PC Injection Molding: Properties, Grades, Applications and DFM Notes

What PC Is — and What It Is Not

Polycarbonate is an amorphous engineering thermoplastic. Amorphous matters more than the chemistry here: PC has no crystalline melting point, so it shrinks little and predictably (0.5–0.7 %), holds tight tolerances well, and stays optically clear. Those three properties are why PC ends up in lenses, medical housings and transparent covers rather than in structural brackets.

What PC buys you over the cheaper styrenics is impact strength. At 600–900 J/m notched, room-temperature PC is roughly three to four times tougher than ABS (150–250 J/m), and it stays ductile far lower: about −30 °C unnotched and −10 °C notched, with no sharp ductile-to-brittle transition of the kind that makes ABS fail suddenly in cold service.

Two things PC is not:

• It is not a solvent-resistant material. Amorphous polymers have no crystalline barrier to keep aggressive chemicals out of the bulk, and the carbonate linkage hydrolyzes. Combined with molded-in stress, this produces environmental stress cracking — the single most common field failure mode for PC parts, and the reason a part that passed every mechanical test can crack three months later next to a cleaning agent.
• It is not a forgiving material to process. PC dries at 120 °C for 3–4 hours to below 0.02 % moisture, melts at 280–320 °C and wants an 80–120 °C mold. Wet resin hydrolyzes in the barrel: molecular weight drops, and the part comes out brittle in a way no post-processing recovers.

If you are choosing between PC, ABS and PC+ABS rather than specifying PC itself, that decision has its own guide: PC vs ABS vs PC+ABS. This page is about specifying and molding PC once PC is the answer.

PC Properties at a Glance

Baseline figures for unfilled general-purpose PC:

The processing window, not the property table, is what decides most PC programs:

PC Grade Families

PC is a base polymer, not a specification. These are the families that actually appear on drawings, with the projects on this site that used them:

Two families worth flagging because they change the tool, not just the resin: flame-retardant PC is more shear-sensitive (FR packages degrade if the melt is overheated or held too long), and glass-filled PC wears the tool — 20 % GF in a 300,000-shot program will erode gates and cavity edges, so hard steel stops being optional.

Where PC Fits in Production

PC earns its price in five situations:

Transparent or light-carrying parts. No other material on this site combines clarity with this impact level. Clear covers, lenses, light pipes and display windows are PC programs or they are acrylic programs, and acrylic loses the impact argument immediately.

High-impact housings where ABS is not enough. If the drop test is specified at −20 °C or the part is a hand-held device with a large unsupported wall, the ABS impact curve falls away exactly where PC does not.

Service temperatures above 100 °C. HDT of 125–135 °C covers appliance interiors, under-dash electronics and equipment that runs warm. ABS at 85–100 °C leaves no margin there.

Medical and diagnostic equipment. USP Class VI PC with traceability is the material of record for analyzer and meter housings that need both clarity and chemical resistance to disinfectants — and it is one of the few materials where the documentation burden (ISO 10993, lot traceability, clean room molding) is as decisive as the properties.

Flame-retardant electrical enclosures with a visible face. FR PC gives V-0 and a Class A surface in the same part, which FR ABS cannot always hold at the wall thicknesses involved.

Where PC is usually the wrong answer: large, thin, complex housings (melt viscosity forces high clamp force or multiple gates), parts exposed to solvents or fuels, and cost-driven programs where PC+ABS delivers ~80 % of the impact at 60–70 % of the cost.

PC Projects From Our Floor

Fourteen PC programs, with the specs that were actually run. This is the largest material group in our case portfolio — PC is not a niche material here:

Three patterns in that table are worth reading before you specify:

• Tool steel follows the grade, not the material. Structural and housing work runs P20 or 718H; optical and medical work runs S136; glass-filled and million-shot programs run S136H at 48–52 HRC. That split is consistent across all fourteen rows.
• Shot life spans 20× — 50,000 on the OBD prototype-plus-production mix up to 1,000,000 on the charger housing. Shot life is a tooling decision, and it is quoted per program.
• T1 lands at 20–35 days for single-cavity through 16-cavity PC tools. PC’s processing window costs nothing in lead time when the tool is designed for it.

DFM Notes Specific to PC

Drying Is Not Optional

PC absorbs moisture and hydrolyzes at melt temperature. The limit is 0.02 % — the tightest on this site — and it needs a desiccant dryer holding a dew point below −30 °C, with dried-air conveying to the press hopper. A hopper dryer alone does not get there.

The failure is silent. Wet PC does not always splay visibly on the first shots; it loses molecular weight, and the part looks fine while its notched impact has dropped by half. If a PC part is brittle in service and the process records show a drying excursion, that is the explanation, and no amount of re-annealing fixes it.

Gate Sizing and Melt Viscosity

PC’s melt viscosity is high, and that single fact drives gate design. Unfilled PC wants gates of 0.8–1.5 mm minimum diameter; going smaller means the melt shears as it enters, which raises stress exactly where the part is most likely to crack. Expect higher injection pressures and a machine with real plastication capacity — a press that runs ABS comfortably may not plasticate PC fast enough to fill the same shot.

Pin-point gates on PC are a known weak point. Where the part is cosmetic or fatigue-loaded, a larger gate with a clean vestige usually beats a small one that leaves a stress concentration at the surface.

Mold Temperature and Surface Finish

PC wants 80–120 °C mold temperature. This is not a comfort setting: below 80 °C the surface texture and gloss drop, weld lines become visible, and molded-in stress rises across the whole part. High-gloss SPI A1–A2 is achievable on PC, but it is achievable at that temperature — which means a mold built for ABS (running at 50–60 °C) generally cannot be re-cut for PC without attention to the cooling circuit and the temperature controller.

Practically, budget for higher mold-cooling demand on a PC tool. It is the recurring cost that surprises buyers comparing PC and ABS tooling quotes.

Shrinkage and Cavity Sizing

At 0.5–0.7 % PC shrinks predictably, and because it is amorphous there is no post-molding crystallization to move the dimensions weeks later. Cavity sizing is therefore not where PC programs go wrong. Where they do go wrong is wall thickness: PC fills thick sections more easily than thin ones, and a designer coming from ABS will often thin a wall past what PC’s viscosity can fill at reasonable pressure.

Internal Stress and Environmental Stress Cracking

This is PC’s defining DFM issue and it deserves its own line in the drawing. ESC requires three things at once: a susceptible polymer, tensile stress, and a chemical. PC supplies the first two by nature — molded-in stress from a cold mold or an undersized gate plus assembly stress from a snap-fit or screw boss is usually enough.

What to do about it:

• Raise mold temperature into the 80–120 °C band and extend hold, so molded-in stress drops before the part leaves the tool
• Anneal thick or heavily loaded parts where the geometry cannot avoid stress
• Specify the chemical environment on the drawing. Cleaning agents, hand sanitizers, adhesives, thread-lockers and mold releases are the usual culprits, and a PC part that will be wiped with an alcohol-based disinfectant daily is a different design problem from one that will not
• Keep snap-fits and press-fits out of sustained tensile load — a snap-fit that holds the part in permanent tension is a crack waiting for the first solvent it meets

Venting and Gas Entrapment

High viscosity cuts both ways on venting: PC is less likely than ABS to flash through a vent gap, but more likely to trap gas because the melt front does not push air out as readily. The practical result is burn marks and gas traps at the last-filled areas, which is why PC tools usually want more venting than the ABS equivalent, not less.

Thick Sections and Low-Temperature Service

PC stays ductile to roughly −30 °C unnotched but only about −10 °C notched, so a part with a sharp internal corner in cold service behaves like a notched specimen whether the drawing calls it one or not. Radius internal corners, and if the part is going below −10 °C, treat every notch — a gate vestige, a thread start, a moulded-in rib intersection — as a design decision rather than a detail.

Troubleshooting: the Defects PC Actually Produces

Related Process Pages

PC programs on this site usually touch these pages:

• Injection Molding — the base process and machine range
• Mold Design — gate, cooling and ejection decisions that PC forces
• Multi-Cavity Mold Manufacturing — 8- and 16-cavity PC tools
• Clean Room Molding — medical PC programs
• Overmolding — PC-to-PC and PC-to-TPE two-shot work
• Precision Molding — optical and tolerance-critical PC parts
• Assembly — joining PC without introducing stress
• Materials Guide — the full property table across all materials on this site

Getting PC Right on Your Part

If you have a PC part in design, send the drawing and the application conditions — service temperature, chemicals it will contact, drop-test spec, and whether clarity is functional or cosmetic. Those four answers change the grade, the gate design and the mold temperature before any tooling is quoted.

We will come back with a DFM review and a quote, and we will tell you if PC is the wrong material for the job.

Request a quote