How to Properly Care for FEP Sheets in Resin 3D Printers

FEP sheets are one of the simplest yet most fundamental parts of resin 3D printers. Although fairly robust, proper handling of the FEP sheets can be quite a finicky matter. After some time, you will also have to replace the FEP sheets to maintain print quality.

In this article, we will be focusing on the basics of FEP sheets, why exactly this material is used in resin printing, and how to properly handle and replace them.

What are FEP sheets?

FEP stands for fluorinated ethylene propylene. This is a copolymer invented by DuPont and is made by combining hexafluoropropylene and tetrafluoroethylene. In terms of chemical composition, FEP is very similar to polytetrafluoroethylene (PTFE) or what we commonly call Teflon.

Like Teflon, FEP has non-stick properties because of its low surface energy. This has made FEP a viable alternative to PTFE in non-stick applications. FEP also has the added benefit of being melt-processable, making it possible to make FEP items via injection molding and similar techniques. Owing to its better workability and flexibility, applications that use FEP films are quite common.

FEP has resistance to caustic agents and dielectric properties that is comparable to that of PTFE. The chemical resistance of FEP also extends to UV radiation, as its chemical structure remains stable even under sustained UV exposure.

Why are FEP sheets used in resin printers?

In resin printers, FEP sheets are installed at the bottom resin tanks. It acts as the mechanical barrier between the UV light source and the resin while allowing the light to pass through. FEP is uniquely suited for this application because of a couple of interesting characteristics:

• FEP does not chemically degrade even after long exposure to both UV radiation and the photopolymer resin
• When the build plate moves after each layer is printed, it needs to pry off the hardened resin from the surface of the FEP sheet. This is facilitated by the low surface energy of FEP. Some users apply a coating of PTFE grease on the FEP sheet to make this process even easier.
• The flexibility of FEP allows it to yield a little, yet not fail, as the build plate moves. A single FEP sheet can typically be used for more than 30 resin printing projects before replacement is needed.
• FEP has excellent optical transparency. At the thickness of standard FEP sheets, light undergoes virtually no refraction as it passes through FEP. This ensures the excellent detail accuracy and high resolution of resin 3D printing.
• The thermal resistance of FEP is not as high as that of PTFE. While it can withstand the ~70 C temperatures of UV curing, it is also these elevated temperatures that can cause the FEP sheet to turn cloudy over time.

Proper handling of FEP sheets

In the proper maintenance of FEP sheets, there are two goals to keep in mind – to retain both the mechanical integrity of the FEP sheet and its optical properties. Proper care is essential as the condition of your FEP sheet will have a direct effect on the quality of your print.

The resin vat of a 3D printer does not need to be cleaned very frequently. There have been many accounts of users leaving the uncured resin in vats for up to eight months and printing with it without issue. If you plan on doing this, make sure that the printer is kept in an area far from any source of UV light and that you stir the leftover resin thoroughly before use.

Cleaning a resin vat will be necessary for only two circumstances – if you want to change to another resin or if you’re troubleshooting a problem. In any case, the uncured resin can be recovered and stored in a bottle. Do NOT mix fresh and used resin together. Make sure to filter any uncured resin before storage.

We recommend using IPA to clean the resin vat and make sure that any uncured resin is removed. This is best done by submerging the entire resin vat inside a large container of IPA several times. Pat the FEP sheet and resin vat dry using a microfiber cloth. Do not use paper towels as they can leave microscopic scratches on the surface of the FEP sheet.

It’s best to re-level the resin vat upon installation and before printing.

Potential problems with FEP sheets

Even with proper care, FEP sheets tend to develop issues after some time. This does not automatically mean that your FEP sheet is beyond salvage. Here are some of the common problems encountered with FEP sheets.

Prints sticking to the FEP sheet

Ideally, you will want your resin print to stick to the build plate and separate from the FEP sheet. However, the reverse can also happen. When this happens, you have no choice but to stop printing to remove the stuck material on the FEP sheet.

The safest way to do this is to use a fingernail as a lever to pry the stuck print from the FEP sheet (make sure to wear nitrile gloves). If this does not work, try to prod it out by tapping on the opposite side of the FEP sheet.

If this still does not work, you can consider using a plastic or rubber spatula as a lever. Whatever you do, do NOT use a metal spatula as this will almost certainly leave the FEP sheet heavily scratched.

Small punctures

There is always a risk of damaging an FEP sheet whenever you clean it, or even during normal operation. This is why it’s always important to take a close look at your FEP sheet before starting every printing project. If you have any suspicion of the FEP sheet having any small holes, then it’s worth investigating.

The best way to do this is to pour a substantial amount of IPA into the resin vat and place it over some paper towels. Water will not work in this method as its surface tension may be too high to penetrate very tiny holes. If there is indeed a hole in the FEP sheet, the paper towels will get soaked in the area where the holes are.

Some online guides will say that these holes can still be repaired by taping them over and simply avoiding the taped areas when slicing models. However, this is almost literally a band-aid solution that is destined to fail eventually.

In our opinion, there is simply too much risk of the resin dripping onto the UV light and ruining it. It would be a lot safer to just replace the FEP sheet.

How to replace your FEP sheet

Over time, an FEP sheet can become cloudy, have heavy scratches, or be punctured. You might also notice that your prints get stuck into your FEP sheet more often. These are tell-tale signs that your FEP sheet needs replacement.

If you own a resin printer, it’s a good idea to always have a handful of replacement FEP sheets on stock. You will also need a standard set of screwdrivers and Allen wrenches, some IPA, and a small knife.

To start, unscrew the resin vat and lift it off from the printer. You will find that the FEP sheet is sandwiched and held taut between two steel frames. You will have to remove all the screws that are holding these steel frames together. Make sure to keep all the screws you remove inside a separate container – they are very easy to lose.

At this point, you will want to soak the steel frames in IPA to remove any residual resin. You can also wash the old FEP sheet with some IPA so that it can be disposed of safely. Once thoroughly cleaned, wipe the steel frames and set them aside for a few minutes to air-dry.

You can then get a new FEP sheet and take care to hold it only on its edges. Make sure to remove the protective films on both sides of the FEP sheet. You will now have to sandwich the FEP sheet between the metal frames in the same fashion as the old sheet that you discarded. There is no need to cut holes in the FEP sheet as it can be punctured by the screws during tightening.

To ensure even tension, it’s a good idea to tighten screws in pairs located at opposite sides of the frame. There is no single proper way to do this, but you should be able to see the FEP sheet as it is being pulled taut. The FEP sheet should be tight but still have a bit of yield. To check this, lightly tap the surface of the FEP sheet. You should hear a sound similar to a small drum.

With that done, just use a small knife to cut the excess FEP sheet from the frame. Just be careful not to let the knife anywhere near the taut section of the FEP. It would be frustrating to have to start over at this point.

You should now be ready to mount the resin vat back into the 3D printer. Don’t forget to level the resin vat again.

Final thoughts

Proper FEP sheet maintenance and replacement are some of the first lessons that you should know if you would like to get into resin 3D printing. FEP sheets are fairly resilient but still need to be handled carefully lest you end up with failed prints.

Proper FEP sheet handling also goes a long way towards extending its lifespan. While FEP sheets aren’t exactly expensive, having to replace them frequently can be a huge time sink.