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What is Plastic Injection Molding?

Plastic injection molding is the process used to manufacture most everyday plastic items. There is a good chance the device you are using to read this article has some injection-molded parts. Plastic injection molding is popular for its speed and minimal per-part cost of production. When it comes to producing millions of units, nothing beats the highly automated nature of injection molding. In addition to this, plastic injection molding wastes very little material and energy and requires minimal labor. However, it must be noted that the molding machinery is expensive and molds can cost hundreds of thousands of dollars, making upfront costs very steep indeed. These costs can only be offset by large production volumes. This article will give an overview of what plastic injection molding is and how it works.

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By Team Xometry
 5 min read
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How Does Plastic Injection Molding Work?

Plastic injection molding is a method used to produce plastic parts at an extremely high rate. As such, it should be no surprise that injection molding is the go-to technology for most plastic consumer products. The plastic injection molding process consists primarily of the following components:

1. Injection Unit

Plastic pellets are fed into the injection unit using a hopper. These pellets fall into a heated barrel which melts the plastic. A screw conveyor moves the plastic through the barrel and also serves to mix it for even heating. In addition to this, modern screws have tapered shapes that force pellets against the side of the barrel to add additional frictional heat. A significant proportion of the heating is achieved through this friction effect and heater bands supply the remainder. Once a predetermined amount of plastic has been melted in front of the screw, it is pushed forward and forced into the mold.


2. Mold 

The mold is effectively a negative of the product and typically consists of 2 or more separate parts. The plastic is injected into the mold’s open cavity, also known as the cavity image, where it solidifies and takes on the shape of the part. Air is allowed to escape through vents machined into the mold. This solidification process can be accelerated using strategically placed cooling channels to cool the mold quicker. The part is then ejected from the mold using ejector pins. The location of these ejector pins can be seen on injection-molded parts as circular indentations. They are often placed on a less visible side of the plastic part. Depending on the complexity of the part being manufactured, molds can be extremely expensive. 


3. Clamp

Plastic injection molding is done at extremely high pressures. As such, when plastic is forced in, the pressure will try to separate the two halves of the mold. To stop this from happening, the halves of the mold must be clamped tightly together. The clamping pressure is typically supplied by a hydraulic ram.

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Plastic Injection Molding Machine

Part & Mold Design

Part designs for plastic injection molding need to follow some basic DFM (Design for Manufacturing) guidelines. These guidelines ensure that injection-molded parts can be easily manufactured with as few defects as possible. Molds are manufactured to very strict tolerances so if the part was poorly designed at the outset, that expensive mold may need to get re-machined before it can be used. Any rework results in significant costs. Some typical DFM guidelines cover the following aspects:


  • Minimum wall thicknesses
  • Draft angles for easy part removal
  • Location of gates for aesthetic features
  • Density and placement of support structures 

Molds consist of at least two sections. They also contain cooling channels that are close enough to the surface of the mold to be able to quickly cool the part. In addition to cooling channels, air release vents need to be machined on the face of the mold to allow air to escape during the injection process. These vents must be extremely narrow so as to allow air to escape but no plastic.

Plastic Injection Molding Materials

The vast majority of plastic injection molding is done with thermoplastics like ABS. Thermoplastics can be re-melted without breaking down chemically. This makes them ideal for injection molding. Their counterparts, thermosetting polymers, can also be used for injection molding but need to follow a slightly different process than what’s outlined in this article. 


Typical thermoplastics used in plastic injection molding are listed below:


  • ABS
  • Polypropylene
  • Polycarbonate
  • Polyamide (Nylon)
  • Polyethylene
  • Acrylic 

Benefits of Plastic Injection Molding

Plastic injection molding has become the preferred method to manufacture many different plastic parts. This is due to its long list of benefits, such as:


  • Part Complexity: Injection-molded plastic parts can have very complex features that would be expensive to manufacture using other processes. The high injection pressures can easily force material into tight spaces and create detailed features and surface patterns. It must be noted, however, that increased part complexity can dramatically increase the price of a mold. 

  • Multi-Material Parts: Plastic injection molding can make use of processes called overmolding and insert molding to create mixed-material parts. This allows designers to, for example, make hand tools with hard plastic cores for structural strength but soft plastic outer grips. Even metal parts can be used as inserts with plastic molded around them.
  • Highly Automated: Plastic injection molding is highly automatable. This fact helps make the resulting parts so cheap to manufacture. Injection molding can be easily scaled to produce millions of parts from a single mold in a cost-effective manner. 

Should I use Plastic Injection Molding for my Parts?

Injection-molded parts have many advantages and as such can have a positive impact on a product’s functionality and cost. However, parts that are not designed according to standard DFM principles can dramatically increase the cost of the mold and may affect the profitability of the whole production. To learn how best to optimize a part for injection molding download the Xometry injection molding design guide or consult a Xometry expert for more details.

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Team Xometry
This article was written by various Xometry contributors. Xometry is a leading resource on manufacturing with CNC machining, sheet metal fabrication, 3D printing, injection molding, urethane casting, and more.