EN
DE
Water absorption rate is one of the key factors affecting the performance of injection-molded products. When plastic materials absorb moisture, their molecular structure, mechanical properties, and dimensional stability may undergo changes, thereby impacting the quality of the final product. For instance, hydrophilic materials (e.g., PA, PET) may experience reduced strength, dimensional expansion, or surface defects upon water absorption, while hydrophobic materials (e.g., PP, PE), despite their low absorption rates, can still encounter issues due to additives or environmental humidity.
Additionally, water absorption indirectly influences the control of injection molding process parameters, such as drying conditions and molding temperature. Therefore, it is essential to fully consider the effects of water absorption during material selection, process design, and post-processing to ensure stable and reliable product performance.
1. Changes in Material Properties
Decline in Mechanical Performance
Highly water-absorbent materials (e.g., PA nylon, PET) may undergo molecular chain hydrolysis after absorbing moisture, reducing tensile strength, impact resistance, and rigidity. For example, PA6 exhibits increased impact strength but decreased rigidity upon water absorption.
Thermal Performance Effects
Moisture vaporizes during high-temperature injection molding, potentially causing material degradation and lowering the heat deflection temperature (HDT).
2. Dimensional Stability Issues
Linear Expansion Effect
Moisture absorption causes material expansion. For instance, PA66 experiences a linear dimensional change of 0.25–0.33 mm/m per 1% water absorption, directly affecting precision part tolerances (e.g., medical component bore deviations up to 0.05 mm).
Post-Molding Shrinkage Fluctuations
Materials with high water absorption may undergo secondary shrinkage/expansion due to environmental humidity changes, compromising geometric accuracy.
3. Surface Quality Defects
Bubbles & Silver Streaks
Insufficiently dried materials can trap vaporized moisture during molding, leading to surface defects like bubbles, silver streaks, or cloudiness. For hygroscopic materials (e.g., PA, PC), moisture content exceeding 0.1% may cause surface flaws, with impact strength dropping by 15–20% in extreme cases.
Reduced Glossiness:
Moisture absorption alters melt flow, potentially causing uneven surfaces, fogging, or rough textures.
4. Processing Efficiency & Cost
Increased Sensitivity to Process Parameters
Extended drying times are required (e.g., PEI needs 120°C for 4–6 hours), and humidity fluctuations beyond ±5% demand adjustments in temperature, pressure, etc.
Higher Rejection Rates
Excessive moisture can cause short shots, flash, and other defects, increasing scrap rates by 8–12% in production (e.g., power tool housings).
5. Special Material Considerations
Hygroscopic Materials (e.g., PA, PVA)
Post-molding conditioning (e.g., boiling water immersion) is needed to stabilize water absorption and prevent brittleness.
Hydrophobic Materials (e.g., PP, PE)
Low water absorption, but moisture-sensitive additives (e.g., colorants) may still cause issues.
Control Recommendations
Pre-Drying Treatment: Use dehumidifying dryers (e.g., 80°C for PA, 4–6 hours) with dew point monitoring (<-40°C recommended).
Process Optimization: Adjust injection temperature (with caution for hydrolysis-prone materials) or packing pressure duration.
Material Substitution: In humid environments, consider modified materials (e.g., moisture-resistant ABS or glass-filled PA66).
Post-Processing: Stabilize hygroscopic materials via conditioning or annealing.
Phone
+86-0769-86066062
info@kehuimold.com
