Injection molding is a molding procedure whereby a heat-softened plastic material is forced from a cylinder into a relatively cool cavity giving the article the desired shape. Injection molding is a manufacturing technique for making parts from plastic material. Molten plastic is injected at high pressure into a mold, which is the inverse of the desired shape. The mold is made by a mold maker from metal, usually either steel or aluminum, and precision-machined to form the features of the desired part. Injection molding is very widely used for manufacturing a variety of parts, from the smallest component to entire body panels of cars.
The process of plastic injection molding
An injection molding machine consists of three basic parts, the mold plus the clamping and injection units. The clamping unit is what holds the mold under pressure during the injection and cooling. Basically, it holds the two halves of the injection mold together.
During the injection phase, plastic material, usually in the form of pellets, are loaded into a hopper on top of the injection unit. The pellets feed into the cylinder where they are heated until they reach molten form (think of how a hot glue gun works here). Within the heating cylinder there is a motorized screw that mixes the molten pellets and forces them to end of the cylinder. Once enough material has accumulated in front of the screw, the injection process begins. The molten plastic is inserted into the mold through a sprue, while the screw controls the pressure and speed.
The dwelling phase consists of a pause in the injection process. The molten plastic has been injected into the mold and the pressure is applied to make sure all of the mold cavities are filled.
Then the plastic is allowed to cool to its solid form within the mold. The clamping unit is then opened, which separates the two halves of the mold. An ejecting rod and plate eject the finished piece from the mold.
A machine used to extrude materials is very similar to the injection-molding machine explained above. A motor turns a thread, which feeds granules of plastic through a heater. The granules melt into a liquid, which is forced through a die, forming a long 'tube like' shape. The extrusion is then cooled and forms a solid shape. The shape of the die determines the shape of the tube.
Advantages of Injection Molding
- High tolerances are repeatable
- Wide range of materials can be used
- Low labor costs
- Minimal scrap losses
- Little need to finish parts after molding
Disadvantages of Injection Molding
- Expensive equipment investment
- Running costs may be high
- Parts must be designed with specific molding consideration.