What Are the Steps of Plastic Injection Molding?

Plastic injection molding is one of the most widely used manufacturing processes for producing complex plastic parts in large volumes with exceptional precision. Central to this process is the injection molding machine, a powerful and versatile tool designed to melt plastic pellets and inject them into a mold cavity under high pressure. Understanding the key steps of plastic injection molding can help manufacturers optimize productivity, maintain quality, and reduce waste.

This article walks you through each critical step involved in the plastic injection molding process, while providing detailed explanations, supporting data, and practical insights.

Step 1: Material Preparation – Selecting and Drying the Resin

Before the injection molding machine begins its operation, the appropriate plastic resin must be selected based on the part's desired properties, including strength, flexibility, chemical resistance, and color. Common thermoplastics used in injection molding include ABS, polypropylene, polycarbonate, and nylon.

Once the resin is selected, it typically comes in the form of small pellets. However, these pellets often absorb moisture from the air. If injected without drying, this moisture can cause internal bubbles, surface defects, or even weaken the final product. Therefore, the drying process is essential and is performed using desiccant dryers or hot-air dryers to bring the moisture content down to the manufacturer's specification—often below 0.02%.

A properly dried resin leads to a more consistent melt, better flowability, and significantly fewer defects, thereby improving overall cycle efficiency.

Step 2: Melting and Conveying – The Role of the Injection Unit

Once the resin is dry, it's fed into the hopper of the injection molding machine. From here, the granules enter the barrel, where they are gradually heated and mixed by a rotating screw. This screw not only pushes the plastic forward but also contributes to its melting by generating frictional heat in addition to the barrel's external heaters.

Temperature control during this phase is critical. The melt temperature must be high enough to ensure good flowability but not so high that it causes degradation. The melt viscosity affects how well the material will fill the mold cavity, and it directly impacts the part's surface finish, dimensional accuracy, and strength.

The injection unit carefully controls pressure, temperature, and speed during this phase, ensuring a homogeneous melt that's ready for injection.

Step 3: Injection – Filling the Mold with Precision

The actual injection phase is perhaps the most dynamic and sensitive part of the molding cycle. Once the plastic has reached the proper melt condition, the screw moves forward like a plunger, injecting the molten plastic into the mold cavity through a nozzle and runner system.

Injection speed, pressure, and time must be precisely controlled to avoid issues such as short shots (incomplete filling), flash (excess material at the parting line), and air traps (bubbles inside the part). Properly designed mold gates and venting systems help eliminate these problems.

The injection molding machine ensures consistent repeatability of this step, with modern machines capable of controlling shot size and pressure to within a fraction of a percent. The ultimate goal is to completely fill the mold cavity before the plastic begins to solidify, thus capturing the finest mold details.

Step 4: Packing and Holding – Ensuring Dimensional Accuracy

After the cavity is filled, the machine enters the packing or holding phase. During this time, additional pressure is applied to compensate for material shrinkage that occurs as the plastic cools. This step is essential for preventing voids, sink marks, and warping.

The holding pressure must be carefully calibrated. Too much pressure can force plastic out of the mold or create stress within the part, while too little can lead to underfilled sections or dimensional errors. Holding time depends on the material and the geometry of the part but typically ranges from a few seconds to several minutes.

A consistent and optimized packing phase ensures that parts meet tight dimensional tolerances and exhibit the required mechanical performance.

Step 5: Cooling – Letting the Part Take Shape

Once the holding phase is complete, the part is allowed to cool and solidify inside the mold. Cooling is one of the longest steps in the injection molding cycle and can account for up to 80% of the total cycle time.

The mold is equipped with a cooling system—usually consisting of water channels—that helps draw heat away from the molded part quickly and evenly. Uniform cooling is vital to prevent internal stresses, warping, or part distortion.

Cooling time is influenced by several factors:

• Part thickness

• Mold material

• Resin type

• Cooling system efficiency

Here's a comparison of typical cooling times based on resin type and part thickness:

Effective mold design and thermal analysis tools can help reduce this time without compromising quality.

Step 6: Ejection and Post-Processing – Releasing the Part

Once the part has cooled and solidified, the mold opens, and ejector pins or plates push the part out of the cavity. Timing here is important—ejecting too early can deform the part, while waiting too long slows down production efficiency.

In some cases, the molded part may stick due to its geometry or surface texture. Special coatings, mold release agents, or air-ejection systems may be used to ensure clean release. Automation tools such as robotic arms are often used to pick and place the part, minimizing human error and maximizing speed.

Post-processing operations may follow, such as:

• Trimming or deflashing excess material

• Drilling or tapping for assembly

• Printing or labeling

• Quality inspection

Parts are then packaged or sent for secondary operations like painting or ultrasonic welding, depending on the product's final use.

FAQs About Plastic Injection Molding

Q1: How long does a full injection molding cycle take?
A typical cycle ranges from 20 to 60 seconds, depending on the part size, complexity, and material used.

Q2: What types of defects can occur during injection molding?
Common defects include warping, sink marks, short shots, burn marks, and flash.

Q3: Is injection molding suitable for low-volume production?
While it's most cost-effective for high-volume runs, rapid tooling and 3D printed molds have made low-volume injection molding increasingly viable.

Conclusion

The injection molding process is a finely tuned sequence of steps that transforms raw plastic into high-precision components. From material preparation to ejection, each phase must be precisely executed and carefully monitored. The injection molding machine plays a central role in maintaining consistency, speed, and accuracy throughout the entire process.

By understanding and optimizing each step—melting, injecting, packing, cooling, and ejecting—manufacturers can significantly improve product quality, reduce defects, and increase production efficiency. Whether you are a manufacturer, engineer, or investor, mastering the science and art behind injection molding will give you a significant competitive edge in the world of modern plastics.