Molding polycarbonate requires higher temperatures and more careful preparation than most common plastics, but the payoff is a finished part with exceptional impact strength and optical clarity. The process centers on injection molding, where dried polycarbonate resin is melted at roughly 320°C (608°F), injected into a steel mold, cooled, and ejected as a solid part. Getting good results depends on nailing three things: thorough drying, precise temperature control, and the right tooling.
Why Drying Comes First
Polycarbonate is hygroscopic, meaning it absorbs moisture from the air. Even small amounts of trapped water cause serious problems during molding. At melt temperatures, moisture triggers a chemical reaction called hydrolysis that breaks down the polymer chains. The result is a weaker part with visible defects: splay marks (silver streaks on the surface), bubbles, and a hazy appearance instead of the crystal-clear finish polycarbonate is known for.
Dry the resin at 120°C (248°F) for 2 to 4 hours before processing. The target moisture content is 0.02% or less. A desiccant dryer works best because it actively strips moisture from the air circulating through the hopper, unlike a hot-air oven that can only dry to the ambient dew point. If you’re running continuous production, use a hopper dryer mounted directly on the machine so resin doesn’t sit exposed to shop air between drying and molding.
Temperature Settings for Injection Molding
Polycarbonate has a high melt temperature compared to materials like ABS or polypropylene. A typical melt temperature is around 290 to 320°C (554 to 608°F), with research setups commonly using 320°C. Running too cold produces short shots and high internal stress. Running too hot degrades the material and causes yellowing.
Mold temperature matters just as much. The standard range falls between 20°C and 60°C (68 to 140°F), but higher mold temperatures within that window produce better results for most polycarbonate parts. A warmer mold lets the plastic flow more easily, reduces internal stress, and improves surface finish. For optical parts like lenses or light covers where clarity is critical, aim for the upper end of that range. For structural parts where cycle time matters more than appearance, the lower end works fine.
Set barrel zone temperatures in a gradual profile, increasing from the feed throat toward the nozzle. This prevents the resin from melting too early and bridging in the feed zone while ensuring it’s fully molten by the time it reaches the mold.
Injection Speed and Pressure
Polycarbonate’s high viscosity (it doesn’t flow as easily as many other plastics) means you generally need moderate to high injection pressures. The exact pressure depends on part geometry, wall thickness, and gate size, but expect to run higher than you would for polyethylene or polystyrene.
Injection speed should be moderate. Filling too fast generates shear heat that can degrade the material near gate areas, while filling too slowly lets the melt front cool before the cavity is packed, leading to weld lines and sink marks. For thin-walled parts, faster fill speeds help the material reach the end of the cavity before it freezes off. For thicker parts, slower speeds reduce the risk of jetting, where a stream of plastic shoots across the cavity and creates visible surface defects.
Pack and hold pressure is critical for polycarbonate because the material shrinks significantly as it cools. Insufficient packing leads to voids inside the part and sink marks on the surface. A common starting point is 50 to 80% of injection pressure for the hold phase, then fine-tune based on part quality.
Choosing the Right Mold Steel
The mold itself needs to withstand polycarbonate’s high processing temperatures and, for clear parts, deliver a flawless surface finish. Two steel grades stand out for different reasons.
- H13 steel is a hot-work tool steel heat-treated to a hardness of HRC 48 to 52. It handles the constant heating and cooling cycles of injection molding without developing surface cracks (a problem called heat checking). This makes it a strong choice for high-volume polycarbonate production where mold longevity matters.
- S136 steel is the go-to for transparent polycarbonate parts that need a mirror finish. Its corrosion resistance keeps the cavity surface pristine over thousands of cycles, which directly translates to optical clarity in the molded part. It’s commonly used for medical components, food-grade products, and lighting covers.
For prototyping or short runs, P20 pre-hardened steel works adequately and costs less, though it won’t hold a polish as well or last as long under polycarbonate’s demanding temperatures.
Cooling Time and Wall Thickness
Cooling accounts for the largest chunk of total cycle time in injection molding, often 60 to 80% of the entire cycle. For polycarbonate, cooling time is directly tied to wall thickness by a square relationship: if you double the wall thickness, cooling time roughly quadruples. This makes wall thickness the single biggest lever you have for controlling production speed.
Designing parts with uniform wall thickness is especially important for polycarbonate. Thick sections cool slower than thin sections, and the resulting uneven shrinkage causes warping and internal stress. Where you need structural rigidity, use ribs instead of thicker walls. Ribs should be about 50 to 60% of the adjoining wall thickness to avoid sink marks on the opposite surface.
Cooling channels in the mold should follow the contour of the part as closely as possible. Conformal cooling (channels that curve to match the part shape, typically made through metal 3D printing) can cut cooling time significantly compared to straight drilled channels, especially for complex geometries.
Gate Design and Part Geometry
Gate location and type have an outsized effect on polycarbonate part quality. Because the material is viscous and sensitive to shear, the gate needs to be large enough to let plastic flow without excessive friction. Undersized gates generate heat that degrades the resin right at the entry point, leaving a visible mark and weakening the surrounding material.
For clear parts, gate placement should direct flow so that weld lines (where two melt fronts meet) end up in non-critical areas. Fan gates and tab gates spread the flow and reduce jetting. Pin gates work for small parts but may leave a visible vestige that requires post-processing.
Sharp corners in the part design concentrate stress, and polycarbonate under stress is prone to crazing (fine surface cracks that scatter light). Use generous radii on all internal corners, ideally at least 25% of the wall thickness. Draft angles of 1 to 2 degrees on vertical surfaces help the part release cleanly from the mold without scuffing.
Common Defects and Fixes
Several issues come up repeatedly when molding polycarbonate:
- Silver streaks (splay) almost always trace back to moisture. Re-check your drying process before adjusting anything else.
- Yellowing or brown streaks indicate thermal degradation. Lower the melt temperature, reduce back pressure, or shorten the residence time (the duration resin sits in the heated barrel before injection).
- Cloudy or hazy parts can result from either moisture or mold surface quality. If drying doesn’t solve it, the cavity may need repolishing.
- Warping points to uneven cooling, inconsistent wall thickness, or demolding too early. Increase cooling time or raise the mold temperature to reduce the temperature gradient between the part’s surface and core.
- Stress whitening or crazing shows up as white marks when the part is flexed or impacted. This signals excessive molded-in stress, usually from low mold temperatures, fast cooling, or aggressive packing pressure.
Ventilation and Safety
Polycarbonate processing generates fumes that can irritate the respiratory system and skin. OSHA identifies skin burns from hot plastic splatter and inhalation of gases and vapors as the primary hazards around injection molding machines. Run proper ventilation and exhaust systems near the machine’s nozzle, hopper, and mold area. Check the material safety data sheet for the specific polycarbonate grade you’re using, as additives and colorants can introduce additional fume hazards beyond the base resin.
Purging the barrel is important when switching to polycarbonate from another material or vice versa. Residual resin left in the barrel can degrade at polycarbonate’s higher processing temperatures and contaminate your parts. Use a purging compound rated for the temperature range, or purge with a high-flow polyethylene before switching over.