## 5 Conclusions  
We investigated the fabrication of polymer pellets for the accurate measurement of THz parameters. PTFE and HDPE pellets were created using various compaction pressures; measurements for THz-TDS, tensile strength, axial relaxation and porosity were performed. Three compaction methods were used to assess their performance: hydraulic press with and without a vacuum, as well as a compaction simulator. We also investigated two different grades of HDPE to assess uniformity between grades and the effect of particle size.  
PTFE was the preferred material for use as a matrix due to its low porosity and high tensile strength. It was found that a compaction pressure > 350 MPa was required to prevent lamination/capping in compaction simulator samples. HPDE grades produce samples with significant porosity at all compaction pressures, making refractive index and loss coefficient extraction less accurate. There were noticeable differences in the extracted optical parameters between grades of HDPE, suggesting that inter-batch material variations reinforce the need for reference samples to be created for each batch of polymer. The particle size in HDPE primarily affected the resulting tensile strength and scattering loss of the material, with smaller particles producing pellets with higher tensile strength and lower loss coefficients. Pellets can increase in thickness after compaction for up to four weeks, meaning that THz-TDS analysis should be carried out after this period to ensure reproducible results.  
Compaction using the hydraulic press resulted in noticeably superior samples, with a vacuum providing no noticeable benefits with the exception of reduced porosity in PTFE pellets immediately after compaction. This performance improvement is primarily due to an increase in dwell time compared with that used for the compaction simulator [33]. The ability to control the aspects of compaction in the simulator, and recording of the in-die parameters such as thickness and diameter provide distinct advantage.  
Conceptualization, K.N.M., M.N. and D.M.; methodology, K.N.M., M.N. and D.M.; software, K.N.M. and D.M.; validation, K.N.M., M.N. and D.M.; formal analysis, K.N.M., M.N. and D.M.; investigation, K.N.M., M.N. and D.M.; resources, K.N.M., M.N. and D.M.; data curation, K.N.M.; writing--original draft preparation, K.N.M.; writing--review and editing, M.N., D.M. and A.N.; visualization, K.N.M., M.N. and D.M.; supervision, M.N., D.M. and A.N.; project administration, K.N.M.; funding acquisition, M.N., A.N. and D.M. All authors have read and agreed to the published version of the manuscript.  
This research was funded by EPSRC and National Physical Laboratory through an industrial iCASE studentship (grant number: EP/S02168X/1). This research was carried out on an EPSRC (EP/S02168X/1) funded compaction simulator and terahertz time-domain spectrometer and in the CMAC National Facility supported by UKRPIF (UK Research Partnership Fund) award from the Higher Education Funding Council for England (HEFCE) (Grant Ref HH13054).  
Not applicable.  
The data presented in this study are openly available in the University of Strathclyde repository at [[https://doi.org/10.15129/600b7d06-0](https://doi.org/10.15129/600b7d06-0) acac-4d9e-9b31-409 fb82b16 ae]([https://doi.org/10.15129/600b7d06-0](https://doi.org/10.15129/600b7d06-0) acac-4d9e-9b31-409 fb82b16 ae) (accessed on 24 February 2022).  
The authors declare no conflict of interest.