Understanding Injection Molding: Step-by-Step Process Explained

Injection molding is a widely used manufacturing process for producing parts made from thermoplastic and other materials. It is suitable for both large and small-scale production, ensuring high uniformity across the parts. The injection molding process involves melting plastic granules, injecting the molten material into a mold cavity under high pressure, cooling it and ejecting the solidified part. This cycle repeats until the desired production quantity is achieved. The pressure used ranges from 70 MPa to 200 MPa, with cycle times typically between 10 seconds and 30 seconds, depending on part size, with cycle times for larger parts often exceeding one minute. Injection molding can produce complex parts in various sizes, from small (50 grams) to large (up to 25 kg).

Injection molding uses various types of polymers, including elastomers, fiber-reinforced polymers, thermoplastics , and thermosetting polymers. The most commonly used materials in injection molding are PVC (Polyvinyl Chloride), ABS (Acrylonitrile Butadiene Styrene), Polypropylene (PP), Polystyrene, Polycarbonate (PC), High-density polyethylene (HDPE), Low-density polyethylene (LDPE), Nylon (PA), Teflon (PTFE), PEEK (Polyetheretherketone). These materials are selected for their diverse properties, making them ideal for a wide range of applications in injection molding.

Main Components of Injection Molding Machine:

Hopper, Barrel, Reciprocating Screw, Heaters, Nozzle, Mold Cavity, Ejector Pins, Crossheads.

The Injection Molding Machine consists of three main functional units:

Injection Unit- Includes Hopper, Barrel, Reciprocating Screw, Heaters, Nozzle.
Mold Unit- Includes Mold Cavity, Mold.
Clamping Unit (Hydraulic/Toggle)-Includes Ejectors, Crossheads.
Each unit plays a key role in the process, from feeding granules/pellets into the hopper to the final ejection of the molded part.

1. Injection Unit:

The Injection Unit is responsible for melting the granules and injecting them into the mold cavity.

Hopper:

A container that holds and dispenses granules into the barrel. It is equipped with a Hopper-Dryer to remove moisture from the granules. Without proper drying, moisture can cause gas formation during melting, delaying the injection process.

Barrel:

A chamber surrounded by heaters, where granules from the hopper are melted. The reciprocating screw inside the barrel moves back and forth to help melt and process the granules. The barrel stores the molten material before injection.

Recirpocating Screw:

This screw moves granules through the barrel, melting them along the way. It features flights around the shaft that push the granules forward and heat them evenly, ensuring uniform melting. The process of the conversion of raw solid material to liquid form is called “Plastification”. The screw operates in three zones: Feeding Zone, Compression Zone (Transition Zone), Metering Zone.

Heaters:

Heating bands or coils are clamped around the barrel to maintain a constant temperature for uniform heating of the granules, ensuring consistent product quality.

Nozzle:

Once the material is melted, the nozzle injects molten plastic into the mold cavity.

2. Mold Unit:

The Mold Unit is responsible for shaping the product according to the mold cavity and cooling it down to solidify. This is done through a process called "curing", which involves using cooling systems to control the temperature.

Mold Cavity:

The molten material injected through the nozzle fills the mold cavity, where it cools and solidifies. Cooling channels inside the mold use water or air to remove heat and speed up the cooling process.

Ejector Pins:

After the molding process, the ejector plates push the finished part out of the mold. The ejector pins extend to remove the part and then retract to close the mold for the next cycle.

3. Clamping Unit:

The Clamping Unit holds the mold in place during the molding process and withstands the pressure and stresses generated. There are two types of clamping systems, depending on the requirements:

Hydraulic Clamping System:

Uses cylinders to secure the mold. This system is suitable for machines below the 160-ton range and provides compact mold locking.

Toggle Clamping System:

Uses a series of toggle links to open and close the mold. This system ensures the mold remains securely closed during the injection process.

Cross Heads:

In the toggle clamping system, the toggle link is connected to the cross-head, which is moved by a driving device, such as a ball spline shaft, to help the cross-head move efficiently.

Working of Injection Molding Machine:

Feeding:

Polymer granules are fed into the hopper, where they enter the barrel due to gravity.

Heating and Melting:

The granules are uniformly heated in the barrel by the reciprocating screw, which melts them into a liquid form.

Injection Chamber:

Once fully melted, the molten material collects in the injection chamber of the barrel. As the screw moves backward, the molten material moves forward.

Mold Preparation:

Before injection, the clamping unit ensures the mold is securely closed.

Injection:

The molten material is injected into the mold cavity through the nozzle. The amount injected, known as "shot size/shot volume," must fill the mold completely, accounting for material shrinkage. The injection speed is also controlled for optimal efficiency and product quality.
Shot Size = Mold Cavity (Product Volume + Sprue Volume + Runner Volume) + Shrinkage

Cooling:

The mold is cooled through water or air circulation in cooling channels, allowing the material to solidify.

Ejection:

Once the part has cooled and solidified, the ejector plates separate, and the ejector pins push the finished product out of the mold.

Cycle Repetition:

The ejector pins retract, and the mold closes for the next production cycle.

Advantages:

Injection molding offers fast, high-volume production with minimal labor and low rejection rates. It can create complex parts with design flexibility, various colors,finishes and allows for the use of multiple materials. The process minimizes waste, ensures precise, high-quality products. It supports recycling of scrap materials and maintains tight tolerances.

Disadvantages:

Injection molding has high initial setup and running costs, especially due to automation and expensive machinery. Small-scale production can be costly and design changes require new molds to be created, adding to expenses.

Automotive Components:

Bumpers, dashboards, trays, engine parts.

Aerospace Components:

Instrument panels, brackets, interiors, housings, fasteners, connectors.

Medical Components:

Handles, test tubes, syringes, dental implants.

Consumer Electronics:

Remote Controls, keypads, smartphone casings.

Home Applicances:

Components for air conditioners, refrigerators, control panels, casings, and internal parts

Industrial Components:

Gears, bearings, housings, control panels, bushings, etc.

Flash:

Flashing happens when too much molten material is injected, causing excess material to spill over the edges of the mold and solidify on the surface.

Prevention: Control shot size and injection pressure for proper mold filling, check the mold for wear and sealing and ensure accurate clamping force to prevent mold separation during injection.

Warping:

Uneven cooling within the mold causes shrinkage at some parts, resulting in distortion due to internal stresses.

Prevention: By using a uniform cooling system to ensure even heat distribution and optimizing the mold design for better heat dissipation.

Jetting:

This occurs when molten material is injected at a high speed, causing it to splatter and solidify before the mold cavity is fully filled, leaving marks on the mold.

Prevention: To improve injection molding, reduce injection speed, adjust mold design for smoother flow paths and optimize gate design for controlled molten material entry.

Surface Delamination:

This occurs when foreign substances or contamination in the molten material prevent proper mixing with the plastic, causing thin layers to form on the surface.

Prevention: Ensure raw material is clean and free from contaminants, use proper drying techniques to prevent moisture and regularly clean the mold while handling materials carefully to avoid contamination.

Sink Marks:

These marks appear in thicker areas of the part because the inner sections cool and shrink faster than the outer sections, leaving indentations on the surface.

Prevention: To prevent sink marks, reduce part thickness in key areas, increase injection pressure for thicker sections and apply packing pressure to minimize shrinkage during cooling.

Compared to other manufacturing methods such as casting,cnc machining , or 3D printing, injection molding is more cost-effective for large-scale production. Though it requires a high initial investment for molds and machinery, the small-scale cost decreases as production volume grows, making it very economical for mass manufacturing. Although, 3D printing is cost-effective for small quantities but becomes more expensive per unit as production is huge. Therefore, traditional methods like injection molding are more economical. Injection molding positively impacts the economy by the production of affordable, high-quality goods. Additionally, it supportssustainability through the use of recyclable materials, waste reduction, and efficient production processes.

At Clarwe, we specialize in high-precisioninjection molding for both prototyping and full-scale production. From complex geometries to tight tolerance requirements, our advanced molding capabilities support a wide range of thermoplastics to meet the exacting standards of industries such as aerospace, automotive, medical, and general engineering. Our ISO-certified processes, in-house tooling expertise, and efficient production workflows ensure high part quality, repeatability, and fast turnaround—whether it’s a one-off design or a multi-cavity production run.

Need a quote for your injection molding project?
Share your part files or specs, and we’ll provide a clear, competitive estimate—fast and accurately.