Injection molding is one of the most widely used manufacturing processes for producing plastic parts at scale. To better understand why it is trusted across industries such as packaging, automotive, electronics, and household goods, it is important to clearly explain how an injection molding machine works and how each stage of the process contributes to consistent product quality.
Before looking at the details, it helps to understand that an injection molding machine is designed to melt plastic material, inject it into a mold under pressure, and then cool it into a finished shape. While the process may appear simple on the surface, each step is carefully controlled to ensure accuracy, repeatability, and efficiency.
At its core, an injection molding machine is industrial equipment used to manufacture plastic parts by injecting molten plastic into a mold. Once cooled and solidified, the part is ejected and the cycle repeats. Because the same mold is used repeatedly, the process is ideal for high-volume production with stable quality.
From a structural point of view, most machines consist of three main systems: the injection unit, the clamping unit, and the power and control system. These systems work together to complete each molding cycle with precise timing and force control.
Before walking through the process step by step, it is useful to briefly understand the role of each major unit.
The injection unit is responsible for melting the plastic material and injecting it into the mold. It includes the hopper, barrel, screw, heaters, and nozzle.
The clamping unit holds the mold closed during injection and cooling. It applies enough clamping force to prevent molten plastic from leaking out under high pressure.
The power and control system, which may be hydraulic, electric, or servo-driven, provides motion, force, and precise control of temperatures, pressures, and speeds throughout the cycle.
With the main structure in mind, the working principle can now be explained step by step.
First, plastic raw material in the form of pellets or granules is fed into the hopper located on top of the injection unit. These materials are typically thermoplastics such as PP, ABS, PE, or PC. From the hopper, gravity feeds the material into the barrel.
At this stage, stable and continuous feeding is important to ensure consistent melting and injection later in the process.
Next, the screw inside the heated barrel begins to rotate. As the plastic pellets move forward along the screw, they are gradually heated by external heater bands and by friction caused by screw rotation.
During this phase, the plastic changes from a solid state into a uniform molten state. At the same time, the screw movement helps mix the material evenly, ensuring stable viscosity. Accurate temperature control here is essential, as overheating or underheating can affect part quality.
Once enough molten plastic is prepared, the screw stops rotating and moves forward like a plunger. At the same time, the clamping unit closes the mold and applies sufficient clamping force.
Then, the molten plastic is injected through the nozzle and runner system into the mold cavity at controlled speed and pressure. Injection pressure, speed, and timing are critical parameters, as they directly influence filling quality, surface finish, and dimensional accuracy.
After the mold cavity is filled, the machine switches to a holding or packing phase. During this time, additional pressure is applied to compensate for material shrinkage as the plastic begins to cool.
Once holding pressure is completed, the cooling phase begins. Cooling channels inside the mold remove heat from the plastic until it solidifies. Because cooling often takes up a large part of the cycle time, efficient cooling design is key to productivity.
When the cooling time is complete, the clamping unit opens the mold. An ejector system then pushes the finished plastic part out of the mold cavity.
At this point, the cycle is complete, and the machine is ready to begin the next injection molding cycle. In automated production lines, parts may be transferred directly to conveyors or downstream processing equipment.
Although the steps are consistent, the final part quality depends heavily on machine parameters. Injection speed, injection pressure, holding pressure, mold temperature, material temperature, and cooling time all play important roles.
Modern injection molding machines allow these parameters to be precisely controlled and repeated, which is why the process is known for high consistency and low defect rates when properly set up.
From an operational perspective, hydraulic injection molding machines use oil pressure to drive movements, while electric machines rely on servo motors. Electric machines typically offer higher precision, faster response, and better energy efficiency, especially for clean and high-accuracy applications.
Hydraulic machines, on the other hand, are still widely used for applications requiring high clamping force and robustness. Both types follow the same basic working principle, but their performance characteristics differ depending on production needs.
Understanding how an injection molding machine works helps manufacturers choose the right equipment, optimize process settings, and avoid common defects such as short shots, flash, or warpage. It also allows buyers to communicate more effectively with machine suppliers and ensure that the selected machine matches their product and production requirements.
In summary, the injection molding process relies on a clear sequence of material feeding, melting, injection, cooling, and ejection. Each stage depends on the machine’s structure, control accuracy, and mechanical stability. When these factors are well designed and properly controlled, injection molding delivers high-quality plastic parts efficiently and repeatedly.
As a professional plastic injection molding machine manufacturer, NPC designs and manufactures reliable hydraulic and electric injection molding machines with stable performance, precise control, and strong structural design. If you are looking for equipment that supports efficient production and long-term operation, NPC is ready to provide technical support and customized solutions for your injection molding needs.
