Drying Plastics Before Injection Molding – Temperature, Time, and Moisture Control

Introduction – Why Drying Is Essential

Moisture is one of the most common — and most preventable — sources of quality problems in plastics injection molding. Many engineering materials, including PA, PC, PET, ABS, PMMA, and PBT, absorb water from the surrounding environment, and even trace amounts of residual moisture lead to serious defects during processing.

Proper drying before injection molding is not optional — it is a process requirement for all hygroscopic materials. Inadequate drying causes silver streaks on the surface, voids, reduced mechanical strength, hydrolytic degradation of polymer chains, and dimensional instability of molded parts. This guide covers the parameters, methods, and best practices for drying plastics in injection molding production.

Hygroscopic vs. Non-Hygroscopic Materials

Classifying plastics by their moisture absorption behavior is fundamental to selecting the correct drying process:

Hygroscopic Materials

Hygroscopic materials absorb water into their molecular structure. Moisture bonds chemically with the polymer chains and cannot be removed by surface heating alone — it requires drying in an atmosphere with a low dew point. Materials in this category include:

• PA (polyamide / nylon) – moisture absorption up to 2.5% by weight; the most hygroscopic of the common engineering plastics
• PC (polycarbonate) – absorption 0.15–0.35%; susceptible to hydrolysis at processing temperatures
• PET (polyethylene terephthalate) – absorption 0.3%; IV (intrinsic viscosity) degrades when moisture exceeds 0.02%
• PBT (polybutylene terephthalate) – absorption 0.08–0.1%; must be dried before every processing run
• ABS (acrylonitrile-butadiene-styrene) – absorption 0.2–0.4%; moisture causes silver streaks and voids
• PMMA (polymethyl methacrylate) – absorption 0.3–0.4%; moisture degrades optical clarity
• POM (polyacetal) – absorption 0.2–0.25%; must be dried to <0.1%

Non-Hygroscopic Materials

Non-hygroscopic materials do not absorb water into their structure — moisture is present only on the surface of the pellets. Brief hot-air drying without dew point control is sufficient:

• PP (polypropylene) – absorption <0.01%; drying recommended only when condensation is present
• PE (polyethylene) – absorption <0.01%; generally requires no drying
• PS (polystyrene) – absorption <0.05%; short drying of 1–2 hours at 70–80°C

Effects of Moisture on Injection Quality

Residual moisture in pellets triggers a cascade of quality problems that scale with the level of contamination:

• Hydrolytic degradation – water molecules cleave ester bonds in polymer chains (PA, PC, PET, PBT). This causes a permanent reduction in molecular weight, mechanical strength, and chemical resistance. The damage is irreversible
• Voids and porosity – steam released during plasticization creates micro-voids inside the part, reducing strength and creating stress concentrators
• Silver streaks – visible lines on the part surface caused by the gaseous phase of water migrating along the melt flow front
• Degraded surface finish – cloudiness, roughness, loss of gloss — particularly critical in optical parts (PMMA, PC)
• Dimensional instability – moisture alters shrinkage behavior, causing dimensional variation between parts
• Process instability – variability in melt viscosity makes it difficult to maintain repeatable injection parameters

Drying Parameters Table

The table below lists recommended drying parameters for the most commonly used plastics. Values are indicative — always verify against the material supplier's datasheet:

Material	Drying Temperature [°C]	Drying Time [hrs]	Max. Moisture [%]	Max. Dew Point [°C]
PA 6 (nylon 6)	80–90	4–6	0.10	-30
PA 66 (nylon 66)	80–90	4–6	0.10	-30
PA 12	70–80	4–6	0.10	-30
PC (polycarbonate)	120–130	3–4	0.02	-40
PET (amorphous)	150–170	4–6	0.02	-40
PET (crystalline)	160–180	4–6	0.02	-40
PBT	110–130	3–4	0.03	-40
ABS	80–85	2–4	0.05	-20
PMMA	80–90	3–4	0.05	-20
POM (acetal)	80–100	2–3	0.10	-20
PPE/PS (Noryl)	100–110	2–3	0.05	-20
PPS	130–150	3–4	0.02	-40
PEEK	150–160	3–4	0.02	-40
TPU	80–100	2–4	0.05	-30

Note: drying times refer to pellets at a standard initial moisture content (after transport/storage). If material has been stored for extended periods under high-humidity conditions, increase drying time by 50–100%.

Types of Industrial Dryers

The choice of dryer depends on the material type, required moisture level, and production throughput:

Hot Air Dryers

The simplest dryer type — heats ambient air and passes it through a hopper containing the pellets. The air is not dehumidified, so the dew point depends on ambient conditions (typically +10 to +25°C). Suitable only for non-hygroscopic materials (PP, PE, PS) or for pre-heating pellets before processing.

Desiccant Dryers

The industry standard for hygroscopic materials. Air flows through a bed of desiccant (molecular sieve or silica gel) that removes moisture. Two desiccant towers operate alternately — one drying the process air while the other regenerates its desiccant. Key parameters:

• Dew point – typically -30 to -40°C (required for PA, PC, PET)
• Airflow – 1.0–1.5 m³/h per kg of material in the hopper
• Regeneration – automatic, 4–6 hour cycle per tower
• Energy consumption – higher than hot air dryers due to regeneration heating

Vacuum Dryers

Drying takes place under reduced pressure, which removes moisture at a lower temperature and in less time — up to 50% faster than desiccant dryers. Ideal for temperature-sensitive materials (TPU, elastomers) and production environments requiring rapid material changeovers. Higher capital cost, but lower energy consumption per kilogram of dried material.

Compressed Air Dryers

Use compressed air expanded to a low dew point (Joule-Thomson effect). Compact and fast to start up, but costly to operate due to compressed air consumption. Used as a supplementary solution or in low-throughput production environments.

Dew Point Control

The dew point of the drying air is the single most important parameter in drying hygroscopic materials. It determines the maximum capacity of the air to draw moisture from the pellets:

• Dew point -20°C – sufficient for ABS, PMMA, POM (materials with moderate hygroscopicity)
• Dew point -30°C – required for PA, TPU (strongly hygroscopic materials)
• Dew point -40°C – required for PC, PET, PBT, PEEK, PPS (materials susceptible to hydrolysis)

Dew point monitoring should be continuous and automatic. Modern dryers are equipped with dew point sensors and alarms that trigger when the threshold is exceeded. If the dew point rises above the target value, the dryer must automatically halt material feed to the injection molding machine.

Practical rule: the dew point of the drying air must be at least 10°C lower than the equilibrium moisture level required for the material. For PC, this means a dew point below -40°C — at -30°C, residual moisture may not reach the required 0.02%.

Moisture Measurement Methods

Monitoring the drying process requires reliable measurement of residual moisture in the pellets:

• Gravimetric analysis (LOD – Loss on Drying) – a pellet sample is weighed, dried in a laboratory oven (typically 2–3 hours at 150°C), and reweighed. The mass difference equals the moisture content. This is the reference method, but it is time-consuming — results after 3+ hours
• Karl Fischer titration – electrochemical titration method, accuracy to 0.001%. ISO 15512 standard. The most precise method, but requires a laboratory and reagents
• IR / NIR analyzers – infrared radiation absorbed by water molecules. Results in a few minutes. Requires calibration for each material type
• Inline capacitive sensors – mounted directly in the drying hopper or on the conveying line. Continuous real-time measurement. Accuracy ±0.01% after calibration

Recommendation: for series production, the best approach is a combination of an inline sensor (continuous monitoring) with periodic verification using Karl Fischer titration (sensor calibration every 1–3 months).

Common Drying Mistakes

Based on TEDESolutions' service experience, the most frequent errors made when drying plastics are:

• Insufficient drying time – starting the injection molding machine before the target moisture level is reached. The minimum drying time is not a suggestion — it is a process requirement
• Excessive drying temperature – exceeding the recommended temperature does not accelerate drying; it causes thermal degradation, pellet agglomeration, and discoloration. Particularly critical for PA and POM
• No dew point control – using a hot air dryer for hygroscopic materials. Air with a dew point of +20°C cannot dry PA to the required 0.1%
• Oversized drying hopper – if the dryer capacity far exceeds the machine's consumption rate, material remains in the hopper too long, leading to degradation at elevated temperatures
• Re-absorption after drying – dried pellets transported through open conveying lines or stored in open containers re-absorb moisture from the air within minutes. PA can absorb 0.5% moisture in 4 hours at 60% RH
• Neglected dryer maintenance – spent desiccant (molecular sieve) gradually loses its drying capacity. Replace every 3–5 years or after 20,000 operating hours

Troubleshooting

Common drying-related problems and their solutions:

Problem	Possible Cause	Solution
Silver streaks on parts	Residual moisture above limit; dew point too high	Extend drying time; check dryer dew point; inspect desiccant condition
Voids in the part	Moisture converted to steam; material degradation	Reduce moisture to the required level; check drying temperature (too high?)
Reduced mechanical properties	Hydrolytic degradation of polymer chains	Check moisture using Karl Fischer titration; replace the material batch if degradation is irreversible
Yellowing of parts (PA, POM)	Drying temperature too high; material residence time in hopper too long	Reduce temperature by 10°C; right-size the hopper to match consumption rate
Dew point will not drop below -20°C	Spent desiccant; system air leak; faulty regeneration heater	Replace desiccant; inspect seals and hoses; test the regeneration heater
Pellet agglomeration in hopper	Drying temperature too high; material has a low softening point	Reduce drying temperature; use a vacuum dryer; agitate the pellets
Variable part properties between batches	Differing initial moisture content between batches; no monitoring in place	Measure moisture of each batch before drying; standardize storage conditions

Best Practices

Proven principles for effective drying of plastics in injection molding production:

• Match the dryer to the material – hygroscopic materials require a desiccant dryer with a dew point of -30 to -40°C. A hot air dryer is not adequate
• Size the hopper for 4–6 hours of consumption – hopper capacity should correspond to 4–6 hours of machine throughput. An oversized hopper means excessive material residence time at elevated temperature
• Monitor dew point continuously – install a dew point sensor with an alarm. Critical for PC, PET, and PBT, where even a brief loss of dew point leads to degradation
• Keep the material circuit closed – use closed conveying lines to transport dried pellets. Any contact with ambient air means re-absorption of moisture
• Maintain a drying log – document temperature, dew point, drying time, and residual moisture for every batch. This data is required in IATF 16949 and ISO 13485 audits
• Service the dryer regularly – replace desiccant every 3–5 years, check air filters monthly, calibrate sensors every 6 months

Tederic injection molding machines of the NEO-T and D-Series offer optional integration with drying and material conveying systems, enabling centralized moisture monitoring directly from the machine controller.

Summary

Proper drying of plastics before injection molding is the foundation of production quality. Key takeaways:

• Hygroscopic materials (PA, PC, PET, PBT, ABS, PMMA) require drying in a desiccant dryer with dew point control
• Dew point -40°C is required for hydrolysis-sensitive materials (PC, PET, PBT, PEEK)
• Parameter ranges – drying temperature 70–180°C, time 2–6 hours, target moisture 0.02–0.10% depending on material
• Hydrolytic degradation is irreversible – moisture above the limit causes permanent loss of mechanical properties
• Continuous monitoring – inline dew point sensors and periodic Karl Fischer verification ensure process control
• Most common mistakes – insufficient drying time, excessive temperature, no dew point control, re-absorption after drying

TEDESolutions supports customers in selecting drying systems and integrating them with Tederic injection molding machines, ensuring optimal processing conditions for every material type.