Resin types and properties

The resins available for 3D printing have increased considerably in recent years with the introduction of resins with new properties focused on specific applications.

These new resins fall into three broad categories: resins for engineering, dental and jewellery applications. References or names sometimes vary between manufacturers, which can be confusing when selecting the right material or finding equivalents between brands.

Resins for engineering applications

Four main types of resins can be found:

• High strength with low deformation at break: Usually referred to as "Tough" or "Rigid". These are resins with similar elastic behaviour to standard resins, but with higher modulus and therefore higher strength. They usually have low values of deformation at break and low impact resistance. Their main property is resilience. They are generally characterised by good dimensional stability and good finishes.
• Moderate deformation at break and good impact resistance: These are the so-called ABS or high impact resins. These are resins with a higher elasticity than the previous ones and a greater deformation at break. Their main property is toughness, so their impact behaviour is usually good.
• Flexible/Elastic: Resins with flexible and elastic behaviour. Generally, both properties are present, however, in flexible resins, flexibility predominates, while in elastic resins, elasticity predominates.

Image 1: Comparison of Flexible 80A resin and Elastic 50A resin. Source: FormLabs.

• High temperature: Resins capable of withstanding high temperatures without deformation. There is a lot of variability between manufacturers, so that while one high temperature resin can withstand up to 80 ºC, others can exceed 120 ºC. For this reason, the manufacturer's specifications on the maximum working temperature should be consulted.

Jewellery focused resins

There are mainly two types: calcinable and high detail.

• Burn-out resins: Designed for the production of burn-out models for the production of moulds. Their quality is determined on the basis of the solid residue left after calcination. There are two different types: those that include waxes in their formulation and those that do not. Those that include waxes tend to leave less residue after calcination, at the cost of slightly lower print quality.
• High detail or model resins: These generally use lower molecular weight oligomers combined with a more reactive formulation. As a result, they have a low viscosity and high reactivity, which allows for more accurate and detailed models.

Resins focused on the dental sector

This is the category with the greatest variety of materials. Although they are always associated with biocompatibility, not all of them are, so the specifications should be consulted for their degree of biocompatibility. There are mainly six categories named according to application:

• For models: These are resins similar to high detail resins. They are intended for the production of evaluation or planning models. They are not developed for patient contact, so some types may not have a biocompatibility certificate. Two types are available: one for generic use or dental parts and one with a gingiva-like finish.

Image 2: 3D printed model with resin. Source: Uni.

• For surgical guides or splints: Intended for the production of surgical guides and discharge splints. They are generally available in transparent colour and provide a high level of detail. They must have at least biocompatibility class IIa for surgical guides and IIb for splints.
• For temporary implants: They are used in the production of temporary veneers and temporary caps. They usually contain ceramic particles and are available in VITA shades. Parts printed with these resins can be shaded and lacquered to achieve the appearance of the patient's original parts. They must have at least Class IIb biocompatibility and are not recommended for use on the patient for more than three months.

Image 3: Temporary implants. Source: Harzlabs.

• For permanent implants: These are rare and costly, as they require class III biocompatibility. They are used in the production of permanent caps and veneers.
• Radio-opaque: These are resins that are opaque to X-rays. They are used in the production of alignment templates for X-ray tomography. It is recommended that they have at least class IIa biocompatibility.
• Calcinable: Similar to jewellery. They are formulated to leave less residue than jewellery, although they are often indistinguishable from the higher quality jewellery-oriented ones.