In plastic mold manufacturing, injection molding and extrusion are two common methods used in producing plastic products. There is a slight similarity between these two methods, which is why most people think they are the same. But they are not; they are way different. In this post, TLD Vietnam will share with you the differences between extrusion vs injection molding.
Extrusion vs Injection Molding: Trace back to the history
Extrusion molding was invented in the early 19th century. It first started in 1820 when Thomas Hancock produced a rubber masticator that was designed to recycle processed rubber waste. And in 1836, Edwin Chaffe created a two-roller machine for mixing additives into rubber.
Despite these early advances, thermoplastic extrusion was not realized until 1935, when Paul Troester and his wife, Ashley Gershoff, successfully carried out the first thermoplastic extrusion process in Germany.
Injection molding, by contrast, is a manufacturing process in which molten material is injected into a mold cavity to form a defined shape. The concept was first introduced in 1795 by Joseph Bramah. In 1820, Thomas Burr advanced the process through the development of the first hydraulic press, improving pressure control and repeatability. However, meaningful industrial application did not occur until 1894, when the process underwent significant refinement.
Further evolution involved the use of copper and brass alloys, contributing to the formal establishment of injection molding as a manufacturing method in the 1930s. Over time, thermoplastic and thermosetting polymers proved more efficient and versatile than metal alloys, leading to the widespread adoption of injection molding as one of the most important production techniques in the modern plastics industry.
Extrusion vs Injection Molding: Definition
The term “extrude” refers to the act of forcing material outward, which accurately describes the principle of extrusion molding. In this process, molten material is continuously pushed through a die to create a product with a defined cross-sectional geometry. Extrusion molding is primarily used to manufacture components that are produced in long lengths and maintain a constant cross-sectional profile.
Similar to injection molding, extrusion is widely applied across multiple industries and is compatible with most thermoplastic materials. Typical products manufactured using this method include pipes, tubes, straws, fence profiles, and window frames.
Injection molding is a manufacturing process used to produce a broad range of plastic components. Although it was developed in the nineteenth century, it remains one of the most effective techniques for producing complex geometries with high-dimensional accuracy while controlling production costs at scale. As a result, the method continues to play a significant role in the global plastics manufacturing market.
The process relies on custom-designed molds to produce consistent, repeatable parts. Comparable to CNC-based manufacturing, injection molding supports a wide selection of materials and surface finishes, which contributes to its extensive adoption across industries. It is commonly applied in high-performance sectors such as aerospace, automotive manufacturing, and medical device production, where components must meet precise technical and regulatory requirements.
Differences between Injection Molding and Extrusion
The following section provides a detailed comparison between injection molding and extrusion, examining their operating principles, advantages and limitations, application areas, and the types of plastics commonly used.
Extrusion vs Injection Molding: Process
In extrusion molding, plastic resin is introduced into the feeder, which is typically surrounded by heated jackets that gradually melt the material into a homogeneous, viscous state. A die with a defined opening is attached to the end of the feeder, and the molten resin is continuously forced through the die to form the extruded profile. As the material advances from the feeder toward the die, it follows a screw-driven flow pattern that promotes uniform distribution. Upon exiting the die, the material cools and solidifies, after which it can be cut to the required length.
In injection molding, plastic resin is fed into a hopper and conveyed from the feed zone into the compression zone, where friction and applied heat transform the pellets into a molten state. A reciprocating screw then drives the melt through a chamber and injects it into a closed mold via a nozzle. Once the mold cavity is filled and maintained under pressure, the material is allowed to cool and solidify. After cooling, the mold opens, and the ejection system releases the finished part for removal or downstream handling.
Extrusion vs Injection Molding: Mold Tooling
Extrusion mold tooling is composed of the feeder and the heated jacket.
Injection molding tool design is primarily made up of two units – the injection unit and the clamping unit.
Extrusion vs Injection Molding: Advantages
Extrusion molding offers a highly efficient approach for producing long, continuous plastic components, particularly when multiple lengths of an identical profile are required. The material remains at elevated temperatures upon exiting the extruder, allowing for further post-extrusion processing. Additionally, extruded products typically have smooth surfaces that require minimal or no secondary finishing.
Injection molding, by contrast, is particularly suited for producing three-dimensional components with complex geometries. The process allows for high precision and the use of various plastic resins. Moreover, completed parts can be strengthened, and material waste can be reduced through recycling, enhancing both efficiency and sustainability in production.
Extrusion vs Injection Molding: Disadvantages
In extrusion molding, the material may experience swelling or distortion after exiting the die. The method also imposes limitations on the types of products that can be produced, as it is generally restricted to linear polymers. This restriction arises from the fact that extrusion relies on a fixed die rather than custom-designed mold cavities to define the final shape.
Injection molding, in contrast, involves longer lead times due to the requirements for mold design, review, testing, and fabrication. Once these preparatory steps are completed, production of the required parts can proceed rapidly. The process also demands skilled personnel familiar with the specific industry to manage design considerations and raw material procurement effectively. Consequently, specialized injection molding facilities are recommended for medical components, while other industries rely on dedicated expertise for their respective applications.
Extrusion vs Injection Molding: Application
Extrusion molding is primarily suited for continuous production processes. Typical applications include products such as hoses, straws, door frames, tubes, pipes, and cable conduits.
Injection molding, by contrast, is versatile and applicable to a wide range of plastic components across various industries. It can be employed for both discrete part production and continuous manufacturing. Commonly produced items include toys, bottle caps, handles, baskets, casings, chairs, vehicle dashboards, and fasteners, among others.
Extrusion vs Injection Molding: Plastic Used
Extrusion molding is limited to thermoplastic materials, as the process is primarily oriented toward producing recyclable products. Commonly used polymers include ABS, PVC, polypropylene, polyurethane, clear acrylic, thermoplastic rubber, and thermoplastic elastomers.
Injection molding, in contrast, accommodates both thermoplastics and thermosetting plastics, enabling the production of both durable and recyclable components. Frequently used materials in this method include ABS, nylon, polypropylene, polycarbonate, acrylic, and polyethylene.
Extrusion vs Injection Molding: Characteristics
Unlike injection molding, which produces fully formed three-dimensional components, extrusion molding is generally limited to two-dimensional profiles. Extrusion also allows for post-processing adjustments, such as cutting cross-sections or shaping parts to specific configurations.
Injection molding, as previously noted, generates three-dimensional components directly from the mold. The molten material solidifies within the mold to produce fully formed 3D parts, providing a key advantage over extrusion, where the final shape may require additional handling or modification.
Extrusion vs Injection Molding: Cost
The production cost of extrusion molding is relatively low, largely due to the ability to collect and reuse excess material. Additionally, the recyclability of the plastics used contributes to a more sustainable process by reducing waste and overall material expenses.
In contrast, injection molding involves higher equipment and tooling costs, which makes it less economical for low-volume production. Cost efficiency improves only when large quantities of parts are manufactured, as the fixed costs are spread over a higher production volume.
Conclusion
Determining which method is superior between injection molding and extrusion is not always straightforward, as each serves specific purposes and offers distinct advantages in plastic part production.
However, some noticeable differences between injection molding and extrusion that cannot be avoided are the fact that injection molding can create complex 3D shapes, while extrusion is best at 2D shapes. Also, injection molding cannot be used to manufacture the typical cross-sections that extrusion molding will produce accurately.
