Flash, burrs and moulding deformation rarely have one fixed shape or size — they vary between batches, moulding tools and even individual cycles. That irregularity is exactly why simple threshold- or template-based approaches fail where a model trained on representative examples handles it better.
In this scenario, the diversity of training data matters most — the model needs to see enough natural process variation to avoid confusing an acceptable deviation with an actual defect. That is why the pilot dataset (up to 500 annotations) deliberately includes examples from different batches where available.
Edges and geometry are also often harder to image cleanly than a flat surface — so assessing lighting and optics is often the first step here, before any model training begins.