CNC Machining
POM (Delrin®)
POM (Polyoxymethylene), widely known as Acetal or Delrin®, is universally recognized as the most machinable engineering plastic. Offering an incredible combination of high stiffness, low friction, and excellent dimensional stability, it easily bridges the gap between metals and standard plastics for precision gears, bearings, and intricate fluid manifolds.
Core Mechanical Properties
Unlike Nylon, which absorbs moisture and swells, POM remains highly dimensionally stable in wet environments. Below are the typical mechanical properties for machined Acetal billets.
| Property | Typical Value | Key Benefit |
|---|---|---|
| Tensile Strength | ~ 65 - 70 MPa | High stiffness and mechanical yield, making it an excellent replacement for brass or aluminum in many structural parts. |
| Coefficient of Friction | 0.20 - 0.35 | Naturally lubricious surface. Perfect for sliding mechanisms, wear pads, and unlubricated gear trains. |
| Moisture Absorption | ~ 0.2% | Significantly lower than Nylon. Tolerances remain tight even when completely submerged in water. |
| Machinability | Excellent | Produces clean, short chips rather than long stringy ribbons, allowing for ultra-fast CNC cycle times. |
Material Comparison: Homopolymer vs Copolymer
This is the most critical distinction in POM machining. "Delrin" is a specific brand of Homopolymer, while general "Acetal" is often a Copolymer. They perform differently in gas-tight and chemical applications.
| Material Grade | Centerline Porosity | Key Characteristics & Use Case |
|---|---|---|
| POM-H (Delrin®) | Yes | Homopolymer Acetal. Offers slightly higher room-temperature tensile strength, stiffness, and creep resistance. Warning: Extruded rods often exhibit "centerline porosity" (micro-voids in the center), making it unsuitable for pressurized gas/fluid manifolds. |
| POM-C (Acetal Copolymer) | No | Copolymer Acetal. Slightly softer than Delrin but offers superior chemical resistance to strong alkalis and hot water. Crucially, it has no centerline porosity, making it the perfect choice for liquid/gas routing manifolds and medical valves. |
| PTFE-Filled (Delrin® AF) | Depends on Base | Blended with Teflon® (PTFE) fibers to create the absolute lowest friction plastic available. Used exclusively for extreme-wear bearings and sliding joints where external lubrication is impossible. |
Hardcore Industrial Applications
Thanks to its dimensional stability and low friction, POM is routinely used to replace small metal parts in high-cycle electromechanical assemblies.
Precision Gears & Bearings
The high fatigue resistance and self-lubricity of POM make it the go-to material for CNC turned bushings, rollers, and highly complex gear trains in printers and automotive assemblies.
Fluid & Gas Manifolds
Because POM machines so cleanly, we can drill deep, burr-free internal intersecting holes. Using Copolymer Acetal (POM-C) ensures leak-proof pneumatic and hydraulic blocks.
Snap-Fit Fasteners
POM possesses excellent "elastic memory", meaning it can be repeatedly flexed and snapped into place without permanently deforming or fatiguing over time.
Expert Machining Tips
POM is a joy to machine, yielding beautifully clean, short chips. However, internal stresses and centerline issues must be managed by our CNC programmers:
- 1
Navigating Centerline Porosity
When machining Delrin (POM-H) rods or thick plates, the very center of the material is often porous. We specifically program toolpaths to avoid placing critical sealing features (like O-ring grooves) directly in the porous centerline, or we switch to POM-C.
- 2
Heat Control & Sharp Tooling
While highly machinable, POM is still a thermoplastic and will melt if pushed too hard with dull tools. We strictly use highly polished, sharp-edged carbide end mills combined with air blasts to clear chips instantly and maintain a pristine surface finish.
- 3
Managing Asymmetrical Machining
Like all extruded plastics, POM holds internal stress. If a design requires removing a massive amount of material from only one side, the part may bow. We employ specific roughing strategies, flipping the part multiple times to balance stress relief.
Post-Processing & Assembly
Because POM is highly resistant to chemicals and possesses natural lubricity, it requires specialized approaches for final assembly.
1. As-Machined / Bead Blasting
POM machines so well that the "as-machined" finish is typically exceptionally smooth and glossy. If a uniform, non-reflective aesthetic is required, we can apply a light glass-bead blast for a premium matte texture.
2. Mechanical Assembly (No Glue)
Due to its extreme chemical resistance and low surface energy, POM is incredibly difficult to glue or paint. Assembly must rely on mechanical fastening. We routinely machine tight-tolerance press-fits or utilize snap-fit geometries.
3. Threaded Inserts & Welding
While POM holds threads well, for high-cycle assembly, we recommend pressing or ultrasonic-welding brass threaded inserts into the plastic. POM can also be successfully bonded to itself using ultrasonic welding techniques.
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