In a previous Blog, I talked about the possibility of a gas-free future for rotomolders. Since then, we’ve witnessed the situation in Ukraine, which has added to the pressures on natural gas supply in many European countries. Even if you are based in a country with a secure gas supply, it’s highly likely that your unit cost of gas (natural or LNG) will be rising. The supply of oil and gas is a global business.
Conventional rotomolding, using gas ovens, is not an economical process, as far as energy utilization is concerned. Rising gas prices will create an extra headache that rotomolders could certainly do without.
There are a number of far-reaching strategies that rotomoulders can apply, to build a more energy-secure future. However, these don’t help much in the short term.
Is there anything helpful that you can do immediately?
YES! Here are 5 “quick fixes” that could help.
Fix 1: UNDERSTAND your cycle
Despite all the equipment that’s available to monitor the temperature inside and outside your molds, many rotomoulders still think that they can “fly blind”. Guessing which Oven Set Point and Cook Time will give you the most optimized process is strictly for those with psychic powers – the rest of us need help!
For anyone wondering which of the many systems out there will work best for their operation, here’s some good news! ARM will be running a webinar on this subject on May 19, 2022. ARM members can find the sign up link in the email newsletter.
In the meantime, there is an excellent webinar on the ARM website entitled “Seven Stages of Rotomolding”. Look for it in the “Members’ Section”. You’ll need a Username and Password to enter this area; if you don’t have one, ARM staff will be happy to help you.
Fix 2: TUNE your process
If you run a carousel machine, what proportion of the time is your oven empty, waiting for an arm to become available? If it’s more than 10% of the time, you haven’t got a well-tuned process!
If you run a two-arm shuttle machine, it’s likely to be even harder to keep your oven full.
An empty oven represents wasted gas, however you run it. If it’s programmed to maintain Set Point Temperature while empty, you will be losing heat, via the oven extraction fan, on a continuous basis. If your burner is programmed to lo-fire when the oven is empty, the whole oven structure will cool down and will need an extra burst of energy to heat it up again, when an arm becomes available.
The normal reason for an empty oven is the lack of an arm to fill it. This is because cooking is almost never the rate-determining step in a roto cycle.
If you’re molding large thick parts, the rate-determining step is most likely to be cooling. There will be a limit to how much you can speed up cooling, before you encounter warpage problems.
If your arm is full of small molds, the unloading / reloading step is probably holding you back. There may be some things you can do – applying extra labor, semi-automation etc – but there will be practical limits to this.
What to do? If you REDUCE the Oven Set Point Temperature, your burner will use less gas. Of course, your cook cycle will lengthen but, if you presently run an empty oven, you can accommodate it.
Making a change like this is really only possible if you can properly measure what’s actually going on. Which brings me back to… Fix 1.
Does this sound counter-intuitive? If you need convincing, there is another relevant webinar in the Members’ Section of the ARM website entitled “Process Tuning 101”.
Fix 3: Upgrade the performance of your BURNER
If your machine is brand new, this is probably not applicable. For the rest of us, this is well worth a look. Perhaps there is an economical retrofit which would make your burner significantly more efficient?
Or maybe your existing burner hasn’t been maintained in a while? If not… give it some love…
Fix 4: Insulate your ARMS
Really? Surprisingly, YES! Especially on a large machine.
To survive the roto process, the arms of a machine need to be robust and heavily built. So, as well as being the thing that moves your moulds around, an arm is also a great big lump of steel. We routinely heat this lump up and then cool it down immediately afterwards, which represents a lot of heat energy effectively doing… nothing. 493K did some studies to evaluate this effect, which they presented at ARM’s Montreal conference in 2010. They found that, once the cycle had settled down, the temperature of an un-insulated arm varied by approx. 80ºF from hottest to coolest state. In contrast, an insulated arm only varied by 20ºF. Taken over a period of time, after repeated cycles, this adds up to a lot of energy, which you simply don’t need to waste
Fix 5: Install a HEAT SINK
I’ve seen several molders do this, to good effect. Quite simply, they line the oven floor with firebrick, or a similarly dense material. This acts as a heat store, which will add back heat to the oven as it cools and reduce the number of times your burner is required to fire.
Obviously, you need to check that you have sufficient clearance to do this, between the fully loaded arm and the oven floor.
Hopefully there is something here that helps. As always, ARM is here to help its members. This topic is just one example of the valuable store of information and wisdom that our website contains. I really do recommend that you visit it soon.
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.