Key takeaways
- PC wins on impact strength, cold toughness and heat deflection, at a higher resin price.
- ABS wins on cost, processing window and surface finish, but yellows outdoors and goes brittle at sharp corners.
- UV performance comes from the grade and the surface system, not from the polymer name.
- Decide on the worst load case and the service environment, then confirm with a moulded trial.
What the choice decides
An enclosure carries the structure, insulates the electrics and gives the product its face. Polycarbonate (PC) and acrylonitrile butadiene styrene (ABS) are both used for equipment housings, terminal boxes, covers and hand-held devices, and both mould, paint and print well.
PC is chosen for impact strength and heat resistance. ABS is chosen for cost, a wide processing window and a good as-moulded finish. PC/ABS blends sit between the two. The argument is rarely settled on a data sheet; it is settled by the drop the product must survive, the sunlight it will see and the wall the design can afford.
Impact and toughness
PC has the highest notched impact of the three families and usually fails in a ductile way, so it deforms and tears rather than shattering. It keeps useful toughness when cold, which matters for outdoor cabinets, tools and portable equipment. ABS is tougher than general purpose polystyrene but turns brittle in the cold, at a sharp corner, or where a weld line sits at the impact point.
A data sheet impact figure is measured on a standard bar. Your housing is not a standard bar. A 0.5 mm internal corner, a gate that fills a thin rib last and a cold mould can turn a ductile material into a brittle one. Radius internal corners, keep the wall even, keep weld lines away from impact points and place the gate so flow does not run straight into a thin section.
UV and outdoor exposure
ABS contains butadiene, and butadiene is attacked by ultraviolet light. Unprotected ABS yellows, chalks and loses impact strength outdoors. An enclosure inside a dark cabinet may never see enough UV to matter, but a housing near a window, on a pole or on a roof will.
PC also yellows under UV, though more slowly, and the yellowing starts as a surface effect before it becomes a strength problem. Stabilised grades, a coextruded cap layer or an opaque paint are the usual answers, and each adds cost or a process step. Judge the system by exposure test, not by colour on the day of moulding: state the exposure hours and the colour shift limit on the drawing.
Heat, fire and electrical behaviour
PC holds its shape closer to a warm transformer or a solar load because its heat deflection temperature is higher. ABS softens earlier and creeps under a sustained load, so a boss carrying a terminal block can relax over time. Flame performance belongs to the grade, colour and wall thickness, so confirm the rating at the wall you will actually mould.
| Requirement | PC | ABS | What it changes in the design |
|---|---|---|---|
| Notched impact | High, ductile, good when cold | Moderate, brittle when cold or notched | PC suits drop risk; ABS needs radii |
| Outdoor UV | Slow yellowing, needs stabiliser | Yellows and chalks quickly | ABS suits indoor parts |
| Heat deflection | Higher, holds load closer to heat | Lower, creeps under load | PC suits warm interiors and loaded bosses |
| Flame and glow wire | Grade and thickness dependent | Grade and thickness dependent | Confirm at the real wall |
| Moisture and dimension | Low absorption, stable | Low absorption, stable | No conditioning allowance needed |
| Processing window | Narrow, needs drying and hot mould | Wide, forgiving, cooler mould | PC raises cycle time and scrap risk |
| Resin cost | Higher and denser | Lower and lighter | PC adds cost per part |
General trends for unfilled grades. A specific grade, colour and wall thickness can change any row.
Processing and tool design
PC must be dried before moulding. Moisture in the melt hydrolyses the polymer, and the result is splay, weak weld lines and a part that fails a drop test with no visible warning. The dryer, dew point and residence time belong in the process sheet, and the moulder should be able to show the drying record.
PC also runs hotter and is more viscous than ABS, so it needs generous gates, gradual section changes and short flow paths. A hot mould improves the surface and reduces moulded-in stress, but it adds cooling time. ABS is more forgiving: it runs cooler, tolerates a wider range and gives a good surface with less mould heating.
Cost per part, not cost per kilogram
PC costs more per kilogram than ABS, and it is denser. PC sits around 1.20 g/cm3 against roughly 1.04 to 1.06 g/cm3 for ABS, so the same moulding weighs about 15% more in PC before any design change. A comparison on resin price alone understates the gap.
The rest sits in the cycle. A hotter mould, longer cooling and a narrower window all raise machine cost per shot. PC scrap is less forgiving too: regrind that has been dried badly or cycled too often loses impact strength. Ask for the grade, the regrind percentage and the colour route in every quotation so two prices are compared on the same basis.
Choosing for a specific enclosure
Start with the worst condition. If the housing will be dropped, knocked or used outdoors in the cold, PC or a blend is the safer start. If it lives indoors, out of sunlight, and the cost target is tight, ABS will usually do the job. Then confirm the detail before the tool is cut.
- Impact and drop requirement, including the lowest service temperature
- UV exposure and the colour shift that would be rejected
- Continuous temperature inside the enclosure, not the room temperature
- Flame, glow wire and tracking requirement at the actual wall
- Finish, print and any solvent or adhesive contact
- Annual volume, colour route and regrind policy
Frequently asked questions
Is polycarbonate always better than ABS for an enclosure?
No. PC is tougher and holds heat better, but it costs more, needs careful drying and is harder to mould. An indoor cover that never sees impact or sunlight is often better served by ABS. The right answer depends on the worst condition the part will meet.
Can ABS be used outdoors?
Only with a grade and surface system designed for it. Unprotected ABS yellows, chalks and loses impact strength in sunlight. A stabilised grade, a paint system or a cap layer can improve the result, but the exposure and acceptance limit should be tested and stated on the drawing.
Why does PC need drying before moulding?
PC absorbs moisture, and wet resin hydrolyses in the melt. The visible result is splay, and the invisible result is a weaker weld line and lower impact strength. The dryer, dew point and residence time should be recorded, because a part can look acceptable and still fail a drop test.
Does a UL 94 rating apply at every wall thickness?
No. A rating applies to a specific grade, colour and thickness. A V-0 rating at 1.5 mm may not hold at 0.8 mm, which is often the thickness used on covers. Confirm the rating at the wall you will mould and keep the yellow card reference with the material record.
Sources and further reading
- ISO 4892-2:2013, Plastics, methods of exposure to laboratory light sources, xenon arc lamps
- UL 94, Standard for safety of flammability of plastic materials for parts in devices and appliances
- IEC 60695-11-10, Fire hazard testing, 50 W horizontal and vertical flame test methods
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
Choosing a grade for an enclosure?
Send the drawing, the service condition and the electrical requirement. The engineering desk will reply with the material questions to close before the tool is cut.
Send a drawing