Tell them apart
By color
Amorphous PEEK looks translucent brown, because light passes through the disordered structure. Semi-crystalline PEEK is an opaque beige, because the crystals scatter light.
Materials article
Amorphous vs Semi-Crystalline PEEK
PEEK is sold on its heat, chemical, and wear resistance. Those numbers assume the plastic is semi-crystalline. The same polymer can also solidify amorphous, and an amorphous PEEK part is a noticeably weaker material. Which one you get depends on how the part was cooled, and on a 3D printer that mostly comes down to how hot the build chamber is.
What crystallinity means
A crystal has an ordered, repeating structure and a sharp melting point, like ice or salt. An amorphous material, like glass, has a disordered structure and softens gradually as it heats up.
Polymers are long chains, so they never line up perfectly. In some regions the chains fold and pack tightly into crystals. In between, they stay tangled and disordered. A polymer with both is called semi-crystalline. Victrex notes that a polymer is never fully crystalline, and that high-performance polymers above about 25% crystallinity behave like semi-crystalline materials. PEEK is typically 25% to 45% crystalline, depending on how it was processed.
How the two behave
The crystals act like reinforcement inside the softer amorphous material. That matters most above the glass transition temperature (Tg), which is about 143°C for PEEK. Below Tg, both forms are rigid. Above it, the amorphous regions turn rubbery, and only the crystals keep a semi-crystalline part holding its shape. PEEK melts at about 343°C.
| Amorphous | Semi-crystalline |
|---|---|
| Softens gradually as it heats | Sharp melting point |
| Easy to form | Hard to form |
| Transparent | Opaque |
| Poor chemical resistance | Good chemical resistance |
| Bonds well with adhesives | Hard to bond with adhesives |
| Poor fatigue resistance | Good fatigue resistance |
| Poor wear resistance | Good wear resistance |
For PEEK specifically, AON3D gives the service temperature gap: semi-crystalline PEEK can be used up to about 240°C, while amorphous PEEK starts to crystallize and deform above about 150°C. Tensile strength, stiffness, creep, and chemical resistance all drop with crystallinity.
Chemical resistance
Crystals block attack
Tightly packed crystals are harder for solvents and fuels to get into. That is why PEEK is used for seals, pump parts, and connectors in oil and gas, and why the amorphous form gives much of that up.
Wear and fatigue
Bearings and gears
Victrex lists wear and fatigue resistance as semi-crystalline strengths. Gears, thrust washers, and seal rings rely on the crystalline form.
Cooling decides the result
Crystals only form while the plastic sits between Tg and its melting point, which gives the chains time to fold into order. Cool it fast and they freeze in place disordered. Victrex puts numbers on this:
- Very fast cooling, above about 700°C per minute, gives amorphous PEEK. Victrex does this on purpose for some clear film, which can then be formed at about 180°C.
- Injection molding in a hot mold, about 170–200°C, gives parts that are about 35% crystalline. A mold that is too cold leaves a darker amorphous skin on the part.
- Annealing in an oven at about 230°C can raise crystallinity to around 40%.
- Laser-sintered PEEK (SLS) can reach 45% or more, because the whole powder bed stays hot.
What this means for 3D printed PEEK
An FDM nozzle lays down a thin bead of melted PEEK, often at 365–440°C, into air that is much cooler. That bead cools very quickly. Whether it has time to crystallize depends on the chamber and bed around it.
Hot chamber
Crystallizes as it prints
AON3D recommends a chamber and bed temperature of 133°C or higher, close to PEEK's Tg. Held there, each layer crystallizes during the build, which is called in-situ crystallization. The part comes off semi-crystalline with its full properties and little distortion.
Cooler chamber
Prints amorphous
Below that, the PEEK freezes amorphous. 3D4Makers publishes an amorphous profile at a 75°C chamber, 110°C bed, and 425°C nozzle, with 0.15 mm layers and 100% infill. Amorphous PEEK shrinks less while printing, which makes large parts easier to keep flat.
The cost
Weaker layers
AON3D warns that a cold print also leaves a big temperature gap between the new layer and the one below. The result is warping, residual stress, and a weaker bond between layers, so strength in the build (Z) direction drops sharply.
Annealing an amorphous print
An amorphous print can be crystallized afterward by heating it between Tg and the melting point. This is sometimes called cold crystallization, and 3D4Makers recommends it after printing to get the part's mechanical and thermal properties back.
There is a catch. The crystals are denser than the amorphous plastic, so the part shrinks as it crystallizes. AON3D notes that this distortion can be significant and works against the near-net shape you printed. If a part will be annealed, plan for it:
- Expect the part to shrink and possibly warp during the heat treatment.
- Leave stock on critical faces and bores, and machine them to size after annealing.
- Keep wall thickness even, so the part shrinks evenly.
Which one does your part need?
Semi-crystalline
Pick it for
Parts that see fuel, oils, or other chemicals. Heat above about 150°C. Wear, sliding contact, or repeated loading, such as bearings, gears, and seals. Aerospace brackets and interior parts where strength per weight counts.
Amorphous
It can work for
Fit checks, fixtures, and prototypes that stay below about 150°C and do not see harsh chemicals. Large prints where low warping matters more than peak strength. Parts that will be bonded with adhesive.
On your order
Say what the part sees
If the part has to meet a heat, chemical, or wear requirement, tell us when you place the order. That tells us whether the part needs the semi-crystalline properties, and whether critical dimensions should be finished after heat treatment.
Sources
- Victrex: Polymer Crystallinity Explained. Crystallinity basics, the property comparison, and molding, cooling, and annealing figures.
- AON3D: PEEK, Best 3D Printing Temperatures to Maximize Crystallinity. Chamber temperature for in-situ crystallization, service temperatures, color, and annealing distortion.
- 3D4Makers: PEEK Filament Print Profile, Amorphous. The amorphous print settings and post-print annealing.