Technical resource
Low Viscosity Polycarbonate for Thin-Wall Molding and Optical Parts
Table of Contents
• Why Standard Polycarbonate Freezes in Thin Cavities
• Molecular Weight and Melt Flow Rate (MFR) Polycarbonate Limits
• Low Viscosity vs High Viscosity Polycarbonate Property Comparison
• How Copolymer Modification Fixes Impact Loss
• Siloxane-Modified Polycarbonate Copolymer for Cold Impact
• Optical Grade Polycarbonate Pellets and Birefringence Control
• Injection Molding Parameters and Moisture Control
• Barrel Temperatures, Drying Specs, and Wall Thickness Data
• Preventing Hydrolytic Degradation in Polycarbonate Processing
• Frequently Asked Questions
Push standard polycarbonate into a 0.6 mm wall cavity and the melt stops halfway. Short shots everywhere.
So the press operator cranks barrel heat past 320°C and bumps injection pressure to the limit. Now the mold fills, sure, but the resin yellows, flash pops out on the parting line, and the parts crack two days later from molded-in stress.
That is where low viscosity polycarbonate changes the job. Instead of fighting thick polymer chains with brute machine force, high flow polycarbonate resin cuts melt viscosity at the molecular level.
It fills long, thin flow paths at lower temperatures and drops cycle time fast.
Why Standard Polycarbonate Freezes in Thin Cavities
General-purpose polycarbonate is tough because its polymer chains are long and tangled. Typical viscosity-average molecular weight (Mv) sits between 20,000 and 30,000 g/mol.
At normal molding temperatures of 280°C to 320°C, its melt viscosity easily tops 1,000 Pa·s. That thick melt works fine for a 3 mm safety shield. Put it into a multi-cavity connector tool or a thin LED light guide, and skin freeze locks the runner before pack pressure even reaches the gate.
Molecular Weight and Melt Flow Rate (MFR) Polycarbonate Limits
To make low viscosity polycarbonate, resin producers shorten the chain length down to an Mv of 15,000 to 27,000 g/mol, or add chain-end flow promoters.
Under ISO 1133 testing at 300°C and 1.2 kg load, standard molding grades show a melt flow rate (MFR) of 3 to 10 g/10 min. High flow polycarbonate resin jumps to 15, 25, or even 40 g/10 min.
What is the catch? Shorter polymer chains untangle easier under impact.
Stiffness and tensile yield strength stay almost the exact same, right around 59 to 65 MPa. But notched and unnotched impact toughness drop when molecular weight goes down too far.
Low Viscosity vs High Viscosity Polycarbonate Property Comparison
Engineers have to balance flow length against impact strength before picking a resin grade. The table below compares verified material data between unfilled low viscosity and high viscosity polycarbonate homopolymers.
Material Property | Low Viscosity Polycarbonate | High Viscosity Polycarbonate | Engineering Note |
Molecular Weight (Mv) | 15,000 – 27,000 g/mol | 20,000 – 30,000+ g/mol | Shorter chains lower melt viscosity |
Melt Flow Rate (300°C/1.2 kg) | 10 – 40 g/10 min (ISO 1133) | 3 – 10 g/10 min (ISO 1133) | Up to 4x faster cavity filling |
Tensile Yield Strength | 59.1 – 65.2 MPa | 59.1 – 65.2 MPa | No loss in static tensile strength |
Flexural Strength | 82.7 – 96.5 MPa | 86.2 – 93.1 MPa | Comparable bending stiffness |
Notched Impact (23°C) | 10.1 – 83.1 kJ/m² | 69.7 – 102 kJ/m² | Standard high-flow grades are notch sensitive |
Unnotched Impact (23°C) | 188 – 226 kJ/m² | 590 – 600 kJ/m² | High viscosity is ~3x tougher unnotched |
HDT at 0.45 MPa | 134 – 138 °C | 138 – 142 °C | 4°C thermal difference under load |
How Copolymer Modification Fixes Impact Loss
Early high-flow resins were brittle in cold weather. If you dropped a thin-wall housing at -20°C, it shattered at the weld line.
Modern polymer chemistry gets around that trade-off without going back to thick, slow-moving melts.
Siloxane-Modified Polycarbonate Copolymer for Cold Impact
Instead of just chopping BPA chains shorter, compounders graft 5 to 20 wt% polydimethylsiloxane (PDMS) blocks into the polycarbonate backbone.
Those flexible siloxane segments act like internal lubricants during melt flow and shock absorbers in the solid part. A siloxane-modified polycarbonate copolymer flows into 0.5 mm walls easily, but still holds notched Izod impact strength above 30 J/m at room temperature and over 20 J/m at -30°C.
Optical Grade Polycarbonate Pellets and Birefringence Control
Optical lenses, VR eyepieces, and automotive light guides have a different problem: internal stress distortion. When thick polycarbonate gets forced into a thin mold under 120 MPa of pressure, the frozen molecular chains line up and bend polarized light.
Low viscosity optical grade polycarbonate pellets let the melt flow in gently at low shear. Fluorene-modified low viscosity grades keep light transmittance above 88%, yellowness index below 3 (ASTM E313), and photoelastic constants under 5 Brewsters, wiping out rainbow stress marks.
Injection Molding Parameters and Moisture Control
Running thin-wall polycarbonate injection molding takes tighter dryer and barrel discipline than standard ABS or polypropylene. Even a great high-flow resin fails if shop prep is sloppy.
Barrel Temperatures, Drying Specs, and Wall Thickness Data
Because polycarbonate melt viscosity is lower in high-MFR grades, molders can drop barrel heat by 15°C to 25°C and cut cooling time by 15% to 30%. The table below lists shop-verified processing settings.
Processing Parameter | Low Viscosity PC (MFR 20–40) | Standard PC (MFR 6–10) | Shop Floor Impact |
Pre-Drying Temperature | 110 – 120 °C for 3–4 hrs | 120 °C for 3–4 hrs | Dehumidifying hopper dryer required |
Max Moisture Content | < 0.02% | < 0.02% | Stops splay and chain scission |
Melt Temperature | 260 – 290 °C | 285 – 315 °C | Lower heat prevents yellowing |
Mold Temperature | 70 – 95 °C (120°C optical) | 85 – 120 °C | Faster part freeze and ejection |
Injection Pressure | 65 – 95 MPa | 85 – 120 MPa | Less clamp tonnage and flash |
Min Wall Thickness | 0.40 – 0.80 mm | 1.20 – 1.80 mm | Fills micro ribs and thin housings |
Preventing Hydrolytic Degradation in Polycarbonate Processing
Water is poison to hot polycarbonate. At room temperature, pellets soak up 0.14% to 0.16% moisture from shop air.
If moisture sits above 0.02% inside a 280°C barrel, water molecules attack the carbonate linkages and chop the polymer chains apart. In low viscosity polycarbonate, where chains are already shorter by design, hydrolytic degradation in polycarbonate turns a tough part brittle in seconds. Always run a closed-loop desiccant dryer with a dew point at or below -30°C.
Frequently Asked Questions
What melt flow rate (MFR) is considered low viscosity polycarbonate?
Under ISO 1133 or ASTM D1238 testing at 300°C and a 1.2 kg load, polycarbonate grades with an MFR between 15 and 40 g/10 min are classified as low viscosity or high flow. Standard injection grades sit between 6 and 12 g/10 min, while extrusion and blow-molding grades run at 3 to 5 g/10 min.
Does low viscosity polycarbonate yellow faster during injection molding?
No, it usually yellows less. Because high flow polycarbonate resin fills the cavity easily at 260°C to 285°C instead of 310°C+, the resin experiences less thermal shear and oxidation inside the screw barrel, keeping optical parts clear.
What is the thinnest wall section achievable with high flow polycarbonate?
Unfilled low viscosity homopolymers and siloxane-modified copolymers reliably fill wall thicknesses down to 0.45 mm to 0.50 mm on small electronic connectors and LED covers, provided injection speed is high and venting is clean.
Can you add regrind when molding low viscosity polycarbonate parts?
Up to 15% to 20% clean, dry sprues can be blended with virgin pellets for non-optical structural parts. However, every heat history cycle shortens the molecular weight slightly, so optical lenses and thin safety housings should use 100% virgin resin.