Crystallinity is set in the tool, not the pellet
PEEK is semi-crystalline, and how much of it crystallises depends on how long the melt spends above its glass transition while it cools. A mold held well above that transition gives the chains time to order; a cold mold freezes a skin that is largely amorphous. The amorphous skin is weaker in chemical resistance, lower in stiffness and not finished moving — it can crystallise later, in service or in an autoclave, and shrink as it does. Small parts have the least thermal mass to hold heat, so they sit on the wrong side of this more easily than large ones.
Unfilled shrinks evenly. Filled does not.
Unfilled PEEK shrinks about the same in every direction, and more than an amorphous resin does. Add 30% glass or carbon fibre and the fibres line up with the flow and refuse to shrink along it, so the part shrinks several times more across the flow than along it. On a micro part that is not a correction factor to average out — it is two different materials depending on which way a dimension runs relative to the gate. Gate position decides which of your dimensions gets the easy direction.
Thin walls freeze before they pack
PEEK is viscous and it freezes fast against steel, so a thin section far from the gate can solidify before packing pressure reaches it. The gate itself is usually the thinnest section in the system and freezes first, which ends packing for the whole part. That is why gate size and location are settled in the tool design for PEEK rather than tuned at the press, and why a gate-seal study belongs in the first sampling run.
Filled grades wear the steel
Glass and carbon fibre at melt temperature are abrasive. Gates, runner corners and any sharp detail they flow past wear first, and at micro scale a gate that has opened up by a few microns changes when it freezes. Filled PEEK is a steel-selection decision made before the tool is cut, not a maintenance note afterwards.
What a medical-grade PEEK certificate means
Implant and medical grades of PEEK are sold with testing behind them — biocompatibility to ISO 10993, USP Class VI, implant-grade specifications. That testing is on the grade as supplied. It does not transfer to a molded part on its own: processing, colourants, mold release and cleaning all sit between the pellet and the device, and qualifying the finished part for patient contact is the device owner’s job under their own quality system. What a molder can do is keep the grade traceable and the process documented, so the qualification has something to stand on.