Are you having material-related quality problems with your purchased plastic parts? Poor control of the plastic resin’s moisture content may be the root cause.
Many engineering grade resins (e.g., Nylon, ABS, Acrylic, Polyurethane, Polycarbonate, PET, PBT) must be dried to prevent visual and mechanical quality problems such as:
- Loss of mechanical properties or brittleness
- Product defects and voids
- Visible streaks or bubbles on the product’s surface
- Cloudiness in clear parts
You may not have thought about plastic pellets being wet, but some can actually draw moisture to themselves at a molecular level, and they can appear dry on the outside when loaded with moisture. These are hygroscopic resins, such as nylons and the others listed above.
When a resin is described as hygroscopic, it means the resin has a strong affinity for moisture and will pull it from the air. All ambient air contains moisture; however, the levels will vary depending on the relative humidity, which is specific to air temperature. Depending on the resin material, it will hold moisture via either adsorption (surface moisture) or absorption (moisture within the pellet).
How Resin Is Dried
Several different types of dryers can be used to dry resin before processing, but increased temperature and dry air are two essential elements for most. It is important to note that the resin will begin to reabsorb moisture once re-exposed if proper measures aren’t taken to keep it dry until processing.
There is a temperature set point that must be reached for a resin to desorb or give up the moisture it is holding. If the temperature isn’t reached, desorption will not occur, no matter how long the temperature is maintained. Too high of a temperature above the set point for a specific resin can cause other issues like degradation of material properties and discoloration.
Super dry air is heated to the appropriate set point and used to dry the resin. A warm, dry environment (a low dew point) will cause the moisture to move from the plastic, which has a high vapor pressure, to the surrounding dry atmosphere, which has a low vapor pressure, as it tries to reach equilibrium. As the drying process goes on, moisture removal continues until the resin is dry enough for processing.
Partner with GreenLeaf Industries
Our engineers have a deep understanding of plastics and injection molding – from drying to processing. We engineer quality into our products and work closely with our customers to meet their expectations.
