Polycarbonate (PC) Injection Molding

Everything runs hotter and drier

Polycarbonate is not a difficult material because it is unpredictable. It is difficult because the process window is narrower and sits higher than most plastics, and because one of the requirements — drying — has consequences that do not show on the surface.

Parameter	Polycarbonate	Compared with ABS
Melt temperature	~280–320 °C	Much higher (~210–270 °C)
Mould temperature	~80–120 °C	Much higher (~40–80 °C)
Drying	~120 °C, 3–4 hours, critical	~80–90 °C, 2–4 hours, modest
Shrinkage	~0.5–0.8%	~0.4–0.7%
Draft	1–2°	0.5–2°
Wall thickness	1.5–4.0 mm	1.0–3.5 mm

Three of those differences are consequential.

Drying is a property issue, not a surface issue

Moisture in a polycarbonate melt causes hydrolysis: the water reacts with the polymer and breaks the molecular chains.

The important thing about hydrolysis is what it does not do. It does not always produce an obvious defect. A part moulded from inadequately dried material can come out looking acceptable — good surface, correct dimensions — while its molecular weight is permanently reduced, which means reduced impact strength, reduced ductility and a greater tendency to crack.

So a polycarbonate part can fail the impact test it was specified to pass, for a reason that is invisible at inspection. That is why drying is treated as a controlled process parameter — around 120 °C for three to four hours, with the dried material sealed or held in a dryer hopper — rather than as a warm-up step.

It is also why regrind must be dried like virgin material. Regrind has more surface area and picks up moisture faster, so it is not exempt from the rule.

A hot mould is a quality decision

The mould temperature of 80–120 °C is high, and it is high for two reasons.

Flow. The polycarbonate melt is viscous, and a cold mould freezes it before it can fill a complex or thin-walled cavity. A hot tool gives the material time to reach the extremities of the part.

Stress. This is the reason specific to polycarbonate. The material stress-cracks — alkalis, ammonia, amines, ketones, esters and chlorinated solvents attack it, and where chemical exposure coincides with stress in the part, fine cracks open and spread into fractures.

A hot mould cools the part slowly, which reduces the moulded-in stress that the cooling process would otherwise lock in. So mould temperature is not only a filling parameter on a PC job; it is a stress control, and it feeds directly into whether the part survives its service environment.

The other stress sources are worth naming, because they arrive from different places:

• Assembly stress — a press fit, an over-tightened fastener, a snap feature forced into place
• Service load — a part bolted down against a surface that is not actually flat
• Chemical exposure — including ammonia-based cleaning agents, which are the default in many workplaces

Where a part is stressed, thick, complex or optically critical, an annealing cycle after moulding is used to relieve what remains. It is decided per drawing, on the basis of what the part has to do.

Low, uniform shrinkage

The compensation for the demanding process is that polycarbonate’s shrinkage is low and relatively uniform.

At roughly 0.5–0.8%, it is low; and because the material is amorphous, there is no crystalline phase forming and pulling the part differentially, so shrinkage is similar in all directions. That means:

• Less warpage than a semi-crystalline material such as polypropylene or nylon
• A first tool that usually lands close to the drawing, needing less correction
• Better dimensional stability in service, because there is no continuing crystallisation after ejection

Shrinkage still shifts within its band with wall thickness, gate location, packing and mould temperature, so it is agreed per drawing in DFM. But the review is less fraught than on a semi-crystalline material.

Design rules

Draft generously. 1–2° typical, more than ABS or acetal need. Polycarbonate is notch sensitive and the hot viscous melt grips the tool.

Radius every corner. This matters more on PC than on most materials. A sharp internal corner is a stress concentration and a crack initiation site, and the material has limited capacity to yield. An internal radius of at least half the wall thickness is the working rule.

Keep the wall uniform, and not too thin. 1.5–4.0 mm covers most parts. Thin walls are harder than with most thermoplastics because the melt is viscous, so the flow length achievable before freeze-off is shorter. Where local thickness is needed, core it out or reduce to roughly two-thirds of the nominal wall.

Think hard about the gate. On a transparent part the gate position is a cosmetic decision — flow marks, weld lines and gate blush are all visible through polycarbonate. On a stressed part it is a mechanical decision, because the weld line is the weak line. On a PC part it is usually both.

Design for the service chemistry. A design that traps a part under stress in a location that will be cleaned with an ammonia-based product is a design that will crack. That is worth checking specifically, because the geometry, the assembly and the cleaning regime come from three different people.

Molding route

Plastic molding runs through our vetted partner network, managed under one quality process, while metal inserts and machined features are done on our own floor in Qidong City, Jiangsu Province, China. Mould tooling is quoted per drawing, and mould size, shot weight and gate count are reviewed before the tool is cut.

Because a polycarbonate part often needs a metal feature — a threaded bush, a bearing bore, a mounting point — the insert arrangement is common here. The mould and the inserts come from the same drawing, so the fit between plastic and metal is planned before the tool is cut.

Moulding holds part-scale tolerances agreed per drawing in DFM; it does not match machined metal. Critical fits are carried by a machined insert rather than by the plastic.

Where molding polycarbonate is the right choice

• Transparent housings, covers and windows produced in volume
• Machine guards and protective shields
• Light covers and diffusers, where the service temperature exceeds what ABS or acrylic can take
• Electrical enclosures needing toughness and transparency
• Medical device housings, where gamma or ethylene oxide sterilisation is used
• Complex transparent geometry with curved surfaces, ribs or mounting flanges
• Parts that combine polycarbonate with a moulded-in metal insert
• Any PC part whose volume has grown enough to justify a tool

Where molding polycarbonate is the wrong choice

The volume does not justify a tool. Below the break-even point, machining from plate is cheaper and absorbs design changes.

The moulding partner cannot dry material properly. This is worth stating plainly: polycarbonate moulded wet is a part with permanently reduced properties and no visible defect. The drying equipment is not optional.

The part meets alkalis, ammonia or amines, and is under stress. Polycarbonate stress-cracks. The chemical environment and the stress state both have to be reviewed, and if either cannot be changed, the material has to.

The part is outdoors long-term and not UV-stabilised. Polycarbonate yellows and embrittles in sunlight unless a stabilised or coated grade is used.

Optical perfection is the requirement. Moulded polycarbonate is good, but acrylic is clearer, harder and less prone to visible flow marks. Where the part must look flawless, that is worth weighing.

Repeated steam autoclaving is the sterilisation route. Polycarbonate hydrolyses under repeated autoclave cycles. Gamma and ethylene oxide are the compatible methods.

The service temperature is above roughly 120 °C. PC softens and creeps; a higher-temperature polymer family is the answer.

What to put on the drawing

Instead of	Write
Plastic	Polycarbonate (PC), for a transparent or high-impact part at volume
PC, grade	State whether it is UV-stabilised, and whether optical quality is required
PC, transparent part	Say which surfaces are visible — gate position is a cosmetic decision
PC, stressed or press-fitted	Say so — consider annealing, and note the chemical environment
PC, chemicals or cleaning regime	Name them — alkalis, ammonia and amines cause stress cracking
PC, sterilisation	State the method — gamma and EtO are fine, repeated steam is not
PC, metal insert	Say so — the insert is machined on our floor and moulded in
PC, low volume or still changing	Machine it instead — tooling is a fixed cost

The three lines that decide most polycarbonate moldings are the wall and draft geometry, what the part touches and how it is cleaned, and whether it is optically critical. The first is the moulding review. The second decides whether the moulded-in stress will become a crack. The third decides the mould polish and the gate position — and both are expensive to change after the tool is cut.

Send the drawing through the request a quote form and the reply comes back within 24 hours with DFM feedback — including a note on drying, wall, draft, gate position and the stress and chemical review, plus a comparison against machining the same part where the volume is borderline.