MJF Nylon Heat Resistance: Will It Survive a Hot Car?
MJF nylon heat resistance in a hot car: dashboards average about 69 °C after an hour in sun, and HP's PA12 HDT is 95 °C under heavy load. Where it creeps.
A dashboard in a Phoenix summer is a brutal test for a printed part. The r/3Dprinting poster had already found out ABS wasn't up to it, and MJF nylon heat resistance is the next thing buyers ask us about.
What Material Can Survive the Heat Inside a Parked Car?
For a part that isn't carrying much load, MJF PA12 has a comfortable margin over the dashboard temperatures researchers have measured. HP's data sheet puts its PA12 heat deflection temperature at 175 °C under a light 0.45 MPa load and 95 °C (XY) under 1.82 MPa.
The poster's part was an ABS bracket holding a polarizing lens on a dash cam. It survived days in the 90s and 100s °F, then sagged after a few hours parked in the sun at 110+. PLA had already warped in 90-degree weather, so they asked what to try next.
How hot a parked car gets
Researchers at Arizona State University and UC San Diego measured cars parked on hot days. In the sun, cabin air averaged 116 °F after an hour and dashboards averaged 157 °F, about 69 °C. Cars in the shade had dashboards averaging 118 °F. A dark part in direct sun can run hotter than the air around it, and a parked afternoon lasts longer than an hour.
Reading MJF nylon heat resistance numbers
The HP PA 12 data sheet gives HDT at two loads: 175 °C at 0.45 MPa, and 95 °C in XY or 106 °C in Z at 1.82 MPa. HP's footnote says the figures are typical values and shouldn't be used for specification purposes. HDT is the temperature where a test bar bends a set amount under that stress. It isn't a rating for how long a part can sit hot.
The gap between those two loads is the whole story. A phone mount that only holds its own weight is close to the light-load case. A clip that's preloaded all day sits hot under stress and creeps, the same slow relaxation that makes press-fit bosses in MJF loosen. Give that clip more section, or design out the preload.
Which of our materials to start with
Our materials page lists MJF Nylon for "strong functional printed parts, housings" and MJF Glass-Filled Nylon for "stiff, dimensionally stable printed parts". Try the glass-filled grade when stiffness at temperature matters. The HP numbers above are for HP's own PA 12, so if you need heat data for the exact grade we'd print, ask us when you quote. The trade-offs are in MJF PA12 vs PA12-GF, and months of sun are covered in MJF PA12 black outdoors.
A data sheet only tells you so much about MJF nylon heat resistance. Print one, leave it on the dash through a hot week with the windows up, and measure it afterward. It's the cheapest test you'll ever run.
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