How 3D Printer Resin Works – A quick Overview

Curtis Satterfield, Ph.D.

Curtis Satterfield, Ph.D.

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A common question I am asked is how exactly does 3D resin go from liquid to a solid? The answer involves a little chemistry and physics and is easy to understand.

The liquid resin used in photocuring resin printers is a mix of chemical monomers and oligomers. When exposed to UV light, the energy from the light causes the monomers and oligomers to cross-link forming polymers. These polymers bond to one another and to the existing polymers on the body of the 3D print.

There are several different types of resin with different properties. All the resins work in a similar manner but have different physical properties when cured. It is also important to know how to work with and properly store resin. Failing to follow proper procedures can drastically reduce the shelf life of your resin.

How 3D Printer 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.

Why Do I need to Post-Process My Prints?

The first step in post processing is to wash the print in a solvent. This step removes any uncured resin still sticking to the print. Due to the viscous nature of the resin it will cling to the print and you will lose details if you cure the print without washing. The second step is the final cure. The resin is not fully cured when it comes out of the printer. Most prints are still soft and pliable as the printer does not fully cure each layer. You will need to use a curing chamber to finish the chemical reaction resulting in a solid print.

Types of 3D Resin and Their Uses

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.

How to Store 3D Printing Resin

Because of its photosensitive nature, 3D resin must be kept in a light proof container. Always store unused resin in a dark place out of direct sunlight. You can leave resin in your printer’s vat. Resin can be left in the vat for a few days provided the cover is kept on the printer. You can also purchase or make vat covers and store your vats in a dark container. This will allow you to swap resin without cleaning the vat each time.

If you leave the resin in the vat for any length of time be sure to properly mix the resin before printing! If the resin has been sitting for an extended period it may be a good idea to filter it into a bottle and shake well before using. If the resin was stored in a covered vat it should be safe to return to the original bottle. Be sure to use a paint strainer to filer out any particulate matter in the resin. I use these paint strainers (Amazon link) with a funnel when pouring resin from the vat into the container.

Unopened bottles of resin can last months or even years if stored properly. Each manufacturer will have a shelf life associated with their specific products. If you have resin that has been sitting in the bottle for a long time you may experience a failed print when attempting to use the product.  

Related Questions

Does 3D Printing Resin Smell?

All 3D Resins have some amount of odor to them. Read my article on what 3D printing smells like for more information. It is important to wear proper protective equipment when working with resin. Always wear a respirator!

What Do I Need to 3D Print with Resin?

Resin printers require more supplies than just a printer and some resin. If you are thinking of getting a resin printer read my article about all the supplies you will need. The short answer is you need more supplies than you may realize.

Precautions when Working with 3D Printer Resin

It is important to wear personal protective equipment (PPE) when working with resin. Resins are toxic and you should avoid breathing the fumes or getting it on your skin. Always wear an appropriate respirator and nitrile gloves (Amazon link) while working with resin. I recommend a respirator made to filter volatile organic compounds like this one from 3M (Amazon link)