Polycarbonate tube and light pipe: extruded forms for lighting and fluid handling are manufactured from amorphous bisphenol A polycarbonate resin that combines a refractive index of 1.586 with visible-region luminous transmittance of 88–91 % through a 3.2 mm section when measured under ASTM D1003-21. In lighting, extruded polycarbonate light pipe functions as a transparent waveguide in which total internal reflection is controlled by sidewall surface quality and refractive index contrast. In fluid handling, polycarbonate tube is used for sight glasses, level indicators, and lower-pressure transfer lines where clarity, dimensional stability, and resistance to impact are required. Extrusion of these forms is sensitive to residual moisture; desiccant drying to <0.02 % moisture and a dew point below -40 °C is standard because hydrolysis at melt temperature cleaves carbonate linkages and raises melt flow rate under ISO 1133-1:2022 while reducing Charpy impact strength under ISO 179-1:2010. The processing window for polycarbonate is therefore narrower than for polyolefin tube extrusion and must be controlled by calibrated melt-temperature and melt-pressure instrumentation.

Extrusion Parameters for Maintaining Molecular Weight Retention in PC Tubing

Production of polycarbonate tube and solid rod is performed on single-screw or co-rotating twin-screw extruders with screw L/D ratios between 24:1 and 30:1. Barrel zone settings are normally 260–300 °C, with the metering section and adapter held between 280 °C and 300 °C; die-head temperatures are set 10–20 °C lower than the metering zone to control die swell and surface haze. A three-zone screw with a compression ratio of 2.25:1 to 2.75:1 and a barrier-flight Maddock section is specified on many polycarbonate lines because it avoids solids-bed breakup and excessive shear heating that would lower molecular weight. Vacuum venting at -0.08 MPa or deeper removes volatiles and residual moisture; insufficient vent vacuum produces microvoids in the tube wall and internal haze that cannot be corrected downstream. Melt pressure at the breaker plate is typically maintained between 10 MPa and 25 MPa, and a positive-displacement melt pump may be installed before the die to hold output variation below 1 % for precision light pipe diameter control. In optical-grade tube extrusion, the mandrel and die land are polished to a surface roughness below 0.1 µm Ra, characterized according to ISO 4287:1997, because die-land scratches transfer directly to the extrudate. Drawdown ratio is limited to <1.15:1 for solid light pipe to preserve sidewall smoothness and reduce flow-induced birefringence. Line speeds for small-diameter tube are typically 2–20 m/min depending on outer diameter and wall thickness. Production-scale vacuum calibration tanks with closed-loop water temperature control at 20–40 °C are used to set final dimensions.

In solid-core polycarbonate light pipe, optical attenuation is not controlled solely by resin transmittance. Sidewall roughness, contamination, and dimensional variation at the polymer–air interface scatter guided rays and reduce the angular acceptance of total internal reflection. Extruded light pipe for luminaire applications therefore requires post-extrusion cutting and polishing of end faces, with end-surface flatness verified by ISO 4287:1997 profile tracing to ensure coupling to LED sources. For runs longer than 300 mm, haze below 1.0 % per ASTM D1003-21 and a yellowness index below 2.0 per ASTM E313-20 are typical acceptance criteria, but published data for long-length attenuation in extruded polycarbonate rods is limited because optical loss depends on source geometry, angular distribution, and surface cleanliness as well as bulk absorption.

What Optical Transmission Limits Apply to Solid-Core Polycarbonate Light Pipes?

Total internal reflection in a cylindrical polycarbonate light pipe is defined by the refractive index difference between the polycarbonate core and the surrounding medium. At 589 nm, the refractive index of polycarbonate is approximately 1.586; against air at 1.000, the critical angle is 39.1°. Light rays intersecting the outer wall at angles exceeding this critical angle remain guided, but this condition is violated by surface wetting, condensation, adhesive residue, or cladding materials with refractive indices below the core but above air. Polycarbonate is inherently absorbing in the ultraviolet and near-ultraviolet below 400 nm, and prolonged exposure to unfiltered UV sources produces yellowing, surface microcracking, and transmission loss. Visible-region transmittance for clear polycarbonate is specified as 88–91 % at 3.2 mm thickness under ASTM D1003-21; this value drops in thick sections and in outdoor lighting without UV-stabilized formulations. For luminaire light pipes, dimensional tolerances are commonly machined to ISO 2768-1 medium class or tighter at LED coupling faces, and the combination of surface quality and diameter tolerance determines output uniformity. Edge-lit and end-lit designs differ in flux distribution, but both require freedom from internal flow lines and weld lines; weld lines in spider-leg die tooling are a known source for nonuniform emission and are minimized by spiral mandrel die geometry in optical-grade tube.

When clear polycarbonate light pipe is extruded at high shear rates, surface melt fracture can appear as ripples or hazy bands that reduce sidewall specularity. Melt fracture in polycarbonate is controlled by reducing shear stress at the die wall below roughly 0.2 MPa; this is achieved by increasing die temperature, reducing line speed, or using broader die lands. Flow lines and weld lines in spider-leg tooling are a known source of nonuniform emission, while internal gel particles from degraded regrind appear as bright or dark points under LED end-fire illumination. Photometric inspection of extruded light pipe therefore includes automated line-scan imaging to detect point defects larger than 20 µm and continuous diameter monitoring at ±0.03 mm for optical-grade rod. Published data for quantitative defect thresholds in polycarbonate light pipe is limited because most acceptance criteria are set by original equipment manufacturer internal specifications rather than public standards.

Fluid Handling Compatibility and Environmental Stress Cracking Resistance

Polycarbonate tube in fluid-handling service must be specified with reference to both chemical attack and environmental stress cracking, because many organic and alkaline fluids that do not dissolve polycarbonate nevertheless initiate brittle failure under hoop stress or flexural strain. Dilute mineral acids, aqueous salt solutions, and some alcohols are tolerated at room temperature under unstressed immersion testing according to ASTM D543-21; ketones, esters, aromatic hydrocarbons, chlorinated solvents, and aliphatic amines are destructive or stress-cracking agents. Brake fluid, ammonia, and amine-based cleaning solutions are specifically incompatible with polycarbonate tube because they can produce surface haze or crazing. Environmental stress cracking resistance is evaluated by ISO 22088-3:2006 using curved specimens under imposed strain. Water absorption after 24 h at 23 °C is approximately 0.15 % by ISO 62:2008; continuous hot water above 60 °C is an operational boundary because hydrolysis and stress cracking accelerate. For food-contact tube, polycarbonate resin must comply with 21 CFR 177.1580 and applicable migration limits in the country of use. For medical fluid pathways, selected polycarbonate grades are evaluated under ISO 10993-1 biological risk assessment, but final qualification depends on the formulated tube, sterilization method, and patient-contact duration. Repeated steam autoclave sterilization above 121 °C is not recommended because it causes hydrolytic degradation and dimensional distortion; gamma irradiation at 25–40 kGy is used for some medical polycarbonate tube but may discolor the polymer.

For pressurized polycarbonate tube, the hydrostatic design basis is determined by long-term creep rupture testing according to ASTM D2837-22 and ISO 9080:2012. The allowable hoop stress is not based on short-term tensile yield alone; time-dependent brittle rupture and environmental stress cracking from the internal fluid can reduce the service pressure below values suggested by ISO 527-2:2012 tensile yield of 60–65 MPa. Polycarbonate tube under sustained pressure may fail by ductile expansion and crazing or by brittle fracture initiating at scratches, machining marks, or overtightened compression fittings. All fluid connections should avoid sharp threads and should use sealing materials that do not contain ketones, esters, or aromatic solvents. Service temperatures above 40 °C require derating of the pressure rating, and operation above 120 °C is prohibited under sustained load. Published data for specific polycarbonate pneumatic or hydraulic tubing configurations is limited; hydrostatic qualification should therefore be performed on the finished extruded geometry with the intended end fittings and service fluid rather than inferred from injection-molded tensile or creep coupon data.

The following matrix summarizes the principal standard designations encountered in specifying extruded polycarbonate tube and light pipe for lighting and fluid-handling applications.

When Polycarbonate Tubing Replaces Borosilicate Glass in Sight-Glass Applications

Polycarbonate sight tube is substituted for borosilicate glass where impact fracture and machining complexity are primary concerns. The thermal expansion coefficient of polycarbonate is 65–70 × 10⁻⁶ K⁻¹, compared with approximately 3.3 × 10⁻⁶ K⁻¹ for borosilicate glass; mounting brackets must allow axial clearance to avoid buckling or cracking under thermal cycles. Maximum continuous service temperature for unfilled polycarbonate sight tube is generally limited to 120 °C under low mechanical load, and lower when hot water, steam, or aggressive cleaning chemicals are present. The Vicat softening temperature for a typical polycarbonate grade is approximately 145–150 °C under ISO 306:2022 B50, but this value does not establish a continuous load service temperature. Notched Charpy impact strength is typically 10–15 kJ/m² when measured under ISO 179-1:2010, but this value is sensitive to surface finish and notch radius. The optical advantage of polycarbonate sight tube is high visible transmission through the tube wall; however, scratch resistance is inferior to borosilicate glass, and repeated cleaning with abrasive wipers produces surface haze that reduces readability. Haze development after abrasion can be quantified by ASTM D1003-21. For sight-glass applications exposed to fluorescent or metal-halide sources with emission below 400 nm, UV-stabilized polycarbonate is required to prevent yellowing and loss of transparency.