PC Injection Molding: The Complete Guide to Polycarbonate Molding
Manufacturing Guide
PC Injection Molding: The Complete Guide to Polycarbonate Molding
Polycarbonate (PC) injection molding is the workhorse process for parts that must be transparent, tough, and dimensionally stable — headlight lenses, medical device housings, electronics enclosures, safety glazing, and hundreds of other products that get hit, heated, and inspected every day. Among transparent thermoplastics, PC is molded far more often than PMMA (acrylic) or AS (SAN): industry experience across Chinese tooling shops puts PC as the second most common material overall behind ABS, precisely because it delivers optical clarity without sacrificing impact strength (First Mold, PC page).
But PC is also one of the least forgiving materials on the machine. It must be dried aggressively, molded hot, and gated carefully — or it returns silver streaks, cracking, and internal stress that no amount of finishing can remove. This guide covers everything a buyer or design engineer needs: process physics, real parameter windows, mold design for optical parts, tolerances, cost and lead-time reality, application mapping, defect troubleshooting, and the DFM checklist we run at Molditquick (东莞国宏精密) before quoting any PC program.
Table of Contents
- What is PC injection molding?
- Why polycarbonate? Key properties and grades
- PC vs PMMA vs ABS: which transparent plastic?
- PC process parameters: temperature, pressure, cycle
- Drying is not optional
- Mold design for polycarbonate molding
- Optical parts: gates, weld lines, and stress control
- PC/ABS blends and modified PC grades
- Tolerances you can actually hold
- Cost and lead times for PC parts
- Applications: electronics, automotive, medical, industrial
- Common PC defects and how to fix them
- DFM checklist before you send the RFQ
- Quality systems and certifications
- How to buy PC injection molding smartly
- FAQ
What is PC injection molding?
PC injection molding (often abbreviated “pc moulding” in RFQs and datasheets) is the process of melting polycarbonate resin and forcing it under high pressure into a closed steel or aluminum mold, where it cools into the final part shape. Like all thermoplastic injection molding, it is a four-phase cycle:
- Fill — screw advances, melt flows through the runner and gate into the cavity.
- Pack/hold — pressure is maintained so the cooling polymer contracts without sinking.
- Cool — the part solidifies enough to hold its shape.
- Eject — the mold opens and ejector pins push the part out.
What makes polycarbonate plastic molding different from molding PP or ABS is the physics of the resin itself. PC is an amorphous polymer with a glass-transition temperature around 145–150 °C — far above the 90–105 °C of ABS. It flows poorly when cold, absorbs moisture aggressively, and shrinks little but anisotropically under stress. Every parameter below exists to manage those three facts.
Why polycarbonate? Key properties and grades
| Property | Typical value | Why it matters |
|---|---|---|
| Light transmission | 85–92 % (2–3 mm) | Near-glass clarity for optics and lighting |
| Impact strength | 600–900 J/m (notched Izod) | Unbreakable in normal use — the reason it replaces glass |
| Continuous service temperature | 120–130 °C | Survives hot environments ABS cannot |
| Heat deflection (HDT, 0.45 MPa) | ~135–140 °C | Good for automotive interior/exterior and LED housings |
| Tensile strength | 55–75 MPa | Stiff enough for structural housings |
| Mold shrinkage | 0.5–0.7 % | Predictable, but stress-sensitive — see tolerances below |
| Density | 1.19–1.20 g/cm³ | Heavier than PP/ABS; relevant to part weight and freight |
| UL flammability | V-2 to V-0 (grade dependent) | Electronics enclosures commonly need V-0 |
Values above are typical published ranges for standard bisphenol-A polycarbonate (e.g., SABIC Lexan, Covestro Makrolon families); exact numbers come from the specific grade datasheet.
Common PC grades you will see in quotations:
- General-purpose / clear: Makrolon 2458, Lexan 143R-class — housings, glazing, lenses.
- UV-stabilized: for outdoor signage and lighting covers.
- Glass-fiber reinforced (10–40 % GF): stiffer, lower CTE, but loses transparency.
- Flame-retardant (V-0): electronics enclosures, EV components.
- Medical/biocompatible: USP Class VI / ISO 10993 grades for devices.
- PC/ABS blends: toughness of PC + flow of ABS (section 8).
- Optical grades: very low haze for lenses and light guides.
PC vs PMMA vs ABS: which transparent plastic?
| Criterion | PC | PMMA (acrylic) | ABS |
|---|---|---|---|
| Transparency | 85–92 % | 92–93 % (best) | Opaque (or translucent grades) |
| Impact strength | Excellent | Low — brittle, chips | Good |
| Scratch resistance | Poor (needs hardcoat) | Good | Poor |
| HDT | ~135 °C | ~90–100 °C | ~100 °C |
| Chemical resistance | Fair (stress-cracking risk) | Fair | Good |
| Mold shrinkage | 0.5–0.7 % | 0.3–0.6 % | 0.4–0.7 % |
| Relative cost | Medium-high | Medium | Low |
| Typical use | Lenses, glazing, housings, medical | Lamp covers, displays, signage | Enclosures, toys, dashboards |
Rule of thumb used on our shop floor: if the part must survive impact, heat, or repeated handling — choose PC. If optical clarity and scratch resistance at low cost dominate, and the part will not be abused — choose PMMA. If it does not need to be transparent at all, ABS is usually cheaper and easier to mold. The choice between PC and PMMA matters most in automotive lighting and displays, where both appear in the same assembly.
PC process parameters: temperature, pressure, cycle
The published process window for PC is narrow and hot. These are the typical supplier-published ranges (SABIC/Covestro-style datasheets) we start from at Molditquick and then fine-tune per grade and part geometry:
| Parameter | Typical range | Notes |
|---|---|---|
| Barrel temperature | 260–310 °C | Rear zone ~260–280 °C, nozzle ~290–310 °C; never exceed grade max |
| Mold temperature | 80–120 °C (optical parts 100–120 °C) | Hot mold = glossy surface, low stress, good weld-line strength |
| Drying | 120 °C for 3–4 h, dew point ≤ −30 °C | Target moisture < 0.02 %; see section 5 |
| Injection pressure | 700–1,400 bar (typical) | High because PC is a viscous, shear-thinning melt |
| Hold pressure | 50–70 % of injection pressure | Controls sink and internal stress |
| Screw speed | 40–60 % of max, moderate | Avoids shear heating that degrades the resin |
| Back pressure | 5–15 bar | Compresses melt, removes trapped air |
| Injection speed | Medium — fast enough to fill thin sections, slow enough to avoid jetting | Jetting is a signature PC defect |
| Cycle time | 30–90 s typical (part dependent) | Wall thickness dominates cooling time |
| Shrinkage | 0.5–0.7 % | Amorphous; directional effects are small but stress-related |
Why the mold must be hot. If the mold runs cold, the skin freezes early and the part comes out with a dull surface, visible flow marks, and high frozen-in stress — which later shows up as stress cracking around bosses and inserts. For transparent PC parts we hold 100–120 °C mold temperature and cool slowly enough to relax orientation. The downside is a longer cooling phase, so cycle times for optical PC parts run longer than for opaque engineering parts.
Drying is not optional
PC is hygroscopic: it picks up atmospheric moisture rapidly. At melt temperature that moisture hydrolyzes the polymer chains and produces gas — which shows up as silver streaks (splay), bubbles, and brittle parts that look fine until they are bent or dropped. The published rule for polycarbonate is drying at 120 °C for 3–4 hours (some grades 100–120 °C per datasheet) to a residual moisture of below 0.02 %. In practice at Molditquick:
- We dry PC in dehumidifying dryers, not simple hot-air ovens — dew point matters.
- Drying time is counted from when the hopper load reaches temperature.
- We do not leave dried PC sitting: 30–60 minutes of exposure to humid air can re-contaminate it enough to cause splay on the next shot.
- For optical programs we run a moisture meter check before startup.
If your molder skips drying or shortens it “to save time,” reject the parts. Silver streaks on a transparent PC lens are a process violation, not a cosmetic lottery.
Mold design for polycarbonate molding
Mold steel and hardness
PC molding is not chemically aggressive, so standard mold steels work — but hot molds and high injection pressures argue for quality tooling:
- P20 / 718H for prototype and medium-run tools (up to ~100–500K shots depending on grade).
- H13 / S136 (420SS) or NAK80 for high-cavitation production tools and optical parts where cavity finish must never degrade.
- Optical cavities are polished to a mirror finish (SPI A1–A3) and hard-chrome or PVD coated where scratch-free life matters.
Mold-class expectations follow the industry ladder: Class 105 prototype tools (<500 shots, ±0.05 mm, 10–18 day lead) up through Class 104 (<100K), Class 103 (<500K), Class 102 (<1M) (RJC Mold). Xometry publishes the same ladder as Class 105 → Class 101 for ultra-high volume (Xometry).
Wall thickness and geometry
- Uniform wall 1.5–3.5 mm is the sweet spot for PC housings; thin-wall electronics parts run 0.8–1.2 mm with glass-reinforced or high-flow grades.
- Avoid wall transitions over 2:1 — PC’s high viscosity magnifies sink and stress at thick/thin junctions.
- Ribs 0.5–0.6× wall thickness, draft 0.5–1° per side minimum, up to 2° for texture.
- Bosses need generous corner radii and core pins; a sharp boss base is a stress-cracking generator.
- Industry-published design envelopes: wall 0.5–5 mm, draft 0.5–2°, ribs 0.5–3 mm, bosses Ø3–10 mm (RapidDirect).
Gating PC
Gate type and location decide transparency and stress:
- Submarine (tunnel) gates and edge gates for most parts.
- Fan or film gates for flat optical panels — spread the melt, avoid jetting.
- Pin-point gates with a long cold slug well stop cold material from entering the cavity.
- Hot runners work well for PC in high-volume tools, but gate vestige and shear history must be validated — degraded melt at the gate reads as haze on a lens.
- Gate into the thickest section to let packing do its job; gate location should also keep the weld line off critical optical or load-bearing zones.
Optical parts: gates, weld lines, and stress control
Transparent PC parts are where the craft lives. Three rules we enforce on every optical program:
- One flow path. Any split flow creates a weld line — a visible, slightly hazy seam that is also a mechanical weakness. For lenses and covers, design the gate so the melt front sweeps across the whole surface without rejoining.
- No cold slugs. A cold shot front pushed into the cavity leaves a visible streak. Deep sprue wells, insulated runner drops, or hot-runner tips remove it.
- Stress relief by process, not prayer. High melt temperature, high mold temperature, and low injection speed with a long pack profile keep molecular orientation low — the difference between a lens that cracks in the field and one that doesn’t.
PC parts that will be glued, solvent-bonded, or snap-assembled should be annealed (e.g., 120–130 °C for 1–2 h per hour of wall thickness where the grade allows) or molded with low stress — otherwise environmental stress cracking (ESC) appears at the bond line weeks later. This is the single most common field failure of polycarbonate molding, and it is almost always a process problem, not a material problem.
PC/ABS blends and modified PC grades
A large share of “PC injection molding” RFQs are actually PC/ABS blends — the material of choice for automotive interior panels, electronics enclosures, and power tools. PC/ABS keeps PC’s impact and heat resistance while improving flow, lowering cost, and making the surface easier to paint or texture. Typical blend ratios run 50/50 to 70/30 PC/ABS; process windows sit between the two parents (barrel ~240–280 °C, mold ~60–90 °C).
Other modified PC families a capable molder should handle:
- Glass-fiber reinforced PC (10–40 %) — stiffer, lower CTE, used for structural brackets and frames; loses transparency.
- PC/PBT and PC/PET blends — chemical resistance plus toughness, common in automotive exterior.
- Flame-retardant PC (V-0) — electronics, EV battery components, chargers.
- UV-stabilized PC — outdoor lighting, signage, glazing.
- Low-clarity / diffused PC — backlight diffusers that hide LEDs while passing light.
If your supplier only offers “clear PC” and nothing else, they are a reseller, not a molder. Blends and modified grades are where real process capability shows.
Tolerances you can actually hold
PC is amorphous, so its shrinkage (0.5–0.7 %) is predictable and directionally uniform compared to semi-crystalline materials — that is a genuine tolerance advantage. What limits PC precision is internal stress, not shrinkage math.
| Feature | Realistic tolerance | Notes |
|---|---|---|
| General linear dimensions | ±0.1 mm (per 100 mm) | Industry-standard injection tolerance |
| Precision/critical features | ±0.05 mm | Held with process control + CMM verification |
| Datum-to-datum on optical parts | ±0.02–0.05 mm | Gate and cooling design dependent |
| Mold cavity machining | ±0.02 mm typical, ±0.005 mm cores | What the tool itself is built to |
Reference values published by competitors: Protolabs quotes machining tolerance ±0.003 in (0.08 mm) plus resin shrinkage ±0.002 in/in (Protolabs); RapidDirect quotes ±0.02 mm cavity tolerance (RapidDirect); First Mold quotes standard ±0.1 mm and precision ±0.05 mm with ±0.005 mm mold-core tolerance (First Mold); Zetar quotes ±0.1–0.2 mm general linear and ±0.05 mm on critical features (Zetar). Standard tolerance tables reference DIN 16901 / ISO 20457 and ISO 2768-m / DIN 7168-m (Boyan, First Mold).
At Molditquick we quote per-feature: general ±0.1 mm, critical features ±0.05 mm, verified on CMM with first-article inspection before mass production — and we flag in DFM any dimension that pushes beyond what the material and gate layout can deliver.
Cost and lead times for PC parts
What drives PC part cost:
- Resin price — PC is 2–4× the cost of ABS or PP; optical and medical grades cost more.
- Drying discipline — hot, long drying means energy and time; it is part of the price.
- Mold cost — optical PC tools need mirror polishing and often hard coatings; expect Class 103–101 tooling for volume.
- Cycle time — hot molds and slow fills extend cycles versus ABS; thicker walls are expensive in PC.
- Yield — silver streaks and stress cracks scrap parts; process-experienced molders quote realistic yields.
Lead-time reference bands (competitor-published): rapid prototype tooling 7–12 days, production tooling 25–30 days (automotive-class tools up to 45 days) (First Mold); prototype tools Class 105 at 10–18 days (RJC Mold); Protolabs ships rapid production parts in 1 day and production quantities in about 7 days from a validated tool, with no MOQ (Protolabs); Zetar quotes 6–12 weeks for standard production tools and 12–16 weeks for complex ones (Zetar).
Molditquick’s own in-house tooling shop means one supplier owns DFM → tool build → sampling → mass production. For a typical PC housing program we quote prototype tools in the 7–15 day band, production tools 4–6 weeks, and first articles within days of tool completion. Volume bands follow the standard ladder — under 1,000 pcs (prototype/low-volume), 5,000, 30,000, 100,000+ (RapidDirect) — and our 21 Sodick injection machines (18 standard + 3 additional) plus in-house wire EDM (9+4 machines) let us ramp without splitting the program across factories.
Applications: electronics, automotive, medical, industrial
Consumer electronics — phone and tablet frames, laptop bezels, charger housings, smart-speaker enclosures, VR headsets. PC/ABS dominates; clear PC appears in lens covers and light pipes. Flame-retardant PC carries the enclosure UL rating.
Automotive & EV — headlight assemblies and lenses, taillight covers, sunroof panels, interior lighting panels, mirror housings, door handles, instrument-cluster windows (First Mold). IATF 16949 supply chains (we hold it) demand PPAP-level documentation for these parts.
Medical devices — IVD analyzer housings, syringe pump covers, surgical instrument housings, respiratory device components. Biocompatible PC grades (USP Class VI / ISO 10993) plus our ISO 13485 system cover device programs.
Industrial — control panel covers, sight glasses, gear covers, lighting enclosures, inspection windows, cooling fan blades, machine guards (First Mold).
Consumer & leisure — safety helmet shells, sports goggles, camping gear, outdoor furniture components, water-filter housings (pressure-rated).
PC overmolding and insert molding — PC is often the substrate in two-shot work: TPE or TPU grips overmolded onto a PC/ABS core for power tools and toothbrushes, or metal inserts molded into PC housings for threaded connections. See our overmolding guide and insert molding guide for the material-compatibility details.
Common PC defects and how to fix them
| Defect | What it looks like | Root cause | Fix |
|---|---|---|---|
| Silver streaks / splay | White streaks near gate | Moisture, or degraded melt | Dry 120 °C/3–4 h to <0.02 %; purge; lower barrel temp |
| Stress cracking | Cracks at bosses/inserts, sometimes delayed | Frozen-in stress + chemical exposure | Raise mold temp, lower injection speed, anneal, redesign radii |
| Jetting | Worm-like folds at gate | High-speed injection into thin cavity | Slow first-stage fill, enlarge gate, redirect gate |
| Weld lines | Visible seams, weak joints | Split flow fronts | Move gate, add overflow wells, raise melt/mold temp |
| Sink marks | Depressions over thick sections | Insufficient packing | Raise hold pressure/time, reduce wall, move gate |
| Voids / bubbles | Internal cavities in thick walls | Shrinkage pulling center | Reduce wall, increase hold, use less-hot melt |
| Warpage | Twist after ejection | Uneven cooling/stress | Balance cooling channels, uniform wall, hot mold |
| Haze / dull surface | Loss of clarity | Cold mold, wet resin | Raise mold temp 100–120 °C, verify drying, polish cavity |
| Flash | Thin fins at parting line | Low clamp force vs pressure | Increase clamp, clean parting, reduce injection pressure |
| Black specks | Contamination in clear parts | Degraded resin in barrel/hot runner | Purge with cleaning compound, check hot-runner dead zones |
| Short shot | Incomplete fill | Low melt temp, poor venting, thin wall | Raise temps, add vents, slow-fast-slow fill profile |
The first move on any PC defect is to check drying logs, then mold temperature, then fill speed. In that order. 80 % of PC quality problems trace to one of those three.
DFM checklist before you send the RFQ
Send your molder a design that is ready to win:
- Uniform wall — target 1.5–3.5 mm; transitions under 2:1.
- Draft — 0.5–1° polished, 1–2° textured.
- Radii at every corner — minimum 0.5× wall; sharp corners invite stress cracks.
- Bosses — outer wall ≥0.6× boss ID; add gussets instead of growing wall.
- Ribs — 0.5–0.6× wall thickness, not thicker than the wall they support.
- Optical zones marked — call out the “must-be-clear” area so gates and weld lines stay off it.
- Tolerance callouts — general ±0.1 mm, critical ±0.05 mm; flag true-position requirements.
- Material spec — grade family (e.g., Lexan/Makrolon equivalent), flame rating, UV or medical requirement, color.
- Surface finish — SPI class; optical polish (A1–A3) where needed.
- Chemical exposure known — tell the molder if parts see solvents, lubricants, or cleaners (ESC risk).
- Secondary operations — hardcoating, painting, pad printing, ultrasonic welding, snap assembly.
Our DFM checklist guide covers the full 40-point version; every Molditquick quote includes a free DFM review that returns the file with manufacturability fixes marked.
Quality systems and certifications
PC programs across regulated industries demand more than a clean part. Molditquick (东莞国宏精密) operates under IATF 16949 (automotive), ISO 13485 (medical devices), and ISO 9001 (general quality) — the three systems that cover electronics, automotive, and medical PC work. What that means in practice:
- IATF 16949 programs ship with PPAP/APQP documentation — control plans, FAI, capability studies.
- ISO 13485 programs follow DMR-style controls, lot traceability, and validated process parameters for biocompatible PC grades.
- First-article inspection on CMM with full dimensional reports before production release.
- In-process inspection (IPQC) and end-of-line AQL sampling on every batch.
- Material certificates (COA) from the resin supplier for every lot.
The 280-person operation — tooling, molding, CNC, and inspection under one roof in a 10,000 m² facility — means the same team that polishes the optical cavity also signs the first-article report. That is the arrangement that keeps PC programs on schedule.
How to buy PC injection molding smartly
- Give the molder the full picture: end use, environment, chemicals, assembly method, volumes, target price. PC is sensitive to all of them.
- Ask for the drying procedure in the quote — if the response does not mention 120 °C/3–4 h drying to <0.02 % moisture, the supplier is not a PC specialist.
- Ask for a DFM review of the gate and weld-line placement before tooling starts — for transparent parts this is the difference between a lens and a paperweight.
- Get first-article + CMM report before production release, and agree the AQL now.
- Plan for the tool — optical PC tools need polish and possibly coatings; budget mold cost and 25–45 day lead accordingly.
- Test with prototype tooling first when optics or stress are involved — a Class 105 tool at 10–18 days is cheap insurance versus a full production tool that needs rework.
- Check certifications against your market — automotive needs IATF 16949, medical needs ISO 13485, most OEM programs ask for ISO 9001 at minimum.
FAQ
1. What is PC injection molding? Injection molding of polycarbonate resin — melting PC at 260–310 °C, injecting it into a mold, and cooling it into a transparent or colored part. “PC moulding” and “polycarbonate plastic molding” are the same process.
2. Why is polycarbonate injection molding expensive? Resin cost (2–4× ABS), mandatory drying, hot molds, slower cycles, and higher scrap risk on optical parts all add up — but the impact strength and heat resistance replace metal or glass in applications where nothing cheaper survives.
3. What temperature for PC injection molding? Typical barrel window 260–310 °C, mold 80–120 °C (100–120 °C for optical parts), drying 120 °C for 3–4 hours to below 0.02 % moisture.
4. What is the shrinkage rate of PC in injection molding? 0.5–0.7 %, fairly uniform because PC is amorphous — an advantage for tolerance control versus semi-crystalline materials.
5. Can PC be injection molded transparent? Yes — it is the most commonly molded transparent engineering plastic, with 85–92 % light transmission. Transparent molding demands hot molds, perfect drying, and gates placed to avoid weld lines and jetting.
6. PC vs PMMA for injection molding — which is better? PC: tougher, more heat resistant, lower scratch resistance, higher cost. PMMA: best clarity and scratch resistance, brittle, cheaper. Choose PC for impact/heat, PMMA for optics on a budget.
7. How do you prevent silver streaks in PC molding? Dry the resin to <0.02 % moisture, purge the barrel, and keep melt temperature within the grade window. Streaks are moisture or degradation, period.
8. Why do PC parts crack around screws and inserts? Frozen-in molding stress plus chemical exposure (ESC). Fix by raising mold temperature, lowering injection speed, annealing, adding corner radii, and telling the molder about any solvents the part will see.
9. What is PC/ABS injection molding? Molding a PC/ABS blend (typically 50/50–70/30) that combines PC toughness with ABS flow and cost — the standard choice for automotive interiors and electronics housings.
10. What tolerances can PC injection molding hold? General ±0.1 mm, critical features ±0.05 mm, with mold cores machined to ±0.005 mm. Reference industry values: Protolabs ±0.003 in (0.08 mm), RapidDirect ±0.02 mm cavity, First Mold ±0.1/±0.05 mm.
11. Can PC be overmolded or insert molded? Yes — PC is a common substrate for TPE/TPU overmolding and metal insert molding, covered in our overmolding guide and insert molding guide.
12. What is the maximum part size for PC injection molding? Industry capability references: Protolabs up to 480×751×203 mm (Protolabs); RJC and First Mold list up to 1200–1500 mm on the long axis (RJC, First Mold). Your real limit is wall uniformity, not machine size.
13. Does PC injection molding need MOQ? Platform molders quote no MOQ (Protolabs); at Molditquick we run low-volume and prototype programs from hundreds of parts — see our low-volume injection molding service.
14. How long does PC injection molding take? Prototype tools 7–15 days, production tools 4–6 weeks (automotive class up to 45 days), first articles days after tool completion; production lead typically 2–4 weeks after approval.
15. What industries use PC injection molding? Electronics (enclosures, lenses), automotive/EV (headlights, glazing, panels), medical (device housings), industrial (guards, sight glasses), consumer (sporting goods, water filters, lighting).
Ready to quote a PC program? Send your 3D model and specification to our injection molding service — we return a DFM review, material recommendation, and costed quotation with tooling and piece-price bands. For material alternatives see the plastic material selection guide and injection molding material comparison, or browse the full materials library.