What happens to PLA in a hot car

Almost everything we print is PLA. It is stiff, accurate and holds a crisp letter edge, and at room temperature it keeps its shape for years. Its one real weakness is heat, and the most common way a buyer meets that weakness is a car.

The number that matters

PLA has a heat deflection temperature of about 55 to 60 degrees Celsius, roughly 131 to 140 Fahrenheit, from filament makers' published test data. That is the temperature at which a test bar under a light load starts to bend. It is not the melting point, which is far higher. It is the point where the plastic stops being rigid and starts behaving like a stiff rubber.

Below it, PLA is fine. Above it, gravity and any small load start to win.

How hot a parked car gets

A closed car in direct sun heats up fast. On a warm day the cabin air passes 50 C within an hour, and on a hot one it goes well past 60 C. Surfaces in direct sun get hotter than the air: a dashboard or a dark seat can be 20 or 30 degrees hotter than the cabin. A dark piece of plastic sitting in a sunbeam on a dashboard heats more than either.

So the honest answer to whether a PLA part survives a hot car is: on the floor, in the shade, in spring, probably. On the dashboard or the rear shelf in July, no.

What a softened part looks like

Heat does not melt a print into a puddle. What happens is quieter:

• Flat plates bow. A plaque or a baseplate lying on an uneven surface takes the shape of that surface.
• Tall thin walls lean. Card tray and bin walls, being thin, droop where they are unsupported.
• Anything under load bends. A box on top of a tray presses it flat. A plaque leaning on something bends around it.
• Fits change. A part that bowed even a millimeter may no longer drop into the box or drawer it was made for.

Once it cools, PLA is rigid again in its new shape. It does not spring back. There is no fixing a sagged part reliably at home; it needs to be reprinted.

Shipping in summer

Our pieces ship by USPS Ground Advantage and spend two to five days in transit. A package in a truck or on a porch in summer can get warm, but parcels are rarely in direct sun inside a vehicle, and a part packed flat in a box with no load on it survives temperatures that would bend a loaded one. If a piece arrives warped, that is a defect and we replace it. Email a photo to [email protected].

When to choose ASA

ASA softens at about 95 to 100 C and is UV stabilized, which means it is formulated to hold its color and strength in sun for years. It is the material in many car exterior parts for that reason. We offer it on the number plaque, which often lives on a mailbox post, a gate or a dock box in full sun.

Should the rest of the catalog be ASA too? For pieces that live in drawers, on desks and in game boxes, no. ASA is slower to print, costs more and gives off more fumes while printing, and none of that buys anything indoors. If you need a specific piece for a hot place, like a sign for a covered porch that gets afternoon sun, email us and we will quote it in ASA.

Other hot places

The car is the famous one, but PLA has the same problem in a few other spots:

• Dishwashers. A wash cycle runs hotter than PLA likes. Wipe pieces by hand.
• Sunny windowsills. A dark plaque in full afternoon sun through glass can pass 55 C even when the room is cool.
• Radiators, warming drawers, above a toaster.
• Closed metal sheds and truck boxes in summer.

The short version

Keep PLA pieces where you would be comfortable sitting in a T shirt and they last indefinitely. For anything that lives in sun or heat, choose ASA or ask us. The materials page has the full comparison and the list of things we will not print at all.