The mold cavity determines the external shape of the product, while the core shapes the interior. When the material enters the chilled cavities, it starts to replasticize and returns to a solid state and the configuration of the finished part. The machine then ejects the finished parts or products. The problem with the injection molding process is the molten plastic begins to cool as it is forced through the runners and into the mold. The rate of cooling depends on the temperature of the melt, the temperature of the mold, and the type of plastic.
Various plastics have different flow and thermal properties and solidify at different rates.
As the plastic cools, it begins to solidify onto the walls of the sprue, runners, gates, and cavities, narrowing the flow path. Therefore, a higher pressure is required to push the plastic through this flow path. The more time the plastic is in contact with the walls, the more solidification occurs. If the plastic has a long flow path in a mold, it will take more time to fill and require a lot more injection pressure to fill it. By adding more gates, the flow paths can be shortened and the fill time and injection pressure can be reduced.