Polycarbonate (PC) is one of the most widely used engineering plastics in the world. The material’s highly desirable properties — including strength, toughness and clarity — make it attractive for the development of many everyday products, such as medical tubing connectors, medical and consumer electronics, eyeglasses, and automotive and lighting lenses. As demand for medical and consumer electronics products grows, more manufacturers are using PC to ensure part appearance and function.

Ironically, the unique properties and chemistry of polycarbonate directly contribute to some of its biggest molding challenges. Black specks, discoloration, general resin degradation and other cosmetic irregularities have plagued polycarbonate molding, requiring certain processing strategies and methodologies to minimize and correct problems.

Here we’ll focus on some hot runner considerations and troubleshooting strategies to improve the efficiency, performance and productivity of polycarbonate injection molding.

Hot Runner Design Considerations

The melt network, or melt channel layout, is a prominent feature of the hot runner design for polycarbonate. All hot runners should be designed to consider geometric balance. This means that when flowing from a single sprue entry to multiple cavity gate exits, the melt undergoes an experience that is as similar as possible.

All features should have an occurrence and orientation that are as similar as possible. Every effort should be made within reason (materials, manufacturing and ultimately cost) to eliminate as much variation as possible.

Three mold attributes that determine the geometrically balanced melt channel layouts are cavity count, cavity arrangement and cavity spacing. Cavity count affects the number of occurrences for melt channel divisions and splits. The cavity arrangement determines where those splits will be.

This eliminates some of those options or multiplication sequences. Cavity spacing affects the length of melt channels and the angles of their intersection. A manifold must survive processing expectations, so all three influence the type of manifold material and total hot runner thickness.

Troubleshooting Strategies

Polycarbonate can have some highly prevalent cosmetic molding issues. Examples of part quality defects and performance challenges include consistent streaks, intermittent flush or “burp,” black specks and pressure limitation.

A gate streak with consistent occurrence can be an indicator of excessive friction or shear in the melt path. This can be due to an obstruction, flow interruption or gate diameter. If this is a sudden or recent occurrence, you may need to refurbish to replace a heater or remove a flow obstruction. If it’s immediate at sampling, you made need to increase gate size or evaluate the nozzle tip and valve pin.

During processing, an occasional streak or burp can appear intermittently caused by a stagnant area in the flow path where resin hangs and doesn’t immediately flush or flow. This molten material whirls and swirls in the same area and eventually degrades, changes viscosity and releases discolored material into the melt stream. This could indicate that you need to refurbish and inspect the manifold to confirm the melt channel surface finish and melt channel splits are made properly.

Black specks result from material degrading at the melt channel wall and flaking into the melt. A common cause is when a manifold is left at processing temperature and stagnant for a long time. The best way to flush heat from the manifold, nozzles and material is to follow a proper shutdown procedure while cooling, which remains active until all thermocouples read room temperature. Otherwise, the material at the melt channel walls degrades and emerges as black flake in the next run.

Because polycarbonate exhibits high viscosity, it’s possible to be limited by the pressure capability of the molding machine. First, check the power draw of all heaters to ensure there is no abnormal heat loss in the system. Next, check the purge pressures at the same process fill time through the machine barrel and through a blocked-open mold. This helps identify where pressure consumption in the entire system is due.

If the cavity requires the highest pressure, it may still be possible to decrease material viscosity by increasing temperature set point (within reason). Some molding machines come with a 2.0-mm pilot hole in the barrel nozzle that any toolroom can enlarge to increase the diameter and relieve significant process pressure. Increasing melt temperature can overcome this restriction, but it’s a major penalty for processing quality that can be resolved quickly with minimal rework costs.

Some hot runner systems are designed for thermally sensitive or corrosive applications — such as very small technical or medical parts made from different grades of polycarbonate that can easily degrade at certain temperatures or residence times — to further manage the molten material (especially PC) delivered to the cavity.

Aerospace-grade materials are used in the construction of these manifolds and nozzles with melt channels precision-drilled for fine surface finish. This manufacturing approach withstands the injection pressures common to polycarbonate molding for millions of cycles.

A manifold designed for a hot runner system should consider a geometrically balanced melt channel layout with minimized risk for stagnation. Thermal FEA ensures an optimal and uniform temperature profile based on the resin and mold design needs. All melt-contact surfaces should be nonreactive, which minimizes the oxidation of the melt at temperature. This can minimize the occurrence of black specks at startup or other chemical sensitivities of the material.

While polycarbonate’s chemistry sometimes presents challenges, understanding the material’s needs and leveraging them through analysis, design and processing can deliver positive results. All aspects of the production work cell are critical and can contribute to the overall success of polycarbonate injection molding.