How does injection molding work?

An injection machine is made up of three primary components – the feed hopper, the screw and the heated barrel. Plastic granules for the part are fed into the heated barrel by a hopper. Plastic that is placed in the hopper is usually in powder or granular form although some resins, for instance silicone rubber, may be a liquid and might not entail heating. The material is then melted utilizing the frictional action of a reciprocating screw accompanied with heater bands. The molten plastic is then injected through the nozzle and into a mold cavity – it may seem easy, but injection molding is actually a very complex process. While in the mold cavity, the material cools and solidifies to the configuration of the cavity. When the part has hardened, the moveable platen that the mold is mounted opens and the part is ejected using ejector pins.

Injection molding machine sizes can run anywhere from 5 tons of clamping force to over 6000. The higher the tonnage, the larger the machine. In fact, injection molding machines are classified based on tonnage, or more precisely the clamping force or pressure.

For example, a machine that is capable of delivering 110 tons of clamping pressure is rated for 110 tons. This pressure ensures that the mold remains closed during the injection process. Too little or too much pressure can cause part quality issues as well as flashing, where excess material surfaces on the edge of the part. The viscosity of the plastic being used is also impacted by pressure. MFI or Melt Flow Index is a measure of the melt of a thermoplastic polymers ease of flow.

The greater the MFI, the higher the required tonnage. For example, let’s say you have a 4 cavity mold and each part is a 5”x 5” x 0.2” square. First you must calculate the projected area of the part. With this mold, the calculation would be 5” x 5” = 25 square inches x 4 cavities = 100 square inches of projected area. Projected area calculation is needed because it is the principal variable that affects clamp tonnage . If we use a 10 MFI Polypropylene, we would usually use a minimum of 2.5 tons per square inch of projected. So for this mold, the 100 square inches of projected area requires an injection machine sized to at least 250 tons.

Another factor in sizing the correct machine is shot size and mold dimensions.

The clamping unit’s main function is to close and open the mold as well as ejecting the parts. Two of the most common types of clamps are the toggle clamps and the hydraulic clamps. Toggle clamps are traversed by hydraulic cylinders. These clamps use mechanical connections to generate greater forces than a hydraulic cylinder of the same size that uses a direct connection.

There are many more basics and fundamentals to the science and technology of injection molding that we could go into in a later blog post. However, its worth mentioning that while the fundamentals of injection molding remain somewhat constant, the more technology we introduce into the existing clamp and injection processes the more complex the considerations become when developing a mold or building a machine for injection molding.