Repair guide · Prusa Nextruder · Oct 21, 2026

Prusa Blob of Death on the MK4S and CORE One: How to Get It Off, What to Buy, and How to Stop the Next One

You came back to a pink fist where the nozzle used to be. On a Nextruder printer that is usually a $6 to $65 repair, not a new print head. Here is Prusa's own removal method, which runs hotter than most guides, the parts to buy at each stage of damage with current prices, and the habits that stop the next one.

By Dreaming3D · Carmel Valley, San Diego · Covers the MK4S, MK3.9S and CORE One family

What a blob is, and why it's usually not a clog

Prusa's description is simple: the first layer lets go of the sheet, sticks to the moving hot nozzle, and the printer keeps extruding into it. Each pass adds plastic to the lump instead of the part, and the lump wraps the nozzle, the heaterblock, the leads and eventually the shroud. Unlike the spaghetti monster, which tends to happen mid-print, Prusa notes that a blob often starts early.

That timing is the whole story. The chart above is our arithmetic for a 0.4 mm nozzle laying 0.45 × 0.20 mm lines. At a first-layer pace of 40 mm/s that's 3.6 mm³ of plastic a second; at an infill pace of 120 mm/s it's 10.8 mm³. If it all sticks, ten minutes at infill pace is about 8 g of PLA, a ball roughly the size of a US quarter. An overnight print can, in the worst case, put 386 g in the same place. Real blobs lose some plastic to stringy waste, so the exact gram count matters less than the slope: the bill grows with every minute nobody looks.

There's a second, rarer kind of blob that has nothing to do with the bed: plastic leaking from the hotend itself and building from the top down. On older Prusa hotends, forum regulars usually trace it to a nozzle that wasn't seated properly after a change. If yours grew from above the nozzle tip while the part stayed stuck to the sheet, treat it as a hotend assembly problem, not a first-layer one, and follow Prusa's nozzle guide for your exact printer. Our nozzle explainer covers how nozzle choice changes everything downstream.

Before you touch it: read the screen

Don't pull on a cold blob. The heater and thermistor leads are inside it, and cold PLA grips them like epoxy. Look at the display first, because it tells you whether the nozzle can still heat at all:

Photograph it from the front and both sides before you start. If you end up calling a repair shop, us included, those photos tell us which parts to bring.

How to remove it: Prusa's method, step by step

Prusa's video shows an older MK3, but it says the same principles apply to all of its FDM printers. We've added the Nextruder-specific details from Prusa's MK4S and CORE One service articles.

1. 1

   Read the screen before you touch anything

   Note the filament and the temperature it was printing at, and photograph the blob. If the printer shows a hotend MINTEMP error (#26208 on the MK4S, #31208 on the CORE One), Prusa says the thermistor is already broken; a hotend preheat error (#26202 / #31202) means the heater cable is. With either one the nozzle can't heat, so skip to a hot-air gun on low and keep it off the printed parts around the hotend.

2. 2

   Give yourself room

   Use Control, then Move Axis, to bring the hotend somewhere you can reach and see underneath. On a CORE One, open the door; on an MK4S, clear the bed so nothing hot drops onto the sheet.

3. 3

   Preheat about 30 °C above the print temperature

   Set the nozzle in the temperature menu, not the preheat presets. Prusa's numbers: 250 °C for a PLA blob, 280 °C for PETG or ABS. The point is to soften the whole lump, not just the skin.

4. 4

   Wait at least five minutes

   Let it soak at temperature. Small blobs often slide off on their own. Big ones need the time for heat to reach the outside.

5. 5

   Switch the printer off, then peel

   Prusa's instruction is to turn the printer off before the pliers go in, to avoid a short. Work in sections and pull away from the heater and thermistor leads, never toward them. The heater cable's insulation stops before the wire enters the cartridge, which is exactly where blobs tear it.

6. 6

   Brush, then inspect where the wires leave the block

   Clear the last film with a small brass brush. Look hard at the heater and thermistor leads where they exit the heaterblock, and at the part-cooling shroud edge. On an MK4S, open the fan door, loosen the four M3x5rT screws and slide the print fan assembly aside to see behind it.

7. 7

   Replace only what failed

   Torn lead or a MINTEMP / preheat error: swap the whole Nextruder hotend assembly ($50.99 from Prusa) rather than splicing a 40 W heater. Melted shroud edge: replace the shroud. Plastic up in the heatsink, or load cell errors after cleanup: that's the heatsink and load cell.

8. 8

   Fit a new sock cold, tell the firmware, run the selftest

   With the printer off and cold, push the silicone sock on from below and stop if it resists; Prusa warns that forcing it can bend the heatbreak. Then set Settings, Hardware, Nextruder Silicone sock to ON, and run the selftest before the next print.

What to buy, stage by stage

Where the plastic reached decides the bill, which is why the chart above stacks costs instead of listing every part. Prices are Prusa's US store unless noted, before tax and shipping, read between Oct 19 and Oct 21, 2026.

Why we'd replace the hotend rather than splice a lead

Prusa points out that the heater cable's insulation ends before the wire enters the cartridge, and that bare stretch is exactly what a blob grabs. You can splice it, but a complete assembly with a new nozzle, heaterblock, 40 W heater and thermistor is $50.99 from Prusa, and a fresh factory joint on a heater beats a field repair. When you order, check the nozzle variant: assemblies are sold with either the standard brass Prusa Nozzle or the high-flow brass CHT, and the MK4S and CORE One ship with the CHT. If your old CHT nozzle came through clean, move it over.

Prusa direct or a US reseller?

Prusa's own price is the lowest we found, and its store lists a 1 to 3 business day preparation time before shipping. US resellers charge more ($64.99 to $78.48 at PartsBuilt, $70.99 at Printed Solid) but stock domestically, which matters when the printer is your income. If you want the printer back this week, the reseller premium is usually cheaper than the downtime.

While it's apart: the single-color extruder refresh

If the hotend is out anyway and the printer has a few thousand hours on it, this is the cheapest moment to replace the extruder's wear parts too. The planetary gearbox assembly and its ring are sold as a set, and you want both, because mixing a new gear with a worn ring is asking for skipped steps. These totals are for a CORE One or CORE One+ running one filament at a time:

The HT Hotend Upgrade is a CORE One family part only: Prusa and its resellers list the CORE One, CORE One+, CORE One+ Gen 2, CORE One L and CORE One L+, not the MK4S and not INDX machines. It reaches 400 °C, swaps without tools, and prints PLA and PETG fine, so it can stay installed. Our $184.26 figure is Prusa's US store on Oct 19; All3DP reported $199.99 at launch, and some European resellers show it backordered into December. Buy it for PEEK-CF, PPSU or PPA-CF work, not as a blob fix. If you're also due for bearings, belts and pulleys, Prusa's CORE One Maintenance Kit is $199.00.

How to stop the next one

Every one of these is free or close to it, and they're in the order we'd do them.

1. Clean the sheet before every print

Prusa's first rule is a clean print surface every time, because a first layer that's only half stuck is how blobs start. Fingerprints from pulling the last part off are enough. Our FDM cleaning and maintenance guide covers sheet care alongside the rest of the routine.

2. Watch the first ten minutes, or have a camera do it

Prusa's advice is to check the print now and then and pause the moment something looks wrong. Given that blobs often start early, the first layers are the ones that count. If you start overnight prints, start them when you can watch the opening minutes, and use Prusa Connect with a camera so a phone glance replaces a walk to the garage.

3. Fit the sock, then flip the switch

A $5.59 set of three is the cheapest insurance in this post. Prusa is clear that it won't stop a blob, but a sock-covered heaterblock sheds plastic instead of bonding to it. After fitting one, go to Settings, Hardware, Nextruder Silicone sock and set it to ON.

4. Leave stuck filament detection on

Since firmware 6.0.0, the MK4/S, MK3.9/S, XL and CORE One family use the load cell to notice filament that's stuck and not moving. Prusa warns that turning it off raises the risk of a clog. It's not a blob detector, and neither is crash detection, which reacts to collisions; neither one checks whether your part is still on the sheet. Keep both on, but don't treat either as a babysitter.

5. Give small footprints help

Tall parts standing on a small contact patch are the ones that let go mid-print and get dragged into the nozzle. A brim or a few mouse-ear discs costs grams of plastic and saves the hotend.

6. On a CORE One+ Gen 2, let the wiper work

The Gen 2 refresh added an integrated nozzle wiper that clears stray filament before a print starts, so yesterday's ooze doesn't get dragged through the first layer. We covered what else changed in our breakdown of Prusa's "+" refresh.

San Diego notes

• Marine layer filament. Coastal garages run humid for much of the year. Wet PETG pops and strings, and a first layer full of steam bubbles doesn't grip well. Dry it before long prints.
• Garage temperature swings. An MK4S on an open bench in a garage cools fast at night; a part that warps and lifts at hour three is a blob waiting to happen. The CORE One's enclosure helps here, which is part of the case we made in our Core One L versus Bambu Lab comparison.
• Parts come by mail. Nextruder parts arrive by mail, so unless you keep spares, every blob starts with a shipping wait. A spare sock set and a spare hotend assembly ($56.58 together from Prusa) are worth more than they cost if the printer earns its keep.

When to call us instead

Removing a blob is a patient-hands job most owners can do. Call a repair shop when the plastic got inside the heatsink or the extruder gearbox, when load cell or selftest errors persist after a clean hotend swap, or when a lead tore inside the cable bundle rather than at the block. We fix Prusa printers on-site across San Diego County, and by video or mail-in everywhere else. Our brand-by-brand failure guide covers what else goes wrong on Prusas once they've got some hours on them.

FAQ

What is the Prusa "blob of death"?

It's a mass of plastic that builds up around the hotend. Prusa's explanation is that the first layer detaches from the sheet, sticks to the moving nozzle, and keeps growing because the printer keeps extruding. It often starts early in a print. Prusa calls it an extruder blob; the community calls it the blob of doom or blob of death.

What temperature should I heat the nozzle to remove a blob?

About 30 °C above the temperature the blob formed at. Prusa's examples are 250 °C for PLA and 280 °C for PETG or ABS. Hold it there for at least five minutes, then switch the printer off before you start pulling plastic with pliers.

What parts do I need to fix a blob on a Prusa MK4S or CORE One?

Often just a silicone sock ($5.59 for a set of three from Prusa). If the heater or thermistor lead tore, or the screen shows a MINTEMP or preheat error, buy a complete Nextruder hotend assembly ($50.99 from Prusa; it includes the nozzle, heaterblock, 40 W heater and thermistor). Add a part-cooling shroud if its edge melted, and a heatsink with load cell only if plastic reached it or load cell errors persist.

Is Prusa's $149 Spare Parts Bundle compatible with the MK4S or CORE One?

No. Prusa sells it for the MK4 and MK3.9, and its own compatibility list marks the MK4S, MK3.9S and the CORE One family as not compatible. Buy the individual Nextruder parts instead.

Does a silicone sock prevent blobs?

No. Prusa says the sock doesn't prevent extruder blobs, but it makes one easier to remove if it happens. After fitting it, set Settings, Hardware, Nextruder Silicone sock to ON so the firmware accounts for it.

Is blob damage covered by Prusa's warranty?

Prusa's knowledge base says damage from a blob isn't covered, because it isn't a hardware or software fault, and that it can be prevented by calibrating the printer and stopping the print when filament starts sticking to the nozzle.

My printer shows a MINTEMP or preheat error after the blob. What broke?

Prusa's guidance: a MINTEMP error means the hotend thermistor is already broken, and a preheat error means the heater cable is. On a Nextruder, the practical fix for either is a new hotend assembly, which includes both.

Will stuck filament detection or crash detection stop a blob?

Don't count on it. Stuck filament detection uses the load cell to notice filament that isn't moving, which is a clog check, not a blob check. Crash detection reacts to collisions. Neither watches whether your part is still on the sheet; a camera and the first ten minutes of your attention do that.

Can Dreaming3D fix a Prusa blob in San Diego?

Yes. We repair Prusa, Bambu Lab, Creality and Elegoo printers on-site across San Diego County, and help by video or mail-in elsewhere. Text a photo of the blob to 858-342-6984 before you start pulling on it, or use the repair request page.

Your turn

Got a blob photo you're proud of, or one you're not? Send it to us on Instagram at @dreaming3dprinting and tell us what let go first. The best ones go in our next update.

Related posts

• Prusa's "+" refresh: what changed on the XL+, CORE One+ Gen 2 and CORE One L+
• 3D printer repair in San Diego: every brand, every failure mode
• How to clean and maintain your FDM 3D printer, 2026
• We fix 3D printers in person in San Diego, and by video or mail everywhere else

Sources

1. Prusa Knowledge Base: Extruder blob
2. Prusa Knowledge Base: Extruder blob (Polish edition, warranty wording)
3. Prusa Knowledge Base: Nextruder silicone sock (CORE One, MK4/S, XL)
4. Prusa Knowledge Base: Stuck filament detection
5. Prusa Knowledge Base: Crash recovery, repeated crash
6. Prusa: Nextruder silicone sock set
7. Prusa: Spare Parts Bundle (for MK4), compatibility list
8. Prusa: MK4/S spare parts (nozzles, idler spring)
9. Prusa: CORE One L spare parts (CHT nozzle)
10. Prusa: HT Hotend Upgrade
11. Prusa: CORE One Maintenance Kit
12. Prusa: Extruder idler
13. PartsBuilt: Nextruder hotend assembly, 0.40 mm brass
14. PartsBuilt: Nextruder CHT hotend assembly (high flow)
15. PartsBuilt: MK4/MK4S parts (shroud, heater)
16. PartsBuilt: Planetary gear kit (PG assembly + ring)
17. Printed Solid: Nextruder Hotend brass
18. All3DP: Prusa's HT hotend for the Core One series
19. 3DJake: HT Hotend Upgrade compatibility and stock

Tags: Prusa, MK4S, CORE One, Nextruder, blob of death, extruder blob, hotend repair, silicone sock, 3D printer repair San Diego, Prusa parts