Camden A. Chatham, PhD, Polymer AM Research Lead at Savannah River National Laboratory, on crystallization-coalescence relationships in laser powder bed fusion.
The “Sintering Window” is a staple figure in articles and presentations focusing on laser-based powder bed fusion of polymers (PBF-LB/P). It is easy to construct: a single differential scanning calorimetry (DSC) experiment at some convenient rate between 10-20 K min-1 reveals the melting and crystallization behaviour of a candidate polymer being evaluated for PBF-LB. So long as one observes separation in the melting and crystallization behaviour (i.e., the “Sintering Window”), the candidate polymer is believed to be “printable.” Without such behaviour the candidate polymer is not printable is the correlating statement. However, this method has permeated the research community without rigorous scientific investigation. Instead, this staple figure originates from early patent literature. The method known as the Sintering Window was first proposed in a 1994 patent issued to DTM Corporation, the first company to make and sell PBF-LB/P machines. Unfortunately, while the Sintering Window provides sufficient details for the patent literature to describe commonalities among certain groups of polymers that printed successfully and others that did not, the method was not defended using the historical polymer science literature, and falls short of being a robust descriptor for identifying candidate polymers.
Other unhelpful and un-verified statements around the Sintering Window which have been regularly repeated in journal articles and conference presentations include (i) “a wider observed separation between melting and crystallization behaviour is better,” (ii) “once scanned by the laser the polymer remains molten for the entire build,” and (iii) the continued use of the word “sintering” to describe particle consolidation during PBF-LB. Collectively, these statements hinder the PBF-LB/P research community to bring new polymers to market. This article highlights the incongruence between these oft-repeated statements and the historic understanding of polymer science. The article measures several key thermal and crystallization related properties of two “PBF-grade” nylon-12s, one “MJF-grade” nylon-12, two generic nylon-12s, and one “PBF-grade” polypropylene. Exploring this breadth of nylon-12 formulations – particularly comparing the generic grades against the PBF grades – elucidates what are physics-rooted trends useful for evaluating neat candidate materials and what may be altered via ‘processing aids’ in the compounding step.
Surprisingly, even though the prevailing narrative around the Sintering Window is that the wider window corresponds to slower crystallization kinetics and that this is more desirable for printing, the PBF-grade materials have a shorter crystallization halftime than the generic nylon-12s. Whatever engineering trade-offs were made during compounding, crystallization halftime was not the highest priority. More likely, the compounders value rapid coalescence regardless of crystallization behaviour.
The primary role of crystallization in coalescence is arresting long-range diffusive motion at the point of physical gelation. Therefore, rheological properties, like zero-shear extensional viscosity and relaxation time, are more prominently featured in the physics-based models and governing equations for particle coalescence. A more recent method for evaluating a candidate polymer for PBF-LB is the 2019 patent issued to chemical company Arkema describing what they call “Open Time.” Instead of focusing on crystallization, Open Time measures the viscosity response to a particular temperature profile corresponding to expected processing conditions. Present work described in the articles shows that the termination of Open Time corresponds with independently measured physical gelation via crossover modulus (for nylon-12) or frequency independent tan(δ) (for PP). Therefore, Open Time indirectly captures crystallization behaviour as it impacts viscosity, which is a more direct method for assessing the ability of a polymer to rapidly coalescence during PBF-LB. In the author’s comparison between new and aged versions of a PBF-grade nylon-12, it was observed that the Open Time method flagged potential issues in printing the aged material (arising from slower coalescence) whereas the Sintering Window did not show significant differences. This is expected by the historical body of polymer science which indicates viscosity and other rheological measurements as some of the most sensitive techniques to changes in molecular weight and molecular architecture (i.e., the molecular scale process of ageing).
In summary, the article provides evidence and argument for the following statements: (1) PBF-LB/P is properly understood as melt processing, not sintering; (2) the polymer does not remain molten for the entire build; and (3) rheometric properties are both more sensitive and more relevant to coalescence than crystallographic properties and should therefore be primarily used when evaluating candidate polymers for PBF-LB.
