Robots that are devised for assisting and interacting with humans are becoming fundamental in many applications, including in healthcare, education, and entertainment. For these robots, the capacity to exhibit affective states plays a crucial role in creating emotional bonding with the user. In this work, we present an affective architecture that grounds biological foundations to shape the affective state of the Mini social robot in terms of mood and emotion blending. The affective state depends upon the perception of stimuli in the environment, which influence how the robot behaves and affectively communicates with other peers. According to research in neuroscience, mood typically rules our affective state in the long run, while emotions do it in the short term, although both processes can overlap. Consequently, the model that is presented in this manuscript deals with emotion and mood blending towards expressing the robot's internal state to the users. Thus, the primary novelty of our affective model is the expression of: (i) mood, (ii) punctual emotional reactions to stimuli, and (iii) the decay that mood and emotion undergo with time. The system evaluation explored whether users can correctly perceive the mood and emotions that the robot is expressing. In an online survey, users evaluated the robot's expressions showing different moods and emotions. The results reveal that users could correctly perceive the robot's mood and emotion. However, emotions were more easily recognized, probably because they are more intense affective states and mainly arise as a stimuli reaction. To conclude the manuscript, a case study shows how our model modulates Mini's expressiveness depending on its affective state during a human-robot interaction scenario.The research leading to these results has received funding from the projects Robots sociales para estimulación física, cognitiva y afectiva de mayores (ROSES) RTI2018-096338-B-I00 funded by Agencia Estatal de Investigación (AEI), Ministerio de Ciencia, Innovación y Universidades and RoboCity2030-DIH-CM, Madrid Robotics Digital Innovation Hub, S2018/NMT-4331, funded by "Programas de Actividades I+D en la Comunidad de Madrid" and cofunded by Structural Funds of the EU. Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature
