The technology behind Denby
07 August 2008
Clay was discovered at Denby in Derbyshire, UK in the early 1800s and pottery has been made on the site since 1809.
Although the principles of production are the same as 200 years ago, today’s processes would not be recognised by Denby’s founding fathers. What was very much a hands-on manual industry based on craft skills and subjective assessments has become highly automated, with every pertinent parameter monitored and measured.
Like cooking, quality begins with ingredients and Denby is passionate about using the best clay. This is made into slip by mixing it with water in the cylinder house. Each mixer takes 1.3 tonnes of clay and 200kg of water and is lined with stone cobbles. It is driven through a 2.2m steel ring and nylon spur gear.
‘The nylon gear teeth require a soft starting effect, so we use a Mitsubishi inverter. There are two gear tooth sensors and a single bolt in the ring,’ explains John Spriggs, site automation engineer. ‘The position of these gives us an A,B and Z quatrature encoder.’
The pulse train feeds into a tiny Mitsubishi FX1S PLC (programmable logic controller), the sole means of control for this mighty machine. The PLC runs the mix sequence, the inverter to set the speed of rotation and a Mitsubishi E50 HMI (human machine interface) for local diagnostics.
‘It amuses me that this monster of a machine is controlled by such small components,’ chuckles John. ‘You might think that they wouldn’t be up to the job, but they’ve been working away for some while now – and there are no plans to replace them yet.’
Part of the slip blending process is the acid dosing station. This was relocated in January 2008 to reduce corrosion and has become relatively complicated because it is in fact two parallel tank systems, yet it too is controlled by a local FX PLC and E50 HMI, a combination that gives the fine recipe control required.
Next stop is an effluent treatment panel, where a slightly larger FX2N does the honours alongside another E50. The slip is then dewatered, and as Spriggs says: ‘This is an energy hungry process, which is why we try to be accurate with the initial water dosing. There is one more pairing in the clay preparation process – controlling a magnetic filter that removes iron and cobalt impurities that if left in would leave splots on the finished product.
‘Denby is the last pottery in the country to mix its own slip, clay and glazes on site. It’s critical to our quality standards. Yet the whole process is controlled by controllers that would sit in the palm of your hand.’
The glaze mixing processes are similar to the slip mixing, except the quantities are an order of magnitude smaller and there is approaching 500 different recipes to cover all the different colours and finishes.
‘We’ve got six tanks in the glaze department, all connected via Mitsubishi MX4hmi software. Overall control used to be via a PC, but that did not take to the rigours of the shop floor, so we have recently replaced it with one of Mitsubishi’s Q series PAC (programmable automation controller)’
With the slip and glazes ready, it’s off to the moulding areas. Denby has dozens of ranges of tableware, cookware, and tea sets, and each range consists of any number of bowls, plates, cups, mugs, trays, casseroles, tureens, lids, teapots, coffee pots, sugar bowls, flasks etc. Despite this many of the processes are highly automated.
Round items are perhaps the most automated in production. Typically the clay is extruded and cut into precision-sized blanks with a moving head wire cutter. These are lifted onto moulds, spun and compressed into shape. They will then be conveyed through a microwave dryer where the pot is dried sufficiently for demoulding. The conveyor carries on, perhaps via an infrared dryer, to a hot air dryer.
Again it is usual to have two identical systems running in parallel. This offers several advantages: two products can be handled simultaneously; large runs can be handled at double speed, and each side in turn can be taken down for maintenance, resetting or repair so that production never grinds to a halt.
‘These discrete part moulding processes are quite different from the bulk liquid processing in the materials preparation stage, something that is reflected in the control cabinets,’ John reflects. ‘Typically a making machine may have a dozen axes of motion in each side, with a mixture of induction, stepper and servo motors, plus any number of sensors, fans, switches etc.
‘For many years we have favoured all-Mitsubishi control solutions, and the A-series mid-to-large range PLCs have been the stalwarts of our making machine control panels. Often we’ll also incorporate an FX or two for dedicated control of a particular function, such as foot polishing – an abrasive process to smooth the items’ bottoms to perfection so that they don’t scratch customers’ dining tables.’
Tiny Mitsubishi Alpha controllers pop up all over the factory for ‘relay replacement’ applications, often programmed by the electrician installing the unit. John also likes to use Alphas on the glaze spray flow lines. Once again, it’s a simple job, but critical for the level of perfection expected by and of Denby.
The final stage in pottery production is firing, mostly using tunnel kilns. Denby has four tunnel kilns, typically with one PLC controlling the burners and another controlling the haulage. Both PLCs will feed into an HMI so that the shop floor staff are kept fully aware of every little nuance of control in this critical stage.
‘The tech spec for a pottery kiln sounds like it could pass muster at NASA,’ says John. Our newest one is 120m long and its expected to run continuously for the next ten years at least. Our specification for kiln use is the Q-series PAC.’
With the newest kiln coming on stream, its predecessor has been taken out of service and, once it has cooled, will have its control system upgraded.
‘It was still running with its original relay controls, but we are going to bring it right up to contemporary spec with a Q-series system. This is an exercise already successfully completed on our oldest tunnel kiln.’
John and his colleagues have the challenge of replicating the achievements of their forebears and keeping the Denby plant up to the highest standard of modern control engineering, whilst never disrupting production or compromising quality or undermining the heritage of the brand.
‘We have several plans in formulation. We will be integrating the different parts of the plant with an MES (manufacturing execution system) – no doubt based on Mitsubishi kit, and we are just discovering the delights of robots for a variety of tasks, including machine loading and unloading. We expect this to be a role where Mitsubishi robots will continue to figure in the future. And of course we have to do our bit towards the environment, so we are having a big push on energy management.’
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