Polycarbonate Injection Molding: Expert Guide to Materials, Processes, and Design

Polycarbonate (PC) is a material prized for its impact resistance and transparency. It might look like glass, but in terms of strength, it’s about as un-glasslike as you could imagine: you can find it in bulletproof windows, riot shields, and other ultra-tough parts designed to be almost unbreakable.

Although it can be processed via extrusion, thermoforming, and even 3D printing, one of the best ways to make polycarbonate parts is injection molding. The molten plastic material is quite viscous, but it can still be precisely injected into mold cavities to make plastic parts of various shapes and sizes.

This article goes over the basics of polycarbonate injection molding, including process parameters, design rules, and popular applications.

What is Polycarbonate?

Clear like glass and strong like some metals, polycarbonates (PC) are thermoplastic polymers whose chemical structures contain carbonate groups. Suitable for a range of manufacturing processes including injection molding and thermoforming, these materials are strong and tough, also offering good thermal resistance and transparency.

Chemically, polycarbonate is a polyester of carbonic acid formed in one of two ways: through a polycondensation reaction between bisphenol A (BPA) and phosgene (COCl₂), or by transesterification using diphenyl carbonate. The repeating structural unit of polycarbonate contains aromatic rings linked by carbonate groups, which gives the material its notable rigidity and transparency.

Because polycarbonate is a thermoplastic, it can be melted and formed, making it ideal for processes like injection molding. PC pellets for injection molding are made by extruding the material into strands and cutting it into uniform pieces. These plastic pellets typically cost a few dollars per kilogram. Comparatively speaking, polycarbonate materials cost more than common thermoplastics like ABS, PE, and PP but are comparable in price to many nylons.

In summary, polycarbonate is a fairly workable plastic known for its clarity, strength, and heat resistance, making it an ideal material for molded products like lenses, safety shields, and electronic housings.

Polycarbonate Material Properties

PropertyValueNoteDensity1.20–1.22 g/cm3Moderate for an engineering thermoplastic; more dense than ABS but less than glass, making PC suitable for optical and structural lightweightingYoung’s modulus2.0–2.4 GPaModerate stiffness; more rigid than PE and PP but less than acrylicsTensile strength55–75 MPaRelatively high for an amorphous thermoplastic; similar to nylon and close to some aluminium alloysGlass transition temperature147 °CVery high for a common thermoplastic, leading to good dimensional stability and impact resistance at high temperaturesWorking temperature range-40 °C to 130 °CRelatively broad operational window; retains toughness at subzero temperatures without going brittle

The Polycarbonate Injection Molding Process

Polycarbonate injection molding is a powerful and cost-effective processes for producing large volumes of high-strength plastic parts with a good level of clarity. Though the polycarbonate injection molding process is similar to other plastics in certain ways, it requires precise temperature control due to the high melting point of the material.

The process in short:

• Drying of pellets
• Filling of hopper
• Melting of pellets in barrel
• Injection into mold cavity
• Ejection of finished parts
• Post-processing

Before the manufacturing process begins, polycarbonate pellets are typically dried at moderate temperatures. This is because PC is highly hygroscopic, and failure to dry the pellets properly can lead to manufacturing defects caused by absorbed water.

Once the PC pellets are prepared, they are fed into the injection molding machine hopper, where a reciprocating screw moves them into the heated barrel. Polycarbonate injection molding temperatures are typically in the 260–320 °C range, significantly higher than low-cost materials like ABS. Because of its relatively high viscosity, the molten polycarbonate is then injected at fairly high pressure into the mold cavity.

The melted polycarbonate fills the mold cavity. As it cools and solidifies, it assumes the shape of the cavity. Once fully solidified, the molding is ejected from the opened mold using ejector pins.

Cycle times for polycarbonate moldings range from about 30 seconds to about two minutes, depending on part size, wall thickness, mold temperature, and other factors.

In some ways, polycarbonate is a material of extremes. Its unusually high level of optical transparency, strength, and heat resistance makes it desirable across many industries. However, those properties demand fairly extreme process parameters, such as high injection molding temperature and injection pressure.

Factors that affect the ideal molding parameters include specific PC material grade, the part geometry and wall thickness, and the type of mold.

• Barrel temperature: Typically 260–320 °C, with general-purpose grades near 270–300 °C and reinforced or flame-retardant grades toward the higher end. The temperature profile should increase gradually from the feed zone to the nozzle to ensure uniform melting without degradation.
• Mold temperature: 80–120 °C, higher than most thermoplastics, to reduce internal stress and improve optical clarity or surface finish. Lower mold temperatures can shorten cycle times (the material cools and solidifies faster) but carry a risk of warping or haze in transparent parts.
• Injection pressure: 15,000–25,000 psi, as PC’s viscosity is relatively high. Thin-walled or complex parts may need higher pressures or faster injection speeds to avoid short shots.
• Injection rate: Moderate to fast, ensuring that enough time is provided for complete cavity filling without running the risk of shear-induced stress or gate burning caused by the high temperatures involved.
• Moisture control: 3–4 hours of 120 °C drying, as PC is highly hygroscopicand even trace moisture causes hydrolytic degradation, resulting in bubbles or reduced strength in the final moldings. Pellets should also be kept in sealed containers or dryers until ready for use.

Nonstandard injection molding techniques can also be used to mold polycarbonate parts. For example, water-assisted injection molding can be deployed to prevent defects in large, thin-walled parts, while compression injection molding is better for thick-walled parts that might otherwise suffer from sink marks.

Polycarbonate Materials for Injection Molding

Polycarbonate injection molding materials come in different grades and can be mixed with certain additives to produce different effects, and some PC plastics are optimized for a particular end-use or industry. Major producers of PC materials include Covestro, SABIC, Mitsubishi Chemical, and LG Chem.

• General-purpose PC offers a balance of strength, transparency, and heat resistance for the broadest range of applications.
• High-flow PC has a higher melt flow index and may use additives like low-molecular-weight polybutylene terephthalate to improve flow, making injection into mold cavities easier.
• Flame-retardant PC typically contains phosphorus-based or sulfur-based additives to suppress fire.
• Food-grade PC is usually approved by an agency like the FDA for direct contact with food substances.
• Healthcare-grade PC is usually approved an agency like the FDA and may be biocompatible and be approved for gamma, EtO, or steam sterilization.
• Weather-resistant PC typically contains UV stabilizers to improve weather resistance and is used in automotive and other industries.

Although it is a strong, neat material, polycarbonate can be strengthened with chopped glass or carbon fibers. Impact modifiers can also be added to improve impact strength and toughness. Some grades include mold-release agents that prevent finished moldings from sticking to the metal mold. Other additives may include fillers, antistatic additives, and dyes to provide locked-in color to finished moldings.

Polycarbonate blends are used to achieve different material properties or reduce the cost of molded parts. Common blends include PC/ABS for improved processability, PC/PBT for improved chemical resistance, and PC/PMMA for improved scratch resistance while maintaining excellent optical clarity.

Molds for Polycarbonate

Molding polycarbonate requires a durable metal mold that can survive high temperatures and pressures over numerous cycles. PC molds are therefore generally made from hardened tool steels such as H13, S7, or stainless grades like 420. These materials can resist thermal fatigue, wear, and corrosion caused by prolonged exposure to hot PC melt.

One difficult when injection molding polycarbonate is that the material has a tendency to adhere to the inside of the mold, making ejection more difficult than it is with other thermoplastics. Because of this, high-polish finishes on the mold are preferred to textured ones, and coating of nickel or chromium may be applied to improve release while maintaining optical clarity of transparent parts.

Aluminum molds can be used for polycarbonate prototyping or low-volume production, but their relative softness compared to tool steels can cause issues. Regardless of mold material, adequate venting is key to preventing stress marks or bubbles.

Polycarbonate Surface Finishing

Several post-processing and surface finishing techniques are used for polycarbonate injection molding. However, the choice of technique depends on the end-use of the part, with optical parts requiring different treatment to mechanical or structural parts.

Molded polycarbonate gives a faithful impression of the mold surface, so it is often advised to create molds with a high-polish finish, as mentioned in the previous section. This is important for optical parts, where unwanted surface texturing can reduce clarity in parts like lenses or light fixture casings. On the other end of the spectrum, parts that require a degree of surface roughness can be made by chemical or laser etching the inside of the mold. Such parts may require a greater draft angle to overcome the added friction and facilitate ejection.

Polycarbonate Injection Molding Surface Finishing Techniques

ProcessFunctionNotesTexturingAdjusts surface roughnessTextures added directly to mold interior; greater draft angle required for very rough texturesSanding and polishingRemoves small blemishes, smooths surfaces, improves optical clarityManual and mechanical techniques both possibleVapor polishingSmooths surfaces, creates higher level of transparencyMethylene chloride or other solvent vapors most effective but controlled conditions requiredPainting and coatingAdds color, UV protection, or scratch resistanceExcellent adhesion possible; hard-coat acrylics are a common coating for transparent partsElectroplating and metallizingAdds metallic layer for aesthetics or EMI shieldingConductive base coat needed; vacuum metallization and electroless nickel plating most commonPrintingAdds logos, symbols, textGood ink adhesion via pad or screen printing

Polycarbonate Injection Molding Design Guide

When designing complex injection molding parts to be made from PC plastic, engineers need to consider the design for manufacturing (DFM) principles of injection molding, as well as the unique properties of polycarbonate.

Wall Thickness

1–4 mm

PC should have a relatively uniform wall thickness to prevent internal stress and optical distortion. Thin sections below 1 mm can be hard to fill due to the high melt viscosity of the material, while thick walls above 4 mm risk sink marks and long cooling times.

Draft Angle

0.5–2° (up to 3–5° for textured surfaces)

Because PC adheres strongly to polished steel and has low shrinkage, generous draft angles are important for proper ejection. Textured or matte surfaces require increased draft to prevent scuffing or stress whitening.

Radii

At least 0.6 mm or ¼–½ wall thicknessSharp corners concentrate stress and can lead to cracking, especially in clear PC parts. Internal and external radii improve mold filling and impact resistance, while radii transitions should be smooth and filleted.

Ribs

Height up to 3× wall thickness; thickness ½ wall thickness

Ribs provide stiffness without adding weight. Because PC cools slowly, ribs that are too thick can cause sink marks on visible surfaces. Radiused rib bases and uniform spacing can help achieve better results.

Bosses

Base thickness ½ wall thickness

Bosses should be cored to avoid thick sections and risking internal voids. Thread-forming screws or inserts are common for assembly of PC parts but these must avoid excessive stress to prevent cracking.

Tolerance

±0.05–0.2 mm

PC parts can achieve high dimensional accuracy thanks to the low shrinkage (~0.5–0.7%) of the plastic. But very tight tolerances should be avoided on optical or large parts, as internal stress and warpage may develop during cooling.

Gates

Edge, fan, or tab gates are preferred for uniform flow and reduced shear stress. For optical parts, use large gates to lower injection velocity and prevent flow lines.

Vents

0.02–0.04 mm

Proper venting prevents gas traps and burn marks, especially given PC’s high viscosity. Vents need to be small enough to prevent flash while also allowing trapped air to escape. Venting near weld lines or thick features is recommended.

Software Assistance

Many CAD tools offer injection molding assistance to make it easier to design molded polycarbonate parts. These features may include:

• DFM analysis: Checks key features like wall thickness, draft angles, undercuts, and gate placement to validate design for injection molding.
• Process analysis: Features like mold flow analysis, shrinkage prediction, and finite element analysis (FEA) give users an idea of how the PC will behave during molding.
• Mold creation: Automatic generation of simple or complex molds based on the part design, including ejector pin placement and other features.
• CAM integration: Integration with CAM software that controls the manufacturing equipment, ensuring a seamless transition from design to production.

Polycarbonate Injection Molding Applications

With its balance of desirable mechanical and optical properties, injection molded polycarbonate is used across a number of industries, finding use in parts like automotive headlamps and medical devices. Potential and real-world applications of molded PC are given below, sorted by industry.

Optical: Clear injection molded plastics like polycarbonate are a safe alternative to glass for products like lenses for sunglasses and eyeglasses, as well as safety glasses for use in laboratories, work sites, and other dangerous environments. The strong, transparent thermoplastic can also be used for parts like camera lenses and augmented reality glasses.

Automotive: In the automotive industry, polycarbonate injection molded parts include clear plastic parts like headlamp lenses. The material is suited to this purpose due to its transparency, impact resistance, and low weight. However, these parts are typically treated with surface coatings to improve their scratch resistance and prevent UV degradation. Other niche uses include EV battery pack cell holders.

Electronics: Being a good electrical insulator and having a high heat deflection temperature, polycarbonate is widely used in electronics. Durable housings and casings are common molded products, while some smartphone bodies like the Samsung Galaxy S21 have been made from the material. Some rugged computers like the Panasonic Toughbook also feature PC bodies for durability. A specific type of injection molding is used to produce data storage discs (CDs, DVDs, etc.) from polycarbonate. Production of data storage requires an injection mold with a stamper containing a negative of the disc’s contents.

Healthcare: With regulatory approval, certain grades of PC can be used for healthcare applications. Examples include protective gear such as goggles and face shields, housings for dialysis machines and other medical devices, diagnostic equipment like testing kits, and drug delivery systems like pens, pumps, and tubing. PC’s clarity and ability to be sterilized at fairly high temperatures also make it suitable for catheter parts.

Construction and architecture: As a clear material that is up to 250 times stronger than glass, polycarbonate is the transparent resin of choice for many clear plastic structural parts. Clear polycarbonate injection molding can produce parts like bank teller shields, guard booth windows, street lamp covers, skylights, electrical meter enclosures, and light diffusers.

Consumer goods: Transparent plastic molding of polycarbonate can be used to make consumer products like water bottles and other drinkware, food processor parts and other kitchenware, baby feeding bottles, phone cases, enclosures for small personal appliances like electric toothbrushes, hard-shell suitcases, and sporting goods.

Pros and Cons of Polycarbonate Injection Molding

Polycarbonate plastic injection molding has important benefits, like producing high-quality transparent parts, but it is not the easiest material to handle due to its high melting point and relatively poor flow characteristics.

Advantages

• Optical clarity: Injection molding with a high-polish metal mold preserves the transparency of polycarbonate, producing optical-grade clear plastic parts.
• Other desirable properties: Molded polycarbonate materials offer good heat and cold resistance, impact resistance, electrical insulation, dimensional stability, and flame retardancy.
• Fast and efficient process: Injection molding is ideal for manufacturing high volumes of PC parts at a low cost-per-part.
• Tight tolerances: The low shrinkage of polycarbonate allows for a high level of accuracy and tight tolerances.
• Recyclability: PC can be recycled via mechanical or chemical recycling, but the latter is best for maintaining the material’s impact resistance.

Disadvantages

• Demanding material: The high melting point and low fluidity of polycarbonate means it requires a high injection molding temperature and high injection pressures, which can be harder to maintain over many cycles.
• Scratch resistance: Given that polycarbonate is often used for optical parts, its susceptibility top scratches can be an issue—though it can be mitigated with additives or protective coatings.
• UV resistance: Polycarbonate’s UV resistance is also lacking, which can be an issue for outdoor parts like car headlamps unless mitigated with a protective coating.
• Moisture sensitivity: PC is highly hygroscopic and requires comprehensive drying before injection molding.

Reliable Polycarbonate Injection Molding with 3ERP

Polycarbonate is a highly versatile and effective thermoplastic for clear plastic parts and beyond. However, because it is harder to work with than other common plastics like ABS or PP, choosing a skilled and reliable injection molding partner is essential.

With 15+ years of experience in the business, 3ERP can carry out polycarbonate injection molding projects of any size, from prototypes to mass production. Our professional expertise allows us to handle DFM, mold making, quality parts production, and surface finishing, overseeing the PC molding process from start to finish.

Request a quote from us if you need strong transparent parts.

Frequently Asked Questions (FAQ)

Can injection molded composite polycarbonate be painted?

Yes, paints and coatings adhere well to polycarbonate and related composites. However, the material has poor resistance to solvents. PC pellets can also be mixed with colored pigments prior to molding using masterbatch, dry blending, or other methods.

How thin can polycarbonate be injection molded?

The minimum polycarbonate wall thickness is around 1 mm. The high viscosity of the thermoplastic makes it difficult to inject into thinner cavities.

Is injection molded polycarbonate stronger than other plastics?

Injection molded PC is very strong and offers high impact resistance, hence its use for demanding applications like riot shields. Its strength is higher than most plastics within its price range, while its impact strength is superior to many metals.

At what temperature is polycarbonate injection molded?

Polycarbonate injection molding machine settings include a barrel temperature of around 260–320 °C and a mold temperature of around 80–120 °C.

Is polycarbonate a good substitute for glass?

Polycarbonate can replace glass for many applications, especially where shatter resistance and safety is a priority, since it is far stronger. Its main disadvantages compared to glass include a tendency to yellow after prolonged UV resistance and poor scratch resistance.