3.3. Cleaning the PEI spring steel print sheet for proper adhesion

The removable spring steel print sheet is one of the Prusa Mk3 “quality of life” features that set it apart from lower and higher cost competitors. Three PEI surfaces are available:

A double-sided powder-coated textured PEI spring steel sheet. Prior to April 2018, the Mk3 could be ordered with this option, but it was withdrawn from sale until backorders could be filled. It is now readily available.
- A double-sided powder-coated satin PEI spring steel sheet. This item was recently added to Prusa’s stock and supply is improving.
A double-sided spring steel sheet with smooth PEI pre-applied with adhesive (stickers).

Note

These notes are based on my experiences with the Prusa i3 Mk3 and Artillery/Evnovo Sidewinder X1 printers. If you are using a different printer, please verify the hardware details are similar.

The PEI sheets for the Mk3 are a wonder. When properly cleaned and cared for, printed parts will stick firmly enough to get a solid print, yet pop off easily when cooled. Regardless of which surface you use, some basic upkeep and cleaning is required to maintain good adhesion.

3.3.1. Understanding the PEI sheet and adhesion

The first thing to understand is that PEI is very different than most other print surfaces. Do some searching on the internet and you’ll find hundreds of articles on using glue stick, hair spray, glass sheets, blue painter’s tape, sugar syrup, and a host of other “solutions” for improving bed adhesion. Ignore these. They are written for printers that require manual bed leveling, ship with warped parts, lack removable print surfaces, use rudimentary print surfaces, or suffer other shortcomings.

The one and only trick to PEI is to keep it clean. How do to this depends on just how contaminated the surface is. The biggest enemy is grease, and the biggest source of that grease is your fingers as you handle the sheet. No matter how you clean your sheet, try to avoid touching the PEI surface as much as possible.

There’s no magic to getting good 1st layer results and excellent bed adhesion with this surface. The only requirements are proper maintenance of the PEI surface and first layer (Live-Z) calibration to get the best results.

3.3.2. The riddle of adhesion

You’ve probably seen some of these guidelines before and may have wondered exactly why these simple-sounding steps work. Here’s an unscientific breakdown.

Isopropyl alcohol 3 and acetone 4 work as solvents 1 breaking apart grease molecules. These solvents do not completely dissolve grease, but do break it down far enough that it can be removed from surfaces easily.
The more concentrated the solvent, the more effective it will be at a given volume. Those little 70% isopropyl wipes do work… on a shiny new PEI sheet. They’re just not effective in dealing with larger amounts of accumulated grease. This is why 91%+ alcohol is recommended.
Acetone is even more effective, breaking up oxidation on the PEI surface, but can make the PEI surface brittle and prone to cracking if over-used.
The amount of grease that any solvent will effectively break down is directly proportional to the volume of solvent to grease. The more you apply, the more grease it can effectively break down. The more concentrated the solvent, the more grease a given quantity can break down.
Alcohol, acetone, and any other solution you apply directly to the bed is just going to move those broken-down grease and grime molecules around. Some will be picked up by wiping with a clean towel, but some will remain on the PEI sheet.
A dunk under the sink with Dawn dish soap (or your local equivalent) is the most effective method of cleaning. Dish soap is a surfactant 2 , not a solvent. It works in a fundamentally different way, bonding both grease and water molecules to wash away contaminants. The reason this is so effective is not because Dawn is “stronger”, but because there’s simply a much greater volume of Dawn and water and the grease is washed away from the PEI sheet. Hot water helps break up grease as well, enhancing the effect. Use water as hot as you can safely get it.
Windex, a wipe of finger grease, talcum powder or specialized release agents can be used to reduce adhesion when using hot, sticky materials that grip the PEI surface too firmly such as PETG. It is still a good idea to clean the PEI before a print to get an even surface, even if you will be applying a release agent for printing with sticky materials.
If you are printing high-temperature materials (e.g. PETG at 260C on a 90C bed), test to make sure it will release once cool without damaging the PEI surface. You may want to use a glue stick or other release agents for these materials. Test on a sacrificial PEI surface if possible. See over adhesion, below.

Note

IANAC - There’s a lot more chemistry and magic to this if you care to do a deep dive. I am not a chemist.

3.3.3. A touch too much

No matter how careful you are with handling the PEI, adhesion will eventually diminish. It’s surprising how much simple handling of the sheet contaminates the surface. Even if you’re careful, there are likely times that you touch the surface without thinking about it. How are you removing prints?

Do you remove the sheet to flex it?
Do you grip the sides of the sheet while popping parts off?
Do you brace the side of sheet with your hand while removing parts?

If so, you are introducing grease at least at the edges. This wouldn’t be a problem, but the next time you wipe the bed with isopropyl alcohol or acetone, you may be inadvertently wiping some of the broken-down residue onto the print area.

During some recent testing I concentrated on avoiding touching the PEI surface. I held the flex sheet exclusively at the edges, only printed on one side and used a plastic scraper to remove printed parts, skirts and prime lines. With a bit of care, I was able to get well over a dozen solid prints without adhesion problems with no additional cleaning. Properly cleaned PEI does make a big difference.

3.3.4. Cleaning the PEI print surface

Note

Prusa only recommends the isopropyl alcohol for maintaining the textured and satin powder-coated sheets. The instructions below apply primarily to the smooth PEI sheets, but can be used on the powder-coated sheets if you have still experiencing adhesion problems. There is effectively no warranty on any of the Prusa spring steel sheets, so if the recommended cleaning regimen is not working, there’s no reason not to try additional measures.

Under normal circumstances, your cleaning routine should be as simple as:

Give the bed a thorough clean with 91%+ isopropyl alcohol. Not necessarily every print, but any time you think you might have touched the sheet or otherwise contaminated it. Apply a generous amount of liquid alcohol to the center of the bed. More is better. An alcohol-moistened towelette or dampening a cloth with alcohol is unlikely to be effective. A wash bottle or other liquid dispenser can be handy. Wipe outwards, moving any accumulated grease to the edges of the PEI and away from your active print area.

Fig. 3.5 Applying isopropyl alcohol for PEI surface cleaning
If and only when isopropyl alcohol fails, try a clean with 100% acetone. Acetone rejuvenates the PEI surface by removing oxidation, but can eventually contribute to cracking of the PEI. Consider a wipe every month or so for conditioning. Here again, apply a small puddle in the center of the PEI and use a clean paper towel to wipe away from the print area.
If isopropyl alcohol and acetone don’t work, there’s a good chance there’s enough build-up of gunk that you’re simply moving it around. Take the sheet to your kitchen sink and give it a good was with Dawn dish soap or your local equivalent (e.g., Fairy in the UK). Use a clean plain paper towel to clean it and another to dry it. Use hot water if available, though this isn’t required. This is, by far, the most effective method of cleaning PEI. The hotter the water, the better. If you have the luxury of a nearby sink, there’s no reason not to wash the sheet off rather than bother with alcohol or acetone.
Finally, all else fails, you can try scuffing the surface. The Prusa PEI sheets are very thin, so recommendations you may find for sanding thick PEI slabs should be avoided. Avoid coarse abrasives like shop sandpaper. I get excellent results with a 3M 7445 Scotch-Brite pads used for polishing glass and chrome (1200-1500 grit). Rub gently, scuffing up the surface slightly without digging in or removing any PEI.

Scuffing the PEI surface with a 3M Scotch-Brite 7445 pad — Fig. 3.6 Scuffing the PEI surface with a 3M Scotch Brite 7445 pad

Warning

There are very different grades of cleaning pads and sponges. 3M 7445 pads variant are very mildly abrasive, roughly equivalent to 1200 grit sandpaper. Scuffs made with this pad will buff out with regular surface cleaning. Green pads and other abrasives, notably most sand paper and kitchen pads, can be considerably more abrasive. Avoid these.

3.3.5. How do I know my PEI surface is clean?

When properly clean, water should sheet off the PEI surface when tilted. If you see any water sticking to spots, give that area another clean. If you drag your finger across a PEI surface, you should feel some friction. If it is slick, give it another wash. (You just touched it, so give that spot another clean. Sorry.)

Finally, and above all else, avoid touching the PEI surface. Seriously. With a good clean and a proper Live-Z calculation, you should have no problems with adhesion. If you’re careful, won’t need to clean the bed every print.

3.3.6. Cleaning materials

The materials you use to clean with can be as important as the solvents and soaps. Using a used dish rag or sponge can introduce grease right back onto your PEI surface.

Use clean plain paper towels. Avoid any with skin softening agents, scents, or other additives.
Blue low-lint paper shop towels are more durable.
Microfiber cloths are great at picking up dust, but can be contaminated with grease. Wash often.
Prusa suggests using plain paper coffee filters as an alternative for quick cleans.

3.3.7. Dealing with over-adhesion

Some materials like PETG grip a bit too well to the PEI print surface. If printing with these materials, you want to use something to lessen the grip. Prusa recommends the following:

3.3.7.1. PETG and other sticky filaments

For “normal” sticky materials, you need to use something to loosen the adhesion. Cleaning the PEI surface as described above is still recommended to ensure an evenly clean surface, but you want to apply a small amount of release agent.

A wipe of the PEI surface with Windex or similar ammonia-based window cleaner will work. Don’t use anything enhanced with vinegar or anti-streaking agents. The key is that this cleaner leaves a very thin residue that helps remove materials that otherwise stick a bit too well. Don’t wipe it all away. You want a thin film of residue to remain.
Simply wiping the print area with your fingers can work well. Not too much, but enough to interfere with the adhesion.

3.3.7.2. High-temperature PETG and other specialty materials

For high-temp sticky materials that print at 250C and higher, you may need a better release agent.

A thin layer of glue stick applied to the print area will help pop parts off. You don’t want to use too much, and it will eventually build up and require a good cleaning under the sink to remove (see above).
I’ve had good luck using MagiGoo on the PEI. The applicator allows applying a precise thin layer exactly where it’s needed. It’s still a bit messy and requires eventual clean-up, but not as bad as glue stick. MagiGoo is a bit expensive, but a small bottle goes a long ways.

Here’s a “lessons learned” advisory, showing both the damage and subsequent attempt at patching my original PEI sheet after printing high-temperature PETG without sufficient release agent, and the same part and material printed on the same surface with a coating of MagiGoo.

Fig. 3.7 Over-adhesion damage and mitigation with MagiGoo

3.3.8. Testing bed adhesion

If you’ve not seen the Prusa PEI surface in action, here’s a quick test showing just how good the Prusa PEI flex surface is. Let’s start with matteeee’s bed adhesion test.

Fig. 3.8 Thingiverse bed adhesion test model

This is an ideal model for testing the limits of adhesion.

The print is attached to the bed with only a small surface area. No brim, raft, or other adhesion aids should be used.
The severe, unsupported, tall print angle applies downward force to pull poorly-attached surfaces away from the print surface.
The tall print will come under maximum pressure and bumping from the nozzle as it moves throughout the print.

Here’s the same model ready for slicing in PrusaSlicer.

Bed adhesion test positioned in |PS| — Fig. 3.9 Bed adhesion test positioned in *PrusaSlicer*

Here are the results of slicing, verifying the lack of bridge, raft, supports or infill.

Bed adhesion test sliced in |PS| — Fig. 3.10 Bed adhesion test sliced in *PrusaSlicer*

Here’s the model printed on a 9 month old, heavily used but cleaned PEI surface using PLA with 0.20mm layer heights with a 0.40mm nozzle.

Fig. 3.11 Bed adhesion test printed on cleaned PEI surface

3.3.9. Part removal

Finally, here’s the process involved in removing the print without damaging the PEI surface or operator injury.

Compare this to the metal tools, hammering, tape and glue often needed to get prints off other printers. Take advantage of the removable flex surface to pop prints off. Do not use metal spatulas or scrapers. If you have to use those, you’re doing it wrong.

You can use a non-metallic scraper to help lift prints that stick a bit. iomaa’s printable removal wedges are handy for removing completed prints without touching the PEI print surface. You can knock skirts and prime lines off with the printed part.

Fig. 3.12 Removing stuck-on parts with a 3D printed scraper

A plastic razor is also useful for removing parts, skirts, and prime lines without risk to the PEI print surface.