How to Prevent Cracking in Large Diameter POM Rods – Causes and Solutions
Why do larger POM rods crack while smaller ones don’t?
POM (polyoxymethylene), also known as acetal, is widely used due to its excellent mechanical properties and durability. Smaller diameter rods, such as up to 50 mm, rarely experience cracking issues. This is because smaller rods cool more evenly and faster, reducing the risk of internal stresses.
In contrast, larger diameter rods, such as 100 mm, often face uneven cooling. The outer layer cools faster than the core, leading to:
- Internal stresses caused by temperature differences.
- Non-uniform crystallization, weakening the material’s structure and increasing the risk of cracks.
Below are the most common causes and economical, practical solutions to address this issue.
1. Uneven Cooling of POM Rods
Problem:
In larger rods, the surface cools much faster than the core during extrusion. This results in material shrinkage and internal stresses, often leading to radial cracks.
Solution:
✔️ Apply gradual cooling:
- Use warm water (60–80°C) instead of cold water during the initial cooling phase.
- Gradually reduce the water temperature in subsequent cooling zones.
- Consider adding an air bath before water immersion to limit abrupt temperature changes.
2. Rapid Crystallization in the Core
Problem:
POM, especially homopolymer types, crystallizes quickly, leading to non-uniform crystalline structures and weakened cores in larger rods.
Solution:
✔️ Lower processing temperatures:
- Cylinder temperature: 190–210°C instead of 210–230°C.
- Die temperature: 10–15°C lower than the cylinder.
✔️ Short-term annealing to relieve stress:
- Test short-term heating at 80–100°C, followed by slow cooling to room temperature.
- The exact duration of annealing depends on multiple factors, so experiments are recommended to optimize parameters.
3. Material Properties – Homopolymer POM
Problem:
Homopolymer POM, while offering higher stiffness, is more brittle than copolymer POM. Large diameters amplify internal stresses, increasing the likelihood of cracking.
Solution:
✔️ Consider switching to copolymer POM, which:
- Has better crack resistance.
- Is less prone to internal stresses.
✔️ Alternatively, add impact modifiers or plasticizers to improve the flexibility of homopolymer POM.
4. Extrusion Process Parameters
Problem:
Excessive pressure or temperature during extrusion can lead to overheating and weaken the material’s structure.
Solution:
✔️ Optimize process parameters:
- Reduce extrusion speed for more even cooling and crystallization.
- Adjust cylinder and die temperatures to ensure a gradual temperature drop.
- Lower die pressure to avoid layer separation inside the rod.
5. Moisture in Raw Material
Problem:
Even small amounts of moisture in POM granules can create microbubbles, weakening the rod structure and leading to cracks.
Solution:
✔️ Dry the material properly:
- Pre-dry the granules at 80–90°C for 3–4 hours.
- Store raw material in dry conditions using desiccant air dryers in the feeding system.
Summary
Cracking in large diameter POM rods is a complex issue caused by:
- Uneven cooling,
- Rapid crystallization,
- Material properties, and
- Improper extrusion parameters.
Proposed Solutions:
✔️ Gradual cooling – Avoid sudden temperature drops by using warm water and stepwise cooling.
✔️ Stress-relieving annealing – Test short-term heating at 80–100°C, followed by slow cooling to prevent internal stress buildup.
✔️ Process optimization – Adjust temperatures, extrusion speeds, and pressures to avoid overheating and stress concentration.
✔️ Material selection – Consider copolymer POM or impact-modified grades to improve durability.
✔️ Drying raw materials – Ensure the material is moisture-free before extrusion.
These practical and economical methods can significantly reduce the risk of cracking in large diameter POM rods and improve product quality.