The currently growing demand for metallic and polymeric products has undoubtedly changed the rules of manufacturing, enabling customers to more functionally define their products based on their needs. Nowadays, a new technique for rapid tooling, Additive Manufacturing (AM), can create customized products with more complex geometries and short life cycles (flexibility) in order to keep up with the new variables imposed by the manufacturing environment. In the last two decades, the migration from subtractive manufacturing to AM has materialized such products with reduced costs and cycle times. AM has been recently promoted to develop polymer molds for product manufacturing. This paper reviews the main findings in the literature concerning polymer molds created by AM compared to conventional (metal) molds obtained by subtractive manufacturing. Information about specific topics is scarce or nonexistent, for example, about the characterization of the most commonly injected materials and molds used in this type of technology, their mechanical properties (part and mold), designs for all types of geometries, and costs. These aspects are addressed in this literature review, highlighting the advantages of this alternative manufacturing process, which is considered a desirable technology worldwide.

The degree of quality of thermoplastic injection-molded parts can be established based on their weight, appearance, and defects. However, the conditions of the injection process may induce effects on the mechanical performance of the injected parts, and the residual stresses can cause cracks or early failures when an external load or force is applied. To evaluate these mechanical behaviors, different experimental techniques have been reported in the literature, where digital photoelasticity has stood out both for being a non-contact technique and for achieving quantitative results through sophisticated computational algorithms. Against this background, our proposal consists of analyzing the overall residual stress distribution of parts injected under different molding conditions by using digital photoelasticity. In this case, the specimens are subjected to bending strength tests to identify possible effects of the injection process conditions. The findings show that, at mold temperatures of 80 °C, flow-induced residual stresses increase with packing pressure. However, these internal stress levels do not affect the external load applied by the mechanical bending test, while the mass injected at higher levels of packing pressure helps to increase the bending strength of the injected part. At lower mold temperatures (50 °C), the mechanical strength of the injected part is slightly reduced, possibly due to a lower effect of the packing pressure.

This study investigates and compares plasma electrolytic oxidation (PEO) coatings produced on wrought Ti6Al4V alloy substrates with those resulting from electron beam powder bed fusion (PBF-EB). For a duration of 1000 s, a phosphate/silicate electrolyte with a current density of 50 A/cm² was employed to fabricate the coatings. Surface and polished cross-sections of the coated specimens underwent SEM and X-ray diffraction (XRD) analyses. The obtained coatings exhibit differences of up to approximately 18% in thickness and formation, as well as in their anatase phase. The anatase phase is present at a level of 54.09% in the substrates processed by PBF-EB and 38.54% in wrought substrates. After 1000 s of PEO, the coatings formed on the wrought substrates exhibited higher porosity and larger pores (>1 μm) compared to those produced on the PBF-EB specimens. The PBF-EB coatings had lower porosity because they contained fewer pores larger than 1 μm. The findings imply that the unique microstructural arrangement of PBF-EB-produced additively made Ti6Al4V materials plays a significant impact in the development and morphological properties of PEO oxide coatings.

Injection molds are production tools that require detailed analysis based on the quality of the resulting part, the impact on cycle times, and the expected production volume. Cooling channels also play a critical role in mold performance and product quality as they largely determine cycle time. Designs that incorporate conformal cooling channel (CCC) geometries that conform to or align with the part contour are currently being explored as an alternative to conventional cooling channel designs in injection molds. In this study, a simulation of CCC geometries was performed and their effects on mold temperatures and warpage were investigated. Two cross-sectional geometries, circular and square, were selected for a three-factor level design of experiments (DOE) analysis. The response variables used were mold temperatures and part warpage. A cup-shaped part with upper and lower diameters of 54 and 48 mm, respectively, a height of 23 mm and a thickness of 3 mm was used for the injection molded part. A comparison was also made between two materials for the injection mold, steel and polycarbonate. The DOE results showed that the distance between the CCC and the injected part and the diameter or side of the square have significant effects on the response variables for both systems (steel and polycarbonate molds). In addition, a comparison between conventional and conformal cooling channels was analyzed using a cup-shaped part and a less rigid part geometry. The finite element simulation results show a 9.26% reduction in final warpage in the cup-shaped part using CCCs compared with the conventional cooling methods in steel. When using parts with lower geometry stiffness, the use of CCCs reduced final part warpage by 32.4% in metal molds and by 59.8% in polymer molds.

Highly ordered TiO₂ nanotubes were obtained by anodization on Ti6Al4V substrates manufactured by electron beam powder bed fusion (E-PBF). Effects of anodization parameters such as anodizing time, stirring, fluoride concentration, and water content were analyzed in an organic electrolyte (ethylene glycol) that contains ammonium fluoride. The ordering of the nanotubes was measured by regularity ratio calculations based on fast Fourier transform (FFT) from SEM images. It was found that for the processed specimens, the highest ordering of the TiO₂ nanotubes was reached at 30 V for 5000 s with a concentration of 9 vol% H₂O and 0.4 wt.% NH₄F, exhibiting nanotubes free of delamination, cracks, and coral-like structures with a regularity ratio (RR) of 1.91. This work offers a simple method for creating homogeneous and organized TiO₂ nanotubes on Ti6Al4V substrates manufactured by E-PBF which potentially improves its functionality in diverse industrial applications such as nanosensors, controlled-release substances, solar cells, water splitting, electrochromic devices, and Li-ion battery anodes. Graphical Abstract

Additive manufacturing (AM) is a relatively new option in mold manufacturing for rapid tooling (RT) in injection processes. This paper presents the results of experiments with mold inserts and specimens obtained by stereolithography (SLA), which is a kind of AM. A mold insert obtained by AM and a mold produced by traditional subtractive manufacturing were compared to evaluate the performance of the injected parts. In particular, mechanical tests (in accordance with ASTM D638) and temperature distribution performance tests were carried out. The tensile test results of specimens obtained in a 3D printed mold insert were better (almost 15%) than those produced in the duralumin mold. The simulated temperature distribution closely matched its experimental counterpart—the difference in average temperatures was merely 5.36 °C. These findings support the use of AM in injection molding and RT as an excellent alternative for small and medium-sized production runs in the global injection industry.