How Resin 3D Printers Work: A Quick Guide

Curtis Satterfield, Ph.D.

Curtis Satterfield, Ph.D.

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You may have heard of resin printers or have seen the amazing prints they are able to create. If you’re new to 3D printing you may be wondering how resin 3D printers work.

A resin printer uses a photocuring UV resin as the print medium. The resin is exposed to a UV light source that cures the resin causing it to harden and stick to the previous layer.  The build plate then rises, and the process is repeated until the print is finished.

For this article we will be focused on Digital Light Projector (DLP) resin printers. DLP is the primary style of entry level resin printers used by hobbyists.

DLP printers use an LCD screen to mask a UV light source that projects up through the image on the LCD screen and hardens the layer of resin. The printer then raises the build plate up and lowers it back down into the resin before the next layer is exposed. This process repeats until the print is finished. In this article we will examine resin 3D printing in detail.

Resin Printer Terms

It will be helpful to review the different parts of a 3D printer. Below is a list of parts and what the part does.

Build Plate – The surface that the hardened resin attaches to during the build process. The print bed is usually made of metal which allows for a good suction force from the resin.

Cover/Lid – The top of a resin printer is made of colored acrylic that is treated to keep out UV light. The most common colors for lids and covers are red, yellow, and orange.

Cure/Curing – Exposing the resin to a UV light source causes the resin to undergo a chemical process and harden and is referred to as curing.

DLP – Digital Light Projector works by shining a light source through a an LCD screen to expose resin in the printing vat.

Exposure Time – The amount of time that a layer of resin is exposed to the UV light. This time is set in the slicer.

FEP Film – Fluorinated Ethylene Propylene film is used on the bottom of resin printing vats. The material is a transparent non-stick material that allows the UV light to pass through and expose the resin. Because the FEP is non-stick the resin has a stronger adhesion to the build plate and so the print will release from the FEP film while staying attached to the build plate.

LCD Screen – The Liquid Crystal Display that is used to mask the UV light source and project the layer shape onto the uncured resin.

Model – Digital object designed for 3D printing. Typically saved as an STL file.

Post Processing – The tasks necessary to prepare the print for use after the printer has finished.

Printer Board – This refers to the electronic circuit board that controls the printer.

Resin – The printing medium for resin printers. Made of photocuring monomers that bond to form polymers when exposed to UV light.

Resin Vat – A tank that holds resin in the printer for the printing process. The bottom consists of a clear sheet of FEP film that allows the UV light to expose and cure the bottom layer of resin in the tank.

STL – This abbreviation can have many meanings such as STereoLithography, Standard Triangle Language, or Standard Tessellation Language. STL files are how most 3D models are stored in digital format. It contains data that describes the surface geometry of the object.

Wash – The act of rinsing a 3d resin print in a solvent such as isopropyl alcohol to remove any residual resin from the print before the final cure.

How a Resin Printer Works

After loading a file onto the printer and pressing print, the build plate will slowly lower down the Z axis into the resin vat.

Elegoo Mars 3D resin printer with labels showing various parts of the printer

After the build plate is in position the UV light will turn on and shine through the image created on the LCD screen. In the image below I have emptied the vat of resin and started an exposure test on my Mars so you can see the UV light passing through the image on the LCD screen.

UV light exposure on the Elegoo Mars resin printer

Exposure test on my Elegoo mars demonstrating how the printer works

The UV light will pass through the unmasked areas on the LCD screen and cure the resin where it is exposed to the light. The first few layers, or bottom layers are overexposed to ensure that the print sticks firmly to the bed. Bottom layer times are typically set to 10-12 times longer than regular layer exposure times. After the layer is complete the light will turn off and the build plate will lift about 5mm and then lower back down.

The build plate rises and lowers back down for two reasons. The first reason is to “peel” the printed layer off the FEP sheet on the bottom of the resin vat. Even though FEP is a not-stick material, the resin still clings to the FEP due to suction and the curing process. Slowly lifting the build plate allows the print to peel off the FEP film without detaching from the build plate. The second reason for lifting and lowering is to allow fresh resin to flow back into the space where the print was peeled off the resin.

This process will continue until all the layers have been exposed and the print is complete.

Finished resin 3D print hanging from build plate

Finished resin print on my Elegoo Mars

Once the printing is complete it is time to move on to post-processing.

Post-Processing Your Resin Prints

After the printer finishes there is still a lot of uncured resin clinging to the model. Most printers come with an adapter to tilt the build plate 45 degrees to allow the excess resin to drip back into the vat. We let the build plate hang and drip for about 30 minutes to get as much resin back into the vat as possible.

After 30 minutes is removed from the printer and we need to wash off the extra resin before we can do the final cure. The print is removed from the build plate and put it into a washing container to remove the excess resin. I use the Anycubic Wash and Cure and I highly recommend it! You can read more about it here.

Anyubic wash and cure machine for resin 3D printing

Anycubic Wash and Cure machine. Source: Anycubic.com

After the model is washed in cleaning solution the supports can be removed. Removing supports pre cure will reduce damage caused by the supports. The prints are rinsed in warm water and the supports are gently removed.

After the print had been washed and the supports removed the model is left to air dry. Curing the print before it is dry can result in residue on the print. The print is then put into a curing chamber for the final cure.

As you can see there are several steps involved post printing. In the next section we will cover all the materials you need to 3D print with resin.

What You Need to 3D Print with Resin

Resin printing consists of a series of steps that includes post-processing a model after the print is complete. Unlike FDM printing a resin print is not ready once the printer finishes. It is still covered in extra resin and not fully cured. The following process is an overview for printing in resin.

 

  1. Prepare your 3D model in the slicing software
  2. Print your 3D model on your resin printer
  3. Remove the Print from the build plate
  4. Wash the model in isopropyl alcohol to remove residual resin
  5. Cure the print with a UV source.
  6. Remove Supports
  7. Clean up any marks left on the print from the supports

The process is straight forward, but you will need extra equipment that didn’t come with your printer. This is the recommended process from most printer manufacturers. I like to remove the supports after the wash step and before curing. In my experience this makes the supports much easier to remove are less likely to leave scars on the print.

 

 

What Else Do I Need to Buy?

This is the elephant in the room when it comes to resin printing. People that are new to the hobby see the detailed prints that are possible with resin and become hooked. Then they see that a new resin printer can be purchased for less than $300 and before they know it, they’ve clicked buy. What many people don’t realize is the number of consumables you need. Unlike and FDM printed parts, resin prints are not finished as soon as the printer stops. The prints need to be washed and cured for the model to be ready for use and for that you need more consumables.

Here is a list of the consumables with links to the items you will need for your resin printer:

That’s a lot of extra supplies just to print a resin model. A washing and curing machine will run half as much as you paid for your printer. And you will go through gallons of cleaning solution and hundreds of nitrile gloves! If you’d like to learn more about these items and why you need them head over to my article “What You Need for Resin 3D Printing.” All the necessary consumables for resin printing can double or triple your cost per model!

I cannot stress the next point enough. Resin is toxic, get proper personal protective equipment! The masks that come with some resin kits are not rated to filter volatile organic compounds. Do your body a favor and go buy a proper respirator!

3D Models and Resin Printing

Resin printing is great for smaller models with high levels of detail. The drawback is that resin printers cost more to operate and can take longer than FDM printers. Many hobbyists use resin printers to print miniatures and accessories for tabletop gaming. The high level of detail can make a 3D printed model indistinguishable from a purchased model that was injection molded.

Below is a picture of two models. The model on the left was bought from a store and is made from injection molded plastic. The model on the right was printed on my Elegoo Mars.

Comparison of store bought model versus 3D printed model

Store bought miniature on the left and 3D printed model on the right.

You can see that both models have a high level of detail. In fact, the resin model will look just like a regular model to the untrained eye.

Preparing Your 3D Model for Resin Printing

In order to 3D print there are a couple steps you need to take before actually pressing print. The first step is to find a model that you would like to print. There are many freely available 3D models that you can download from online. Most models will be saved as an STL file. In order to download the models and the software for 3D printing your computer will need high-speed Internet access.

If you want to create your own 3D models using Blender or a CAD program such as AutoDesk Fusion360 you will need a computer to run the software. After you create the model or part in your CAD program you will need to save it as an STL and import it into your slicer.

Regardless of the method used to get your 3D models you will need to import them into your slicer to process. There are slicers specific to resin printers and ChituBox is one of the most common. A model is imported into ChituBox and then must be oriented and supported. Once the supports are added to the model the software slices the model and prepares the G-Code file for the resin printer.

The slicer creates a file consisting of one image for each layer of the model. The images are then displayed on the LCD screen in the print area and a UV light shines through to expose and harden the resin. The image below was taken from a model sliced in ChituBox. On the left you can see the first layer of the model to be printed. On the right is the corresponding image that will be displayed on the LCD screen. The white area is where the light will shine through exposing and hardening the resin.

Image showing first layer of a resin print in Chitubox

Proper supporting of 3D models is important with resin prints. As the resin model lifts out of the vat, the layer must be peeled off the FEP film in the vat. The peeling force applies great strain on the model and without proper supports the model can warp or pull off the build plate.

In the image below you can see the model on the left was properly supported and was successful. The model on the right was not properly supported and failed.

Results of manual versus auto supports

Other important settings in the slicer include the exposure time and lift speeds. The image below shows these settings in ChituBox.

ChituBox print settings
  • Layer height will have a direct impact on quality. Lowering the layer height can increase detail but it will increase the overall print time. I recommend making a test print of the same model at both 0.05 and 0.025mm then compare the outcome. If you are happy with the details from the .05 stick with it. If you like the detail on the .025 model and don’t mind the longer print times, go with .025.
  • Bottom layer count and bottom exposure time are settings that help your print stick to the build plate. Bottom exposure time should be around 10-12 times your layer exposure time. If you have issues with build plate adhesion, check out my article here.
  • Exposure time is how long the light turns on and exposes non bottom layers. This is where you want to enter the value you got form the resin calibration test. Be sure this is set correctly as it will affect your print quality!
  • Bottom lift and lifting distance tells the printer how far to raise the build plate between layers. 5mm works well for most machines and you shouldn’t need to adjust this setting.
  • Bottom lift and lifting speed determine how fast the build plate raises between layers. Decreasing lift speed will allow the print to peel from the FEP more slowly. Slowing down your lifting speed can reduce the risk of failed prints or the model breaking free from supports. The downside is that decreasing lift speed will increase your overall print time. I set my lift speeds to 60mm/min and find it works well for my Elegoo Mars.
  • Retract Speed tells the printer how fast to lower the build plate back down into the resin vat. The default is 150mm/min and should be left at that value.

 

After the model has been sliced by the software it is ready to be transferred to the printer for 3D printing. Before looking at how the printer works let’s talk about how the resin works.

How 3D Resin Works

Unlike fused deposition modeling (FDM) printers where the material starts as a solid, resin printer material starts as a liquid. While the liquid resin is not exposed to UV light it will stay in a liquid state. In order to turn the liquid into a solid it must undergo a chemical process. The process uses UV light as a catalyst causing the chemical composition to change. This change results in the liquid resin changing from a liquid state to a solid state.

To understand how the process works we need to examine the molecular structure of the resin. The resin contains molecules called monomers. Monomers are molecules that can bond with other identical molecules and form a polymerPolymers are substances with molecular structures that are primarily similar molecules bonded together. Many synthetic materials such as plastics and resins are comprised of polymers.

The larger molecular mass of the polymer produces physical characteristics such as toughness and elasticity. The formation of the molecular assemblies results in semi-crystalline structuresThis is known as polymerization and results in three-dimensional networks of polymer chains.

To turn cause the chemical reaction that links the monomers into polymers, the printer uses a UV light source, either a laser or UV LED array, to shine on the resin. Using a laser, the shape of each layer is traced onto the resin causing it to harden. For home use printers, an LCD screen is used to mask out the print layer. The UV light exposes through the clear areas on the screen resulting in a chemical reaction that hardens the resin. Once a layer has been hardened the build plate raises and the next layer is exposed. This process continues until the entire print is finished.

Once the print is done the model is then removed from the build plate and washed with a solvent to remove and residual resin. The model is then placed in a curing chamber to finish curing the resin.

Because of resins photosensitive nature continuing to expose a print to UV sources will result in degradation of the resin over time. If a print must be exposed to UV light it is necessary to treat the print, so the resin does not continue to cure. A simple way to accomplish this is by painting the object.

Precautions When Working with Resin

3D printing resin is toxic, and you need to avoid breathing the fumes and getting it on your skin. Always wear an appropriate respirator and gloves when handling resin. Until your models are fully cured do not handle them without gloves!

You will need a lot of disposable nitrile gloves for resin printing. I recommend getting a pair of kitchen cleaning gloves to use as they are more durable and will last a long time. A pair of these gloves cost about the same as a box of 100 nitrile disposable gloves and will pay for themselves in no time.

You also need a quality respirator that can filter out volatile organic compounds. The masks that come with resin printer setups are not of the highest quality. I highly recommend getting a proper respirator that meets industry standards for filtering VOCs such as this 3M Respirator.

Types of Resin

Standard resin is your basic and usually lowest priced resin. It can come in a variety of colors and is opaque in appearance. Standard resin typically has low viscosity allowing the resin to flow freely during the printing process. Standard resins are designed to reduce part shrinkage when cured. Standard resins also tend to be brittle when cured and may not work well for functional parts requiring high strength.

Transparent resin is another form of standard resin. Transparent resins differ from other standard resin as their appearance is transparent instead of opaque. An interesting aspect of transparent resin is that the UV light can past through the current layer into preceding layers of the print. Because the transparent resin does not absorb the UV light as readily, exposure times for transparent resin tend to be longer. Transparent resin shares the same mechanical characteristics of standard resin.

Casting resin is used in various industries like jewelry making and metal casting. This resin is designed to be “burned out” in a process like lost wax casting. Castable resins tend to be made with waxes that melt when exposed to high temperatures. These resins are designed for this specific purpose and do not work well for other uses.

Tough resin is designed to have higher strength than standard resin. Tough resin is designed to mimic the mechanical characteristics of ABS plastic. Tough resin is ideal for functional prototypes as it produces strong parts that are less likely to shatter. With tough resin you typically need your parts to have a minimum wall thickness. Trying to print models with thin walls can result in failed prints.

Flexible resin is ideal for medical devices, anatomical models, special effects props, and anything that needs to bend and move. The main difference between flexible resin is how much flex the final part has.

For most home uses standard, transparent, and tough resin will be the most common. Many people also find success mixing their 3D resins to improve the final qualities of the print. For example, you could mix a little flexible resin with standard resin to reduce the amount of brittleness in the final print. If you are interested in mixing 3D resins check out the article I wrote on the topic.

What Influences Printing Times

The overall print time is influenced by how many layers the model has and the exposure time per layer. With monochrome LCD printers you can have exposure times as low as two seconds. With RGB screen printers you may need anywhere from 6-12 seconds per layer depending on the resin.

Resin Printers are designed to print with extremely thin layer heights such as 20 or 50 microns. Even though resin printers print a full layer at once because they print such thin layers, they can still be slower than an FDM printer. For example, a 3mm tall object sliced at .2mm for an FDM print has 15 layers. A 3mm object sliced at .05mm layer height will have 250 layers. At 7-10 seconds per layer, the time adds up.

The lift speed that you set in your slicer will also affect the total print time for the print. While lowering the lift speed decreases the chance of the print ripping off the build plate or supports, it increases the overall print time.

Exposure Times and Resin Prints

The most important factor in getting good prints from your resin printer is the layer exposure time. Over or underexposing your prints will result in losing details on your model. You need to ensure that you have the proper layer time dialed in for the resin that you are using. Most manufacturers have a guide on exposure times for their resins. And many printer communities have online spreadsheets showing exposure time for different resin brands on a single printer model.

Using data from the manufacturer or an online guide is ok, but I recommend running a resin calibration test to find the optimal print time for your printer. I use the Multi Printer Resin Exposure Test found here. In short, you will use 3 files from the Git repository. Two files are G-Code files that are needed to configure your printer for the test. The third file is the actual model that you will print to calibrate the resin.

The test will print a small card that has several images and numbers on it. Across the top of the card will be numbers showing exposure time in seconds. Under each number will be a set of symbols that you use to compare and determine which layer exposure time is right for the resin you are using.

The image below is a calibration test I did for Elegoo standard on my Elegoo Mars. The exposure times are outlined in red and the calibration images are outlined in magenta. This picture skews the results a little as looking at the print in person 7.5 had the best balance while on the picture 10 appears to be a better setting. Ideally, you want the positive and negative spaces to be equal on both sides of the column.

Resin calibration print with highlighted times and exposure test

Resin calibration card I made on the Elegoo Mars

Should I Buy a Resin Printer?

At this point you may be considering purchasing a resin printer. If you are looking to print smaller highly detailed models then a resin printer may be right for you. If on the other hand you are looking to print larger models without as much detail FDM might be the way to go. If you are unfamiliar with how FDM printing works you can read my guide here. If you are interested in purchasing a resin printer go read my recommendations for resin printers here.