What Was Happening
A customer in Richmond TW9 contacted us about their Prusa MK3S+, which had stopped producing reliable first layers. Every print — regardless of the model, filament, or settings — was peeling off the bed within the first few layers. The customer described the first layer as looking “loose and stringy” rather than the smooth, squished lines they were used to seeing.
The printer had been working perfectly for over a year. The problem started approximately three weeks before the customer contacted us, shortly after they replaced the nozzle. The customer had tried cleaning the PEI bed with isopropyl alcohol, raising the bed temperature, using glue stick, and slowing the first layer speed — none of which made any difference. They suspected the new nozzle was faulty, but the extrusion itself appeared normal when printing in mid-air.
Our Diagnosis
We started by examining the first layer during a live test print. The nozzle was clearly too far from the bed — the filament was being deposited as loose, rounded threads rather than squished, overlapping lines. This pointed directly to a Live Z offset problem.
We checked the Live Z value and found it set to -0.580 mm. On a Prusa MK3S+ with a standard 0.4 mm nozzle, typical Live Z values range from -0.600 to -1.200 mm depending on the specific hardware. A value of -0.580 mm was almost certainly too high (too far from the bed), which explained why the first layer wasn’t being pressed into the surface firmly enough to adhere.
The customer had replaced the nozzle but had not re-run the first layer calibration afterwards. A new nozzle — even the same model and size — can have a slightly different tip geometry and protrusion length. This changes the relationship between the nozzle tip and the PINDA inductive probe that measures the bed height. The old Live Z value, calibrated for the previous nozzle, was no longer correct.
We also inspected the smooth PEI sheet. After more than a year of regular use, the surface had lost much of its texture. PEI sheets have a microscopic surface roughness that provides mechanical grip for the first layer. Over hundreds of prints, this roughness wears down — the sheet looks clean but has become too smooth to grip effectively. Cleaning with IPA removes oils but doesn’t restore worn surface texture.
Finally, we tested the filament moisture level. The PLA spool had been stored open on the printer for several weeks. A short extrusion test showed occasional small bubbles and a slightly rough surface texture — signs of moisture absorption. Wet filament produces micro-bubbles during extrusion that weaken the first layer bond.
How We Fixed It
We replaced the worn smooth PEI sheet with a new one. The new sheet was wiped with isopropyl alcohol to remove any manufacturing residue before use. The spring steel plate was re-seated on the magnetic bed and checked for flatness — it sat evenly with no warping at the corners.
Next, we recalibrated the PINDA probe height. On the MK3S+, the bottom of the PINDA probe should sit approximately 1 mm above the nozzle tip when the nozzle is touching the bed. We adjusted the probe position to match this specification using the Prusa calibration card.
With the probe correctly positioned, we ran the Prusa first layer calibration routine. This prints a single-layer zigzag pattern whilst the user adjusts the Live Z value in real time. We lowered the Live Z from -0.580 mm to -0.720 mm — the point where the first layer lines were gently squished together with no gaps between them and a slightly textured but not transparent top surface. This is the correct first layer appearance for PLA on a smooth PEI sheet.
We then dried the customer’s PLA filament in a filament dryer at 50°C for six hours. After drying, a test extrusion showed clean, smooth filament with no bubbling — confirming the moisture had been removed.
With all adjustments complete, we printed a test model — a multi-part mechanical assembly the customer was working on. The first layer went down perfectly: smooth, firmly adhered, with consistent line width across the entire bed. The full model printed without any warping or lifting at the corners.
The Result
The Prusa MK3S+ was producing perfect first layers again within minutes of the calibration. The customer’s multi-part PLA mechanical assembly — which had failed repeatedly over the previous three weeks — printed successfully in full. Every part had consistent dimensional accuracy and clean bottom surfaces. The customer was able to snap-fit the parts together as designed, which confirmed the dimensional accuracy was back within tolerance. The entire repair took approximately two hours onsite, including the filament drying time.
We also saved the updated calibration values so the customer could quickly re-check first-layer behaviour after future nozzle changes.
Why This Happens
The Prusa MK3S+ uses an inductive PINDA probe to measure the distance to the steel build plate. The probe doesn’t measure the distance to the nozzle tip — it measures to the plate surface. The Live Z offset bridges the gap between what the probe measures and where the nozzle actually is. When the nozzle is replaced, the tip position changes by a fraction of a millimetre. This small change is enough to turn a perfect first layer into one that doesn’t stick.
Most Prusa users learn the first layer calibration routine once during initial setup and then forget about it. The printer works well for months or years, and when they eventually replace a nozzle, they don’t realise that the calibration needs to be repeated. The Prusa documentation does mention this, but it’s easy to miss, and the symptom — sudden loss of bed adhesion — doesn’t obviously point to the nozzle change as the cause, especially when the new nozzle appears to extrude normally.
PEI sheet wear compounds the problem. A new PEI sheet is forgiving — it grips well even if the Live Z is slightly off. A worn sheet has much less margin for error, so a small Live Z discrepancy that would have been fine on a new sheet becomes a print failure on a worn one.
Local Help in Richmond TW9
We provide 3D printer setup and calibration for customers in Richmond TW9 and surrounding areas. Prusa printers are well-built machines, but they still need periodic recalibration — especially after hardware changes. We can visit onsite or you can bring the printer to our Putney base, which is a short trip from Richmond via the A316.
If adhesion drops again, we can run a quick onsite first-layer recalibration rather than waiting for repeated failed prints.
Prevention Tips
- Re-run the first layer calibration every time you change the nozzle. This is the single most important maintenance step on a Prusa printer. A new nozzle always changes the Live Z offset, even if it’s the same brand and diameter.
- Replace the PEI sheet when cleaning no longer restores adhesion. A smooth PEI sheet typically lasts 6–12 months of regular printing. If IPA and occasional warm water washes stop working, the surface is worn — not dirty. Consider switching to a textured PEI sheet, which lasts longer and provides better grip for most materials.
- Store filament in sealed bags with desiccant when not in use. PLA absorbs moisture from the air within days, especially in humid environments. Wet filament weakens first layer adhesion and degrades print quality throughout the entire model.
- Keep the PINDA probe height at 1 mm above the nozzle tip. Check this occasionally — the probe can shift if the mounting screw loosens. An incorrect probe height throws off the entire bed levelling mesh and Live Z offset.
- Print a first layer test square after any hardware or firmware change. It takes two minutes and catches calibration drift before it ruins a long print.