Moisture Content

Dryer is the equipment that reduces resin moisture before molding, preventing hydrolysis (chemical degradation), splay (silver streaks), bubbles and unstable dimensions. It is mandatory for hygroscopic resins (PA, PC, PET, ABS, PBT). ## Dryer types - Hot-air: ambient air heated to 80 – 90 °C. Inexpensive but limited to non-hygroscopic resins. Cannot drive moisture below ambient. - Desiccant: dry air regenerated with molecular sieves (zeolites) or silica gel. Standard -40 °C dew point. Industry standard. - Vacuum: accelerated moisture removal under vacuum. Drying time 1/3 of desiccant. Expensive but fast. - Compressed-air dryer: chilled compressed air + filtration. For small volumes. ## Typical parameters by resin | Resin | Drying T | Time | Dew point | |---|---|---|---| | ABS | 80 – 90 °C | 2 – 4 h | -25 °C | | PA 6, PA 66 | 80 °C | 4 – 8 h | -40 °C | | PC | 120 °C | 4 – 6 h | -40 °C | | PET | 160 – 175 °C | 4 – 6 h | -40 °C | | PBT | 120 °C | 3 – 4 h | -40 °C | | PMMA | 80 – 90 °C | 2 – 4 h | -25 °C | ## System components - Drying hopper with diffuser / internal cone - Air heater (electric) - Circulation blower - Inlet / outlet filters - Regenerable desiccant bed - PID temperature control + dew-point sensor ## Common mistakes - Drying temperature too low: moisture not driven down enough - Too high: degradation / hopper sticking - Insufficient time: especially when switching from virgin to regrind (more hygroscopic) - Dew-point sensor out of service: desiccant saturated or regeneration failed - Loss of dry air between dryer and machine hopper (uninsulated)

Humidity is the amount of water vapor present in the air, and hygrometry is its measurement. In injection molding it matters because the surrounding air is the source from which a hygroscopic resin absorbs moisture: the higher the humidity, the faster a pellet picks up water that later causes defects. Humidity is the air; moisture content is what ends up in the plastic. ## How it is expressed - Relative humidity (RH, %): water vapor in the air compared with the maximum that air can hold at that temperature. Most plant readings are RH. - Dew point (°C): the temperature at which the air would saturate. Desiccant dryers are rated by the very low dew point of the air they deliver (e.g. −40 °C), which is what actually pulls water out of the resin. ## Why it matters in molding - Material pickup: open bags, ambient hoppers and humid plants let hygroscopic resins (PA, PC, PET, PBT, ABS) re-absorb moisture quickly — sometimes within minutes — undoing prior drying. - Drying performance: a dryer works by delivering air with a low dew point; high ambient humidity and leaks raise that dew point and weaken drying. - Condensation: cold tooling or inserts in a humid shop can collect surface water, causing splay and surface marks. ## How it is managed Control the plant and machine-side environment: keep dried resin in a sealed hopper, minimize exposure time, monitor dryer dew point rather than just temperature, and re-dry regrind and any material left open. Match the target to the material data sheet; non-hygroscopic resins tolerate humidity far better than hygroscopic ones. ## Related terms - See also: moisture, moisture content, dryer, regrind, resin ## What is the difference between humidity and moisture in molding? Humidity is water vapor in the air; moisture is the water the resin actually holds. Humidity is the cause — high air humidity drives a hygroscopic resin to absorb moisture, which is the effect inside the plastic. ## How does humidity affect injection molding? Humid air lets hygroscopic resins re-absorb water quickly and raises a dryer's delivered dew point, so parts can show splay, bubbles and reduced strength even after drying if exposure is not controlled. ## What is dew point and why does it matter for drying? Dew point is the temperature at which air would saturate with water; the lower it is, the drier the air. Desiccant dryers are rated by dew point because dry, low-dew-point air is what removes moisture from the resin.

Moisture is water held by a plastic resin — both on the surface of the pellet and absorbed inside it. In injection molding it is the single most common cause of cosmetic and strength defects, because at melt temperature that water turns to steam and can chemically attack the polymer. It is distinct from ambient humidity (water in the air) and is quantified as moisture content (% or ppm). ## Why it matters - Hygroscopic resins (PA/nylon, PC, PET, PBT, ABS, TPU) actively absorb water from the air; molding them wet causes hydrolysis — the water breaks polymer chains, permanently lowering strength and toughness even if the part looks fine. - Cosmetic defects: steam at the flow front leaves splay (silver streaks), bubbles, voids and poor surface. - Process noise: moisture changes apparent viscosity shot-to-shot, so a wet lot will not repeat like a dry one. ## How it is controlled Dry the resin before molding to the supplier's target on the material data sheet — typically in a desiccant dryer (not just hot air) for hygroscopic grades, at the specified temperature and time. Keep dried material in a closed hopper, limit exposure after drying, and re-dry regrind, which re-absorbs water fast. Non-hygroscopic resins (PE, PP, PS) only carry surface moisture and usually need little or no drying. ## Related terms - See also: moisture content, humidity, dryer, virgin resin, regrind ## Why is moisture a problem in injection molding? At melt temperature the water flashes to steam, causing splay, bubbles and voids; in hygroscopic resins it also triggers hydrolysis, breaking polymer chains and permanently reducing the part's strength. ## What is the difference between moisture and humidity? Humidity is water vapor in the surrounding air; moisture is the water the resin actually holds on and inside its pellets. High humidity is what drives a hygroscopic resin to pick up moisture. ## How do you remove moisture from plastic resin? Dry it before molding — hygroscopic grades need a desiccant dryer at the data-sheet temperature and time; keep the dried resin in a sealed hopper and re-dry regrind, which reabsorbs water quickly.

Moisture Analyzer is the instrument that measures water content of resin before processing, verifying that drying was sufficient. It is the last line of defense against moisture defects: splay, bubbles, hydrolysis (chemical degradation) and unstable dimensions. ## Measurement principles - Loss On Drying (LOD): heats the sample to constant weight. Inexpensive, slow (15 – 30 min) - Karl Fischer: chemical titration with water-specific reagent. Very precise (±10 ppm). Reference standard. - Coulometric: automated Karl Fischer variant, fast (3 – 8 min) - Capacitive / dielectric: in-line, continuously monitors moisture in the drying hopper ## Typical maximum levels - PA 6 / PA 66: <0.15 – 0.20 % - PC: <0.02 % - PET: <0.005 % (50 ppm) - ABS: <0.10 % - PBT: <0.04 % - PP, PE: no routine measurement needed (non-hygroscopic) ## Ideal shop-floor procedure 1. Sample from the hopper just above the feed throat 2. Sample size: 5 – 10 g 3. Measure with Karl Fischer (lab) or LOD analyzer (production) 4. Log each batch in the process record 5. Frequency: at every material change and once per shift ## Commercial equipment - Karl Fischer: Metrohm, Mettler Toledo - Compact LOD: Sartorius, A&D, OHAUS - In-line (dielectric): Process Sensors, Aboniq ## Common mistakes Calibration not verified with a standard sample, oven contamination (previous residues), inadequate sample size, and not purging the sampling line before drawing (ambient moisture).

Resin is the polymer raw material that injection molding turns into parts — the base plastic, usually compounded with additives and supplied as pellets. In the molding world "resin", "polymer" and "material" are used almost interchangeably for what goes into the hopper. ## Families of molding resin - Commodity: high volume, low cost — PP, PE, PS, PVC. - Engineering: better mechanical/thermal performance — ABS, PA (nylon), PC, POM, PBT. - High-performance: extreme heat/chemical resistance — PEEK, PEI/ULTEM, PPS, LCP. A resin can be thermoplastic (re-meltable, the norm for injection molding) or thermoset (cures permanently). Grades are often filled (glass fibre, mineral) to boost stiffness and reduce shrinkage. ## How resin behaves in molding Each grade has a material data sheet giving melt temperature, mold temperature, drying conditions and shrinkage. Key practical points: - Drying: hygroscopic resins (PA, PC, PET) must be dried or moisture causes splay and weak parts. - melt window: too cold and it won't fill; too hot and it degrades. - Regrind: clean scrap can return as regrind mixed with virgin resin, within limits. ## Related terms - See also: pellet, thermoplastic, virgin resin, regrind, material data sheet ## What is resin in injection molding? It is the plastic raw material — a polymer plus additives, supplied as pellets — that is melted and injected to form parts. ## What is the difference between resin and plastic? In practice they are used interchangeably; "resin" emphasises the raw polymer feedstock, while "plastic" often refers to the finished material or part. ## What types of resin are used in injection molding? Commodity (PP, PE, PS), engineering (ABS, PA, PC, POM) and high-performance (PEEK, PPS) thermoplastics, often available glass-filled.