SLA 3D printers use mirrors, to aim a laser beam across a print area. Curing and solidifying a photopolymer resin as it goes.

SLA 3D printers use mirrors, to aim a laser beam across a print area. Curing and solidifying a photopolymer resin as it goes.

SLA 3D printers use mirrors, to aim a laser beam across a print area. Curing and solidifying a photopolymer resin as it goes.

SLA 3D Printing - Stereolithography

SLA 3D Printing - Pros

High Detail
Smooth Surface finish
Great tolerance

High Detail
Smooth Surface finish
Great tolerance

High Detail
Smooth Surface finish
Great tolerance

SLA 3D Printing - Cons

Visible build lines
Supports needed
Post processing often neccesary

Visible build lines
Supports needed
Post processing often neccesary

Expensive Resin
Longer printing times
Low impact strength

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What is Stereolithography?

 

Sterelithography machines use an ultra-violet light. They build models by changing liquid resin solid, as well as, photopolymerization. This process is repeated layer by layer until the solid model is complete. In addition, mirrors direct the laser beam accross the build platform which either moves up or down. This forms each layer. Furthermore, highly intricate detail, can be achieved in a short space of time. There are two types of SLA printers: bottom-up and top-down

SLA Materials

 

SLA machines use thermoset photopolymers to build models. This viscous liquid is then solidified by a laser. Standard resins cost around $50 per litre. Whereas, high detailed and castable resins can reach up to $400. In addition, color variation is limited. Also, SLA resins have a limited shelf life of one year.

 

UV curing of parts in stereolithography is paramount. Otherwise parts will not achieve thier top properties. Correct UV exposure times are listed with each type of polymer.

Same print of castle chess piece, in differen SLA resins.
Common SLA resin properties

Standard

 

Smooth Surface Finish

 

Brittle

Transparent

 

Transparent

Castable

 

Castable with low ash after burnout

Durable

 

High Stiffness

 

ABS or PP like

High temp

 

Temperature resistant

 

Thermoforming tooling

Flexible

 

Rubber-like flexability

 

Not suitable for high accuracy.

Top-down stereolithography printer

Stereolithography Perameters

 

SLA printers use three changable perameters to build. These are:

 

– Layer height

 

-Light source Resolution

 

-Part orientation/ Support location

 

Most printers auto-change the perameters depending on the material being used. Layer heights of as little as 25 microns can be achieved. Therefore giving SLA parts a very smooth finish.

SLA Supports

 

Vat polymerization requires supports, like most 3D printing processes. In addition, Parts are printed at an angle, due to the peeling stage. Each new layer is solidified and must be temporarily seperated from the vat. Thus, allowing this layer to cool. After which, it is dipped back in to the liquid, ready for the next layer. As a rule, the print will fail if the build platform can not be seperated from the vat.

 

There is only one vat. Therefore, support structures must be removed by hand, because they are always printed in the main build material.

Stereolithography bottom up machine in progress
SLA curling example
SLA Dimensional Accuracy

 

SLA falls victim to curling, much like FDM 3d printing. Large flat surfaces are the most prone. Each layer shrinks during solidification. Consequently, stress is caused when the next layer is added, to the already shrunk layer.

 

Supports play a vital roll in anchoring the risky sections to the build platform. In addition,  limiting large flat areas, as well as, part orientation are also important.

 

Dimensional deviation also occurs during the seperation stage. The picture on the left show, the forces causing the soft print, to bend slightly. Subsequently, accumilating layer by layer. In conclusion, highly flexable resins are not suitable for highly accurate prints. 

SLA Post-Processing

 

Stereolithography Printers are capable of printing very fine detail. They can achieve feature sizes as small as 0.3mm. However, most prints will require supports. And in addition, must be removed post process. Supports are added automatically by splicing software. A good CAD designer will change the orientation, to position supports in the least significant areas.

 

SLA resins have many finishes that can be applied after printing. Aside from support removal and sanding, you can go even further to achieve a smoother finish. Such as, polishing, painting or even a mineral oil finish.

stereolithography post processing before and after

Support removal

 

Support structures are built in the same material as the model. Thus, they need to be removed. They are printed in a tower like form, with a narrow tip. As a result, they are easier to remove. In this case, they are removed by hand, using pliers. After which, small marks are visible.

Sanding

 

Sanding down, of the support marks is neccesary to achieve an over all smooth finish. In this case, 600 grit paper is ideal. However, 800 – 1000 grit is reccomended for a injection mold like smooth finish. Wet sanding offers the best preperation for painting.

UV Curing

 

 

Post curing in stereolithography, helps get the best out of your parts. Moreover, making them stronger and perform better. This is because, the photo-polymarisation reaction is still happening. Therefore, full mechanical properties have not yet been achieved.

Support removal SLA
post-processing SLA sanding
UV curing SLA
Stereolithography strengths and weaknesses

SLA’s Greatest assets are the high levels of detail and smooth surface finish. As a result, SLA is ideal for producing injection mold-like prototypes. In addition, the high levels of intricacy and detail, see SLA being used in dentistry and jewelery. SLA is regularly used for prototypes that need to show finer details of the model.

Resins house the biggest limitations in SLA 3d printing. In particular, their mechanical properties. They are generally brittle, making them un-suitable for end use. Furthermore, being photo sensitive, they degrade over time. In particular, through loss in color and becoming ever more brittle. Coating can be applied, to extent a parts life. However, this will only slow the process.