Heat-Set Inserts in Carbon Fiber Nylon: Hole Sizes and Temps That Actually Hold

The bracket printed clean. You grabbed a screw, and the threads stripped before the fastener even seated. Tapping carbon fiber nylon works exactly once: the thread is cut into layer lines, and layer lines lose to a steel screw every time. If your parts get assembled, disassembled, and torqued, brass heat-set inserts are the fix. Here is how we do it in the shop.

Why Carbon Nylon Needs Inserts

A tapped hole in PA-CF puts the thread right on the layer boundary, which is the weak axis of any FDM part. Carbon-filled nylon is stiff too, so instead of flexing and seating, a hard screw just strips the soft thread. A brass insert replaces that weak thread with metal and spreads the load across the surrounding walls. You also get a thread that survives real service: parts can come apart and go back together without betting on luck.

Hole Size Is the Number That Matters

Start with a hole 0.2–0.4 mm smaller than the insert's outer diameter, then test on a spare corner before you commit. CF nylon shrinks as it cools and every roll prints a hair different, so the first test hole tells you more than any chart. Insert drops in too easy? It will spin the first time you torque it — go smaller. You have to hammer it in or the boss cracks? Open the hole up a few hundredths. If the printed hole comes out tight, drill it to size with a sharp bit instead of forcing the insert.

Iron Temperature and Press Technique

Use a soldering iron with an insert tip and run it around 250°C for PA6-CF and PA12-CF. Push straight down with steady pressure and do not twist — the knurl needs to melt its way in, not tear its way in. When the insert sits flush, hold it a second so heat soaks into the plastic around the knurl, then lift. A depth stop keeps inserts perpendicular, and a crooked insert is a failed part. Carbon-filled melt is stiffer than plain nylon, so if it fights you, nudge the iron up ten degrees instead of pushing harder.

Walls, Bosses, and Orientation

Give every hole at least three walls around it and design the boss at roughly twice the insert's diameter. Print holes upright where the part allows, so the layers wrap the hole instead of running straight through it. And remember that CF nylon is brittle where pure nylon bends: a slightly tight hole that pure nylon would shrug off will crack a carbon nylon boss. When in doubt, err on the loose side of the fit and let the knurl do the holding.

What We Use

Most of our insert-and-fastener parts run in TINMORRY PA6-CF — it takes a press without complaining and the brass seats cleanly. For parts that live in humid air, we switch to Inslogic PA12-CF, which soaks up less moisture and holds a tighter hole tolerance. Both spools ship with the exact settings we run in the shop.

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