Views: 0 Author: Site Editor Publish Time: 2026-04-20 Origin: Site
Polycarbonate (PC) is one of the most versatile engineering thermoplastics on the market. Known as "bulletproof plastic," it offers exceptional impact strength, optical clarity, dimensional stability, and a wide service temperature range.
But here's the catch: PC is notoriously sensitive to processing conditions.
Unlike commodity plastics like PP or ABS, PC demands strict control over moisture, temperature, and pressure. Get it wrong, and you'll face hydrolysis degradation, internal stresses, surface bubbles, or—worst of all—parts that crack right out of the mold.
The good news? Once you understand PC's unique characteristics, you can produce high-quality, stress-free parts consistently. Let me walk you through everything you need to know.
If you remember only one thing about PC processing, make it this: PC must be bone-dry before molding.
PC is extremely sensitive to moisture. Even trace amounts of water will cause hydrolysis—a chemical reaction that breaks down polymer chains, reducing molecular weight and destroying mechanical properties.
Drying Condition | Visible Signs | Part Quality |
|---|---|---|
Poor | Foamy extrudate, popping sounds, white smoke from nozzle | Dark color, extensive silver streaks, bubbles, extremely brittle, cracks on ejection |
Marginal | Dull, cloudy extrudate with small bubbles | Poor surface finish at gate, reduced impact strength |
Proper | Clear, glossy extrudate | Good mechanical properties, optical clarity |
Parameter | Value |
|---|---|
Temperature | 110–120°C (230–248°F) |
Time | 4–8 hours (minimum 4 hours) |
Maximum bed thickness (hot air oven) | 30 mm (1.2 inches) |
Target moisture content | <0.02% |
Pro tip: Use a dehumidifying dryer—not just a hot air oven. PC absorbs moisture quickly from ambient air, so keep it sealed between drying and processing.
PC has a unique characteristic: melt viscosity is highly sensitive to temperature, but relatively insensitive to shear rate.
What does this mean in practice?
To increase melt flow → raise temperature, not injection pressure
PC has no clear melting point; it softens gradually over a range
Temperature is your primary tool for managing flow
Zone | Temperature Range |
|---|---|
Rear (hopper) | 50–80°C (122–176°F) |
Middle | 260–280°C (500–536°F) |
Front | 270–300°C (518–572°F) |
Nozzle | 270–300°C (518–572°F) |
General processing range: 270–320°C (518–608°F)
Maximum safe temperature: 320–340°C (608–644°F)
Above 340°C: Decomposition occurs → darkening, silver streaks, black specks, bubbles, and severe property degradation
Pro tip: For glass-filled PC grades, aim for the higher end of the range (290–310°C) to ensure adequate flow.
Mold temperature directly affects part quality, surface finish, and—most importantly—internal stress levels.
Mold Temperature | Effect |
|---|---|
Too low | Poor flow, incomplete filling, high residual stresses (leading to cracking), surface defects (waves, dullness, flow marks), visible weld lines |
Too high | Longer cycle times, potential sticking issues, surface gloss problems |
Application | Temperature Range |
|---|---|
Standard molding | 70–100°C (158–212°F) |
Typical range | 80–100°C (176–212°F) |
Complex/thin-wall/high-gloss parts | 100–120°C (212–248°F) |
Glass-filled PC | 100–130°C (212–266°F) |
The science: Higher mold temperatures slow down the cooling rate, allowing molecular chains to relax. This reduces frozen-in stresses and minimizes the risk of environmental stress cracking (ESC) later.
PC's high viscosity requires higher injection pressures than many other engineering plastics.
Application | Recommended Pressure |
|---|---|
General molding | 80–120 MPa (11,600–17,400 psi) |
Thin-wall / long flow length / complex shapes | 120–175 MPa (17,400–25,400 psi) |
Important: Don't rely solely on pressure to improve flow. Remember: temperature is your primary flow control tool for PC. High pressure without adequate temperature will just create molded-in stress.
Unlike some materials, injection speed has relatively little effect on PC's final properties. However, proper speed selection still matters:
Part Type | Recommended Speed |
|---|---|
Standard parts | Medium speed |
Thin-wall parts | High speed |
Thick-wall parts | Low to medium speed |
Multi-stage injection | Slow → Fast → Slow (preferred approach) |
Caution with hot runner systems: Start with slow injection until a small amount of resin enters the cavity to avoid trapping gas.
Packing pressure is critical for eliminating sink marks and voids—but it's a double-edged sword.
Issue | Solution |
|---|---|
Sink marks / voids | Increase packing pressure or time |
High internal stress (gate area) | Decrease packing pressure |
Best practice | High melt temperature + low packing pressure |
Parameter | Value |
|---|---|
Packing pressure | 50–70% of injection pressure |
Packing time | 3–10 seconds (thicker parts require longer) |
Switchover point | Switch to packing when mold is 80–90% filled |
How to verify packing time: Perform a gate seal study. Increase packing time until the part weight stabilizes—that's your optimal setting.
Because PC has high melt viscosity, aggressive screw speeds can overload the drive and cause poor mixing.
Parameter | Recommended Range |
|---|---|
Screw speed | 30–70 rpm |
Back pressure | 10–15% of injection pressure (typically 5–15 bar / 70–220 psi) |
Pro tip: Lower screw speeds reduce shear heating and minimize the risk of degradation. If you need faster recovery, consider a larger screw diameter rather than increasing speed.
PC's high viscosity and stress sensitivity demand specific mold design practices:
Feature | Recommendation |
|---|---|
Runners | Short, thick, circular cross-section; highly polished |
Gates | Large diameter (minimum 1.5 mm); avoid sharp corners |
Gate location | Position away from stress-sensitive areas |
Vent depth | <0.03–0.06 mm (0.001–0.0024 inches) |
Draft angle | 0.5°–1° (30′–1°) |
Defect | Likely Cause | Solution |
|---|---|---|
Bubbles / silver streaks | Moisture in resin | Increase drying time/temperature |
Sink marks | Insufficient packing | Increase packing pressure/time or raise mold temperature |
Cracking / crazing | High internal stress | Raise mold temperature, reduce packing pressure, anneal parts |
Burn marks / dark streaks | Melt degradation (too hot or too long residence) | Lower temperature, reduce cycle time, purge with PS or PE |
Short shots | Poor flow | Raise melt temperature, increase injection speed/pressure |
Visible weld lines | Low mold temperature | Raise mold temperature, improve venting |
If your application demands low-stress parts (optical components, precision parts), post-mold annealing is highly recommended:
Parameter | Value |
|---|---|
Temperature | 125–135°C (257–275°F) |
Time | 2 hours (adjust for thick sections) |
Cooling | Slow, natural cooling to room temperature |
Never purge PC with:
LDPE
POM (Acetal)
ABS
PA (Nylon)
These can degrade or leave residues that contaminate PC. Use PS (Polystyrene) or PE (Polyethylene) for purging.
Parameter | Recommended Range |
|---|---|
Drying | 110–120°C for 4–8 hours → <0.02% moisture |
Melt temperature | 270–320°C (max 340°C) |
Mold temperature | 80–120°C (higher for stress-sensitive parts) |
Injection pressure | 80–120 MPa (up to 175 MPa for thin walls) |
Packing pressure | 50–70% of injection pressure |
Screw speed | 30–70 rpm |
Back pressure | 5–15 bar (10–15% of injection pressure) |
PC injection molding isn't difficult—but it is unforgiving. The material's exceptional properties come with a price: strict discipline in drying, temperature control, and stress management.
Master these fundamentals, and you'll produce parts that leverage PC's full potential—high impact strength, optical clarity, and long-term dimensional stability.
Have questions about specific PC grades or applications? Drop a comment below.
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The Secret to Extending Injection Mold Life: A Complete Guide to Surface Treatment Technologies
ABS vs. PC: Which Engineering Plastic Should You Choose?
