Identifying Unknown Filaments

More often than not, filaments are stored beautifully, labelled and identified - whether it's a spool of ABS or a packaged Masterspool refill, knowing what the plastic is should be available at first glance to make selecting the right print settings a breeze (especially if you use the optimal settings on any of our Plastics pages)!

Reality is not always such an organised ideal though! Whether you're a maker, tinkerer, designer and enthusiast you will no doubt be familiar with the odds and ends that seem to collect when it comes to 3D printing! Unlabelled filaments are not only an organisation nightmare but a risk to your 3D printer! Printing with the wrong settings may lead to failed prints at best, or burning filament jamming your hot end at worst!

This article will give you some guidelines on tests you can do at home to identify those unknown filaments so they can be put to good use instead of cluttering your work-space or being resigned to 'waste'.

Test Methods

Visual Inspection: It sounds obvious but the first step in any identification process is a visual inspection, it may save you a lot of time! What exactly should you be looking for?

  • Is it transparent, translucent or opaque? Very crystalline plastics (e.g. POM) will appear opaque. Semi-crystalline plastics (e.g. PET) will appear cloudy or translucent. Amorphous plastics (e.g. PETG) may be nearly transparent. Remember that transparency will also be influenced by additives used by manufacturers.
  • Can you see any recycling codes? The SPI recycling code system can be used to identify certain plastics.
1 Polyethylene Terephthalate (PET)
2 High Density Polyethylene (HDPE)
3 Polyvinyl Chloride (PVC)
4 Low Density Polyethylene (LDPE)
5 Polypropylene (PP)
6 Polystyrene (PS)
7 Other (the letter below the symbol will indicate the plastic type)

Density Test: This is the easiest and safest way to identify the broader families of plastics and is probably the best way to identify plastics at home! As different plastics have broadly different densities, a simple measurement (and some high school maths) can be used to identify the plastic!

  1. Cut off a small sample of the filament. Ideally cut off exactly 1cm to make the calculations easier.

  2. Find the volume of the sample using the following equation:

    Where r = radius which is half of the diameter. This is either given with the filament or can be found by measuring with a pair of calipers.

    h = height and is the length of the sample that you cut.

    π = 3.14159

  3. Find the mass of the filament sample. This is best done with sensitive electric scales. If you don't have access to sensitive scales then use a large enough sample to accurately weigh.

  4. Find the density by dividing the Mass by the Volume.

  5. Compare the result to the graph of densities below to find the best match.

    Note: Densities will vary depending on manufacturing process, additives and other factors. The graph below is a guideline of the generally accepted densities of common resins.

Burn Test: The burn test is a simple test where a small sample of the unknown thermoplastic is burned in a controlled manner in order to identify a set of observable properties:

  • Does the material continue burning in air after the ignition source is removed?
  • Flame colour.
  • Smell
  • Does the material melt and drip?
  • Colour of smoke?
  • Does the smoke react with litmus paper?

Despite the information the burn test can provide, it is not recommended for use at home as many plastics release toxic fumes when burnt, as well releasing as ultrafine particles that should not be inhaled.

There is no way of perfectly identifying an exact filament at home but with these simple tests it's certainly possible to find out what type of plastic the filaments is. Combine this information with a filament management tool like RollingUp and you should have no problem finding out what that mystery filament is!

More information about individual plastics can be found throughout the Plastics section of

Other Resources

Prospector - Plastics

SPI Resin Identification Codes