Ask Dr. Nick: How Can We Optimize Rotomolding Machine Cycles?

Question:
How can we optimize rotational molding machine cycles? Are there software tools, numerical methods, or training resources that can help improve cycle efficiency?

Dr. Nick:
“Optimization of machine cycles” can be looked at in two ways.

The first interpretation would be optimization within each machine; balancing the three processing stages (cook, cool and unload/reload). Depending on your style of machine, plus what products you are making on each arm, this can be relatively easy or extremely difficult.

The essence of in-machine optimization is to determine the “rate determining step” (RDS) for your particular process/product combination. As a general guide:

• If you’re molding large, thick-walled parts, your RDS will be cooling.
• If you’re molding lots of small parts on a big machine, your RDS is likely to be unloading/loading.
• If you’re molding polyethylene, your RDS is very unlikely to be cooking.

Once you know which RDS applies, find ways of shortening that part of the cycle. Then reconsider if your RDS has changed and, if it has, find ways of shortening that new part of the cycle.

Note that there’s no point in speeding up cooking if your cooling (or unloading/loading) can’t keep up. You’ll simply end up with arms in a waiting mode.

The second interpretation would be optimization between machines; you might look at this to optimize your utilization of manpower and minimize operator waiting time. I’ve seen this described as “multi-machine manning,” and significant savings can be made.

Using a process optimization method, like Lean Six Sigma, you can measure what’s happening today and develop methods of improving and controlling new working practices.

For ARM members looking for more information on in-machine optimization, my webinar Process Tuning 101 covers this subject in more detail. ARM also offers training through its Academy, including programs on Operator Training, Health & Safety, Wall Thickness & Control, and Crosslinkable Polyethylene (XLPE).

You might also consider approaching some of the suppliers of rotomolding machines, who may have some level of process optimization built into their control software.

Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.