Polyoxymethylene (POM), a high-performance engineering plastic, is widely used in precision machinery, automotive parts, and electronic components due to its excellent mechanical strength, low friction coefficient, and dimensional stability. However, the material's properties pose significant challenges in CNC machining of POM. This article systematically analyzes typical issues encountered in CNC machining of POM and proposes solutions that comply with international standards, providing technical reference for industry professionals.
Polyoxymethylene (POM), also known as aldehyde polymer, polyoxymethylene, and polyacetaldehyde, is an engineering thermoplastic. With a density between 1.410 and 1.420 g/cm³, POM not only exhibits excellent hardness, strength, and rigidity, but also boasts chemical resistance, dimensional stability, mechanical strength, and a low coefficient of friction, making it a reliable material in engineering applications. Its low friction properties make POM an ideal material for rotating or sliding components such as bushings(View CNC turning POM bushing case studies), bearings, and gears.
Properties of POM Plastics
Low Friction and Wear Resistance: POM plastics have a low coefficient of friction, which results in excellent self-lubricating properties. This reduced frictional resistance enables sliding or rotating motion of fluids, resulting in improved productivity due to low friction and high efficiency.
Chemical Resistance: POM is well-suited for products that come into long-term contact with chemical solvents, such as pump parts, seals, and fuel system components. This is primarily due to POM's excellent resistance to a wide range of chemicals, solvents, and fuels. It withstands contact with a wide range of organic chemicals, alcohols, oils, and greases without significant degradation. Low Water Absorption and Dimensional Stability: POM plastic exhibits excellent dimensional stability, maintaining its size and shape even under conditions of humidity and temperature fluctuations. Due to its extremely low water absorption, POM is less susceptible to dimensional changes such as swelling and deformation caused by moisture. Applications requiring tight tolerances and consistent efficiency also rely heavily on POM's dimensional stability.
Processability: POM plastic exhibits excellent processability, enabling precise and efficient manufacturing processes. It is easy to mold, machine, turn, and drill, making it easy to create complex parts and designs. For this reason, POM resin is often chosen for applications requiring complex geometries and high precision.
Excellent Creep/Impact Resistance: POM plastic exhibits excellent creep resistance, meaning it can withstand sustained mechanical stress without deforming. This property allows POM components to continue functioning properly even under sustained load or strain.
The main challenges of CNC machining POM
Although POM has excellent mechanical properties and wear resistance, it still faces some technical challenges during processing.
- Cracking. POM materials are prone to cracking during machining, especially during high-speed CNC machining or deep cuts. This is caused by the release of internal stress in the material or uneven cooling. To minimize cracking, proper cooling and machining parameters are crucial. Ensure uniform cooling rates during machining and avoid sudden temperature fluctuations. Using appropriate cutting fluids and adjusting cutting speeds can also help mitigate the risk of cracking.
- Deformation issues. POM plastic is highly hygroscopic, so changes in humidity and temperature during processing can cause deformation. POM material expands when heated and may contract upon cooling, especially when processing large or complex parts. Furthermore, excessive cutting forces during POM processing can cause deformation, especially when processing thin-walled or microstructures.
- Tool wear. POM's low hardness generates relatively little heat during machining, but its viscous nature can lead to rapid tool wear. Choosing the right tool material and coating, along with optimal cutting conditions, can effectively extend tool life. Using high-quality carbide or coated tools, combined with precise cutting parameters, can help reduce tool wear and improve machining quality.
- Surface quality. Surface quality is a major challenge in CNC machining of POM plastics. Due to the material's characteristics, scratches, wiredrawing, and other uneven textures may appear on the surface. To achieve a smoother surface, it's necessary to adjust cutting parameters, use appropriate tools, and employ various machining processes, such as finishing or post-processing, to improve the surface finish.
Causes and solutions for deformation during POM CNC machining
Deformation is a common challenge when CNC machining POM (polyoxymethylene). The following are four main causes of deformation and their solutions:
1. Cutting heat. POM has poor thermal stability and is heat-sensitive. If it is not adequately cooled during machining, it will deform rapidly. To address cutting heat, the following measures can be taken:
· Keep the tool sharp to reduce heat generated during cutting;
· Reduce the amount of cuts per pass and divide the cutting process into multiple passes;
· Increase the amount of coolant used.
The goal is to minimize heat generation or quickly remove the heat generated during cutting to avoid thermal deformation.
2. Internal stress. Engineering plastics have a high coefficient of thermal expansion. When machining with large allowances, removing internal stresses can cause deformation. To address this issue, the following measures can be taken:
· Select and prepare the material appropriately;
· When removing large amounts of material, use symmetrical machining methods to offset stress and deformation generated during machining;
· Adjust the machining allowance and try to maintain a thicker material allowance to reduce deformation when internal stress is released.
3. Clamping Deformation
During CNC machining, POM material may deform during clamping. When the clamp is released, the workpiece returns to its original shape. To avoid this, deformation can be reduced by strengthening the contact surface between the workpiece and the clamp. Specific methods include:
· Changing the clamping method. For example, using a bench vise with a soft pad or securing with glue;
· For larger workpieces, a vacuum suction cup can be used, but the workpiece surface must be flat;
· After securing one side with glue and smoothing the other, use a suction cup to secure the workpiece surface.
4. Elastic Deformation
POM material has a high degree of elasticity. During machining, the portion of the material in contact with the tool will bend internally during cutting. After the tool is removed, the machined portion may deform slightly. To reduce dimensional deviation caused by material elasticity, the following measures can be taken:
· Performing multiple tool compensation adjustments based on the cutting effect;
· Repeated cutting when machining smaller amounts of material can help reduce dimensional deformation caused by material elasticity.
By reasonably adjusting the processing technology and parameters, the deformation problem of POM material in CNC processing can be effectively controlled to ensure the processing quality and accuracy.
The success of CNC machining POM relies on a deep understanding of material properties and precise control of process parameters. By optimizing tools, changing clamping methods, and integrating international testing standards, the yield rate of finished products can be significantly improved.