Last updated: 2026-09-05 · 5 min read
Polycarbonate is one of the most capable engineering resins you can put in a mold — optically clear, tough, stiff and serviceable well above the temperatures where ABS gives up — and it punishes careless molding more visibly than almost any other common material. Mold PC well and it is a transparent, dimensionally stable part straight off the tool; mold it carelessly and it is a brittle, yellowed, stressed part that cracks in the field. The three levers that decide which one you get are drying, mold temperature and stress-aware design, and this article walks through each with the parameters and the design notes that matter.
Key facts
<>
Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.
Why does moisture ruin polycarbonate parts?
PC belongs to the family of resins that chemically attack themselves when melted wet. At molding temperature, water molecules in the pellet cleave the carbonate linkages in the polymer backbone — a reaction called hydrolysis — and every chain that breaks is a chain that will never carry load the same way again. Drying is therefore not a cosmetic step that removes splay marks; it is the step that protects the molecular weight you are paying for. A wet PC batch can look perfectly glossy coming out of the tool and still fail a drop test or a hinge-cycle test, because the damage is chemical, not visual.
The symptoms arrive in sequence as moisture rises: first surface defects such as splay and silver streaks, then foaming and gas burns at the gate, then the silent one — parts that pass appearance but lose impact strength. That last stage is the expensive one, because it survives inspection and reaches the field. It is also the stage most often argued about afterward: did the resin arrive wet, did the dryer hold temperature, did the hopper sit open on a humid shift? A molding partner who logs dryer status per lot can answer those questions from records; one who guesses is reconstructing a failure they caused or absorbed, either way without evidence.
Which molding parameters matter most for PC?
| Parameter | What it does on PC | Watch the limit |
|---|---|---|
| Resin drying | Protects molecular weight; typical datasheet guidance runs a few hours at moderate temperature in a desiccant dryer. | Regeneration of the desiccant and hopper hygiene matter as much as time and temperature — a wrong recipe dries nothing. |
| Melt temperature | PC melts hot; enough heat fills thin sections and clear geometries cleanly. | Stay inside the grade’s window — overheating degrades and yellows the resin, and degradation is cumulative with residence time. |
| Mold temperature | A warm tool improves surface replication, reduces flow-mark visibility on clear parts and relieves packed-in stress. | Very warm tools lengthen cycles; the honest trade is cosmetic quality against cost per part. |
| Injection speed | Staged profiles fill complex geometry without jetting or burn marks. | Too fast shears and burns at the gate on a clear part; too slow leaves cold-flow weld lines you can see. |
| Packing and hold | Sizes the part and sinks the gate region — and on PC, overpacking builds stress you will see under crossed polarizers. | Hold pressure profiles belong to the process sheet, set against the actual part, not copied from a similar job. |
| Residence time | Long, hot sits degrade PC even when every individual setting is right. | Shot size matched to barrel capacity; purging or rejecting material after long stops, not molding on it. |
Two habits separate a disciplined PC process from a lucky one. First, the process sheet is written against the grade — the exact grade, not the family — because two PC formulations with the same appearance can carry different melt windows and different drying requirements. Second, the process is validated where it is weakest: moisture before the shot, stress after it. Both are measurable, both belong in the T1 report, and neither requires asking anyone to take it on faith.
How should you design parts for polycarbonate?
PC rewards the same design hygiene as other thermoplastics and adds a few notes of its own. Most of these are inexpensive to honor at the DFM stage and expensive to retrofit into steel:
One design note deserves its own sentence: stress in PC is not an abstract concept, it is an image. Put a clear molded part between two crossed polarizing filters and the frozen-in stress lights up as colored bands — fountain-flow patterns, gate shadows, corner concentrations, all of it. Buyers who learn to look at their clear PC parts this way stop arguing about whether stress is “a problem” and start asking the only useful question: is the stress pattern acceptable for this application, or does the design need another iteration?
Why do clear PC parts show defects that opaque parts hide?
The same physics runs in every resin, but opacity is a forgiveness mechanism. On a black or pigmented housing, a faint flow line, a slight color shift at a weld front, or a million tiny silver streaks simply do not register. Clear PC has nowhere to hide: the flow pattern of the entire fill is printed on the surface, and the internal stress map is printed through the volume. This is why clear-part programs need their own acceptance discipline. The cosmetic standard should be agreed on physical limit samples at a defined viewing distance and lighting, not on adjectives; the defect vocabulary — flow marks, blush, specks, sink, gate blush, stress whitening — should be written down with examples before the first trial; and the inspection method should name who looks, from how far, at what angle, under what light.
Black specks deserve a special warning. On clear PC they are the most common rejection reason and the hardest to root-cause after the fact, because every stage can introduce them: degraded resin left in the barrel, carbonized buildup at a runner corner, a contaminated dryer, even poor hopper hygiene. Shops that mold clear PC as a discipline keep dedicated purging routines and dedicated tools for it. Shops that treat a clear job as an ordinary job with a prettier resin discover, at T1, that they have a speck problem and no clean answer for where it lives.
How does UV exposure change the resin conversation?
Standard PC yellows and embrittles under sustained sunlight — the chemistry is well understood and shows up as a loss of transmission and a drop in impact strength over years outdoors. For an outdoor enclosure, that fact forces a decision early in material selection rather than a warranty claim late in the field. The options, in rising order of commitment: pick a UV-stabilized grade, which buys years but not immunity; design for a coating or co-extrusion, which protects the bulk resin; step to a weatherable blend or a different family such as ASA, which trades some of PC’s stiffness and clarity for outdoor life; or relocate the part indoors where none of this applies. What is not an option is molding a standard indoor grade, calling it suitable, and hoping the application stays shaded — the resin datasheet for each grade states the weathering position plainly, and it is worth reading before the RFQ rather than after the first warranty season.
When does a PC blend or a different resin make more sense?
PC rarely competes with commodity resins on price, so the honest selection question is always what the application actually needs. Where impact with paintability matters more than clarity, a PC/ABS blend or plain ABS may carry the program at a friendlier cost per part; where clarity and stiffness both matter, PC stands nearly alone; where sustained heat pushes past PC’s ceiling, the conversation moves to higher-temperature amorphous resins; where outdoor life dominates, ASA and weatherable blends enter. Blends also change the molding conversation — PC/ABS processes a bit differently, dries a bit differently and tolerates a bit less heat than neat PC — so a resin substitution late in a program is never just a purchasing event. Change the grade after tooling and you owe a new material review, a new process window and usually a new T1; change it before the deposit and it costs a meeting.
What acceptance evidence should PC parts carry at T1?
- Dryer and moisture records for the lot molded. The lot number, the dryer setpoints, and a moisture measurement at the hopper — so the chemical integrity of the resin is documented, not assumed.
- The process sheet as actually run. Melt and mold temperatures measured at the tool, not just set on the controller, plus the speed and hold profiles — the difference between what a machine displays and what the melt experiences is where PC problems hide.
- Full dimensional results on the agreed features. PC holds size well, so a first-article report that dodges dimensions is telling you something else is wrong; every critical callout measured and reported against the drawing.
- A cosmetic verdict under the agreed conditions. Limit samples beside actual parts, at the defined viewing distance and lighting, with the defect vocabulary from the pre-trial agreement — and photographs of anything borderline.
- Stress documentation on clear parts. Crossed-polarizer photographs of the actual molded parts, so the stress conversation carries an image instead of an adjective, and annealing trials documented separately if the drawing calls for them.
- A written residual-risk note. What the trial could not test — long-term weathering, load at temperature, chemical exposure — named explicitly, so acceptance at T1 is not silently stretched into a claim it was never making.
Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.
Clear PC is the resin that ends fake arguments fastest, because everything is visible at once. I have watched buyers reject a part for a speck nobody could find again under the inspection lamp, and I have watched buyers accept a part with a stress band across a hinge boss because nobody thought to put it under polarized light. Both mistakes come from the same place: no agreed standard for what acceptable looks like. Give me the viewing conditions, the limit samples and the load path, and the T1 conversation takes twenty minutes. Without them it takes a week and ends with somebody’s opinion.
— Arthur, Tooling Engineer at aCatMould
Frequently asked questions
How long does PC need to dry?
The datasheet for your grade governs, but typical guidance is on the order of a few hours in a desiccant dryer at moderate temperature. Sealed packaging straight from the supplier may need less; resin exposed to shop air needs the full window and a moisture check at the hopper. The measurement costs less than the scrap.
Can I tell if parts were molded wet?
Sometimes. Severe moisture leaves splay, silver streaks or foaming that show immediately. Mild moisture can leave parts that look flawless but lost impact toughness — that is why dryer logs and hopper moisture readings belong in the T1 evidence instead of a visual verdict.
Why are my clear PC parts stressed even though the process is in spec?
Because stress comes from geometry and packing as much as from settings: sharp corners, thick-thin transitions, gated bosses and overpacking all freeze stress into the part. In-spec settings reduce it; design and process together decide whether the remaining pattern is acceptable for the application.
Does annealing fix PC stress?
It reduces it. Annealing at the temperature and duration the resin supplier recommends relaxes much of the frozen-in stress and can restore dimensional stability for demanding assemblies — at the cost of a second operation, possible minor size shift, and fixtures. Treat it as a designed step, not a rescue for a bad process.
Is PC suitable for outdoor enclosures?
Not in an unstabilized grade. Standard PC yellows and loses toughness under sustained UV, so outdoor programs start with a UV-stabilized grade, a protective coating, or a weatherable resin such as ASA — decided during material selection, with the datasheet, before tooling.
What causes black specks on clear PC parts?
Contamination or degradation somewhere between the bag and the part: carbonized resin in the barrel or runner, a dirty dryer, or poor purging discipline after a stop. On clear parts, speck control is a dedicated routine — clean purge practices, matched shot sizes and documented barrel hygiene — not a one-time cleanup before the trial.
Key takeaways
- PC’s toughness and clarity are real only if the molecular weight survives molding — drying is the step that protects it, and moisture damage is chemical, not cosmetic.
- Write the process against the exact grade: melt window, mold temperature and drying recipe come from the datasheet, validated at the tool rather than read off the controller.
- Mold temperature on PC is a quality lever — warm tools fill clear parts cleanly and reduce frozen-in stress, at a cycle-time cost you should price honestly.
- Clear parts need their own acceptance discipline: limit samples, agreed viewing conditions, a written defect vocabulary and polarizer photographs for stress.
- Outdoor exposure rules out unstabilized PC — choose UV-stabilized grades, coatings or a different resin family during selection, not after the first warranty season.
If your program involves clear covers, lens-grade surfaces, insert-loaded bosses or an outdoor enclosure in PC, send us the 3D model and the cosmetic drawing before the deposit. You will get the material conversation in writing — grade, drying plan, cosmetic standard, stress acceptance and the evidence list for T1 — from a named tooling engineer, before any steel is ordered.
Sources
MatWeb — material property database for polycarbonate grades and drying guidance
ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
ASTM D638 — Standard Test Method for Tensile Properties of Plastics, ASTM International
Autodesk Moldflow — injection molding simulation for filling and stress prediction, Autodesk
Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · MoldMaking Technology