Injection molding

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Injection Molding

1. Definition

What & How:
Injection Molding is a manufacturing process commonly used in creating plastic components. In brief this process involves injecting liquid plastic into a two piece mold. This plastic is allowed to cool within the mold and then the solidified plastic is ejected and you have your part.

Many things are made using injection molding


Steps of Injection molding: There are several stages that make up a typical injection molding system.

1. The hopper - The plastic enters the machine as either plastic beads or some other form of plastic granule. This is stored into the hopper until it is needed.
2. Injection Barrel- Typically the hopper feeds into the injection barrel by way of gravity. The injection barrel is heated in order to begin melting the plastic on its way to be injected. Any color is typically added to the plastic at this stage.
3.The Injector - The plastic is driven through the barrel through one of two different means. Either a reciprocating screw drives the plastic forward into the injector, or there is a hydralic ram. With current technologies the reciprocating screw drive has a much higher precision for injection and is used more frequently, especially with smaller components.
4. The Mold - The mold is the component that contains an empty volume (cavity) in the negative shape of the object desired. This is where the plastic is injected to recieve it's permanent shape. Typically the mold is cooled so that the plastic hardens evenly and quickly. The mold contains two parts which seperate to allow the part to be retrieved after the process is done.
5. Ejector Pin - Some components do not simply fall out of the mold when completed so an additional ejector pin is added in these cases to knock the part out of the mold


A diagram of injection molding


The principle cost in injection molding is the price of creating the mold itself. This cost will vary depending on the complexity of the shape you want to create, the longevity you desire from the mold, and the overall size of the piece.

Design Considerations

Similar to casting the process of injection molding will be used when the volume of parts justifies the need for the creation of a mold.

The sides of a part on each half of the mold cannot be parallel. Instead the shape must be formed at an angle known as a draft. Although injection molding allows for complicated shapes the shape must still be simple enough to be removed from a two sided mold. Often plastic will shrink when it cools and that must also be taken into account when designing the mold and injection process.

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