Injection molding is frequently used for substances that are liquid at relatively low temperature. This includes plastics and polymers but also low temperature metals. Higher temperature metals often use vacuum molding or low pressure casting. The benefits of injection molding are efficiency and quality.
Many plastics and polymers are liquid at a higher temperature but do not flow like water. For this reason, it is impractical to pour them into molds in an industrial process. Pressure injection helps to overcome a viscous consistency while also controlling the control of flow and preventing spillage. Even with thin liquids, an injection mold is ideal as a closed system.
The best advantage of injecting is precise metering. With suction, the material must fight gravity and might not fill the cavity completely. With injection, the sensors know the exact volume filling the molds and can stop at just the right amount.