An injection molding machine consists of 3 main parts: the injection unit, the mold - the heart of the whole process - and the clamping/ejector unit.

In this section, we examine the purpose of each of these systems and how their basic operation mechanics affect the end-result of the Injection molding process.

Watch a large injection molding machine in action while producing 72 bottle caps every 3 seconds in the video here:

The injection unit

The purpose of the injection unit is to melt the raw plastic and guide it into the mold. It consists of the hopper, the barrel, and the reciprocating screw.

Here is how the injection molding process works:

1. The polymer granules are first dried and placed in the hopper, where they are mixed with the coloring pigment or the other reinforcing additives.

2. The granules are fed into the barrel, where they are simultaneously heated, mixed and moved towards the mold by a variable pitch screw. The geometry of the screw and the barrel are optimized to help build up the pressure to the correct levels and melt the material.

3.The ram then moves forwards and the melted plastic is injected into the mold through the runner system, where it fills the whole cavity. As the material cools down, it re-solidifies and takes the shape of the mold.

4. Finally, the mold opens and the now solid part is pushed out by the ejector pins. The mold then closes and the process repeats.

The whole process can be repeated very fast: the cycle takes approximately 30 to 90 seconds depending on the size of the part.

After the part is ejected, it is dispensed on a conveyor belt or in a holding container. Usually, injection molded parts are ready to use right away and require little to no post-processing.

Manufacturing the mold

The mold is like the negative of a photograph: its geometry and surface texture is directly transferred onto the injection molded part.

It usually makes up the largest portion of the start-up costs in injection molding: the cost of a typical mold starts at approximately $2,000-5,000 for a simple geometry and relatively small production runs (1,000 to 10,000 units) and can go upwards to $100,000 for molds optimized for full-scale production (100,000 units or more).

This is due to the high level of expertise required to design and manufacture a high-quality mold that can produce accurately thousands (or hundreds of thousands) of parts.

Molds are usually CNC machined out of aluminum or tool steel and then finished to the required standard. Apart from the negative of the part, they also have other features, like the runner system that facilitates the flow of the material into the mold, and internal water cooling channels that aid and speed up the cooling of the part.

Learn more about CNC machining in the manufacturing and design guide 

Recent advances in 3D printing materials have enabled the manufacturing of molds suitable for low-run injection molding (100 parts or less) at a fraction of the cost. Such small volumes were economically unviable in the past, due to the very high cost of traditional mold making.