PC injection molding produces parts that must be strong, clear, and dimensionally stable at the same time. Polycarbonate rewards good process control and punishes shortcuts: moisture turns into silver streaks, hot melt degrades into yellowing, and uneven cooling locks stress into the part until it crazes in service.
Why Polycarbonate Is Chosen
PC combines high impact strength, a wide service temperature range, good electrical insulation, and optical clarity. Those properties suit housings, lenses, light guides, and structural covers. The trade-offs are high melt viscosity, sensitivity to moisture, and a tendency to stress cracking when molded with residual stress.
Grade Selection
Optical grades offer low haze for lenses and windows. Flame-retardant grades serve electrical enclosures, and glass-filled grades raise stiffness at the cost of transparency. PC-ABS blends improve flow and impact for housings that do not need clarity. The grade should be selected against the service environment, since UV exposure and cleaning chemicals both attack PC.
Drying and Melt Preparation
PC absorbs moisture, and any residual water hydrolyses the melt, producing silver streaks and reducing molecular weight. Drying at 120 degrees Celsius for three to four hours is typical, verified with a moisture analyzer rather than by timer alone. Regrind should be limited and dried with the same discipline.
PC Injection Molding: Process Settings
Melt temperature usually falls between 280 and 320 degrees Celsius, with high injection speed to fill thin sections before the melt freezes. Holding pressure should be sufficient to pack without over-stressing the part, and screw speed moderate to limit shear heating. Excessive residence time degrades the melt, so shot size should be matched to the barrel capacity.
Cooling and Residual Stress
PC needs uniform cooling on both faces. A cavity that is colder than the core bends the part toward the hot side and leaves stress that appears as crazing after assembly or chemical contact. Annealing at 110 to 120 degrees Celsius relieves stress in lenses and parts that will be machined or coated.
Tool Design Notes
- Polish flow paths in optical parts; any scratch becomes visible
- Provide generous draft, since PC grips steel when hot
- Use a hot runner with a balanced melt path for multi-cavity tools
- Vent the cavity thoroughly to avoid burn marks at the end of fill
- Size gates to avoid high shear and jetting in transparent parts
Common Defects and Causes
Splay and silver streaks point to moisture. Yellowing indicates overheating or long residence time. Crazing after assembly suggests residual stress or contact with a solvent. Sink marks follow thick ribs and insufficient packing, and warpage follows uneven cooling or excessive orientation.
Quality Checks
Dimensional inspection covers critical fits; optical tests cover transmission and haze. A polariscope or birefringence check reveals stress distribution in transparent parts. Impact and drop tests confirm that the part behaves as designed rather than as the process allowed.
Frequently Asked Questions
Why do PC parts crack after assembly?
Usually molded-in stress combined with a solvent, a tight fit, or a screw boss that is too thin.
Can PC be molded clear without annealing?
Thin, simple parts often can. Thick lenses and parts subject to chemical contact benefit from annealing.
What causes yellow streaks in PC?
Overheating, excessive residence time, or a blocked hot-runner tip degrading the melt.
Conclusion
PC injection molding depends on dry resin, a controlled melt, and even cooling to deliver clarity and strength together. For related reading, see polycarbonate reference, our PC translucent molding guide, and cycle time and output guide. Contact TCHD to review your part.