Drying Guide for Engineering Plastics: Why Moisture Control Determines Part Quality

The Hidden Cost of Wet Plastic

Inadequate drying is the single most common cause of injection molding defects across all engineering plastics. It causes splay marks, silver streaks, brittleness, reduced mechanical properties, dimensional instability, and in the worst case, catastrophic molecular weight loss that renders the part unusable. Yet many processors still rely on rules of thumb and visual inspection rather than verified drying parameters. The cost of a single scrap batch can exceed the cost of a desiccant dryer by a factor of 10. This guide covers the science of polymer drying, the specific requirements for each major engineering plastic, and practical methods for verifying dryness before processing.

The Science of Moisture in Polymers

Polymers absorb moisture in two ways: surface adsorption (water molecules adhering to the pellet surface) and absorption (water molecules diffusing into the amorphous regions of the polymer). Surface moisture is easily removed by hot air drying. Absorbed moisture requires desiccant drying because the water molecules are bound within the polymer matrix. The critical difference is between hygroscopic polymers (which absorb moisture into the bulk) and non-hygroscopic polymers (which only have surface moisture). Hygroscopic polymers include PEEK, PEI, PSU, PPSU, PA6, PA66, PC, ABS, and PET. Non-hygroscopic polymers include PP, PE, PS, and POM (though POM must be handled carefully due to its chemical sensitivity to moisture). When a hygroscopic polymer is processed without adequate drying, the absorbed moisture vaporizes at melt temperature, producing steam bubbles. The volume expansion from water to steam is approximately 1,600× — the resulting gas bubbles create splay marks, silver streaks, and internal voids. More critically, moisture can hydrolyze the polymer chains — water molecules react with the polymer backbone, breaking the molecular chains and reducing the molecular weight. This is irreversible and destroys the mechanical properties of the part. A PEEK part molded from wet resin can have 30-50% lower tensile strength than a properly dried part, even if the surface appearance is acceptable.

Drying Parameters by Polymer

PEEK: 150°C for 3-4 hours, target moisture <0.02%. PEI (Ultem): 150°C for 4-6 hours, target <0.02%. PSU: 120-130°C for 3-4 hours, target <0.05%. PPSU: 130-150°C for 3-5 hours, target <0.05%. PC: 120°C for 3-4 hours, target <0.02%. PA66: 80°C for 3-4 hours, target <0.2%. PA6: 80-100°C for 4-6 hours, target <0.2%. ABS: 80°C for 2-4 hours, target <0.05%. PET: 120-150°C for 4-6 hours, target <0.005%. PPS: 120-150°C for 2-3 hours, target <0.05%. LCP: 120-140°C for 2-3 hours, target <0.01%. POM: 80-100°C for 2-3 hours, target <0.15%. These parameters are for desiccant dryers with a dew point of -30°C or lower. Hot air dryers are not suitable for any hygroscopic polymer — they cannot achieve the low dew point required to remove absorbed moisture.

Verifying Dryness

The most reliable method for verifying polymer dryness is the dew point meter. A desiccant dryer with a properly functioning desiccant bed should produce air with a dew point of -30°C to -40°C. If the dew point rises above -10°C, the desiccant bed is saturated and must be regenerated. The moisture content of the polymer can be measured by a moisture analyzer (Karl Fischer titration or loss-on-drying). For production environments, the practical test is the 'purge test' — purge 5-10 shots of material through the nozzle and inspect the melt for splay, bubbles, and steam. If the first 5 shots show splay that clears as the material is purged, the material at the barrel surface was wet and the remaining material is dry. If the splay persists, the material in the drying hopper is still wet. A trained operator can identify wet material by the sound of the melt exiting the nozzle — wet material produces a sputtering, popping sound from steam generation, while dry material flows smoothly.

References & Industry Standards

• ASTM International. Standard Specifications for Engineering Plastics & Thermoplastics. astm.org
• ISO. ISO 1043 — Plastics — Symbols and Abbreviated Terms. iso.org
• National Institute of Standards and Technology (NIST). Polymer Properties Database. nist.gov
• UL Prospector. Plastics & Elastomers Material Database. ulprospector.com
• MatWeb — Material Property Data. matweb.com