As a representative of ARM, I was recently asked in a discussion with some other ARMO groups about whether electromagnetic induction heating could be suitable for rotomolding.
The issue was raised because of how the Emissions Reduction Plan of New Zealand may limit rotomolders’ ability to use gas to power their rotomolding machines. Today there is increased pressure, around the world, to limit carbon dioxide emissions. Therefore alternative heating systems for rotomolding machines is becoming a pressing subject; it’s worth thinking about now because, in the future, manufacturers in other territories may face similar limitations to the ones being proposed in New Zealand.
Although rotomolding is somewhat of a niche within general industrial manufacturing, we use A LOT of gas, relative to what we do. It has been estimated (in a study by Queens University Belfast) that only about 8% of the energy used by a conventional roto machine heats and melts polymer. Approximately 7% is used to heat up the mold. Where does the rest go? According to the same study, this is what happens:
- 25% Is wasted due to incomplete combustion
- 24% Goes up the flue
- 12% Escapes thru gaps in walls and roof
- 10% Escapes thru door opening and closing
- 9% Is required to heat the arm (mostly lost during cooling)
- 5% Is from convection losses
85% Total Wasted Energy
This study was undertaken several decades ago and modern machine designs are probably somewhat more efficient. In addition, as you load more molds on your arm, the heat utilization tends to improve; obviously there are other practical limitations to this.
Electrical heating of the mold surface has already been developed by at least two European companies; Persico and AMS. These companies use electrical elements embedded in the mold wall; this helps much more of the input energy to be “useful” (ie heating polymer and mold).
No electrical heater is 100% efficient. Standard resistive heaters are reckoned to be about 65-70% efficient. In contrast, electromagnetic induction heaters are often claimed to be more like 90% efficient.
Another potential advantage is faster warm-up. Induction heaters will work much more rapidly when heating molds made of a ferrous metal, so this could be an excellent option for fabricated steel molds. Aluminum molds will heat up eventually, but at a slower rate.
There is another significant benefit if you scrap your oven and go for direct tool heating. Suddenly all sorts of automation add-ons may become practical. In conventional roto machines, such items will tend to be relatively expensive, because their electronic and pneumatic components need to survive the aggressive heating and cooling cycles involved.
Here’s a simple example. Injection molds and blow moulds will routinely have automated part ejectors incorporated into their construction. Whilst these devices can be incorporated into rotational molds, they have much more robust construction and will also have more stringent maintenance needs. Hence higher cost. All this disappears if the components are not being continually heated to 500ºF and then cooled down to 100ºF.
As well as part ejection, we can also carry out automated parting line opening, in-mold pressurization, inert atmospheres (for sensitive materials), drop boxes for multi-layers, in-mold temperature monitoring and even in-mold visualization using webcams. Everything is much easier if it’s carried out at ambient temperature.
Process automation also means that some things may be much easier than they are with our current oven-based systems. For example, we can more easily manipulate mold rotations and heat distribution.
Labor savings are another obvious advantage. A few years ago I was molding flowerpots on a conventional roto machine that was situated alongside a fully automated system. The sound of parts dropping off the back of the auto machine, effectively with zero labor involvement, was an interesting contrast to the toil and sweat I was involved with next door!
To be fair, there are some challenges with the current generation of automated machines.Understandably, the cost of molds is significantly higher, due to their added complexity. For some, an even bigger barrier is that none of today’s systems can use existing legacy tooling; this is likely to be a particular problem for custom molders, who have the challenge of convincing their OEM clients to invest in new, more expensive, tooling.
Let’s remember that, historically, the main driver in the growth of our process has been that molds for roto have traditionally been of simple construction and relatively low cost. If you end up with a tooling cost that approaches injection or blow molding prices, why would you choose roto over these other, much quicker, alternatives?
I suspect that, like many new technologies, these disadvantages will be further addressed as the market for automated roto machines expands. I am confident that the machinery pioneers who have blazed this trail for us will get their just rewards in the end.
So could we perhaps turn a climate necessity into a major opportunity? This industry that we all love has proved itself to be extremely resilient and adaptable to change.
One of my jobs, as ARM Technical Director, is to signpost the future and suggest pathways to success. Hopefully this Blog will move some of your thoughts towards a future where we use much less gas.
If you have anything to add to this debate, please do contribute your ideas and opinions.
Dr Nick Henwood serves as the Technical Director for the Association of Rotational Molders. He has 30 years-plus experience in rotomolding, specializing in the fields of materials development and process control. He operates as a consultant, researcher and educator through his own company, Rotomotive Limited, based in UK.