xiehe model

Plastic injection molding is the process of making plastic parts via the injection of molten plastic — typically a thermoplastic — into a metal mold, which is usually made from steel or aluminum.

The machine feeds raw material into the mold itself, the geometry of which is effectively a negative impression of the final part and which usually 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 very wide range of parts, injection molding has some design constraints. Parts must have fairly narrow walls, should generally avoid overhanging features, and must have some degree of draft (tapered sides) so the 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) 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 state. Less common are elastomers.

Injection molding is the most popular manufacturing process for producing plastic parts. With an injection molding machine, raw plastic material, and a mold, it is possible to make a huge variety of parts, large and small, durable or disposable, for a number of industries and applications. So how does injection molding work?

Injection molding is a forming process — as opposed to 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:

1. 01 Melt
2. 02 Inject
3. 03 Cool
4. 04 Eject

The simple explanation:

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

Injection molding starts with pellets (granules) of plastic material being poured into a hopper. The pellets are then moved from the hopper to a barrel, where they are heated until they reach a molten state.

Once the material is melted, it is 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 in a way 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, but the cooling process can also be accelerated using cooling lines around the mold filled with circulating water.

When the material is fully cooled and solidified, the operator opens the mold, and the molded part can then be ejected. Depending on the rigidity of the material, the use of ejector pins may help to get the part out of the mold without breaking it.

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

For any injection molding project, important decisions need to 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. Normally, product designers will have certain injection molding materials in mind 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 important to discuss proposed quantities of molded parts at the outside, because the number of shots determines the kind of mold to be used: a prototype mold or a high-volume production mold.
3. Mold flow analysis: Injection molding mold flow analysis software provides a simulation report, predicting factors like part warpage and cooling channel efficiency and ultimately helping manufacturers avoid mistakes. In the event of a negative report, the design can be changed to improve the injection molding 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 produce highly detailed metal molds in a short space of time.
5. Create the moldings: Once the mold is ready to go, the injection molding process can begin as explained in the “What is the injection molding process?” section.