3D printing is a pretty effortless process of manufacturing. However, it takes time. So when we talk about improving 3D printing, besides enhancing the quality and dimensional accuracy, it makes sense to improve the print time.

Back in the old days, when printers used RGB screens and tiny displays, the main limiting factor was the resin exposure itself. However, with the rise of monochromatic large-format displays, the exposure itself is nearly negligible in the whole printing process. There are two reasons for that – first is that the LCD is more transparent. Therefore, the exposure now takes between 1.5–3 seconds compared to 8-12 seconds for the old printers. However, the peeling process prolonged from 2-3 seconds to often 5-20 seconds. The reason for that is mainly the large print area where three effects are playing against us: the resin viscosity that makes it harder to squeeze it into a layer, larger FEP films deforming more before they release from the newly cured resin (so we have to lift higher) and also, we need a longer time for the resin to flow in to fill the void after the previous layer. Therefore, even though the exposure is nearly 4 times as fast, the printers are at most 2× faster. With some heavy tuning, my old Mars can print the fastest of all my machines. The exposure is dialed down by massive power, and the rigid construction and small area require only a 2mm lift distance to successfully peel.

We gain negligible speed improvement even if we reduce exposure to half on modern printers. Instead, we should focus on speeding up layer peeling. Imagine the speed improvement if we managed to eliminate the peeling completely! We could achieve a print speed of up to 100 mm per hour with 50µm layers! If you are interested, you can join me on the journey to achieving continuous printing on a consumer-grade resin printer that I persuaded nearly a year ago.

How to achieve continuous resin printing

Continuous resin printing isn’t particularly new; there are the DLP printers by Carbon and many scientific papers (unfortunately, most of them are behind a paywall, so I don’t link them). Recently, Chinese company Carima introduced their line of continuous resin printers. All these printers can print really fast. There is also Hitry Rocker 1 and many industrial resin printers that use deep resin tanks to cure the tank’s surface and sink the model into the resin to avoid peeling.

Carbon printers use a patented “continuous liquid interface production” (CLIP). The bottom of the resin tank is transparent and also permeable to oxygen. As we learned in many past blog posts, UV resin doesn’t cure when oxygen is present (this is why stains from resin remain sticky and why some people cure under-water or under glycerol). When the bottom of the tank is saturated with oxygen, a thin layer of resin in contact with the oxygen never hardens and forms a “dead zone.” Therefore, the model never sticks to the bottom of the resin tank, and you can continuously pull up without peeling cycles. However, to properly refresh the oxygen layer, you need access to the bottom of the resin tank. Therefore these printers use a DLP projector.

The Carima printers, on the other hand, use resin tanks similar to what we know from consumer-grade printers. However, they use (at least for me) unknown film that yields very low peeling force; thus, it doesn’t need the peeling cycle. They don’t seem to use oxygen inhibition at all.

Scientific papers also show that you can achieve continuous printing with a thin liquid oil interface on the bottom of the tan or by electrowetting. Nevertheless, all these approaches struggle with two problems:

• Resin curing produces heat, and you have to be able to cool down the resin (which is even harder when there is no peel cycle),
• And since you don’t lift the printed model more than by a single layer, you struggle with resin replenishment under the model.

This is why you always see fast demos on thin lattice structures with minimal cross-section and plenty of free space. You don’t produce much heat with such models, and it is easy to replenish the resin under the model as the model has a small radius (distances from the perimeter into the middle of the area).

The other approach applied by many industrial machines and recently by Rocket 1 is actually quite ingenious ­until you get into the details. Instead of lifting the build plate up, they sink it into a deep resin tank. The model is formed by curing the resin on the surface of the resin tank (either by laser or a DLP projector). What is, however, the most challenging is to preserve a constant surface of the liquid, mitigating any capillary action and also fighting the oxygen inhibition as you cure resin that is in direct contact with air.

Somewhere in between lies the Formlabs Form 3 printer that tackles the peeling problem by flexing the resin tank’s bottom during the exposure sweep. In my opinion pretty clever. But it comes at the cost of more complex mechanical construction. And it is questionable whether we can call this printer “continuous printing”; it is more of a “continuous peeling” printer.

Since most of these printers cost tenths or hundreds of thousands of USD, they are not widely spread among users and small businesses. Also, cheap printers do not adopt these methods as they are often patent-protected. So I wondered how hard it could be to continuously print on a standard affordable MSLA resin printer that uses LCD? Let’s find out.

The journey toward achieving continuous printing on a consumer-grade printer

Of the existing methods, I liked the oxygen inhibition the most as, in theory, it should require the slightest modification to the printer. We “just need to switch FEP for something that inhibits curing.”

After a lot of research, I devised a plan for using thin silicone film to replace FEP. Why? Silicones are very permeable to oxygen. Various resources claim it to be 100—10000× more permeable to oxygen than FEP. However, how do you form silicone into a thin film? I tried numerous methods, but the most successful one was the simplest one – just squeezing platinum-cured silicon between two rigid plates with thin spacers on the side. The technique is so simple and primitive that I refused to believe in its effectiveness. I tried it after I read about it on the Royal Society of Chemistry blog. I managed to manufacture 30×30cm pieces of 75–200µm thick film with more than 8/10 success rate. I used GMS A30 silicone. The resulting films are cloudy when viewed from a distance. However, it doesn’t seem to affect its transparency when the pattern is close to it, as shown in the picture below.

I stretched the film onto a resin tank and printed a tiny test model using my typical printing profile. To my surprise, the first 10 layers printed just fine. However, after that, the model “got stuck to the film” – the lifting distance started to be insufficient to properly peel as the peeling force increased. The film is also much more flexible and softer than FEP (after all, it is Shore 30A hardness). I observed here that when the film is exposed to air, it soaks some oxygen. That oxygen is, however, consumed/taken away after 10 peel cycles. This is why the film starts to stick, in my opinion. However, when you leave it for 30 minutes on the air, it can again print.

Therefore, what we need is to replenish the oxygen layer. However, we cannot increase the distance between the LCD of the printer and the bottom of the resin tank (just like, e.g., the Carbon printers do it) as we lose the ability to project images (I briefly showed how even a sub-millimeter gap can damage print quality in a previous blog post). Also, the flexible film needs to be supported. Otherwise, it will bend under the resin weight. Remember, we print 50µm layers, so we need everything flat in this range.

My plan was to create an ultimately thin (at most 200µm) inflatable mattress stretched on the bottom of the resin tank and inflated with air to constant pressure. The positive pressure would replenish the oxygen layer as it can penetrate the silicone. I say inflatable mattress as we need to make it flat, not bulged in the middle. Therefore, there should be two sheets connected by many small joints that prevent bulging – just like on an inflatable mattress. If I manage to make the joints with straight walls, they even shouldn’t distort the pattern displayed by LCD much.

So how do we manufacture this? My initial idea was to create a sacrificial insert in the shape of the pocket inside the mattress and cast it inside a silicone sheet. After casting, the insert would be dissolved and washed away. The first choice was to print this pattern on an FDM printer out of ABS and dissolve it in acetone. The first challenge was how to print the pattern. I ended up with a G-code generator that generated the pattern from individual strokes. The resulting mesh was nice and featured really cool moire patterns.

To ensure the proper thickness of the final mattress, I first made a solid film on top of which I placed the printed pattern and cast the second side. To my surprise, the casting turned out great! However, what I struggled with was dissolving the pattern. 3D-printed ABS dissolves very poorly in acetone, and I only managed to clean only about 5 mm of the pattern with it. I was injecting the acetone with a syringe and needle inside the pocket. So this experiment didn’t work out. I also tried creating vax inserts. However, I failed to make them small enough and rigid enough.

Then I realized that I didn’t need a thick insert but only a separator of the top and bottom layers. It won’t be ideal as the space between two joints would bulge a little as it inflates, but it should be small enough to be usable. Therefore, I tried using dry photoresistive film for manufacturing PCBs as the separator. The nice thing about it is that I can expose the pattern using the printer itself – that is, with a high resolution. Unfortunately, you cannot laminate it onto the silicone sheet as it won’t stick. Therefore, I tried exposing and developing the film without any base media. It worked to some extent. The film is very thin and fragile, so it can easily break and lower the success rate. Also, the film, without any support, tends to warp, which showed to be a dealbreaker. Otherwise, it was easy to wash out, and I made small samples (about 3×3 cm), but as the film warped, it often made thin sections that easily torn (see pictures below). With these problems, I abandoned this technique.

After these failures, I had another idea – we don’t need pressure. What we need is continuous delivery of fresh oxygen to the film. And this can also be achieved by negative pressure. You don’t need to blow air into the mattress, but it is sufficient to suck the air out. And for that, you don’t need a mattress – only one-half of it (a sheet with a lot of small bumps), as the negative pressure, should ensure it stays attached to the LCD. And manufacturing such things is easy as you can resin print a mold. The results are perfect, and the original hypothesis that the bumps won’t affect the pattern seems to be correct.

I also made an attachment to the resin tank with two radial fans sucking air from beneath the film. With this setup, I started evaluating what it could and could not do.

Results

First of all – is there a cure inhibition? Yes, it is. When I take a drop of resin, squeeze a build plate to it, expose it and lift it, the top of the resin is “wet” – there is a thin layer of uncured resin. The uncured film seems to be very thin. It seems like a greasy surface that you can wipe into a paper towel. Which is not the case with FEP. With FEP, all resin is completely cured, and both cured resin and the FEP film are “dry.”

Is the cure inhibition permanent? Yes, it is. I can fill the resin tank with resin (so no oxygen can reach the film from the top). I managed to print 150 layers without a problem. So the oxygen replenishment seems to be working.

Does continuous resin printing work with this film? Unfortunately and disappointingly, no. There are two problems. The first one is a minor imperfection, and the second one is a show-stopper.

First, the whole film is very soft and bends a lot. Therefore, I need large lift distances to actually peel the layer. This could be solved by embedding threads (e.g., nylon) into the film, adding rigidity. However, the second reason is fundamental and has no solution.

When I measured my film’s peeling forces, I found that they were roughly 5× larger than the peeling forces on FEP. Why? I think the capillary action. The thin layer of liquid bonds well to the model and film. However, when all resin cures, there is no capillary action; thus, the peeling forces are lower. I measure roughly the same peeling force when I just put drops of resin and squeeze them with a build plate and then peel without exposure.

This is a fundamental flaw in the whole experiment. We wanted to achieve cure inhibition to lower the peel force, but instead, we increased it. Therefore, no revolution in consumer-grade MSLA resin printers is happening. But I learned a lot, and there are some foundations for future research.

However, these experiments make me wonder – how do Carbon printers work? What’s the difference? And is there a chance they would work better with a rigid piece of FEP, PFA, or ACF that doesn’t bend? I don’t know, and I don’t have a Carbon printer to find out.

Final remarks

The whole experiment was performed in early 2022. Since it was a failure and didn’t yield usable results, I initially decided not to publish it. However, more and more, I realize it is necessary to share even unsuccessful experiments and ideas. Others can learn, spot flaws, and improve. I hope this could be the case. Nevertheless, I didn’t take many pictures during the experiments, and most of my samples got lost, so please, excuse the rather poor documentation of this experiment.

This is also the place where I would like to thank all my Patreons and GitHub sponsors. When I have a long period between posts, it usually means that I went into a dead end like this in my research on improving resin printing. Your continuous support allows me to continue, and also, every dead-end brings us closer to understanding and opens new ideas to persuade.

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