# Thermo-Viscoelastic Characterization of 3D Printing Polymers
## 1 Introduction  
Rapid tooling (RT) is the process of utilizing 3D printing technology to produce tooling for the desired part. 3D printing technology and rapid tooling are potential resources in reducing the lead time to produce a prototype [1, 2]. Many studies [1, 2, 3, 4] showed that rapid tooling (RT) has the capability to greatly reduce the cost and time needed to develop plastic injection models for prototyping and short injection runs. For example, traditional injection molding tooling is expensive and time-intensive to produce. Molds made of polymer using 3D printing have the advantage of being cheaper than metal ones [3, 4]. Another advantage of 3D printing technology as a manufacturing technology for developing plastic injection mold inserts is the ability to create complex geometries without the limitations of traditional machining. Prior research on polymer additive manufacturing mold inserts has shown that a variety of materials are capable of producing short runs of some parts [3, 4, 5, 6]. [PERSON] et al. [3] found that lower barrel temperature and injection pressures increase the lifespan of the polymer RT mold inserts. [PERSON] et al. [7] studied an energy-saving LSR injection mold with conformal heating and conformal cooling hybrid channels. However, despite the ability to print an accurate mold capable of withstanding temperatures and pressures of plastic injection molding, low thermal conductivities and high coefficients of thermal expansion (CTE) are the main limiting factors in the efficacy of the 3D-printedpolymer models. It is necessary to hold longer times to allow the injected plastic to cool due to the low thermal conductivity of the plastic mold. Moreover, the high CTEs of polymers may yield greater thermal strains within the mold after injection and their mechanical properties are changed above the glass transition temperature. In addition, polymeric materials are well known to be viscoelastic. The viscoelastic nature of polymer materials may cause some significant reliability problems [8]. Therefore, in order to access the reliability and life span of the 3D-printed polymer mold, it is needed to understand the thermo-mechanical behavior of 3D printing materials under service conditions. However, little work has been reported for the time-temperature-dependent behaviors of polymeric materials used in 3D printing. Recent studies indicated that it is possible to develop a composite material through the combination of different shapes/sizes of filler particles and it is expected to improve the compressive strength and thermal conductivity, consequently increasing the hybrid mold performance [9]. [PERSON] et al. [10] reviewed various types of polymeric materials, their properties and their global impacts when those materials were used in additive manufacturing, but no thermo-viscoelastic properties of those were reported. 3D printing technology is an important technology for rapid tooling. After rapid development and improvement in recent decades, 3D printing technology has become widely used in a variety of applications, from simple to complicated industrial applications with less cost and better efficiency [11]. Stereolithography (SLA) is the oldest form of 3D printing. In 1984, the first exemplary exploration was conducted by [PERSON] and is known as stereolithography, in which layers are added by curing photo-polymers with ultra violate (UV) lasers [5; 11]. SLA relies on the controlled emission of UV radiation on a vat of liquid resin, turning into a solid polymer according to a pre-defined model loaded into the slicer software, and its process has been virtually unchanged since its development in the 1980s. Digital light processing (DLP) is another type of resin-based 3D printing. The process for DLP is very similar to the one of SLA except that it projects an entire layer at a time using an array of micrometer-sized mirrors, each of which can rotate to control the point of emission of UV radiation.