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Plastic Injection Molding Machine: How Does It Work?

Why is a plastic injection molding machine important? There are many reasons, which is why it’s also crucial to fully understand the process. Read on.

Plastic Injection Molding Machine: How Does It Work? - KB Delta

Plastics remain one of the most commonly used formable materials worldwide. The global plastics industry was birthed in 1907 when Bakelite – i.e., the first synthetic or thermosetting plastic – was produced. However, the rapid growth of this global production industry was only realized in the 1950s.

According to Plastic Oceans International up to 380 million tons of plastic are produced yearly. Some reports even indicate that as much as 50 percent is utilized for single-use purposes.

As global plastic production became even more popular, several technological innovations and plastic creating methodologies were invented and put to use. One of them is the plastic injection molding machine via which the injection molding process takes place.

This guide briefly covers what a plastic injection molding machine is as well as how it works.


What is Plastic Injection Molding Machine?

The plastic injection molding machine is a highly complex piece of equipment that efficiently creates a solid part via injecting molten materials through the reciprocating screw system into a mold.

This piece of machinery is a combination of multiple components consisting of a material hopper temperature controlling devices, a barrel that contains a reciprocating screw, the injection mold itself, and a clamping unit.

The first plastic injection molding machine was invented in 1872 by John Wesley Hyatt and his brother, Isaiah. John Wesley Hyatt is also reputed to be the first individual to inject hot celluloid into a mold in order to produce billiard balls in 1868.

The machine – and process remained virtually unchanged until 1946 when James Hendry constructed the first screw plastic injection molding machine. This machine completely revolutionized the plastics industry, as up to 95 percent of all plastic injection molding machines use this method to heat, mix, and efficiently inject plastic into molds.


How Does a Plastic Injection Molding Machine Work?

The plastic injection molding machine consists of 3 basic sections (for clarity): injection, mold, and clamp. Let’s treat each section to see how the final plastic product is made.


  • Injection

Resin pellets, shavings, or flakes – and color – are fed to a barrel or reciprocating, motor-driven screw feed through a hopper. The feeding can occur automatically, manually, or through a vacuum.

A heating element encloses the barrel where the raw materials are subjected to high temperature and pressure until they melt and become malleable. The shear action of the screw also contributes to this process.

The screw conveys the molten polymer resin to the sprue orifice or nozzle. The resin is prevented from flowing backward, thanks to a check ring set in place.

This arrangement perfectly ensures that the molten polymer resin arrives in a homogenous state at the point of injection and the right pressure.


  • Mold

The molten polymer resin is transported – via runners that efficiently direct the flow away from the nozzle – into the split mold assembly that comprises a platen-mounted moving core as well as a stationary cavity.

The runners end within gates situated at the boundary of the cavity. The type, location, and size of these gates profoundly influence the overall quality of the final molded component or part.

Molds efficiently incorporate internal channels via which liquids are pumped right after the injection phase in order to assist part cooling just before ejection.

As regards the cooling phase of the process – during which the plastic parts form the desired shape – the plastic may shrink just a little. The mold halves only open when the required cooling time is reached.

The estimation of the cooling time depends significantly on the properties of the plastics involved in the process.

Custom-made molds are filled out according to some specifications. This is why it is possible to create multiple identical copies that can be efficiently customized in a wide variety of ways.


  • Clamp

The hydraulically actuated clamp ensures the two parts are tightly closed together in order to efficiently sustain the extraordinarily high pressures often generated during the injection stage.

Most plastic injection molding machines are defined – especially in terms of size – by the clamping force it achieves, which is often expressed in tons. When the part is properly cooled, the clamp releases it via ejection, and the plastic injection molding machine is readied right away for the next cycle.

The basic concept behind the plastic injection molding process – which seems simple but highly complicated – has been around for more than 70 years. It also involves the use of the right equipment and expertise.

However, this original idea – which involves injecting molten plastic resin into molds – has dramatically evolved over the years into an elegant, versatile, and highly efficient tool for creating multiple everyday items fast.


The Injection Molding Process

Injection molding is currently the most used manufacturing process for fabricating plastic components. It may have been invented during the 19th century, but it remains one of the best and most efficient ways of producing highly complex parts without exceeding budgets.

A wide range of products we commonly use today is manufactured via injection molding, which varies greatly in application, complexity, and size.

The injection molding process is employed for producing a wide variety of thin-walled plastic parts, including open containers such as buckets, plastic housing (a thin-walled enclosure with several bosses and ribs on the interior) used in several household appliances, power tools, consumer electronics, automotive dashboards, etc.

Injection mold is also used in producing everyday items such as small plastic toys, toothbrushes, valves, syringes, etc.

The thermoplastics commonly used in injection molding include:


  • Nylon PA
  • Acrylonitrile-Butadiene-Styrene (ABS)
  • Polypropylene
  • Polycarbonate


Components of a Plastic Injection Molding Machine

Here are some of the plastic injection molding machine components or parts:


  • Material hopper (this s the funnel into the barrel)
  • Feed zone
  • Screw-type plunger
  • Nozzle
  • Injection chamber
  • Stationary platen (often made of steel and holds part of the mold)
  • Barrel
  • Rotating screw
  • Heating unit
  • Movable platen (often made of steel and holds part of the mold)
  • Injection ram
  • Clamping cylinder


All these components work seamlessly together to hold the mold where the final shape of the plastic is formed.



The plastic injection molding machine remains one of the best equipment for designing high-quality and unique parts efficiently. It allows plastic manufacturers to enjoy the overall flexibility of making complex and highly durable shape products at cost-effective rates.



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