Almost all polymer printing uses thermoplastics, but the way each process handles the polymer differs enough that the same nominal material behaves differently depending on which machine made the part. Understanding what each process does to the polymer explains most of the differences in strength, surface and accuracy that buyers notice.
Extrusion: melting and re fusing
The filament is melted and deposited, and each bead fuses to its neighbours while partly cooled. The polymer never reaches a fully homogeneous state, so the part is a stack of welded beads rather than a solid. That is the source of both the low cost and the anisotropy, and no amount of parameter tuning removes it entirely.
Sintering: coalescing powder
The chamber is held just below melting and the laser adds only enough energy to fuse the powder locally. Particles coalesce over the whole cross section, so the resulting part is far more homogeneous than an extruded one and behaves similarly in every direction. It is also porous at a microscopic level, which is why sintered parts absorb dye so well and are not watertight without sealing.
Photopolymers are not thermoplastics
Resin printing does not use thermoplastics at all. It cures liquid monomers into a thermoset network, which cannot be remelted and behaves differently in creep, toughness and long term stability. That is a genuinely important distinction when someone specifies a printed part in a nominally familiar material and gets photopolymer behaviour.
Choosing on requirement
For cheap shape checks, extrusion. For functional parts that must behave, sintering. For fine detail and appearance, photopolymer. For heat resistance beyond common filaments, high temperature resins or specialist filaments in a heated chamber. And whenever the requirement is genuinely demanding, ask what the process does to the polymer rather than reading the material name.
Questions people ask about 3d printing thermoplastic
Are all 3D printed plastics thermoplastics?
No. Extruded and sintered parts are thermoplastic. Resin printed parts are thermosets formed by curing liquid monomers, and they behave differently.
Why does the same material behave differently between processes?
Because the processes leave the polymer in different states: welded beads in extrusion, coalesced particles in sintering, a cured network in resin printing.
Which process gives the most consistent properties?
Powder sintering, because the cross section coalesces rather than being welded bead to bead, so parts behave similarly in every direction.
Are sintered parts watertight?
Not without sealing. The microstructure is slightly porous, which is also why they take dye so readily.