4042 Closed Loop Process Control for Powder Bed Fusion of Thermoplastics

Problem

Current thermoplastic PBF machines operate largely in an open loop scripted build process, with the only feedback provided by individual pyrometers reading powder surface temperatures on the feed bin and build surfaces and driving quartz heating lamps to pre-build set points. These systems have little or no way of responding to process transients other than to shut down and scrap the build. Unchecked variations in temperature in the part or powder during the layer spreading process can cause thermal stresses and associated part curl, which could potentially damage the machine. Even in the most carefully monitored equipment, the lack of compensation for temporal and spatial temperature variation across the build surface can have catastrophic results.

Objective

The objective of the project is to provide feedback temperature measurements at three critical phases of the PBF process and use these measurements to actively drive the thermal history of the thermoplastic along the trajectory associated with the desired mechanical performance. The project aims to improve part quality and performance predictability, while reducing sensitivity to variations in build conditions across different machines and even within a single build process.

Technical Approach

Part quality in PBF is closely tied to the optimization of the thermal time history of the material being converted from powder feedstock to finished 3D object. The technical approach is based on careful measurements of the thermal history of the build material at three critical phases in the PBF process, as well as control of key process parameters based on these measurements to produce desirable conditions even when variations or disturbances in the build process are present.

The first feedback control target is in the powder supply phase, prior to spreading a new layer. Temperature distributions are measured across the supply powder, incorporating a multi-zone heating system to ensure the powder provided to the build surface is of uniform temperature and at the optimal temperature to minimize part distortion. The second measurement of the build cycle is the laser-sintering phase. In order to provide feedback for the laser system, this approach extends single point temperature monitoring to full build surface temperature distribution monitoring, enabling a path to local and regional compensation for temperature variations from desired powder sintering/melting quality at each layer of the build. Finally, the project incorporates thermal measurements in the part cake and utilizes feedback control—both heating and cooling—to match prescribed cooling profiles, which can be optimized for structural properties in the produced components.

Project Participants

Project Principal

Other Project Participants

• Stratasys Direct Manufacturing
• Boeing
• Integra Inc.

Public Participants

• U.S. Department of Defense
• National Science Foundation
• U.S. Department of Energy