Why do some polymer materials need to be dried before injection molding? The chemical structure of a particular polymer determines whether it will absorb moisture due to their polar chemical structure. A number of polymers such as polystyrene, polyethylene, polyvinyl chloride and polypropylene are nonhygroscopic for their nonpolar chemical structures and do not absorb moisture. However, due to their more complex chemistry, materials such as polycarbonate(PC), acrylonitrile-butadiene styrene(ABS)terpolymers, PC-ABs blends, nylon, thermoplastic olyurethanes, polyesters and polymethylmethacrylate(PMMA)are hygroscopic and absorb moisture a problem arises at the polymer processing temperature which can exceed 200C boil off the water at 100C in the polymer that are hygroscopic polymer materials Fig 1.21 shows the moisture can either be external (surface)or internal (inside)of workpiece.
The effect that water on a molded workpiece may appear on the surface because the bubbles generated from the boiling of the moisture get trapped in the polymer, then cool, and solidify in the mold. This creates bring on the defect as water reaction such as polycarbonate and nylon-based materials, those splay marko or siherssmaky. in the surtace troubleshooting tends to reduce its physical and mechanical properties. The results in reversing the polymer-forming reaction in the polymer, leading to chain scission or depolymerization can make a polymer difficult, if not impossible, to process to obtain a desired product.
Hygroscopic material can absorb more moisture from the air than other plastic resins can. Hygroscopic volume of pellet is determined by the resin type, the ambient relative humidity and time, and will reach a moisture balance point with the surrounding environment. For example ABS
is usually 0.3%, whereas nylon is typically 0. 15%. It is very important to remove moisture not only
from the pellets exterior surface but from the pellets interior as well for drying plastics materials
process. There are four steps to driven moisture off the materials. The first is heating Heat can drive
the moisture to the surface of the pellet. The second is airflow that acts as"besom"to sweep the
moisture away from the surface of beads. The third is dry air that acts as a"sponge"or recipient to
remove the moisture of the pellet surface. The fourth is time which effects drying to take place.All
of these steps are important in drying which will greatly affect plastic materials properly. The delivery of air to the hopper must be as that it can absorb water from the moist pellets. The drier the air, the more effective it will be in extracting moisture from the resin. Tab. 1.5 lists typical drying conditions for thermoplastic materials.
