Custom PC Injection Molding Factory

What PC actually is

Polycarbonate is an amorphous engineering thermoplastic built on carbonate linkages. That chemistry gives it an unusual combination: high impact strength, good dimensional stability and transparency, with a heat deflection temperature well above ABS. It is the resin behind clear machine guards, medical device housings, electrical enclosures and automotive lighting components.

It is also a demanding resin to mould. The melt temperature is high, the mould has to run hot, and the resin is hygroscopic and notch-sensitive. A PC part that looks correct can still be full of moulded-in stress and fail weeks later. That is why PC is a material where the tool and the process deserve more attention than the purchase price.

The amorphous structure is the reason PC holds a tolerance well and shrinks comparatively evenly. It is also the reason it has no sharp melting point: it softens gradually, which makes the processing window wide enough to work in but narrow enough that a cold mould shows up immediately as stress and haze.

Processing window

Run PC at a melt temperature of roughly 280–320 °C and a mould temperature of 80–110 °C. The high end of the melt window is for thin walls and long flow paths; the low end for thick sections where residence time is already long. Above 320 °C the resin begins to degrade, and the first sign is a yellow tint and a drop in impact strength rather than a visible defect.

The mould temperature is not optional. Below roughly 80 °C the melt freezes too early, the pack is lost, and the part carries high moulded-in stress. The symptoms are haze in a clear part, warpage, and a part that cracks at a sharp corner under load. Raising the mould temperature costs cycle time, and it is the single most effective quality lever on a PC tool.

Drying is critical. PC absorbs moisture, and at 300 °C that moisture hydrolyses the polymer. Three to four hours at 120 °C in a desiccant dryer is the normal instruction. Wet resin produces silver streaks, a brittle part, and a drop in molecular weight that no process adjustment can recover. The dryer has to be verified, not assumed: a controller reading 120 °C does not prove the dew point is low enough.

Residence time matters as much as temperature. A large barrel running a small shot leaves the melt hot for too long, and the degradation is cumulative. Size the shot to the barrel and purge thoroughly between grades, especially when moving from a lower-temperature resin to PC.

Shrinkage and dimensional tolerance

Design the tool to 0.5–0.7 % shrinkage. PC is amorphous, so flow-direction and cross-flow shrinkage are close, and the correction after first article is usually a uniform offset. The value moves with wall thickness, packing and mould temperature: a hot mould and a long hold push it toward the low end, a cold mould and a short pack toward the high end.

The bigger dimensional risk in PC is not shrinkage but stress relaxation. A part moulded with high internal stress can measure correctly on the day and move after annealing or after a few weeks in service. For a part that has to hold a tight tolerance, specify the measurement condition and consider annealing before the final inspection, so the number you approve is the number the part will keep.

Wall thickness drives differential shrinkage. A thick boss joined to a thin wall will sink at the boss and pull the surrounding face, and no shrink factor fixes that. The design and the cooling layout have to solve it together: core the boss, add gussets, and put cooling where the mass is.

For optical parts the tolerance is not only dimensional. Birefringence, haze and gate marks are dimensional defects in the sense that they decide whether the part passes. A PC lens usually needs a hot mould, a slow fill and a gate placed outside the optical zone, and the tool has to be polished to the optical requirement before the first shot rather than after.

Where it wins, where it gets replaced

PC wins where the part needs transparency plus toughness, or stiffness plus heat resistance. Clear guards, lenses, medical housings, electrical enclosures and structural covers are the classic cases. It also wins where a part has to take an impact at low temperature, because PC retains more impact strength than most commodity resins as the temperature falls.

The cheaper substitution is ABS or PC/ABS. For an opaque indoor housing that does not exceed roughly 80 °C, ABS does the job at a lower material price and a much easier process. PC/ABS sits between the two and is often the right answer for an automotive interior part that needs heat resistance without the full cost of PC.

The upgrade is a high-temperature resin such as PPSU or PEI when the part has to survive repeated steam or sterilization, or a polysulfone when the service temperature goes beyond what PC can hold. Those resins bring higher melt temperatures and more demanding tooling again.

The common misuse is a part in continuous contact with a solvent, a fuel or an aggressive cleaner. PC stress-cracks, and the crack starts at a gate or a sharp corner where the stress is already high. Another misuse is a thin, long-flow part moulded in a cold tool: the part fills, looks acceptable, and fails on impact because the weld lines are weak and the moulded-in stress is high.

PC is also a poor choice when the cost target is a commodity housing and the appearance requirement is a textured matte surface. ABS achieves that surface more easily and at a lower cost, and the PC premium buys nothing the part needs.

What PC asks of the mould

The defining demand is heat. PC wants a cavity at 80–110 °C, so the cooling circuit has to be able to hold that temperature evenly, and the tool has to be insulated from the platens rather than conducting heat away. A circuit designed for a 50 °C ABS tool will not hold a PC mould, and the result is a part with high stress and a warped geometry.

Steel selection follows the heat. A pre-hardened P20-type block at 30 HRC can run a short PC campaign, but it softens and loses polishability if the mould is run hot for a long run. Production PC tooling normally uses a hardened grade such as H13 at 48–52 HRC, or a stainless grade when the part is optical or medical. Hardness here is about holding a polish and resisting indentation at temperature, not about abrasion, because PC carries no filler.

Venting is the second demand. PC is viscous and fills at high pressure, so air has to leave the cavity at the last-filled points. Vent the parting line at the flow front and add vents at deep ribs and bosses. A starved vent produces a burn mark and a short shot that reads like a pressure problem, and the usual response - raising the pressure - makes the flash worse without fixing the fill.

Gating is the third. PC shear-thins, so a small gate creates a high-shear region that shows as a stress mark and a weak point. Use a generous gate, a fan or a valve gate for a cosmetic part, and place it where the mark is hidden and where it can feed the thick section. For optical parts the gate belongs outside the optical zone, full stop.

Ejection and draft belong together. A hot PC part is still soft when it leaves the cavity, so ejection marks are easier to make and harder to remove. Polish the core, add draft of at least 1° and more on textured walls, and use a generous ejector area rather than a few small pins on a visible face.

Cooling layout should follow the mass, not the outline. PC's low thermal conductivity means a thick section stays molten long after a thin wall has frozen, and the part warps toward the hot side. Baffles and bubblers in the thick sections, and a separate circuit for the core where the mass is, do more for cycle time and flatness than lowering the mould temperature, which is the wrong lever here.

Slides and inserts need clearance for the higher thermal expansion at mould temperature. A fit that is correct cold can bind hot, and a slide that binds marks the part and wears the tool.

Cost and cycle-time effect

PC costs more per kilogram than ABS, and it also costs more per part in cycle time because the mould runs hot and the part needs longer to cool. The two effects compound, so the unit cost gap is larger than the material price difference suggests.

The tooling cost is higher as well: hardened steel, a hot-oil or high-temperature water circuit, and more polishing for a clear part. That is why PC is usually specified for a part that genuinely needs it rather than as a general upgrade.

The yield side can offset some of the gap. PC holds a tolerance well and does not need the cosmetic rework that a soft ABS part sometimes does, and a clear part that passes inspection first time is worth more than the resin saving. The honest comparison is total cost per accepted part, not price per kilogram.

Where the volume is high and the part is opaque, the cheaper material usually wins. Where the part is clear, hot or impact-critical, the PC premium is buying a function that ABS cannot provide at any yield.

Design guidelines

• Wall thickness. Keep walls in the 2.0–3.5 mm range with gradual transitions. PC is notch-sensitive, so an abrupt step is both a sink risk and a stress riser.
• Corners and fillets. Use an internal fillet of at least 0.5 × wall, and 0.75 × where the part sees impact. A sharp internal corner is where a stress crack starts.
• Draft. At least 1° on smooth walls, 1.5–2° on textured walls, more on deep draws. A hot part drags more than a cold one.
• Ribs and bosses. Rib thickness around 0.5–0.6 of the wall, boss wall around 0.6, with gussets instead of a thicker boss. Core the boss so the section stays uniform.
• Gates. Avoid a small pin gate on a cosmetic or optical face. Use a fan or valve gate and place the vestige outside the visible area.
• Chemical contact. List every cleaner, adhesive and solvent the part will meet, and check compatibility before the tool is cut. This is a design input, not a finishing detail.
• Tolerances. PC holds roughly ±0.05–0.1 mm on small features and better than ABS across long dimensions, but only if the part is measured after stress has relaxed. State the measurement condition.

Toepassingen

Polycarbonate is chosen for clear guards, lenses, light covers, medical device housings and electrical enclosures, and for structural parts that have to stay stiff and tough at temperatures that would soften a commodity resin. The common thread is a function that depends on either transparency or retained impact strength at temperature.

It is also used for parts that need dimensional stability across a temperature swing, because its amorphous structure shrinks evenly and its heat deflection temperature is high. When a housing has to pass a drop test and a heat test at the same time, PC is often the only resin that does both without a metal insert.

Where the part is opaque, indoor and below roughly 80 °C, ABS or PC/ABS usually delivers the same function for less. Where the part sees solvent, a strong UV load without stabilization, or a cost target that leaves no room for a hot mould, PC is the wrong specification and no process tuning will change that.