Can Polycarbonate (PC) Be Injection Molded?

Can polycarbonate (PC) be injection mold? Short answer: Absolutely yes. Polycarbonate (PC) is one of the most widely used and highly valued thermoplastics in injection molding.

Through polycarbonate injection molding, manufacturers produce everything from virtually unbreakable eyewear lenses and riot shields to automotive headlamp covers and intricate electronic housings. PC is sought after for its exceptional optical clarity, outstanding impact resistance, and impressive heat tolerance.

However, achieving consistent results with PC requires precise process control. The material is hygroscopic (it absorbs moisture easily) and needs high processing temperatures, which make it more demanding than common plastics like ABS or polypropylene.

This detailed guide explores why polycarbonate is a top choice for injection molding, the critical parameters that govern its process, and solutions for common molding challenges.

Why Polycarbonate Excels in Injection Molding

Polycarbonate is an amorphous thermoplastic polymer, meaning its molecular chains lack a regular crystalline structure. This structural property gives PC its renowned transparency, strength, and dimensional stability. It’s often selected for parts that require performance beyond what standard plastics can deliver.

Key Properties That Make PC Ideal for Injection Molding

Property	Description	Common Applications
High Impact Strength	Up to 250 times stronger than glass;   virtually unbreakable.	Safety helmets, machine guards, riot   shields.
Optical Clarity	Naturally transparent with excellent   light transmission.	Eyeglasses, LED diffusers, automotive   headlights.
Heat Resistance	Withstands continuous use up to 135°C   (275°F).	Electrical connectors, under-hood   automotive parts.
Dimensional Stability	Low shrinkage (0.5–0.8%) maintains shape   accuracy.	Precision parts, medical components.
Electrical Insulation	Excellent dielectric properties.	Electrical housings, switches, electronic   covers.
Lightweight	Lighter than glass but significantly   stronger.	Skylights, aircraft parts, vehicle   windows.

This unique combination of toughness, clarity, and heat resistance makes injection molding the most practical method to mass-produce high-performance PC parts.

The Polycarbonate Injection Molding Process

While the general injection molding cycle—melting, injecting, cooling, and ejecting—is the same for most plastics, PC requires extra attention to drying, temperature, and pressure control.

1. Pre-Drying: The Most Critical Step

PC is hygroscopic, so it absorbs moisture from the air. Even a trace amount of water (above 0.02%) can cause hydrolysis, which breaks down the polymer chains during molding.

• Problem: Moisture turns into steam at high melt temperatures, causing silver streaks, splay marks, and, more importantly, brittleness due to molecular degradation.

• Solution: Always pre-dry PC pellets in a desiccant dryer at 100°C to 120°C (212°F to 248°F) for 3–4 hours. This ensures the moisture content stays below the safe limit.

2. Temperature Control: Managing the Heat

PC has a high melting point and high viscosity, demanding elevated processing temperatures.

Parameter	Recommended Range	Purpose
Melt Temperature	260°C – 320°C (500°F – 608°F)	Ensures the resin is fully molten for   proper flow.
Mold Temperature	80°C – 120°C (176°F – 248°F)	Prevents premature solidification and   internal stress.

• Zoned Heating: The barrel should be heated progressively from the feed zone to the nozzle to avoid premature degradation.

• Avoid Overheating: Although high heat is needed, excessive temperature or long residence time may cause yellowing or discoloration.

3. Injection and Holding Pressure

PC’s viscosity also affects its pressure requirements:

• Injection Speed: A fast injection speed helps fill the mold before the material cools, essential for thin-walled or glossy parts.

• Injection Pressure: A high injection pressure ensures full mold filling and prevents short shots.

• Holding Pressure: Proper packing pressure must be maintained to compensate for shrinkage during cooling, preventing sink marks and voids.

Common Challenges in PC Molding and How to Solve Them

Even with its many advantages, polycarbonate is not an easy plastic to mold. Below are some common issues and their remedies.

1. Flow-Related Defects

Defect	Likely Cause	Solution
Weld Lines	Melt fronts meet but don’t fuse properly.	Raise melt/mold temperature and injection   speed.
Short Shots	Material solidifies before cavity fills.	Increase temperature, injection pressure,   or improve venting.
Flow Marks	Uneven flow causes streaks on the   surface.	Increase injection speed and mold   temperature for better flow.

2. Internal Stress and Warping

Because PC parts are rigid, internal stress can easily lead to warping or cracking.

• Cause: Uneven cooling across the part leads to differential shrinkage.

• Solutions:

• Maintain uniform mold temperature (up to 120°C).

• Design parts with consistent wall thickness and rounded corners (avoid sharp edges).

• Use annealing (post-molding heat treatment) for stress relief in optical or precision parts.

3. Advanced Molding Techniques

For high-end PC parts requiring optical or structural perfection, advanced methods can be applied:

• Rapid Heat & Cool (RH&C) Molding: Temporarily heats the mold before injection to achieve flawless surfaces and eliminate weld lines—ideal for optical lenses.

• Compression Injection Molding: Injects material into a partially open mold, which then closes to compress the resin, reducing stress and sink marks in thick parts.

Design Tips for Successful PC Injection Molding

Optimizing part design is as critical as fine-tuning the molding machine. The following design rules ensure defect-free and structurally sound PC parts.

Design Feature	PC Design Guideline	Purpose
Wall Thickness	1.0 – 3.8 mm (0.04 – 0.15 in)	Balances flow and cooling; uniform walls   reduce defects.
Corner Radii	≥ 3 mm (0.125 in)	Prevents stress concentration and   cracking.
Draft Angle	0.5° – 3° per side	Aids ejection without scratching the part   surface.
Rib Thickness	50% – 60% of the adjoining wall	Provides strength without causing sink   marks.

Conclusion

Yes—polycarbonate can absolutely be injection molded, and when processed correctly, it produces parts with exceptional durability, optical quality, and dimensional accuracy.

Although PC molding requires careful attention to drying, temperature control, and pressure management, the rewards are unmatched. From automotive lighting and safety equipment to medical and electronic housings, PC remains a top choice where clarity, strength, and performance are paramount.

When properly handled—with precise pre-drying, controlled high temperatures, and sound part design—polycarbonate injection molding delivers mass-producible, cost-effective, and premium-quality results.

You can explore a visual overview of PC molding processes in the Paulson Training video: “Injection Molding Using Polycarbonate.”