Signs Of Bad PLA Filament: What To Do About It!

PLA is the most popular filament used by both professional and amateur printers. It is popular because it is very easy to use, requiring low extrusion temperatures of around 180 degrees Celsius. It also helps that it does not have a strong toxic odor!

Made from mostly organic material, it is the most biodegradable filament. Its ease of use makes it very beginner friendly. While it has a less steep learning curve, there are a few facts about PLA that once you know them will ensure your prints turn out well.

So, How do you tell when PLA has gone bad and what causes it to go bad? How long before PLA goes bad and what can you do to fix it? Why is PLA brittle?

How Can You Tell If PLA Filament Is Bad?

The clearest indication that PLA filament has gone bad is a poor-quality print. Poor prints typically have a rough or grainy surface, have a streaky color, or will be stringy. They also tend to be brittle and break easily if bent or dropped.

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There are several other ways to tell if filament has gone wrong before you start printing. Knowing the signs will save you spools of wasted filament beforehand.

  1. Check the filament diameter with a caliper. If it has become larger by more than 0.05mm then that is a sign the filament is most likely wet.
  2. If there isn’t a pair of calipers handy to check the diameter, running a few lengths of filament through the printer can also give a warning sign. If the filament jams the nozzle or you can’t feed it into the printer, then it has most likely expanded.
  3. Check the color. It should be consistent throughout the spool.
  4. Check the texture. The texture of good filament is smooth. If it is grainy or a little bumpy it could be wet.
  5. Steam coming from the printer is usually a result of running wet filament through it.
  6. The filament snaps when handled.
  7. Popping sounds when the filament is run through the printer.

For more on this, we recommend that you check out our post ” Why Does My Filament Keep Breaking? Fix PLA Filament Snapping!

How Long Does It Take For PLA To Go Bad?

PLA has a lifespan of between 2 to 3 years. It can, however, last significantly longer as thermoplastics have a shelf life of up to 15 years. This is conditional on how the filament is stored. How filament is stored determines whether it has a longer or shorter shelf life.

But what causes PLA to go bad? Filament is spoiled by three things. These are moisture, light, and dust.

Filament degradation caused by moisture

PLA, like just about all other filament types, is hygroscopic. It absorbs moisture. This is more pronounced in humid environments. Filament that is not stored properly will absorb the ambient moisture and swell up. This is how you get printers producing steam or filaments not fitting through the extruder.

Filament degradation caused by light

UV light either from direct sunlight or artificial light will break down the chemical compounds of filament and make it weaker.

Filament degradation caused by dust

A lesser talked about factor in filament degradation is dust. It might not directly affect the filament itself but it will affect the quality of the print if the dust gets mixed in with the filament during extrusion.

So what should you do when your filament goes bad?

How Do You Fix A Bad PLA Filament?

The best way to remedy bad filament is to dry it. In most cases, bad filament is due to moisture absorption. Drying can salvage most filament and make them reusable. There are several ways to dry filament including using an oven, a food dehydrator, or a dry box.

Drying with an oven is the most commonly used method because nearly everyone has access to an oven. The main disadvantage to using an oven is you do not have as much control over the oven temperature at all times.

Generally, you should make sure the temperature does not exceed 50 degrees Celsius when drying in an oven. You should dry it for between 2 to 8 hours depending on the weight. One kilogram is dried for about 8 hours before it is ready for use.

A dry box is a much easier fix. You don’t need to worry about temperature settings or someone needing to use the oven while you’re drying your filament!

Is PLA Supposed To Be Brittle?

PLA filament is inherently brittle. The organic compounds in its composition give it a weaker structure that makes it prone to brittleness. This is before it is extruded into a print. It also produces prints that are more brittle compared to other filaments like ABS.

What is PLA made from?

Most PLA is made from renewable biomass products. This means it consists of lactic acid units similar to what you find in foods like corn. This is how PLA got the name polylactic acid.

Lactic acids are extracted from plants that contain glucose through a process known as wet milling. A lot of PLA is made from cornstarch. The starch is mixed with other chemicals to produce dextrose or sugar as you and I know it as.

The sugar is fermented to produce lactic acid monomers which are used to PLA either through the process of polymerization or condensation. Polymerization is the most commonly used method.

Advantages of PLA

The use of organic material to produce PLA has both advantages and disadvantages.

One of the advantages is that it produces a filament that is easy to work with because of its low extrusion temperature.

The other advantage is it produces a filament that is more biodegradable than petroleum-based filament. Because it has plant material as its base ingredient, PLA breaks down quicker and easier than other plastics. This is not to say you can just bury it in your backyard and use it as compost, however!

PLA has other chemical additives mixed into it and will not biodegrade on its own. The process has to be speeded up under certain industrial conditions in a specialized facility. Source

Disadvantage of PLA

PLA has a low glass transition temperature. This is why PLA is easier to print with. However, a low glass transition temperature is also what makes PLA brittle and weaker than other plastics. Its strength is also its weakness.

The reason why PLA has a low glass transition temperature is that the lactic acid in PLA has low crystallinity. Source


I am a very well experienced techie civil engineer who's extensively interested in 3D printing technology and even more captivated by the potential of 3D printing livable structures

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