Polycarbonate (PC) Injection Molding: Material Guide

A polycarbonate mold that fails on the first trial usually fails in the tool design, not the resin. A gate sized for ABS shear rates on a resin that must run at 280-320°C produces gate blush on every shot. A wall section that ABS would cool evenly holds molded-in stress in PC and cracks in the field weeks after delivery. Both failures are visible on the drawing long before the steel is cut — and both are the reason this guide reads polycarbonate from the mold side, not the chemistry side.

Polycarbonate (PC) is the engineering thermoplastic behind headlamp lenses, face shields, medical device housings, and automotive glazing. It is an amorphous polymer of bisphenol A that transmits light at 88-90% on a 2-3 mm wall (typical published data) and carries the highest notched Izod impact of the common unmodified thermoplastics — 600-900 J/m (ASTM D256). That combination is why mold shops quote it daily, and why tooling built for PC has to earn its price: the same chemistry that delivers clarity and toughness punishes careless tooling and wet resin.

This guide is written for the people who buy and build the tool: mold buyers, tooling engineers, and manufacturing engineers. It walks from the properties that dictate mold decisions, through the design rules for PC tooling, into the processing window the mold must survive, and ends with defects traced to their mold-side root causes. Processing numbers follow typical published values for injection-molding grades. Test methods are cited where they matter. For the broader picture of how a PC tool fits into a full program, see how we run mold making at DieStrike.

The Snapshot

• PC is an amorphous polymer of bisphenol A: transparent, with 88-90% light transmission on a 2-3 mm wall (typical published).
• Mold shrinkage is 0.5-0.7% — low and uniform, because PC is amorphous. Glass-filled grades drop to 0.1-0.4%.
• HDT at 1.82 MPa: 120-130°C (ASTM D648); glass transition Tg ≈ 147°C.
• Notched Izod impact runs 600-900 J/m at 23°C (typical published range, ASTM D256) — the highest of the common unmodified thermoplastics.
• Resin must be dried to under 0.02% moisture: 120°C for 3-4 hours in a dehumidifying dryer; wet resin hydrolyzes and embrittles the part.
• Melt at 280-320°C into a mold at 80-120°C (100-120°C for optical and structural parts); PC is high-viscosity, so gates and runners are sized for low shear.
• DieStrike builds PC-capable tooling to IATF 16949 / ISO 9001 systems, with CMM-verified dimensions on every mold.

What Is Polycarbonate (PC)?

Polycarbonate plastic is an amorphous thermoplastic produced from bisphenol A (BPA) by polycondensation with phosgene or by melt transesterification with diphenyl carbonate. The repeat unit pairs rigid aromatic rings with carbonate linkages: -O-C6H4-C(CH3)2-C6H4-O-CO-. The aromatic rings give stiffness, toughness, and dimensional stability; the carbonate linkage gives melt processability. Commercialized in the 1950s, PC became the default material for parts that need transparency, impact strength, and heat resistance in the same shot.

Because PC is amorphous, it has no crystalline melting point and no sharp transition from solid to melt — it softens progressively above its glass transition near 147°C (typical published). For the mold designer, that single fact matters more than any datasheet row: amorphous resins shrink less and more uniformly than semicrystalline ones, which is why PC tooling compensates predictably and warps far less than PP or nylon tooling. Transparency follows from the same structure: crystalline regions would scatter light, and PC has none.

PC is a family rather than a single grade. Standard grades carry a density of 1.19-1.22 g/cm³ and a refractive index near 1.585; optical grades add guaranteed haze and birefringence limits; flame-retardant grades reach UL94 V-0 at 1.5-3.0 mm wall; medical grades carry ISO 10993 documentation; glass-filled grades trade transparency for stiffness and wear the mold faster. The grade specification matters as much as the material choice, because a grade change moves both the processing window and the shrinkage number the tool was cut to.

Datasheet test data comes from standard methods. ASTM D638 covers tensile, D256 covers Izod impact, D648 covers HDT, and D1003 covers haze and transmission. ISO 527, ISO 180, ISO 75, and ISO 13468 are the equivalents. Always compare grades using the same standard — numbers from different methods are not interchangeable.

PC Properties That Drive Mold Design

Material selection and tool design are one decision, not two. The table below maps the PC properties that matter most to the specific mold decisions they drive. If a datasheet number is missing at RFQ time, this is the list to ask for.

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Values are typical published ranges for injection-molding grades; actual numbers vary by grade and test method.

Shrinkage and dimensional stability. Amorphous resins shrink less and more uniformly than semicrystalline ones, and PC sits at 0.5-0.7% mold shrinkage. Uniform shrinkage means the part holds its shape across wall-thickness variations, and gross warpage risk is lower than with PP or nylon. The visible risk is local: sink marks where a rib or boss sits behind a surface, and internal voids where a thick section packs poorly. Glass-filled grades behave like a different material — shrinkage of 0.1-0.4%, anisotropic flow, faster mold wear — so the grade must be locked before the cavity is cut, not after.

Heat resistance. An HDT of 120-130°C at 1.82 MPa is what keeps PC in headlamp lenses, sunroof panels, and under-dash electronics where ABS and acrylic soften. For the tool, the HDT sets expectations, not exotic steel: an 80-120°C mold runs on standard P20 or 718-class inserts, and the cooling layout — not the steel — decides whether you hold a 40-second or a 60-second cycle. What the high HDT does demand is a hot mold: running PC cold trades cycle time for molded-in stress and cracked parts.

Transparency and surface quality. PC molds from high-gloss to matte texture on the same tool, but transparent parts are a different discipline: every flow mark, weld line, and scratch is visible. Gloss follows mold temperature — a hot mold at 100-120°C replicates the polished cavity faithfully, a cold mold gives dull, hazy surfaces — and the cavity finish must be specified to match: SPI A-1/A-2 polished steel for optical parts, EDM texture only where haze is acceptable. For the comparison against the other transparent workhorse, see the acrylic (PMMA) material guide.

Impact and electrical behavior. Notched Izod values of 600-900 J/m at 23°C are typical (ASTM D256) — the highest of the common unmodified thermoplastics — and the material fails by ductile yielding rather than shattering. Standard grades keep useful ductility down to roughly -20°C; impact-modified grades extend it below -40°C. PC is also a serviceable insulator with dielectric strength in the 15-30 kV/mm range (typical published values). Flammability is UL94 HB for standard grades; flame-retardant grades reach V-0 at 1.5-3.0 mm wall, at the cost of some flow and a higher price. For the mold, the impact number is a wall-thickness and gating instruction: the part must be gated so weld lines avoid loaded zones.

Mold Design Considerations for PC

PC is demanding on the machine and demanding on the tool — most PC rejects trace back to a mold design decision, not a bad shot. The rules below are the ones that matter when the steel is being cut for a PC part.

Shrinkage compensation. Compensate cavities for 0.5-0.7% shrinkage, split by wall-thickness zone — thick sections shrink differently and carry more molded-in stress. Because PC shrinkage is uniform, the compensation is predictable, but the split between zones still matters on parts with ribs and bosses. The five shrinkage compensation rules apply directly to PC parts and cover the zone-by-zone method.

Wall thickness and ribs. Nominal walls of 2-4 mm are the working range for most PC parts; sections over 4 mm cool slowly, concentrate molded-in stress, and invite voids and sink marks — the classic PC failure trap. Ribs should run 50-60% of the nominal wall thickness at the base — thicker ribs guarantee sink marks on the opposite surface. Radius rib bases at 0.5-1x the wall thickness to avoid stress concentrations, and keep transitions gradual so the uniform shrinkage stays uniform. Thin-wall parts below 2 mm need high-flow grades and their own gating conversation; the thin-wall warpage guide covers the tooling side.

Gate design. PC is high-viscosity, so gates are sized for low shear, not for easy degating. Edge and fan gates are preferred over pin gates on structural and optical parts: high shear at the gate is a direct source of molded-in stress and gate blush, and a blush is visible on every transparent shot. Gate land length stays short, and the gate cross-section is generous relative to wall thickness. Gate placement decides where weld lines land — place gates so flow fronts meet at a vent or a non-cosmetic, non-loaded zone, because PC weld lines are both weak and visible. The gate design best practices guide covers the placement and sizing trade-offs in detail.

Runner system. Cold runners are standard for PC: full-round runners in the 5-10 mm diameter range for typical parts, with the sprue bushing sized generously to the runner. Hot runners work well for high-volume PC tools — the resin holds its window — but keep the manifold temperature balanced across cavities, because PC gloss, haze, and weld-line quality are sensitive to melt-temperature differences between cavities. The hot runner vs cold runner trade-off is covered in detail.

Venting. Vents of 0.02-0.05 mm depth are the typical guideline for PC. Deeper vents flash; shallower vents trap air and cause burn marks at the end of fill. Because PC runs at a 280-320°C melt, trapped air burns faster and more visibly than in most resins — the cavity and the parting line need real venting paths, and deep, blind pockets may need vacuum venting.

Draft and ejection. Minimum draft of 0.5-1° per side on untextured walls; textured surfaces need 1-2° per degree of texture depth. Ejection is where PC stress cracks start: small pins concentrate stress and cause whitening and crazing at the pin location, so spread the ejector area with larger pins or lifters, and polish every ejector that touches a cosmetic or optical surface. The draft angle mistakes list is worth a pass before the steel is cut, because a draft correction after hardening is expensive.

Cooling. Mold temperature is 80-120°C — hot for a thermoplastic — and the cooling layout decides the cycle. Conformal or drilled channels sized to the part geometry, with coolant flow that keeps the cavity and core within a few degrees of each other, protect both cycle time and dimensional stability. The cooling channel design tips cover the layout rules for uniform mold temperature.

Mold steel. Standard PC tooling runs on P20 or 718-class prehardened steel (28-36 HRC) for most production volumes. Optical and high-gloss parts move to S136 or 420 stainless, hardened and polished to SPI A-1/A-2 — a rough cavity surface nucleates stress cracks and shows every flaw in a transparent part. Glass-filled PC grades wear the mold faster, so high-cavity, high-volume tools step up to H13 or comparable for core life.

Injection Molding PC: The Processing Window

PC runs on a standard reciprocating-screw injection machine with an L/D of 20:1 or higher. The process is not exotic — the discipline is: dry the resin before anything else, hold the melt window, and run a hot mold. Skip any of the three and part quality collapses faster than with almost any other engineering plastic.

Drying is step one and it is non-negotiable. PC is hygroscopic and picks up roughly 0.15-0.25% moisture at 50% relative humidity (typical published). The molding limit is 0.02%. Dry with a dehumidifying dryer at 120°C for 3-4 hours at a dew point of -40°C or lower, and keep dried resin covered — PC can re-absorb meaningful moisture in hours, not days. Above 0.02%, moisture hydrolyzes the polymer chains at melt temperature: molecular weight drops and the part loses impact strength before any visible defect appears. The machine side owns the dryer; the mold side still feels it — splay on the first shot and field failures after delivery.

Melt temperature: 280-320°C. The practical sweet spot for most grades is 290-310°C. Do not exceed roughly 330°C even briefly, and keep residence time under about 10 minutes — PC degrades slowly but measurably at the top of the window, showing up as haze and black specks. For the tool, a hot barrel means high-temperature components, proper thermal-expansion allowance in slides and lifters, and gate lands that do not erode.

Mold temperature: 80-120°C. Optical and structural parts belong at 100-120°C: a hotter mold reduces molded-in stress, stress whitening, weld-line visibility, and surface defects, and it replicates a polished cavity faithfully. The low end gives shorter cycles and a matte, hazy surface — and a cold mold on PC is the fastest route to stress cracks.

Pressures and speeds. Typical injection pressure runs 700-1,400 bar (7,000-14,000 kPa), with high holding pressure to pack out shrinkage. Back pressure stays moderate — 0.3-1.0 MPa — and injection speed should be moderate, because high shear at the gate is a direct source of molded-in stress and gate blush. A screw with an L/D of at least 20:1 handles PC well.

Shrinkage: 0.5-0.7%. The number moves with grade, wall thickness, and process conditions: higher mold temperature and higher hold pressure push shrinkage toward the low end. Because the shrinkage is uniform, the visible risks are sink marks near ribs and voids in thick sections rather than gross warpage. Cavities are compensated with the datasheet range, then tuned on the first molding trial.

Venting. Vents of 0.02-0.05 mm depth are the typical guideline for PC. Deeper vents flash; shallower vents trap air and cause burn marks at the end of fill.

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Windows are typical published processing ranges. Always start from the resin supplier's data sheet for the exact grade.

Common PC Applications

PC parts cluster where transparency meets impact or heat. Automotive glazing — sunroof panels, headlamp lenses, interior trim — demands the 120-130°C HDT and stone-chip impact, and the tooling follows our automotive injection molding practice: hot molds, polished cavities, low-shear gating. Medical device housings and IV components use ISO 10993 grades that must survive gamma or EtO sterilization without yellowing; see the medical device industry page for the regulatory tooling chain. Electrical enclosures, battery housings, and EV charging components run flame-retardant PC or PC/ABS blends for the UL94 V-0 rating and impact performance — covered on the consumer electronics and EV and energy pages. Where a part must take a hit, hold shape at temperature, and pass a flame test, PC earns its tooling cost.

PC Defects: Mold-Side Root Causes and Fixes

Most PC rejects trace to a mold design decision, not a bad shot. The table below maps each common defect to its mold-side root cause and the tooling fix — the process fixes that also work are noted where they matter.

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Moisture splay deserves its own line because it is the one PC defect that is rarely a tool problem: silvery streaks radiating from the gate mean the resin was wet — moisture flashing to steam in the barrel, the hydrolysis signature. The fix is drying discipline — 120°C for 3-4 hours, target under 0.02% — and splay that reappears mid-run points at a leaking hopper dryer, not at the mold.

Stress cracking deserves a second line because it is the PC defect that surfaces after delivery. Molded-in stress from thick walls, cold molds, or sharp corners sits in the part as a loaded spring; the first chemical trigger — brake fluid, ammonia cleaner, a ketone — cracks it. The mold-side chain: uniform 2-4 mm walls, corner radii of 0.5-1x wall, low-shear gating, hot molds, and spread ejector area. Parts that obey these rules rarely crack in service.

PC vs ABS: How to Choose

PC and ABS are the two most specified engineering thermoplastics for housings and structural parts — they are often confused because both are workhorse resins. This polycarbonate vs ABS comparison covers cost, processing, and mechanical performance. PC wins on transparency, impact, and heat. ABS wins on price, processability, and surface finish.

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Typical published ranges for standard grades. PC/ABS blends sit between the two on every row.

ABS runs at melt temperatures of 210-250°C against PC's 280-320°C, and it tolerates a wetter hopper. The processing gap is why many molders quote ABS parts more cheaply than PC parts of the same geometry. The material choice also decides the mold conversation: ABS accepts cold runners, pin gates, and aggressive draft angles; PC needs low-shear gating, hot molds, and stress-managed geometry.

If the application allows either material, run the cost comparison on the part, not the datasheet. For the full ABS property set, processing windows, and grades, see our ABS material guide.

Note the middle path: PC/ABS blends trade some transparency and heat resistance for better processability and lower cost. They are standard in automotive interior trim and electronics housings, and they change the tooling conversation too — blend shrinkage, flow, and mold-temperature needs sit between the two pure resins.

Frequently Asked Questions

Q1. What is polycarbonate plastic?

Polycarbonate (PC) is an amorphous thermoplastic made from bisphenol A. It is transparent (88-90% light transmission), tough (notched Izod 600-900 J/m), and heat resistant (HDT 120-130°C at 1.82 MPa). It is injection molded into lenses, face shields, automotive glazing, and medical devices — parts where the mold must be designed for low shear and stress-managed geometry.

Q2. Is polycarbonate plastic safe?

It depends on the grade and the application. Food-contact grades comply with regulations such as FDA 21 CFR 177.1580 and EU 10/2011; medical grades carry ISO 10993 biocompatibility data. General-purpose grades are not certified for food or medical contact. BPA is a component of standard PC, which is why infant-bottle restrictions reshaped the food-contact market; BPA-free copolyesters now fill that segment. Verify the grade certificate before specifying, and never assume "PC" means "food safe".

Q3. What is the melting point of polycarbonate?

PC is amorphous, so it has no sharp melting point. Its glass transition is about 147°C, and the 1.82 MPa HDT is 120-130°C. Processing melt temperature runs 280-320°C. For practical purposes, PC parts stay dimensionally stable to roughly 115-130°C continuous, depending on grade.

Q4. Can polycarbonate be injection molded?

Yes. PC injection molding is a standard process on conventional machines. The requirements: dry the resin to under 0.02% moisture (120°C for 3-4 hours), melt at 280-320°C, and mold at 80-120°C. Design the mold for low shear and uniform walls. PC is forgiving of the machine and unforgiving of wet resin.

Q5. Is polycarbonate stronger than ABS?

Yes on the properties that matter for impact and heat. Notched Izod is roughly 600-900 J/m against ABS's typical 150-400 J/m. HDT is 120-130°C against 85-105°C.

PC also transmits light; ABS is opaque. ABS counters with lower cost and easier processing. PC/ABS blends sit between the two.

Q6. What is the shrinkage of polycarbonate?

Mold shrinkage is 0.5-0.7% for standard grades and 0.1-0.4% for glass-filled grades (typical published). It is low and consistent for an engineering plastic, which is why PC molds hold tight tolerances. The cavity is sized up by the shrinkage rate, and warpage risk is low because PC is amorphous.

Q7. Does polycarbonate need to be dried before molding?

Yes, unconditionally. PC absorbs up to about 0.15-0.25% moisture at 50% relative humidity; the molding limit is 0.02%. Wet resin hydrolyzes at melt temperature, cutting molecular weight and impact strength. Dry at 120°C for about 3-4 hours in a dehumidifying dryer at -40°C dew point, and protect the hopper.

Q8. Why do molded polycarbonate parts crack?

Two mechanisms. Molded-in (internal) stress from thick walls, cold molds, or sharp corners. And environmental stress cracking when a stressed part meets a chemical trigger — fuels, solvents, ammonia cleaners. Fixes: uniform 2-4 mm walls, radii of 0.5-1x wall, mold at 100-120°C, and annealing at 120-130°C for stressed parts.

Q9. Is polycarbonate transparent?

Yes. Standard grades transmit 88-90% of visible light on a 2-3 mm wall, with low haze. Optical grades add guaranteed transmission, haze, and birefringence limits for lenses. Glass transmits slightly more light, but PC carries far more impact energy and weighs about half as much.

Q10. Is polycarbonate recyclable?

Yes. PC is identified under resin code 7 (Other). Recycled PC streams exist for industrial and non-food applications. Molding regrind can be reused at controlled ratios — commonly up to 20-30% in non-critical parts — when drying is handled (typical practice). Optical, medical, and food-contact parts normally require virgin resin.

The Bottom Line

Polycarbonate earns its place on three points. Transparency comes from an amorphous structure with no crystalline regions to scatter light. Impact toughness is the highest of the common unmodified thermoplastics. Heat resistance — HDT 120-130°C at 1.82 MPa — holds where ABS and acrylic soften. The limits are just as clear: a moisture target under 0.02%, a melt window of 280-320°C, and a hot mold at 80-120°C define where it works.

The tooling chain is short. Compensate cavities for 0.5-0.7% uniform shrinkage, split by wall zone. Size gates for low shear so the cavity packs before freeze-off and the gate does not blush. Vent at 0.02-0.05 mm, draft at 0.5-1° minimum, spread the ejector area, and balance the cooling layout before the steel is cut — every one of those decisions shows up in the first trial shot and in the field. Run the part drawing through our DFM checklist before quoting, and when you shortlist toolmakers, our guide on how to choose a mold manufacturer covers what to verify in the shop.