulk tanker and integrated into the suction port, ensures that plastic granules are transferred into the central raw material silo with dry air.
During raw material transfer without a dehumidifier, humid air drawn from the outdoor environment is sent into the silo, essentially sending water vapor into the silo as well. Water vapor disrupts the humidity control regime in the silo and causes blockages in the central raw material conveying system that transfers granules from the silo to process machines due to day/night temperature differences. For this reason, before raw materials are sent into the silo, the internal volume of the silo should be conditioned with dry air using a dehumidifier. Subsequently, materials sent by the bulk tanker should also be sent into the silo with dry air, ensuring that plastic granules in the silo are kept entirely in dry air. The use of dehumidifiers in silo conditioning is extremely important for stable production quality, both to prevent blockages in the descent and return lines of central raw material feeding systems and to minimize the drying time used on process machines.
Mold Area Dehumidifier Usage
In injection and blow molding processes in the plastic industry, due to the cold water supplied to the molds, when mold surface temperatures fall below the ambient dew point temperature (not dry bulb), sweating (condensation) occurs on the mold surface. Condensation on the mold surface can negatively affect the surface quality and dimensional accuracy of plastic parts.
The method generally used by machine operators is to raise the set temperature of the cold water sent to the mold to prevent sweating. This way, sweating is prevented and stable process progression is attempted. However, as the cooling water set temperature is raised, the cycle time in the process extends and production capacity decreases. In such cases, a dehumidifier is used to send dry air to the mold area to prevent mold area sweating, ensuring that the outer surface of the mold is surrounded by air with a low dew point. This way, even in the summer season when absolute humidity and dew point rise, the desired cold water supply to the molds is maintained without sweating, and production stabilization is achieved.
Silica Gel Rotor Chemical Dehumidifiers
Especially with advancing technology and growing demand for efficiency, electric injection machines are becoming increasingly widespread. With the "parallel movements" designed to shorten "cycle time" to the extent these machines allow, the cooling cycle time within the total cycle time can be further shortened with the cold water sent to the mold and the non-condensing mold surface. For dehumidifiers that perform rapid cooling and dry air blowing to work effectively, the airflow should ideally be kept under control and directed directly to the mold surface. However, situations such as machines with IML (In-Mold Labeling) features or systems with side/top robot entry can make this control difficult.
In IML (In-Mold Labeling) systems, ensuring that labels are placed properly inside the mold by the robot while simultaneously blowing dry air from the dehumidifier to prevent condensation on the mold surface requires a delicate balance. To achieve this balance, some important points should be considered in the design of the air blowing system:
Dry air directed from the dehumidifier to the mold area can be focused on specific zones through specific diffusers/nozzles or channels. Very high air velocities can cause labels to shift or be misplaced. Flow-adjustable valves or regulators can be used to control air velocity.
Completely enclosing the mold area may not always be possible, but partial partitioning or protective panels can be used for effective airflow direction. The width of the air blowing cross-section affects the spread of airflow. A wider opening can spread the airflow over a larger area at a lower speed, reducing the direct impact on labels. Airflow can be synchronized with robot entry/exit or labeling operations. Sensors and automation systems can adjust air blowing according to these operations to increase effectiveness.
Since each production environment is unique, the design of the air blowing system should be tailored to specific requirements. Additionally, conducting tests specific to the production environment is important to verify the system's effectiveness and its impact on the label placement process.
In some cases, flexible air blowing systems can be used. These systems can change the direction or intensity of airflow when needed.
The dry air blowing system to the mold area can be used not only in injection processes but also in blow molding operations in the plastic industry. Since similar problems can be encountered in both procedures, the application of dry air blowing with a dehumidifier has similar advantages in the blow molding process as well. Sending dry air to the mold area using dehumidifiers in both plastic production processes brings the following benefits:
- Moist or wet molds negatively affect the surface quality of plastic parts. Mold drying performed with a dehumidifier prevents such problems, enabling the production of higher quality and more precise parts.
- The dehumidification process accelerates the removal of moisture from the mold area, allowing production processes to proceed faster. It enables shorter cycle times and higher product output.
- Molds protected by dehumidifiers have extended service lives, and reprocessing and maintenance costs are reduced. Additionally, reduced defective production prevents material waste and lowers costs.
- Dehumidifiers and drying systems that can be adjusted for different plastic types and mold designs increase production flexibility, enabling efficient production of various products.
Thanks to dehumidifiers used at different points of the process starting from the bulk tanker, the drying phase of the material that begins at the silo and ends as the final part continues throughout its entire journey. This reduces the waiting time for raw material drying of 2-4 hours expected on the process machine for production.
Combating moisture in the plastic industry is not just a quality control measure but also a strategy that increases the efficiency and sustainability of production processes. Dehumidifiers supplied by NKT - Humidity Control Technologies enhance the competitiveness of companies in the plastic industry while enabling the production of more robust and reliable products in the long term. At NKT – Humidity Control Technologies, with our expert engineering team, we provide our customers with the most suitable dehumidifier solutions, offering sustainable, high-performance recommendations that adapt to advancing technologies and constantly changing market needs.