Polycarbonate Injection Molding: Processing Guide for PC and PC/ABS
Polycarbonate Injection Molding: Processing Guide for PC and PC/ABS
Polycarbonate injection molding punishes process errors faster than almost any other engineering thermoplastic. Get the melt temperature wrong by 30°F, skip the drying step, or pull parts with zero draft, and you will scrap the run. Follow the parameters in this guide and you can hold tolerances to ±0.002 in. on structural PC parts and produce optical-grade lenses with haze values below 1% per ASTM D1003.
Polycarbonate Properties That Drive Processing Decisions
PC is an amorphous thermoplastic with a glass transition temperature (Tg) of approximately 302°F (150°C). Its notched Izod impact strength runs 12 to 18 ft-lb/in per ASTM D256, which is why it dominates safety glazing, helmet shells, and structural housings. It is also hygroscopic to a degree that matters: PC absorbs up to 0.35% moisture by weight at ambient conditions, and that absorbed water causes hydrolytic degradation at melt temperatures.
The key polycarbonate properties to keep in front of your team during mold design are listed below.
- Tensile strength: 8,500 to 9,500 psi (ASTM D638)
- Flexural modulus: 330,000 to 380,000 psi (ASTM D790)
- Heat deflection temperature: 270°F at 264 psi (ASTM D648)
- Coefficient of thermal expansion: 3.8 x 10-5 in/in/°F
- PC shrinkage: 0.005 to 0.007 in/in (0.5% to 0.7%) for general-purpose grades
- Optical transmission: 88% to 90% for unfilled, natural grades
PC shrinkage is isotropic and predictable for unfilled grades, which is a major advantage over glass-filled nylons or semi-crystalline resins with directional shrinkage differentials. Your tool steel dimensions can be calculated with confidence using 0.006 in/in as the nominal, with steel-safe cuts on cores to adjust at first article.
PC Grades: Choosing the Right Resin Before Tool Design Starts
Selecting the wrong PC grade before cutting steel is one of the most expensive mistakes we see from new programs. Resin selection affects gate sizing, mold steel selection, surface finish specs, and secondary operations. The four main categories you will encounter are general purpose, glass-filled, flame-retardant, and pc optical grade.
| Grade Type | Common Resins | Melt Flow (g/10 min) | Typical Applications | Key Processing Note |
|---|---|---|---|---|
| General Purpose | Makrolon 2405, Lexan 141 | 4 to 22 | Housings, brackets, covers | Barrel temp 540°F to 590°F |
| Glass-Filled (10% to 30%) | Makrolon 8035, Lexan 500R | 3 to 8 | Structural frames, gears | H13 or 420SS cores; shrinkage drops to 0.001 to 0.003 in/in flow direction |
| Flame Retardant | Makrolon FR2005, Lexan 940 | 5 to 18 | Electronics, enclosures | Lower melt temp ceiling; avoid over 580°F |
| Optical Grade | Makrolon OD2015, Panlite AD-5503 | 15 to 65 | Lenses, light pipes, covers | Mirror-polished A1 steel; hot runner only |
| PC/ABS Blend | Cycoloy C1200HF, Bayblend T85 | 10 to 30 | Automotive trim, consumer electronics | Lower melt temp 450°F to 510°F vs. pure PC |
Glass-filled grades are extremely abrasive. Use H13 or 420SS on all core and cavity surfaces when running 20% GF or higher. P20 will show measurable wear within 50,000 shots on a 30% GF PC, based on tool audits our project managers have run on offshore programs sourced from Ningbo and Shenzhen.
PC Molding Conditions: Drying, Barrel, and Mold Parameters
PC molding conditions are non-negotiable on the drying side. You must dry PC to below 0.02% moisture content before processing. Run a desiccant dryer at 250°F (121°C) for a minimum of 4 hours for standard grades, and 6 hours for optical grades. A moisture analyzer check on pellets before the run costs under $5 in consumables and prevents thousands of dollars in scrap. Do not use a hot-air dryer; desiccant only.
Melt temperature ranges vary by grade and shot size, but the table above gives you starting points. For general-purpose PC, barrel temperatures are typically set in zones from front to rear: 565°F / 575°F / 560°F / 540°F with a nozzle at 570°F. Back pressure should run 50 to 150 psi hydraulic. Screw speed stays low, 40 to 70 RPM, to minimize shear degradation.
Mold temperature is where many processors leave performance on the table. Run mold temperature at 180°F to 220°F (82°C to 104°C) for structural PC. For pc optical grade parts, push mold temperature to 250°F to 280°F (121°C to 138°C) to prevent sink marks, birefringence, and cosmetic flow marks. Use a pressurized water system or oil temperature controller rated for those ranges. A mold running at 160°F on an optical lens will produce parts with haze above 3%, which most optical specs reject.
Injection Speed and Pressure
PC is a high-viscosity resin even at correct melt temperatures. Injection speed should be moderate, not aggressive. Use 1 to 3 in/sec for structural parts and 0.5 to 1.5 in/sec for optical parts to avoid jetting, weld line weakness, and residual stress. First-stage injection pressure typically runs 10,000 to 18,000 psi plastic pressure. Pack pressure runs 50% to 70% of injection pressure, held for 5 to 15 seconds depending on wall thickness.
For walls thicker than 0.150 in., extend pack time and consider sequential valve gates on hot runners to fill from the heavy section outward. This is standard practice in our shops for automotive lens carriers and thick-wall medical housings.
Stress Cracking Prevention in PC Parts
Environmental stress cracking (ESC) is PC’s most common field failure mode. Crazes and cracks appear when molded-in residual stress meets a chemical trigger: solvents, mold releases, cleaning agents, certain plasticizers, and UV exposure. Parts with residual stress above 1,500 psi surface stress are at high risk per published industry data from Covestro’s PC processing guidelines.
The primary controls for ESC prevention are thermal, mechanical, and chemical. On the thermal side, anneal structural PC parts at 250°F (121°C) for 2 to 4 hours after molding. This reduces residual stress by 40% to 60% in most part geometries according to Covestro’s published annealing data. On the mechanical side, use draft angles of 1.5° minimum per side on all vertical walls, and radius all inside corners to a minimum of 0.030 in. to reduce stress concentration.
Never use mold release spray directly on PC cavities. Silicone-based releases are ESC triggers. If release is needed, apply a semi-permanent PTFE-based coating to the tool and eliminate spray from the press-side process entirely.
Chemical exposure control is the third leg. Work with your customer’s product team to identify all chemicals that will contact the part in service, assembly, and cleaning. Test coupons at your molded stress level before approving the production resin and gate combination.
Processing PC/ABS Blends
PC/ABS blends give you a processing window that is easier to hit than pure PC while retaining much of PC’s impact performance. The ABS phase lowers melt viscosity, reduces mold temperature requirements, and improves flow in thin walls below 0.080 in. The trade-off is a modest reduction in heat resistance: heat deflection temperature for PC/ABS typically runs 200°F to 235°F versus 270°F for pure PC.
Dry PC/ABS at 180°F to 200°F for 3 to 4 hours in a desiccant dryer. Melt temperature runs 450°F to 510°F, and mold temperature runs 130°F to 180°F. These are meaningfully lower than pure PC conditions, which matters for cycle time. A 3mm wall section that cycles in 28 seconds on pure PC can often cycle in 22 seconds on a PC/ABS blend, a 21% reduction that adds up fast on high-volume programs.
Gate design for PC/ABS follows the same logic as pure PC: avoid restrictive submarine gates on thick-wall parts, use edge or fan gates for cosmetic A-surfaces, and size your gate land to no more than 50% of the nominal wall thickness. Runner diameters should be 0.250 in. minimum to avoid freeze-off and excessive shear heat in the runner system.
Mold Design Checklist for PC and PC/ABS Tools
Before you approve tool drawings from any supplier, verify these design parameters are in the tool design record.
- Steel: P20 for general purpose PC under 500,000 shots; H13 or 420SS for GF grades or high-volume optical tools
- Polish: SPI A1 or A2 for optical surfaces; SPI B1 for cosmetic structural parts
- Draft: 1.5° minimum per side for smooth surfaces; 3° minimum per side for textured surfaces (0.001 in. depth of texture requires 1° of additional draft per Mold-Masters application data)
- Gate: 0.060 in. to 0.125 in. diameter pin gate for hot runner; 0.100 in. to 0.200 in. edge gate for cold runner
- Cooling: Conformal or baffled circuits targeting delta-T below 10°F across the cavity face
- Venting: 0.0005 in. to 0.001 in. vent depth on PC; PC is sensitive to burn marks from trapped gas at end-of-fill
- Ejector pins: Minimum 0.125 in. diameter on structural walls; add ejector sleeves on tall cores above 1.5 in.
When sourcing tools offshore, confirm that your Chinese tool shop has a calibrated CMM on site and can provide a first-article inspection report with cavity steel dimensions versus nominal. We require this on every tool we source for customers running PC programs.
Frequently Asked Questions
What is the correct drying temperature and time for polycarbonate before injection molding?
Dry standard PC grades at 250°F (121°C) for a minimum of 4 hours in a desiccant dryer. Optical grades require 6 hours at the same temperature. Target moisture content below 0.02% by weight. Do not recycle dried material that has sat in a hopper for more than 2 hours without reheating.
What PC shrinkage rate should I use to size my mold cavities?
Use 0.005 to 0.007 in/in (0.5% to 0.7%) for unfilled, general-purpose PC grades. Glass-filled grades drop to 0.001 to 0.003 in/in in the flow direction and 0.003 to 0.006 in/in transverse. Design cavity steel to nominal shrinkage and leave cores steel-safe for post-sampling adjustment.
Can I run polycarbonate in a cold runner mold?
Yes, but expect higher scrap rates on runner regrind and tighter process windows. Cold runner systems for PC require runner diameters of 0.250 in. or larger and insulated nozzles to prevent freeze-off. For optical grade or high-cosmetic PC, use a hot runner. The tool cost premium for a hot runner on a 4-cavity optical mold is typically $8,000 to $15,000, which you recover quickly in reduced scrap and cycle time.
What causes splay or silver streaks in PC parts?
Splay on PC parts is almost always a moisture problem. If your dryer check is clean, look at excessive screw speed or back pressure creating shear degradation, or a leaking nozzle check valve pulling air into the melt. Confirm barrel temperatures are in range before blaming the resin. Splay from degraded PC will also show a slight yellowing or brown discoloration at the gate area.
How do PC/ABS blends compare to pure PC for structural applications?
PC/ABS retains 80% to 90% of pure PC’s notched Izod impact strength while offering easier processing and better compatibility with ABS-based color concentrates. The primary trade-off is heat resistance, with heat deflection temperature running 35°F to 70°F lower than pure PC. For applications that see sustained temperatures above 200°F, stick with pure PC or a high-heat PC grade.
Run your wall thickness, gate location, and clamp tonnage through our injection molding consulting team before finalizing tool design. Early review catches problems that cost $3,000 to fix on a drawing and $30,000 to fix after the tool ships from China.