Dehumidifying or drying plastics in the processing phase is a vital part of injection molding. Drying plastic resin is used to minimize or eliminate complications that may be caused by too much moisture in a plastic material. The extent to which moisture affects the quality of a molded part is determined by the specific plastic resin being processed and the intended purpose of the part. This article will discuss two categories of resins as well as the advantages of drying plastic material.
Hygroscopic vs. Non-Hygroscopic
Each type of resin has a set of processing characteristics that have a certain affinity to gather moisture. These two groups of polymers discuss the difference between hygroscopic and non-hygroscopic polymers.
These polymers include Nylon, ABS, Acrylic, PET, PBT, Polyurethane, Polycarbonate, and many more. These resins absorb moisture internally and release moisture through the air. Any resin moving from storage to the molding machine often requires drying due to hygroscopic properties. When the wet hygroscopic pellet is surrounded by a dry and hot environment for a sufficient amount of time, the vapor pressure outside the pellet is lower than the vapor pressure within the pellet. The moisture within the pellet begins to migrate toward the area of low vapor pressure outside the pellet. Eventually, the pellet becomes dry. Below are some characteristics of hygroscopic polymers.
- They have a strong affinity to attract moisture.
- Internal moisture cannot be removed with hot air alone.
- Will absorb moisture into their molecular structure if exposed to ambient air. Must process quickly after drying.
These polymers include PVC, Polypropylene, Polystyrene, Polyethylene, and many more. These resins do not absorb moisture internally into the pellet. However, moisture can be collected on the surface of the pellet. Applying heat becomes an important part of removing surface moisture when this happens. Below are characteristics of non-hygroscopic polymers.
- Any moisture collected is on the surface of the pellet (adsorption).
- Typical moisture collection is due to condensation.
- Moisture is easily removed by passing a sufficient stream of warm air over the material.
Advantages of Drying Plastics
The moisture contained within the plastic may seem like a small aspect of processing, however if not controlled it can make it nearly impossible to produce quality plastic parts. Resin drying prior to processing maintains the performance characteristics of your resin and ultimately your competitive position. Some advantages of drying plastics include:
- Preventing Cosmetic Problems: Known as splay or silver streaking.
- Preventing Hydrolysis: A chemical reaction that breaks the covalent binds in the polymer chain, reducing molecular weight of the polymer and significantly reducing mechanical properties.
- Preventing Part Failure: When drying, if the maximum level of moisture appropriate for processing is not reached, premature part failure and structural defects can occur.
Again, we dry hygroscopic resins to get the moisture out. More importantly, it’s to ensure maximum polymer performance. We create parts for medical and other high-liability applications. We understand that if a wet resin is processed, we will be leaving a “fingerprint” on the part. If the part fails, tests can be done to see if the polymer chains are the proper length.
So how do you get this moisture out? The most predominate method in this industry is the use of a Hot Air Desiccant Dryer. Make sure to refer to our other blog discussing the different types of dryers, Which Resin Dryer is Right For You? From concept, to prototype, to completed product, Midstate Mold is committed to the success of your project. Contact us today!