Feeling the heat - processing thermoplastics for re-use
A new thermal fluid-based heating and cooling system that could work effectively at the upper and lower ends of the heating cycle has been developed. Chris Horsley from Babcock Wanson explains
The world is littered with waste plastics. The UK alone uses more then five million tonnes of plastic each year, with just 20-25% recovered or recycled. The vast majority of this 20-25% applies to thermoplastics that can be repeatedly melted down and re-used. Thermosets such as epoxy-based plastics can be depolymerised, however, they cannot be re-melted or re-shaped once cured and continue to head for landfill.
Recognising the potential material source these plastics represent, 2K Manufacturing set about working out a way to reuse them – the result is Ecosheet. Made from unwanted co-mingled polymer waste they are an environmentally sustainable alternative to plywood boards used in homes and gardens. Although slightly more expensive than plywood for the initial outlay (about 10%), their lifespan is significantly longer meaning lifetime costs are reduced and the boards can also be repeatedly recycled.
Such a product has required specialist technology to produce. 2K’s solution was the powder impression moulding system, developed in conjunction with Babcock Wanson UK. Babcock Wanson’s remit was the design and build of the heating and cooling process to operate the Powder Impression Moulding (PIM) technology. Here, mixed plastics are ground into powdery flakes and the powder is spread over a polymer skin, covered with another layer, and then heated to just below their melting point so that they bond. During the process, air is blown through to create a spongy-looking core. The sheet hardens and gains strength in the cooling process. The amount of temperature change in a short cycle limits the fluids that can be used – steam or high-pressure hot water are not ideal to achieve such temperatures.
Instead, a thermal fluid heater was chosen thanks to high operating efficiency and total output. In principle, a thermal fluid heating system is much like a domestic hot water system. It consists of a heater connected to a mild steel flow and return pipework, which can provide heat to one or more users or systems. Instead of water as a heat transfer medium the system contains thermal fluid (usually a mineral oil). Since it is a closed-circuit, no-loss system, the heat losses are minimal, increasing the overall plant efficiency.
The thermal fluid system has been designed with a separate cooling circuit (using the same fluid) employing air blast coolers, each fitted with two axial fans to provide the cooling load. The ability to use a single specialist fluid for both the heating and cooling of the moulds is an essential part of the process. A single flow path in each mould is used with the hot or cold fluid being pumped through the flow path to achieve accurate temperature control throughout the cycle. Without this feature it would be necessary to have two distinct flow paths, which would mean increased costs. The alternative is to use a heating and cooling medium that would be allowed to mix during the changeover period, such as steam and water. In this case, such a solution would not be practical due to the high temperatures and steam pressures required.
Controlling fluid temperature
One of the key challenges in this installation was to source this single fluid capable of both heating and cooling the moulds. To reach these extremes of temperature while maintaining good heat transfer properties and no pressurisation, using just one heat transfer fluid is not easy. The organisation researched worldwide and used thermodynamic modelling to select suitable fluids. Eventually it found a specialist fluid that has performed extremely well.
[img_assist|nid=41900|title=|desc=|link=none|align=right|width=350|height=226]The process of refining, improving efficiency and reducing energy consumption is ongoing. When the plant is running at full capacity it will produce up to 160,000 boards a year. But when you realise the UK buys up to 30 million sheets of plywood per year – all shipped from overseas, including Brazil, Africa, China and Russia, at considerable environmental cost – there is plenty of scope for expansion.