- 16 November 2006 -

Pure water for blemish-free UPVC

For manufacturers of UPVC products, the slightest blemish or scratch on highly polished surfaces means that a valuable product must be scrapped and reworked. Filters provided by UK-based Industrial Purification Systems have to supply cooling water that is pure enough to leave the material alone.

Liverpool, UK-based Industrial Purification Systems currently leads the field in the provision of filter technology for UPVC window/conservatory extrusion industry, and serves over 94% of the UK UPVC capacity.

The UPVC window and conservatory extrusion process dictates that cooling water be supplied in large volumes and at a constant temperature in order to maintain profile accuracy. In recent years there has been significant capital invested in the need to achieve ever increasing quality standards to give the ultimate finish. This in turn has led to the spotlight being placed on water quality.

Industry-specific problems

The extrusion industry has a number of unique and complex processing problems where water used can be contaminated from a number of sources, some of which are controllable, others less so.

Many processes use plastic polymer as a base product and, on occasions, this fine dust can be spilt, releasing it into the general atmosphere. Such contamination generally ends up in the water return system, which is often located at floor level. This results in high levels of contamination within the water. If left, this contamination would, within a matter of hours, present itself as a major production problem.

Water is generally used in the wet calibration section which is continually flooded with pressurised chilled water, usually between 10-14 o C. The volume used for extruder plants is considerable and can typically range from 40,000 litres per hour to 500,000 litres per hour.

The solids, combined within the water, are passed along through the chillers, and it is common for chillers heat exchanges to become blocked. Small particulates of raw polymer can also get trapped between the profile and the calibrator tool, resulting in scratching. If particles as small as 20 microns - too small to be visible - should then settle in between the molten UPVC as it is being shaped and the stainless steel plates used to shape it, then scratches will result. This is not just a single scratch but often multiple parallel tracks running the length of the profile resulting in many metres of profile having to be scrapped.

A damaged section might be 40 metres long, and will need to be cut into pieces and then reduced to granules in order to be reprocessed. This in turn produces plastic swarf, which further exacerbates the problem as this is washed away by the water to the cooling side of the process. This cycle can be unpredictable and variable in the amount of solids found in the system at any specific time.

Exact filtration required

All of the process water flow needs to be filtered to exact standards, usually to 50 micron - in some production plants the filtration has been upgraded to 20 micron. However, many filter systems fail to meet the performance demanded by manufacturers and their clients.

"It's a major problem - but there's a relatively simple solution," says Steve Cupples, MD of Industrial Purification Systems. "You have to filter the impurities out of the water before they get into the wrong places and ruin the product. The difficulty is that the volumes of water involved are considerable (typically 120,000), but also the volumes of dust and contaminant particles in the water are unusually high, rendering many traditional filtration systems useless.

Conventional screen filters clog up too easily, and have to be constantly cleaned, which is inefficient. In a large scale operation where contamination levels are high to begin with, it can be necessary to clean the filters several times an hour to get the water to a basic level of cleanliness... and then you still have to keep filtering it to get rid of the dust that accumulates in the water."

The solution could be a modular filtration system called SpinClean which IPS first introduced to its clients in 1990. It is capable of trapping particles as small as 7 microns and as large as 600 microns - about four times the thickness of a human hair - and is expandable to cope with volumes of water from 3000 litres per hour up to 20 million litres a day.

Each filter module contains a number of grooved nylon discs of either 2 or 3 inches diameter, which are compressed tightly together as water is forced to take a 'tortuous path' over them, squeezing through the grooves on the disc surfaces. These grooves get narrower at the centre, meaning that at some point, a particle will find itself stuck in a groove, and even if it breaks up into smaller particles, the grooves continue to narrow until the smaller particles are caught too.

"Whereas a conventional screen filter has only one chance to stop a particle, SpinClean has 32," says Cupples. "Even if particles break up, the debris is retained in the filter matrix, so there's no escape until the cleaning cycle cuts in."

At regular int ervals, several times an hour if necessary, the filter modules are cleaned in an automated sequence. Each module is automatically decompressed, releasing the discs which then spin rapidly, using centrifugal force to dislodge trapped impurities as the m odule is backflushed and drained.

Once a module's cleaning cycle is com plete, which takes no more than 10 to 15 seconds, it resumes filtration, and the next module in the sequence is cleaned, meaning that production is uninterrupted, as at no point does filtration cease. Waste water is minimal, usually less than 1% of flow.

In recent years many of IPS clients have incorporated the IPS backwash recovery system, which enable a "ZERO" loss capability, all the backwash water is re used. This means no loss of valuable cooling capacity o r treatment chemical, the company says.

Application of SpinClean technology

Veka Plastics in Burnley asked IPS to install SpinClean in 2002, and MD Peter Eatough says that the benefits have been clear from the beginning.

"We get much less surface damage than before the system was introduced. Our energy wastage has been dramatically reduced and our efficiency and quality has increased accordingly. In an industry so highly competitive and price sensitive, it's vital to save every single penny we can in production efficiencies."

At another client's plant, prior to installation, the plant had a scrap rate of 15% per line, which was 370 kilos of scrap, or 4.6 kwatts, per hour being lost. The plant has 31 lines, which equates to 24,000 kW of energy being wasted each week. Following water filtration, the scrap rate has been reduced to less than 5%, which equates to only 8,000 kW. This is an energy saving of 16,000 kW per week. Costwise, at a scrap rate of 15%, approx. 500 tonnes of product is lost per year and at a selling cost of £10 per ton, this is effectively £5000 profit down the drain, IPS says.

Cupples says that SpinClean is a flexible system, adaptable to the specific needs of almost any industrial process which uses large volumes of water. "This system is so well suited to the needs of UPVC manufacturers that we have already supplied it to 95% of the UK marketplace," claims Steve. "But really that's hardly scratching the surface. There's no underestimating the massive contribution to energy efficiency and cost saving that effective filtration systems such as SpinClean can make to all kinds of industries."

Since its launch into the market just ten years ago, the company has further developed this technology to enable capture of the backwash water and then re process it for return to the system resulting in a 'zero loss' system.

Recent installations of this complete new system include the Eurocell Profiles, Coba Plastics and Swift Frame plants. The benefits are massive and a return on investment can be seen in less than six months.

David Bradley from Synseal Extrusions of Sutton in Ashfield is a user of SpinClean , and has recently increased his SpinClean capacity with a fourth unit for a new production area, as he strongly believes that this technology meets the quality and efficiency demanded by the extrusion industry. Scrap rates are minimised or completely eliminated due to surface scratching. It reduces chemical use and by doing so produces less organic load. A chillers reliability is maintained as blockages are eliminated by filtration. Tool life is extended due to reduced water contamination and the stability of process temperature due to cooling of "clean" water rather than di rty water gives tighter tolerance of product also resulting in less scrap. .