Technology Overview (IM)

Injection molding processes make plastic parts via the injection of molten plastic — typically a thermoplastic — into metal injection molds, usually made from steel or aluminum.

The machine feeds raw material into the mold itself, effectively a negative impression of the final part, which consists of two sections: an injection (A) mold and an ejector (B) mold.
The space between the two sections is the mold cavity, into which material is injected.
Although capable of producing a wide range of parts, injection molds have some design constraints. Plastic injection molded parts must have narrow walls. They should avoid overhanging features and have some degree of draft (tapered sides) so that the molded part can be ejected from the mold.

Injection molding is principally used with plastics and thermoplastics in particular. Thermoplastics are polymers that soften at an elevated temperature (at which point they can be freely injected into a mold) and then return to a solid state after cooling. Injection molding also works with thermosets, which can be cured to make a solid but cannot then be melted back into a liquid. Less common are elastomers.

Injection molding is the most popular manufacturing process for producing plastic parts. An injection molding machine with raw plastic material and various molds can make many diverse production parts, large and small, durable or disposable, for many industrial applications. So how does injection molding work?

Injection molding is a forming process — Injection molding is a forming process — rather than a subtractive (cutting) process like CNC machining or an additive process like 3D printing — that uses a mold as a forming device. The process is suitable for materials like thermoplastics, which are heated until they reach a molten state and then injected into a metal mold where they cool and take the form of the mold’s interior or cavity.

The simple explanation:

The injection molding process can be divided into four stages: melting the material, injecting it into the mold, cooling the material (or allowing it to cool) until it hardens, then ejecting the final part from the mold. Put simply.

In principle, injection molding is a relatively easy-to-understand process. Executing it, however, is a little more complex.

Injection molding starts with pouring pellets (granules) of plastic material into a hopper. These pellets are then moved from the hopper to a barrel and heated until they reach a molten state.

The melted material is then forced through the barrel by a reciprocating screw until there is enough material near the exit point of the barrel to fill the mold. This quantity of material is known as a shot.

After passing through a check valve, the shot of liquid material is forced from the barrel into a channel in the mold called a sprue, then through a network of smaller channels called runners and into the mold cavity. These runners are usually organized to deliver material to the right areas of the mold with adequate force.

The material immediately begins to cool and harden once it reaches the mold. The cooling can also be accelerated using cooling lines around the mold filled with circulating water.

When the material is cooled and solidified, the operator opens the mold, and the molded part can then be ejected. Depending on the rigidity of the plastic material, using ejector pins may help remove the plastic part from the mold without breaking it.

The sprue and runner are trimmed from the part — sometimes leaving a small mark — before the injection molded part is ready for post-processing or delivery.

In any injection molding project, crucial decisions must be made before the molding process begins. The workflow often goes like this:

1. Select the material: Material selection is the first step when preparing for injection molding. Product designers will typically consider specific injection molding materials when designing a new product. And if they are unsure, making rapid prototypes is the best way to test out different material options.

2. Confirm the quantity: It is crucial to discuss the proposed number of molded parts to be produced at the outset. The number of shots will determine the type of mold used: a prototype mold or a high-volume production mold.

3. Mold flow analysis: Injection molding mold flow analysis software provides a simulation report. The report predicts factors like part warpage and cooling channel efficiency and ultimately helps manufacturers avoid mistakes. In the event of a negative report, the design can be changed to improve the process.

4. Create the mold: Mold making is a specialist process. These days, molds are often made via CNC machining and EDM since these processes can quickly produce highly detailed metal molds.

5. Create the moldings: Once the mold is ready to go, the injection molding process can begin.