In injection molding production, the most insidious enemy of quality is moisture. It cannot be seen, it appears at the moment of molding and it quietly raises the scrap rate. In this article we explain which plastics need to be dried and why, and how to choose the right dryer.
What Does Moisture Do to Plastic?
As damp granulate turns into melt, the water evaporates and creates two kinds of problems:
- Visual defects: Silver streaks on the surface, flow marks, bubbles, a matte surface
- Structural damage: In materials such as PET, PC and PA, water breaks the polymer chains at high temperature (hydrolysis). The part looks sound from the outside, but its impact strength and mechanical life fall seriously.
A visual defect is noticed and corrected; hydrolysis, on the other hand, comes to light in the field, months later, when the part breaks. That is why drying is not an "option" but a process requirement.
Hygroscopic and Non-Hygroscopic Plastics
Plastics fall into two groups according to how they relate to moisture:
Hygroscopic (absorb moisture into their structure)
PA (nylon), PC, ABS, PET, PBT, PMMA, TPU, PLA, ASA... These materials do not just hold moisture on the surface, they absorb it into the granule. They start taking up water from the ambient humidity the moment the bag is opened. Surface hot air does not reach internal moisture — a dry air (desiccant) dryer is essential.
Non-hygroscopic (moisture stays on the surface)
Polyolefins such as PE, PP and PS do not absorb moisture; only condensation on the surface can occur. For this an economical [hot air dryer](/en/category/ferlin-hotdry) is sufficient.
How Does a Dry Air Dryer Work? What Is the Dew Point?
A dry air dryer passes air through a desiccant (moisture-retaining) bed to lower its dew point to levels of down to –40 °C, then heats it and passes it through the granulate in the hopper. The very dry air draws the moisture out of the granule.
Two concepts that are often confused:
- The process temperature is positive (e.g. 80 °C) — the temperature at which the air is blown onto the granulate
- The dew point is negative (e.g. –40 °C) — a measure of how dry the air is
Air can be both hot and very dry at the same time; it is this combination that does the drying.
Typical Drying Values by Material
- ABS: 80 °C · 2–3 hours
- PA6/PA66: 80 °C · 3–5 hours (a high-temperature version is an advantage for PA66)
- PC: 120 °C · 2–4 hours
- PET: 160–180 °C · 4–6 hours (critical in preform production)
- PMMA: 80 °C · 3 hours
- TPU: 80–90 °C · 2–3 hours
- High-performance plastics such as PPS, PEEK and PEI: 140–170 °C — require a high-temperature dryer
The values must be verified against the manufacturer's datasheet; excessive time or temperature also yellows the material and drives off the additives.
Choosing the Right Dryer
Ferlin's three dryer families meet three different needs:
1. [EasyDry](/en/category/ferlin-easydry) — dry air (desiccant), 50–150 °C. The standard for hygroscopic materials including PA, PC, ABS and PET. Hopper volumes of 30–1500 L. 2. [EasyDry HT](/en/category/ferlin-easydry-ht) — dry air, 50–170 °C high-temperature version. For technical plastics such as PA66, PC, PPS and PEEK. 3. [HotDry](/en/category/ferlin-hotdry) — hot air, an economical solution for non-hygroscopic materials such as PE/PP/PS. 150–1500 L.
The sizing rule: the hopper volume must cover your hourly consumption multiplied by the drying time. Example: 25 kg/h of PA consumption × 4 hours of drying = at least 100 kg (~150 L) of hopper.
Common Mistakes
1. Leaving dried material standing in an open bucket — PA takes up moisture again within 30 minutes 2. "Thinking you are drying" with a desiccant whose regeneration phase has failed — watch the dew point display 3. Trying to dry PET with a hot air dryer — the internal moisture does not come out and the IV value falls 4. Choosing a hopper larger than needed and cooking the material for hours longer than necessary
For a dryer capacity calculation suited to your material, consult our technical team — we produce a model recommendation for your consumption free of charge.