Taking charge of dust control

It was in 600BC that the philosopher and mathematician Thales of Miletus first described rubbing amber on the fur of a cat and consequently being able to pick up feathers. This is the first known account of the natural force of static electricity and the word electrostatic (meaning electricity at rest) was introduced.

When a material holds a net electrical charge, either positive or negative, it is said to have a static charge. In many cases this charge will decrease slowly with time, the actual length of time being dependent on the resistance of the material. The two extreme examples can be taken as plastics and metal. Plastics generally have high resistivities, allowing them to maintain static charges for long periods. On the other hand, metals have low resistances and an earthed metal object will hold its charge for only a very short period of time. At rest, static charge poses a potential contamination problem. Once a surface is charged, it will attract and hold small particles in the air having an opposite electrical charge.

Particle attraction

Static causes problems in many manufacturing environments, and even in the most stringent cleanrooms static charge attracts particulates from people, processes and equipment. It is important that appropriate measures are taken to ensure that static is kept to a minimum, if not completely eliminated.

The control of static is a major issue within cleanroom environments. In the electronics industry, for example, voltages as low as five volts can cause catastrophic failure of components, or worse, latent damage that results in field failure. This can be very costly in terms of repair and manufacturers’ reputations.

The problem is predominantly addressed by the use of conductive or static dissipative materials in conjunction with ionisation. Ionisation inhibits the build-up of static charges by delivering balanced ionised air to the target surfaces. This, in turn, prevents electrostatic discharges (ESD) and the electrostatic attraction of airborne particles. If static is not controlled, it results in contamination and damage to components, which may include semiconductors, PCBs, medical devices and thin film products directly, as well as interference with the operation of vital production equipment.

It is not just electronics cleanrooms that suffer from static. Sovrin Plastics, one of the UK’s leading plastics injection moulders, has introduced stringent static control technology from Meech International to ensure that all its products reach the strict quality standards demanded by its customers around the world.

Sovrin Plastics, based in Slough, has been established for more than 40 years. The company has extensive experience of technical tool-making, moulding, production and assembly work.

Its impressive 6,500m2 site houses six Class 7 (10,000) cleanrooms (1600m2), technical plastic injection moulding and state-of-the-art CAD/CAM and CNC tooling divisions that enable Sovrin to offer a complete design and manufacturing package for cost-effective, quality engineered solutions in plastic injection moulding.

Higher standards

Over the years the company has seen its market share grow substantially, with many new opportunities arising. One of these was the supply of medical and pharmaceutical products for the Japanese market. This new project required Sovrin to achieve even higher quality standards than were already being maintained throughout the company.

The challenges posed by opening up this new market demanded that Sovrin look afresh at its total production and assembly processes to create products that would pass the most rigorous inspection under magnification.

Sovrin’s products are manufactured and assembled in Class 10,000 cleanrooms, but even in these conditions particles, for example skin particulate and fibres, can be deposited despite the use of full protective clothing. The plastic injection process that moulds the products creates a static charge on the items and this attracts any particles that are present, which then cling to the plastic surface. In order to reduce particulate contamination to the lowest possible level, Sovrin elected to apply a complete static control policy to the production process.

One of Meech’s team of technical advisors visited the plant in order to gain a full understanding of the issues and recommend the most appropriate solutions. The phased implementation included the provision of Meech Series 200 ionising blowers to be positioned over each conveyor belt to remove static charge from the moulded parts as they are ejected from the moulds.

Ion balance

Meech 200 Series ionising blowers have been specifically designed to generate very large quantities of both positive and negative ions to give fast and effective control of static charge, while their “whisper” quiet operation makes them ideal in the cleanroom environment. The Meech overhead blowers installed at Sovrin provide good workstation coverage with automatic ion balance. They provide very rapid decay times and are easy to clean and maintain with replaceable titanium emitters. Visual and audible alarms warn if the pins are dirty or the ions are out of balance. Automatic shutdown options are also included.

Once the parts are ejected from the mould, the conveyor then deposits them into stainless steel containers that are lined with anti-static polyethylene bags. Each bag is sealed inside another bag for transportation to the assembly facility. Assembly work is carried out in Class 10,000 cleanroom conditions, but the potential for particulate contamination is significantly reduced by use of Meech bench-mounted ion nozzles.

These are positioned to blow ionised air across the products to neutralise any static electric charge. Any displaced particles are trapped on tacky mats placed both on the benches and on the floor. These mats are changed every two hours.

Meech 271 Flexible Ion Nozzles are compact, hands-free ionisers designed specifically for assembly and cleaning tasks. Their exceptionally quiet operation makes them ideal for bench-top use. They are also easy and safe to use. With low air consumption, rapid decay times and replaceable titanium emitters, the nozzles are cost-effective and easy to maintain.

The result of the implementation of these static control solutions means that Sovrin is now answering the quality requirements of the strict Japanese marketplace.

“Contamination will always be a concern,” says Paul Basten, Sovrin’s production manager, Cleanrooms. “However, being able to call on the expertise at Meech International means we are confident that any issues can be overcome.”

As business grows for Sovrin it is installing new injection moulding equipment for a major pharmaceutical project. Meech will again be the partner of choice.

There is no doubt that static in cleanrooms is a major issue. However, by implementing a planned and structured static control strategy the harmful effects can be kept to a minimum while both product integrity and company reputations can be maintained. Furthermore, it has the added benefits of increasing productivity, reducing the number of rejects and raising quality levels.