|Researcher: Darrell Bunyan|
Supervisor: Dr. Richard J. Heath and Prof. John R. Tyrer
Selective Laser Sintering or SLS can be used for the consolidation of thermoplastic powders where it can be used to create various shaped parts. However, its use has been limited to prototyping rather than rapid manufacturing as the parts formed by SLS contain porosity and hance do not achieve the optimal mechanical properties.
Thermoplastics melt at low temperatures but have poor thermal conductivity and then partially melt forming highly viscose melt fluid, compared to metal powders. Therefore, the commercial powders employed suffer incomplete melting, limited flow causing partial bonding between particles (and that due to thermal expansion) and the risk of thermal degradation. Commercial grades of SLS grade thermoplastic powders are relatively limited, for instance with few polymer choices and those being of regular spherical form, particle size of approximately 60 microns.
The Effect of Particle Size
Research elsewhere has shown smaller particle size powders can reduce porosity, but obtaining a variety of regular particle sizes has proved difficult. In our research, we have taken the lead from observations made in metals’ SLS: metal powders readily melt to low viscosity liquids, meaning that there is extensive (usually complete) melting, because of flow through the layers of powder deposited originally. Here there is excellent heat transfer, leading to loss of particles’ original boundaries and air largely being eliminated. So, how to do this with thermoplastic powders?
Low Viscosity Polymers
Generally, low viscosity polymer melts indicate prolonged heating to relatively high temperature – but achieving this state is difficult because of both the intense energy flux of a laser and the associated risk of thermal degradation.
Alternatively low viscosity melts are obtained using low molecular weight thermoplastics, some thermoplastic blends or thermosetting oligomers. We have studied a mixed nylon blend and a low molecular weight polypropylene, comparing against a standard Nylon 12 SLS grade (image a). Both the novel polymers have been used to produce low porosity product, as illustrated by image b of SLS polypropylene.
Figure: (a) Standard 60 micron grade of Nylon 12 after sintering, showing retention of particle form. (b) Polypropylene after sintering showing loss of particle form and development of fuller fusion with continuous crystal structure.
Profiled Laser Beams
Further studies have examined the effect of a profiled laser beam, so controlling heat input into polymer particles, with some reduction in porosity.
The project has shown that it is possible to produce low porosity thermoplastic products by SLS, where material properties have been considered, indicating more complete melting with low melt viscosity are effective. Concerns exist with respect to using low molecular weight thermoplastics, with inferior mechanical properties.
Future work: to examine a fuller range of thermoplastics, as well as thermosettable oligomers, as well as examining means of producing these powders with a variety of uniform spherical particle sizes.
- R.J. Heath, D. Bunyan, J.R. Tyrer, M. Gibson, “Application of holographic beam shaping in selective laser melting for the fabrication of functional polymers components”, 28 th Int Congress: Applications of Lasers & Electro-Optics (ICALEO), Orlando, Florida November 2009.
The Department of Materials, formerly named the Institute of Polymer Technology and Materials Engineering has roots going back virtually 40 years and throughout this time we have been contributing to the advancement and application of knowledge in Materials Science and Engineering by means of teaching, scholarship and research. Our philosophy is based on the engineering application and use of materials which, when processed, are altered in structure and properties. This encompasses design considerations and business implications.