Nothing ruins a Monday like opening the chamber and finding one solid brick where your parts should be. That’s SLS global melting, and one Fuse 1+ owner on the Formlabs forum fought it for months. Their printer had 1,900 hours on it and ran Nylon 12 at 50% refresh with a −1.5 °C bed offset.
What causes SLS global melting?
The printer thinks the bed is colder than it is, so it keeps adding heat. Formlabs’ melting guide lists the usual suspects: a contaminated IR sensor, a missing IR sensor O-ring on the Fuse 1+ 30W, a damaged build chamber plug, or a failing heater. A clouded sensor reads low, and the machine cooks the powder to compensate.
How do you tell which one it is?
Look at the brick. Formlabs splits it three ways. Melting in preprint that’s shiny and not stuck to the bed points to a dirty or badly seated IR sensor. Matte and welded to the bed points to the chamber’s electrical connection, so check the plug for bent or sunken pins. Melting late in a long print usually means the sensor clouded over during the job, and Formlabs adds the exhaust filter, fan and line to the cleaning list.
That last pattern matched the forum case. Their builds melted only after more than 1,500 layers.
What does replacing parts at random cost you?
A lot. The owner replaced the quartz tubes and frame, the recoater motor, gaskets, the optical cassette, the motor and temperature board, the IR sensor and its flow body. A tech came out and found no air leak. Formlabs ended up replacing the whole printer, and the root cause never came back to the forum. A second shop in the same thread swapped the air preheater, the RTD twice and the control board, and saw errors 258/3 and 261/3. Their repair service eventually blamed electrical noise.
Also worth knowing: another member pointed out that 100% fresh powder can melt on its own. Age a batch before you run diagnostic prints.
When should you stop troubleshooting?
Once the obvious items are clean and you’re ordering boards, you’re paying for downtime and parts at the same time. Start a log the first time it happens, with the layer number, print hours, chamber ID and refresh rate, plus photos of the sensor. That’s what gets a service plan to approve a tech visit or a swap instead of another round of mail-order parts.
Related on the Solvi blog: SLS Orange Peel: Why It Happens and How to Fix It; SLS Cooldown: How Hot Can You Unpack a Build?; SLS Powder Cleanup: How to Keep Dust Under Control.
Sources: Formlabs forum “Fuse 1+ Global Melting” thread; Formlabs: Melting (SLS); Formlabs: Cleaning the IR sensor.