However, POM may be unstable and undergo degradation when exposed to high temperatures or acidic conditions. Its molecular structure contains a significant amount of oxygen, which makes enhancing its flame retardancy challenging. The material also possesses a continuous operating temperature range of approximately -40°C to 120°C.
POM plastics exist in two primary forms: homopolymer (POM-H) and copolymer (POM-C). POM-H is renowned for its exceptional hardness and rigidity, whilst its melting point (172–184°C) is approximately 10°C higher than that of POM-C (160–175°C). POM plastic exhibits a density range of 1.410–1.420 g/cm³, with crystallinity levels of 75–85%. Modified POM variants are also available, featuring higher melting points than the standard version.
Advantages and Disadvantages of POM CNC Machining
In this section, we shall discuss the characteristics of POM/Delrin/Acetal plastic for CNC machining, including its advantages and disadvantages.
Advantages of CNC POM Machining
POM/Delrin is a high-performance crystalline plastic, making it a viable alternative to metal in certain applications. Key advantages of POM include:
1. Thermal and Electrical Insulation
POM exhibits excellent thermal insulation properties alongside robust mechanical characteristics, proving highly suitable for electronic components. Its capacity to withstand considerable electrical stress makes it an ideal high-voltage insulator. Low moisture absorption further enhances its suitability for maintaining moisture protection in electronic assemblies.
2. Mechanical Strength
POM exhibits a tensile strength range of 7000-9000 psi, coupled with exceptional hardness and toughness. Furthermore, its lower density compared to metals makes it a suitable alternative for manufacturing lightweight components requiring resilience under high-pressure conditions.
3. Fatigue Resistance
Within a temperature range of -40°C to 80°C, POM demonstrates outstanding durability and high fatigue resistance. Fatigue resistance. Moreover, its fatigue resistance remains largely unaffected by exposure to water, chemicals, or solvents. These remarkable properties make POM the perfect material for designing components subjected to repeated impacts and stresses.
4. Impact Resistance
POM possesses exceptional toughness, capable of withstanding sudden impacts without deformation. Furthermore, its impact resistance can be further enhanced through specialised treatments.
5. Dimensional Stability
Dimensional stability is a parameter assessing a material's ability to retain its original shape under processing conditions such as temperature and pressure. POM exhibits minimal deformation during machining, making it an excellent material for achieving precise tolerances.
6. Friction Coefficient
In mechanical systems, moving components require lubrication to minimise friction between them. However, POM components operate smoothly even without lubrication due to their inherent smoothness. This property proves highly advantageous when designing systems where external lubrication could cause product contamination, such as in food processing machinery.
7. Durability
Its exceptional tensile strength and durability qualify POM plastic for high-stress applications. Due to its strength and resilience, it frequently serves as a substitute for aluminium and steel alloys.
8. Moisture Absorption
POM exhibits minimal moisture absorption, maintaining structural integrity even under the most humid conditions or applications (e.g., submerged).
9. Creep Resistance
Owing to its exceptional toughness, POM withstands immense pressure without deformation. Consequently, it is the material of choice across industries when designing components requiring high durability and creep resistance.
Disadvantages of CNC POM Machining
Although it possesses superior machinability compared to most other plastics for producing CNC-machined components, CNC POM machining also presents certain drawbacks:
1. Low Adhesion
Due to POM's chemical resistance, adhesives do not bond well with POM, making bonding challenging.
2. Flammability
POM lacks self-extinguishing properties and will continue burning until oxygen is depleted.
3. Heat Sensitivity
CNC machining POM at elevated temperatures carries a risk of deformation.
Challenges and Techniques in CNC Machining of POM
The primary issues encountered during CNC machining are material deformation and cracking. These problems lead to two types of fracture: direct cracking during the machining process and hidden cracks caused by internal stresses. The latter develop gradually and may inflict significant damage, leading to complications.
Before addressing the aforementioned issues, it is essential to discuss material selection and prototyping. Employing high-quality materials helps prevent noticeable deformation. Therefore, prioritising modified POM materials for CNC machining is crucial. POM material quality can be inconsistent, with deformation levels often unstable between batches due to impurities, pitting, and crack-like structures.
Regardless of the POM material type, always conduct trial runs before CNC machining. Trial runs are also required when using different batches of the same material.
Machining Tools
POM exhibits low hardness and minimal cutting forces, permitting machining with standard high-speed steel and carbide tools provided they possess sharp edges to minimise cutting heat. PCD turning tools may also serve as an alternative for CNC turning operations.
POM Machining
Should the selected POM material be of inferior quality or subject to stringent dimensional tolerances, annealing is advisable following rough machining to alleviate internal stresses. This method effectively minimises deformation during finishing operations.
The following process parameters are provided for reference only, as variations exist between POM material types and manufacturers:
Following rough machining, POM undergoes oil bath annealing in hot oil or air bath annealing in an oven. Adjusting the annealing temperature is crucial; it should be approximately 10-20°C (140-150°C) below the product's heat deflection temperature. For oil bath annealing, annealing time increases by 40-60 minutes per 5mm increase in wall thickness. For air bath annealing, time increases by 20-30 minutes per 5mm increase in wall thickness. After annealing, allow the POM to cool naturally to room temperature.
An alternative annealing technique termed the "grounding method" involves annealing CNC parts at 100°C. Following rough machining, immerse the components in boiling water for 5 to 6 hours when ambient temperature is below 80°C, then allow them to cool naturally to room temperature (ideally at a constant temperature). Alternatively, the natural ageing method may be employed, provided sufficient time is allocated for annealing.
Solutions for POM Deformation in CNC Machining
To enhance deformation consistency and maintain tolerances within narrower ranges, ensure workpiece blanks are dimensionally standardised and uniform during CNC machining.
1. Modify Clamping Methods
POM material deforms under clamping pressure but recovers to its original state upon release. Increasing the contact surface area can be achieved by modifying the clamping method, such as using vice pads or adhesive to secure the workpiece. For larger plates, ensure they are flat before securing them in place with vacuum cups. It is advisable to first secure one side with adhesive, then clean and use the vacuum cup to fix the smooth surface for rough machining. In some industries, chilled suction cups are also employed for securing.
2. Minimising Cutting Heat
POM exhibits high thermal sensitivity and readily deforms during machining if insufficiently cooled. Employ sharp tools to minimise heat generation during cutting. Additionally, reducing cutting depth, employing multiple passes, and increasing coolant usage further prevent overheating and rapidly dissipate any heat generated during cutting.
3. Execute multiple passes
POM exhibits significant elasticity. Upon tool contact, the material may deform inward, causing distortion in the cut and pressed part as the tool withdraws. Therefore, multi-pass compensation adjustments based on actual cutting results are crucial. Dividing the cutting task into multiple passes and reducing the cutting depth per pass minimises any dimensional distortion caused by material elasticity during machining.
4. Counteracting Internal Stresses
Engineering plastics possess a greater coefficient of thermal expansion than metals. When machining with substantial allowances, deformation may occur due to stress relief. To prevent this, the correct selection and handling of materials, as previously outlined, is paramount. Where significant material removal is required, using thicker stock, controlling allowances, and employing symmetrical machining methods is advisable. Furthermore, it is vital to verify the rationality of the part design itself to mitigate any stresses and deformations induced during machining.
During the transfer and storage of finished components, prioritise temperature control to prevent distortion. Where feasible, maintaining a constant ambient temperature is advisable. Furthermore, ensure adequate protection of component surfaces to guard against scratches or damage.
Why does CNC machining cause cracking?
Although the degree of deformation mentioned earlier is a significant factor contributing to cracking, it is not the sole cause. The following factors may also lead to cracking in POM material during operation:
1. Substantial material removal during CNC machining can cause POM plastic to crack.
2. Direct drilling with oversized drill bits generates excessive cutting forces, which may compress and fracture the POM material.
3. Inadequate chip evacuation during deep hole drilling in CNC machining, particularly if the drill bit is not repeatedly retracted to remove debris, can result in compression cracks.
4. Cracking may result from inadequate cooling during drilling, causing elevated cutting temperatures and forces.
5. High feed rates induce internal stresses within POM material, leading to fracture.
6. Failure to promptly replace worn drill bits causes them to harden, resulting in cracking when drilling POM.
Considerations for POM Machining Processes
CNC Milling and Threading
Beyond tooling differences, considerations for CNC milling and threading are generally similar. Avoiding deformation during clamping, using sharp tools, setting lower feed rates, and ensuring adequate cooling are crucial to prevent issues in these processes.
CNC Turning
Cooling is essential to prevent melting and wear during CNC machining. Prior to employing coolant, solid lubrication or compressed air cooling is recommended as the preferred method. Furthermore, high rotational speeds and excessive feed rates and engagement should be avoided. Tools with slightly larger rake angles and clearance angles are acceptable provided the cutting edges remain sharp. High-Speed Steel Turning Tools
Commonly employed, with front angles approximately 25°–40° and rear angles approximately 10°–20°. It is advisable to minimise chuck clamping force.
Drilling
Avoid commencing directly with large-diameter drill bits; instead, initiate with a smaller hole and ream at low speeds. Drill bits must be maintained in sharp condition. Subsequent drilling parameters should consider the following: Rake angle: 60°–90°; Helix angle: 10°–20°; Front angle: 0°; Rear angle: 10°–15°. Additionally, limit feed direction forces and perform timely retraction (5–6mm depth) to facilitate in-drill cooling and chip evacuation. When drilling through holes, reduce feed rate to prevent axial material displacement by the drill.
Conclusion
In summary, POM or Delrin constitutes an excellent plastic material for CNC machining in the production of mechanical components and industrial products. Whilst POM is readily machinable, it is prone to deformation during processing. However, such deformation and associated issues can be minimised through the application of recommended machining practices, including adequate cooling, minimising tool wear, and ensuring tool sharpness.
