Which Plastics Manufacturing Process is Right for Me?

Previously published on fastradius.com on May 20, 2020

Plastics are a family of highly versatile manufacturing materials — some are soft and flexible; others are stiff and impact resistant. This wide range of physical properties guarantees that, regardless of application, the manufacturing of plastic parts is generally time- and cost-efficient.

Regardless of the end-use — whether parts are being used to create innovative armchair designs, power tool components, or prosthetic limbs —  it’s important that designers and engineers know how to select the right manufacturing process for their needs.

Types of Plastic Product Manufacturing

Given that plastics come in different shapes, sizes, durabilities, and colors, it should come as no surprise that there are a range of manufacturing processes that enable the production of plastic parts. Here’s a quick rundown on a few of the most common.

3D Printing

This process uses CAD designs and models to create three-dimensional parts by adding a layer of the production material at a time until the part is completely formed. The primary advantage of 3D printing is that, compared to other manufacturing processes, it has far fewer design limitations, which makes it an especially economical method for creating pieces with complex geometries.

Unlike CNC machining, for instance, where engineers may need to consider how the mill head will reach tight spots, as well as the size of the tools needed to create fine details in the part’s geometry, 3D printing allows for complicated design features — such as curved internal channels — to be created without tooling.

Additive is capable of intricate designs that are difficult or impossible to make efficiently with other manufacturing processes. It also allows for a part to be created as one solid piece, rather than as multiple components that are subsequently assembled.

CNC Machining

This process involves using computer-controlled cutting tools such as mills, lathes, and drills in tandem with rotation to create plastic parts.

Unlike additive manufacturing processes, CNC machining is a subtractive process, meaning that material is gradually removed from the block that will eventually become the finished part, called the workpiece. There are two broad categories of CNC machining: milling, which uses spinning tools and a fixed workpiece; and lathing, which uses fixed tools and a spinning part.

CNC machining can be an effective alternative for creating parts that are hard to mold or that require tight tolerances, rendering the method ideal both for prototyping and creating certain end-use products, including bushings and gears. However, part complexity can quickly increase the cost per part when using machining processes, and certain geometric shapes — such as curved interior channels — can be difficult if not impossible to create using subtractive manufacturing.

Injection Molding

As the name suggests, this process involves injecting pressurized molten thermoplastic material into hardened steel or aluminum molds. These molds, which can be geometrically complex, result in parts with tight tolerances, iterative accuracy, and high-quality surface finish — all of which makes injection molding an effective process for high-volume production runs.

While injection molding is expensive to tool up and start running — creating the molds can be costly and take up several months — no other plastic product manufacturing process is able to match their ability to swiftly produce high-volume production runs, which drastically reduces the cost per part.

Extrusion Molding

The extrusion molding process is similar to injection molding in that it requires melted plastics to create parts. However, rather than injecting the molten material into a mold, extrusion systems force the plastic through a die that gives the plastic a fixed shape.

Plastic pipes, straws, gutters, door jambs, hoses, and other symmetrical parts can be made efficiently with extrusion systems. Due to their simpler shapes, extrusion-made parts often have low production and tooling costs.

Urethane Casting

The urethane casting process involves creating a silicone mold from a master pattern of the finalized part design. Once the mold has set and the master pattern is removed, the mold can then be used to produce individual copies of the part with high levels of accuracy and precision, as well as good surface finish.

Cast urethane silicone molds are less durable than the hard-tooled ones used for injection molding, but they can be created far more quickly and cost-effectively. This makes it urethane casting a good choice for low- to medium-volume production with tight turnarounds.

Vacuum Forming

Vacuum forming uses a vacuum to push sheets of heated, malleable plastic across single-surface molds. Commonly used with thermoplastics like high-impact polystyrene sheeting, this process is used to create durable products in a variety of shapes, including protective coverings, street signs, and packaging for taste- or odor-sensitive products.

Roto-Molding

The rotational molding process involves rotating a hollow mold filled with a powdered resin as the mold is heated and cooled, thereby ensuring that the walls are evenly coated and free of sags or deformations. It’s an effective method for creating highly stable parts with uniform wall thicknesses, such as plastic tanks and containers, that do not typically require additional surface finishing.

Factors to Consider When Manufacturing Plastic Parts

Because there is a wide range of manufacturing processes available, it’s important to choose the one best-suited for the part. Here are a few considerations to keep in mind.

Material

Part application is a primary consideration when selecting a viable material, and determining the most beneficial physical characteristics for the part in question can help narrow down the material choices.

However, it’s important to note that some materials are best-suited to specific manufacturing processes. Elastic, low durometer polyurethane parts, for instance, are easy to produce with urethane casting, but extremely difficult to work with when it comes to 3D printing. In some cases, the desired material characteristics can even be improved by strategically selecting a manufacturing method.

Form

Factors such as tight tolerance requirements, unique internal features, or a high level of geometric complexity will help determine which manufacturing processes are most economical and efficient. If the part demands a specific process, making design for manufacturing (DFM) revisions may be necessary in order to optimize the cost-effectiveness of production.

Lead Time

Final product deadlines will help in determining the best manufacturing method, as production times may vary significantly across processes. Depending on the part and the machine, some 3D printers can produce completed, viable parts within a few hours. On the other end of the spectrum, while the actual injection molding process is very fast — in many cases capable of achieving cycle times of less than a minute — tooling the molds can take months to finalize, which extends the production schedule.

Volume and Cost

One last major factor to keep in mind is the production volume required. Injection molding systems demand steep up-front costs, which tends to make them economical for high-volume runs but cost-inefficient for prototyping. Other processes, such as CNC machining, have lower initial costs but can easily cost more per part depending on the amount of manual labor involved. Finding the right balance between production volume and cost-effectiveness is key.

Getting Your Plastic Parts Order Started

The variety of plastic manufacturing processes available speak to plastic’s versatility. The right process for any given part will be dependent on a number of factors, including the part’s application, design, production volume, and lead time.

At SyBridge, we work closely with our customers to make sure their parts are made with the most effective and efficient manufacturing process. If you have questions about how we work, or if you’re ready to start production, contact us today.