SUNLU FilaDryer S4 Review

The SUNLU FilaDryer S4 makes sense if you regularly need to dry several rolls of filament and have room beside your printer. Reported tests show that it can turn badly printing TPU into usable material, while four-spool capacity saves you from drying each roll separately. The catch is that extra capacity brings a large box, audible fans and more filament routes to arrange.

I’d choose the base S4 for batches of moisture-affected filament that can share one temperature. I’d pass if you mainly print trouble-free PLA or need drying temperatures above its 70°C ceiling. This review covers the base S4; the newer S4 Pro has different capabilities.

Will your spools fit in the S4?

The S4 holds up to four standard 1kg rolls, with two loaded through each top lid. Its body measures 458 × 218 × 312mm, roughly 18 × 8.6 × 12.3 inches. You also need room above it to open the lids and lift rolls out. A shelf that barely clears the closed box won’t be convenient.

SUNLU lists the internal space as 450 × 210 × 211mm. That’s the chamber size, not a promise that any four spools will fit. Measure each spool’s outside diameter and full width, including any rim adapters. A weight label tells you how much filament is on it, not how large the spool is.

Reported loading checks found standard 1kg and 750g rolls workable, while one bulk 3kg spool wouldn’t fit. Smaller rolls deserve a separate check: they may fit inside but sit less securely when the printer pulls filament from them. One test also found Eryone cardboard spools troublesome when nearly empty. That doesn’t rule out cardboard generally; it makes the shape and remaining weight worth checking.

Before starting a long print, make sure each loaded spool turns freely without rubbing its neighbor or catching on the housing. If an unusual roll fits but won’t feed reliably, you can still dry it first and use your printer’s normal holder afterward. All four rolls need enough room to turn, not just enough room to close the lids.

What the S4 can improve in your prints

Filament is the plastic strand your printer melts to build a part. Moisture absorbed by that plastic can interfere with printing. Drying is worth considering when a previously dependable roll starts producing rough surfaces, bubbles or excessive strings between parts of a print. Those symptoms can also have other causes, so a dryer isn’t an automatic fix.

The strongest reported S4 example involved flexible TPU: the same roll and printer instruction file produced a much better test boat after five hours at a 55°C setting. Keeping the print settings unchanged makes that a useful before-and-after result. However, it came from a pre-release unit, not a controlled comparison of today’s retail dryers.

The same testing found only modest improvement with matte PLA. A very old NinjaFlex TPU roll remained unusable despite many hours of drying. Don’t buy the S4 on the assumption that it will rescue every neglected spool.

PETG showed a useful but incomplete recovery in another test. An almost-full Amazon Basics roll that had never produced a successful print was dried overnight. The next test boat was usable, but still had some stringing and bubbles. The drying temperature and exact overnight duration weren’t reported, so this is evidence of possible improvement, not a PETG schedule to copy.

A separate PLA experiment couldn’t establish a clear printing benefit: deliberately wetting an Anycubic roll hadn’t made its prints noticeably worse in the first place. Without a worse starting print, that experiment couldn’t show how much drying helped. It doesn’t prove that PLA never needs drying.

For you, the useful distinction is between improving a print and seeing a lower humidity number. The S4 measures the air inside its chamber, not the water remaining throughout a wound roll. Warm air can show lower relative humidity without proving that the whole roll is ready.

If your PLA already prints cleanly, don’t expect a dramatic upgrade. If a moisture-sensitive roll has deteriorated, drying offers a sensible next step. Compare the same small print before and afterward, keeping its settings unchanged. If the faults remain, check the printer settings and filament path rather than assuming another cycle must solve them.

Setting the S4 for drying and storage

The base S4 has one shared drying temperature, with an advertised range of 35–70°C. You can select a material preset or adjust the temperature yourself. Use the filament maker’s drying instructions to choose the setting, and check that the spool can tolerate it too. Different colors can share a cycle; different plastics can share one only when their drying requirements allow it.

The controls use Set to select an option and Up or Down to change it. Timed drying runs for the selected period. Humidity maintenance is meant to restart heating as the chamber becomes more humid, which is useful when you keep rolls inside between prints.

SUNLU’s current product documentation describes an adjustable restart threshold of 30–50% relative humidity, with heating stopping at 25%. Early units were described differently. Check the instructions supplied with your S4 before relying on automatic operation, particularly if buying older stock. The product page has a User Manual download under Downloads; make sure you’re looking at S4 documentation rather than S4 Pro instructions.

Treat the five-hour TPU result as an example, not a universal timer setting. Material, starting moisture, spool size and drying equipment all matter. For example, Bambu specifies 65°C for eight hours in a blast drying oven for PETG HF. That tells you what that particular filament requires under those conditions; it doesn’t establish an identical result in the S4.

Heating the chamber is also different from drying the roll. One reported S4 session reduced chamber humidity from 45% to 26% in about four hours, but didn’t measure moisture inside the filament. Neither a warm-up claim nor a low display reading is a reliable finish signal by itself.

The nylon preset doesn’t mean the S4 can dry every nylon. Polymaker’s Fiberon PA6-GF25, for example, lists a 100°C oven drying schedule, beyond the S4’s range. Use a different schedule only if the filament maker recommends it for your drying equipment.

After drying, sealed storage with suitable desiccant—material that absorbs moisture—helps protect your rolls. Humidity maintenance requires power; don’t assume an unplugged S4 provides the same protection as a sealed storage container.

Feeding your printer from the S4

The S4 has eight filament outlets across its top and sides, giving you options for positioning it near a printer. The listed accessories include two 1m PTFE guide tubes and four short 8cm pieces. These smooth tubes guide the filament between the dryer and printer; check whether the supplied lengths suit your layout.

Keep bends broad and avoid pulling filament sharply upward into a high-mounted sensor. A reported setup with a Neptune 4 Pro developed excessive tension when feeding without the guide tube. Another test found direct feeding worked properly only from the dryer positions nearest the printer. These are layout problems to plan around, not proof that an entire printer model is incompatible.

Try each position you intend to use. Check that the spool turns easily and that the tube won’t become taut as the printer’s moving parts travel. If you need to move the S4 farther away to make room, check the route again. A longer tube can solve reach while adding resistance.

Printed spools and accessories need their own heat checks. One owner reported a PLA+ printed spool warping after an hour in an S4, but gave no temperature; replies described different experiences. That’s a reason to check ratings, not evidence of a recurring dryer failure.

Bambu’s manufactured reusable spools have version-specific temperature ratings: up to 70°C for the low-temperature version and 90°C for the high-temperature version. Those ratings don’t transfer to a downloaded spool printed in PLA, or to its locks and adapters. Use components rated for your chosen cycle rather than assuming every printable accessory is suitable.

Sharing a room with the S4

Expect to hear the fans. Reported impressions range from a steady hum to noise intrusive enough that placement mattered. That makes the S4 easier to recommend for a printing room than for the desk where you take calls or a bedroom where you want to dry filament overnight. The available sound observations don’t provide a consistent distance, room background or retail revision for a dependable decibel comparison.

Power figures need similar care. One hands-on report recorded instantaneous demand above 360W. That’s a momentary power reading, not the average consumption of an entire drying cycle. SUNLU’s own pages also use differing power figures, so a single headline wattage won’t tell you the running cost.

If you want to know what a cycle costs, measure its electricity use in kilowatt-hours with an appropriate plug-in energy meter, then multiply that by your electricity rate. Heating time, temperature and how often humidity maintenance restarts will affect the total. Drying several rolls together saves you from loading them one at a time, but it doesn’t necessarily use less electricity per roll than a smaller dryer.

Is the S4 worth buying for your setup?

I’d buy the base S4 if you regularly have three or four rolls waiting to dry, their requirements fit its temperature range, and you can give it a permanent spot with workable filament routes. That’s where its capacity becomes useful every week instead of taking up space for an occasional rescue attempt.

For one active roll, a smaller dryer such as the SUNLU S2 is cheaper and takes less room. For rolls that are already dry and mostly waiting to be used, sealed containers with desiccant may solve the storage problem without another powered appliance.

The closest alternative to check before buying is now the S4 Pro. SUNLU lists it with independent settings for two zones and an 85°C maximum. If you need to dry different materials at different temperatures together, those features address a real limitation of the base S4. They don’t establish better drying results or quieter operation. When the two cost about the same, I’d favor the Pro for that flexibility; a cheaper base S4 remains reasonable for batches sharing one setting.

SUNLU lists a one-year warranty for the base model, but the available evidence doesn’t establish its long-term failure rate. Buy for the capacity and supported drying needs you have now. If you can’t name the rolls that would regularly fill it, the smaller option is probably the better fit.