Plastics today are nothing like their early day. An industry that began with plastic buttons and combs has advanced to enable the manufacture of consumer packaging in all shapes and sizes, including sophisticated aircraft parts, intricate components for medical devices, or rugged industrial gears and guards. Modern manufacturing capabilities can transform plastics into parts seemingly without limits. However, different technologies and materials yield dramatically different results.
Design engineers often are faced with a critical decision at the beginning of their projects: is plastic injection molding or plastic machining a better choice for their application? Which process is going to supply a part or product that meets specifications, supplies proper performance and the necessary lifespan, with a price point that will not bust the budget? Comparing and contrasting the two processes can help determine which manufacturing method will be the best fit for your project.
Looking for high volume with loose specifications or tight tolerances?
Plastic injection molding is a high-volume manufacturing process used for large quantities of identical parts. The process involves injecting molten plastic material or resin into a mold cavity where it cools and hardens into the desired shape. Once tooling is created, the production process itself is highly automated, so it requires little labor beyond setup. Injection molding is typically used for high-volume production runs for its cost efficiencies, repeatability and speed of production.
What this process does not offer is tight tolerances or uniform surface finishing. It also requires tooling to create the molds that enable production. Tooling can be expensive; many companies have turned to overseas mold manufacturers to control costs, but that carries its own complexities in longer shipping times and even potentially risks intellectual property rights.
In contrast, plastic machining uses a cutting tool to remove material from a solid block of plastic to create the desired shape, otherwise known as subtractive manufacturing. Compared to injection molding, plastic machining and fabrication is more labor intensive and time consuming. However, this machining process allows for greater design flexibility and is the method of choice when comparing the two for creating parts with very tight tolerances. Plastic machining typically is recommended when a part has low volume requirements or for parts that cannot be made using injection molding.
Factors to consider when making the decision and when designing a part will often include:
Flexibility:
Plastic machining offers greater flexibility compared to injection molding due to the ability to work from a CAD design or file instead of being constrained by a mold. Once the mold is cast, changes cannot be made to the injection molding process without great expense in both time and money. Plastic machining offers the opportunity for design modifications and much easier customization. In addition, plastic machining can create parts with complex geometries, including internal features that would be difficult or impossible to achieve with injection molding.
Precision:
The tolerance for plastic machining can be as low as plus or minus 0.002 of an inch on vertical mills, which cannot be achieved with injection molding at the present time. Machining has the advantage of achieving tighter tolerances and more precise surface finishes due to the manufacturing process. In fact, depending on the cutting tool used, machining can produce a broad range of surface finishes, from rough to mirror-like. The cutting tools used in machining remove material in small increments and at different rates, which allows for greater precision.
In contrast, when molten plastic enters a mold, the process of cooling and solidification can cause greater variations in the final part. Certain industries, such as aerospace, medical devices, or automotive/heavy equipment parts require greater accuracy and tight tolerances.
Material options:
Injection molding can draw from a wide range of materials or resins; however, some are difficult or impossible to mold. Plastic machining can work with highly specialized or unique materials with special properties, such as PEEK, Ultem and Vespel. These materials offer unique characteristics such as high temperature resistance, chemical resistance and superior mechanical properties.
Strength and durability:
Several industries and applications require parts with greater structural integrity than what plastic- machined parts can supply. Machining can create solid parts with greater strength and durability compared to parts fabricated by injection molding. Sheet thickness used in machining can range from a fraction of an inch to several inches, which supplies greater integrity or rigidity for a durable part compared to an injection molded part with a wall thickness of perhaps an eighth of an inch.
For example, construction equipment or agricultural machines often require parts that can withstand harsh or extreme environmental conditions, chemical or UV exposure, or they must support heavy loads. Machining allows for the use of high-strength plastics such as NYLATRON®, an oil-filled sheet of nylon that is self-lubricating, to supply the necessary strength and durability for these applications.
The part’s durability can offer a longer lifespan as it withstands environmental stress or repeated use, which results in a greater return on investment for the part and less downtime for repair or replacement.
Part size:
The part size for an injection -molded piece is limited by the mold size. The larger the mold, the more expensive it is to manufacture and as a result the more time consuming and costly the setup. It could require an entire day and the assistance of heavy cranes to position the tooling required for a larger injection-molded piece.
In contrast, plastic machining can create parts within the same size range offered by injection molding, but within a shorter time and in a more cost-effective fashion.
Production volume:
For low-volume production runs, plastic machining is far more cost-effective than injection molding. The lack of tooling allows for greater cost savings for short production runs. It also enables faster setup times and shorter lead times, which makes it a great option for smaller production runs.
Injection molding provides the means to create parts with little complexity and repeated, large- volume production runs.
There is a process for every application. The staff at HP Manufacturing is ready to consult with customers to help determine the best process and best methods to achieve your business goals. We specialize in high-precision plastic fabrication of custom components, assemblies, fixtures and more. We are organized to provide open communication, detailed feedback and attention to detail. In fact, customers point to our communication and quality processes as characteristics of working with HP that help set us apart. Call HP Manufacturing today about your next project.
