Replacing Metal: Cost Savings with Injection Molding

Amid supply chain shortages and increased environmental consciousness, metal replacements are becoming increasingly popular—the composite materials market alone is projected to reach USD 126.3 billion by 2026 at a CAGR of 7.5%. Making the switch away from metal can be daunting, especially when faced with unfamiliar manufacturing processes for composite materials and plastics. But no need to fret—the following breakdown explains injection molding, a common manufacturing process for thermoplastic composites and other plastics.

What is Injection Molding?

Injection molding is a forming process commonly used in the manufacture of plastic products which yields highly consistent parts in a short period of time and at a low cost per part. In contrast, the more widely used subtractive process of machining is typical in the production of metal products. While there is a lower cost to begin machining, the cost per part is significantly higher, making injection molding the superior choice for many high-volume manufacturing applications of suitable materials.

How it’s Done

In short, material is melted, inserted into a mold, allowed to cool, and then removed from the mold in the shape of the part and/or product needed. This is done using a machine with three parts: an injection unit, a mold, and a clamp. Pellets of the material being injection molded are placed in the hopper which then feeds into the barrel of the injection unit. Once inside, a screw in the unit propels the pellets forward as heating bands around the barrel warm the material. After a certain amount of molten material is in front of the screw, it pushes forward with tremendous force, injecting the material into the empty part of the mold known as the cavity. The plastic quickly solidifies, the mold opens, and the part is ejected into a bin below. Then, the mold closes back up, and the process is repeated.

Heating Process

The reciprocating screw has some extremely important features that assist in the heating process. First, the material only fills the space surrounding the screw's shaft, leaving a thinner, more evenly heated layer of material in its place. Secondly, "flights" wrap around the shaft of the screw, transporting the raw material forward through the barrel as the screw rotates. The screw motion agitates the melting pellets within the flights to create a homogenous mixture, while the flights themselves also help to mix the plastic. Third, the screw action heats the material from the inside out. The shaft’s diameter increases along the screw so that the distance between the wall and the shaft decreases, squeezing out air as the pellets are moved forward and shearing them by pressing against the barrel’s wall. This shearing creates friction which supplies a majority of the heat needed to melt the plastic—between 60 and 90 percent—with the rest coming from the heater bands. The friction force generated also assists in heating the plastic throughout. The molten plastic then flows past the front of the screw through indentations called flutes. When there’s enough plastic to fill the mold at the front of the screw, it rams forward like a plunger injecting the plastic into the mold.

Part Ejection

As molten plastic is injected, it forces air out of the mold through vents, which are shallow channels on the mold’s surface. To speed the plastic’s solidification, a coolant (typically water) flows through channels inside the mold just beneath the surface of the interior. After the injected part solidifies, the mold opens—however, as it opens, the volume increases without introducing air, creating tremendous suction that holds the mold together. To prevent any damage from occurring due to this force, at first the mold only opens several millimeters, allowing air to rush in and break the vacuum. Once the vacuum is broken, the mold quickly opens the rest of the way so the part can be removed.

Injection Molding vs. Machining

The following chart breaks down the differences between injection molding and machining in terms of different properties relevant to manufacturing:

Features and Capabilities

Speed and Complexity

One of the main advantages of injection molding is the speed of production. Once designs are developed into molds, however complex, that mold is used repeatedly to produce the design at a rapid rate. In contrast to machining, where additional complexity requires additional time, injection molding generates the entire part in seconds. Not only is time saved in the production of the part itself, traditionally secondary operations can be performed simultaneous to production—processes like decoration and texturing are typically done after a metal part is produced, sometimes at a different location, but can easily be included in the mold of the part itself through injection molding.

Materials and Sustainability

In the metal replacement market, thermoplastics are often the primary materials used in injection molding processes. These materials have the unique advantage of being recycled at a much higher rate than metals—up to 15% of pellets included in the injection molding process can be previously used pieces of the same material. In addition, there is a high degree of predictability to the process—engineering tools such as mold flow analysis can be used to predict the way material will fill the mold when inputted from different locations, and what effect that has on the strength of different places on the part.

Challenges and Limitations

Design Challenges

The ejection of the part creates a design requirement known as a “draft angle,” which is a tapering of the inner walls of the mold. If a part has walls that are exactly ninety degrees, it becomes very difficult to eject because its inner walls scrape the inside half of the mold, and the vacuum is difficult to break because air cannot readily enter. However, if the walls are slightly tapered—even just one or two degrees—it becomes much easier for the part to be removed because once the part moves slightly, the walls are no longer in contact with the inside half and air can rush in.

Cost and Volume

One of the major concerns with injection molding is the high cost of tooling, the process of constructing the mold. This high up-front cost deters many, especially when compared to machining. However, the more expensive mold can then rapidly create the part with unparalleled consistency. This yields a lower cost per part when compared to machining, especially for high-volume and/or high-complexity applications. The cost of the mold can also complicate prototyping methods for injection molding when only a few, potentially unfinalized pieces need to be molded from a highly expensive tool. Even in this case, there are lower cost alternatives for injection molding prototypes before a finished design goes into production at full volume.

Alpine Advanced Materials’ skilled composite engineering team follows our proven injection molding process to produce purpose-built parts that deliver the highest performance. Learn more about our manufacturing capabilities.