Specific Resin Issues
Nylon and PET are two examples of hygroscopic materials and require careful attention to drying parameters. Certain grades of nylon, if exposed directly to water, can absorb up to 8% moisture by weight – much higher than most other resins (and take longer to dry).
Water bonds with the nylon polymer, making it particularly challenging to dry. Understanding a nylon’s initial moisture content and moisture capacity will determine how much drying is necessary to prepare the material for processing. When molded with too much moisture, nylon will exhibit defects including drool, flash or foam and can cause the pressure to drop in a molding machine or extruder. Nylon that is too dry when processed can produce parts that will be brittle or break.
PET materials are all hygroscopic and should generally be dried to a moisture content of .005% or less before processing. More specific drying guidance for various PET types depends on the material type – virgin or regrind – and the process.
PET appears in two states: crystalline (white) and amorphous (clear). When PET, primarily virgin PET, is in a crystalline state, it can be dried easily. Drying virgin PET at 300 to 350ºF for four to six hours at a dew point of -40º F (or lower) will bring the moisture content to the optimal level of .005% or less.
Amorphous PET, however, will soften and clump together (agglomerate) unless heated properly. Regrind PET is amorphous, for instance, as is some scrap generated by blow molding or thermoforming. Amorphous PET tends to cling to the walls of the drying hopper. To properly heat regrind PET, processors can agitate the material while heating to 180º F for 20 to 30 minutes. The material will change its appearance from clear to white as it crystallizes.
Amorphous and crystallized scrap are often combined and dried together by blow molders. If blended at a rate of less than 20% amorphous material, drying can be achieved without clumping. In thermoforming, if amorphous scrap is more than 20% of the mix, it must be crystallized before it is dried. If the percentages are higher or if the application is critical, a separate crystallizer can be incorporated to condition the material.
Combined Materials
In addition to combining virgin and regrind materials, processors often combine polymer resins of varying properties or additives. Some rules to follow when blending polymer resins include:
- When combining polymer resins with different drying times, dry the materials separately before blending and processing.
- If any additive(s) used in the resin combination have processing parameters similar to the basic resin, the modified resin can be dried and processed according to the supplier’s specifications.
- When dry blending materials, watch that they don’t separate when moved from the blender to the drying hopper. Uneven drying or bad parts can result from this separation.
Size and Density
Another key material consideration when drying resins is pellet size.
Smaller pellets – 1/8-inch diameter, for instance – have a larger ratio of surface area to volume, so they heat faster and dry faster. The moisture has a shorter distance to travel to the pellet surface.
Smaller pellets also have less free air between them, further reducing the required drying time. In some cases, smaller diameter pellets will pack in the drying hopper and a higher-pressure blower might be required to properly circulate the air through the material.
Larger pellets require more time because it takes longer for moisture to migrate from the center to the surface.
Lighter materials require a larger drying hopper to achieve the same drying time as a heavy material. Different materials exhibit different bulk density per cubic foot of material. For example:
PET: 50 to 55 pounds
ABS: 40 pounds
Polyurethane: 35 to 38 pounds
Regrind runners/parts: 25 to 40 pounds
Regrind bottles/sheets: 10 to 40 pounds
