Chapter 39 is built in two halves: primary forms — polymers as granules, powders, liquids — in headings 3901 to 3914, then semi-finished products and articles through to 3926. Knowing which half a good belongs to settles most cases.
The trap is that "made of plastic" is not a classification criterion. Plastic furniture belongs to Chapter 94, a toy to Chapter 95, a shoe to Chapter 64. And the boundary with rubber in Chapter 40 turns on vulcanisation, not on how flexible the finished article feels.
Every number opens on the lines beneath it, down to the ten-digit classification number.