How We Made It - Injection Molding Fun Facts
When an injection mold is made, distribution tubes must be cut into the steel to allow the liquefied plastic resin to flow into the cavity for each component to be molded. The mold stays closed long enough for the plastic to harden. This is called the “mold cycle”. After the mold opens the plastic is pulled out of the mold either by hand, via a robotic arm, or pushed out with “push out pins” that are part of the mold design. The plastic that filled the mold is often (but not always) one piece that includes all of the components that were just molded as well as the plastic that had flowed through the distribution tubes. The plastic that filled the distribution tubes (and will be discarded or ground up for other molding purposes) is called the “sprue”. The sprue is separated from the components in several ways, one of which is shown in this video. The cutting action is designed to produce a very smooth surface where the distribution tube met the edge of the component (called the “gate”). Poor quality mold design and/or poor quality workmanship often result in a sharp or rough surface where the molded component is separated from the sprue.