Thanks to Prof. Dr. Tim Osswald and Alec Redmann for sharing insights on the project in a speed talk at K-show 2019!
Optimizing the thermal curing cycle of EPX82
Industrial 3D printing processes create functional, end-use parts with mechanically isotropic properties and smooth surface finishes. Prof. Dr. Tim Osswald, Alec Redmann and the team at the University of Wisconsin-Madison worked together with the California-based company Carbon Inc. to optimize the thermal curing cycle of EPX 82 resin used in the their Digital Light SynthesisTM (DLSTM) process.
What is the Carbon DLSTM process?
Digital Light Synthesis (DLS) provides advantages over the traditional SLA process, allowing for the production of highly customizable geometries with excellent material properties and increased printing speeds. DLS uses ultraviolet light and oxygen to continuously grow objects from a pool of resin instead of printing them layer-by-layer. It also enables the use of high-performance materials which require a second thermal curing stage where the full material properties are developed.
Objectives of Kinetics Optimization
At the beginning of the project, the heating cycle for EPX 82 was 750 minutes long. The aim of the study was to optimize the thermal curing cycle time using kinetic analysis and to validate the final mechanical properties of parts produced with the new curing cycle. For this, Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), NETZSCH Kinetics Software and Dynamic Mechanical Analysis (DMA) were used.
Step 1: Thermogravimetric analysis to determine volatilization
The NETZSCH TG 209 F1 Libra® was used to conduct thermogravimetric analysis in order to determine if there is volatilization or degradation of the material, and at what temperatures it occurs. Figure 1 clearly displays that there are no shorter molecules in the material that are evaporating. Furthermore, degradation happens at nearly 375°C.
Step 2: Differential scanning calorimetry to understand the thermal cure
In a second step, the NETZSCH DSC 214 Polyma was used to monitor the curing reaction at different heating rates. Six different temperatures programs were applied. In the higher temperatures ranges, different events can be seen. Based on the DSC measurements, the thermal cure was broken up into three different curing events in order to continue with further analyses.
Step 3: Analysis with Kinetics Software
Subsequently, the previous analyses were taken together to simplify the generation of reaction models. The software helps characterize the activation energy, which is an important part of the curing reaction. From this, the slopes of the measurement curves indicate how easy or difficult it is for a material to cure at different rates of conversion.
The Kinetics software identifies all relevant constant variables. It is now possible to simulate the curing reaction of the material and vary the maximum conversion and heating rates. By combining the information, an optimized process can be established.
As the last step, the mechanical properties of the parts produced with the simulated processes have to be analyzed.
Step 4: Use the DMA to characterize the properties
Finally, glass transition temperatures of samples cured using the different temperature cycles were analyzed with a NETZSCH GABO EPLEXOR® in order to determine important quality control parameters.
It can be observed that all defined cycles have relatively similar stiffness results – with the exception of the fastest cycles (black dotted line). The low glass transition temperature indicates that the sample did not fully cure during the fast heating cycle. Instead, only the second-fastest cycle can be used as the part displays the same material properties as the original cycle.
Putting intelligence behind the process
The optimization of the thermal curing cycle of EPX 82 reduced the cycle time from 750 minutes to 202 minutes, which is a reduction of 73 % (9 hours). The users of the DLS process will not only benefit from the time savings achieved by the optimization, but also from energy savings.