This book is comprised of six chapters on injection molding. In Chapter One the main objective of the study is to recycle post-consumer high-density polyethylene (HDPE) and reuse it in fast-moving consumer goods (FMCG) products. Chapter Two aims to compare the advantages and disadvantages of injection molding and 3D printing and convey the latest developments in these methods. Chapter Three presents an overview of bioplastics’ characteristics, and processing in injection molding. Chapter Four studies the effect of Moringa Oleifera leaves on thermo-mechanical properties of low-density polyethylene. Chapter Five explains issues such as the advantages and usability of natural fiber reinforcement polymer composites in terms of environmental sustainability. In the last chapter, different rubber processing methods are explained. Also, the basic concept of rubber injection molding and the factors affecting this process are discussed with relevant literature.