Recently I had an extended correspondence with a rotomolder and I’ve included the whole email string, because it raises some common issues. For context, this operation houses rotomolding and injection molding within the same building.
Question: We use polyethylene in our rotomolding process. Occasionally, we have a mold failure which results in some product leaking from the mold, causing it to degrade due to excessive heat and resulting in smoke and fume (“a smoker”).
We wish to do some air monitoring when this occurs, but we need to fully understand the constituents in this fume, so we know exactly what we are testing for.
Dr. Nick: Polyethylene (PE) molecules are comprised solely of carbon and hydrogen atoms, linked together in long, chain-like structures. When PE degrades, the products will be compounds of these two species plus oxygen, which is present (at about a 21% level) in air.
When PE burns (combusts) in an excess of oxygen, (ie lots of air present), the products will be carbon dioxide gas (CO2) and water vapour (H2O). This will be the typical situation if you spill a small amount of PE from a vent whilst the mold is still inside an oven (especially powder). Some of the powder will become airborne, will be sucked into the burner duct by the circulation fan, will ignite when it reaches the burner flame and will breakdown into its products of combustion.
If the spill is larger, powder may will melt on the floor of the oven and will degrade differently (see below). It will also cause a nasty mess on the floor, which will need to be cleaned up, when safe to do so (ie when the oven is stone-cold and fully isolated from electricity and gas).
If an excessive quantity of powder flows into the burner duct, it will create an explosion hazard. Explosions have happened in rotomolding plants in the past, with catastrophic results. Such an incident would normally be caused by a relatively big spillage, for example tens of lbs of powder spilling out if a mold completely opens whilst inside an oven.
If there is insufficient oxygen available, the products of combustion will be different: carbon monoxide (CO), free carbon (C) and water vapour. The free carbon appears as smoke. CO is a poison and even low levels can be dangerous. Hopefully, you’re monitoring CO levels in your plant already; if not, get a meter and alarm installed immediately; this is a basic Health & Safety issue. The bigger danger of a CO release is from poor combustion in your gas burner. Even for my small gas-fired lab machine, I have a CO detector installed at my workstation.
PE can also degrade without combustion, if it is heated to a temperature above its melting point. This happens routinely in rotomolding, if a part is over-cooked. In this case, the degradation products are mainly a family of more complex compounds; they are known as “carbonyls”, because they all contain a carbonyl (C=O) bond. The most common members of this carbonyl family are aldehydes and ketones.
Aldehydes have distinctive odours and the characteristic smell that we typically experience when we approach a rotomolding machine is mainly a low-level mixture of aldehyde compounds.
I would say that detecting low levels of such complex organic compounds is outside the practical scope of most industrial facilities and would involve relatively expensive levels of instrumentation. From a safety point of view your top priority, in my opinion, should be adequate carbon monoxide monitoring.
About a month later, I received an update:
Question:Thanks for your reply, it was very comprehensive. We have since had comprehensive CO monitoring installed.
However, we still get the occasional “smoker”, and it always raises concerns. What is the best way to deal with one, once we have one? What do others do?
Dr. Nick: For dealing with an occasional occurrence, I guess you have two basic options:
1. Local high-volume extraction, close to the likely source
2. Taking additional measures to minimise the causes.
In terms of Option 2, you previously stated that there was occasional leakage of material from a mold during rotation in the oven. For this, and other good reasons, tightly fitting mold parting lines are a sensible, and achievable, objective. You can undertake the necessary rectification work, ie re-making any parting lines that have gaps in them. Gaps in parting lines will also cause defects on the molded part.
The other main potential source is the escape of powder from the rotating mold via mold vents. The direct solution to this is to use some type of media in the vent, to prevent any powder escape.
Choice of vent media is an operational choice, and you have several possibilities:
• Wire wool is a common choice but, in my opinion, not a good one. Wire wool will heat up during its time in the oven and is more likely to clog with melted powder. There have also been reports of hot wire wool causing in-mold explosions.
• Some form of temperature-resistant non-metallic wool is a better option. Standard fibreglass insulation wool may not be sufficiently temperature-resistant: maximum operating temperature is 250ºC / 480ºF. Mineral wool (eg “Rockwool”) can resist heat up to 760°C / 1,400°F, so it’s a better choice. You’ll need very little to form a suitable plug in the vent.
• Wool media should be considered a consumable item; the wool plug should be refreshed on every cycle. Partially blocked, or fully blocked, vents will create multiple problems.
• More permanent insert systems are available, which have high temperature silicone rubber components. These are convenient, but potentially more expensive. Although they will last for a number of cycles, the silicone rubber tends to embrittle eventually, and the components then need replacing.
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.
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