Root-Cause Analysis of Warpage in Reinforced PC
Reinforced PC (PC + glass fiber) is widely used in precision products such as electronic and electrical housings, automotive components, and lighting fixture brackets. Warpage of reinforced PC has always been a hot and difficult topic in injection molding. Unlike semi-crystalline materials such as reinforced PA, PC is an amorphous polymer, so in theory there is no randomness of crystallization shrinkage; yet the warpage of glass-fiber-reinforced PC is often more severe than expected. This article analyzes the deeper causes of warpage in reinforced PC from the perspective of the combined action of amorphous plastic characteristics and glass-fiber orientation, and provides a detailed performance comparison of the two common grades PC+GF10 and PC+GF20.
The Particularity of Warpage in Reinforced PC
The amorphous (non-crystalline) structure of PC means it has no crystallization phase-transition shrinkage like nylon; its molding shrinkage mainly comes from thermal shrinkage and molecular orientation relaxation. Pure PC has a very low shrinkage rate (0.5%-0.7%) and good isotropy, so its warpage risk is small. However, after adding glass fiber, the orientation effect of the fiber in the flow direction introduces strong anisotropy. Through the superposition of dual orientation of molecular chains and glass fiber, the difference between the flow-direction shrinkage and the transverse-direction shrinkage of PC+GF can reach 3-5 times, far exceeding that of pure PC.
In addition, the melt viscosity of PC is very high (more than 10 times that of PA6), and in this high-viscosity environment the glass-fiber orientation is even harder to adjust through the subsequent holding-pressure stage. When the oriented glass-fiber skeleton is frozen in the part, the resulting internal stress is ultimately released in the form of warpage.
Comparison of Warpage Differences Between PC+GF10 and PC+GF20
From a quantitative comparison of the warpage risk of reinforced PC: the shrinkage difference coefficient of PC+GF10 (transverse shrinkage / flow-direction shrinkage) is usually 3-4.5, while this coefficient of PC+GF20 can reach 4-6. This means that PC+GF20 has a higher glass-fiber content, a stronger orientation effect of the fiber network, and correspondingly a higher warpage risk. Under the same mold design and process conditions, the warpage of PC+GF20 is usually 1.3-1.8 times that of PC+GF10.
However, PC+GF20 has a clear advantage in rigidity: its flexural modulus can reach 4000-5500 MPa (2800-3500 MPa for PC+GF10), and its heat distortion temperature is also 5-10 C higher. For structural parts requiring high rigidity (such as LED street-lamp housings and outdoor air-conditioner unit brackets), PC+GF20 is the better choice.
Mold-Design Countermeasures for Warpage in Reinforced PC
For the warpage problem of reinforced PC, there are several core countermeasures at the mold-design level: Preferably choose a center-gating or symmetrical multi-point gating scheme to make the melt flow path symmetrical and balanced, reducing excessive glass-fiber orientation caused by one-way long-range flow. The gate size should be large enough (a width >= 3 mm and a thickness >= 70% of the part wall thickness are recommended) to reduce flow resistance and reduce shear orientation of the glass fiber in the gate area. Cold runners should be as short as possible to avoid pre-orientation of the melt as it flows through the runner. For large-size flat products, consider using a hot runner system to better control the balance of the gating system. The cooling channel design must ensure uniform temperature in each area of the cavity, with the temperature difference controlled within 10 C.
Process-Adjustment Strategies for Warpage in Reinforced PC
At the process level, the following parameters have a clear effect on controlling warpage of reinforced PC: The mold temperature is controlled at 90-120 C; the higher the mold temperature, the more uniform the cooling, the more orientation relaxation, and the smaller the warpage. But note that too high a mold temperature will lengthen the cycle and increase the risk of post-shrinkage. The injection speed adopts a staged control strategy: medium speed (50-80 mm/s) at the beginning of filling to avoid jetting flow marks, high speed (80-120 mm/s) in the main filling stage to maintain the melt-front temperature and reduce orientation, and reduced speed at the end of filling to reduce mold impact. The holding pressure is set at 50%-70% of the injection pressure, and the holding time is based on complete gate freezing. Use a longer cooling time (PC has a low thermal conductivity and requires a longer cooling time).
PC+GF10 vs PC+GF20: Comprehensive Material-Selection Advice
If the product does not require high rigidity and warpage is the primary concern, choose PC+GF10 first. If the product has clear rigidity requirements (such as the need to withstand large external forces or a long span) and warpage can be controlled through mold and process means, then choose PC+GF20. If the warpage requirements are extremely strict (such as housings for precision optical components), consider mineral-filled PC or glass-fiber-free PC with metal inserts. In terms of cost, PC+GF20 is usually 8%-12% more expensive than PC+GF10, and this must be weighed comprehensively during material selection.
