Comparison of injection molding processes for crystalline and amorphous plastics
The molecular structure differences between crystalline plastics (such as PP, PA, and POM) and amorphous plastics (such as ABS, PC, and PMMA) lead to significant differences in their injection molding process parameters, primarily reflected in the setting of the molding temperature range. Crystalline plastics have a clear melting point and a narrow melting and solidification temperature range (typically 10-30°C). For example, the melting point of PP is approximately 165°C. In actual processing, the melt temperature must be controlled between 180-220°C. Temperatures that are too low will result in incomplete crystallization, while temperatures that are too high may cause degradation. In the production of a certain PP plastic barrel, a melt temperature fluctuation of ±5°C can cause the crystallinity of the plastic part to vary by ±2%, resulting in a dimensional fluctuation of 0.3mm. Non-crystalline plastics have no fixed melting point and undergo a gradual transition from glassy state to highly elastic state to viscous flow state. ABS has a wide processing temperature range of 180-250°C. Even temperature fluctuations of ±10°C have little impact on the performance of the plastic part. For example, the shell of an ABS toy is produced within the range of 200-230°C, and the impact strength remains at around 21kJ/m².
Mold temperature has a significant impact on molding quality between the two types of plastics, driven by the exothermic nature of the crystallization process. Crystalline plastics require higher mold temperatures (typically 50-120°C) to promote orderly molecular chain alignment. For example, if the mold temperature for PA66 is below 80°C, rapid crystallization can lead to internal stress in the part. A PA66 gear exhibited warpage of 0.5mm at a mold temperature of 60°C, but this warpage decreased to 0.1mm at 100°C. Furthermore, mold temperature uniformity is even more critical for crystalline plastics. An uneven water channel layout in a POM bearing sleeve mold resulted in local mold temperature variations of up to 15°C. This variation in the crystallinity of the part resulted in a 12% dimensional accuracy error. Amorphous plastics require lower mold temperatures (typically 30-60°C), primarily avoiding internal stress by controlling the cooling rate. For PC lenses, maintaining a mold temperature of 80°C is sufficient; a higher temperature would prolong cooling time and reduce production efficiency.
The process settings during the holding phase are a key differentiator in the shrinkage compensation effects of the two types of plastics. Crystalline plastics experience significant volume shrinkage (4-10%) during solidification due to the dense molecular arrangement, requiring longer holding times and higher holding pressures. For example, the holding time for POM is typically 70% of the cooling time, and the holding pressure is 80% of the injection pressure. A POM gear mold reduced the shrinkage rate of the plastic part from 2.5% to 0.3% by extending the holding time from 10 seconds to 15 seconds. Non-crystalline plastics, on the other hand, have a lower shrinkage rate (1-5%), so the holding time can be shortened to 50% of the cooling time, and the holding pressure can be 60-70% of the injection pressure. A PC lampshade already meets dimensional requirements at a holding pressure of 60MPa; increasing it to 80MPa would lead to cracking due to increased internal stress.
Differences in melt flow properties necessitate different injection speed and pressure parameters for the two types of plastics. Crystalline plastics exhibit excellent fluidity after melting (high melt flow rate), but they cool and solidify quickly, necessitating a faster injection speed (50-100 mm/s) to shorten filling time and prevent premature solidification. A PP lunch box uses an injection speed of 80 mm/s, keeping the filling time within 2 seconds without missing parts. However, if the speed is reduced to 30 mm/s, the filling time increases to 4 seconds, with noticeable missing parts appearing in corners. Amorphous plastics have higher melt viscosity, and excessively fast injection speeds can easily cause turbulence and shear overheating. The optimal injection speed for ABS is typically 30-60 mm/s. An ABS television housing exhibited surface ripples at 70 mm/s, but a reduction to 50 mm/s resulted in a smooth appearance. Furthermore, the injection pressure for crystalline plastics is generally lower than that for amorphous plastics. PP injection pressures are typically 80-120 MPa, while PC often requires 120-160 MPa to fill the cavity.
The post-processing processes for the two types of plastics also differ due to their varying performance characteristics. Post-processing for crystalline plastics primarily addresses dimensional instability caused by incomplete crystallization. For example, PA66 parts require annealing in hot water at 100°C to eliminate internal stress and promote secondary crystallization. A PA66 tie rod improved its dimensional stability from 0.2mm/24h to 0.05mm/24h after 4 hours of annealing. Post-processing for amorphous plastics focuses more on eliminating internal stress. PC lenses are often baked at 120°C for 2 hours to reduce the internal stress level from level 3 to level 1, thus preventing cracking during later use. Regarding recycled material utilization, the recycled content of crystalline plastics (such as PP) can typically be as high as 30% with minimal impact on performance. However, the recycled content of amorphous plastics (such as PC) should be kept below 20%, as otherwise, the molecular weight decreases, significantly reducing impact strength. For example, when the recycled content of a PC water cup exceeded 25%, the drop test pass rate dropped from 98% to 75%.
