In the recycled plastics industry, PET bottle washing lines are complex and widely distributed: feeding, sorting, crushing, pre-washing, hot washing, rinsing, dewatering and drying. A problem at any stage can affect overall throughput and product quality. Many recyclers struggle with scattered I/O points, difficult retrofits, unstable water level/flow control and high maintenance costs.

In a recent upgrade project, one PET recycling plant adopted a Siemens PLC plus RB series remote I/O architecture. The line uses 1×RB-1110, 3×16DI, 4×16DO, 4×AI and 4×AO, providing centralized acquisition and control of motor start/stop, valve status, water level and flow signals along the line.

The analog modules focus on flowmeter and level measurements. Based on target levels in the hot washer and tanks, the system automatically adjusts inlet and outlet valves to form a closed-loop “visible level, controllable flow” strategy. Digital I/O handles interlocks for crushers, washers, pumps and conveyors so that when one section fails, the line can stop or bypass in an orderly way instead of collapsing as a whole.

With this retrofit, the plant has:

• Centralized monitoring of all process stages, reducing on-site inspections;

• More stable control of hot washing levels and flows, making cleanliness and energy consumption easier to quantify;

• Easier future expansions, since new equipment can be integrated simply by adding RB modules close to the field devices.

The RB series remote I/O has helped this PET bottle washing line move from “experience-based tweaking” to “data-driven control”, laying a solid foundation for future capacity increases and refined management.