At Carbon, we are constantly looking to improve the reliability, precision, and performance of the Carbon Digital Light Synthesis™ (Carbon DLS™) 3D printing process, which enables the printing of a wide variety of elite-performing materials from tough, rigid parts to high tear-resistant elastomers. Here, we introduce inert baking as a new post-process option to boost the impact toughness and temperature resistance of some of our strongest resins—EPX 82, EPX 86FR, and our new EPX 150. We also discuss inert baking’s benefits and use beyond this resin family.
Problem Statement
Carbon’s dual-cure resins incorporate heat-activated chemistries to expand the properties that can be achieved in photopolymer resin systems. For many of these resins, a high-temperature bake is required for a complete cure and maximum strength, temperature, and chemical resistance. However, baking in an ambient air environment at elevated temperatures leads to decomposition and oxidative crosslinking during the bake that can limit toughness and lead to the yellowing of light-colored parts. Our solution is to displace oxygen during the baking process with an inert gas, a process called inert baking.
EPX 82 offers a best-in-class combination of functional toughness, impact strength, and temperature resistance. By baking it in an oxygen-free environment, we can make this industry-leading resin even better.
Figure 1. A 1-lb ball dropped from 21 inches high onto an EPX 82 air-baked part (left) vs an EPX 82 inert-baked part (right)
EPX 150 is the newest resin in our high-performance rigid material catalog and benefits similarly from inert baking. EPX 150 offers even better temperature and chemical resistance than EPX 82 and better toughness and overall usability compared to CE 221. It’s great for applications like electrical connectors and spray nozzles that require high temperature and chemical resistance, and dimensional stability under load. EPX 150 is capable of surviving hundreds of cycles of steam sterilization, which makes it a perfect option for your evaluation in use for medical instruments and trays.
Recommended Equipment
Inert baking uses the same workflow right up to the bake—same resin, same printing process, same wash. Inert baking requires an oven that is capable of maintaining a low-oxygen environment and a high-purity nitrogen source to displace air during the bake.
Carbon recommends the following oven for use with its EPX resins. More details on installation, usage, maintenance, and specifications are available in the “Inert Baking” course on Carbon Academy.
- Yuanyao Inert Oven model YNO-225, 300°C max temp (8.0 cu ft). Smaller options are also available.
- Sales Contact: sales07@yuanyao-tech.com, “Carbon Inert Oven” configuration available starting at $8,350 USD
This oven runs at a slight positive pressure (1.0–2.0″ WC) to enable a low-oxygen environment. The Yuanyao oven comes with a data logger and an internal compressor that enables rapid cooling post-bake.
Carbon has found that <1,000–ppm oxygen content during baking gives the best results, with benefits greatly diminished at 20,000–ppm (2%) oxygen. The reported results have been achieved with house nitrogen containing ~700–ppm oxygen as measured by an optional oxygen analyzer. This nitrogen source is used with a flow rate of 85–60 SCFH to ensure sufficient air turnover.
Carbon recommends the following concentrator, but high-purity gaseous or liquid nitrogen or argon can achieve the same performance.
- Parker Hannifin DB-5 (DB-05-PPM), capable of purity and throughput to support two YNO-225 ovens
Benefits for the EPX Family of Materials
Increased toughness and impact strength
The following table summarizes the tensile and impact properties of inert baked EPX 82, EPX 86FR, and EPX 150 samples. As shown, an increase in elongation at break is observed for all samples baked in the inert oven. The impact energy for different geometries (Charpy, Izod, and Gardner) all increased significantly, up to 300%.
Table 1. Tensile, impact, and thermal properties of air vs. inert baked EPX 82, EPX 86FR, and EPX 150
| Resin | EPX 82 (Air) | EPX 82 (Inert) | EPX 86FR (Air) | EPX 86FR (Inert) | EPX 150 (Air) | EPX 150 (Inert) |
|---|---|---|---|---|---|---|
| Tensile Properties ISO 527-2, Type 1A, 5 mm/min | ||||||
| Tensile Modulus (MPa) | 2,800 | 2,800 | 3,440 | 3,182 | 2,900 | 2700 |
| Yield Strength (MPa) | 80 | 84 | 95 | 96 | 79 | 76 |
| Ultimate Tensile Strength (MPa) | 80 | 84 | 95 | 96 | 79 | 76 |
| Elongation at Break (%) | 5 | 8 | 10 | 14 | 4 | 5 |
| Impact Properties | ||||||
| Unnotched Charpy, ISO 179-1/1eA (kJ/m2) | 25 | 76 | 33 | 64 | 29 | 37 |
| Unnotched Izod, ASTM D256 (J/m) | 370 | 840 | 439 | 847 | 250 | 576 |
| Gardner, ASTM D5420, GC, 3.2 mm (J) | 0.5 | 1.5 | – | – | 0.7 | 0.9 |
| Thermal Properties | ||||||
| Heat Deflection Temperature (ASTM D648 0.455 MPa) (°C) | 130 | 130 | 135 | 135 | 153 | 155 |
Why do we see this improvement? EPX 82, EPX 86FR, and EPX 150 incorporate core-shell rubber tougheners (CSR), which can partially degrade at the surface of the part in high temperatures during the bake. This can be observed and monitored using infrared (IR) spectroscopy. CSR peaks are seen at about 960 and 910 cm-1 in an IR spectra for a freshly-printed EPX 82 part (black curve). These peaks disappear after curing in air (red curve) but are preserved by baking the parts in an oxygen-free environment (green curve).
Use Beyond the EPX Resins
Carbon also recommends inert ovens to support volume production of EPU 44 and 46. Inert baking reduces yellowing at high temperatures, allowing light-colored parts to be produced with shorter, higher-temperature bakes to improve baking cycle times. Further, an inert environment during the baking process allows larger volumes of parts to be baked while remaining below the limiting oxygen concentration for flammable volatiles that may evolve.
RPU 130, MPU 100, and RPU 70 have also been tested in inert baking, with minimal benefit or change in properties. These resins are compatible with the inert oven recommended previously, but differences in air flow between inert ovens and air ovens may lead to subtle differences in part accuracy, so Carbon does not recommend using these ovens interchangeably in production.
Conclusion
The improvements in material properties offered by inert baking can vastly improve performance in most applications in which EPX 82, EPX 86FR, and EPX 150 are already used. They will perform better in functional impact testing compared to the same part cured using an air bake. This allows parts printed with EPX 150 to be autoclavable, chemically resistant, and tough, which is a best-in-class combination for multiple applications. Inert-baked parts printed with EPX 82 and EPX 86FR will exhibit increased functional toughness, impact strength, and temperature resistance.
3D as It’s Meant to Be
Interested in utilizing Carbon to accelerate product development? Reach out to us at sales@carbon3d.com to learn more!