Detailed CNC Machining Methods for POM, PA, PC, and PP
These materials are thermoplastics, meaning they are sensitive to heat and mechanical stress. The key to successful machining is managing these factors to prevent melting, poor surface finish, and most critically, deformation.
1. POM (Polyoxymethylene / Acetal / Delrin®)
- Characteristics: Excellent stiffness, low friction, good dimensional stability. Known for its “clean” machining.
- Detailed Method:
- Tools: Use sharp, positive rake angle tools with polished flutes. High-speed steel (HSS) or carbide is suitable.
- Cutting Parameters: High cutting speeds with moderate feed rates. This promotes clean shearing rather than melting.
- Coolant: Not always necessary for POM, as it has a low melting point. Use compressed air to remove chips and prevent heat buildup. If needed, a mist coolant is effective.
- Chip Evacuation: POM produces long, stringy chips. Use tools with good chip breakers and high-pressure air to clear the cutting zone to prevent recutting of chips.
2. PA (Polyamide / Nylon)
- Characteristics: Tough, abrasion-resistant, and hygroscopic (absorbs moisture from the air). This moisture absorption is a primary cause of dimensional change.
- Detailed Method:
- Material Preparation: This is critical. Machine nylon soon after drying it. Store material in a dry environment and keep it sealed until machining.
- Tools: Very sharp carbide tools are preferred due to nylon’s abrasiveness. A high positive rake angle is essential.
- Cutting Parameters: Use high cutting speeds and high feed rates. A slower feed rate can generate more heat.
- Coolant: Use a flood coolant or mist to manage heat and wash away chips. Compressed air is also effective.
3. PC (Polycarbonate)
- Characteristics: Very tough and impact-resistant but prone to internal stress and notching. It is susceptible to stress cracking from contact with certain chemicals.
- Detailed Method:
- Tools: Sharp, polished carbide tools with zero rake or slightly positive rake angles are best to avoid gouging and reduce stress.
- Cutting Parameters: Lower cutting speeds are crucial to prevent heat generation, which can cause melting and increase internal stress. Use moderate feed rates.
- Coolant: Always use a coolant (flood or mist) to keep the material cool. Avoid chemical coolants that can cause stress cracking; use clean water-soluble coolants or compressed air.
4. PP (Polypropylene)
- Characteristics: Soft, flexible, has excellent chemical resistance, and a very low melting point. It is prone to warping and is difficult to hold to tight tolerances.
- Detailed Method:
- Tools: Extremely sharp tools with a high rake angle and a keen cutting edge are necessary to cut the material rather than stretch it.
- Cutting Parameters: Use high speeds with high feed rates. A slow feed will allow heat to build up and cause the material to gum up on the tool.
- Coolant: Coolant or compressed air is mandatory to keep the temperature down and prevent deformation.
How to Avoid Excessive Stress and Deformation During Machining
The strategies below address the root causes of stress: heat and mechanical force.
- Use Sharp Tools: This is the most important rule. A dull tool rubs instead of cuts, generating excessive heat and bending the part, which induces internal stress. Inspect and replace tools regularly.
- Optimize Feeds and Speeds:
- High Speed, High Feed (for POM, PA, PP): A sharp tool combined with a high feed rate ensures the tool is cutting the material, not rubbing it. This shears the material cleanly and carries heat away with the chip.
- Lower Speed (for PC): PC is more sensitive to heat; a slightly slower speed helps manage temperature rise.
- Employ Effective Cooling:
- Compressed Air: Excellent for clearing chips and providing cooling without a chemical interaction, which is vital for stress-sensitive PC.
- Coolant/Mist: Use flood coolant or mist to dissipate heat effectively, especially for deep pockets or long operations. Ensure the coolant is compatible with the material (e.g., avoid certain oils with PC).
- Minimize Material Engagement:
- Use climb milling (down milling) whenever possible. The tool engages with the maximum chip thickness first and exits with zero, reducing tool deflection and heat generation.
- Take lighter finishing passes (small depth of cut) to remove a small amount of material with minimal stress.
- Secure Workholding Properly:
- Avoid excessive clamping force, which can distort the part. Once the machine releases it, it will spring back to its original shape, ruining the tolerances.
- Use soft jaws that are machined to the part’s contour for even pressure distribution.
- Use strategic support fixtures for thin-walled sections to prevent vibration and deflection during cutting.
- Manage Internal Material Stress:
- The raw plastic stock itself can have internal stress from its manufacturing process (extrusion, casting). Annealing the material before machining can relieve these stresses. This involves heating the material block to a specific temperature below its melting point and then cooling it slowly in a controlled oven.
- Allow for Stress Relief After Machining:
- After the bulk of the material is removed, internal stresses can re-balance and cause the part to warp. It is good practice to perform roughing operations, remove the part from the machine, let it sit for a few hours, and then re-secure it for finishing passes. This allows any stress to relax before the final dimensions are cut.
