PLA is one of the most popular materials for hobbyist 3D printing. It is easy to print and can produce great-looking parts. However, many makers discover a frustrating issue: a perfectly printed PLA object can deform or fail when left inside a hot car. This article explains why PLA prints often warp in high heat and how you can prevent these failures. It also clears up common misunderstandings and gives practical options for better results.
PLA and Heat: Understanding the Limits
PLA has low heat resistance: Standard PLA (polylactic acid) has a relatively low heat tolerance compared to many other plastics. It often begins to soften around 55 to 60 °C. This softening point is closely related to the glass transition temperature, often called Tg. Below this temperature range, PLA is rigid and holds its shape well. Above this range, PLA becomes more rubbery and can lose stiffness quickly.
Glass transition is not the same as melting: A common misunderstanding is that a print is safe as long as it does not reach the melting point. PLA typically melts much higher than its glass transition range, but the print can still deform long before melting. Once the temperature gets near or above Tg, the part can soften enough to bend under load. This is why a PLA part can look strong at room temperature and still fail in a hot environment.
Heat problems can happen at moderate temperatures: Many PLA products and technical notes warn against using PLA parts in sustained heat. In practice, temperatures above about 50 °C can be risky if the part must keep its shape. Indoors, this is rarely a problem. In closed spaces that heat up, it becomes a serious limitation.
How Hot Can a Parked Car Get?
Car interiors heat fast: A parked car can reach high temperatures due to trapped heat and sunlight. Even on mild days, the inside of a closed car can rise far above the outdoor air temperature. On warmer days, car cabin air can approach or exceed 50 to 60 °C. Surfaces in direct sun, such as dashboards, can become much hotter than the cabin air.
Not just direct sunlight: A PLA part does not need to sit in direct sunlight to soften. If the air inside the car gets hot enough, the entire interior warms up over time. A print in a glovebox or under a seat may still be exposed to temperatures near PLA’s softening range if the car stays closed in the sun.
Why this matters for PLA: The car environment can reach temperatures that overlap with PLA’s glass transition range. That means the conditions for softening are common, not rare. Many users are surprised because the outdoor temperature does not feel extreme, but the car interior can still become hot enough to deform PLA.
What Happens to PLA Prints in Hot Cars?
Warping and sagging: The most common failure is warping or sagging. When PLA softens, it can droop under its own weight. This is especially likely for thin walls, long spans, or parts with unsupported overhangs. A phone mount, a clip, or a bracket can lose its original shape in a short time if the car interior gets hot.
Creep under stress: Another common issue is creep, which means slow deformation over time while the part is under load. Heat makes creep much worse. A PLA part that is clamped, screwed tight, or holding tension may slowly change shape as it warms up. This can cause loose fits, misalignment, or failure of snap features.
Layer-related weakness can show up: When softened, PLA can also show weakness along layer lines. If the part is loaded while warm, layers may separate more easily. This is not because the printer suddenly printed badly. It is because the plastic is no longer stiff enough to resist the forces it was designed to handle.
Myths and Misconceptions About PLA in Hot Cars
- Myth: PLA is fine if the print looks strong. A strong-looking PLA print at room temperature can still deform when heated. Heat resistance is a material property, not a print quality setting.
- Myth: The part is safe if it is not in direct sun. A closed car interior can heat the entire space, not only sunny spots. Ambient heat can be enough to soften PLA.
- Myth: PLA only deforms when it melts. PLA can soften near Tg and bend under load without ever melting.
- Myth: More infill fixes heat resistance. Thicker walls and higher infill can slow heating and reduce flex, but they do not change PLA’s softening temperature. Once the part reaches the softening range, it can still deform.
- Myth: All PLA blends are heat safe. Some PLA blends can be tougher or more impact resistant, but many still soften in a similar temperature range. You should not assume a blend will survive a hot car unless it is clearly designed for higher heat use.
How to Prevent Heat Failures
You can avoid most hot-car failures with a few practical choices. The best solution depends on how the part is used and how much heat exposure is expected.
Choose a More Heat-Resistant Material
Switching material is often the most reliable fix: If a part must live in a car, PLA is usually not the best choice. Consider these general options:
- PETG: Often a simple upgrade from PLA. It typically softens at higher temperatures than PLA and is commonly used for parts that need more heat tolerance.
- ABS or ASA: These materials usually handle higher temperatures than PLA and are often chosen for automotive-like environments. They can be harder to print well because they may warp during printing, so an enclosure and good settings can help.
- Nylon and other engineering filaments: These can offer high heat performance, but they often require careful printing and dry storage.
Optional tool note: A basic filament dryer can help here, especially for filaments that absorb moisture easily.
Avoid Long Heat Exposure When Possible
If the item is not permanently installed in the car, the simplest prevention is to remove it when you park for long periods. Keeping the part out of the car during peak heat is often enough to avoid softening. Parking in shade can reduce heating, but it does not guarantee safety if the car still gets hot inside.
Design for Heat and Stress
Design choices can reduce the risk of deformation, especially when combined with better materials. These changes do not make PLA heat proof, but they can improve the result:
- Reduce long thin sections that can droop when warm.
- Add ribs or supports to increase stiffness.
- Avoid designs that rely on constant tension, such as tight clips.
- Limit load on the part, especially in warm conditions.
- Use mechanical fasteners carefully and avoid overtightening if the part might heat up.
Consider Annealing (Optional)
Annealing can improve heat resistance in some cases: Annealing is a process where you heat a finished PLA part under controlled conditions so the plastic structure changes. This can increase heat tolerance, but it can also cause shrinkage or warping. Results vary by filament type, part shape, and how the part is supported during heating. If you try annealing, expect testing and adjustments. For many beginners, switching to a more heat-resistant filament is simpler and more predictable.
Key Takeaways
- PLA can begin to soften around 55 to 60 °C, which is within the temperature range a parked car can reach.
- A PLA print can deform without melting. Softening near Tg is enough to cause warping and creep.
- Direct sunlight is not required. Hot cabin air can soften PLA even in shaded areas inside the car.
- More infill and thicker walls can help slightly, but they do not change PLA’s temperature limits.
- For car use, a more heat-resistant filament is usually the best solution. PETG can be a step up, and higher-temperature materials can be better if you can print them reliably.
- If PLA must be used, reduce heat exposure, reduce stress on the part, and consider annealing only if you are ready for trial and error.
PLA is excellent for many projects, especially indoors. Hot cars are a special case where PLA’s heat limits become important. Once you understand the temperature ranges and the softening behavior, it becomes easier to choose the right material and avoid surprise failures.