Why dry plastics?
Drying plastic materials ensures the manufacture of quality products !
The manufacturing of plastic moulded parts requires the removal of excess moisture. Water absorbed or bound by plastic materials tends to cause the formation of bubbles or surface defects (scratches, superficial tears...) and in some cases a degradation of mechanical characteristics (fragile part).
In most plastic processing plants, material dryers are standard equipment.
Non-hygroscopic plastics
The exception to this rule would be facilities that exclusively process polyolefins such as polyethylene and polypropylene (obviously, without talc or glass fibres). These are among the few families of polymers that do not require drying as these material families are not hygroscopic (they do not absorb water).
Materials like polyethylene are non-polar, whereas water is a highly polar substance that acts like a tiny magnet with a negatively charged end and a positively charged end. Water has no affinity for non-polar polymers. Exposing polyethylene or polypropylene to water is like putting oil in water. The two substances immediately separate because they have nothing in common chemically.
Hygroscopic plastics
But almost All other commercial polymers are capable of absorbing some moisture from the atmosphere., as they have a certain level of polarity. The amount of moisture a given polymer can absorb depends on the chemistry of the polymer and the atmospheric conditions to which it is exposed.
Polymers such as HIPS (polystyrene) are only slightly polar and, even when saturated, can only hold 0.071% moisture.
At the other end of the spectrum is polyamide (nylon), a highly polar polymer which, when saturated, can absorb 8 to 9% of water (100 times more than HIPS).
Often confused by some transformers, Steaming and drying are 2 different processes that should be used in a targeted manner, depending on the material and its hygroscopic nature.
At Hybster, from day one, we've chosen to use a centralised drying system, which supplies all our injection moulding machines via a vacuum-operated feed network, with colouration at each machine, and a recycled materials integration system. We'll tell you more about it next time!
All ovens and dryers are equipped with a system for measuring the humidity and temperature of the material, set to agreement with the material supplier's recommendations.
❓ Frequently Asked Questions
Plastic materials need to be dried before injection moulding to prevent defects caused by moisture.
Many polymers absorb ambient moisture. During injection moulding, the water present vaporises in the molten polymer, creating defects: porosity, silver streaks, and a drop in mechanical properties due to hydrolysis. Pre-drying eliminates this moisture before processing and guarantees the dimensional and aesthetic quality of the part.
Which plastics are hygroscopic?
Highly hygroscopic polymers include PA (polyamide), PET, PBT, PC, PMMA, PU, and PEEK. Conversely, PP, PE, PVC, and PS are less sensitive to ambient moisture. For hygroscopic polymers, drying before injection is mandatory and systematic.
What is the maximum admissible humidity level for PA and PC?
For polyamide (PA6, PA66), the moisture content must fall below 0.15% prior to injection (ideally 0.08%). For PC, below 0.02%. For PET, below 0.005%. Above these levels, defects appear and properties deteriorate. A dew point test can be used to check this level prior to production.
How long does polyamide need to be dried before injection?
The drying of polyamide typically takes 4 to 8 hours at 80°C in a dehumidifying dryer (air dew point -40°C). Previously undried PA may require 12 hours. A heated ambient air dryer alone is ineffective: it migrates moisture but does not remove it sufficiently.
What defects appear if the material has not been dried?
Without drying, silver streaks are observed on the surface, internal bubbles, visible surface marks, reduced mechanical strength, altered colour, and uneven shrinkage. For highly hygroscopic polymers (PC, PA), the defects are systematic and disqualifying without prior drying.
What temperature should be used for polymer drying?
Drying temperature depends on the polymer: 60-80°C for ABS and PC, 80°C for PA, 100°C for PET, 120°C for PEEK. Exceeding this temperature will prematurely degrade the material. Drying always uses dehumidified air (dew point -30 to -40°C), not just hot, unprepared air.
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