𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞 : 𝐂𝐫𝐚𝐳𝐞 → 𝐂𝐫𝐚𝐜𝐤 → 𝐁𝐫𝐞𝐚𝐤

𝐓𝐨𝐝𝐚𝐲'𝐬 𝐊𝐍𝐎𝐖𝐋𝐄𝐃𝐆𝐄 𝐒𝐡𝐚𝐫𝐞

𝐂𝐫𝐚𝐳𝐞 → 𝐂𝐫𝐚𝐜𝐤 → 𝐁𝐫𝐞𝐚𝐤

When we see a broken plastic part, our attention immediately goes to the fracture. That is the visible failure. The moment when the component can no longer perform its function.

But did the failure really start there? In many polymers, the answer is no.

The process often begins with a craze, a localized region where microvoids and fibrillar structures start to develop under stress. At this stage, the part is usually still functional. Sometimes, the only visible sign is a slight whitening of the material.

As loading continues, these regions can grow and eventually develop into cracks. Once a crack forms, the remaining life of the component becomes significantly shorter, until the final stage is reached: fracture.

What makes this particularly interesting from an injection molding perspective is that the origin of the problem is not always found in the application itself.

Residual stresses from molding, molecular orientation, weld lines, sharp geometries, material selection, and processing conditions can all influence when and where the first craze appears.

This is why failure analysis should never start with the question:

"Why did the part break?"

A better question is:

"When did the damage actually begin?"

Because the fracture is often only the final chapter of a story that started much earlier.

*Image source:

Figure adapted from "Correlating processing induced orientation with tensile properties for mass polymerized acrylonitrile butadiene styrene test specimens", RSC Applied Polymers (2024), Fig. 4 – Principle of crack and craze formation and shear yielding in rubber-toughened thermoplastic polymers.

source : Krstina Janković

#injectionmolding #polymerprocessing #INPACT