Makrolon Material: Reading the Polycarbonate Four-Digit System, 2205 to 6557 requires the reader to treat the grade code as a controlled vocabulary, not a material data sheet. Polycarbonate (PC) is an amorphous thermoplastic formed by the reaction of bisphenol A with carbonyl chloride or diphenyl carbonate; its glass transition temperature lies near 145–150 °C, and its melt-processing window is commonly 280–320 °C. Makrolon is a registered trade name of Covestro, and the four-digit identifiers from 2205 through 6557 designate combinations of melt-flow family, stabilization package, release behavior, and application-specific purity. Grades are qualified under test standards including ISO 1133-1:2022, ISO 527-2, ISO 4892-2, and ASTM D1238. The numerical sequence does not function as a single physical property scale; a 2205 and a 6557 may overlap in certain rheological values while differing in regulatory target and additive composition.

What Does Each Digit Position Encode in the 2205 to 6557 Range?

The first two digits identify the product platform and melt-flow family. In the 2xxx and 3xxx series, lower prefixes such as 22, 24, and 26 are typically high-flow injection-moulding grades, while 28, 31, 32, and 35 migrate toward lower melt-volume rate and higher melt strength for extrusion, thermoforming, and injection blow moulding. The 6xxx series is a high-purity and medical-related family; its position in the four-digit system is not a direct extension of the viscosity code and must be read separately. The third digit commonly separates additive or stabilization packages, but the meaning is series-specific. In many standard grades, 0 denotes general-purpose stabilization, while 5 indicates UV-stabilized or specialty-modified systems. The fourth digit defines the grade suffix, release package, or color subclass; it is the least suitable field for inferring mechanical or rheological performance. For colored 6xxx variants, published data for this specific configuration is limited, and the Covestro datasheet is the only reliable specification source.

Because the fourth digit controls release and color subclass, replacing a 2405 with a 2407 can change ejector force, demoulding consistency, and UV resistance without shifting the first two digits. Comparative MVR measurements must use identical ISO 1133-1:2022 conditions: 300 °C barrel temperature, 1.2 kg load, and dried granulate. Sorting by prefix is a screening operation; specification release requires full datasheet traceability.

Melt Rheology and the Practical Limits of Prefix-Based Sorting

The first two digits are a pre-selection filter, not a quality-control boundary. High-flow 22xx and 24xx grades are normally evaluated on injection-moulding machines with hot-runner manifolds and cold sprue bushings. The melt is shear-thinning, but the slope of the viscosity curve is lower than that of polyolefins; therefore shear heating at gate lands and hot-runner tips becomes significant at high fill speeds. Capillary rheometry at 300 °C, 400 s⁻¹, and 1000 s⁻¹ provides more useful tooling data than a single MVR point. Commercial Makrolon grades in the 2205 to 6557 range span an MVR window from high-flow grades above 20 cm³/10 min to low-flow extrusion grades below 6 cm³/10 min under ISO 1133-1:2022. The 6xxx medical grades can overlap the flow range of 2xxx and 3xxx products; the prefix should not be used to predict viscosity alone.

Mechanical properties across this span are narrow relative to rheology. Unmodified PC grades typically exhibit a tensile modulus of 2300–2400 MPa and tensile yield stress of 60–65 MPa under ISO 527-2. The stress-strain response depends on strain rate and conditioning; tensile elongation at yield is approximately 6% for many standard grades. Notched impact performance is strongly affected by radius, mould temperature, and residual stress, so direct comparison between a 2205 and a 3505 requires the same specimen geometry and conditioning protocol. The presence of a UV stabilizer, internal release agent, or nucleation package affects long-term aging more than initial tensile properties. When a grade is changed within the same first two digits but with a different fourth digit, the melt-flow and mechanical data may remain similar, while contact compliance and optical aging behavior may not.

Residual moisture controls processability. PC hydrolyzes at melt processing temperatures when moisture exceeds approximately 0.02 wt%, producing splay, molecular weight loss, and reduced impact. Production dryers are set at 120 °C for 4–8 h with dehumidified air at a dew point below -30 °C. In plants where ambient relative humidity exceeds 60%, extended drying or Karl Fischer moisture verification is required. A dew-point sensor at the dryer outlet is a better startup control than a melt-pressure transducer because it detects the most common root cause of lot-to-lot processing variance. Hopper residence time, return-air temperature, and regeneration cycle must be recalibrated when switching from a fast-flow 22xx grade to a lower-MVR 35xx grade because the plasticating rate differs.

Injection moulding parameters for unfilled PC typically use melt temperatures of 280–320 °C and mould temperatures of 80–120 °C. Optical parts require the upper half of the mould-temperature range to reduce frozen-in stress and birefringence. Screw L/D ratios of 18:1 to 24:1 and compression ratios of 2.0:1 to 2.5:1 are specified in processing guides. High-flow 22xx grades tolerate shorter residence times, but hot-runner dwell above 320 °C can produce yellowing and chain scission. Changing from a 2605 to a 2607 can alter ejection because the release package in the suffix is different; mould-release spray levels or mould coating may require adjustment. Additives based on alkaline amines should be avoided in regrind streams, as they can accelerate hydrolysis and transesterification during prolonged residence.

Extrusion of lower-flow 31xx, 32xx, and 35xx grades for sheet and profile uses melt temperatures from 260 °C to 300 °C and roll-surface temperatures from 80 °C to 120 °C. Barrel profiles are set with a reverse-temperature slope to avoid overheating the first compression zone. Melt-pressure stability at the die is evaluated at ±1 bar on production extrusion lines; larger swings indicate wet granulate, bridging, or worn screw elements. For twin-screw compounding of glass-filled or flame-retardant PC variants, the first two digits of the base resin must be selected for the required final MVR after additive loading, because reinforcers reduce melt flow.

When the 6xxx Medical and High-Purity Grades Replace 2xxx General-Purpose Resin

Selection of 6555 or 6557 over 2205 or 2407 is normally driven by regulatory exposure and purity, not by tensile strength. Medical-grade Makrolon compounds are evaluated under ISO 10993-1 for biological risk; food-contact grades are assessed against FDA 21 CFR 177.1580 and EU Regulation (EU) No 10/2011. The 6xxx prefix does not itself constitute biocompatibility; the specific grade, colorant, and manufacturing site must be qualified in the device or packaging master file. Sterilization is a processing step that changes residual stress: autoclaving at 121 °C can relax orientation and alter dimensions; gamma irradiation can produce yellowness; electron-beam irradiation at doses above approximately 50 kGy can induce chain scission. Published data for this specific configuration is limited for colored 6557, so validation doses must be studied on actual finished parts.

Drying of 6xxx grades is more critical than for standard opaque grades because hydrolytic byproducts may influence biological test results. Moisture content is verified by Karl Fischer titration rather than inferred from dryer dew point alone. In production, 6557 parts are often processed with mould temperatures above 90 °C to improve surface replication and reduce leachable migration. The fourth digit and color subclass affect surface friction in ultrasonic welding; weld protocols should be revalidated by tensile pull testing to ISO 527-2 when switching to a different suffix within the same medical family.

Optical lenses and covers produced from low-stress grades are evaluated by ISO 13468 for transmittance and ASTM D1003 for haze. The tool surface must remain above 100 °C to limit birefringence and post-mould shrinkage. Automotive lighting lenses using UV-stabilized 2407 or 2807 grades are subjected to ISO 4892-2 or ASTM G155 with delta yellowness index as the acceptance criterion. Electrical enclosures in PC use IEC 60695-2-12 glow-wire testing at 850 °C or 960 °C depending on the end-product specification and wall thickness. Thin-wall consumer electronics parts using 2205-type grades fill flow-length-to-thickness ratios above 150:1 on production tooling; cavity pressure sensors validate gate freeze time and avoid short shots during hot-runner valve sequencing. In medical applications with 6557, ultrasonic weld strength is measured by tensile pull testing to ISO 527-2 because the release package can change energy director collapse. Changing from a 2405 to a 2407 on an automotive lens line without re-qualifying UV retention can produce yellowness index drift within 500 h of xenon arc exposure under ISO 4892-2.