How Proper Drying and Dosing of Granulate Reduces Production Scrap
When silver streaks, bubbles or color variation show up on parts, the first instinct is usually to adjust the machine — temperatures, pressures, speeds. Statistically though, the two most common culprits are not on the machine at all: moisture in the granulate and unstable additive dosing. The good news is that both are solved by auxiliary equipment costing a fraction of the machine price, paid back through reduced scrap.
What Moisture Actually Does to the Material
Hygroscopic polymers — PA, PC, ABS, PET, PMMA — absorb moisture from the air during storage. At melt temperature that water turns to steam and causes two kinds of damage: visible (silver streaks, bubbles, dull patches) and invisible, which is worse — hydrolysis of the polymer chains. The part looks fine, then fails at assembly or at the customer, because its mechanical properties have degraded.
That is why drying is a process parameter, not an option: every hygroscopic material has a prescribed temperature, time and target dew point. Plastic Systems dehumidifiers and dryers cover the whole range — from machine-side DWH-Mini units to central systems — with dew-point control that guarantees the material is actually dry, not merely warm.
Dosing: the Silent Generator of Color Variation
The second scrap source is an unstable masterbatch-to-resin ratio. A volumetric feeder doses by volume — and bulk density varies batch to batch, so the 2% set in the morning becomes 1.6% or 2.4% by the afternoon. The result: visible shade differences between runs and color complaints.
Gravimetric feeders (DG series) weigh each component and correct deviations automatically. Besides consistent color, that typically means 10–20% lower masterbatch consumption — with expensive additives, the feeder pays for itself faster than any other part of the line.
A System, Not Individual Devices
The biggest effect comes when drying, dosing and material conveying work as one system under supervisory software: material dried centrally, conveyed in closed lines without contact with humid air, and dosed gravimetrically at each machine with parameters logged per recipe.
Perotec Systems designs and supplies complete Plastic Systems drying and dosing systems in Serbia — from a single unit to a central system for the whole hall. Contact us with your material list and throughputs and we will propose a configuration.
Frequently Asked Questions
How do I know whether scrap is caused by moisture in the granulate?
Typical signs are silver streaks on the part surface, bubbles in the cross-section, brittle parts failing at weld lines, and foamy melt at the nozzle. If problems appear after a material batch change or on humid days — drying is the first suspect.
How long should granulate be dried before molding?
It depends on the polymer: PA6 typically 4–6 hours at 80°C, PC 3–4 hours at 120°C, ABS 2–4 hours at 80°C, PET up to 6 hours at 160–180°C. Both time and temperature matter — drying too short leaves moisture, too long degrades the material.
Is a gravimetric feeder worth it compared to a volumetric one?
With more expensive masterbatches and larger runs — as a rule, yes. Gravimetric dosing typically cuts masterbatch consumption by 10–20% because it doses by actual weight instead of volume, with automatic correction and recipe logging.