Root-Cause Analysis of Reinforced PC Warpage
Reinforced PC (PC+glass fiber) is widely used in precision products such as electronics and electrical housings, automotive components, and lighting brackets. Warpage of reinforced PC has long been a hot and difficult issue in injection molding processes. Unlike semi-crystalline materials such as reinforced PA, PC is an amorphous polymer and theoretically has no randomness from crystalline shrinkage, yet the warpage of glass fiber reinforced PC is often more severe than expected. This article analyzes the deep-seated causes of reinforced PC warpage from the perspective of the combined effect 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 Specificity of Reinforced PC Warpage
The amorphous (non-crystalline) structure of PC means it has no crystalline 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, with low warpage risk. However, after adding glass fiber, the orientation effect of glass fiber in the flow direction introduces strong anisotropy. Through the superposition of dual orientation of molecular chains and glass fibers, the shrinkage differential between the flow direction and the transverse direction of PC+GF can reach 3–5 times, far exceeding that of pure PC.
In addition, PC has a very high melt viscosity (more than 10 times that of PA6). In such a high-viscosity environment, glass fiber orientation is even more difficult to adjust during the subsequent packing phase. When the oriented glass fiber skeleton is "frozen" into the molded part, the resulting internal stress is ultimately released in the form of warpage.
Warpage Difference Comparison: PC+GF10 vs PC+GF20
From a quantitative comparison of reinforced PC warpage risk: the shrinkage differential coefficient (transverse shrinkage / flow-direction shrinkage) of PC+GF10 is typically 3–4.5, while that of PC+GF20 can reach 4–6. This means PC+GF20 has a higher glass fiber content and a stronger orientation effect of the glass fiber network, with correspondingly increased warpage risk. Under the same mold design and process conditions, the warpage amount of PC+GF20 is typically 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 deflection temperature (HDT) is 5–10 °C higher. For structural parts requiring high rigidity (such as LED streetlight housings and air-conditioner outdoor unit brackets), PC+GF20 is the better choice.
Mold Design Countermeasures for Reinforced PC Warpage
For reinforced PC warpage, the following are core mold design countermeasures: prioritize center gating or symmetric multi-point gating to make melt flow paths symmetric and balanced, reducing excessive glass fiber orientation caused by one-directional long-range flow. Gate dimensions should be sufficiently large (recommended width ≥ 3 mm, thickness ≥ 70% of the part wall thickness) to reduce flow resistance and minimize shear orientation of glass fiber in the gate region. Cold runners should be as short as possible to avoid pre-orientation of the melt as it flows through the runner. For large flat parts, consider using a hot runner system to better control the balance of the gating system. Cooling water channel design must ensure uniform temperature across all cavity areas, with a temperature differential controlled within 10 °C.
Process Adjustment Strategies for Reinforced PC Warpage
At the process level, the following parameters have a clear effect on controlling reinforced PC warpage: control mold temperature at 90–120 °C—the higher the mold temperature, the more uniform the cooling, the more orientation relaxation, and the smaller the warpage. Note, however, that excessively high mold temperature prolongs the cycle and increases the risk of post-shrinkage. Use a segmented injection speed control strategy—medium speed (50–80 mm/s) at the start of filling to avoid jetting flow marks, high speed (80–120 mm/s) in the main filling stage to maintain melt front temperature and reduce orientation, and reduced speed at the end of filling to minimize mold-filling impact. Set packing pressure at 50%–70% of injection pressure, with packing time based on complete gate freeze-off. Use longer cooling time (PC has low thermal conductivity and requires longer cooling time).
PC+GF10 vs PC+GF20: Comprehensive Material Selection Recommendations
If the product has low rigidity requirements and warpage is the primary contradiction, prioritize PC+GF10. If the product has clear rigidity requirements (such as needing to withstand large external forces or long spans), and warpage can be controlled through mold and process means, then choose PC+GF20. If warpage requirements are extremely stringent (such as housings for precision optical components), consider mineral-filled PC or glass-fiber-free PC combined with metal insert solutions. In terms of cost, the price of PC+GF20 is typically 8%–12% higher than that of PC+GF10, so a comprehensive trade-off is needed during material selection.
