Both plastic injection molding and plastic machining work well for making components from plastic. Injection molding uses solid plastic pellets that are melted, and then injected into a mold, which then forms into a mold’s exact dimensions as it cools. In contrast, plastic machining most often involves advanced machinery like CNC (computer numerical control) machines that cut a solid plastic block into the desired shape. Both plastic injection molding and plastic machining have their advantages and disadvantages; ultimately, the best method will depend on the application, the volume, and the upfront cost appropriate for your project.

Comparing Plastic Injection Molding to Machining of Parts

Several issues should be considered when comparing plastic injection molding and plastic machining. Though the costs of molds can be an expensive upfront investment, injection molding is generally less expensive per component made, as it takes much less labor and creates less waste material. On the other hand, Machining is better for smaller production runs that require less upfront tooling investment and can support a higher per-piece cost.

For similar reasons, plastic machining allows greater flexibility in design, so it’s better for prototyping, while mass-produced parts are usually better made via plastic injection molding, as it’s more cost-effective for larger production runs. Both methods create components with tight tolerances, with plastic injection molding getting within .005 inches (.127 mm) tolerances, while machining allows parts to get within less than .001 inch (.0254 mm) of specifications. For most applications, this difference in precision doesn’t matter, though certain industries like aerospace, medical equipment or telecommunications require very tight tolerances.

Benefits of plastic machining include:

• Fewer cosmetic imperfections
• More flexibility in design and material
• Produces small production runs more cost-effectively
• Quicker turnaround times
• Very close tolerances

Now, let’s compare plastic injection molding to plastic machining in greater detail.

Design

When it comes to designing parts, the shape often will delineate which technique is more appropriate. For example, achieving sharp or square internal corners in a component is extremely difficult with CNC machining. While plastic injection molding generally can accommodate such design aspects seamlessly, machining requires special workarounds that add to the expense.

On the other hand, if there’s a high likelihood of design changes, machining is usually the better option, as each custom mold often costs tens or even hundreds of thousands of dollars. CNC machines also allow industrial designers to make modifications more easily, while changes to injection molds can be time-consuming and expensive.

Material

The plastic composites from which a component is made must be considered when deciding on a fabrication method. CNC machining works well for tougher, higher-performing plastics that require machining to shape, like polyetheretherketone (PEEK), polyoxymethylene (POM), or polyvinyl chloride (PVC). Plastic injection molding, however, works well on an array of thermosets and thermoplastics, along with flexible materials like elastomers, whereas machining such polymers can result in a loss of precision.

Precision & Tolerances

In most cases, plastic machining will result in greater precision and tighter tolerances. When parts must fit exactly, even slight physical discrepancies can make components operate poorly or even make them nonfunctional. CNC machining tools allow workpieces to be cut more precisely, whereas making molds for plastic injection molding with such tolerances increases expense dramatically.

Surface Finish

Plastic injection molding can require specific features to be included in part designs that otherwise would not be required with machining.  For instance, Injection Molding utilizes ejector pins that remove cooled components, which oftentimes require specific relief from critical surfaces to mitigate slight surface blemishes. Though these are largely just cosmetic, CNC machining allows for extremely smooth finishes. When aesthetics matter, machining can generally provide more predictable control.

Costs & Efficiency

For the most part, fabricating parts using plastic injection molding methods is more cost-effective and efficient than machining plastic. Injection molding costs, however, depend on the size and complexity of the molds and the number of molds that need to be made. To reach certain production goals, molds with multiple cavities often must be made. In contrast, CNC machining requires only a plastic block or rod and a CAD (computer-aided design) file to shape a workpiece, making it the less expensive option for smaller production runs.

Speed

Regarding speed from design to production, CNC machining a component can be done sooner. Creating a CAD drawing takes just days before work can start, shortening the lead time necessary before production begins. Molds, on the other hand, can take weeks or months to produce. More recently, advances have been made to 3D printing technologies that allow disposable molds to be printed quickly but offer very limited shot life to produce components. This new technology is making headway in bridging the speed gap between injection molding and machining.

Volume

The greater the quantity produced via plastic injection molding, the lower the cost per component. Though the initial costs of molds make it expensive initially, the prices per piece drop sharply the bigger the production run. Though CNC machines can also make parts in volume, they’re most cost-effective for smaller production runs. Typically, plastic injection molding makes more sense economically when thousands or tens of thousands of parts need to be made.

Benefits of Plastic Injection Molding

To summarize the major benefits plastic injection molding provides manufacturers, let’s look at where these techniques excel at producing components made from plastic. Injection molding is extremely efficient and versatile, especially when mass-producing parts.

Benefits of plastic injection molding include:

• High levels of efficiency make the production of plastic parts quicker to mass-produce, with many components taking from 15 seconds to two minutes to make, though this also depends on size and complexity.
• Complex designs can be produced with uniformity for high-volume part production, though such aspects as boss design, corner transitions, gate positioning, proper placement of vents, rib design, weld lines, and wall thicknesses must be carefully factored into the molds.
• Augmented part strength is a key factor for manufacturers making components in volume, though flexibility or rigidity necessary for a part’s operation should be considered during fabrication; often, environmental conditions to which a component will be exposed will affect the material used.
• Material types that can be used are almost limitless, as technology has developed plastic resins to work with injection molding techniques that can offer a part greater impact or tensile strength, elasticity, heat reflection, or water absorption.
• Coloring parts during injection molding can be easily accomplished by adding liquid color or solid plastic additives, as well as by blending different colored resins or utilizing pre-colored resins.
• Significantly reduces waste in higher volume production runs, as leftover plastic can be easily recycled and introduced back into production.
• Lower labor expenses result from higher outputs of parts, with automated tools available to optimize operations with nominal guidance needed.

While CNC machining has its place in plastic component fabrication, plastic injection molding still offers the most economical means for mass-producing plastic parts.

Choosing Spaulding for Plastic Injection Molding

Spaulding Composites Inc. remains an innovative company with a century and a half of experience working with composite materials in industrial applications. We take pride in adapting our capabilities, providing flexible solutions, and making quick decisions for our customers, all with a view toward longer-term possibilities. Using the most cutting-edge technology for control, production, and testing purposes, Spaulding Composites seeks out the best US-sourced raw materials, using long-term procurement contracts to keep our prices competitive. We also constantly seek to improve our lead time while maintaining process reliability, real-time control, and traceability of our products. Spaulding is well-known for manufacturing components made from thermoset composites for various industries, including aircraft parts, night vision optics, pneumatic equipment parts, and components for transmissions in tractors and heavy trucks. Our company has also obtained an ISO 9001:2015 certification for manufacturing injection molding parts and thermoset composite materials. To learn more about Spaulding Components products and services, we invite you to contact us without delay.