Stereolithography Technology in 3D Printing

Introduction to SLA Technology: Stereolithography commonly known as SLA is the earliest 3D printing technology ever invented. Charles (Chuck) W. Hull was granted patents over SLA in 1986. The stereolithography technique for 3D printing belongs to a family of Additive Manufacturing technologies known as the vat photo-polymerization technique which is otherwise called resin 3D printing. Vat photopolymerization is one of the prominent 3D printing processes in which liquid resin is cured into hardened plastic on exposure to the light sources – laser or projector.

The 3D printers that are equipped with the SLA technology use a thermoset material called resin. These resins are light-reactive meaning they react to light when exposed to certain wavelengths. The short molecular chains present within the resin are joined together when exposed to light, which then results in polymerization of monomers and oligomers. These monomers and oligomers when polymerized form into solidified rigid or flexible geometries. When these are cured further to perfection, we obtain a stunning 3D printed object.

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Stereolithography Technology in 3D Printing – Process Explained

Once the 3D printable file format is fed into the 3D printer, the model is then sliced into a series of fine layers by the printer’s software. The first cross-section of the STL file is then drawn on to a VAT UV curable resin. Soon after the completion of the first layer, the platform is again loaded with a fresh layer of resin, covering the build surface. The next layer is then traced out, curing and bonding the layer below. This process is repeated layer after the layer until the model is fully grown.

Once the printer is done with printing all the layers as instructed by the 3D design, the machine platform is raised allowing the excess resin to drain. The model is then removed and safely washed in isopropyl alcohol (IPA) to remove any supporting structures or uncured resin from it. It is then placed in the UV curing oven for further hardening. This helps the model in gaining its fullest strength. Once the final hardening is done, it is then painted if necessary.

*A graphic representation of the basic mechanics of stereolithography 3D printing.*

Working Principle of SLA Technology –

Stereolithography uses a photo-polymerization technique to produce 3D models using UV curable resin. Since the resin is cured in vat via a light source, the method is called vat photopolymerization.

Vat photopolymerization:

Photopolymerization is a technique in which a liquid polymer is hardened when exposed to a particular bandwidth of light. Since the photopolymerization is carried out in a vat, the process is termed as Vat Photopolymerization.

SLA 3D Printing Workflow | Stereolithography Workflow –

The workflow of a 3D printer equipped with stereolithography technology is as follows.

#1.Design –

For building a stunning 3D object using stereolithography, the first stepping stone is to produce a 3D model by using CAD software. Or one can even replicate an object by scanning it with a 3D scanner. Either way, you will have a printable file format (STL or OBJ).

Now that we have the digital data on how your 3D model should be printed, go ahead and put the file to your 3D Printer. Each 3D printer in the market comes with a software interface. The interface is different for different manufactures but with most 3D printers, the software should be intuitive and doesn’t demand any expertise in operating it. Feed-in your 3D printable file to the printer and let it do the rest. The software then slices down the digital model into a number of printable layers and sends the same as printing instructions to the printer. This can be wireless or through a cable connection, depending on your printer’s make and specs.

Expert Tip:

You can do little work on your model at this stage to craft off any unnecessary structures in the designs or hollow parts of your model to conserve printing material.

#2.Printing –

After the digital model is fed into the printer, the printing process should start in the stipulated time once the printer is done with the preparation time. During the preparation time, the printer goes through a calibration process and then heats up the necessary components to carry out the job. You don’t have to worry as these processes are automated and don’t require your attention.

Sit back, relax, and grab a coffee or two until your printing job is done!

#3.Post-printing, Warping, Curing –

Unlike Selective Laser Sintering (SLS) or Binder Jetting techniques, Stereolithography (SLA) uses supports to complete the printing of an assigned model. These supports are to be washed off with a solvent to resin. The most commonly used solvent is isopropyl alcohol (IPA). Once the excess material from the model is rinsed off with IPA, the model is then subjected for a post-treatment to strengthen it.

After the model is rinsed and air-dried, it is then fed into a UV curing oven for final hardening. This is where the model achieves its fullest strength.

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Top-down vs Bottom-up approaches in SLA 3D Printing –

Based on the printing approach, the stereolithography 3D printers are essentially classified into two kinds, the top-down and bottom-up. Most SLA 3D printers on the market follow the top-down approach while there are some who had gone for bottom-up. However, it isn’t judgemental of which approach is the best as each of it has its own ups and downs.

Top-down Approach Advantages & Disadvantages

Pros

As there is no necessity for separation of the model from the build plate for every single layer printed, this approach is promisingly faster compared to the bottom-up.
High-reliability factor in general.
Support structures are less needed compared to that of the bottom-up approach as there is no need for printing the model at an angle.
Print failure chances are less as the force on the 3D part is comparatively less.

Cons

It requires more resin.
Machine sizes are usually large.
The thickness of the resin between the surface and the top of the 3D model is to be cautiously taken care of.

Bottom-up Approach Advantages & Disadvantages

Pros

The machine size is smaller compared to that in the case of the top-down approach.
Resin required is less as the model is pulled out of the vat as it grows.
Controlling the thickness of the layer is much easier compared to that of the other alternative approach.

Cons

SLA 3D printing must be printed at an angle hence there is a high chance of getting more support structures which ultimately consume more resin.
There is a greater chance of print failure due to the part’s weight.
The resin vat is to be replaced more often in order to avoid losing print quality.

Advantages and Disadvantages of Stereolithography Over Other Techniques –

Advantages of stereolithography:

High accuracy with almost no visible layers. As the prints that SLA can produce are of high dimensional accuracy, this technique is ideal for print jobs that require high precision such as dental, jewelry and hearing aid, etc.
It can produce a smoother surface finish on models.
SLA printers can produce objects with high water tightness and air tightness characteristics. This makes this technique more advantageous for automotive uses and biomedical research.

Disadvantages of stereolithography –

Parts printed using the SLA are prone to degrade when exposed to sunlight. However, the usable life span can be extended with a special coating on the surface.
The lifespan of the SLA models is less as they tend to lose their mechanical properties over time.
The printing speeds are slow compared to that of DLP. This is because of the fact that the SLA 3D printers cannot trace the cross-section of a model in a single frame, unlike DLP.
Polymer materials used in stereolithography are brittle and are not as strong as those used in the other 3D printing technologies.

Applications of SLA 3D Printing | Stereolithography Applications –

Because of the versatility offered by the SLA 3D printed objects, they comfortably fit most needs that demand precision. The applications of SLA 3D printing are endless. Some of which are stated below.

Manufacturing –

SLA can be used in the manufacturing industry for prototyping tooling and directly 3D printing custom tools. Can be used to make moulds and other essential accessories at far lower costs.

Dental –

3D printing has brought a revolution in dental technology. Using this it has now become easy to replicate dental models with high precision without having to compromise on the quality.

Healthcare –

With the help of 3D printing technology, the development of new surgical cutting and drill guides, prosthetics, as well as the creation of patient-specific replicas of bones, organs, and blood vessels, have become easier at low unit costs with superior fit and repeatable results.

Jewelry –

Replication of jewellery has now become easy with the advancement of 3D printing technologies. Jewellery professions can now prototype designs with the aid of CAD and 3D printing.

Entertainment –

3D modelling has now seen miles in the entertainment industry especially in the areas of motion pictures, video games and even for creating some kind of special effects in the filmmaking industry.

Engineering and Product Design –

Almost everything you imagine can be 3D printed and unfortunately, there is no limitation for imagination. With the advent of 3D printing, engineers are now seeing a world full of possibilities.

Watch how stereolithography (SLA) 3D printing works.