The short answer
For many under-hood parts, start with heat-stabilized glass-filled nylon 66 or nylon 6 (typically 30–35% glass). Use a hydrolysis-resistant PA66 GF grade for radiator end tanks and coolant parts, glass-filled PPA where heat or coolant is beyond nylon, PPS GF40 for the hottest coolant and fuel parts, glass-filled PBT for sensors and connectors, and talc- or glass-filled PP for cooler covers and shrouds. Then check the grade against the automaker's material specification.
Under-hood parts fail in four ways: they lose strength after months at temperature (heat aging), they are attacked by hot coolant, they swell or crack in oils, fuels and road salt, or they fatigue under vibration. The resin choice depends on which of these the part sees, and how hot it gets.
How hot does it get?
Temperatures vary a lot by engine, location and airflow, so treat these as rough guides and use measured part temperatures where you can.
- Engine and appearance covers: mostly radiant heat from the engine; peak temperatures often fall around 100–150°C (212–302°F), higher near the exhaust.
- Air intake manifolds: on naturally aspirated engines the intake air is relatively cool and the heat comes mostly from the cylinder head. Manifolds with an integrated charge-air cooler run much hotter.
- Coolant pipes, thermostat housings and radiator end tanks: coolant commonly runs around 90–120°C (194–248°F) under pressure, and test specifications often go to about 130–135°C (266–275°F).
- Charge-air ducts: on the hot side of a turbocharger, air can reach roughly 180–230°C (356–446°F) on many engines. After the intercooler it is much cooler.
For why the HDT on a data sheet is not a long-term limit, see HDT vs. continuous use temperature.
Typical resins by part
| Part type | Typical resin | Why |
|---|---|---|
| Engine cover, appearance cover | Heat-stabilized PA6 or PA66, glass and/or mineral filled; filled PP on cooler engines | Stiff, good surface, takes radiant heat; mineral fill reduces warp |
| Air intake manifold | Heat-stabilized PA6 GF30 or PA66 GF30–35 | Burst strength and good vibration-weld strength; PA6 gives a good weld and surface |
| Hot charge-air ducts and resonators (turbo side) | Specially heat-stabilized PA66, PA46, PPA or PPS, glass-filled | Must hold pressure and shape at roughly 180–230°C (356–446°F) in many designs |
| Cold charge-air ducts (after the intercooler) | PA6 or PA66 GF, blow-molded or injection-molded grades | Lower heat; cost and pressure resistance drive the choice |
| Radiator end tanks | Hydrolysis-resistant PA66 GF30–35 | Long-term resistance to hot water-glycol; good burst and weld strength |
| Coolant pipes, connectors, thermostat housings | Hydrolysis-resistant PA66 GF, PPA GF, or PPS GF40 for the hottest parts | PPA and PPS absorb less coolant and hold dimensions better at temperature |
| Water pump parts, coolant valves | PPS GF40 or PPA GF | Tight tolerances in hot coolant; low swelling |
| Oil pans, oil filter housings, valve covers | Heat-stabilized PA66 GF30–35; PPA or PPS for hotter oil service | Hot oil resistance, stiffness, sealing at gaskets |
| Sensors, connectors, ignition parts | PBT GF (often hydrolysis-resistant); PA66 GF; PPS for hot spots | Low moisture uptake and steady dimensions for seals and terminals |
| Fan shrouds, battery boxes, reservoirs, splash covers | Talc- or glass-filled heat-stabilized PP; long-glass PP for large parts | Lowest cost and weight where heat and load are moderate |
Typical choices only. Actual materials depend on the engine, the part's measured temperature, the fluids and the automaker's specification. Grades vary widely, even within one resin family.
Heat aging: why "heat-stabilized" matters
Nylon held hot in air oxidizes. A standard grade can look fine for weeks and then turn brittle. Heat-stabilized grades contain stabilizer packages (copper-halide systems are common in black nylon; organic systems are often preferred for electrical parts and light colors) that slow this down. Specialty grades push PA66 into the 180–210°C (356–410°F) range for charge-air parts.
Producers and automakers judge this with long-term oven aging: molded bars held at a set temperature for 1,000, 2,000 or 3,000 hours or more, then tested. The temperature, duration and pass level come from the specification. Look at elongation and impact retention as well as tensile strength, because elongation usually drops first. A grade that keeps 80% of its tensile strength can still crack in service if its elongation has collapsed.
HDT is not a guide here: 30% glass-filled PA66 has an HDT of about 240–255°C (464–491°F), far above any temperature it can hold for years. See PA66 GF30 properties.
Coolant: the hydrolysis problem
Water-glycol coolant at 110–135°C (230–275°F) is one of the hardest environments for glass-filled nylon. It attacks the polymer and the bond to the glass fibers, and strength and weld-line strength fall over time. Hydrolysis-resistant PA66 grades were developed for this service, and they are the usual choice for radiator end tanks and many coolant parts. See glass-filled nylon.
For hotter coolant or tighter dimensions, PPA absorbs less coolant and keeps its stiffness, and PPS absorbs almost none. Standard glass-filled PPS can still lose a meaningful share of its strength after long hot-water exposure, because the water attacks the glass-to-resin bond, so producers offer hydrolysis-rated PPS grades for coolant parts.
Oils, fuels and road salt
- Engine oil and transmission fluid: heat-stabilized PA66 GF is widely used for oil pans and covers. Aged oil turns more aggressive, so the hottest oil-wetted parts may move to PPA or PPS.
- Fuel: nylon swells somewhat in fuels, more with alcohol blends; PPS and PPA hold dimensions better. See best plastic for fuel and oil.
- Road salt: PA6 and PA66 can stress-crack in calcium chloride from road salt and in zinc chloride, which forms where salt splash reaches galvanized steel. Long-chain nylons and PPA resist this better.
Vibration and fatigue
Engine-mounted parts see vibration for their whole life, at temperature. Fatigue strength of glass-filled nylon drops as it warms and absorbs moisture, and it is lowest at weld lines and across the fiber direction. Vibration-welded manifolds depend on the weld strength of the grade, which differs between PA6 and PA66 and between grades. Ask for fatigue and weld data at temperature, and see best plastic to replace metal for design notes on creep and stiffness.
Automaker specifications
Automakers and many Tier 1 suppliers publish their own material specifications covering heat aging, fluid resistance, and for some parts odor and emissions. Production parts are usually tied to grades approved against the spec on the drawing. If you are second-sourcing a grade or switching resins, start from the spec number your customer calls out, and confirm the new grade is approved to it before you plan a switch.
What to send us for a quote
- The part, the grade on the drawing and any material specification it calls out
- Measured or expected part temperature, and the fluids it touches
- Color (black or natural) and whether laser marking is needed
- Annual volume and ship-to location
We source heat-stabilized and hydrolysis-resistant nylon, PPA, PPS, PBT and filled PP. See automotive.
Common questions
What plastic is used for under-the-hood parts?
Heat-stabilized glass-filled nylon 66 and nylon 6 cover a large share of under-hood parts, from engine covers and intake manifolds to radiator end tanks. PPA and PPS take over where heat, coolant or fuel exposure is beyond nylon, PBT is common for sensors and connectors, and talc- or glass-filled PP covers cooler covers and shrouds.
What is the best plastic for radiator end tanks?
Hydrolysis-resistant, heat-stabilized nylon 66 with 30–35% glass is the long-standing choice. Standard nylon loses strength in hot water and glycol, so the grade must be one the producer rates for long-term coolant exposure.
Why does glass-filled nylon fail in engine coolant?
Hot water-glycol coolant slowly attacks nylon and the bond between the nylon and the glass fibers (hydrolysis). Strength and weld-line strength drop over time, faster at higher coolant temperatures, which is why hydrolysis-resistant nylon, PPA or PPS is used for hot coolant parts.
How hot does a plastic air intake manifold get?
It depends on the engine. Manifolds on naturally aspirated engines usually run well within the limits of heat-stabilized PA6 or PA66 GF30, while hot charge-air parts on turbocharged engines can see roughly 180–230°C (356–446°F), which calls for specially stabilized PA66, PA46, PPA or PPS.
What does a heat-aging test for automotive plastics look like?
Molded test bars are held in an oven at a set temperature, often somewhere around 150–210°C (302–410°F), for 1,000–3,000 hours or more, then tested for retained tensile strength, elongation and impact. Automaker and Tier 1 material specifications set the actual temperatures, times and pass levels.
Is PPS better than nylon for coolant parts?
For the hottest coolant and fuel parts, often yes: glass-filled PPS absorbs almost no water, keeps its dimensions and resists glycol and fuels at temperatures where nylon degrades. Standard glass-filled PPS can still lose strength in long hot-water exposure, so use a grade the producer rates for hydrolysis.
Can polypropylene be used under the hood?
Yes, for cooler, lightly loaded parts such as covers, fan shrouds, battery boxes and fluid reservoirs. Talc- or glass-filled, heat-stabilized PP is common there; it is not suitable near the exhaust or for hot structural parts.
Tell us the part and its temperature
A polymer chemist reviews every request, then matches your grade or finds a qualified equivalent and prices it.
What to send for an accurate quote
- The grade name, from any supplier, or a data sheet
- Color (natural, black or a custom match)
- Fill type and percentage, if any
- Melt flow, if you know it
- Quantity for this order and what you use in a year
- Ship-to city and state or province