Polycarbonate (PC) Injection Molding: Properties, Grades, Drying & Process Guide

Polycarbonate (PC)—supplied under benchmark trade names such as Lexan (SABIC) and Makrolon (Covestro)—is the premier amorphous engineering thermoplastic when an injection-molded part must combine extreme impact toughness (650–850 J/m Notched Izod), glass-like optical clarity (88%–90% light transmission), high heat resistance (125–138 °C HDT), and tight dimensional stability (0.5%–0.7% isotropic shrinkage).

However, polycarbonate is far less forgiving on the press floor and in CAD than ABS or acrylic (PMMA):

1. Invisible Hydrolytic Degradation: If pellets enter the 300 °C barrel with more than 0.02% (200 ppm) moisture, water cleaves the carbonate polymer chains (hydrolysis). A mildly wet PC part can emerge from the mold looking dimensionally acceptable while having lost 50%–70% of its impact strength, shattering like glass on its first drop test.
2. Molded-In Stress & Chemical Stress Cracking: Because molten PC has high viscosity, injecting it into a cold mold (< 70 °C) freezes high internal tensile stress into the walls. When that stressed part later contacts threadlockers, isopropyl alcohol wipes, or industrial cleaners, it fails via Environmental Stress Cracking (ESC).

Use this guide alongside material selection, UL 94 flammability ratings, and mold surface finish to specify the right polycarbonate grade and process audit controls.

1. Polycarbonate Grade Comparison Matrix: Optical vs FR-PC vs Glass-Filled vs PC/ABS

Specify the exact functional sub-grade on your drawing rather than calling out generic “Polycarbonate”:

Property / Specification	General-Purpose / Optical PC (e.g., Makrolon 2405 / Lexan 141R)	Flame-Retardant FR-PC (UL 94 V-0 / 5VA)	20% Glass-Filled PC (PC-GF20)	PC/ABS Alloy (e.g., Bayblend / Cycoloy)
Specific Gravity (g/cm³)	1.20	1.21 – 1.25	1.35	1.12 – 1.18
Light Transmission (@ 3.0 mm)	88% – 90% (Refractive Index 1.586)	Transparent or Opaque (grade-dependent)	Opaque	Opaque
Tensile Yield Strength (MPa)	60 – 66 MPa	60 – 65 MPa	100 – 115 MPa	52 – 62 MPa
Flexural Modulus (GPa)	2.3 – 2.4 GPa	2.3 – 2.5 GPa	5.5 – 6.0 GPa	2.3 – 2.7 GPa
Notched Izod Impact (23 °C, J/m)	650 – 850 J/m (ductile yield)	500 – 700 J/m	100 – 160 J/m	450 – 650 J/m
Heat Deflection Temp (HDT @ 1.8 MPa)	125 – 132 °C ($T_g \approx 147^\circ\text{C}$)	120 – 130 °C	138 – 142 °C	105 – 125 °C
UL 94 Flame Rating	UL 94 V-2 (@ 0.75–1.5 mm)	UL 94 V-0 (@ 1.5 mm) / 5VA (@ 2.5–3.0 mm)	UL 94 V-1 or V-0	UL 94 HB or V-0
Mold Shrinkage (mm/mm)	0.5% – 0.7% (isotropic)	0.5% – 0.7% (isotropic)	0.2% – 0.4% (flow)	0.5% – 0.7% (isotropic)
Primary Buyer Use Case	Safety glazing, light pipes, medical fluid housings, rugged instrument windows	EV battery modules, smart meters, power supply enclosures, lighting ballasts	Structural camera chassis, precision die-cast metal replacement frames	Thin-wall electronics housings where pure PC is too viscous to fill

(Note: For outdoor lenses or solar enclosures, specify a UV-stabilized grade (e.g., Lexan 143R / Makrolon LED/UV), indicated by the "f1" outdoor suitability rating on its UL Yellow Card.)

2. Four Engineering Limitations of Polycarbonate (and How to Mitigate Them)

1. Low Surface Hardness (Scratching & Abrasion): Uncoated polycarbonate has a Pencil Hardness around 2B–HB, meaning wiping a dusty PC lens with a cloth leaves permanent micro-scratches.
   • Fix: Specify a secondary polysiloxane or UV-cured acrylic hard-coat (3–8 µm thick) for optical windows and automotive headlamp lenses, or switch to PMMA (acrylic) if impact loads are light.
2. Severe Notch Sensitivity at Sharp Corners: While a smooth PC bar absorbs 750 J/m of impact energy, a sharp internal corner (R < 0.1 mm) drops its effective impact resistance by over 80%, shifting failure from ductile yielding to brittle shattering.
   • Fix: Enforce an internal corner radius of R ≥ 0.5 × nominal wall thickness (minimum R 0.5–0.8 mm) across all bosses, ribs, and snap-fit bases in CAD.
3. Environmental Stress Cracking (ESC) & Solvent Attack: Polycarbonate is rapidly attacked by ketones (acetone, MEK), aromatic hydrocarbons, esters, ammonia/amines, alkaline cleaners, and uncured anaerobic threadlockers (Loctite 242/262).
   • Fix: Never use standard liquid threadlockers on brass inserts mounted in PC bosses (they wick into micro-cracks and shatter the boss within 48 hours). Use pre-applied dry patch screws, mechanical locking fasteners, or specify a PC/PBT (Xenoy) or PC/polyester alloy when chemical/fuel resistance is required.
4. Hydrolysis in Hot Water or Steam Sterilization: Standard general-purpose PC degrades if repeatedly steam-autoclaved above 121 °C (hydrolytic cleavage drops molecular weight after 5–20 autoclave cycles).
   • Fix: For medical reusable instruments, specify a medical lipid-resistant or radiation/ethylene-oxide (EtO / Gamma) sterilizable grade, or upgrade to PSU (Polysulfone) / PPSU (Radel) for 1,000+ steam-autoclave cycles.

3. Polycarbonate Processing Window & Mandatory Shop-Floor Controls

Published processing windows from Covestro (Makrolon) and SABIC (Lexan) reflect polycarbonate’s high glass transition temperature ($T_g \approx 147^\circ\text{C}$) and high melt viscosity:

Processing Parameter	High-Flow Optical / Thin-Wall PC	Standard Medium-Viscosity PC	10%–30% Glass-Filled PC
Desiccant Drying (Dew Point ≤ -30 °C)	120 °C (250 °F) for 3–4 hours (max 48 h in hopper)	120 °C (250 °F) for 3–4 hours	120 °C (250 °F) for 3–4 hours
Max Moisture Before Molding	≤ 0.02% (200 ppm) (Target ≤ 0.015%)	≤ 0.02% (200 ppm)	≤ 0.02% (200 ppm)
Melt Temperature Window	280–305 °C (535–580 °F)	295–320 °C (560–610 °F)	300–330 °C (570–625 °F)
Mold Temperature Window	80–100 °C (175–210 °F)	85–110 °C (185–230 °F)	95–120 °C (205–250 °F)
Perimeter Vent Depth	0.030–0.060 mm (0.0012–0.0025 in)	0.040–0.075 mm (0.0015–0.003 in)	0.040–0.075 mm

Three Audit Questions to Ask Your Molder Before Running PC:

1. “How do you verify < 0.02% pellet moisture before starting the press?” Do not accept “we set the dryer timer for 4 hours.” Require moisture verification or an active dew-point monitor (≤ -30 °C to -40 °C) on a closed desiccant hopper loader. Remember that hydrolyzed PC parts often have zero visual splay when only slightly wet (0.03%–0.05%), yet fail drop impact testing.
2. “Can your thermolators hold 85–110 °C mold temperature?” Running polycarbonate in a cold water-chilled mold (40–50 °C) to shave 4 seconds off cycle time locks severe frozen-in tensile stress (> 10–15 MPa) into the part walls. Hot molds (85–110 °C) allow polymer chains to relax before freezing, improving impact toughness, optical birefringence, and chemical stress-cracking resistance. (Suppliers can verify low internal stress on transparent PC parts via a polarized light cross-polarizer filter or an ethyl-acetate/toluene glacial acetic acid solvent-dip test).
3. “What cavity steel and gate design are used on transparent optical PC parts?” Transparent lenses and light pipes require ESR 420SS (Stavax / 1.2083 at 50–52 HRC) diamond-polished to SPI A-1 or A-2 (Ra < 0.012–0.025 µm), paired with wide fan or tab gates (at least 75%–90% of wall thickness) directed against a core wall to prevent jetting snakes and flow-hesitation ripples.

Polycarbonate (PC) vs PMMA (Acrylic) vs ABS

• PC vs PMMA (Acrylic): Choose PC when the window or lens must survive impact, vandalism, or > 90 °C temperatures (PC has ~15×–25× the Notched Izod impact strength of standard PMMA). Choose PMMA when optical light transmission (92% vs 89%), scratch hardness, and 10-year outdoor non-yellowing UV stability outrank impact strength—and at ~30%–40% lower resin cost.
• PC vs ABS: Choose PC for transparency, high impact toughness down to -40 °C, high heat (130 °C HDT), or inherent UL 94 V-2 / V-0 flame resistance. Choose PC/ABS when the part is opaque and needs easier thin-wall flow and lower cost while keeping high impact toughness.

Buyer FAQs

Why must polycarbonate be dried to < 0.02% moisture before injection molding?

Polycarbonate is hygroscopic (absorbing ~0.15%–0.35% moisture from ambient air) and undergoes hydrolytic chain cleavage (hydrolysis) when melted at 280–320 °C in the presence of water. If pellet moisture exceeds 0.02% (200 ppm), water molecules sever the polymer backbone, reducing molecular weight and causing parts to shatter brittlely under impact—often without showing obvious silver splay on the surface. Polycarbonate must be dried in a dehumidifying desiccant dryer at 120 °C (250 °F) for 3–4 hours.

What mold temperature is required for polycarbonate injection molding?

Standard unfilled polycarbonate requires a hot mold temperature of 80–110 °C (175–230 °F), while glass-filled and high-heat optical grades require 95–120 °C (205–250 °F). Molding polycarbonate in a cold tool (< 70 °C) freezes high residual tensile stress into the polymer skin, leading to premature warpage, optical birefringence distortion, and rapid environmental stress cracking (ESC) when exposed to cleaners or adhesives.

Is polycarbonate naturally UL 94 V-0 flame retardant?

Unmodified general-purpose polycarbonate is naturally self-extinguishing to UL 94 V-2 at standard wall thicknesses (0.75–3.0 mm), which is already higher than unmodified ABS or polypropylene (UL 94 HB). To achieve UL 94 V-0 (zero flaming drips, ≤ 10 s burn time at 1.5 mm) or UL 94 5VA (at 2.5–3.0 mm), you must specify a flame-retardant polycarbonate grade (FR-PC) formulated with anti-drip PTFE and halogen-free sulfonate/siloxane or brominated flame retardants.

Why do polycarbonate screw bosses crack after assembly?

Polycarbonate screw bosses crack after assembly due to Environmental Stress Cracking (ESC)—a combination of high hoop tensile stress from an oversized self-tapping screw (or ultrasonic insert press-fit) and chemical attack from anaerobic threadlockers (e.g., Loctite), cutting oils, or alcohol cleaners. Prevent boss cracking by sizing the boss OD to 2.0–2.5× the screw OD, adding a 0.5–0.8 mm base fillet radius, molding at 85–110 °C tool temperature to minimize residual stress, and strictly prohibiting liquid threadlockers on polycarbonate.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Optical transmission, UL 94 flame ratings, and chemical resistance are grade- and thickness-dependent; always verify specifications against the manufacturer’s datasheet and UL Yellow Card.