The Post-Curing Process

Post-curing, is a light and heat dependent process, that helps 3D prints reach the highest possible strength and become more stable. This additional step to the 3D printing process is an optional step, and should be considered in select applications.  With certain printers, such as the Form 2, printing study models with standard resins does not need this step for most orthodontic uses like vacuum form retainers and diagnostic models.  Post-curing is particularly important for ‘functional’ resins, which in orthodontics would qualify as occlusal splints, surgical guides, and models that would go in a pressure pot.  Adding a UV post-cure chamber to the printing process will finalize the polymerization process and stabilize the resin’s mechanical properties. Specifically, dental resins double in strength with UV post-curing.

From the Formlabs UV curing comparison study: “using a 405 nm light source during post-cure was superior to an equivalent level of lower wavelength light, such as 365 nm. Post-curing at higher temperatures results in a shorter time to full cure. Additionally, higher temperatures lead to higher mechanical properties.”

Post-Cure Hardware

Depending on the size and volume of the appliances or models you are manufacturing, you can consider different types of post-cure hardware. For smaller prints, a low cost 36W nail salon or crafting curing chamber works well.  There are post-curing accessories available from Envisiontec.  Otherwise, the chamber from Kudo 3d has received the best reviews.

When Do I Need to Use a Biocompatible Resin in Orthodontics?

Biocompatibility is required by the FDA for the manufacture of any ‘medical devices’ that will come into direct contact with the skin or oral tissue. Dental and orthodontic models that are used to fabricate clear aligners, or other appliance do not have to be printed with bio-compatible materials.

Direct printed indirect bonding trays, occlusal splints, denture bases, or other printed items that qualify as a medical devices and will come into direct contact with tissue should be printed using a bio-compatible resin.

Currently, bio-compatible resins for use in orthodontics are manufactured by FormLabs, Stratasys, and Envisiontec just to name a few suppliers.

What Does It Mean for A Resin to be Bio-compatible?

Typical 3D printed materials undergo ASTM International standards testing to evaluate material properties like tensile strength, flexural modulus, etc. However, bio-compatible materials must undergo a completely separate set of tests that are according to ISO standards, which can get expensive very quickly. These tests can cost upwards of $100,000 per resin, depending on the medical or dental indications for the resin.

There may not actually be any material difference between standard resins and bio-compatible resins from the same company; however, to produce medical devices with a resin, for the company be able to assert ‘medical claims,’ and to market directly to the dental and medical market the ISO testing is mandatory.

To be classified as a bio-compatible materials, 3D printed resins will undergo biologic evaluation according to ISO 10993-1: 2009 to address cytotoxicity, genotoxicity, and delayed hypersensitivity, and USP plastic class VI, which includes the test for irritation, acute systemic toxicity and implantation.

Resins may also undergo additional ISO testing for certain material properties pertinent to their intended use. For example, a bio-compatible dental resin might undergo ISO 20795-1:2013 testing, which applies to denture base polymers where the manufacturer might be making advertising claims regarding impact resistance.

Looking into the Form 2 3D printer?

I have been 3D printing with several Formlabs printers for over 3 years and wanted to share my experience. My journey started with the original Form 1, followed by the Form 1+, and now the Form 2 is the latest version. The Form 2 printer has improved the process significantly, both in reliability and user experience.  Taking into account the versatility, reliability and price point, I urge you to consider this printer if you wish to get into the 3D printing arena.

The Form 2 is a bottom up, laser based SLA printer. It uses a sliding peel process with wiper, and an automated cartridge system. Compared to its predecessors, the Form 2 has an enclosed optical path that keeps the mirrors protected from dust and contaminants.   It offers Wi-Fi, Ethernet and USB connectivity, plus a touch-screen control. Its build volume is now 5.7 x 5.7 x 6.9 in.

Formlabs has developed a whole line of dental and orthodontic options for 3D printing, including biocompatible resins (Dental SG & Dental LT) that can be used for direct printed splints, implant guides and IDB guides. Since 2013, I have been using their Grey resin to 3D print dental arch models for a variety of different lab fabricated appliances.

Although the Form 2 is not the fastest printer on the market, we have used it in a print farm configuration, running multiple 3D printers each day to scale our lab output. Each printer can run 8-12 dental arches per build, depending on arch size. At a price point of $3499, compared to other 3D printers in the dental market, the return on investment on this printer is excellent.

The Form 2 is available directly from Formlabs. It is essentially plug and play with intuitive software included.  The Preform software and their Dashboard feature have a short learning curve and expand on the benefits of this equipment.

Stay tuned for future posts on our Form 2 setup and workflow.

Why a Print Farm for Orthodontics?

Developing a multi-printer lab facility allows you to scale the manufacture of 3D printed models and appliances. As volume grows, a 3D print farm maximizes the print area, workflow and overcomes the barriers to production in-house. Instead of investing in a single, large and expensive 3D printer, one can buy multiple, smaller and less expensive printers that run in batches.

When you have a single 3D printer, even if it very fast, you will run into challenges in maximizing the print area and waiting for a batch to finish before another can begin. Consider each 3D printer has a build area.   For example, if the build area is 5” x 5” in your printer, adding a second printer increases your print area to 10”x10”. Add another, and you have 15”x15”. Continue to expand, and your volume can increase.

The multiple print areas also allow for the use of different resins when different parts need to be made. For example, 3D printing study models and printing intraoral appliances require different resins. A large single printer could handle the volume of both parts together, but cannot handle the different materials at the same time. When you have different printers you can handle the batches without the lab having to change the resin and run the printer again.

Another advantage of a print farm is safety in numbers. What if your 3D printer goes down? Having multiple 3D printers in your lab gives added safety that you won’t have to outsource your printing while your only printer is getting repaired.

How to Plan and Implement

It first begins with selection of your 3D printers. It is often easier to setup a multiple printer facility having the same printers multiplied. This allows for ease of training and use by your lab team, and a simplified process to manufacturing. Diversifying your farm with multiple types of printers could allow for different speeds of prints and different workflow depending on the urgency of the desired batch.

When planning a print farm, select a work area with adequate space, ventilation, and proper lighting. Avoid direct sunlight, UV light, dust and high traffic areas since they can affect the printing process. The printers should be in a stable, flat surface free of debris. They also need shelving that allows for the lids to open.

Multiple printers need to be labeled to prepare the print queue.   Large batches of prints can get more difficult to track and retrace. If there is defective print, proper labeling allows the lab tech to trace the affected printer. Tanks and platforms should also be labeled if different types of resins are being used.

For more details, see the Formlabs whitepaper

Layout and key components

1. Download – An stl file is downloaded after the patient is scanned
2. Preparation – The lab computer station cleans up the model and prepares the print layout in the 3D printer software
3. 3D printers – Printers are conneted via Ethernet, USB or WiFi
4. Washing area – Isopropyl alcohol tanks are used to wash the prints. This needs consideration for proper ventilation.
5. Drying area – A ventilated area is used where a tray of prints are placed under a fan.
6. Post-curing (if needed) – With some resins, an additional post-cure step is required using UV light.
7. Finishing – remove supports, polishing and trimming
8. Packaging area – prepare models for appliance fabrication or package appliance for delivery

Resolution and 3D printing

When someone asks the question, what is the resolution of a 3D printer? There’s not a one-number answer. Resolution is a factor of polymerization kinetics, the properties of the photopolymer, and optics of the light source, and your print quality is in turn impacted by the resolution in all three dimensions: the two planar 2D dimensions (X and Y) across the build platform, and the Z dimension (the height). To even begin to have a conversation about 3D printer resolution, you will have to consider at least two numbers: the minimum feature size of the XY plane and the Z-axis resolution (layer height). 

Furthermore, the planar (X and Y) and Z dimensions are generally controlled via very different mechanisms, so their resolutions will also be different and they need to be considered separately. There is also no standard that all 3D Printer manufacturers adhere to when discussing their resolution, so you begin to see why there is quite a bite of confusion about what the term “resolution” means in 3D printing.

3D Print Resolution & Minimum Feature Size

The Z-axis resolution, or layer height, for all 3D printers is easily determined and therefore widely reported even though it is less related to the overall output quality, but it is far less important to dimensional accuracy than the XY resolution and minimum feature size.

The XY resolution with DLP-SLA printers will typically be reported as a range and will tell you how small the pixels can be made by focusing the projector. As you’ll remember from our previous post, the DLP projector has a set pixel resolution so the pixels will be larger when focused on a larger surface area. Sometimes manufacturers will only give you the finest resolution and you’ll have to ask for the full range. An accurate spec sheet for printer properties for a DLP printer will look like the below image:

MSLA or LCD-masked SLA printers are a bit different since the XY resolution is determined by how the LCD photomask was manufactured and the size of the pixels on the LCD so you will in fact get a single number and not a range.

In both DLP-SLA and MSLA printers, the relationship between ‘minimum feature size’ and resolution is relatively uncomplicated. Since the layer images are projected, the XY resolution and the minimum feature size are equivalent. However, with laser SLA printers, ‘minimum feature size’ and XY resolution are related, but they are not exactly the same.

Since layers are drawn with laser SLA technology, the XY resolution determined by the pointing precision of the galvos motors (the mirrors that point the laser), and how finely they can move the laser in each layer (finer than 10microns for Form2 printer). While the ‘minimum feature size’ in a laser SLA 3D printer is equivalent to the laser spot size.

This makes comparing the XY resolution of laser SLA printers to DLP and LCD-masked printers challenging, especially considering many manufactures don’t publicize the pointing precision of proprietary galvos. Unlike DLP and MSLA, which have a fixed matrix of pixels relative to the build area, laser-based SLA machines are able to focus the laser beam on any XY coordinate. This means that laser SLA 3D printers using high-quality optics can more accurately reproduce the surface of a part even when the laser spot size is larger than the DLP pixel size. 

To illustrate this point about laser SLA, we can examine a dimensional accuracy study for the FormLabs Form2 printer. They examined 84 surgical guides printed with Dental SG resin and compared the printed guides with the original .STLs of the intended models and found dimensional accuracy within ± 100 microns over 93% of the surface area.For orthodontic applications, 100microns is excellent dimensional accuracy. The difference map below shows an example 3D scan of a printed surgical guide using their Dental SG resin overlaid with the original 3D model. Even though the laser spot size of the Form2 is 140microns, you can see the high dimensional accuracy produced in much finer detail than the laser spot size.

Make sure you are comparing apples to apples when examining the resolution of 3D printers for your orthodontic practice. Hopefully, this information should help.

Comparing Laser Based SLA, DLP-SLA, & MSLA 3D Printers for Digital Orthodontics

The word “stereolithography” comes from the Greek words “stereo,” meaning solid, and “(photo)lithography,” which is a form of ‘writing’ with light. Stereolithography with your 3D printer does exactly that: drawing solids with light one layer at a time.

There are three main categories of SLA processes, laser-based stereolithography (laser SLA), digital light processing stereolithography (DLP-SLA), and masked SLA (MSLA). For all these processes, a vat of photo-reactive liquid resin is selectively exposed to light in order to form very thin solid layers that stack up to create one solid object.

Even though laser-based SLA, DLP-SLA, and MSLA are all types of stereolithography and utilize similar technology, they can produce significantly different outputs. Understanding the subtle difference involved in each 3D printing process will help you know what to expect from your final builds, how to better design your production workflows and maximize the potential each type of machine, and make more informed purchasing decisions. Let’s dive into what exactly that means.

Types of SLA Printers

Laser-based SLA was the original means of stereolithography that was developed in 1986 by Charles Hull, co-founder of 3D Systems. The technology works by using a UV laser to draw each layer of the object and uses two mirrors driven by a motor, known as galvanometers or ‘galvos’ (one on the X axis and one on the Y axis), to rapidly aim the laser beam across the print area, solidifying resin as it moves along. In order to create a solid object the design must be broken down, layer by layer, into a series of points and lines that are given to the ‘galvos’ as a set of coordinates and the laser traces them out. Laser SLA Orthodontic 3D printer examples: FormLabs Form2.

DLP-SLA uses a digital projector to flash a single image of each layer across the entire platform at once. Because the image of each layer is digitally displayed, it is composed of numerous square pixels, resulting in a layer formed from small rectangular bricks called voxels that stack up along the Z axis. DLP-SLA Orthodontic 3D printer examples: Park Dental Research/Orchestrate Juell Flash OC, Envisiontec Vida, etc.

MSLA utilizes an LED array as it’s light source together with an LCD photomask in order to shape the light image from the LED array. Like DLP, the LCD photomask is digitally displayed and composed of square pixels. The pixel size varies based on how the LCD photomask is manufactured, and individual pixels are deactivated on the LCD to allow the LED light to pass through to form the resulting layer. Thus, the XY accuracy is fixed and does not depend on how well you can zoom/scale the lens as is the case with DLP. MSLA Orthodontic 3D printer examples: Structo Orthoform (4Qube is the US distributor).

Layers draw by a laser and those build with voxels are quite different so it is difficult to compare laser SLA with DLP-SLA and MSLA by numerical specifications alone, but you can begin to understand the slight differences in the end products.

Print Size and Speed with Laser SLA, DLP-SLA, and MSLA

Since each entire layer is exposed all at once, rather than drawn out with a laser, both DLP-SLA and MSLA can achieve significantly faster print times than laser SLA. This increased speed is most noticeable with styles of 3D printing that allow for continuous light exposure, and when building certain kinds of objects. Hollowing out dental models with DLP-SLA and MSLA printers, saves material, but does not make a meaningful impact on print times.

For large, fully dense prints, where the print will fill up most of the platform, each layer is exposed significantly faster with DLP-SLA or MSLA than it would be if drawn out by a laser. Also when printing very small and finely detailed objects, where it would take a laser a long time to move along all the coordinates, DLP-SLA and MSLA can also build at a faster rate.

Although DLP-SLA and MSLA can build significantly faster, the speed of an individual printer becomes almost irrelevant when operating a 3D print farm at scale for digital orthodontics, capacity and redundancy are more important. Additionally, both DLP-SLA and MSLA can introduce tradeoffs in resolution and surface finish with certain types of prints.

DLP-SLA resolution depends on the quality of the projector, which defines how many pixels/voxels are available. For example, full HD is 1080p. The light source from the projector in must be focused to an image size in order to achieve a given X-Y resolution for each layer on the build platform. This can become a problem when printing numerous detailed objects together using small voxels. The projector only has a set number of pixels so in order to print in the greatest detail the projected image must be shrunk down. This can be managed with various means of optical correction, but as a general rule finer details on a DLP-SLA printer are best achieved by using a fraction of the overall build area, and large prints can only build at the coarsest resolutions for the projector.

With MSLA, the pixel size of the LCD photomask is set based on how it is manufactured so individual pixels can only be turned on or off to block the light from the LED array. They cannot be shrunk to attain finer detail and surface finish on small individual objects, and the light from the LED array cannot undergo any optical correction without a lens. The LCD units that mask the LED light are vastly less expensive than DLP projectors, which can be advantageous, but those cost savings are not always passed on to the consumer.

Laser-SLA print speeds tend to be on par with FDM-style (material extrusion) printers, but offer superior resolution and output quality and a wider range of materials compared to FDM printers. The general rule is that longer the path the laser must trace, the longer the print time, so hollowing out orthodontic models can reduce print times substantially. As opposed to DLP and MSLA, the print resolution of a laser SLA machine is completely independent of the build volume. A single print can be any size and any resolution at any location within the build area.

Variations in Surface Finish

Naturally, because 3D printed objects are made of layers, 3D prints often have visible, horizontal layer lines. However, because both DLP-SLA and MSLA render images using rectangular voxels, there creates an effect of vertical voxel lines.

In the photo below, you can see these vertical voxel lines on the surface of DLP-SLA, MSLA sample print on the right as compared to the laser SLA print on the left.

Because the voxel unit is rectangular in shape, voxels also have an effect on curved edges. A good way to conceptualize this is to think about how you would build a round shape out of Lego bricks—the edges will appear stepped on both the Z axis and the X-Y plane. This is known as Voxel Effect and it can be seen in detail in the photo below.

For the purposes of printed orthodontic models that will be used to fabricate retainers, clear aligners, and other appliances, surface finish and voxel effect of the models themselves make little difference. Only the calibration of the machine and overall resolution of the printed model would matter. However, when it comes to direct 3D printed of intra-oral appliances like occlusal splints, dentures, pontics, etc. with the wave of biocompatible materials on the way, the quality of the surface finish may be very important to avoid having to do significant post-processing.

By understanding the differences in 3D printing technology and their outcomes, it’s much easier to select the correct 3D printing solutions and design an appropriate workflow for your practice. Register for free membership for access to a basic 3D printing workflow for your orthodontic practice using time-efficient cloud based tools.

*Images & Graphics adapted from FormLabs, I’m not an artist

When purchasing a 3D printer for your orthodontic practice, it is important to understand how the various styles of 3D printers differ from one another and how this can impact print speed and quality.

There are 3 basic styles of 3D printing:

• Bottom Up Printing, where resin is cured through a window in the bottom of the vat by a light source from below. In this style of printing, the build platform is raised out of the resin vat and a ‘peel’ step is required between each layer in order to detach the cured material from the bottom of the vat. This ‘peel’ step is by far the slowest part of SLA printing with most modern light sources.
  Printer Examples: Formlabs Form2, Envisiontec Vida, Structo Orthoform, etc. 
• Top Down Printing, where resin is cured by a light source above the vat and the build platform is lowered down into the resin vat. Rather than a series of alternating light exposures and ‘peel’ steps this style of printing uses continuous light exposure (like a movie) to cure the resin and can achieve exceptionally high print speeds due to the lack of a ‘peel’ step.
  Printer Examples: Juell Flash OC
• Continuous Liquid Interface Production (CLIP), where resin is cured through an oxygen permeable window in the bottom of the vat by a light source from below. The oxygen layer over the window keeps the resin from sticking to the vat and continuous light exposure can be utilized because no ‘peel’ step is required. CLIP essentially is a bottom up printing technique without the ‘peel’ step, so top down style print speeds can be achieved. CLIP technology is currently very cost-prohibitive.
  Printer Examples: Carbon M1

Most consumer SLA printers, and those in use in orthodontics, will utilize bottom-up-and-peel techniques for several reasons:

• Less resin is required to fill the vat if a part is being pulled out of the resin versus being lowered into the resin so less resin has to be purchased at once.
• Since a smaller resin vat is needed, bottom up printers can be built smaller and they tend to have fewer mechanical parts, such as leveling devices submerged in resin.
• Manufacturers see the resin vats and windows used for bottom up printers as a profit-generating consumable since they will have to be replaced somewhat frequently due to wear and tear.

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A Smarter Way to Compare the Price of 3D Printers for Orthodontics

If you are looking to buy a 3D printer, you may feel overwhelmed by the abundance of choices on the market and these companies will engage in all kinds of smoke and mirrors to justify overpaying for a spec sheet that is the same as other printers so buyer beware. Without getting into all the nitty-gritty of 3D printing just yet (we will be doing a deeper analysis on the various types of 3D printers and printing software in future posts), I wanted share the most effective means of evaluating the true upfront costs of any 3D printer, which is to consider it’s price as a function of hourly or daily throughput. 

When you are operating at scale like an orthodontic lab, the overall speed of the individual printer is actually not that important because you will be batching prints and incorporate a lead time into production. You can set up a print farm with smaller, cost-effective 3D printers that has built in redundancy in case a printer breaks and a larger overall capacity than paying the same amount for a more expensive, “faster” 3D printer.  All 3D printers currently operate at build speeds that allow for same day delivery, so this argument is a red herring. Additionally, ‘slower’ printers with larger build platforms will perform better with daily throughput because they will continue to work overnight.

By examining the cost of printers in terms of the investment required to achieve a certain level of hourly or daily throughput, you’ll have a much better sense of the capacity you are purchasing. You can look at a quick cost comparison of commonly available models on the orthodontic market below:

Hopefully, this information allows everyone to start onboard this technology in a cost effective fashion so it can start benefiting your patients!