Everything You Need to Know About Polycarbonate Injection Molding

Polycarbonate (PC) injection molding is a widely used manufacturing process for producing high-performance plastic parts that require excellent strength, impact resistance, and optical clarity. Known for its durability and dimensional stability, polycarbonate is commonly applied in industries such as automotive, electronics, medical devices, and optical equipment. By combining the unique material properties of PC with precise injection molding techniques, manufacturers can create complex, tight-tolerance components with consistent quality and scalable production efficiency.

What is Polycarbonate?

Polycarbonate (PC) is an amorphous engineering thermoplastic widely used for injection molding applications that demand high impact strength, dimensional stability, and thermal resistance. It typically features a tensile strength of around 60–70 MPa and can maintain mechanical performance at continuous service temperatures up to approximately 120°C. Due to its low shrinkage rate and predictable molding behavior, PC is well suited for tight-tolerance and complex part designs.

Compared with other common plastics, polycarbonate offers a unique balance of toughness and transparency. ABS provides good impact resistance and easier processability but lacks optical clarity and high heat resistance. PMMA (acrylic) delivers excellent transparency and surface finish but is more brittle and less impact-resistant than PC. As a result, polycarbonate is often selected when both mechanical durability and optical performance are required in a single component.

Mechanical Properties

Polycarbonate (PC) offers outstanding mechanical strength and toughness, with tensile strength typically ranging from 60 to 70 MPa. Its high impact resistance allows PC parts to withstand sudden loads and repeated mechanical stress without cracking, even in low-temperature environments. This makes polycarbonate well suited for protective housings, structural components, and safety-related parts.

Thermal Performance

Polycarbonate maintains excellent dimensional stability at elevated temperatures. It can operate continuously at temperatures up to approximately 120°C, with short-term heat resistance even higher. This thermal stability reduces the risk of warpage or deformation during both injection molding and end-use conditions.

Optical Transparency

One of the most distinctive characteristics of polycarbonate is its high optical clarity. With light transmission reaching up to 90%, PC provides glass-like transparency combined with significantly higher impact resistance. This makes it an ideal material for lenses, covers, light diffusers, and optical equipment components.

Electrical and Flame-Retardant Properties

Polycarbonate exhibits good electrical insulation properties, making it suitable for electronic and electrical applications. Many PC grades also offer inherent flame resistance and can meet common flammability standards, which is critical for components used in consumer electronics and industrial equipment.

Dimensional Stability and Processability

As an amorphous thermoplastic, polycarbonate features low and predictable shrinkage during injection molding. This allows manufacturers to achieve tight tolerances and consistent part quality, especially for complex geometries and precision components.

The Polycarbonate Injection Molding Process

Polycarbonate injection molding requires precise control of material handling and process parameters due to PC’s sensitivity to moisture and high processing temperature. When properly managed, the process delivers high-strength, dimensionally stable, and optically clear components.

Material Drying

Polycarbonate is highly hygroscopic and must be thoroughly dried before molding. Moisture in the resin can cause hydrolysis during processing, leading to surface defects, reduced mechanical strength, and poor optical clarity. PC pellets are typically dried at 120–130°C for several hours to achieve the required low moisture content.

Melting and Injection

PC requires relatively high melt temperatures, generally in the range of 260–320°C. During injection, controlled speed and pressure are essential to ensure complete cavity filling while minimizing internal stress. Uniform melt flow is especially important for transparent or thick-walled parts to avoid flow lines and birefringence.

Mold Temperature Control

Proper mold temperature plays a critical role in part quality. Mold temperatures for polycarbonate are commonly maintained between 80–120°C to improve surface finish, reduce residual stress, and enhance dimensional stability. Higher mold temperatures are often used for optical-grade components.

Cooling and Solidification

Cooling must be carefully managed to prevent warpage and internal stress. Although polycarbonate cools more slowly than many commodity plastics, gradual and uniform cooling helps preserve mechanical properties and optical performance. Uneven cooling can lead to sink marks or distortion.

Part Ejection and Post-Processing

Once solidified, parts are ejected from the mold with sufficient draft angles to prevent surface damage. Post-processing options such as polishing, coating, or annealing may be applied to improve appearance, reduce stress, or enhance functional performance depending on the application.

Polycarbonate Materials for Injection Molding

Polycarbonate (PC) injection molding materials are available in a variety of grades and can be modified with specific additives to achieve different performance characteristics. Certain PC grades are optimized for particular applications or industries. Leading producers of polycarbonate include Covestro, SABIC, Mitsubishi Chemical, and LG Chem.

General-Purpose PC offers a well-rounded combination of strength, transparency, and heat resistance, making it suitable for a wide range of applications.

High-Flow PC has a higher melt flow index and may include additives like low-molecular-weight polybutylene terephthalate (PBT) to improve flow, facilitating easier injection into mold cavities.

Flame-Retardant PC contains phosphorus- or sulfur-based additives to reduce flammability.

Food-Grade PC is typically approved by regulatory agencies such as the FDA for direct contact with food.

Healthcare-Grade PC meets stringent standards for biocompatibility and may be approved for sterilization via gamma radiation, ethylene oxide (EtO), or steam.

Weather-Resistant PC incorporates UV stabilizers to enhance durability in outdoor or automotive applications.

While polycarbonate is inherently strong, its properties can be further enhanced with chopped glass or carbon fibers. Impact modifiers improve toughness and resistance to breakage, while mold-release agents help prevent finished parts from sticking to the mold. Additional additives can include fillers, antistatic agents, and dyes to provide long-lasting color in molded parts.

Polycarbonate can also be blended with other polymers to achieve specific properties or reduce costs. Common blends include:

PC/ABS – improves processability

PC/PBT – enhances chemical resistance

PC/PMMA – increases scratch resistance while maintaining excellent optical clarity

These options make polycarbonate a highly versatile material for injection-molded components across industries such as automotive, electronics, medical devices, and consumer products.

Molds for Polycarbonate

Injection molding polycarbonate (PC) requires careful consideration of mold design and materials to ensure high-quality finished parts. Polycarbonate’s high strength, viscosity, and sensitivity to heat and moisture make mold selection and preparation critical.

1. Mold Material

Hardened Steel: The most common choice for high-volume production, offering durability and resistance to wear.

Pre-Hardened Steel: Suitable for medium-volume runs; easier and faster to machine than fully hardened steel.

Aluminum: Used for prototyping or low-volume production due to faster machining and lower cost, though less durable than steel.

2. Mold Design Considerations

Wall Thickness: Maintain uniform wall thickness to prevent warping, sink marks, and residual stress.

Radii and Draft Angles: Rounded corners and adequate draft angles improve part release and reduce stress concentrations.

Gate Design: Proper gate type and placement ensure smooth flow of molten polycarbonate into the mold cavity.

Cooling Channels: Efficient cooling is critical because PC has a slow heat transfer rate. Well-designed channels reduce cycle times and minimize warpage.

Polycarbonate Surface Finishing

Surface finishing is an important aspect of polycarbonate (PC) injection-molded parts, affecting aesthetics, durability, and functionality. Because polycarbonate is naturally transparent and has excellent impact resistance, the choice of surface treatment can enhance appearance and performance.

Common Surface Finishing Options

Polishing: Achieves high gloss and optical clarity, ideal for lenses, display covers, and transparent components.

Texturing: Adds a matte or patterned finish to hide scratches, fingerprints, or molding imperfections. Common textures include fine, medium, or heavy matte patterns.

Coatings: Specialized coatings can enhance scratch resistance, UV resistance, or chemical resistance. Clear protective coatings maintain transparency while improving durability.

Dyeing or Coloring: Polycarbonate can be colored during molding using masterbatches or liquid dyes, producing vibrant and uniform colors.

Considerations for Surface Finishing

Mold Surface Quality: The mold cavity’s finish directly affects the final part. Highly polished molds are required for optical clarity, while textured molds are used for matte finishes.

Post-Processing: Some finishing methods, like polishing or coating, may be applied after molding for critical applications.

Additives: Anti-scratch or UV stabilizers can be added to the PC resin to enhance surface performance without extra post-processing.

Polycarbonate Injection Molding Design Guide

Designing parts for polycarbonate (PC) injection molding requires careful attention to material behavior, process limitations, and end-use requirements. Polycarbonate’s high strength, transparency, and thermal stability make it ideal for complex parts, but proper design ensures quality, reduces defects, and improves manufacturability.

1. Wall Thickness

Uniformity: Maintain consistent wall thickness to minimize warpage, sink marks, and internal stress.

Recommended Range: 1.5–4 mm is typical for most PC parts; thinner walls (<1 mm) are possible for small features but may require high injection pressures.

Tips: Gradually taper transitions between thick and thin areas to reduce stress concentrations.

2. Draft Angle

Purpose: Facilitates easy ejection of the part from the mold and prevents scratching or deformation.

Recommended Angles: 

0.5–1° per side for textured surfaces

1–2° per side for polished or optical surfaces

Tips: Larger draft angles are needed for taller or more intricate parts.

3. Radii

Corner Radii: Rounded corners reduce stress concentrations and improve flow.

Recommended Values:

Internal corners: 0.5–1× wall thickness

External corners: 0.5–1× wall thickness

Tips: Avoid sharp corners, especially for transparent parts, to prevent cracking or visual defects.

4. Ribs

Purpose: Add stiffness and structural support without increasing wall thickness.

Design Guidelines:

Rib height: ≤3× wall thickness

Rib thickness: 0.5–0.6× nominal wall thickness

Spacing: ≥2× wall thickness apart

Tips: Use fillets at rib bases to reduce stress and avoid sink marks.

5. Bosses

Purpose: Support fasteners or assembly features.

Design Guidelines:

Boss diameter: ≥2× wall thickness

Wall thickness at boss base: 0.6–0.8× nominal wall thickness

Add fillets at the base for stress relief

Tips: Avoid placing bosses too close to edges or thick sections to prevent sink marks and warpage.

6. Tolerance

Dimensional Control: Polycarbonate parts can shrink 0.5–0.7%; tolerances should consider shrinkage, mold design, and part geometry.

Typical Tolerances: ±0.1–0.2 mm for small features; ±0.3–0.5 mm for larger features.

Tips: Tight tolerances may require secondary machining or post-processing.

7. Gates

Purpose: Point where molten PC enters the mold cavity.

Design Considerations:

Gate type: edge, pin, or submarine gates depending on part geometry

Placement: near thick areas to ensure uniform flow

Size: balanced to avoid jetting or weld lines

Tips: Minimize number of gates to reduce aesthetic defects but ensure complete cavity filling.

8. Vents

Purpose: Allow trapped air or gases to escape during injection to prevent burn marks, voids, or incomplete filling.

Design Guidelines:

Vent depth: 0.02–0.05 mm

Placement: at the end of flow paths or near corners

Tips: Proper venting reduces flash and improves surface finish.

Pros and Cons of Polycarbonate Injection Molding

Polycarbonate (PC) is a high-performance thermoplastic widely used in injection molding for industries like electronics, automotive, medical devices, and consumer products. While it offers many advantages, there are also considerations to keep in mind.

Pros

High Impact Resistance: PC is extremely tough and can withstand significant force without cracking or breaking.

Transparency: Naturally clear like glass, making it ideal for lenses, display covers, and light panels.

Thermal Stability: Maintains strength and dimensional stability over a wide temperature range.

Design Flexibility: Can be molded into complex geometries with tight tolerances.

Electrical Insulation: Excellent dielectric properties, suitable for electronic housings.

Surface Finish Options: Supports polished, textured, and coated finishes for aesthetics and functionality.

Chemical Resistance: Resistant to many common chemicals and cleaning agents.

Blendable: Can be combined with other polymers (ABS, PBT, PMMA) to improve flow, toughness, or reduce cost.

Cons

Hygroscopic Material: Absorbs moisture, requiring careful drying before molding to avoid bubbles and defects.

High Processing Temperature: Requires injection temperatures of 280–320°C, which may limit mold materials and increase energy consumption.

Sensitivity to Stress Cracking: PC can crack when exposed to certain chemicals or high stress areas.

Cost: More expensive than general-purpose plastics like ABS or polypropylene.

Shrinkage and Warpage: Requires precise mold design and process control to minimize dimensional variations.

Scratch Susceptibility: Untreated PC surfaces can be easily scratched; coatings or blends may be required for better scratch resistance.