Analyse und Lösungen für häufige Defekte beim Polycarbonat-Spritzgießen

Polycarbonate (PC) is one of the most versatile engineering thermoplastics in Spritzgießen—transparent, impact-resistant, and thermally stable. But anyone who has actually run PC on a production line knows it is also one of the most finicky materials to process. Its high melt viscosity, extreme moisture sensitivity, and tendency to trap internal stress mean that even small process deviations can produce visible defects: discoloration, silver streaks, bubbles, flow marks, and stress cracking.

In this article, we walk through the six most common PC molding defects we encounter in production—discoloration and black spots, silver streaks and bubbles, flow marks, cold material spots, and internal stress cracking. For each defect, we explain the physical mechanism, how to diagnose it from the defect pattern, and the specific process and tooling adjustments that resolve it. These insights come from two decades of hands-on PC molding experience across automotive, medical, and consumer electronics applications.

What Makes Polycarbonate Injection Molding So Challenging?

Polycarbonate is one of the most difficult engineering thermoplastics to injection mold. Its unique molecular structure—linear chains with benzene rings, isopropylidene groups, and carbonate linkages—creates three core processing challenges that set it apart from easier materials like polypropylene or ABS.

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First, PC has no sharp melting point. Instead, it gradually softens across a wide temperature range (230–320 °C), which means melt viscosity remains high¹ throughout normal processing. Unlike semi-crystalline polymers that thin out dramatically above their melting point, PC behaves more like a Newtonian fluid—its viscosity is more sensitive to temperature changes than to shear rate. Small temperature deviations of just 10–15 °C can push the melt from processable to degraded.

Second, PC is extremely sensitive to moisture. Even trace amounts of water (above 0.02% by weight) cause hydrolytic degradation at processing temperatures, breaking polymer chains and reducing mechanical properties. This means thorough predrying at 120 °C for 3–4 hours is mandatory—not optional. In our experience processing over 400 materials at the Shanghai factory, moisture-related defects account for roughly 60% of all PC molding problems we troubleshoot.

Third, the high viscosity of PC melt means that Spritzgussform design must accommodate higher injection pressures, larger gates and runners, and deeper exhaust grooves than what you would need for commodity plastics. Under-sized flow channels create excessive shear heat, which paradoxically causes thermal degradation even when barrel temperatures are set correctly. Understanding these three constraints—high viscosity, moisture sensitivity, and shear sensitivity—is the foundation for preventing every defect covered in this article.

What Causes Discoloration, Yellowing, and Black Spots in PC Parts?

Discoloration is the most common visual defect in PC injection molding, caused by thermal degradation of the melt. The root cause is typically excessive barrel temperature, overly long residence time, or dead zones in the plasticizing system where degraded material accumulates and intermittently releases into the melt stream.

Pure PC resin has excellent thermal stability and can tolerate temperatures up to 300 °C without significant decomposition. The problem arises when processors use modified PC blends, recycled material, or PC compounded with flame retardants and fillers. These additives narrow the processing window considerably. For example, PC/ABS blends typically require barrel temperatures around 250 °C, while PC/PBT blends for lighting products need approximately 280 °C—each combination has its own thermal ceiling that, once exceeded, triggers irreversible yellowing or carbonization.

Black spots are a particularly frustrating variant because they can appear intermittently—sometimes two or three shots in a row, then disappearing. This pattern almost always indicates dead material trapped somewhere in the plasticizing system: screw check ring gaps, nozzle tip interfaces, or barrel wall scoring. The trapped material carbonizes over time, then breaks loose in chunks. When PC decomposition products accumulate beyond a critical threshold, they also catalyze further decomposition, creating a cascading effect—especially severe in flame-retardant grades.

The fixes are systematic. First, verify barrel temperature setpoints against the material grade and reduce the feed and compression zone temperatures by 5–10 °C increments until discoloration stops. Second, ensure thorough drying: 120 °C for 3–4 hours using a dehumidifying dryer, never exceeding 10 hours to avoid material aging. Third, inspect the plasticizing system for dead zones—remove and clean the nozzle, check ring, and screw if air-shot melt shows discoloration even at correct temperatures. Finally, purge the barrel with a thermally stable material (PS or PE) before and after each production run, and never leave PC sitting at processing temperature during extended stops; drop the barrel to 160 °C (PC glass transition²) or below for thermal holds.

Why Do Silver Streaks and Bubbles Appear on PC Products?

Silver streaks (also called gas streaks) and bubbles are surface and internal defects caused by gas trapped in the melt during cavity filling. In PC injection molding, the four gas sources are water vapor, entrained air, thermal decomposition gas, and solvent gas—water vapor and decomposition gas account for the vast majority of cases.

Silver streaks form when gas dissolved in the pressurized melt escapes to the product surface as cavity pressure drops after filling. The escaping gas leaves tiny elongated bubbles that sparkle under light, always aligned with material flow direction. Bubbles, on the other hand, are gas pockets trapped in the wall thickness—particularly visible in transparent PC parts. Vacuum bubbles are different: they form not from gas but from volumetric shrinkage when insufficient holding pressure leaves a void in thick sections.

How to Diagnose the Gas Source Behind Silver Streaks

Diagnosing which gas is responsible requires reading the defect pattern. Randomly scattered surface bubbles point to water vapor—the most common culprit in PC because the material is so hygroscopic. Fine, dense bubble clusters concentrated near the gate in a radial or fan-shaped pattern indicate entrained air, typically from excessive screw retraction speed or insufficient back pressure. Discoloration accompanying the silver streaks points to decomposition gas from overheated melt. The diagnostic process matters because each gas source requires a different fix.

For moisture-driven silver streaks, the solution is straightforward: ensure drying at 120 °C for 3–4 hours with a dehumidifying dryer. Verify effectiveness by air-shooting—the extruded melt should be continuous, smooth, and free of white vapor. For air entrainment, reduce screw speed, increase back pressure, and extend melt time during the cooling phase. For decomposition gas, lower barrel temperature section by section starting from the nozzle, and check for excessively long residence times (using oversized equipment for small parts is a common culprit).

Vacuum bubbles require a different approach because they are a shrinkage phenomenon, not a gas issue. Increase holding pressure and extend holding time to pack more material into the thick section. Position the gate at the thickest wall to ensure pressure transmission. Increase mold temperature locally at the void location to slow solidification and allow shrinkage compensation. For transparent products, post-mold slow cooling in hot water can also reduce vacuum bubble formation.

What Are Fingerprint Marks and Turbulence Lines—and How Do You Fix Them?

Fingerprint marks and turbulence lines are flow defects caused by PC melt viscosity being too high relative to injection speed and mold temperature. The melt fills the cavity in a stick-slip pattern, leaving wavy lines perpendicular to flow direction (fingerprints) or radial streaks near the gate (turbulence).

Fingerprint marks develop when injection speed and pressure are too low for the melt viscosity. The front of the melt stream contacts the cold mold wall, solidifies, and shrinks. The hot melt behind it pushes the shrunken skin forward, then that layer also cools and shrinks. This alternating advance-freeze cycle creates the characteristic wavy pattern that looks like a human fingerprint. The effect is most visible on large, flat PC surfaces—think display covers or control panels.

Turbulence marks are related but distinct. They appear as irregular flow lines radiating from the gate, caused by the melt hitting the cavity wall at high velocity and skidding across the cold surface before stabilizing into laminar flow. This defect is particularly common when gate design creates a sharp velocity transition—such as a small gate feeding into a large, thick cavity. The key distinction: fingerprints run perpendicular to flow, while turbulence lines run parallel.

Both defects share the same solution set. Increase the nozzle and front barrel temperatures to reduce melt viscosity—this is the single most effective adjustment. Raise mold temperature, especially at the location where marks appear; for appearance-critical PC parts, a mold temperature controller set to 100–120 °C is standard practice. Increase injection speed to shift the filling pattern from stick-slip to continuous flow; multi-stage injection allows you to adjust speed section by section, targeting the problem area without causing flash elsewhere. On the mold side, enlarge gates and runners to reduce flow resistance, and ensure adequate venting and cold-slug wells.

How Do Cold Material Spots Form and How Can You Prevent Them?

Cold material spots are foggy, bright, or worm-shaped marks near the gate caused by partially solidified melt entering the cavity. They form when the melt front loses too much heat at the nozzle tip, runner, or gate before cavity filling begins—or when excessive holding pressure forces already-cooled runner material into the part.

There are two distinct mechanisms. The first is forward cold material: the melt at the nozzle tip and runner entrance cools between shots because the nozzle contacts the cold mold plate. When injection begins, this chilled material enters the cavity first. On thin-walled parts, it spreads into smoky or paste-like cloudy patches. On thick-walled parts, it forms a curved scar resembling an earthworm. The second mechanism is back-pressure cold material: excessive holding time and pressure squeeze already-cooled material from the runner and gate into the part, creating a small circular bright spot near the gate.

Prevention is straightforward but requires attention to detail. Install a cold-slug well at the end of each runner—this traps the forward cold material before it enters the cavity. Increase nozzle temperature to reduce heat loss at the tip. Increase mold temperature to narrow the gap between melt and mold surface temperatures. Reduce injection speed at the start of filling to avoid melt fracture at the gate, then increase speed for the main fill. Optimize gate position, size, and shape to avoid sharp velocity transitions. For holding-pressure cold spots, shorten holding time and reduce holding pressure to the minimum needed for dimensional stability. Also ensure thorough material drying—residual moisture in the cold-slug can worsen the visual defect.

Why Does Internal Stress Crack Transparent PC Products?

Internal stress in PC products is frozen-in molecular orientation and uneven cooling stress. It can cause warpage, reduced optical clarity, and delayed stress cracking days or weeks after molding—transparent PC parts are the canary in the coal mine.

Two primary mechanisms create internal stress. Orientation stress comes from polymer chains being stretched during flow and then frozen in place before they can relax back to a random coil configuration. Higher injection pressure, faster injection speed, and longer holding time all increase orientation by applying more shear to the melt. Temperature stress comes from the large temperature differential between the hot melt core and the cold mold wall. Because PC has high specific heat capacity and low thermal conductivity, the surface solidifies long before the interior—creating compressive stress on the outside and tensile stress on the inside.

The practical consequence is that a PC part may look perfect immediately after molding but develop micro-cracks within days, especially when exposed to organic solvents (cleaning agents, adhesives) or elevated temperatures. In our production environment, we have seen transparent PC lenses crack during assembly simply because the operator used an alcohol-based cleaning wipe—the internal stress was already at the failure threshold, and the solvent lowered it just enough to initiate cracking.

Reducing internal stress requires a holistic approach. Increase melt temperature to reduce viscosity and orientation during flow. Increase mold temperature to allow slower, more uniform cooling and give oriented molecules time to relax. Reduce injection pressure to the minimum needed for complete filling. Minimize holding time—over-packing is a major contributor to orientation stress. Use variable-speed injection: fast fill to avoid flow defects, then slow speed for holding to reduce molecular alignment. For parts with metal inserts, preheat inserts to approximately 200 °C to reduce the thermal mismatch. Finally, post-mold heat treatment at 120 °C for approximately 2 hours allows chain segments to regain mobility and relax frozen deformation—this is standard practice for optical-grade PC components.

What Processing Parameters Should You Monitor to Minimize PC Defects?

There are six parameters that matter most for PC defect prevention: drying, barrel temperature, injection speed, hold pressure, and mold temperature. Getting these right eliminates the vast majority of discoloration, silver streaks, bubbles, flow marks, and internal stress issues.

Drying is non-negotiable. PC requires moisture content below 0.02%³—achieved by dehumidifying dryer at 120 °C for 3–4 hours. Drying beyond 10 hours risks material degradation, especially for flame-retardant grades. Verify drying effectiveness by air-shot inspection before starting production. This single step prevents most silver streaks and surface bubbles.

Barrel temperature must be set as a profile, not a single number. For pure PC, a typical profile runs 250 °C (feed) → 270 °C (compression) → 285 °C (metering) → 290 °C (nozzle). Each modified grade has its own window—PC/ABS at roughly 20 °C lower, PC/PBT at similar or slightly higher temperatures. The key is to start at the lower end of the recommended range and increase only if flow marks or short shots appear. Never set all zones to the same temperature; a proper gradient ensures gradual plasticization without premature melting in the feed zone (which blocks air escape) or under-preheating (which traps air in the melt).

Melt residence time deserves special attention. Using oversized equipment for small PC parts is a common mistake—the large shot-to-barrel-capacity ratio means material sits at processing temperature far too long, accumulating thermal damage. As a rule of thumb, the shot weight should be at least 30–40% of the barrel capacity. If you must run small parts on large machines, use a smaller-diameter screw or accept that frequent purging and color changes are unavoidable. Finally, mold temperature matters more for PC than for most plastics. Running cold molds (below 80 °C) accelerates skin solidification, increases internal stress, and amplifies flow marks. For transparent or appearance-critical parts, 100–120 °C mold temperature with a temperature controller is the industry standard.

What Are the Most Common Questions About PC Injection Molding Defects?

Häufig gestellte Fragen

What is the ideal drying temperature for polycarbonate before injection molding?

Polycarbonate should be dried at 120 C using a dehumidifying dryer for 3 to 4 hours to achieve moisture content below 0.02% before injection molding. This is a non-negotiable requirement for successful PC processing—skipping or short-cutting the drying step is the single most common cause of surface defects. Drying beyond 10 hours risks material degradation, especially for flame-retardant grades which are more thermally sensitive. Always verify effectiveness with an air-shot test before production begins—the extruded melt should be continuous, smooth, and free of white vapor. Using a standard hopper dryer without dehumidification is insufficient for PC.

What causes black spots in polycarbonate injection molded parts?

Black spots in PC injection molded parts are typically caused by carbonized material trapped in dead zones of the plasticizing system—such as screw check ring gaps, nozzle tip interfaces, or barrel wall scoring. The trapped material degrades over time and intermittently breaks loose into the melt stream, producing dark spots that appear randomly across several shots then disappear. Regular disassembly and cleaning of the plasticizing system, combined with proper barrel purging procedures using PS or PE before and after each production run, prevents this issue effectively. Never leave PC material sitting at processing temperature during extended machine stops.

What is the recommended injection molding temperature for polycarbonate?

For pure polycarbonate, the recommended barrel temperature profile is 250 C (feed zone) to 285-295 C (nozzle), with mold temperature at 80-120 C. The key is to use a temperature gradient across zones rather than a single setpoint, ensuring gradual plasticization without premature melting in the feed zone. Modified grades have different windows: PC/ABS blends run approximately 20 C lower, while PC/PBT blends may require similar or slightly higher temperatures. Always start at the lower end of the recommended range and increase only if flow defects appear.

How do you prevent internal stress in transparent PC products?

Preventing internal stress in transparent PC parts requires a multi-pronged approach. Use higher melt temperature to reduce viscosity and molecular orientation during flow. Increase mold temperature to 100-120 C for slower and more uniform cooling that gives oriented molecules time to relax. Minimize injection and holding pressure to the minimum needed for complete filling. Use variable-speed injection with fast fill followed by slow pack. Post-mold heat treatment at 120 C for approximately 2 hours is standard practice for optical-grade components to relax frozen molecular orientation.

Why do silver streaks appear on PC injection molded parts?

Silver streaks on PC parts are caused by gas escaping to the product surface during or after cavity filling, leaving tiny elongated bubbles that sparkle under light. The most common gas source is water vapor from inadequately dried material—this accounts for the majority of cases. Thermal decomposition gas from excessive barrel temperature is the second most common cause. Fine dense silver streaks concentrated near the gate in a radial pattern indicate entrained air from excessive screw speed or insufficient back pressure. Proper drying at 120 C for 3-4 hours eliminates most moisture-driven cases.

Can polycarbonate injection molding defects be fixed by adjusting only the machine parameters?

Many PC defects—particularly silver streaks, discoloration, and flow marks—can be resolved through machine parameter adjustments alone, primarily drying conditions, barrel temperature profiling, and injection speed optimization. However, recurring defects like persistent cold material spots or turbulence marks often require mold modifications such as enlarged gates, additional venting channels, or cold-slug wells. Internal stress reduction may also require design changes to wall thickness uniformity and insert preheating. The most effective approach combines parameter optimization with proper mold design from the start.

What is the difference between bubbles and vacuum bubbles in PC parts?

Bubbles in PC parts are gas-filled voids caused by water vapor, entrained air, or thermal decomposition gas trapped during cavity filling. They are present immediately upon mold opening and do not grow over time. Vacuum bubbles are fundamentally different—they are shrinkage-induced voids that form during cooling when holding pressure is insufficient to compensate for volumetric shrinkage in thick sections. Vacuum bubbles may appear or enlarge after demolding as the interior continues to cool and contract. The diagnostic difference determines whether you address gas content through drying and temperature, or packing through holding pressure.

How Can You Get Expert Support for Your PC Injection Molding Project?

ZetarMold is the manufacturing partner for polycarbonate injection molding projects that demand zero defects. Our engineering team brings 20+ years of experience processing PC and 400+ materials to every project, from material drying and temperature profiling to gate design and stress relief. Our in-house tooling facility, MOLDFLOW simulation capability, and ISO-certified quality systems mean your PC parts are engineered for quality from the DFM stage onward.

Need a quote for your polycarbonate injection molding project? Get competitive pricing, DFM feedback, and production timeline from our engineering team. Request a Free Quote today, or explore our Injection Molding Complete Guide for a comprehensive overview of the process.

1. melt viscosity remains high: melt viscosity remains high refers to pC melt viscosity is more sensitive to temperature changes than to shear rate, behaving similarly to a Newtonian fluid during processing. ↩

2. PC glass transition: PC glass transition refers to polycarbonate has a glass transition temperature (Tg) of approximately 147-150 C, which determines minimum thermal hold temperature. ↩

3. moisture content below 0.02%: moisture content below 0.02% refers to pC requires moisture content below 0.02% (200 ppm) before processing to prevent hydrolytic degradation at molding temperatures. ↩