In terms of the most common consumer-grade 3D printing technologies, filament (FDM) printing and resin (SLA) printing are the two most prominent.
You are most likely familiar with how FDM printing works.
On an FDM system, a thermoplastic filament is passed into a heated nozzle, where it extrudes shapes onto a build platform.
As the deposited layer cools, it hardens, and the process repeats, extruding a new layer of molten plastic onto the previous layer.
Resin printers use thermoset plastics, rather than thermoplastics.
Rather than suring hard by freezing like a thermoplastic, they cure when exposed to light, which triggers a polymerization reaction in the resin.
We refer to these resins as photopolymers, because of this curing mechanism.
Similar to FDM printing, resin printing generally uses a moving z-axis, where the cured layer is deposited.
After a layer is hardened, the print bed moves up (or down) the z-axis, and the next layer is deposited and cured onto the preceding one. Rinse, and repeat.
Resin printing is largely categorized according to the type of light source used to initiate curing.
The most common types are:
- SLA (stereolithography)
- LCD (liquid crystal display)
- And DLP (digital light projection)
We will look at these in detail in the next section.
In terms of the advantages of resin printing, photopolymer 3D printing is known for its high accuracy and detail, as well as its ability to produce parts with smooth surfaces and fine features.
This makes it a popular choice for creating prototypes, models, and parts with intricate details like miniatures.
Additionally, most resin processes are quicker than filament based processes, because on a per-layer basis resin printers print almost instantaneously.
However, on the downside, it can be more expensive than other 3D printing technologies, and the resins used in photopolymer 3D printing may have limitations such as lower strength compared to some other materials.
Overview of the resin printing process
Regardless of which specific process is used for the resin printing, the photopolymer reaction works on the same principles.
Photopolymerization is a process in which light is used to initiate and drive a chemical reaction in a polymerizable material.
It involves the conversion of a liquid monomer or oligomer into a solid polymer by the addition of light energy.
The process occurs in three stages:
- Absorption: The monomer or oligomer material absorbs light energy, which causes the molecules to become excited.
- Initiation: The excited molecules react with a small amount of a chemical initiator, which triggers the start of the polymerization reaction.
- Propagation: The reaction continues as the excited monomer molecules combine with each other to form long polymer chains.
Different resins are activated at different light wavelengths.
Most low cost LCD systems use a UV lightsource with a wavelength of 405 nm, while higher end resin printers use 385 nm light sources.
385 nm printers are currently exclusively DLP based.
Now, let’s take a look at the three most common photopolymer-resin printer technologies.
SLA
SLA (stereolithography) is the oldest form of resin printing technology.
In the traditional sense of the term, SLA typically uses a laser to begin the curing process.
The laser fires its beam at a motorised mirror, which reflects the laser beam into the resin vat, curing it layer by layer.
The beam shape in an SLA system is round, so the smallest individual feature will have a round edge, which viewed under a microscope.
LCD
LCD (liquid crystal display) processes, are also known as mSLA, or masked SLA.
Rather than using a laser to scan along the shape of the g-code (which takes longer), the mSLA process uses an LCD screen to flood the entire layer with light.
The mask is caused by dark pixels on the LCD screen, which prevents certain sections of the layer to remain uncured, which the part geometry is cured by the active / illuminated pixels.
One thing to note, while LCD printers are incredibly cheap to buy nowadays, the LCD screen is a consumable, and may need replacing in as little as 500 hours of printing.
Take this into account if you think that an LCD printer is a bargain.
For certain applications, they are great value, but if you’re thinking of running a larger LCD printer 24 hours a day, you may find yourself with a monthly bill of several hundreds of dollars in recurring LCD replacement costs.
If you are thinking of doing such high throughput printing, then you are entering the realm of industrial printing, and therefore you may wish to consider an industrial grade printer such as a laser-based SLA or a DLP system.
Because the pixels on the LCD are square shaped, the printed voxels are also square, although this is not noticeable without a microscope.
DLP
DLP printing is a type of 3D printing technology that uses a projector to reflect all the light to a pixel via a micromirror (of which there are hundreds on a chip).
Therefore, there is no light convergence, which allows for crisper black/white contrasts.
LCD printers, on the other hand, converge all the light to a pixel, which can cause bumps and shadows on the edge of models.
Apart from the light source, these two technologies also vary in terms of printing speed and accuracy.
DLP printers are generally faster as their lights are more intense, allowing for quicker curing of the resin.
They also perform well when it comes to small-scale accuracy, making them suitable for creating intricate details.
On the other hand, high-end LCD printers tend to be more accurate when printing larger models.
This is because the LCD screen can provide a higher resolution image, resulting in finer details and smoother surfaces.
Overall, the choice between these two technologies depends on the specific requirements of the project and the desired outcome.
DLP-based systems last a lot longer than LCD-systems in terms of the light source.
While an LCD screen may need replacing every 500–1000 hours, a DLP light source can last up to 20,000 hours, and this is especially true for those using the Texas Instruments DLP technology.
While DLP is a higher-end technology, there are lower cost printers available, such as the Anycubic Photon D2, which retails for around $600.
Types of Resin Materials
- Standard resin: Cheap and cheerful, these are the most common type of resin used in resin printers. It is activated by UV light and cures (hardens) when exposed to it. At the lower end of the cost spectrum, they can be purchased for around $30 a liter, and are good for general prototyping or for examining the form and fit of parts.
- Dental resin: This type of resin is specifically formulated for use in dental applications and is biocompatible and safe for use in the mouth.
- Flexible resin: These elastomerics resins are formulated to be flexible and elastic, making them suitable for printing flexible parts and prototypes for parts such as living hinges, and even seals and gaskets.
- Castable resin: This type of resin is formulated for casting, allowing you to create high-quality metal parts for jewelry, toys, and other products. They are designed to mimic jewelers’ resins, and are used for pattern making in the investment casting process.
- Transparent resin: This resin is formulated to be transparent, allowing you to create clear parts and prototypes that let light pass through.
- High temperature resin: High temperature resins are formulated to withstand high temperatures, making them ideal for making prototypes or end use parts that will be exposed to heat. One such application is mold making. They often have a high HDT (heat deflection temperature) meaning they resist deformation under high temperatures.
- Engineering resin: This resin is formulated to provide high strength, stiffness, and toughness, making it suitable for engineering applications such as prototyping and functional testing. Engineering resins are often much higher in price.
- Bio-based resin: This type of resin is made from renewable and sustainable sources, making it an environmentally friendly option for 3D printing. Many of these are based on soybean oil which is converted into acrylates to be used in 3D-printing resins.
- Filled resins: Some resins benefit from the addition of fillers, such as carbon, making them resin-based composites. The addition of filler can increase the strength and stiffness of the bulk material.
Examples of applications and industries
Medical
3D printing is used extensively in medicine, particularly for surgical planning, when surgeons are preparing for complex and dangerous surgery.
Custom resins have been fabricated for all manner of medical applications, including targeted cancer treatments, and biosensors that could one day be implanted inside human beings.
While commonplace resin implants may be a while off yet, they are often used as surgical guides, molds, jigs, fixtures, and other end-use devices.
Many resins are heat-resistant, meaning they can be sterilized, and others are biocompatible, meaning they do not kill human cells.
Dentistry
Dentistry is an industry that requires a large amount of customization, because teeth are all different!
Traditionally, dental products (such as implants) are milled with a CNC machine, which is time-consuming and expensive.
Thanks to resin printing (and 3D scanning), dentists are turning more to 3D printing these days, with many clinics having 3D printers on site for chairside manufacturing.
Resins offer great material properties, biocompatibility, and high resolution – perfect for a range of dental products.
Microfluidics
Microfluid devices require smooth interiors in order to have predictable flow through the channels.
Resin based printer technologies offer higher resolutions and therefore better surface finishes, which are well suited to microfluidic devices.
There are also many transparent resins available, which allow the engineer to see what is going on inside the device.
Prototyping
Sometimes you need a prototype part made quick, and you need it accurate.
In cases like this, FDM printing may not be up to the job.
When designing parts with tight dimensional tolerances, and you don’t want to wait for a CNC machine, then resin printing offers a solution.
While most standard resins may not compare to FDM plastic parts in terms of durability and strength, they provide good dimensional accuracy and excellent surface finishes, which may be exactly what you need for a prototype, or form and fit testing.
Molding
Molding comes in many forms, whether it’s injection molding or casting or anything in between.
As the item being molded takes the surface finish of the mold in question, it is critical to the part’s finish that the mold has a smooth surface also.
This is where resin printing comes in.
Many resins are also machinable, meaning an even higher surface finish quality can be achieved. In addition, many molding processes require high temperature resistance, as they use molten thermoplastics as the feedstock.
There are resins on the market now that have high temperature resistance that are well suited for low production run injection molding.
There are even soluble resins that allow highly complex parts to be molded that would not be possible without 3D printing.
Conclusion
In conclusion, resin printing is a photopolymer 3D printing process that uses light to cure liquid resin into solid polymer layers.
The process occurs in three stages: absorption, initiation, and propagation.
Resin printing is generally quicker than filament-based printing and is known for producing highly accurate and detailed prints with smooth surfaces and fine features.
However, it can be more expensive than other 3D printing technologies, and the resins used may have limitations in terms of strength compared to other materials.
The three most common resin printer technologies are SLA, LCD, and DLP.
Each technology has its advantages and limitations, and the choice of technology will depend on the specific requirements of the print job and of course, the cost.
Just remember, there are lots of consumables involved in resin printing, not just the resin itself.
Many resin systems require the tanks themselves to be replaces, as the constant use of resin can rescue the transparency, affecting the print quality.
Other systems may require changing of the plastic layer on the build plate after repeated use, so be sure to read system-specific reviews before making a purchase.