Mastering Plastic CNC Machining: 8 Typical Plastic Machining Properties
Introduction: Why Understanding Plastic Properties Is the First Step to Successful CNC Machining
In precision manufacturing, the technical complexity of CNC machining plastic materials is often underestimated. As a senior process engineer at Neway, I have witnessed numerous machining failures resulting from neglecting the fundamental characteristics of plastics. Unlike metals, plastics have unique thermal, mechanical, and chemical properties that directly influence process selection and final product quality. Successful plastic machining requires not only advanced equipment, but also a deep understanding of the material itself.
In our plastic CNC machining services, we always adhere to a “material-first” philosophy. Every engineering plastic has its own unique personality, and only by fully understanding these characteristics can we develop the optimal machining strategy. From thermal expansion coefficient and moisture absorption to elastic modulus and thermal sensitivity, each factor can be a key determinant of machining success or failure.
Property 1: Thermal Expansion Coefficient — The Dimensional “Trap” Triggered by Temperature
The coefficient of thermal expansion (CTE) of plastics is typically 5–10 times higher than that of metals, and this must be taken very seriously in CNC machining. Take common ABS as an example: its CTE is approximately 80 × 10⁻⁶/°C, while aluminum is only about 23 × 10⁻⁶/°C. This means that even slight temperature changes during machining can result in significant dimensional deviations.
In actual production, we control the impact of thermal expansion through multiple measures. First, we use sharp tools and optimized cutting parameters to minimize heat generation. Second, we apply compressed air or mist cooling for effective heat dissipation, while carefully selecting the cooling method to avoid inducing internal stress in plastics that are sensitive to thermal shock. Most importantly, we allow parts to cool sufficiently in a controlled-temperature environment after machining before performing final inspection, ensuring that the delivered parts maintain their design accuracy at their actual operating temperature.
Property 2: Hygroscopicity — The Hidden Dimensional Killer in the Air
Moisture absorption is a characteristic inherent to many engineering plastics, with nylon (polyamide) being a typical example. Nylon can absorb up to approximately 8% of its weight in moisture from the air, which not only affects its dimensional stability but also reduces its mechanical properties. We once encountered a case where nylon gears passed assembly checks right after machining, but became excessively tight after two weeks in storage — a direct result of moisture-induced swelling.
In our machining system, material pre-treatment is the first step to ensuring quality. For highly hygroscopic materials such as nylon, we perform strict drying before machining, typically at 80–100°C for 4–8 hours. The machining environment is kept within a controlled humidity range to prevent reabsorption during processing. For especially precise parts, we also recommend lower-hygroscopic alternatives, such as POM, which is renowned for its excellent dimensional stability.
Property 3: Elastic Modulus and Elastic Recovery — Challenges from Flexibility
The elastic modulus of plastics is usually only 1/100 to 1/10 that of metals, making plastic parts much more prone to elastic deformation during machining. When cutting forces are applied, the material deflects; once the tool passes, it elastically recovers, resulting in discrepancies between actual and programmed dimensions. This effect is particularly evident when machining thin walls and slender features.
To address this, we have developed dedicated process strategies. For fixturing, we utilize low-stress custom fixtures that evenly distribute clamping forces and prevent localized deformation. For tooling, we use sharp tools with large rake angles to reduce cutting forces. For parts that are especially prone to deflection, we employ stepwise machining strategies with multiple light passes, allowing the material to gradually release internal stress as it approaches its final dimensions. This approach is especially critical in our machining of complex multi-axis plastic components.
Property 4: Thermal Sensitivity — Fine Control Near the Melting Range
Most thermoplastics have relatively narrow melting temperature ranges, making them highly sensitive to temperature during machining. Excessive heat can cause melting and built-up edge, or even thermal degradation, leading to harmful fumes or compromised properties. For example, polycarbonate (PC) can develop stress whitening, silver streaks, or bubbles if machining temperatures are not properly controlled.
Our solution utilizes cutting tools specifically designed for plastics, featuring large chip flutes and special coatings to minimize cutting temperatures. In terms of parameters, we often use high spindle speeds with moderate feed rates to maintain both efficiency and thermal control. For particularly sensitive materials, we monitor machining temperatures in real-time and adjust parameters accordingly. This refined thermal management is especially important in our precision machining services.
Property 5: Poor Thermal Conductivity — The Risk of Local Heat Build-Up
The thermal conductivity of plastics is typically only 1/100 to 1/1000 that of metals. As a result, heat generated during machining is difficult to dissipate, tending to accumulate in the cutting zone. This build-up affects dimensional accuracy and drastically reduces tool life. Our statistics once showed that, under the same cutting conditions, tool life in plastic machining could be only one-third that of aluminum machining.
To solve heat dissipation issues, we apply several strategies. First, we optimize tool design by using polished cutting edges and dedicated geometries to minimize frictional heat. Second, we employ improved toolpaths, utilizing intermittent cutting strategies that allow tools time to cool between engagements. For deep cavity machining, we use directed compressed air cooling to remove heat from the cutting zone. These measures play a critical role in our CNC milling operations.
Property 6: Internal Stress — The “Memory” of the Forming Process
Many plastic parts are CNC machined from injection-molded blanks or extruded stock, which already contain residual internal stresses from their forming processes. When CNC machining removes material, the original stress balance can be disturbed, resulting in part deformation. This is especially common in the prototyping stage, where off-the-shelf plates or rods are used, whose stress state may differ significantly from that of the final molded product.
Our countermeasures include the strict selection of materials and the design of efficient processes. During material preparation, we may use polarized light inspection or similar methods to evaluate residual stresses and select materials with lower stress levels. In process planning, we adopt symmetrical machining strategies to ensure uniform stress release. For parts that already exhibit deformation, we can apply controlled heat treatment to relieve stress, with precise control of temperature and time, thereby preventing degradation of material properties.
Property 7: Hardness and Wear Resistance — A Challenge to Tool Life
While most unfilled plastics are relatively soft, reinforced plastics pose significant challenges to tool life. Glass fiber or carbon fiber-reinforced materials — such as certain grades of PEEK — are highly abrasive and can rapidly wear down conventional tools. In our tests, when machining 30% glass fiber-reinforced nylon with standard HSS tools, tool life was often less than 30 minutes.
For wear-resistant plastics, we have established a dedicated tool management system. We primarily use diamond-coated tools or polycrystalline diamond (PCD) tools, whose hardness is sufficient to withstand abrasive fibers. In terms of cutting parameters, we choose conditions that allow cutting in a slightly softened state of the matrix rather than ploughing directly through fibers. At the same time, we implement strict tool life monitoring to ensure tools are replaced before dulling affects machining quality.
Property 8: Material Anisotropy — Directional Differences in Strength
Fiber-reinforced plastics typically exhibit pronounced anisotropy, meaning their mechanical properties vary with direction. This arises from the orientation distribution of reinforcing fibers within the matrix. Ignoring anisotropy during design and machining may result in inconsistent performance in different loading directions, or even premature failure.
Our solution involves developing differentiated machining and design strategies that account for material anisotropy. First, we characterize fiber orientation trends in the material. Then, in process and fixture planning, we ensure that high-stress regions align with the primary fiber direction wherever possible to exploit maximum strength. In toolpath design, we avoid aggressive cutting perpendicular to fiber orientation to reduce the risks of delamination or edge chipping. This refined control is particularly important when machining structural components for the automotive industry.
Neway’s Plastic CNC Machining Solutions: Property-Driven Process Optimization
At Neway, we transform our in-depth understanding of plastics into systematic machining solutions. We have developed a comprehensive materials database that contains detailed properties and recommended machining parameters for over 50 engineering plastics. For every new project, our engineers begin by analyzing material characteristics and then develop a targeted process plan.
Our fixturing systems are specifically designed for plastic parts, utilizing modular, low-stress fixtures that securely hold components without damaging their surfaces. We maintain constant temperature and humidity in the machining environment, supported by real-time monitoring to ensure consistent process conditions. Throughout the entire manufacturing flow — from raw material inspection to final product verification — we enforce clear and stringent quality standards.
For parts with special requirements, we also provide professional post-processing services. For example, precision polishing can deliver mirror-like surface finishes, while UV coating can enhance surface hardness and scratch resistance. These value-added services are especially popular for appearance-critical components in consumer electronics.
Machining Characteristics and Application Recommendations for Typical Engineering Plastics
Different engineering plastics have distinct machining characteristics and require tailored strategies. ABS is known for its excellent overall machinability and is suitable for many general applications; however, machining temperatures must be controlled to prevent surface melting. As a representative high-performance plastic, PEEK requires higher cutting temperatures and specialized tooling, but its outstanding mechanical strength and thermal resistance make it a top choice for medical devices and other demanding applications.
When selecting materials, we recommend that customers consider not only functional requirements but also the feasibility of machining. Our engineering team can recommend the most suitable material and design an optimal machining solution tailored to your specific application, ensuring the best part performance while keeping costs under control.
FAQ
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