Citation: Sameni, F.; Ozkan, B.; Zarezadeh, H.; Karmel, S.; Engstrøm, D.S.; Sabet, E. Hot Lithography Vat Photopolymerisation 3D Printing: Vat Temperature vs. Mixture Design. Polymers 2022, 14, 2988. https:// doi.org/10.3390/polym14152988 Academic Editors: Rafiq Ahmad and Amir Ameli Received: 20 June 2022 Accepted: 21 July 2022 Published: 23 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). polymers Article Hot Lithography Vat Photopolymerisation 3D Printing: Vat Temperature vs. Mixture Design Farzaneh Sameni 1,2 , Basar Ozkan 1,2 , Hanifeh Zarezadeh 3 , Sarah Karmel 3 , Daniel S. Engstrøm 1 and Ehsan Sabet 1,2, * 1 Wolfson School of Mechanical, Electrical and Manufacturing Engineering, Loughborough University, Loughborough LE11 3TU, UK; f.sameni@lboro.ac.uk (F.S.); b.ozkan@lboro.ac.uk (B.O.); d.engstrom@lboro.ac.uk (D.S.E.) 2 Additive Manufacturing Centre of Excellence Ltd., Derby DE23 8YH, UK 3 Photocentric Ltd., Peterborough PE1 5YW, UK; hanifeh@photocentric.co.uk (H.Z.); sikarmel@gmail.com (S.K.) * Correspondence: e.sabet@lboro.ac.uk or e.sabet@am-coe.com Abstract: In the vat photopolymerisation 3D printing technique, the properties of the printed parts are highly dependent on the degree of conversion of the monomers. The mechanisms and advantages of vat photopolymerisation at elevated temperatures, or so called “hot lithography”, were investigated in this paper. Two types of photoresins, commercially used as highly accurate castable resins, with different structural and diluent monomers, were employed in this study. Samples were printed at 25 ◦ C, 40 ◦ C, and 55 ◦ C. The results show that hot lithography can significantly enhance the mechanical and dimensional properties of the printed parts and is more effective when there is a diluent with a network T g close to the print temperature. When processed at 55 ◦ C, Mixture A, which contains a diluent with a network T g = 53 ◦ C, was more readily impacted by heat compared to Mixture B, whose diluent had a network T g = 105. As a result, a higher degree of conversion, followed by an increased T g of the diluents, and improvements in the tensile strength and dimensional stability of the printed parts were observed, which enhanced the outcomes of the prints for the intended application in investment casting of complex components used in the aero and energy sectors. In conclusion, the effectiveness of the hot lithography process is contained by a correlation between the process temperature and the characteristics of the monomers in the mixture. Keywords: vat photopolymerisation; hot lithography; polymerisation shrinkage; degree of conversion; glass transition temperature 1. Introduction Additive manufacturing (AM), otherwise known as 3D printing, is a manufacturing technique in which 3D parts are produced directly from a CAD model in a layer-by-layer fashion [1]. Stereolithography or vat photopolymerisation is one of the most commonly used, and was the first commercialised AM method in which a photosensitive resin (pho- toresin) was used as the feedstock material loaded in a tray, which is often referred to as the “vat”. In bottom-up vat photopolymerisation 3D printing, the vat has a transparent film on its bottom plate that is located on a screen from which a light source (usually between 355–470 nm) illuminates the photoresin and cures it layer-by-layer to manufacture the desired 3D part [2–5]. Liquid crystal display (LCD) 3D printing, also known as masked stereolithography (MSLA), is a lithography-based AM technique in which LCD screens are used to cure photoresins in a layer-wise fashion [6]. The type of photoinitiator used in the liquid photoresin system depends on the wavelength of the light source used. Photoinitia- tors can be UV sensitive (<400 nm) or visible-light sensitive (400 to 790 nm). The efficiency of the polymerisation process depends on the structure and reactivity of the monomers and the photoinitiators [1,7]. (Meth)acrylate monomers, which are commonly used in vat photopolymerisation systems are polymerised through free radical polymerisation [8]. Polymers 2022, 14, 2988. https://doi.org/10.3390/polym14152988 https://www.mdpi.com/journal/polymers