We explain how this rapid prototyping technology works, and how it is now available to makers and hobbyists.

If you have been involved with the 3D printing hobby for more than a few months you are sure to have heard the buzz around a seemingly “new” rapid prototyping technology often referred to as DLP or SLA 3D printing. In reality, both are in effect, a variation of the same technology, and neither of them are new.

SLA or Stereolithography Apparatus 3D printing for rapid prototyping was first developed in the 1970s and patented in the 1980s. Essentially, this technology uses light directed towards a build surface, which cures a very thin layer of a photoreactive liquid onto the build platform. The platform then moves away by a small amount and the process repeats until a 3D model is created.

This technology, for the most part, has only been used in the engineering industry due to the very high barrier to entry, which stems from the exorbitant cost of the machines and the photocurable material it uses. That has all changed drastically in the last few years with the machines now retailing for as little as $300 and the photosensitive resin dropping to around $50 for 500mL.

These affordable prices have put this impressive rapid prototyping technology into the hands of the average hobbyist and maker. As such, we figured it was a good opportunity to introduce our readers to the technology explain how it works, why it’s so popular right now, and what the risks are with using it.

HOW DOES IT WORK

There are three main types of Stereolithography 3D printers that are commonly available on the market today.

LASER SLA: LASER SLA uses a high precision LASER and mirror to aim the light at a precise location on an interface. The thin layer of resin cures on the build surface at the same wavelength as the LASER, and leaves just a 25-micron gap. The machine can then simply move the cured resin away from the interface and back to its new position where the process is repeated many times until the model is constructed.

This is an incredibly precise method that can produce an outstanding print resolution, however, it is very slow because the optics are constantly moving the LASER’s thin beam of light. This means that this style of machine, whilst available to the hobby market, remains significantly higher priced than the other forms of Stereolithography.

DLP: Digital light processing (DLP) stereolithography addresses the speed and cost issues with standard LASER SLA printing by using a light projector and in some cases mirrors to reflect the desired light onto the build area. This means an entire layer can be printed in one movement, drastically reducing the time taken to build a part.

The mirror and/or projector being fixed in place also significantly reduces the cost of the machines as they no longer need high precision positional control and motors, etc.

Masked SLA: Masked SLA takes this technology one step further, by removing the projector and mirror and replaces them with an LCD and a bright LED light.

A vat of resin sits above the LCD, separated by a very thin layer of Fluorinated Ethylene Propylene (FEP) plastic. The LCD displays the desired shape by turning off individual pixels where the resin needs to be cured, while all of the other pixels are illuminated.

The machine has a bright LED beneath this LCD, and light can only travel past the LCD via the pixels that are not illuminated. This light passes through the LCD and FEP sheet, allowing the resin squished between the build platform and the FEP sheet to be partially cured. The build platform then lifts to break the surface tension between the cured resin and the FEP sheet, and repositions itself for the next layer to be cured. Like the other methods, this process is repeated many times until the complete model has been printed.

Like DLP, the masked SLA process is many times quicker than the LASER SLA technology, owing to the fact it creates an entire layer with one pass. However, this technology introduces its own unique issues. The most obvious issue is that the LCD is a very fragile device, which is separated from the build platform by a very thin sheet of FEP film. When this film ruptures, the LCD will come into direct contact with the photosensitive resin. Naturally, this contact is likely to destroy the LCD.

Furthermore, the light used by the vast majority of these machines is a near UV light, ranging between 350 – 420nm. This near UV light will rapidly destroy the LCD, meaning the LCD module becomes a consumable item, which we assume is why all of the masked SLA machines on the market come with no more than a 3-month warranty on the LCD.

Another unique issue to the masked SLA system stems from the way an LCD works. An LCD creates an image by turning on or off individual square pixels arranged in a grid. This makes true curves impossible to recreate using this technology due to the 3D shape being constructed via square pixels. As a result, the shape is created using many squares or voxels. This, in effect, is minimised by using very high-resolution LCD displays and a digital processing technique called anti-aliasing, which averages out the pixels on the edge boundaries to provide a smoother look.

However, whilst the effect is persistent with the masked SLA technology, it is insignificant compared to the similar effect we see with Fused Deposition Modelling (FDM) technology. With FDM, the effect is usually only present in the Z axis, while with Masked SLA, the voxel effect is visible in all axis.

To demonstrate the effect between Masked SLA and FDM, we printed this Holiday Christmas Deer by yeg3d via Thingiverse (http://thingiverse.com/thing:571949). We can see the effect is many times more obvious with FDM printing due to the layer heights being much larger.

NOTE: This Christmas Deer print was done without anti-aliasing to show the effect. With anti-aliasing enabled the effect is drastically reduced.

HOW DOES IT DIFFER TO FDM?

One of the most obvious points of differences between FDM and Masked SLA is the significantly higher resolutions that a masked SLA machine can replicate. Generally speaking, your FDM printer, with its 400-micron nozzle diameter, will print each layer at around 100 to 300-microns thick. With masked SLA 3D printing, the layer height drops to an incredibly tiny 25 to 100-microns thick. For comparison, a human hair is on average 65-microns.

This means, not only is a masked SLA printer capable of printing models with significantly higher resolution, but it is also many times more capable of printing extremely high detail that FDM printing simply isn’t able to recreate.

Take, for example, this “Bluejay Guardian” tabletop miniature designed by Thingiverse user M3DM (http://thingiverse.com/thing:3689651). This tiny model is only a few centimetres tall and is incredibly detailed. Each feather on the wings is carefully modelled with detailed creases in her clothes and hair. With masked SLA 3D printing, the vast majority of this detail is replicated, even the tiny little bird in her hand as we can see here.

In order to compare the technology, we printed the Bluejay guardian model at the draft setting of 50-microns for the masked SLA printer and at 100-microns on an FDM printer.

We were impressed with how the FDM printer performed, however, the differences when comparing the masked SLA and FDM print are significant. It’s easy to see that the level of detail is many times greater using the masked SLA machine.

Another very interesting point of difference between FDM and masked SLA is how support structures work. When we first started printing masked SLA prints, we would position the models exactly the same way we were used to positioning models for FDM printing. i.e. We would put the model flat on the build surface to reduce overall support material requirements. This often-produced unsightly bases, as can be seen on many of our first prints. We quickly discovered, to produce significantly better prints, it was better to have the entire model free from touching the build platform and fully suspended by support material. This produced much more consistent models without the large clumps of excess cured resin at the bottom of the model. Of course, this need for support material significantly reduces the efficiency of using the machine and increases the wastage, as the support needs to be physically removed and discarded.

This image shows you how we positioned our models on the build platform for the best results. We found having the parts mounted on 45° angles (roughly) produced the best results and reduces the amount of support material required.

We also found that it was better to remove the support material before the final curing step, as the part becomes quite a bit brittle when fully cured. This way, you can clean the support blemishes easily with a hobby knife and cure the final result. Another thing we discovered during our experiments was that you can and should hollow out the models you're printing on masked SLA machines. These machines do not print internal infill so without hollowing the model they are printed at 100% infill by default. Not only is this incredibly wasteful, it also has the ability to prevent a model from curing properly when using non-transparent resins.

To clarify what this final curing process is; unlike 3D printing with FDM technology, the SLA process requires a few more steps before you get your final product. Like FDM, the model is sliced in a program that creates the toolpath for FDM and the layer illumination for SLA. The machine then creates the model as instructed by the slicer program.

In FDM, this is essentially the end of the process (ignoring support removal, of course). In SLA, however, there are a few more steps. The first is you need to clean the model in a solvent to wash all of the uncured resin from the model. This is usually done in 95% Isopropyl Alcohol, but we found using methylated spirits works just fine for a significantly reduced cost. After carefully washing the part, you then need to then expose it to light at the specified wavelength to fully cure it.

It’s advised that this should be done with a curing box containing a light source matching the wavelength the resin cures at. In most cases, the resin seems to be designed to cure at near UV wavelengths, as such, it should be ok to simply put the part in the sun to do this. However, this can easy overexpose the part and cause it to warp, discolour, and in extreme cases, fracture. As such, it’s best to have a specifically designed curing station that exposes the model to the correct wavelength light.

It’s also worth mentioning that the part will always be sensitive to light at, and around, the specific curing wavelength. This means parts that are likely to be regularly exposed to UV light need to be painted or coated to prevent the UV light from overexposing the model.

This part shown here has been left exposed to indirect natural UV light for too long, causing it to become discoloured, changing to an orange/brown colour. The enclosure in the foreground was printed on an FDM printer for comparison.

SAFETY

The resin used in the SLA additive manufacturing process is a toxic substance and needs to be handled with significantly more care than other hobby level additive manufacturing materials. Whilst it is true that the resin used in SLA 3D printing is no more toxic than common household items, such as oven or drain cleaner, the average person is unlikely to have any significant contact with these chemicals. As such, we feel it’s important to ensure people new to the technology are aware of the inherent risks.

The material safety datasheet for the resin we used states that when handling the material, the user should be wearing protective gloves, as well as wearing eye protection. Naturally, we fully agree with this recommendation, however, this resin can get EVERYWHERE. During our testing, we experienced several mild chemical burns due to accidental cross-contamination.

In one such instance, a drip from a finished but unwashed print, fell onto my sock unnoticed. Another time, I accidentally brushed my exposed forearm against the liquid resin. In both of these cases, the initial contact went unnoticed until a burning sensation was felt sometime later. The result was a very minor chemical burn, however, each person is very unique with their sensitivity to chemicals, and repeated exposure to the resin should be avoided at all costs.

We strongly recommend you follow the directions in the MSDS for the resin you’re using, the printer, or any tools that have been previously used on the printer.

NOTE: In our testing, we were using a clear resin, which was near impossible to notice when cross-contamination occurred. For this reason, we would strongly recommend that you avoid clear resins when first using this technology until you get acquainted with chemical safety practices.

We strongly recommend the use of nitrile/vinyl gloves. Do not use latex gloves.

Suitable chemical protective eyewear that sits flush against your face is also a must. Do not simply use safety glasses designed to protect against impacts, as these offer little protection from liquid chemicals and/or fumes.

This technology produces some quite noxious fumes during the printing process and should not be done in a living area of your home or in enclosed spaces. If you’re intending on using this technology in your home, ensure you have sufficient fume extraction that will exhaust the fumes outside.

There are also significant ecological concerns with this resin material. With a particular concern toward aquatic flora and fauna. At absolutely no point should uncured resin make its way to landfill or worse into a waterway.

Therefore, your gloves and the alcohol/methylated spirits you use to clean your parts and tools needs to be carefully disposed of, and should by no means be allowed to enter waterways or thrown into your household waste. We recommend washing your used gloves in a container of methylated spirits to remove all uncured resin before disposing of them. This will ensure that the resin does not enter the landfill.

The contaminated methylated spirits can be left to evaporate leaving the dried resin behind. This should then be collected and stored in a container labelled “Polyurethane acrylate” where it should be fully cured before being disposed of.

WHO CAN BENEFIT FROM THIS TECHNOLOGY?

Whilst this technology remains incredibly popular for industrial uses, especially in industries such as dentistry, architecture and rapid prototyping, the technology has also exploded in the hobbyist sphere. Many tabletop gamers and model builders have been embracing this technology for the incredibly high detail models it can reproduce.

We were interested in giving the technology a try to see how it can be useful in our workflow creating electronics enclosures. Whilst the technology was more than capable of producing high quality designs, the added cost and complexity of the workflow did not, for us, translate into a worthwhile investment.

For the most part, the enclosures we design and build are not complex designs that would benefit from the much higher resolution possible with this technology.

These images show you our Servo Tester and Calibrator from Issue 24 and Rage Maker from Issue 17 printed on the masked SLA machine. Whilst the results are very impressive, with nearly zero visible layer lines, we feel the added complexity and cost outweigh the benefit when creating our usual electronic enclosures. The benefit, however, does indeed increase when dealing with smaller enclosures, such as the Rage Maker project. This smaller enclosure was much easier to create on the masked SLA machine compared to an FDM printer, owing to the thin walls and higher need for close tolerances. As such, if you’re designing enclosures that require very high accuracy, then this technology is certainly leagues ahead of the FDM style of additive manufacturing.

Likewise, if you are creating very high detailed models, such as jewellery or tabletop miniatures, etc., this technology is likely to revolutionise your current workflow and produce amazing results.

With the machines and materials becoming much more affordable, we’re excited where this technology is headed.