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Why I Wrote The Sheet Mechanic (And Why Calculations Aren’t Enough)

For engineers who already know the math—but still lose projects. For the last few years, I’ve been sharing technical guides here on Mechanical Design Handbook —how to size a motor, how to calculate fits, and (as you recently read) how to choose between timing belts and ball screws. But after 25 years in industrial automation, I realized something uncomfortable: Projects rarely fail because the math was wrong. They fail because: The client changed the scope three times in one week. A critical vendor lied about a shipping date (and no one verified it). The installation technician couldn’t fit a wrench into the gap we designed. University taught us the physics. It didn’t teach us the reality. That gap is why I wrote my new book, The Sheet Mechanic . This is not a textbook. It is a field manual for the messy, political, and chaotic space between the CAD model and the factory floor. It captures the systems I’ve used to survive industrial projec...
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Engineer's Guide to POM (Delrin): Properties & Uses

Precision machined white Delrin (POM) worm gear and bushings showing smooth surface finish

Figure 1: POM (Delrin) is chosen for parts requiring high stiffness, low friction, and excellent dimensional stability.

The "Metal Replacement" Plastic

In the world of mechanical design, POM (Polyoxymethylene), also known as Acetal or Polyacetal, is often referred to as the bridge between metals and plastics.

While standard plastics (like PVC or Polyethylene) are soft and flexible, POM is rigid, tough, and machinable. It is the go-to material for engineers looking to replace small metal components—like gears, bushings, and fasteners—to reduce weight and cost without sacrificing precision.

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The Two Types: Homopolymer vs. Copolymer

Not all POM is the same. When selecting material, you will encounter two distinct variations. Understanding the difference is critical for failure prevention.

1. POM-H (Homopolymer)

Most commonly known by the trade name Delrin® (Dupont).

  • Pros: Higher tensile strength, stiffness, and creep resistance than copolymer. Slightly harder surface.
  • Cons: Prone to "Centerline Porosity" (voids in the center of thick rods). Less resistant to hot water (hydrolysis) and strong alkalis.

2. POM-C (Copolymer)

Common trade names include Celcon® or Ultraform®.

  • Pros: Excellent resistance to hot water and chemicals. No centerline porosity (better for machining deep parts).
  • Cons: Slightly lower mechanical strength compared to Delrin.

Why Engineers Choose POM

1. Tribology (Low Friction and Wear)

POM is naturally "slippery." It has a low coefficient of friction against metals, making it self-lubricating. This makes it ideal for Gears, Bearings, and Sliding Ways where adding external oil or grease is impossible (e.g., food processing machines or printers).

2. Dimensional Stability

Unlike Nylon (PA6), which absorbs moisture from the air and swells up, POM has almost zero water absorption.
Why this matters: A precision gear made of Nylon might lock up on a humid day. A gear made of POM will hold its tolerance perfectly regardless of humidity.

3. Machinability

POM is one of the easiest plastics to machine. It cuts like brass—producing short, clean chips rather than long stringy birds-nests. It allows for tight tolerances (+/- 0.002") on CNC mills and lathes.

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Typical Industrial Applications

Because of its high stiffness and solvent resistance, POM is used in high-value industries:

  • Automotive: Fuel sender units, door lock actuators, and window lift mechanisms (due to fuel/oil resistance).
  • Medical: Insulin pens and inhalers (due to smooth sliding action and biocompatibility).
  • Consumer Electronics: Keycaps for mechanical keyboards and internal gears in printers.
  • Food Industry: Conveyor links and milk pumps (POM-C is widely FDA compliant).

Design & Processing Tips

For CNC Machining:
Keep tools sharp! Dull tools cause heat buildup. POM is sensitive to heat; if it gets too hot during machining, it can release formaldehyde gas. Always use coolant when drilling deep holes.

For Injection Molding:
POM has high shrinkage (around 2-3%). Designers must account for this in the mold design. Avoid uneven wall thickness to prevent warping and sink marks.

Summary of Properties

Property Value (Approx)
Density 1.41 g/cm³
Tensile Strength 60 - 70 MPa
Melting Point 165°C (Copolymer) / 175°C (Homopolymer)
Working Temp -40°C to +100°C

References

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