POM polyoxymethylene is commonly called acetal, Delrin, or in some regions, Celcon. Why is it sometimes called “sai gang” (steel-like)? Because it feels hard, has a smooth surface, and after machining gives a metallic finish. Its strength and stiffness are among the highest of all engineering plastics, and in many applications it can truly replace brass, aluminum, or even low-carbon steel. However, many designers encounter problems like cracking, shrinkage, and noise during their first use of POM. This article avoids complex formulas and focuses on hands‑on experience.

1. Is “Sai Gang” different from “Chao Gang”?

No. Both are common Chinese nicknames for POM polyoxymethylene. They refer to the same material. Some old Taiwan‑owned factories used “Sai Gang”, while later mainland factories started using “Chao Gang” (super steel). There is also a non‑standard distinction that calls homopolymer POM “Sai Gang” and copolymer POM “Chao Gang”. For purchasing, always rely on the material certificate that says “POM” or “polyoxymethylene”.

2. First Selection Step: Homopolymer or Copolymer?

Bottom line: If you have no special requirements, copolymer POM is the safer choice due to its wider processing window and better resistance to decomposition.

3. Quick Reference of Common Grades

• DuPont™ Delrin® homopolymer series: Delrin 100 (high viscosity, thick walls), Delrin 500 (general purpose, gears/bearings), Delrin 150 (ultra‑high viscosity).

• Polyplastics Duracon® copolymer series: M90 (general injection grade, most popular), M25 (high flow, thin walls), M270 (medium‑high flow, complex parts).

• Celanese Hostaform® copolymer series: C9021 (general purpose, hot water resistant), C13021 (high stiffness), C2521 (low friction / wear resistant).

• Chinese domestic POM: Yuntianhua M90, Shenhua MC90 – cost‑effective for general conditions.

4. Four Most Common Processing Mistakes

1. Shrinkage and warpage: POM shrinkage is 1.5%–2.5%, much higher than ABS or PC. Pay attention to gate location and packing time during mold design. Thick‑walled parts tend to have sink marks – keep wall thickness below 4mm or use a high‑flow grade.

2. Thermal decomposition and odor: If POM stays too long in the barrel and exceeds 220°C, it releases formaldehyde gas (irritating odor), causing yellowing and silver streaks on the surface. If the machine stops for more than 10 minutes, purge the barrel.

3. Weak weld lines: Avoid long, thin weld lines in mold design, otherwise parts may break along the weld line under impact. Increasing melt or mold temperature helps.

4. Excessive fuzzing during machining: When turning or milling POM, long stringy burrs often appear. Solution: use sharp carbide tools, high speed with low feed. If needed, use cryogenic (liquid nitrogen) or water cooling.

5. Key Points for Metal Replacement Design

When replacing metal parts with POM, do not simply copy the metal dimensions and shape. Keep in mind:

• Stiffness conversion: POM modulus is about 2.6 GPa, while steel is about 200 GPa. To achieve the same bending stiffness, the moment of inertia of the POM part must be increased by a factor of 7–8 – either by thickening the wall or adding ribs.

• Thermal expansion fit: POM‘s linear coefficient of thermal expansion is about 10 times that of steel (1×10⁻⁴ /K vs. 1×10⁻⁵ /K). When assembled with a metal shaft, the clearance changes significantly at high temperatures – allow expansion room.

• Threaded connections: Direct threading in POM yields low strength. Use metal inserts or self‑tapping screw designs instead.

6. Wear and Lubrication Options

POM inherently has low friction. For extremely low friction (e.g., high‑speed gears), consider:

• PTFE‑modified POM – friction coefficient can drop to 0.05–0.10

• Silicone‑oil‑modified POM – for self‑lubricating bearings

• Glass‑fiber reinforced POM (10%–30% GF) – improves stiffness and creep resistance, but friction coefficient rises slightly

7. Quick Summary

POM acetal (Delrin/Celcon) is a highly cost‑effective engineering plastic. As long as you avoid common processing and design pitfalls, it can replace metal in many applications, reducing weight, lowering noise, and eliminating the need for lubrication.