Within 3D printing materials, resins are a particular case. While filaments and powders are based on thermoplastic materials intended to be shaped by heat (either through extrusion or sintering), resins are based on a reactive liquid mixture of oligomers, crosslinking agents, and initiators.

This difference is especially important because stimuli such as light, temperature, and oxygen can initiate and promote the polymerization reaction. This implies, not only that resin requires better preservation than filaments, but also that it has an expiration date.

In the presence of light, the oligomers present in the resin begin to bond together in a process known as polymerization. When a sufficient amount of oligomers is bonded, the resin solidifies, however, until solidification occurs, the resin's viscosity increases.

Effect of Light on Resins

3D printing resins include photoinitiators, molecules that, in the presence of light, initiate the polymerization reaction causing the oligomers to bond together. Although the initiators used are especially sensitive to blue and UV light (405 nm, 385 nm, and 365 nm), this does not mean they cannot be activated by other types of light, they simply react more slowly.

This is why to preserve resins in the best possible condition, they should be protected from all types of light. It is recommended to remove the resin from the vat when not in use for printing and store it in the container supplied by the manufacturer. Most resin 3D printers include an amber or red cover to protect the interior from blue light entry.

Image 1: Resin Tank. Source: FormLabs.

Although it is an effective solution to protect the resin during short periods of time, it does not prevent degradation in the medium and long term. Some printers include interchangeable tanks with a lid to store the resin, however, this solution is only recommended if a new print will be made in a short period of time.

Effect of Heat

Although temperature does not have as much influence as light, and generally does not cause spontaneous polymerization, it does affect reactivity. Generally, the polymerization reaction is favored in the presence of high temperatures, causing it to occur more rapidly. It is recommended to always store resins at temperatures below 20 ºC, with the optimal temperature between 10 ºC and 15 ºC.

For this reason, it is recommended to always print at temperatures above 20 ºC.

Effect of Moisture on Resins

Something that is usually not considered when working with resins is the influence of moisture. The first acrylic resins for 3D printing had low hygroscopicity, so moisture was not a problem, however, the emergence of new technical resins, some with greater ability to absorb water such as ABS or Tough types, makes it advisable to store these resins in low humidity environments.

Generally, moisture in a resin does not produce visible effects on the parts and is not usually the cause of printing failures, however, it can alter the final mechanical properties of the part.

Resin Recovery after Printing

During printing, the resin in the tank partially degrades due to light leaks and heat generated by the lighting source. This generally causes an increase in resin viscosity, as well as the appearance of solid residues and impurities.

Image 2: Resin Filter. Source: Filament2Print.

To recover the resin and extend its useful life, it must be filtered after each print to remove solid residues and impurities. It is also recommended to mix it with new resin before the next print to reduce its viscosity and improve its reactivity.

Biocompatibility Certified Resins

Resins with biocompatibility certification require special preservation because, in addition to ensuring their good preservation, contamination with other resins or impurities must be avoided to guarantee their certification. The following recommendations should be followed:

• It is very important to handle resins in an aseptic environment and always wear gloves and a mask.
• It is essential to always use independent tanks and platforms for each resin. If this is not possible, the 3D printer should only be used with materials that have the same degree of biocompatibility.
• If other accessories or tools, such as filters or spatulas, are used, one per type of resin or disposable tools should be used.
• For filtering, the use of fiber-based filters (paper, fabrics, PP, etc.) should always be avoided due to the risk of fiber detachment and contamination. Ideally, a stainless steel sieve or mesh of at least 200 mesh should be used.

Expiration Date

It is common for resins to include a preferred use-by date or expiration date. This date is indicative and does not imply that a resin cannot be used after this date, but rather that it is the date until which the manufacturer can guarantee the good condition of the resin if the container has not been opened.

Once the resin container is opened, this date becomes irrelevant, as its shelf life will depend on its use and proper preservation.