- 1. Why PC is difficult, and why it is worth it
- 2. Grades
- 3. Drying: the non-negotiable step
- 4. The processing window
- 5. Residual stress: how to see it and how to remove it
- 6. Design rules
- 7. Optical parts: the extra requirements
- 8. Defects and causes
- 9. Flow length, wall thickness and the pressure ceiling
- 10. Medical and food-contact PC
- 11. Inspecting a transparent part
- 12. Frequently asked questions
Why PC is difficult, and why it is worth it
Polycarbonate has an unusual combination: it is transparent, it has an impact strength of 600 – 900 J/m notched, it holds its properties to 120 °C, and it is dimensionally stable. Nothing else in the commodity engineering range does all four. That is why it is used for machine guards, lenses, medical housings, electrical enclosures and power-tool bodies.
The price of that combination is a narrow processing window and two failure modes that are unforgiving. PC must be dried to below 0.02 % moisture — one of the tightest limits of any common polymer — and it must be moulded with low residual stress, because polycarbonate stress-cracks in contact with almost every solvent it meets, including some mould releases, some adhesives, and some cleaning agents.
The two numbers that decide whether a PC job runs: moisture below 0.02 % at the hopper, and residual stress low enough to survive a 30-second immersion in carbon tetrachloride or a suitable stress-crack test. Get both right and PC is straightforward. Get either wrong and you will spend weeks chasing a defect that is not where it appears.
Grades
| Grade family | Key property | MFR (g/10 min) | Used for |
|---|---|---|---|
| Optical / lens | Haze < 1 %, low birefringence | 6 – 15 | Lenses, light covers, display windows |
| General purpose | Balanced clarity and flow | 10 – 22 | Housings, guards |
| High flow | Thin walls, long flow | 22 – 40 | Thin-wall electrical parts |
| Flame retardant | UL94 V-0 at 1.5 mm | 8 – 20 | Power supplies, chargers |
| UV stabilised | Retains clarity outdoors | 8 – 18 | Outdoor lenses, signage |
| Scratch resistant (hard coat) | Surface hardness 2 – 3 H | 8 – 15 | Touch screens, displays |
| Glass filled, 10 – 30 % | Modulus 6 – 10 GPa | 5 – 15 | Structural brackets |
| PC/ABS blend | Better flow, lower cost | 10 – 25 | Housings where clarity is not needed |
| Medical / ISO 10993 | Biocompatibility | 8 – 20 | Device housings, fluid paths |
Table 1 — PC grade families. Optical grades are a separate specification, not a premium version of general purpose.
Drying: the non-negotiable step
Polycarbonate hydrolyses. Water at moulding temperature breaks the carbonate linkage, reducing molecular weight permanently. A single wet shot does not just produce a cosmetic defect — it degrades the material, and the part that comes out has lower impact strength even after the streaks are gone.
| Moisture content | Visual result | Impact retention |
|---|---|---|
| < 0.015 % | Clear, no streaks | 100 % |
| 0.02 % | Clear on most parts | 95 – 100 % |
| 0.03 % | Faint silver streaking near the gate | 85 – 95 % |
| 0.05 % | Visible streaks across the part | 70 – 85 % |
| 0.10 % | Bubbles, splay, cloudy patches | 50 – 70 % |
| 0.20 % | Gross foaming, unusable | < 40 % |
Table 2 — PC moisture versus outcome. The impact loss is permanent and invisible.
| Dryer type | Temperature | Time | Notes |
|---|---|---|---|
| Desiccant wheel (recommended) | 120 °C | 3 – 4 h | Dew point −20 °C or lower; the standard for PC |
| Vacuum dryer | 110 – 120 °C | 1.5 – 2.5 h | Faster; good for small batches |
| Hot air oven | 120 °C | 4 – 6 h | Only if the air is genuinely dry; ambient humidity re-wets the resin |
| Hopper dryer, short | 120 °C | 2 h minimum | Not sufficient for a cold-start; only for holding dried resin |
Table 3 — Drying routes. A hot-air oven in a humid climate can plateau at 0.05 % and never get lower.
Myth: "the streaks disappeared after an hour, so it's fine now." Silver streaks disappear when the surface moisture drops below about 0.03 %. The molecular weight damage from the first wet shots remains in the barrel until it is purged, and the parts made in that period have reduced impact. Purge and restart after a moisture event — do not just wait for the streaks to go.
The processing window
| Parameter | Typical setting | Notes |
|---|---|---|
| Melt temperature | 280 – 310 °C | Below 270: unmelt and high stress. Above 320: degradation and yellowing |
| Barrel profile | 260 / 280 / 295 / 300 °C | Feed zone cool to avoid bridging |
| Nozzle | 290 – 300 °C | Use a reverse-taper or free-flow nozzle |
| Mould temperature | 80 – 110 °C | The single biggest lever on stress and surface |
| Injection pressure | 80 – 140 MPa | High viscosity; expect high pressures |
| Injection speed | Moderate to fast | Fast fill reduces stress but can cause jetting |
| Hold pressure | 50 – 70 % of injection | Long hold; PC packs slowly |
| Back pressure | 0.5 – 1.5 MPa | Helps homogenise; watch for over-heating |
| Screw speed | 40 – 70 rpm | Low peripheral speed |
| Residence time | < 8 min | Long residence at 300 °C yellows the melt |
| Regrind | 0 – 20 % | Must be dried again; each pass reduces molecular weight |
Table 4 — A starting window for optical-grade PC. Confirm against the grade datasheet.
Mould temperature is where most PC problems are solved or created. At 60 °C a PC part looks fine and is full of frozen-in stress: it will craze around inserts, crack when assembled, and fail a solvent test. At 90 – 100 °C the surface replicates better, the stress relaxes before the part ejects, and the cycle is longer but the yield is far higher. The extra 10 seconds per shot is almost always cheaper than the sorting.
Residual stress: how to see it and how to remove it
| Test | Method | Pass criterion |
|---|---|---|
| Solvent immersion | Carbon tetrachloride or a proprietary stress-crack reagent, 30 s to 3 min | No crazing within 60 s |
| Solvent wipe | Wipe with acetone or ethyl acetate, leave 5 min | No crazing |
| Polarised light | View between crossed polarisers | Uniform low-order colour; strong bands indicate stress |
| Heat relaxation | Measure, hold at 120 °C for 1 h, re-measure | Dimensional change < 0.2 % |
| Assembly test | Torque the screw to specification, hold 24 h | No cracking at the boss |
Table 5 — PC stress tests. The solvent immersion test is the industry standard and takes one minute.
Annealing removes stress after the fact and is standard practice for machined or heavily loaded PC parts. For moulded parts it is a band-aid: the right fix is a hotter mould, a slower fill and better packing.
| Annealing schedule | Temperature | Time |
|---|---|---|
| Thin parts, < 2 mm | 120 °C | 1 h |
| 2 – 4 mm | 120 °C | 2 h |
| 4 – 8 mm | 120 °C | 3 – 4 h |
| Glass filled | 125 °C | 2 – 4 h |
| Cool-down rate | — | ≤ 10 °C/h to avoid re-introducing stress |
Table 6 — Annealing. Slow cooling matters as much as the hold time.
Design rules
| Feature | Rule | Why PC is different |
|---|---|---|
| Wall thickness | 2.0 – 3.5 mm; avoid < 1.5 mm | High viscosity makes thin walls hard to fill |
| Wall uniformity | Within 20 % | Thick sections cause voids and stress |
| Internal radius | ≥ 0.5 × wall, preferably 1.0 × | PC is notch sensitive — sharp corners are crack starters |
| Draft | 1.5° minimum | High shrinkage force; parts grip the core |
| Gate | Large, edge or direct; avoid small pin gates | High shear degrades PC at the gate |
| Metal inserts | Design for 0.3 – 0.5 % differential expansion | PC expands 65 – 70 × 10⁻⁶/K vs 23 for aluminium |
| Threads | Moulded threads only in low-stress areas | Thread roots are stress concentrators |
| Snap fits | Strain ≤ 1.0 % for repeated flexing | PC tolerates a single snap better than repeated |
| Sharp edges | Break all external edges 0.3 mm | Handling damage becomes a crack |
Table 7 — PC design rules. Notch sensitivity is the theme.
Optical parts: the extra requirements
A lens or a light guide is a different specification from a housing, and the differences are not obvious from a datasheet.
| Requirement | What it needs | If it is missed |
|---|---|---|
| Tool polish | SPI A-1 or A-2, diamond polish | Every tool mark is visible in the transmitted light |
| Gate location | Outside the optical aperture | Gate mark and flow lines in the field of view |
| Flow front | Single, uninterrupted front | Weld lines visible as a bright or dark line |
| Mould temperature | 100 – 120 °C | Flow marks and internal stress birefringence |
| Material grade | Optical grade, low birefringence | Rainbow patterns in polarised light |
| Drying | 0.01 % or better | Micro-bubbles that scatter light |
| Packing | Low hold pressure, slow | Orientation and birefringence at the gate |
| Handling | No contact with the optical surface | Scratches that cannot be polished out |
Table 8 — Optical PC requirements. Birefringence is the one that is invisible until the part is between polarisers.
Defects and causes
| Defect | Cause | Fix |
|---|---|---|
| Silver streaks / splay | Moisture | Dry to 0.02 %; check dew point and hopper seal |
| Crazing after assembly | Residual stress plus solvent or stress | Raise mould temperature; anneal; check assembly torque |
| Yellowing | Over-temperature or long residence | Lower melt 10 °C; reduce shot-to-barrel ratio |
| Flow marks on clear parts | Cold mould or slow fill | Mould to 100 °C; increase injection speed |
| Bubbles in thick sections | Voids from shrinkage, or moisture | Increase hold pressure and time; dry |
| Weld line visible | Low mould temperature, slow front | Raise mould; raise speed through the weld |
| Crack at the gate | High shear, small gate | Enlarge the gate; slow the first stage |
| Cloudy patches | Moisture or degraded material | Dry; purge; check for dead spots in the hot runner |
| Black specks | Carbonised material in the hot runner or nozzle | Purge with PC purge compound; inspect hot-runner tips |
Table 9 — PC defects. Note that four of the nine trace back to moisture or residence time.
Flow length, wall thickness and the pressure ceiling
PC has high melt viscosity, which limits how far it can flow through a thin wall before the pressure runs out. This is the constraint that most often forces a design change, and it is worth checking before the tool is cut.
| Wall thickness | Practical flow length (unfilled PC) | Flow ratio |
|---|---|---|
| 1.0 mm | 70 – 90 mm | 70 – 90 : 1 |
| 1.5 mm | 130 – 170 mm | 85 – 115 : 1 |
| 2.0 mm | 200 – 260 mm | 100 – 130 : 1 |
| 2.5 mm | 280 – 360 mm | 110 – 145 : 1 |
| 3.0 mm | 360 – 450 mm | 120 – 150 : 1 |
| 4.0 mm | 480 – 600 mm | 120 – 150 : 1 |
Table 9 — Flow length for unfilled PC at 300 °C melt and 100 °C mould. Glass-filled grades reach 60 – 75 % of these figures.
If the required flow length exceeds the table, you have four options: thicken the wall, add a gate in the middle, raise the melt temperature (limited by degradation), or change material. Adding a second gate is often the cheapest, but it creates a weld line — which on a transparent part is a visible defect and must be placed outside the optical area.
The wall-thickness trap: designers often thin a PC wall to save material and cycle time, then find the part will not fill. Going from 2.5 mm to 2.0 mm cuts the part volume by 20 % but cuts the flow length by nearly 30 % and raises the required injection pressure by 50 – 70 %. On PC the two effects usually cancel out and the part fails to fill at the pressure limit.
Medical and food-contact PC
Polycarbonate is widely used in medical devices, and the qualification requirements are specific.
| Requirement | What it means | Watch out for |
|---|---|---|
| ISO 10993 | Biocompatibility for the intended contact duration | The grade, not the polymer, is qualified |
| USP Class VI | Biological reactivity | Often quoted instead of ISO 10993 — they are not equivalent |
| Gamma sterilisation | 25 – 50 kGy | PC yellows slightly; specify a gamma-stable grade |
| EtO sterilisation | Ethylene oxide | Requires aeration time; PC absorbs EtO |
| Autoclave | 121 – 134 °C steam | PC is marginal — 20 – 50 cycles before crazing |
| Food contact, EU | EU 10/2011 | BPA migration limits apply; check the grade's declaration |
| Food contact, US | FDA 21 CFR 177.1580 | Grade-specific |
Table 10 — PC medical and food qualification. Autoclave is the requirement that usually rules PC out.
The BPA point is worth stating plainly: polycarbonate is made from bisphenol A, and BPA is restricted in food-contact applications for infants in the EU and has been voluntarily withdrawn from baby bottles in many markets. For food-contact PC in any infant-related application, expect a regulatory conversation. For medical device housings and industrial lenses, PC remains standard.
Inspecting a transparent part
| Defect | How to see it | Accept / reject basis |
|---|---|---|
| Flow marks | Hold at an angle to a light source | Any visible mark in the optical aperture is a reject |
| Weld line | Backlight, viewed through the part | Reject in the aperture; acceptable outside |
| Birefringence | Crossed polarisers | Compare against a limit sample |
| Micro-bubbles | 10× loupe under raking light | Any bubble in the aperture is a reject |
| Gate mark | Direct inspection | Outside the aperture by design |
| Scratches | Raking light | Handling damage — reject |
| Haze | Haze meter | Per the drawing; typically < 1 % for optical |
Table 11 — Transparent part inspection. Crossed polarisers are the only way to see birefringence.
Note that three of these checks need equipment that a general inspection bench does not have: polarisers, a haze meter and a controlled light source. A supplier who says they can mould optical PC should be able to show you all three.
Frequently asked questions
Can PC be moulded without a desiccant dryer?
Not reliably. PC needs 120 °C of dry air at a dew point of −20 °C or better. A hot-air oven in a humid factory stabilises around 0.05 – 0.1 %, which produces streaked, weakened parts. If you do not have a desiccant dryer, you cannot run PC.
Why does my PC part crack a week after moulding?
Residual stress plus a solvent. The two usual solvents are a mould release agent (silicone or a solvent carrier) and an assembly adhesive or cleaning fluid. Stress cracking is delayed because the solvent needs time to diffuse. Run the solvent immersion test on a moulded part — if it crazes within 60 seconds, the moulding is over-stressed.
What is the maximum wall thickness for PC?
Practically 4 – 5 mm. Beyond that, the core stays molten while the skin freezes and you get voids and internal stress that annealing will not fully remove. For thick sections, consider a different material or a two-piece design.
Can PC be ultrasonically welded?
Yes, but it needs more energy than ABS and the joint design matters more. Amorphous PC welds well with a step joint and a generous energy director. Glass-filled PC is much harder — the fibres interfere with the melt at the interface.
How much does PC shrink?
0.5 – 0.7 % for unfilled, 0.2 – 0.5 % for 20 – 30 % glass filled. The unfilled figure is more sensitive to mould temperature and wall thickness than ABS, so the tool needs to be corrected against parts moulded at production conditions, not at a trial setting.
Is PC suitable for outdoor use?
Standard PC yellows under UV within 1 – 2 years. UV-stabilised grades with a coextruded or coated UV cap last 5 – 10 years. For anything with a service life requirement outdoors, specify the UV grade and consider a hard coat as well.
What causes a cloudy patch on a clear PC part?
Usually moisture that was trapped in a slow-filling area, or degraded material from a hot-runner dead spot. If the cloudiness is always in the same place, suspect the hot runner; if it moves, suspect moisture.
PC or PMMA for a lens?
PMMA has better optical clarity and UV resistance and is cheaper; PC has 10 – 20× the impact strength and better heat resistance. For a lens that will be dropped, PC. For a lens that must be optically perfect and will be treated carefully, PMMA. For a lens that must do both, PC with a hard coat.
Moulding a PC part and fighting stress or clarity?
Send the drawing and the failure mode. We will tell you whether it is a drying problem, a mould-temperature problem or a design problem — and which of the three is cheapest to fix.
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