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Understanding the Plastic Injection Molding Process

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Understanding the Plastic Injection Molding Process

Injection molding is one of the most widely used methods of processing plastic materials. It is a complex manufacturing process in which plastic is melted, injected, and set into the shape of a metal mold using a hydraulic or electric machine. This method is beneficial for the mass production of products with intricate shapes and is commonly applied in automotive, home appliances, and medical devices.

This article will dive into the fundamentals of plastic injection molding — including its definition, advantages, and step-by-step procedure.

What Is Injection Molding?

Injection molding is a fabrication procedure that enables the production of large quantities of parts, and it entails injecting molten materials into a mold. Materials used in this process typically include metals, glasses, elastomers, and confections, though thermoplastic and thermosetting polymers are the most commonly utilized.

Plastic injection molding has the following advantages:


Manufacturers can select the mold shape and thermoplastic utilized for each component. Thus, the injection molding process can create a wide range of items, including extremely complex parts.


Injection molding machines can uniformly produce thousands of pieces per hour once the operation has been set up and tested. Using electric injection molding machines also contributes to the process being relatively energy efficient.

Quality and Consistency

Plastic injection molding can consistently manufacture strong, ductile, and finely detailed components quickly as long as the process parameters are tightly controlled.


Once the mold has been built, the production cost per component will be reduced, particularly if large quantities are produced.

How Does Plastic Injection Molding Work?

Plastic injection molding is performed in highly specialized injection molding machines (IMMs) fitted based on mold clamping force in tons. These machines can range from micro IMMs that generate exceedingly small parts in small molds to very large IMMs with clamping forces surpassing 3,000 tons.

Here is the step-by-step procedure for injection molding:

Step 1: Choosing the Right Thermoplastic Material and Mold

Selecting the appropriate thermoplastic materials and molds is critical because they are essential in creating and forming the final components. To make the best choice, manufacturers must consider how the thermoplastic and mold react to one another, as certain plastic types may not be suitable for particular mold designs. For instance, if a manufacturer requires a lightweight part with dielectric properties, polycarbonate will be appropriate, but only if the mold does not need to operate at temperatures above 135 ℃ or extremely high pressures.

Step 2: Feeding and Melting the Thermoplastic

Molding machines typically include a feeder at the top, a cylindrical heated barrel in which a large injection screw sits, a gate at the end of the barrel, and the mold tool to which the gate is connected. The process begins with supplying raw thermoplastic pellets into the feeder. Then, these pellets are processed gradually into the machine’s barrel as the screw turns, steadily warming and melting the thermoplastic.

Step 3: Injecting the Molten Plastic into the Mold

When the molten plastic reaches the barrel’s end, the gate closes, and the screw retracts — allowing a predetermined amount of plastic to pass through. At the same time, the two mold tool components are clamped together and held under high pressure. Once the proper pressure is applied to the tool and screw, the gate opens, the screw advances, and the molten plastic is injected into the mold.

Step 4: Holding and Cooling

After most of the plastic has been injected, it is held under pressure for a predetermined time. This process is called holding time, which can span from milliseconds to minutes, depending on the material. This holding time is critical for ensuring that the plastic packs out the tool and forms properly.

Then, the screw retracts after the holding phase, releasing pressure and enabling the part to cool in the mold. This step is called cooling time and can range from a few seconds to several minutes. It ensures that the component sets properly before being ejected and completed on the manufacturing line.

Step 5: Ejecting and Finishing

Pins or plates eject the parts from the tool after the component is formed. These parts fall into a chamber or onto a conveyor at the machine’s base. Then, other devices and equipment can complete finishing processes — such as polishing, dying, or removing excess plastic. Once these processes are completed, the components will be packaged and distributed to manufacturers.

Exceptional Plastic Injection Molding Services From Lerner Molded Plastic

Lerner Molded Plastics offers custom plastic injection molding to customers in a wide range of industries across the United States! We also take pride in having over three decades of plastic manufacturing expertise and a state-of-the-art injection molding facility.

Our company’s professionals specialize in multi-cavity injection molding for complex components. This production method allows the construction of multiple parts per production cycle, resulting in increased efficiency and shorter production times without compromising product quality.

Contact us today to learn more about our capabilities, or request a quote to get started with us!