Companies around the world use CNC machining to create high-quality parts from materials as diverse as ceramics, wood, and composites. Metals and plastics are particularly prominent in mass production, with metals holding an advantage due to their wider machinability.
Machinists are adept at adjusting speeds, feeds, and tool selection in metal machining. However, machining plastics can present significant challenges. Machining methods that are effective for metals like steel, brass, aluminum, and titanium often fail to produce the same excellent results in plastics. The result can be an inadequate finish and dimensional inaccuracies, ultimately leading to the rejection of many parts.
Therefore, in CNC machining, developing appropriate machining strategies tailored to the characteristics and challenges of different materials is crucial to ensure high-quality and acceptable end products.
Any tool chatter, friction, or poor chip formation can cause scratches and marks on the plastic surface, affecting the surface finish. These defects are not only aesthetically pleasing but can also affect the function and performance of the part.
Any factor that causes the plastic to compress or expand during machining affects the machinist's ability to meet precise tolerances when the part returns to its original shape after machining. This dimensional variation, which can stem from temperature fluctuations, machining speeds, or the material's inherent properties, can result in a part failing to meet the desired specifications.
Softer plastics tend to "flow" around the tool rather than forming chips, similar to dipping your finger into wet clay. Harder plastics may break unpredictably near the edge, leaving small holes.
The stress applied to the plastic part during the clamping process can cause elastic deformation. When the clamped portion returns to its original shape after machining and unclamping, the plastic part may be out of tolerance.
If plastic chips are not quickly removed from the cutting area, they can "stick" to the grooves of drill and milling machines. Poor chip evacuation reduces cutting efficiency and causes friction, which rapidly increases the temperature of the surrounding plastic.
Many plastics have a low heat deflection temperature, which can cause them to soften rapidly and deform easily, especially when clamped, cut, or simply under the force of gravity.
Polymers experience significantly greater thermal expansion than commonly machined metals. Overheating can cause expansion and subsequent contraction, potentially preventing tolerances from being achieved.
Different polymers have different solvent sensitivities, causing them to dissolve or swell. Machine shops should understand the chemical compatibility of the polymers they use.
Some plastics absorb significant amounts of water, causing them to swell. Humidity in the air can also cause some plastics to swell beyond their capacity.
Transparent polymers (such as acrylic, ABS, and polycarbonate) can appear hazy or discolored if left with a rough surface finish after machining. Some polymers are even susceptible to degradation under UV exposure.
Because different plastics can present different challenges during processing, selecting the right plastic material for your application is crucial. Some plastics may have high moisture absorption rates, while others may release toxic gases when overheated. Understanding these characteristics can help optimize processing and ensure product quality.
Many machinists who are familiar with metals but not plastics often tend to over-clamp parts to prevent any movement during machining. Over-clamping can leave marks on the clamped plastic or stress the material, causing it to expand and deform in various directions. When enough material is removed during machining, the part can become weaker overall or even deform due to clamping stresses. To minimize clamping forces, it's recommended to use the absolute minimum force required to secure the part. Additionally, consider developing a fixture that minimizes stress on the plastic workpiece, such as a CNC-machined fixture designed specifically for machining Delin plastics.
A dull tool, incorrect cutting edge angle, or even groove design can affect cutting results. Using the right cutting tool can significantly impact cutting speed and efficiency, as well as a high-quality surface finish that avoids scratches and gouges on the plastic surface. Using the wrong tool can also cause chips to accumulate at the cutting point, increasing friction and rapidly raising temperatures around the tool.
Selecting the optimal cutting tool may require some research and experimentation to determine the best results for a specific polymer. This investment will improve processing efficiency and product quality, thereby reducing subsequent trimming and rework.
Excessive heat buildup can cause numerous quality issues with plastics, leading to scrapped parts. Therefore, keeping parts cool is crucial. As mentioned earlier, using the wrong cutting tool and allowing chips to accumulate at the cutting point can lead to rapid heating. With plastics, it's generally better to cut quickly and then move away from the cutting point rather than linger and allow heat to accumulate.
Generally speaking, two or more high-pressure coolant jets should be sufficient to keep the plastic and mold cool while quickly removing plastic debris. This cooling measure effectively reduces temperatures, ensures a smooth machining process, and improves the quality of the final product.
Unfortunately, each plastic behaves very differently. Cutting parameters and tools for one plastic may result in surface roughness and poor dimensional consistency for another. Sometimes, a machinist needs to increase cutting speed to counteract mold vibrations in the plastic, while in other cases, slowing down may produce better results.
To consistently produce precision-machined plastic parts, machinists need to research and experiment with specific plastics to understand how they react to machining. For example, the YouTube channel CNC Machining Giants offers helpful tips for machining several common plastics. This accumulated knowledge will help optimize the machining process and improve product quality and consistency.
The tips presented in this article by no means cover all the knowledge needed to overcome the challenges of processing plastics. Each polymer is highly sensitive to different factors, requiring specialized expertise to produce high-quality results. Fortunately, Hongsinn's machinists and engineers are experienced in processing a wide range of materials, including various metals and plastics.
We provide high-quality CNC machining services to help you produce parts on time and on budget. With our expertise, you can be sure of a smooth machining process and a high-quality product that meets your needs.
