Chapter 3 · Understanding FDM Printing Filaments
With the base materials in mind, we can turn to an actual spool of filament. The material name is only a starting point: diameter, net weight, printing recommendations, performance information, and product condition all affect your choice.
3.1 Product Positioning and Key Specifications
A spool of 3D printing filament has two roles:
For the user, it is a product that can be loaded into a printer and used to make things.
For the manufacturing process, it is an input material with diameter, mass, material, and processing requirements.
To understand filament, start with the basic specifications, then consider processing and performance:
1. Filament Diameter
This is the diameter of the filament strand and the first specification to check for printer compatibility.
1.75 mm is a common desktop size and must match the printer’s feed system and configuration.
2.85 mm is also used on some machines and should not be interchanged casually with 1.75 mm. A larger diameter does not automatically mean higher precision or faster printing; actual capability still depends on the equipment, hotend, and process.
2. Package Size and Weight
Net weight is the mass of the filament itself, excluding the spool and packaging. Common sizes include 1 kg, 500 g, and 200 g. At the same net weight, filament length also varies with density and diameter, so spools of different materials need not contain the same length.
Gross weight includes the filament, spool, and packaging. Distinguish gross from net weight when comparing purchasing, shipping, and material usage, and use the values on the product label.
Spool-free refills reduce repeated use of spools but require a compatible reusable spool and correct assembly.
3. Material Properties
This refers to the filament’s basic chemical composition, such as PLA, PETG, ABS, or TPU. Different materials shape properties such as strength, toughness, and temperature resistance. See the previous chapter for more detail.
4. Technical Specifications
To help users work with a product, specification sheets commonly provide two types of technical information:
Printing parameters guide process settings, such as nozzle temperature, bed temperature, and print speed. Recommended values are starting points; adjust them for the printer, model, and goal. Chapter 4 explains this further.
Performance data describes the physical behavior of a material or test specimen, such as tensile, flexural, and impact properties. First establish whether the data concerns raw material, injection-molded specimens, or FDM-printed specimens. Results obtained with different methods, orientations, or conditions cannot be ranked directly against each other.
The following sections explain these measures individually, rather than judging a material solely by how high or low a number is.
1. Melt Flow Index
What is it? The commonly used term “melt flow index” usually refers to melt mass-flow rate, or MFR: the mass of material flowing through a standard die in 10 minutes at a specified temperature and load. It is commonly expressed in g/10 min.
What does it tell you? It describes flow under the specified test conditions. Check the temperature, load, and method before comparing results.
Under comparable conditions, a higher value generally indicates easier flow. However, better flow does not directly mean faster printing, nor can it alone predict bridging, stringing, or interlayer bonding.
A lower value does not automatically mean greater stability or strength either. Actual printing also depends on the hotend, temperature, flow rate, cooling, and other factors.
Material development therefore needs both flow testing and real printing validation; one melt flow value cannot replace an overall process assessment.
2. Tensile Strength
What is it? The maximum stress a material withstands during a tensile test taken to fracture, expressed in MPa.
What does it tell you? It helps assess resistance to tensile failure. The load a real part can carry also depends on its shape, print orientation, loading mode, and service environment.
3. Flexural Strength and Flexural Modulus
What are they? Flexural strength is the maximum stress a material withstands under bending before fracture or a specified deformation, expressed in MPa. Flexural modulus describes resistance to elastic deformation in bending, also expressed in MPa.
What do they tell you? Flexural strength describes a specimen’s behavior under a bending load, while flexural modulus more directly reflects resistance to bending deformation in the elastic range. Strength, stiffness, and toughness are different measures.
A higher flexural modulus generally means less elastic bending under the same conditions.
A lower flexural modulus generally means easier bending. Suitability for a purpose also depends on recovery after deformation and other properties.
For example, a rigid housing is not necessarily drop resistant, and a flexible, tough protective part is not necessarily suitable for holding a precise shape. Names such as PLA and ABS alone cannot establish which is always stronger.
4. Impact Strength
What is it? A material’s resistance to fracture under a sudden impact load, expressed in kJ/m².
What does it tell you? It measures toughness or resistance to impacts and drops.
Under comparable test conditions, better impact performance helps assess resistance to sudden impact, but it does not directly mean that finished parts of every shape will be more drop resistant.
When comparing impact data, check the specimen, notch, orientation, and test method. Printed parts also need validation appropriate to their intended use.
3.2 Product Condition and Common Irregularities
Filament can change condition or develop irregularities during production, transport, and use. The following explains several phenomena worth recognizing, but a printing problem does not establish one unique filament-related cause.
1. Packaging and Moisture Absorption
Sealed packaging helps reduce moisture exposure during transport and storage. A damaged bag may increase exposure, but a loose package does not prove the filament is damp: moisture uptake also depends on time, environment, and material. Bubbles, popping sounds, and surface irregularities during printing can provide clues for further inspection.
Drying reduces moisture already present; sealing and desiccant mainly help limit renewed absorption. Whether to dry, and the temperature and duration, depend on the specific product and spool requirements. If problems persist after appropriate drying, continue checking settings and equipment.
2. Uneven Diameter
Locally thick, thin, or oval filament can affect feeding or extrusion. Evaluate diameter against the product specification. Measure at several positions and in different directions, and retain the irregular section for further investigation.
3. Uneven Color and Perceived Color Differences
Actual color variation may relate to production conditions, but apparent differences may also result from print gloss, geometry, lighting, or screens. Identify what is being compared before judging the cause. See PART 2, Chapter 5 for the full method.
4. Contamination and Residue
Foreign matter or some residues can interfere with extrusion, causing clogs or visible defects. However, a failure to extrude can also originate at the spool, feed path, or equipment. See PART 2, Chapter 2 for the detailed checks.
These points connect material condition with printing behavior. Complete troubleshooting belongs in PART 2; symptoms alone should not be used to confirm or rule out a filament irregularity.
3.3 Material Modification and Product Differences
The base material determines some physical properties, but formulation and processing improvements can still differentiate products. Understanding modification helps explain why two products both called PLA can have different finishes and printing behavior.
Modification mainly takes two directions:
Appearance and Texture: Different additives and colorants can change the visual and tactile effects of filament. For example, a PLA base can be used to produce matte PLA with a refined, non-glossy finish, or silk PLA with a distinctive metallic sheen and smooth feel. These options greatly expand a model’s decorative and artistic possibilities.
Function and Processing: Formulations can be adjusted for flow, toughness, heat resistance, or other goals. High-speed ranges, for example, may be optimized around a particular processing window, but usable speed still depends on the hotend, nozzle, layer height, and line width. A “high-speed” label alone cannot determine it.
Modification provides more choices, but it also calls for comparison of specific products: changing the appearance or one property does not guarantee that other properties stay the same.
PART 3 will explore the visual use of color, gloss, and different finishes further.
3.4 Why Filament Is Usually Sold by Weight
Slicing and printing involve filament length and material mass, but filament is commonly sold by net weight. Understanding why requires connecting mass, length, volume, and density.
Measuring by weight simplifies purchasing, production, packaging, and stock management. It is a common measurement approach, not a claim that one kilogram of every material provides identical value or prints the same number of parts.
1. Production and Management
Raw materials, production losses, and inventory can usually be accounted for by mass, and weighing the finished product makes it easier to check net content. Weighing is more direct than measuring the length of an entire spool.
2. The Physical Relationship
Filament mass = length × cross-sectional area × density. At the same diameter and net weight, a denser material will generally provide a shorter length of filament.
Equal net weights of PLA and ABS therefore need not have the same length; the actual difference depends on formulation, density, and dimensions.
3. Use and Cost
The same geometry and sliced structure require approximately the same material volume, but mass changes with density: mass = volume × density.
When comparing print costs, consider not only price per kilogram but also actual material volume, support and flushing waste, print time, and success rate.
This connects filament specifications with print results: the price and weight of a spool alone do not establish the final cost of a finished part.
