Makrolon Polycarbonate Viscosity Bands: Matching a Code to a Job

Polycarbonate (PC) is an amorphous thermoplastic whose melt viscosity is governed primarily by weight-average molecular weight and molecular weight distribution. In Makrolon nomenclature, the numerical viscosity code corresponds to melt volume-flow rate (MVR) determined at 300 °C under 1.2 kg according to ISO 1133-1:2022. A low-viscosity code such as Makrolon 2405 or 2407 is associated with an MVR of 19 cm³/10 min; Makrolon 2805 and 2807 occupy the 10 cm³/10 min band; Makrolon 3105 and 3107 fall within the 6 cm³/10 min band. Higher-viscosity extrusion grades are below 6 cm³/10 min. Matching a Makrolon polycarbonate viscosity band to a production job therefore requires simultaneous evaluation of flow length, wall thickness, molding or extrusion method, impact requirement, drying capacity, and regulatory exposure.

What Melt Volume-Flow Rate Defines Each Makrolon Band?

Each viscosity band is associated with a process window and a wall-thickness range rather than a single property. The MVR measured at 300 °C/1.2 kg is not a direct injection-molding parameter; it is a low-shear index. Shear viscosity at 1000 s⁻¹ may be lower by an order of magnitude but follows the same molecular-weight trend. Grade assignment must account for shear-thinning behavior, cavity length-to-thickness ratio, packing pressure transmission, and weld-line strength.

On a 1300 kN hydraulic injection molding machine producing a 0.8 mm wall connector with a flow length of 110 mm from a single edge gate, the 19 cm³/10 min band is selected because the cavity length-to-thickness ratio exceeds 130:1. Melt temperature is held at 285–300 °C with a mold temperature of 90–100 °C. Injection specific pressure is maintained between 80 MPa and 120 MPa; cavity pressure transducers at the end of fill record 250–350 bar. Substitution of the 10 cm³/10 min grade results in short shots below 300 °C or requires injection pressure above 130 MPa, increasing flash risk. The high-flow grade shortens fill time but lowers notched impact measured under ISO 180/1A. Where UL 94 V-0 is required at 0.8 mm, flame-retardant high-flow grades are processed at melt temperatures not exceeding 300 °C to limit polycarbonate degradation.

For medium-viscosity Makrolon 2805/2807, the 10 cm³/10 min band at 300 °C/1.2 kg matches 1.5–3.0 mm wall appliance housings and medical device enclosures where post-sterilization ductility is limiting. Melt temperatures of 270–290 °C and mold temperatures of 80–100 °C are used. In an 8-drop hot-runner mold with 180 mm flow length, packing pressure of 60–80 MPa is held for 6–10 s to compensate for volumetric shrinkage at rib intersections. The 10 cm³/10 min band retains sufficient molecular weight to survive steam autoclave cycles at 121 °C for 20 min, provided residual pellet moisture is below 0.020%. Tensile yield stress is typically 60–65 MPa and tensile modulus 2300–2400 MPa per ISO 527-2:2012.

Extrusion-grade PC demands a lower MVR and higher melt strength

Sheet and profile extrusion require sag resistance and draw-down consistency. Makrolon high-viscosity grades with MVR ≤6 cm³/10 min are selected. A single-screw extruder with 30:1 L/D, 60 mm diameter, and a barrier screw with melt pump is operated at barrel zones 240–270 °C and die temperature 260–280 °C. Melt pressure before the screen pack is 120–180 bar. The lower MVR increases melt strength and permits 3–20 mm solid sheet calendering without excessive sag. High-viscosity PC should not be processed in reciprocating-screw injection units with short barrels because screw recovery time becomes limiting and melt residence time increases. The lower flow code is not a processing defect in extrusion; it is a functional requirement for parison stability and sheet gauge uniformity.

When stress-cracking resistance overrides cycle time, high-viscosity grades prevail

Polycarbonate exposed to plasticizers, amines, or certain alcohols can fail by environmental stress cracking. Higher molecular weight grades with lower MVR reduce craze initiation and slow crack growth. Under constant strain of 1.0% in isopropanol at 23 °C, a 6 cm³/10 min grade may exhibit longer time to failure than a 19 cm³/10 min grade when evaluated per ISO 22088-2:2006; published values for specific Makrolon formulations are formulation-dependent. Selection of Makrolon 3105/3107 over 2405/2407 is common for clips, snap-fit arms, and filter housings. The limitation is slower mold filling and longer screw recovery. Melt residence time should not exceed 6–8 min at barrel temperatures above 310 °C to prevent molecular weight loss, yellowing, and effective MVR drift.

Drying, Residual Moisture, and Residence-Time Limits in Viscosity Band Selection

Moisture control is independent of viscosity band but interacts with residence time. Polycarbonate must be dried to residual moisture below 0.020% before melt processing. Desiccant dryers with dew point of −40 °C to −50 °C are specified; at 120 °C, drying time for virgin pellets is 4 h, while cold regrind may require 6–8 h. Higher-viscosity grades and extrusion grades are more sensitive to moisture because hydrolysis reduces molecular weight and shifts the effective MVR upward, undermining the assigned band. In-line moisture analyzers using Karl Fischer titration per ISO 15512:2019 verify pellet moisture. When relative humidity exceeds 60%, open-container transfer from dryer to hopper should not exceed 30 min, or a closed conveying system with dried air purge is required.

Automotive headlamp bezels and reflectors use medium or high-viscosity Makrolon because of heat resistance and metallization requirements. A 2.5 mm wall reflector with a flow path of 250 mm from a sub-gate is molded in the 10 cm³/10 min band at melt temperature 300–320 °C and mold temperature 120–130 °C to minimize surface defects. Vacuum metalizing requires outgassing below 0.01% by weight; high-MVR grades may contain more low-molecular-weight species and volatiles, causing haze after metallization. The 6 cm³/10 min or 10 cm³/10 min band is selected. Outgassing is validated per ASTM E595-15; adhesion is checked per ISO 2409:2020. Published data for this specific configuration is limited; production validation remains necessary.

Medical device housings requiring repeated sterilization use the 10 cm³/10 min band because of the balance between mold filling and ductility after gamma irradiation. Gamma doses of 25–50 kGy reduce molecular weight and shift MVR upward; starting with a lower-MVR grade provides margin. Validation includes tensile elongation at break per ISO 527-2:2012 after irradiation and notched impact per ISO 180/1A. High-flow 19 cm³/10 min grades may lose a greater share of original ductility after 50 kGy; medium and high viscosity grades retain more, but formulation-specific data from supplier technical bulletins must be consulted. Selection for skin contact or food contact requires compliance with FDA 21 CFR 177.1580 and EU 10/2011.

Dispersion of colorants and fillers is influenced by the assigned viscosity band. A low-viscosity 19 cm³/10 min grade wets pigments faster and dissipates shear heat more uniformly in a 27 mm co-rotating twin-screw extruder with L/D 40:1. A 6 cm³/10 min grade requires either 10–20 °C higher melt temperature or screw speed reduction to 250 min⁻¹ to hold motor load within the extruder limit. The resulting masterbatch letdown ratios must be revalidated per ISO 1133-1:2022 because shear history modifies the final MVR.

Extrusion blow molding of 20 L containers uses a high-viscosity band because parison sag must remain below 15% of length before mold closure. Accumulator-head machines with 80 mm extruders and parison programmers control wall distribution. The selected grade has MVR ≤6 cm³/10 min at 300 °C/1.2 kg. Melt temperature at the die is 230–250 °C to raise viscosity and maintain parison stiffness. High-flow PC would thin at the pinch-off and fail under drop impact. The lower flow code is not a processing defect in this context; it is a functional requirement for melt strength during parison formation.

A viscosity band mismatch manifests as short shots or flash in injection molding, or as sag and gauge variation in extrusion. The processing window for low-viscosity Makrolon is often ±5 °C in melt temperature and ±20 bar in holding pressure at thin-wall sections below 1.0 mm. For high-viscosity grades, the window is wider in melt temperature but narrower in screw recovery time. Cavity pressure monitoring is recommended to maintain process capability index Cpk >1.33. If a molding cell lacks closed-loop process control, high-flow grades are more tolerant of fill-speed changes than high-viscosity grades.