Engineers and product developers face unique challenges and opportunities when designing plastic components for machining, especially if they are transitioning from metal to plastic. Understanding the nuances of plastic machining and leveraging its benefits requires thoughtful design and engineering approaches. Here, we’ll explore critical factors to consider when engineering plastic components, especially for those converting from metal.

Material Selection for Functionality

Material selection for component functionality is often a primary consideration when engineering a plastic component. Various plastics can be used, each with distinct mechanical, thermal, and chemical properties. The material you select for your component can impact cost and performance. Some materials are generally better suited for CNC machining because of the stress and heat the components encounter during machining. The number of materials is vast, so we touch on just a few materials here:

Nylon – Nylon’s versatility and performance characteristics make it a preferred choice for CNC machined parts in various industries. It is strong, has good impact resistance, and is widely available in block or bar form. Lightweight Nylon is often used to replace metal gears, bushings, gaskets, and bearing surfaces for weight reduction and fuel efficiency gains. From electrical connectors and insulating parts to pulleys and rollers, parts benefit from Nylon’s excellent mechanical properties, impact strength, chemical resistance, abrasion resistance, and low coefficient of friction. Bearings and bushings are often machined from Nylon because of its low coefficient of friction and good wear resistance.

Polycarbonate —Polycarbonate is a popular choice for machined products due to its exceptional impact resistance and clarity, making it ideal for applications requiring durability and transparency. Common uses include safety guards, machine enclosures, lenses, and transparent components in industries like aerospace, automotive, and industrial equipment. Its ability to withstand high impacts without shattering ensures reliability in demanding environments.

Polytetrafluoroethylene (PTFE)—This thermoplastic offers excellent mechanical properties, low deformation under load, and high tensile strength. Its chemical resistance and thermal stability make it an excellent choice for seals, gaskets, and fluid control applications.

Polyoxymethylene (POM)—Also called Acetal, this material is a strong ridged plastic that results in high-precision parts because of its dimensional stability, even at elevated temperatures, and ease of machining. Because of its low coefficient of friction, it is also used for bearings, bushings, and gears.

Polyetheretherketone (PEEK)—PEEK is a high-performance engineering thermoplastic that is popular for replacing metal because of its properties: high strength-to-weight ratio, high temperature resistance, and creep and fatigue resistance. It also offers great insulating properties, chemical resistance, and low friction. Plus, it is biocompatible and sterilizable. Products across industries, whether medical, aircraft, industrial, or electrical components, benefit from PEEK.

Poly-Texx®— Poly-Texx is a proprietary line of high-performance, self-lubricating bearing-grade composites developed by Ensinger. These materials are engineered to excel in demanding applications across various industries, offering enhanced durability, reduced friction, and maintenance-free operation. Each Poly-Texx material is tailored to meet specific application requirements, providing solutions that enhance performance and reliability in challenging environments.

Dimensional Stability and Tolerance Compared to Metals

When designing your plastic component, keep in mind that plastics behave differently than metals in terms of machining and dimensional stability. Plastics exhibit greater thermal expansion than metals, meaning they expand and contract more with temperature variations. Therefore, tolerances must account for these changes to ensure proper fit and function. Additionally, some plastics, like nylon, are hygroscopic and absorb moisture, leading to dimensional changes. To address this, parts must be preconditioning parts, or materials less prone to moisture absorption should be chosen.

Another critical factor is machining allowances. Plastics can experience more tool-induced stress during machining than metals, resulting in warping or deformation. Designing parts with machining-friendly geometries can help mitigate these issues and ensure the final product meets the desired specifications. By keeping these factors in mind, you can achieve better dimensional stability and tolerance in your plastic components.

Design Adjustments for Plastic Components

While plastic offers flexibility, designing for machining requires rethinking certain design features as opposed to metal. While some design principles apply universally to machining, plastic materials introduce unique considerations that must be factored in to optimize the process and the final product. Here are the primary design considerations specific to machining plastics:

Rounded Corners—Generous radii on edges should be used to reduce stress concentrations. Rounded internal edges allow the cutting tools to run more efficiently.

Wall Thickness —Thin walls can lead to deflection during machining, resulting in dimensional inaccuracies. Walls thicker than 1.5 mm allow for faster, easier cutting.

Tolerances—Plastics have a higher thermal expansion rate compared to metals, meaning tolerances must account for temperature fluctuations in the part’s operating environment. For most applications, tolerances of ±0.005 inches are adequate, while tighter tolerances (±0.001 inches) should be reserved for critical features. If the part experiences temperature changes during use, design for wider tolerances.

Hole Depth—Deep holes are difficult to machine accurately in plastic and may lead to deformation. Limit the depth-to-diameter ratio to 5:1 or less. For slots or grooves, provide relief areas at the ends to reduce tool stress and improve precision.

Surface Finish—Consider whether the surface finish impacts the part’s performance or aesthetic. For example, polished surfaces may be necessary for optical clarity (as with acrylic), while a rougher finish might suffice for internal components. Surface finish is a key requirement for medical, aerospace, and semi-conductor machined parts.

Threads and Fasteners— Avoid machining threads directly into the plastic. If the design requires threading, it’s better to include metal inserts in your drawing to reinforce the threads and prevent stripping during assembly. For components requiring screws, ensure the drawing includes sufficiently robust bosses to prevent cracking under load.

Feature Placement—Unsupported features like thin tabs or projections should be avoided, as they are prone to flexing or breaking during machining. Arrange holes, slots, and other features to minimize tool travel distance. This reduces stress on the part and improves machining efficiency.

Material-Specific Design Adjustments—Each type of plastic has different properties (e.g., thermal expansion, brittleness, flexibility). Select materials early in the design process and tailor the design to their characteristics. Also, consider post-machining behavior. Some plastics may retain residual stresses after machining, leading to dimensional changes. Include allowances for annealing or stress relief if required.

Collaboration with Your Machining Partner

Choosing the right machining partner can make or break your project, and early collaboration is key. Look for a partner with experience in plastics and offers engineering support and design for manufacturability (DFM) services. A knowledgeable partner can help optimize designs for machining, recommend materials, identify potential issues, and save you money. If your CNC machining partner has a proven track record in machining plastics, they will know how to handle the intricacies of your project. If early prototypes are available, you can validate designs to ensure manufacturability. Finally, ensure they have systems in place to maintain tight tolerances and consistent quality.

Designing and engineering plastic machined components demands a different mindset than working with metals. By focusing on material selection, understanding machining design limitations, and collaborating with the right partner, you can unlock the many advantages plastics offer, from cost savings to innovative design possibilities.

Choose Ensinger for Your CNC Machining Needs

At Ensinger, we deliver reliable CNC machining with the precision and accuracy you need. Our skilled team, advanced technology, and comprehensive approach to quality ensure your parts meet exact specifications, even for complex, tight-tolerance designs.

We recently upgraded our CMM (Coordinate Measuring Machine) to include the capability to measure surface profiles. This enhancement allows for more precise and detailed measurements of surface characteristics, which is crucial for ensuring the quality and performance of machined plastic components. The upgraded CMM machine provides advanced measurement capabilities that can capture intricate surface details, leading to improved accuracy and consistency in the manufacturing process.

A Guide to Designing and Engineering Plastic Machined Parts