Comparing Plastic Injection Molding and CNC Machining

When it comes to designing and manufacturing the plastic parts for your products, you&#;ve got several manufacturing methods at your fingertips. Before you get started prototyping your part and committing to how your plastic parts will be produced, it&#;s important to explore your options and find out which method will work best for your needs, from material selection to the speed and scale of parts production. In this comparison, we explore two methods: plastic injection molding and CNC machining.

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What is Plastic Injection Molding?

Plastic injection molding produces parts by injecting molten materials &#; most commonly thermoplastics &#; into molds, also known as tools. Before being injected, the plastic is heated in the barrel, injected into the mold, moves along channels known as runners, and then into the part itself. Once completed, the mold opens, and the part is ejected, ready to start the process again. Loaded into hydraulic or electric machines, a well-made mold, and the right plastic resin can produce precision parts at a very fast rate.

What is CNC Machining?

While injection molding is about filling, CNC machining is about taking away. CNC &#; which stands for computer numerical control &#; machining is about using computer-controlled cuts removal material from a solid block a layer at a time. This information is pulled from a CAD or similar digital file, allowing for alterations to the files between production. The computer breaks these files down into a series of commands which are fed to one or more milling tools that work on the material to produce the final product. This allows quick transition between digital designs and physical parts.

Injection Molding vs CNC Machining: Comparing the Two Processes

Like with many comparisons between injection molding and other processes (see Manufacturing Comparisons below), injection molding has a higher lead time and costs, but once it gets up to speed can out-perform CNC machining in terms of volume and costs. Some factors to consider.

• Lead Costs: CNC machining requires the creation of a CAD file and its conversion to being usable for the mill. Injection molding requires the creation of a mold, which takes lead time and money.
• Speed: CNC machine can go quickly from CAD to creating products, but once in place injection molding can produce parts quicker.
• Volume: When it comes to price per part, injection molding wins when it comes to volume. While factoring in start-up CNC milling is less expensive for a few parts up to a hundred, from hundreds to thousands of parts, plastic injection molding emerges as the choice.
• Materials: Injection molding covers plastics, particularly the two camps of thermoplastic and thermoset resins, covering everything from everyday plastics like ABS to nylon. CNC machining can use plastics or other materials, but due to the milling process softer materials suffer.

Other Manufacturing Comparisons to Look At

In addition to CNC machining, when it comes to comparing injection molding against other manufacturing processes, it&#;s important to consider all your options. Here are some more related articles:

When it comes to finding the finding the best manufacturing process for your plastic parts, we here at Midstate Mold and Engineering believe having more information to make decisions is always better. Interesting in learning more about your options with injection molding? Feel free to contact us or to directly request a quote today. With more than 180 years of combined experience in injection mold design, tooling, and parts production, we can assist you with every step of the process, from prototype design to making the mold to state-side parts production.

Use this guide to decide if plastic CNC machining or injection molding is right for your project!

While there are several methods for manufacturing plastic products, those made with engineering and high-performance plastics are typically either machined or injection molded. CNC machining plastic is much different from injection molding, so understanding the processes, as well as the advantages and limitations of each, can help you decide which manufacturing process is best suited to your project or if both are needed. This guide explores the two techniques.

The Primary Differences between CNC Machining and Injection Molding

While both methods can provide high-quality parts, it&#;s important to understand how each process converts raw materials into a component that is ready for secondary operations or shipping.

CNC stands for computer numerical control. CNC machines used for plastic processing are typically advanced multi-axis systems that are computer-controlled for precision and accuracy. Machining is considered a subtractive manufacturing method because the machine, whether mill, lathe, or router, removes material from a stock shape or injection molded blank. The number of axes depends on the machine and its function. But generally speaking, the more axes a machine has, the more complex part it can produce with reduced manual handling.

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The shape of the part determines which machine, or machines, is used. A CNC lathe, which rotates the workpiece against the cutting tools to make a shape, is ideal for symmetrical objects like spheres, cylinders, or cones and can also create features such as drill holes, bores, and threads. With a mill, the cutting tool rotates around the workpiece, allowing it to make complex shapes. Routers operate on a three-axis spindle at a higher speed than a mill.

Comparatively, injection molding is a formative technology. The process involves melting a polymer and injecting the molten plastic into a mold, where it cools and solidifies to form the desired part. In addition to traditional molding, overmolding and insert molding are options for more complex products. Overmolding is when one plastic material is molded over another. Insert molding is when plastic is molded over non-plastic parts, such as a soft-touch handle on a metal scalpel.

Other differences that are important to consider are design, cost and volume, and materials:

• Design Considerations  &#; Working with your plastic fabricator from the design phase is beneficial because the part design features can differ depending on the method used to create your part. For example, removing a part from an injection mold requires appropriate draft angles. Walls must be of similar thicknesses to avoid uneven shrinkage and warpage. Neither is required for CNC machining. For CNC parts, the depth of the cavity should be four times its width, a limitation not found with injection molding. Similarly, the overall size of the part is limited by the size of the machine.
• Cost and volume &#; Upfront costs are higher with injection molding. However, for large volumes, the ROI is seen in speed, repeatability, and long tooling lifespan when properly maintained. CNC machining does not have upfront costs for tooling, but there is a point as volumes increase that it is no longer cost-effective.
• Materials &#; CNC machining works best with hard materials. Soft plastics may melt from the friction of machining. Some engineering and high-performance plastics require annealing or curing, which requires special equipment that not all injection molders will have.

Consideration for CNC Machining Plastic

Like any manufacturing process, there are pros and cons to CNC machining plastics. One should consider the impact of each on the product.

Advantages

• Precision &#; CNC machining can produce parts with tight tolerances.
• Customization &#; CNC machining allows for the creation of products with complex design features that may be challenging or more costly for injection molding, including:
  • Undercut removal
  • Holes that penetrate completely through a part
  • Complex internal threads
  • Variable wall thicknesses
  • Non-uniformed cross-sections, such as tapered or conical sections
• Material flexibility &#; A wide range of materials can be machined, including PVC and PTFE.
• Less set up &#; Since no molds must be built, CNC machining can be faster for lower volumes.
• Modifications &#; Design iterations aren&#;t a problem since programs can easily be updated.
• Cost &#; Since no molds are required, CNC machining is most cost-effective for small to medium volumes.

Disadvantages

• Plastic knowledge required &#; You must work with a machine shop that specializes in plastics. Plastics have a higher coefficient of thermal expansion than metal, and since machining creates heat, the material will expand and, in some cases, melt. Understanding the polymer is critical for ensuring proper heat regulation.
• Material waste &#; Since CNC machining is a subtractive process, material waste is inherent.
• Limited capabilities &#; Being a subtractive process, it is limiting or requires more steps for products that benefit from overmolding or insert molding.

Consideration for Plastic Injection Molding

Likewise, plastic injection molding has its own advantages and disadvantages.

Advantages

• High volume efficiency &#; Plastic injection molding provides faster production options for high volumes since multiple cavity molds can create several parts at once.
• Complex geometries &#; Like CNC machining, some features are better suited to injection molding than CNC machining, including:
  • Thin walls
  • Intricate internal channels
  • Small features
  • Complex contours
  • Fine surface textures
  • Over molding
  • Insert molding
  • Fine details and lettering
• Repeatability &#; With a precision machined mold or inserts, injection molding produces exact parts consistently.
• Cost &#; Injection molding is most cost-effective for large runs since properly maintained molds have long lives.

Disadvantages

• Design Changes &#; Once the mold is built, design changes require a new mold, which can be expensive.
• Upfront investment &#; Building molds requires a sizeable upfront investment.
• Limited capabilities &#; Some design features, such as internal threads and undercuts, can be challenging and require special considerations for mold design, which can add cost, or require secondary operations.

Considerations for Plastic Injection Molding and Finish Machining

Having to choose between plastic injection molding and machining isn&#;t always necessary. Sometimes, molding the part and then doing post-mold machining, often referred to as near-net injection molding, makes sense. A near-net shape is molded to the approximate shape of the part and machined to the customer&#;s dimensions and tolerances to create the final part. It dramatically reduces the waste, and a less precise mold can be used. Post machining can also be used to remove flash, prepare the part for further processing, or achieve tighter tolerances.

Advantages

• Material waste reduction &#; Much less material is wasted with near-net molding and machining than with machining alone.
• Tight tolerance &#;Tighter tolerances can be achieved than injection molding alone.
• Complex geometries  &#;  A part can be injection molded, and challenging features can be machined.
• Cost savings &#; Less material waste, especially with expensive high-performance plastics, and shorter machining times reduce costs over machining alone. Molds are less precise and, therefore, less costly to produce.

Disadvantages

• Project dependent &#; It may not be cost-effective for projects using low-cost polymers or those with loose tolerances or simple designs.
• Volumes &#; Volumes must be enough to offset the cost of the mold but not so high that machining becomes cost-prohibitive.

Consult with Experts

The best advice is to consult a plastic manufacturer experienced with injection molding and CNC machining plastics if you are unsure which process is best for you.

At Ensinger, we have a long-time reputation for being plastics experts. We have in-house engineers with the tools and skills to ensure your product is designed for manufacturability and functionality. We combine material and process expertise, whether CNC machining or injection molding, to provide high-quality parts that consistently meet your expectations. Contact us for a consultation on your next project.

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